Network control of user equipment security key updates for mobility

Abstract

Embodiments include methods for a user equipment (UE) configured for mobility between cells of a radio access network (RAN). Such methods include receiving, from a first RAN node, a mobility configuration comprising: candidate configuration(s) for respective mobility candidate cell(s), and one or more counter lists. Each counter list includes counter value(s) usable for deriving secondary node (SN) security keys for communication between the UE and a third RAN node that provides a secondary cell group (SCG) for the UE. Such methods include receiving from the first RAN node a mobility command for execution of a mobility procedure to one of the mobility candidate cells provided by a second RAN node. Such methods include, in response to the mobility command, selecting a first counter value from one of the counter lists and, based on the first counter value, deriving the SN security keys for use after execution of the mobility procedure.

Description

[0001] NETWORK CONTROL OF USER EQUIPMENT SECURITY KEY UPDATES FOR MOBILITY

[0002] TECHNICAL FIELD

[0003] The present disclosure relates generally to wireless networks, and more specifically to techniques for improving mobility of user equipment (UEs) across multiple cells in a radio access network (RAN), specifically in relation to when and how UEs update security keys used for communication with the RAN in conjunction with mobility procedures.

[0004] BACKGROUND

[0005] Currently the fifth generation (5G) of cellular systems is being standardized within the Third-Generation Partnership Project (3GPP). 5G is developed for maximum flexibility to support many different use cases including enhanced mobile broadband (eMBB), machine type communications (MTC), ultra-reliable low latency communications (URLLC), side-link device- to-device (D2D), and several other use cases.

[0006] Figure 1 illustrates a high-level view of an exemplary 5G network architecture, consisting of a Next Generation Radio Access Network (NG-RAN, 199) and a 5G Core (5GC, 198). The NG-RAN can include one or more gNodeB’s (gNBs) connected to the 5GC via one or more NG interfaces, such as gNBs (100, 150) connected via respective interfaces (102, 152). More specifically, the gNBs can be connected to one or more Access and Mobility Management Functions (AMFs) in the 5GC via respective NG-C interfaces and to one or more User Plane Functions (UPFs) in 5GC via respective NG-U interfaces. The 5GC can include various other network functions (NFs), such as Session Management Function(s) (SMF).

[0007] In addition, the gNBs can be connected to each other via one or more Xn interfaces, such as Xn interface (140) between gNBs (100, 150). The radio technology for the NG-RAN is often referred to as “New Radio” (NR). With respect to the NR interface to UEs, each of the gNBs can support frequency division duplexing (FDD), time division duplexing (TDD), or a combination thereof. Each of the gNBs can serve a geographic coverage area including one or more cells and, in some cases, can also use various directional beams to provide coverage in the respective cells. In general, a DL “beam” is a coverage area of a network-transmitted reference signal (RS) that may be measured or monitored by a UE.

[0008] NG RAN logical nodes (e.g., gNB 100) include a Central Unit (CU or gNB-CU, e.g., 110) and one or more Distributed Units (DU or gNB-DU, e.g., 120, 130). CUs are logical nodes that host higher-layer protocols and perform various gNB functions such controlling the operation of DUs. DUs are decentralized logical nodes that host lower layer protocols and can include, depending on the functional split option, various subsets of the gNB functions. Each CU and DU can include various circuitry needed to perform their respective functions, including processing circuitry, communication interface circuitry (e.g., transceivers), and power supply circuitry.

[0009] A gNB-CU connects to one or more gNB-DUs over respective Fl logical interfaces (e.g., 122 and 132 shown in Figure 1). However, each gNB-DU can be connected to only one gNB-CU. The gNB-CU and its connected gNB-DU(s) are only visible to other gNBs and the 5GC as a gNB. In other words, the Fl interface is not visible beyond gNB-CU.

[0010] Access stratum (AS) security in the 5G network includes integrity protection and ciphering of radio resource control (RRC) signaling radio bearers (SRBs) and user data radio bearers (DRBs). Each gNB applies four different AS security keys: one for integrity protection of RRC signaling (KRRCint), one for ciphering of RRC signaling (KRRCenc), one for integrity protection of user data (Kupint) and one for ciphering of user data (Kupenc). All four AS keys are derived from the KgNB key of the gNB, which is based on the KAMF key handled by upper layers as specified in 3GPP TS 33.501 (vl8.0.0). RRC also handles configuration of AS security parameters such as integrity protection algorithm, ciphering algorithm, and parameters used by the UE to determine AS security keys.

[0011] 3 GPP Release 12 (Rel-12) introduced Long-Term Evolution (LTE) dual connectivity (DC), whereby a UE can be connected to two network nodes simultaneously, thereby improving connection robustness and / or capacity. In particular, a master node (MN) provides a master cell group (MCG) for the UE and a secondary node (SN) provides a secondary cell group (SCG). Each cell group includes a primary cell (PCell) and may include one or more secondary cells (SCells).5G / NR also supports DC, including NR-DC that is similar to LTE -DC except that both the MN and SN use the NR interface to communicate with the UE. In addition, 5G / NR supports various multi -RAT DC (MR-DC) scenarios in one of the MN and SN uses the NR radio interface and the other uses the LTE radio interface to communicate with the UE. AS security in the SCG is based on a separate key of the SN, denoted S-KgNB.

[0012] Seamless mobility is a key feature of 3GPP radio access technologies (RATs). In general, a RAN (e.g., NG-RAN) configures a UE to perform and report radio resource management (RRM) measurements to assist network-controlled mobility decisions, such as for handover from a serving cell to a neighbor cell. Seamless handovers ensure that the UE moves around in the coverage area of different cells without excessive interruption to data transmission.

[0013] Conventionally, serving cell change is triggered by layer 3 (L3, e.g., RRC) measurements and involves RRC signaling to change PCell and PS Cell (when dual connectivity is configured) and to release / add SCells. L3 serving cell change - also referred to as handover (HO) - also involves LI and L2 resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching. Certain L3 mobility operations may also involve changes to integrity protection and ciphering algorithms as well the AS keys KSNB, KRRCint, KRRCenc, KuPint and K jPenc-

[0014] Even so, HO and other L3 mobility operations can have various robustness problems. For example, a HO command is normally sent when UE’s connection is degraded, such as at or near cell borders. As such, the HO command may need to be segmented (e.g., to allow for redundancy to protect against errors) and / or retransmitted one or more times before it reaches the UE. The HO command may not reach the UE in time (or at all) before the degraded connection is dropped. Failure of HO to a target cell may lead to the UE declaring radio link failure (RLF) in the serving cell and reestablishing its connection in another cell.

[0015] 3GPP Rel-16 and Rel-17 support conditional HO (CHO) and other conditional mobility procedures. A main principle is that transmission and execution of a mobility (e.g., HO) command are separated. This allows the mobility command to be sent to UE when the radio conditions are still good, thus increasing the likelihood of successful reception. The UE executes the mobility command later based on an associated execution condition. These conditional mobility procedures are facilitated by a conditional reconfiguration framework in which the network provides a UE with one or more reconfigurations, each with associated execution condition(s). 3GPP Rel-18 also supports subsequent L3 mobility, in which a UE may perform multiple L3 mobility procedures without intermediate reconfiguration by the RAN.

[0016] As specified in 3GPP document RP -223520, Rel-18 includes a Work Item on further NR mobility enhancements, including in the technical area of layer-l / layer-2 (L1 / L2) based intercell mobility, also referred to as L1 / L2 triggered mobility (LTM). Conventionally, serving cell change was triggered by layer 3 (L3, e.g., RRC) measurements and involves RRC signaling to change PCell and PSCell (e.g., when dual connectivity is configured), as well as release / add SCells (e.g., when CA is configured). L3 inter-cell mobility also involves complete LI and L2 resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching. Thus, a goal of Rel-18 L1 / L2 mobility enhancements is to facilitate serving cell changes via L1 / L2 signaling to address these problems and / or difficulties.

[0017] In LTM, a UE is pre-configured by its serving RAN node with one radio resource control (RRC) configuration per LTM candidate cell, sometimes referred to as an “LTM candidate configuration”. The UE performs measurements on configured LTM candidate cells and transmits corresponding measurement reports to the RAN node, based on which the RAN node triggers execution of a LTM cell switch procedure by the UE to one of the configured LTM candidate cells. The RAN node may trigger the LTM cell switch procedure by sending the UE an LTM cell switch command. According to 3 GPP agreements, a UE may perform multiple LTM cell switch procedures without being reconfigured by the RAN. For example, after the UE has performed an LTM cell switch from a serving cell to a first target cell (e.g., a first LTM candidate cell), the UE may perform another LTM cell switch from the first target cell to a second target cell (e.g., a second LTM candidate cell) without receiving another RRCReconfiguration message in the first target cell. This second LTM cell switch is often referred to as “subsequent LTM.”

[0018] The Rel-18 L1 / L2 mobility enhancements also support the split CU / DU architecture of Figure 1, including intra-DU and inter-DU / intra-CU LTM cell switches. In the inter-DU / intra- CU scenario, the candidate cell is served by a neighbor DU to the (serving or source) DU that currently provides the UE’s PCell (or PSCell, for SCG change in DC). In the intra-DU scenario, the candidate cell is served by the same DU that currently provides the UE’s PCell (or PSCell, for SCG change in DC). In either case, the LTM candidate cell configurations and other LTM- related configurations for a UE (e.g., measurement and reporting, early UL / DL synchronization configuration, etc.) are only used within a single CU and one or more DUs of a single gNB.

[0019] SUMMARY

[0020] According to 3 GPP agreements, Rel-19 will support inter-CU LTM as well as conditional LTM, which is analogous to L3 CHO. Both of these features - along with subsequent LTM and subsequent L3 mobility - require the support of new security-related functionality that is not part of Rel-18 or prior releases.

[0021] For example, these type of mobility operations create some additional complexity for security in the UE’s SCG. In order to derive S-KSNB, the UE needs to use a secondary key counter (sk-counter) that is generated and sent to the UE by the MN the provides the UE’s MCG. The sk- counter is single-use information, such that it is invalid after being used by the UE to derive an S- KgNB. However, this creates difficulties for subsequent LTM and subsequent L3 mobility operations that are expected to be performed without intermediate intervention by the MN. Similar problems and / or difficulties may exist for subsequent CHO and subsequent conditional LTM.

[0022] An object of embodiments of the present disclosure is to facilitate a UE’s unambiguous determination of whether security key changes are needed in relation to a previously received configuration, such as by providing, enabling, and / or facilitating solutions to overcome exemplary problems summarized above and described in more detail below.

[0023] Embodiments include methods (e.g., procedures) for a UE configured for mobility between cells of a radio access network (RAN, e.g., E-UTRAN, NG-RAN).

[0024] These exemplary methods include receiving, from a first RAN node via a serving cell, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells; and one or more counter lists, with each counter list including one or more counter values usable for deriving a plurality of SN security keys for communication between the UE and a third RAN node that provides an SCG for the UE. These exemplary methods also include receiving from the first RAN node a mobility command for execution of a mobility procedure from the serving cell. The mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure. These exemplary methods also include, in response to the mobility command, selecting a first counter value from one of the counter lists and, based on the first counter value, deriving the plurality of SN security keys for use after execution of the mobility procedure.

[0025] In some embodiments, the plurality of SN security keys include a master SN security key (e.g., KSN or S-KSNB) and the following derived from the master SN security key: a first SN security key for integrity protection of signaling, a second SN security key for ciphering of signaling, a third SN security key for integrity protection of user data, and a fourth SN security key for ciphering of user data. In some embodiments, the mobility command also includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0026] In some embodiments, these exemplary methods also include transmitting to the second RAN node via the target cell, a message indicating that the mobility procedure is complete. In some of these embodiments, these exemplary methods also include deriving a plurality of MN security keys usable to secure communication with the second RAN node after execution of the mobility procedure. For example, one or more of the MN security keys may be used to derive the plurality of SN security keys.

[0027] In some embodiments, the one or more counter lists include a single counter list usable for all MNs for the UE, and the first counter value is selected from the single counter list. In other embodiments, the one or most counter lists include a first counter list usable for the first RAN node and a second counter list usable for the second RAN node, and the first counter value is selected from the second counter list.

[0028] Other embodiments include exemplary methods (e.g., procedures) for a first RAN node configured to facilitate mobility by UEs between cells of a RAN. In general, these exemplary methods can be complementary to the exemplary methods for a UE summarized above.

[0029] These exemplary methods include sending, to a UE via a serving cell, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells; and one or more counter lists, with each counter list including one or more counter values, with each counter value being usable for deriving a plurality of SN security keys for communication between the UE and a third RAN node that provides an SCG for the UE. These exemplary methods include sending to the UE a mobility command for execution of a mobility procedure from the serving cell. The mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure. The one or more counter lists facilitate UE derivation of the plurality of SN security keys for use after execution of the mobility procedure to the target cell.

[0030] In some embodiments, these exemplary methods also include sending to the second RAN node a notification of the mobility procedure for the UE from the serving cell to the target cell provided by the second RAN node. The notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0031] In some embodiments, these exemplary methods also include receiving from the second RAN node a notification of a second mobility procedure for the UE from a serving cell provided by the second RAN node to a target cell provided by the first RAN node. The notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0032] Other embodiments include exemplary methods (e.g., procedures) for a second RAN node configured to facilitate mobility by UEs between cells of a RAN. In general, these exemplary methods can be complementary to the exemplary methods for a UE and for a first RAN node, summarized above.

[0033] These exemplary methods include sending to a first RAN node a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells provided by the second RAN node. These exemplary methods include receiving from the first RAN node a notification of a mobility procedure for a UE from a serving cell provided by the first RAN node to one of the mobility candidate cells as a target cell. These exemplary methods include receiving from the UE a message indicating that the mobility procedure is complete. The message includes status information for counter values usable by the UE for deriving a plurality of SN security keys for communication between the UE and a third RAN node that provides an SCG for the UE.

[0034] In some embodiments, the status information includes one or more of the following::

[0035] • a first counter value used and discarded by the UE in conjunction with the mobility procedure, or an identifier thereof;

[0036] • a list of counter values used and discarded by the UE, or identifiers thereof;

[0037] • a number of unused counter values available to the UE;

[0038] • a list of unused counter values available to the UE, or identifiers thereof;

[0039] • an indication that no unused counter values are available to the UE; • a request for additional counter values; and

[0040] • a request for a specific number of additional counter values.

[0041] In some embodiments, the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0042] In some embodiments, the mobility configuration includes a second counter list including one or more counter values and each counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node, but only while being served by the second RAN node.

[0043] In some embodiments, these exemplary methods also include sending to the UE a second mobility configuration comprising a further counter list that includes one or more further counter values. Each further counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node. In some of these embodiments, the further counter list facilitates UE update of a counter list provided by the first RAN node and stored by the UE.

[0044] In some embodiments, the first and second RAN nodes are different CUs of a single RAN node. In other embodiments, the first and second RAN nodes are different RAN nodes. In various embodiments, the mobility procedure (and the second mobility procedure) can be any one of various conditional or non-conditional mobility procedures, including LTM and L3 mobility procedures.

[0045] Other embodiments and variants of the exemplary methods summarized above are described herein. Other embodiments include UEs (e.g., wireless devices) and RAN nodes (e.g., base stations, eNBs, gNBs, ng-eNBs, etc.) configured to perform operations corresponding to any of the exemplary methods described herein. Other embodiments include non-transitory, computer- readable media storing program instructions that, when executed by processing circuitry, configure such UEs and RAN nodes to perform operations corresponding to any of the exemplary methods described herein.

[0046] These and other embodiments described herein may provide various advantages, benefits, and / or solutions to problems. For example, embodiments may facilitate multiple UE mobility operations (e.g., LTM cell switch) that require both SN and MN security key updates, without need for intermediate reconfiguration or other RAN intervention. As such, embodiments may facilitate necessary updates for encryption and integrity protection as the UE moves between cells in the RAN, thereby increasing security of signaling and user data carried by radio bearers secured in this manner. These and other objects, features, and advantages of embodiments of the present disclosure will become apparent upon reading the following Detailed Description in view of the Drawings briefly described below.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 shows a high-level view of an exemplary 5G / NR network architecture.

[0049] Figure 2 shows a logical architecture for an NG-RAN node arranged in a split CU / DU architecture.

[0050] Figure 3 shows exemplary NR user plane (UP) and control plane (CP) protocol stacks.

[0051] Figure 4 shows a signaling diagram for an exemplary CHO procedure.

[0052] Figure 5 illustrates security key derivation for HO and other UE mobility procedures.

[0053] Figure 6 shows a signaling diagram for an exemplary LTM cell switch procedure.

[0054] Figure 7 illustrates a communication system in which some embodiments of the present disclosure may be implemented.

[0055] Figures 8-11 show signaling diagrams for exemplary LTM cell switch procedures, according to various embodiments of the present disclosure.

[0056] Figure 12 shows a flow diagram of an exemplary method for a UE (e.g., wireless device), according to various embodiments of the present disclosure.

[0057] Figure 13 shows a flow diagram of an exemplary method for a first RAN node (e.g., base station, eNB, gNB, DU, etc.), according to various embodiments of the present disclosure.

[0058] Figure 14 shows a flow diagram of an exemplary method for a second RAN node (e.g., base station, eNB, gNB, DU, etc.), according to various embodiments of the present disclosure.

[0059] Figure 15 shows a communication system according to various embodiments of the present disclosure.

[0060] Figure 16 shows a UE according to various embodiments of the present disclosure.

[0061] Figure 17 shows a network node according to various embodiments of the present disclosure.

[0062] Figure 18 shows a virtualization environment in which various embodiments of the present disclosure may be virtualized.

[0063] DETAILED DESCRIPTION

[0064] Some of the embodiments contemplated herein will now be described more fully with reference to the accompanying drawings. Other embodiments, however, are contained within the scope of the subject matter disclosed herein, the disclosed subject matter should not be construed as limited to only the embodiments set forth herein; rather, these embodiments are provided as examples to convey the scope of the subject matter to those skilled in the art. In general, all terms used herein are to be interpreted according to their ordinary meaning to a person of ordinary skill in the relevant technical field, unless a different meaning is expressly defined and / or implied from the context of use. All references to a / an / the element, apparatus, component, means, step, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise or clearly implied from the context of use. The operations of any methods and / or procedures disclosed herein do not have to be performed in the exact order disclosed, unless an operation is explicitly described as following or preceding another operation and / or where it is implicit that an operation must follow or precede another operation. Any feature of any embodiment disclosed herein can apply to any other disclosed embodiment, as appropriate. Likewise, any advantage of any embodiment described herein can apply to any other disclosed embodiment, as appropriate.

[0065] Furthermore, the following terms are used throughout the description given below:

[0066] • Radio Access Node: As used herein, a “radio access node” (or equivalently “radio network node,” “radio access network node,” or “RAN node”) can be any node in a radio access network (RAN) that operates to wirelessly transmit and / or receive signals. Some examples of a radio access node include, but are not limited to, a base station (e.g., gNB in a 3 GPP 5G / NR network or an enhanced or eNB in a 3GPP LTE network), base station distributed components (e.g., CU and DU), a high-power or macro base station, a low-power base station (e.g., micro, pico, femto, or home base station, or the like), an integrated access backhaul (IAB) node, a transmission point (TP), a transmission reception point (TRP), a remote radio unit (RRU or RRH), and a relay node.

[0067] • Core Network Node: As used herein, a “core network node” is any type of node in a core network. Some examples of a core network node include, e.g., a Mobility Management Entity (MME), a serving gateway (SGW), a PDN Gateway (P-GW), a Policy and Charging Rules Function (PCRF), an access and mobility management function (AMF), a session management function (SMF), a user plane function (UPF), a Charging Function (CHF), a Policy Control Function (PCF), an Authentication Server Function (AUSF), a location management function (LMF), or the like.

[0068] • Wireless Device: As used herein, a “wireless device” (or “WD” for short) is any type of device that is capable, configured, arranged and / or operable to communicate wirelessly with network nodes and / or other wireless devices. Communicating wirelessly can involve transmitting and / or receiving wireless signals using electromagnetic waves, radio waves, infrared waves, and / or other types of signals suitable for conveying information through air. Unless otherwise noted, the term “wireless device” is used interchangeably herein with the term “user equipment” (or “UE” for short), with both of these terms having a different meaning than the term “network node”.

[0069] • Radio Node: As used herein, a “radio node” can be either a “radio access node” (or equivalent term) or a “wireless device.”

[0070] • Network Node: As used herein, a “network node” is any node that is either part of the radio access network (e.g, a radio access node or equivalent term) or of the core network (e.g, a core network node discussed above) of a cellular communications network. Functionally, a network node is equipment capable, configured, arranged, and / or operable to communicate directly or indirectly with a wireless device and / or with other network nodes or equipment in the cellular communications network, to enable and / or provide wireless access to the wireless device, and / or to perform other functions (e.g., administration) in the cellular communications network.

[0071] • Node: As used herein, the term “node” (without prefix) can be any type of node that can in or with a wireless network (including RAN and / or core network), including a radio access node (or equivalent term), core network node, or wireless device. However, the term “node” may be limited to a particular type (e.g., radio access node, IAB node) based on its specific characteristics in any given context.

[0072] The above definitions are not meant to be exclusive. In other words, various ones of the above terms may be explained and / or described elsewhere in the present disclosure using the same or similar terminology. Nevertheless, to the extent that such other explanations and / or descriptions conflict with the above definitions, the above definitions should control.

[0073] Note that the description given herein focuses on a 3 GPP cellular communications system and, as such, 3GPP terminology or terminology similar to 3GPP terminology is oftentimes used. However, the concepts disclosed herein are not limited to a 3GPP system and can be applied to any communication system that may benefit from them.

[0074] Figure 2 shows a logical architecture for an NG-RAN node (e.g., gNB or ng-eNB) arranged in the split CU / DU architecture, such as gNB 100 in Figure 1. This logical architecture separates the CU into control plane (CP) and user plane (UP) functionality, called CU-C (or CU-CP) and CU-U (or CU-UP) respectively. Furthermore, each of the NG, Xn, and Fl interfaces is split into a CP interface (e.g., NG-C) and a UP interface (e.g., NG-U). Moreover, the CU-U and CU-C can communicate via an El interface. Each DU may be connected to only one CU-C, and each CU-U may be connected to only one CU-C. However, a single DU may be connected to multiple CU- Us under the control of the same CU-C, or a single CU-U may be connected to multiple DUs under the control of the same CU-C. Note that the terms “Central Entity” and “Distributed Entity” in Figure 2 refer to physical network nodes. Figure 3 shows an exemplary configuration of NR UP and CP protocol stacks between a UE (310), a gNB (320), and an AMF (330). Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Convergence Protocol (PDCP) layers between UE and gNB are common to UP and CP. PDCP provides ciphering / deciphering, integrity protection, sequence numbering, reordering, and duplicate detection for both CP and UP, as well as header compression and retransmission for UP data.

[0075] On the UP side, Internet protocol (IP) packets arrive to PDCP as service data units (SDUs), and PDCP creates protocol data units (PDUs) to deliver to RLC. The Service Data Adaptation Protocol (SDAP) layer handles quality-of-service (QoS) including mapping between QoS flows and Data Radio Bearers (DRBs) and marking QoS flow identifiers (QFI) in UL and DL packets. RLC transfers PDCP PDUs to MAC through logical channels (LCH). RLC provides error detection / correction, concatenation, segmentation / reassembly, sequence numbering, reordering of data transferred to / from the upper layers. MAC provides mapping between LCHs and PHY transport channels, LCH prioritization, multiplexing into or demultiplexing from transport blocks (TBs), hybrid ARQ (HARQ) error correction, and dynamic scheduling (in gNB). PHY provides transport channel services to MAC and handles transfer over the NR radio interface, e.g., via modulation, coding, antenna mapping, and beam forming.

[0076] On the CP side, the non-access stratum (NAS) layer between UE and AMF handles UE / gNB authentication, mobility management, and security control. RRC sits below NAS in the UE but terminates in the gNB rather than the AMF. RRC controls communications between UE and gNB at the radio interface as well as the mobility of a UE between cells in the NG-RAN. RRC also broadcasts system information (SI) and performs establishment, configuration, maintenance, and release of DRBs and Signaling Radio Bearers (SRBs) and used by UEs. Additionally, RRC controls addition, modification, and release of carrier aggregation (CA) and dual -connectivity (DC) configurations for UEs, and performs various security functions such as key management.

[0077] After a UE is powered ON it will be in the RRC IDLE state until an RRC connection is established with the network, at which time the UE will transition to RRC CONNECTED state (e.g., where data transfer can occur). The UE returns to RRC IDLE after the connection with the network is released. In RRC IDLE state, the UE’s radio is active on a discontinuous reception (DRX) schedule configured by upper layers. During DRX active periods (also referred to as “DRX On durations”), an RRC IDLE UE receives SI broadcast in the cell where the UE is camping, performs measurements of neighbor cells to support cell reselection, and monitors a paging channel on PDCCH for pages from 5GC via gNB. An NR UE in RRC IDLE state is not known to the gNB serving the cell where the UE is camping. However, NR RRC includes an RRC_INACTIVE state in which a UE is known (e.g., via UE context) by the serving gNB. RRC INACTIVE has some properties similar to a “suspended” condition used in LTE.

[0078] As mentioned above, RRC also handles configuration of AS security parameters such as integrity protection algorithm, ciphering algorithm, and parameters keySetChangelndicator and nextHopChainingCount used by the UE to determine its AS security keys upon reconfiguration with sync (with key change), RRC connection re-establishment, and RRC connection resume. The integrity protection algorithm is common for SRB1, SRB2, SRB3 (if configured), SRB4 (if configured) and DRBs configured with integrity protection, with the same keyToUse value. The ciphering algorithm is common for SRB1, SRB2, SRB3 (if configured), SRB4 (if configured) and DRBs configured with the same keyToUse value. Neither integrity protection nor ciphering is used for SRBO.

[0079] RRC integrity protection and ciphering are always activated together, i.e. in one message / procedure. RRC integrity protection and ciphering for SRBs are never de-activated. However, it is possible to switch to a 'NULL' ciphering algorithm (neaO). The 'NULL' integrity protection algorithm (niaO) is used only for SRBs and for the UE in limited service mode, as specified in 3GPP TS 33.501 (V18.0.0). When used for SRBs, integrity protection is disabled for DRBs. In case the 'NULL' integrity protection algorithm is used, 'NULL' ciphering algorithm is also used. Additionally, lower layers discard RRC messages for which the integrity protection check has failed and indicate the integrity protection verification check failure to RRC.

[0080] The AS applies four different security keys: one for integrity protection of RRC signaling (KRRCint), one for ciphering of RRC signaling (KRRCenc), one for integrity protection of user data (Kupint) and one for ciphering of user data (KuPenc). All four AS keys are derived from the KSNB key of the gNB, which is based on the KAMF key handled by upper layers as specified in 3GPP TS 33.501 (vl8.0.0). The integrity protection and ciphering algorithms can only be changed with reconfiguration with sync. The four AS keys change upon reconfiguration with sync (if masterKeyUpdate is included), and upon RRC connection re-establishment and RRC connection resume.

[0081] For each DRB or SRB an independent counter (COUNT) is maintained for each direction and is used as input for ciphering and integrity protection. It is not allowed to use the same COUNT value more than once for a given security key. As specified in 3GPP TS 33.501 (vl8.0.0 clause 6.9.4.1, the network is responsible for avoiding reuse of COUNT with the same RB identity and with the same key, e.g. due to the transfer of large volumes of data, release and establishment of new RBs, and multiple termination point changes for RLC-UM bearers and multiple termination point changes for RLC-AM bearer with SN terminated PDCP re-establishment (COUNT reset) due to SN only full configuration while the key stream inputs (i.e. bearer ID, security key) at the MN have not been updated. In order to avoid such re-use, the network may e.g. use different RB identities for RB establishments, change the AS security key, or an RRC CONNECTED to RRC IDLE / RRC INACTIVE and then to RRC CONNECTED transition.

[0082] In order to limit the signaling overhead, individual messages / packets include a short PDCP sequence number (PDCP-SN). In addition, the hyper frame number (HFN) is used as an overflow counter mechanism. HFN needs to be synchronized between the UE and the network. Further details are specified in 3GPP TS 38.323 (vl8.0.0). For each SRB, the value provided by RRC to lower layers to derive the 5-bit BEARER parameter used as input for ciphering and for integrity protection is the value of the corresponding srb-Identity with the MSBs padded with zeroes.

[0083] As described in 3GPP TS 38.501 (vl8.4.0) section 6.9.2.1.1, whenever an initial AS security context needs to be established between a UE and a gNB, the AMF and the UE derive KgNB and a Next Hop parameter (NH) from KAMF provided by the AMF. A NH Chaining Counter (NCC) is associated with each KSNB and NH parameter, and each KSNB is associated with the NCC corresponding to the NH value from which KSNB was derived. At initial setup, KSNB is derived directly from KAMF, and is then considered to be associated with a virtual NH parameter with NCC value equal to zero. At initial setup, the derived NH value is associated with the NCC value one. The UE and the gNB use KSNB to secure the communication between each other, including derivation of the four keys mentioned above.

[0084] For DC, when the MN establishes security context between an SN and the UE for the first time for a given AS security context shared between the MN and the UE, the MN generates the security key KSN for the SN and sends it to the SN over the Xn-C interface. When the SN is a gNB, its security key may also be referred to as S-KgNB. To generate KSN, the MN associates a counter (SN Counter) with the current AS security context. The SN Counter is used as freshness input into KSN derivations as described in the 3GPP TS 38.501 (vl8.4.0) section 6.10.3.2.

[0085] The MN maintains the value of SN Counter for a duration of the current 5G AS security context between UE and MN. The MN sends the SN Counter value the to the UE via RRC signaling when the UE is required to generate a new KSN, which is then used to derive further keys that are used between the UE and SN for ciphering and integrity protection of user data and signaling, as discussed above. The UE does not need to maintain SN Counter after it has computed KSN since the MN provides the UE with the current SN Counter value when the UE needs to compute a new KSN.

[0086] Seamless mobility is a key feature of 3GPP radio access technologies (RATs). When the UE moves between the coverage areas of two cells, a serving cell change needs to be performed at some point. Currently, serving cell change is triggered by L3 (e.g., RSRP) measurements and involves RRC signaling to change PCell and PSCell (e.g., when dual connectivity is configured), as well as release / add SCells (e.g., when CA is configured).

[0087] In general, a RAN (e.g., NG-RAN) configures a UE in RRC_CONNECTED state to perform and report radio resource management (RRM) measurements to assist network- controlled mobility decisions, such as for handover from a serving cell to a target cell (e.g., PCell change). Upon the reported measurements meeting a certain condition or threshold, the serving RAN node may send a handover command to the UE, indicating a target cell for the handover. In NR, the handover command is an RRCReconfiguration message with a reconfigurationWithSync field. The procedure to perform a handover is sometimes also referred to as “L3 mobility”, as it is controlled by layer 3 (L3, i.e., RRC) and the messages exchanged are part of L3.

[0088] These reconfigurations are prepared in advance by a target RAN node serving the target cell, upon a request from the UE’s serving RAN node. This request is transmitted over the Xn interface in case the serving and target RAN nodes are part of the NG-RAN. The reconfiguration in the handover command considers the UE’s existing RRC configuration in its current serving cell (also referred to as “source cell”), which are provided in the inter-node request. In some cases, the reconfiguration can be provided as a “delta” to the UE’s existing configuration in the source cell, which reduces the size of the handover command.

[0089] The reconfiguration provided by the target RAN node contains all information the UE needs to access the target cell, e.g., random access configuration, a new cell radio network temporary identifier (C-RNTI) assigned to the UE in the target cell, and parameters enabling the UE to calculate security keys that it can use when communicating with the target cell (including sending a handover complete message).

[0090] In general, UE nobility in RRC CONNECTED state is network-based since the network has the most information about conditions such as cell loading (UEs and / or traffic), available node resources (e.g., processing), available frequencies, etc. However, there will be scenarios when the network fails to handover the UE to the “correct” neighbor cell in time, which can cause the UE will declare radio link failure (RLF) or handover failure (HOF).

[0091] As briefly mentioned above, conditional handover (CHO) was introduced in 3GPP Rel- 16 to improve robustness of UE handover. The key idea in CHO is separation of transmission and execution of the handover command. This allows the handover command to be sent to a UE earlier when the radio conditions are still good, thus increasing the likelihood that the message is successfully transferred. The execution of the handover command is done later in time based on an associated execution condition. The execution condition is typically based on a threshold. For example, a signal strength of candidate target cell becomes X dB better than the serving cell (so called “A3 event ”). A preceding measurement reporting event could use a threshold Y that is selected to be lower than X used as the handover execution condition. This allows the serving cell to prepare the handover upon reception of an early measurement report and to provide the RRCConnectionReconfiguration with mobilityControlInfo (for LTE), or a RRCReconfiguration with either a reconfigurationWithSync or a CellGroupConfig (for NR) at a time when the radio link between the source cell and the UE is still relatively stable.

[0092] As used herein, a cell for which conditional handover (or other conditional mobility procedure) is configured is called a “candidate target cell” or “potential target cell”. Similarly, a RAN node controlling a candidate / potential target cell is called “candidate target node” or “potential target node”. Once the conditional mobility execution condition has been fulfilled for a candidate / potential target cell and mobility execution towards this cell has been triggered, this cell is no longer “potential” or a “candidate” in the normal senses of the words, since it is now certain that the mobility operation will be executed towards it. Rather, the candidate / potential target cell can then be referred to as the “target cell”.

[0093] Figure 4 shows a signaling diagram for an exemplary CHO procedure. The signaling shown in Figure 4 is between a UE (410), a source RAN node (420), and a target RAN node (430). For example, the source and target nodes can be gNBs and / or components of gNBs, such as CUs and / or DUs.

[0094] This procedure involves two different measurement thresholds: a low threshold and a high threshold. The two thresholds can be expressed as different levels of a particular metric, e.g., signal strength, signal quality, etc. For example, the high threshold could be that the quality of the mobility reference signal (MRS) of the target cell or beam becomes X dB stronger than the MRS of the UE’s serving cell (e.g., provided by the source RAN node), with the low threshold being less than the high threshold (i.e., target exceeds source by lower amount). As used in this context, MRS denotes a reference signal used for any mobility -related purpose. For example, in NR, MRS can be either SSB (SS / PBCH block) or CSI-RS. As a further example, for NR operating in unlicensed spectrum (referred to as NR-U), MRS can be a discovery reference signal (DRS) in addition to any of the signals mentioned above.

[0095] The UE can be provided with a measurement configuration including the low threshold (not shown in the figure). Upon performing measurements that meet the low threshold, the UE can send a measurement report to the serving node (operation 1). While performing the measurements and evaluating the low threshold, the UE continues operating in its current RRC configuration. In operation 2, based on this report, the source RAN node can decide to request an early handover of the UE to the target RAN node (e.g., to a cell indicated in the measurement report). For example, this early handover request can include a HandoverPreparationlnformation IE such as described above.

[0096] The target RAN node performs admission control for the UE and responds with a CHO request acknowledgement (operation 5) that includes RRC configuration, similar to conventional handover. In operation 6, the source RAN node then sends the UE ^RRCReconfiguration message that includes a “CHO Configuration”, which can include the high threshold. After responding with an RRCReconfigurationComplete message (operation 7), the UE continues to perform measurements and whenever the high threshold condition is met for a target cell, it can detach from the source cell and, after performing a RA procedure and synchronizing with the target cell, send the target RAN node an RRCReconfigurationComplete message (e.g., operations 8-9). Even so, the UE can remain in the source cell for an extended amount of time in case the high threshold condition is not fulfilled.

[0097] In operation 10, the target RAN node sends a HANDOVER SUCCESS message to the source gNB indicating the UE has successfully established the target connection. Upon reception of the handover success indication, the source RAN node stops scheduling any further DL or UL data to the UE and sends an SN STATUS TRANSFER message to the target RAN node indicating the latest PDCP SN transmitter and receiver status (operation 11). The source RAN node now also starts to forward User Data to the target RAN node (operation 12). Upon receiving the handover complete message (operation 9), the target RAN node can start exchanging user data with the UE. The target RAN node also requests the AMF to switch the DL data path from the UPF from the source RAN node to the target RAN node (not shown). Once the path switch is completed the target RAN node sends the UE CONTEXT RELEASE to the source RAN node (operation 13).

[0098] When a UE successfully connects (e.g., completes RA) to a target cell during a CHO or a conventional handover, it releases all the conditional reconfigurations that it has stored. The RAN node serving the target cell may then provide the UE with new conditional reconfigurations if desired.

[0099] On handovers (including CHO) and transitions from RRC INACTIVE to RRC_CONNECTED, the basis for KSNB used between the UE and the target NG-RAN node (e.g., gNB) - called KNG-RAN* - is derived from either the currently active KSNB or from the NH parameter. Deriving KNG-RAN* from the currently active KSNB this is referred to as a “horizontal key derivation” while deriving KNG-RAN* from the NH parameter is referred to as “vertical key derivation.”

[0100] These horizontal and vertical key derivations are illustrated by Figure 5, which shows exemplary security key derivation for HO and other UE mobility procedures. Since NH parameters are only computable by the UE and the AMF, the AMF provides NH parameters to RAN nodes in a way that forward security can be achieved. On handovers with vertical key derivation, the NH is further bound to the target PCI and its DL frequency (ARFCN-DL) before it is used for KSNB in the target RAN node. On handovers with horizontal key derivation, the currently active KSNB is further bound to the target PCI and its frequency ARFCN-DL before it is used for KSNB in the target RAN node.

[0101] During HO or CHO, any DRB or SRB connections between the UE and the SN are released. As such, the SN and the UE delete the ciphering and integrity protection keys used for UP / DRB and CP / SRB traffic between SN and UE, since these keys will be refreshed by the new KSN derived by the UE’s new MN that serves the HO target cell. When the MN is executing an SN Addition procedure (i.e. initial offload of one or more radio bearers to the SN) an SN Modification procedure that requires an update of the KSN, the MN derives KSN as defined in 3 GPP TS 33.501 (vl8.0.0) section 6.10.3.2. The MN maintain the SN Counter as discussed above.

[0102] The CHO procedure discussed above can be generalized into a generic conditional reconfiguration framework, wherein a UE may be configured in advance with other types of reconfigurations that can be executed by an RRCReconfiguration message (in NR) or an RRCConnectionReconfiguration message (in LTE) when associated execution condition(s) is(are) triggered. Each such message is prepared by a candidate target RAN node, associated with a candidate target cell, and includes execution conditions that can be represented by one or more identifiers of measurement configuration(s). This conditional reconfiguration framework can be applied to the following mobility operations:

[0103] • CHO (e.g., target candidate RRCReconfiguration message contains a reconfiguration with sync for the MCG);

[0104] • Conditional PSCell Addition (CPA e.g., target candidate RRCReconfiguration message contains an SCG configuration which contains a reconfiguration with sync for a cell to be the PSCell of the SCG);

[0105] • Conditional PSCell Change (CPC, e.g., target candidate RRCReconfiguration message includes an SCG configuration that contains a reconfiguration with sync for a new target candidate cell to be the PSCell of the SCG);

[0106] • Conditional PSCell Release (e.g., source RRCReconfiguration message to be conditionally applied contains an SCG release indication); or

[0107] • Conditional PSCell Suspend (e.g., source RRCReconfiguration message to be conditionally applied contains an SCG suspend indication).

[0108] An SN-initiated intra-SN CPC procedure was specified in 3GPP Rel-16. In this procedure, a UE operating in MR-DC receives a conditional reconfiguration that includes an RRCReconfiguration message containing an SCG configuration (e.g., a secondaryCellGroup field of a CellGroupConfig information element) with an associated execution condition (e.g., an A3 / A5 event configuration). When the UE detects that the execution condition is fulfilled (i.e., finds a neighbor cell better than current PSCell by a configured amount), the UE performs PSCell change. The intra-SN solution for Rel-16 is only for scenarios where the (candidate) target PSCells are provided by the UE’s current SN. Similar to CHO, when a UE successfully connects (e.g., completes RA) to a target cell during intra-SN CPC, it releases all the conditional reconfigurations that it has stored.

[0109] 3GPP Rel-17 introduces support for Conditional PSCell Addition (CPA) and inter-SN CPC. The CPA procedure is used to add a PSCell / SCG to a UE currently configured with only an MCG, when associated execution conditions are fulfilled. CPA is initiated after the MN requests and receives an SCG configuration from a candidate target SN (T-SN), which the MN then provides to the UE as part of a conditional reconfiguration together with the associated execution condition(s). An inter-SN CPC procedure can be initiated by the MN or by the source SN (S-SN), with the MN handling the signaling toward the T-SN and the UE in either case.

[0110] For CPC and CPA, if there is more than one candidate SN, the MN derives a different KSN for each candidate SN by using different SN counter values in a similar manner as discussed above for non-conditional procedures. Further details are provided in 3GPP TS 33.501 (vl8.0.0).

[0111] 3GPP Rel-18 includes support a feature known as subsequent conditional PSCell addition / change (CP AC), but may be more generally thought of as a type of subsequent L3 mobility. In subsequent CP AC, a UE performs a further (or subsequent) PSCell / SCG change or addition after an initial PSCell / SCG change or addition, without the need of being reconfigured by the RAN. This is realized by the UE retaining its received CP AC configurations for candidate cells after applying one of them during CPA or CPC execution. This is contrary to the Rel-17 handling of conditional reconfigurations (e.g., CPC configurations), where the UE releases its other CPC configurations after applying one of the during execution of a PSCell change. The Rel- 18 UE can perform one or more subsequent PSCell / SCG changes based on one of those retained CPC configurations, without additional reconfiguration signaling by the RAN. Relative to Rel-17, Rel-18 subsequent CP AC reduces interruption time and signaling overhead for subsequent PSCell / SCG changes, especially for frequent PSCell / SCG changes that may occur while a UE is operating in NR frequency range 2 (FR2).

[0112] 3GPP Rel-18 also includes an NR mobility enhancement known as L1 / L2 based intercell mobility or L1 / L2 triggered mobility (LTM). Current L3 -based inter-cell mobility procedures involve LI and L2 resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching. Thus, Rel-18 LTM is intended to facilitate serving cell changes via L1 / L2 signaling that reduce latency, signaling overhead, and interruptions.

[0113] In LTM, a UE is pre-configured by its serving RAN with one RRC configuration per LTM candidate cell, sometimes referred to as an “LTM candidate cell configuration”. This configuration may be an RRCReconfiguration message or a portion thereof, such as one or more lEs / fields / parameters (e.g., CellGroupConfig IE). The UE performs measurements on configured LTM candidate cells and transmits corresponding measurement reports to the RAN, which triggers the execution of a LTM cell switch procedure by the UE to one of the configured LTM candidate cells. This triggering is done by transmitting an LTM cell switch command to the UE in lower layer signaling (e.g., DCI or MAC CE). Based on this command, the UE connects to the associated LTM candidate cell and uses the previously received RRC configuration for this cell.

[0114] The split CU / DU architecture shown in Figure 1 also supports LTM, including for intra- DU and inter-DU / intra-CU cell changes. In the inter-DU / intra-CU scenario, the candidate cell for LTM is a cell served by a neighbor DU to the (serving or source) DU that currently provides the UE’s PCell (or PSCell, for SCG change in DC). In the intra-DU scenario, the candidate cell for LTM is a cell served by the same DU that currently provides the UE’s PCell (or PSCell, for SCG change in DC).

[0115] Figure 6 shows a signaling diagram for an exemplary intra-gNB LTM cell switch procedure, which may be intra-DU or inter-DU / intra-CU. Although the operations are shown with numerical labels, this is done to facilitate explanation rather than to require or imply any particular operational order, unless expressly stated otherwise.

[0116] In operation 1, the UE (610) sends MeasurementReport message to the gNB (620). Based on this message, the gNB decides to configure LTM for the UE and initiates preparation of one or more LTM candidate cells. In operation 2, the gNB sends an RRCReconfiguration message to the UE including LTM candidate cell configurations of one or more candidate cells. In operation 3, the UE stores the received LTM candidate cell configurations and transmits an RRCReconfigurationComplete message to the gNB.

[0117] Since a goal of LTM is to reduce interruption time for UE data transmissions, the UE needs to be ready to communicate with an LTM candidate cell upon (or shortly after) receiving the L1 / L2 signaling for mobility execution from the source cell. For example, the UE must be able to transmit UL data or a scheduling request (SR) to the LTM candidate cell and / or monitor a DL control channel (e.g., PDCCH) from the LTM candidate cell. In other words, UE needs to know the cell that it is moving to so it can apply the corresponding configuration, including the correct timing alignment and / or TCI state for the cell. Likewise, in the case of inter-DU LTM, when the source DU transmits the L1 / L2 signaling for mobility execution, the target DU needs to be prepared for scheduling UL and DL transmissions for the UE in the target cell, and for receiving SR from the UE.

[0118] Accordingly, the UE performs operations 4a-b before receiving any LTM cell switch command. In operation 4a, the UE performs early DL synchronization with the configured LTM candidate cells. In operation 4b, when UE-based TA measurement is configured, UE acquires the TA value(s) of the candidate cell(s) by measurement. Otherwise, in operation 4b, the UE performs early TA acquisition with the candidate cell(s) as requested by the network. This is done via contention-free random access (CFRA) triggered by a physical DL control channel (PDCCH) order from the source cell, following which the UE sends a RA preamble towards the indicated LTM candidate cell. In order to minimize the data interruption of the source cell due to CFRA towards the LTM candidate cell(s), the UE doesn’t receive RA response (with TA) from the LTM candidate cell; instead, TA for the LTM candidate cell is indicated in a subsequent LTM cell switch command. Similarly, the UE doesn’t maintain a TA timer for the LTM candidate cell but relies on the RAN to guarantee the TA validity.

[0119] In operation 5, the UE performs LI measurements on the configured LTM candidate cells and transmits LI measurement reports to the gNB. The UE performs such LI measurement as long as the LTM candidate cell configurations received in operation 2 remain applicable.

[0120] In operation 6, the gNB decides to trigger an LTM cell switch for the UE to one of the configured LTM candidate cells ( “target cell”) and transmits an LTM cell switch command, which is a MAC CE that includes an identifier (e.g., index) of the corresponding LTM candidate cell configurations provided to the UE in operation 2. The MAC CE may also include an identifier of a beam (e.g., a TCI State ID) by which the UE should access the target cell.

[0121] The gNB selects the identified beam based on the LI measurements reported by the UE. These are typically per-beam measurements, such as LI reference signal received power (RSRP) for synchronization signal / PBCH blocks (SSBs). These measurements may not be layer 3 (L3) filtered, so they may change relatively frequently as UE radio conditions change. As such, it may be challenging for the gNB to determine the optimal beam to indicate to the UE in the LTM cell switch command.

[0122] Upon receiving the LTM cell switch command, the UE monitors PDCCH on the indicated beam of the target cell. In other words, the UE considers the TCI state for the indicated beam / TCI state ID to be “activated” when performing the LTM cell switch. The UE also applies the configuration identified in the MAC CE.

[0123] In operation 7, if UE does not have valid TA of the target cell, the UE performs a RA procedure towards the target cell,. The UE performs CFRA if the LTM cell switch command contains the necessary information, as specified in clause 6.1.3.xy of 3GPP TS 38.321 (v!7.7.0). In operation 8, the UE completes the LTM cell switch procedure by sending RRCReconfigurationComplete message to the gNB via the target cell. If the UE has performed a RA procedure in operation 7, the UE considers that LTM cell switch execution is successfully completed when the RA procedure is successfully completed. For RACH-less LTM, the UE considers that LTM cell switch execution is successfully completed when the UE determines that the gNB has successfully received its first UL data. The UE determines successful reception of its first UL data by receiving a PDCCH addressing the UE’s C-RNTI in the target cell, which schedules a new transmission following the first UL data. The PDCCH carries either a DL assignment or an UL grant addressing the same HARQ process as the first UL data.

[0124] To trigger LTM by a UE, the network sends the UE an LTM Cell Switch command an indication of an LTM candidate cell and an indication of a beam based on which the UE should access the indicated LTM candidate cell. In 5G / NR, the beam indication is given as a transmission configuration indicator (TCI) state identifier (ID) associated with the LTM candidate cell, which may be indicated by an LTM candidate configuration ID. In response, the UE performs the LTM cell switch, accesses the indicated cell / beam, and transmits a complete message.

[0125] According to 3 GPP agreements, a UE may perform multiple LTM cell switch procedures without the need of being reconfigured by the RAN. For example, after the UE has performed an LTM cell switch from a serving cell to a first target cell (e.g., a first LTM candidate cell), the UE may perform another LTM cell switch to a second target cell (e.g., a second LTM candidate cell previously configured) without receiving another RRCReconfiguration message in the first target cell. This second LTM cell switch may be referred to as “subsequent LTM” or “subsequent LTM cell switch”.

[0126] According to 3 GPP agreements, Rel-19 will support inter-CU LTM as well as conditional LTM, which is analogous to L3 CHO described above. In conditional LTM, an LTM candidate configuration for an LTM candidate cell includes an execution condition, such as “LTM candidate cell becomes offset better than PCell” or “beam of LTM candidate cell becomes offset better than serving beam”. Instead of executing LTM in response to an LTM cell switch command, the UE executes an LTM cell switch to an LTM candidate cell when the associated execution condition is fulfilled. As such, an LTM candidate cell with an associated execution condition may be referred to as a conditional LTM candidate cell, and corresponding LTM procedure may be referred to as a conditional LTM cell switch. It is expected that Rel-19 will support both intra- and inter-CU conditional LTM.

[0127] However, inter-CU LTM, conditional LTM, subsequent LTM, and subsequent CHO require the support of new security -related functionality that is not part of Rel-18 or prior releases. For example, these type of mobility operations create some additional complexity for security in the UE’s SCG. In order to derive S-KSNB (also known as KSN), the UE needs to use a secondary key counter (called sk-counter or SN counter) that is generated and sent to the UE by the MN the provides the UE’s MCG. This sk-counter is a single-use piece of information, meaning that it is no longer valid after used by the UE to derive an S-KgNB. However, this creates difficulties for subsequent LTM and subsequent L3 mobility operations that are expected to be performed without intermediate intervention by the MN. Similar problems and / or difficulties may exist for subsequent CHO and subsequent conditional LTM.

[0128] Accordingly, embodiments of the present disclosure address these problems and / or issues by flexible and efficient techniques to control security key change during a mobility procedure in a UE’s MCG while the UE’s SCG is maintained. In general, these techniques enable the UE to perform multiple mobility procedures in the MCG that involve both MN and SN security key changes, without the need for intermediate reconfiguration or other intervention by the RAN.

[0129] At a very high level, embodiments can be categorized into four different options summarized below. All options involve a first MN that provides a first MCG for a UE before a mobility procedure, a second MN that provides a second MCG for the UE after the mobility procedure, and an SN that provides an SCG for the UE both before and after the mobility procedure. While these options are explained in the context of LTM, they are equally applicable to L3 mobility procedures such as HO.

[0130] In a first option, the first MN configures one or more LTM candidate cells for the UE, along with a list of sk-counters to be used by the UE in order to calculate an SN security key (e.g., S-KgNB) from the MN security key (e.g., KSNB). The first MN triggers an LTM cell switch by the UE to the second MCG, which requires changes to the UE’s MN and SN security keys. The first MN notifies the second SN about the LTM cell switch and includes with the notification a list of available sk-counters that the UE can use to determine its new SN security key.

[0131] In a second option, the first MN configures one or more LTM candidate cells for the UE, along with a list of sk-counters to be used by the UE in order to calculate an SN security key (e.g., S-KgNB) from the MN security key (e.g., KSNB). The first MN triggers an LTM cell switch by the UE to the second MCG, which requires changes to the UE’s MN and SN security keys. When the UE accesses the LTM candidate cell of the second MCG (indicated in the LTM cell switch command), the UE sends the second SN a list of available sk-counters that the UE can use to determine its new SN security key for subsequent mobility procedures (including LTM cell switch).

[0132] In a third option, the first MN configures one or more LTM candidate cells for the UE, along with a list of sk-counters to be used by the UE to calculate an SN security key (e.g., S-KSNB) from the MN security key (e.g., KSNB), but only while the UE remains connected to the first MN. The second MN - which prepared the one or more LTM candidate cells that the first MN configures in the UE via the first MCG - also includes in or with the LTM candidate configuration for each LTM candidate cell configuration a list of sk-counters to be used by the UE to calculate an SN security key (e.g., S-KSNB) from the MN security key (e.g., KSNB), but only while the UE remains connected to the second MN. In this option, the UE maintains a list of sk-counters for each MN that has provided an LTM candidate configuration.

[0133] In a fourth option, the first MN configures one or more LTM candidate cells for the UE, along with a list of sk-counters to be used by the UE to calculate an SN security key (e.g., S-KSNB) from the MN security key (e.g., KSNB), but only while the UE remains connected to the first MN. When a LTM cell switch is triggered to one of the LTM candidate cells in the second SCG provided by the second SN, the second MN prepares and sends the UE a new list of sk-counters to be used by the UE to calculate an SN security key (e.g., S-KSNB) from the MN security key (e.g., KSNB), but only while the UE remains connected to the second MN. This option allows subsequent mobility while the UE is connected to the same MN, but requires UE reconfiguration when the MN changes.

[0134] Embodiments of the present disclosure may provide various advantages and / or benefits. For example, embodiments may facilitate multiple UE mobility operations (e.g., LTM cell switch) that require both SN and MN security key updates, without need for intermediate reconfiguration or other RAN intervention. As such, embodiments may facilitate necessary updates for encryption and integrity protection as the UE moves between cells in the RAN, thereby increasing security of signaling and user data carried by radio bearers secured in this manner.

[0135] In the present disclosure, the following terms may be used interchangeably: “L1 / L2 based inter-cell mobility”, “L1 / L2 mobility,” “LI -mobility,” “LI based mobility,” “Ll / L2-centric inter-cell mobility,” “L1 / L2 inter-cell mobility,” “inter-cell beam management,” “inter-DU L1 / L2 based inter-cell mobility”, and “L1 / L2 triggered mobility” (or LTM). These terms refer to a scenario in which a UE receives lower layer (i.e., below RRC, such as MAC or PHY) signaling from a network indicating for the UE to change of its serving cell (e.g., PCell) from a source cell to a target cell.

[0136] The content of the lower layer signaling may be referred to as “LTM cell switch command”. Exemplary lower layer signaling includes LI DL control information (DCI) and L2 MAC control element (CE). Compared to conventional RRC signaling, lower layer signaling reduces processing time and interruption time during mobility and may also increase mobility robustness since the network can respond more quickly to changes in the UE’s channel conditions.

[0137] The term “LTM candidate cell” refers to a cell for which the UE is configured for LTM, specifically a cell the UE can move to in a LTM cell switch procedure in response to receiving an LTM cell switch command. An LTM candidate cell may also be referred to herein as “candidate cell”, “(LTM) candidate, “mobility candidate”, “non-serving cell”, “additional cell”, “(LTM) target candidate cell”, “(LTM) target candidate”, and comparable terms. A UE may perform and report measurements (e.g., CSI measurements) on an LTM candidate cell, based on which the UE’s serving RAN node may make an informed decision about which beam (or TCI state) and / or cell to switch the UE. An LTM candidate cell may be a candidate to be a target PCell or PSCell, or an SCell of a cell group (e.g., MCG SCell). In the case of LTM fast recovery, when a failure is detected and the UE selects an LTM candidate cell, the UE performs an LTM cell switch towards the selected LTM candidate cell (e.g., by applying the associated LTM candidate cell configuration) rather than performing RRC re-establishment.

[0138] The change of serving cell (e.g., PCell) may also lead to a change in SCell(s) of the same cell group, e.g., in case an LTM cell switch command triggers the UE to change to another cell group configuration of the same type (e.g., another MCG configuration). For example, an LTM cell switch may include a change in SpCell (e.g., PCell for MCG, PSCell for SCG) and a change (e.g., addition, modification and / or release) in SCells of the same cell group. This may happen when the command triggers the UE to change to another cell group configuration of the same type (e.g., another SCG configuration).

[0139] Before the UE receives the LTM cell switch command, the UE is configured by the network with one or more “LTM candidate cell configurations” via an RRCReconfiguration message. The terms “(LTM) candidate configuration”, “(LTM) candidate target cell configuration”, and “(LTM) target candidate (cell) configuration” may be used interchangeably with LTM candidate cell configuration.

[0140] An LTM candidate cell configuration may be included in an RRC IE such as CellGroupConfig, SpCellConfig, or SCellConfig and / or an embedded RRCReconfiguration message for an LTM candidate cell. An LTM candidate cell configuration includes configuration parameters the UE needs to operate in that LTM candidate cell when it performs an LTM cell switch procedure, e.g., upon reception of the LTM cell switch command. As some more specific examples, an LTM candidate cell configuration can include a PCell configuration and one or more SCell configurations of an MCG, or a PSCell configuration and one or more SCell configurations of an SCG. The exact content and / or structure of the IE and / or embedded message for an LTM candidate cell configuration may be called “RRC model for the candidate configuration” or more simply “RRC model”.

[0141] A UE may receive an LTM candidate cell configuration in a complete form or as a delta (or difference) relative to a reference configuration (which may be signaled separately). In the latter case, the actual LTM candidate configuration is a combination of the delta configuration and the reference configuration.

[0142] The lower layer signaling from the RAN may include an identifier (or index) associated with an LTM candidate cell configuration. The identifier may be sent together with an LTM cell switch command, indicating for the UE to perform an LTM cell switch to the associated LTM candidate cell.

[0143] The term “mobility configuration” refers to a data structure that is used for or related to a UE mobility procedure, and may include one or more of the following elements (non-exclusive):

[0144] • a candidate configuration, i.e., for a mobility candidate cell;

[0145] • lower layer information, such as PHY configuration, MAC configuration, RLC configuration, cell group configuration, and / or serving cell configuration;

[0146] • higher layer information, such as RRC parameters (e.g., timer values), PDCP configuration, radio bearer configuration, or measurement configuration

[0147] • configuration of measurements for future candidate cells;

[0148] • measurement reporting configuration;

[0149] • CSI resource configuration;

[0150] • CSI reporting configuration;

[0151] • configuration for early DL synchronization, e.g., for early TCI state activation;

[0152] • a configuration for early UL synchronization, e.g., for PDCCH ordered preamble transmission and reception of timing advance (TA);

[0153] • configuration for execution of a mobility procedure according to the candidate configuration, such as indications whether to perform RA, RLC reestablishment, MAC reset, PDCP recovery, etc. as well as RA resources, UL configured grants, timer values, etc.; and

[0154] • information related to security key refresh, e.g. RRC MasterKeyUpdate IE and / or RRC RadioBearerConfig IE that includes SecurityConfig field with SecurityAlgorithmConfig.

[0155] The term “part of a mobility configuration” may refer to a subset of the elements in the above list, and / or a subset of items comprising any of the elements present (e.g., subset of configurations for DL pre-sync).

[0156] The term “mobility procedure” refers to a UE procedure for changing serving cell from a source cell to a target cell, which was a mobility candidate cell prior to execution of the mobility procedure. Examples of mobility procedures include L3 procedures such as HO, PSCell change, SCG change, SN change, CHO, CPC, CPA, and CP AC, as well as L1 / L2 procedures such as intra-CU LTM, inter-CU LTM, and conditional LTM. When the UE performs a mobility procedure to a candidate cell, the UE applies the received candidate configuration (of the mobility configuration) associated with that candidate cell when communicating with that candidate cell. The UE may also use other parts of the mobility configuration in preparing for the mobility procedure, such as performing and reporting measurements, early UL / DL synchronization, etc. A non-conditional mobility procedure may be triggered by a command from the RAN, while a conditional mobility procedure may be triggered by UE measurements meeting an execution condition associated with a candidate cell.

[0157] The term “subsequent mobility procedure” refers to a further or follow-on mobility procedure performed by the UE after an initial mobility procedure from a source cell to a candidate / target cell, without a reconfiguration or other intermediate intervention by a serving RAN node. The mobility configuration used for the subsequent mobility procedure may have been received by the UE while in in the source cell, prior to the initial mobility procedure.

[0158] The phrase “LTM cell switch procedure” refers to the process of a UE switching (or changing) from a source cell to a target cell (i.e., an LTM candidate cell) using LTM. An LTM cell switch procedure may also be referred to as “L1 / L2 based inter-cell mobility execution”, “LTM execution”, “dynamic switch”, “LTM switch”, “(LTM) cell switch”, “(LTM) serving cell change”, or “(LTM) cell change”. Similarly, the phrase “switching to an LTM candidate cell configuration” means that the UE applies an LTM candidate cell configuration such that the associated LTM candidate cell becomes its new special cell (SpCell, e.g., PCell for LTM in MCG or PSCell for LTM in SCG) or its new SCell. In other words, an LTM candidate cell can be a candidate for the UE’s PCell, PSCell, or SCell.

[0159] Furthermore, an LTM cell switch may involve a UE switching (or changing) from a source cell group to a target cell group using LTM. For example, this may involve a change in the SpCell for a cell group (e.g., PCell for MCG, PSCell for SCG), a change in SCells of the cell group (e.g., addition, modification, and / or release of one or more SCells), and / or a swap between SpCell and SCell roles for two cells in the same cell group.

[0160] The terms “CHO,” “CHO execution,” and “CHO execution procedure” refer to the process of a UE evaluating certain conditions configured by the RAN and, upon the fulfilling of such criteria, switching (or changing) from a source cell to a CHO candidate cell (which becomes a target cell) without further involvement of the source cell (e.g., signaling). In switching to the CHO candidate cell, the UE applies an CHO candidate configuration such that the CHO candidate cell becomes the UE’s new special cell (SpCell, e.g., PCell for LTM in MCG or PSCell for LTM in SCG) or its new SCell. In other words, a CHO candidate cell can be a candidate for the UE’s PCell, PSCell, or SCell. Further, when the CHO candidate cell is the PSCell, CHO may also be referred to as CPA, CPC, CP AC, or subsequent CP AC. Furthermore, CHO execution witch may involve a UE switching (or changing) from a source cell group to a target cell group using CHO. For example, this may involve a change in the SpCell for a cell group (e.g., PCell for MCG, PSCell for SCG), a change in SCells of the cell group (e.g., addition, modification, and / or release of one or more SCells), and / or a swap between SpCell and SCell roles for two cells in the same cell group.

[0161] The term “security key” may refer to an integrity protection key for CP, an integrity protection key for UP, an encryption key for CP, an encryption key for UP, or an intermediate key used for derivation of any of these key (e.g., KSNB, S-KSNB, KSN, etc.). The term “security configuration” refers to one or more parameters used to control security key derivation performed by a UE, and may include one or more of the following:

[0162] • a first identifier associated to a candidate cell;

[0163] • a second identifier associated to a source cell the UE is connected to when the mobility procedure is executed;

[0164] • a key set change indicator;

[0165] • a next hop chaining counter (NCC); and

[0166] • a non-access stratum (NAS) container (NASC).

[0167] An example security configuration is the masterKeyUpdate IE.

[0168] The terms “security key refresh,” “security key update,” “AS key refresh,” and similar terms refer to a procedure by which a UE changes or updates one or more AS security keys, including during a mobility procedure such as an LTM cell switch procedure. A security key refresh may include at least one of the following operations:

[0169] • UE receives a masterKeyUpdate IE included in a mobility configuration, e.g., in the candidate cell configuration;

[0170] • When a NAS indication (e.g., NASC) is received masterKeyUpdate IE, the UE forwards the NAS indication to UE NAS layer and updates its NAS security context according to 3GPP TS 33.501 clause 6.9.2.3.4;

[0171] • When a key set change indication (e.g. keySetChangelndicator) is received and / or is set to ‘true’ (e.g. within masterKeyUpdate IE), the UE derives or updates KSNB based on KAMF, as specified in 3GPP TS 33.501;

[0172] • UE derives or updates KSNB for the candidate cell configuration based on the current KSNB or the NH, using the NCC value indicated in the received masterKeyUpdate IE, as specified in 3GPP TS 33.501;

[0173] • UE derives KRRCenc and Kupenc associated with a ciphering algorithm (e.g. cipheringAlgorithm indicated in securityAlgorithmConfig), as specified in 3 GPP TS 33.501; • UE derives KRRCint and Kupint associated with an integrity protection algorithm (e.g. integrityProtAlgorithm indicated in securityAlgorithmConfig), as specified in 3 GPP TS 33.501;

[0174] • UE receives a security algorithm configuration included in a mobility configuration, based on which the UE derives UP security keys and / or CP security keys for encryption and / or integrity protection;

[0175] • UE uses its current security algorithm configuration, based on which the UE derives UP security keys and / or CP security keys for encryption and / or integrity protection;

[0176] • UE applies the provided ciphering algorithm and associated security key during a PDCP entity re-establishment procedure; and

[0177] • UE applies the provided integrity protection algorithm and associated security key during a PDCP entity re-establishment procedure;

[0178] • UE derives the security key(s) when it receives a mobility configuration, which it may user for a subsequent mobility procedure.

[0179] Figure 7 illustrates a communication system in which some embodiments of the present disclosure may be implemented. UE 701 is a wireless device (e.g., smartphone) that is capable of connecting to first RAN node 702 over a wireless interface 704 and to second RAN node 703 over a wireless interface 705.

[0180] First RAN node 702 provides a first cell 707, which may be referred to as the UE’s source cell in the context of mobility (e.g., LTM orL3 HO), or the UE’s serving cell, Special Cell, SpCell, PCell, or PSCell in the context of carrier aggregation (CA) and / or DC. Second RAN node 703 provides a second cell 708, which may be referred to as neighbor cell to the serving cell or, in the context of mobility, as target cell, candidate cell, LTM candidate cell, or inter-CU LTM candidate cell for the UE.

[0181] First RAN node 702 and second RAN node 703 may be gNBs of an NG-RAN, and may be interconnected over an interface 706, which may be an Xn or Xn-C type of interface. However, first RAN node 702 and second RAN node 703 are not necessarily interconnected.

[0182] In the context of UE mobility, first RAN node 702 may be referred to as a source RAN node since it provides the source cell for UE mobility. Likewise, second RAN node 703 may be referred to as target or candidate RAN node since it provides the target or candidate cell for UE mobility. In some cases, such as during intra-gNB or intra-CU mobility, first RAN node 702 and second RAN node 703 may be a single RAN node.

[0183] In case of a distributed CU / DU RAN architecture, first RAN node 702 and / or second RAN node 703 may be divided into a CU and one or more DUs. As shown in Figure 7, first RAN node 702 may include CU 709 and DU 710, which may be referred to as serving CU / DU or source CU / DU for the UE. Likewise, second RAN node 703 may include CU 712 and DU 710, which may be referred to as target CU / DU or candidate CU / DU for the UE. In some cases, such as during intra-gNB or intra-CU mobility, source CU 709 and target CU 712 may be a single CU.

[0184] CU 709 and DU 710 may be connected over an interface 711, which may be an Fl type of interface in case of NG-RAN. Likewise, CU 712 and DU 713 may be connected over an interface 714, which may also be an Fl type of interface in case of NG-RAN.

[0185] First RAN node 702 and second RAN node 703 may be connected to third network node 715 over interfaces 716 and 717, respectively. Third network node 715 may be a RAN node, such as gNB in an NG-RAN. In such case, interfaces 1016 and 1017 may both be an Xn or Xn-C type of interface. Although not shown in the figure, UE 701 may be connected to third network (or RAN) node 715 via a wireless connection.

[0186] UE 701 may be configured with DC, such as NR-DC. In this case, first cell 707 is part of a first MCG provided by first RAN node 702 while second cell 708 is part of a second MCG provided by second RAN node 703. The first MCG may be a source MCG for a UE mobility procedure while the second MCG may be a target MCG for the UE mobility procedure; in this context, first RAN node 702 may be referred to as the UE’s source MN while second RAN node 703 may be referred to as the UE’s target MN. In this context, third network node 715 may control the UE’s SCG, which includes a third cell 718 (e.g., PSCell). As such, third network node 715 may be referred to as the UE’s SN.

[0187] In this system, the UE’s mobility procedure (e.g., LTM cell switch) in the MCG requires a security key updates for both MN and SN, even though the UE maintains the SCG. However, if the UE should be able to perform one or more subsequent mobility procedures that require further security key updates for both MN and SN, the UE needs to have sk-counter values to be used for SN key derivation at each update. If no sk-counter is available at the UE or the SCG for a subsequent mobility procedure, the procedure will fail and the UE will initiate either an RRC reestablishment procedure or an SCG failure information procedure.

[0188] Embodiments address these issues and / or problems by facilitating distribution of sk- counter values to be used for SN key derivation at each update. Embodiments are described using the example mobility procedure of inter-CU LTM, but are equally applicable to other mobility procedures such as HO, CHO, conditional LTM, etc.

[0189] Figure 8 shows a signaling diagram of an LTM cell switch procedure that utilizes embodiments of the first option summarized above. In particular, the procedure involves a UE (810), the UE’s SN (820), the source MN (MN1, 830) for the procedure, and the target MN (MN2, 840) for the procedure. The UE receives an LTM configuration from MN 1 that provides the UE’ s MCG. The LTM configuration includes LTM candidate configurations for respective one or more LTM candidate cells, at least one of which is provided by MN2. The LTM configuration also includes one or more security configurations, and may also include other typical content of an LTM configuration discussed above. Each security configuration includes information needed by the UE to perform a security key update in conjunction with an LTM cell switch procedure to one of the configured LTM candidate cells. For example, the security configuration includes a list of sk-counters that the UE may use an SN security key update in conjunction with an LTM cell switch for the MN, regardless of whether the current SCG is kept or changed by the new MN.

[0190] In some embodiments, the security configuration can be separate from the one or more LTM candidate configurations, but possibly in the same message (e.g., RRCReconfiguratiori). In other embodiments, each LTM candidate configuration may include a security configuration, including a list of sk-counters applicable to the corresponding LTM candidate cell.

[0191] For example, MN1 provides the UE with the following ordered list of sk-counters in the LTM configuration:

[0192] • Sk-counterl;

[0193] • Sk-counter2;

[0194] • Sk-counter3.

[0195] The UE performs LI measurement reporting to MN1 according to the LTM configuration. Based on the measurement reporting, MN1 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provide by MN2 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN.

[0196] In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN2, and selects the first entry in the list (Sk-counterl) and uses it to derive the SN security keys from the MN security keys for MN2. The UE may use these derived keys to secure communication with SN via the SCG. The UE then discards the used value Sk-counterl such that the ordered list can be represented as follows (with strikethrough indicating used / discarded values):

[0197] • — Sk-counterl;

[0198] • Sk-counter2;

[0199] • Sk-counter3.

[0200] Upon transmitting the LTM cell switch command to the UE, MN1 notifies MN2 of the UE’s LTM cell switch to the target cell served by MN2 with no SN change, and includes an indication of the UE’s available sk-counters, i.e., sk-counter2 and sk-counter3, based on MNl’s knowledge that the UE discarded the used value Sk-counterl. The UE performs LI measurement reporting to MN2 according to the LTM configuration. Based on the measurement reporting, MN2 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provide by MN1 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN. In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN1, selects the second entry in the list (Sk-counter2), and uses it to derive the SN security keys from the MN security keys for MN 1. The UE may use these derived keys to secure communication with SN via the SCG. The UE then discards the used value Sk-counter2 such that the ordered list can be represented as follows:

[0201] • — Sk-counterl;

[0202] • — Sk-counter2;

[0203] • Sk-counter3.

[0204] Upon transmitting the LTM cell switch command to the UE, MN2 notifies MN1 of the UE’s LTM cell switch to the target cell served by MN1 with no SN change, and includes an indication of the UE’s available sk-counters, i.e., sk-counter3, based on MNl’s knowledge that the UE discarded the used value Sk-counter2. Based on the UE only having one available sk- counter value remaining, MN 1 sends the UE another LTM configuration that includes a security configuration with three additional sk-counter values. After receiving this information, the UE’s ordered list of values can be represented as follows:

[0205] • — Sk-counterl;

[0206] • — Sk-counter2;

[0207] • Sk-counter3;

[0208] • Sk-counter4;

[0209] • Sk-counter5;

[0210] • Sk-counter6.

[0211] While Figure 8 shows MN1 sending an LTM configuration with additional sk-counter values, it is also possible that MN2 sends the UE an LTM configuration with additional sk-counter values prior to the LTM cell switch back to the target cell served by MN1.

[0212] Figure 9 shows a signaling diagram of an LTM cell switch procedure that utilizes embodiments of the second option summarized above. In particular, the procedure involves a UE (910), the UE’s SN (920), the source MN (MN1, 930) for the procedure, and the target MN (MN2, 940) for the procedure.

[0213] The UE receives an LTM configuration from MN 1 that provides the UE’ s MCG. The LTM configuration includes LTM candidate configurations for respective one or more LTM candidate cells, at least one of which is provided by MN2. The LTM configuration also includes one or more security configurations, and may also include other typical content of an LTM configuration discussed above. Each security configuration includes information needed by the UE to perform a security key update in conjunction with an LTM cell switch procedure to one of the configured LTM candidate cells. For example, the security configuration includes a list of sk-counters that the UE may use an SN security key update in conjunction with an LTM cell switch for the MN, regardless of whether the current SCG is kept or changed by the new MN.

[0214] In some embodiments, the security configuration can be separate from the one or more LTM candidate configurations, but possibly in the same message (e.g., RRCReconfiguratiori). In other embodiments, each LTM candidate configuration may include a security configuration, including a list of sk-counters applicable to the corresponding LTM candidate cell.

[0215] For example, MN1 provides the UE with the following ordered list of sk-counters in the LTM configuration:

[0216] • Sk-counterl;

[0217] • Sk-counter2.

[0218] The UE performs LI measurement reporting to MN1 according to the LTM configuration. Based on the measurement reporting, MN1 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provide by MN2 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN.

[0219] In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN2, selects the first entry in the list (Sk-counterl), and uses it to derive the SN security keys from the MN security keys for MN2. The UE may use these derived keys to secure communication with SN via the SCG. The UE then discards the used value Sk-counterl such that the ordered list can be represented as follows:

[0220] • — Sk-counterl;

[0221] • Sk-counter2.

[0222] Upon transmitting the LTM cell switch command to the UE, MN1 notifies MN2 of the UE’s LTM cell switch to the target cell served by MN2 with no SN change. Additionally, to complete the LTM cell switch, the UE sends an RRCReconfigurationComplete message to MN2 via the new MCG. The UE includes status of the UE’s sk-counters, i.e., sk-counterl has been used and sk-counter2 is available for use. More generally, the UE may include one or more of the following information in the RRCReconfigurationComplete message:

[0223] • last used sk-counter value, or an identifier thereof;

[0224] • list of sk-counter values that have been used, or identifiers thereof;

[0225] • number of available sk-counter values;

[0226] • list of available sk-counter values, or identifiers thereof; • indication that no sk-counter values are available;

[0227] • a request for additional sk-counter values; and

[0228] • a request for a specific number (X) of additional sk-counter values.

[0229] The UE performs LI measurement reporting to MN2 according to the LTM configuration. Based on the measurement reporting, MN2 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provide by MN1 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN. In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN1, selects the second entry in the list (Sk-counter2), and uses it to derive the SN security keys from the MN security keys for MN 1. The UE may use these derived keys to secure communication with SN via the SCG. The UE then discards the used value Sk-counter2 such that the ordered list can be represented as follows:

[0230] • — Sk-counterl;

[0231] • — Sk-counter2.

[0232] Upon transmitting the LTM cell switch command to the UE, MN2 notifies MN1 of the UE’s LTM cell switch to the target cell served by MN1 with no SN change. Additionally, to complete the LTM cell switch, the UE sends an RRCReconfigurationComplete message to MN 1 via the new MCG. The UE includes status of the UE’s sk-counters, i.e., sk-counter2 has been used and no sk-counter values are available for use. The UE may also include other relevant information mentioned above.

[0233] Based on the UE only having no available sk-counter values, MN1 sends the UE another LTM configuration that includes a security configuration with three additional sk-counter values. After receiving this information, the UE’s ordered list of values can be represented as follows:

[0234] • — Sk-counterl;

[0235] • — Sk-counter2;

[0236] • Sk-counter3;

[0237] • Sk-counter4;

[0238] • Sk-counter5.

[0239] Although the example shown in Figure 9 uses an RRCReconfigurationComplete message to convey the UE’s sk-counter status information, other existing (e.g., UEAssistancelnformatiori) or newly-defined RRC messages may also be used for this purpose.

[0240] While Figure 9 shows MN1 sending an LTM configuration with additional sk-counter values, it is also possible that MN2 sends the UE an LTM configuration with additional sk-counter values prior to the LTM cell switch back to the target cell served by MN1. Figure 10 shows a signaling diagram of an LTM cell switch procedure that utilizes embodiments of the third option summarized above. In particular, the procedure involves a UE (1010), the UE’s SN (1020), the source MN (MN1, 1030) for the procedure, and the target MN (MN2, 1040) for the procedure.

[0241] The UE receives an LTM configuration from MN 1 that provides the UE’ s MCG. The LTM configuration includes LTM candidate configurations for respective one or more LTM candidate cells provided by MN1. Additionally, the LTM configuration includes a “nested” LTM configuration for MN2, which includes LTM candidate configurations for respective one or more LTM candidate cells provided by MN2. Each LTM configuration also includes one or more security configurations, and may also include other typical content of an LTM configuration discussed above.

[0242] Each security configuration includes information needed by the UE to perform a security key update in conjunction with an LTM cell switch procedure to one of the configured LTM candidate cells associated with the specific MN. For example, the security configuration in the LTM configuration for MN1 includes a list of sk-counters that the UE may use for an SN security key update in conjunction with an LTM cell switch to a candidate / target cell provided by MN1 (e.g., in an MCG1), while the LTM configuration for MN2 includes a list of sk-counters that the UE may use for an SN security key update in conjunction with an LTM cell switch to a candidate / target cell provided by MN2 (e.g., in an MCG2). These may be used regardless of whether the current SCG is kept or changed by the target MN.

[0243] In some embodiments, the security configuration can be separate from the one or more LTM candidate configurations, but possibly in the same message (e.g., RRCReconfiguratiori). In other embodiments, each LTM candidate configuration may include a security configuration, including a list of sk-counters applicable to the corresponding LTM candidate cell.

[0244] For example, MN1 provides the UE with the following ordered lists of sk-counters in the LTM configurations:

[0245] The UE performs LI measurement reporting to MN1 according to the LTM configuration for MN1. Based on the measurement reporting, MN1 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provide by MN2 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN. In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN2, selects the first entry in the list (Sk-counterl) for MN2, and uses it to derive the SN security keys from the MN security keys for MN2. The UE may use these derived keys to secure communication with SN via the SCG. The UE then discards the used value Sk-counterl such that the ordered list can be represented as follows:

[0246] Upon transmitting the LTM cell switch command to the UE, MN1 notifies MN2 of the UE’s LTM cell switch to the target cell served by MN2 with no SN change. Additionally, to complete the LTM cell switch, the UE sends an RRCReconfigurationComplete message to MN2 via the new MCG. In some embodiments, the UE includes status of the UE’s sk-counters, such as discussed above in relation to other embodiments.

[0247] The UE performs LI measurement reporting to MN2 according to the LTM configuration for MN2. Based on the measurement reporting, MN2 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provided by MN1 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN. In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN1, selects the first entry in the list (Sk-counterl) for MN1, and uses it to derive the SN security keys from the MN security keys for MN1. The UE may use these derived keys to secure communication with the SN via the SCG. The UE then discards the used value Sk-counterl such that the ordered list can be represented as follows:

[0248] Upon transmitting the LTM cell switch command to the UE, MN2 notifies MN1 of the UE’s LTM cell switch to the target cell served by MN1 with no SN change. Additionally, to complete the LTM cell switch, the UE sends an RRCReconfigurationComplete message to MN 1 via the new MCG. In some embodiments, the UE includes status of the UE’s sk-counters, such as discussed above in relation to other embodiments. MN1 then sends the UE another LTM configuration that includes a security configuration with three additional sk-counter values. After receiving this information, the UE’s ordered list of values can be represented as follows:

[0249] In these embodiments, the UE needs to maintain a mapping between lists of sk-counters and associated MNs. One way to realize this mapping is to assign each list of sk-counters with an identifier, which does not need to be necessarily unique over all the MNs and does not need to identify the MN as such. Rather, this identifier needs to uniquely identify the LTM candidate cell configurations for which a particular list of sk-counters is valid. This identifier can consist of one or more (including a combination) of the following:

[0250] • Cell ID;

[0251] • Serving cell ID;

[0252] • NR cell global identity (CGI);

[0253] • gNB ID;

[0254] • physical cell identity (PCI);

[0255] • distributed unit (DU) ID;

[0256] • central unit (CU) ID;

[0257] • securityCellSetld; and

[0258] • servingSecurityCellSetld.

[0259] While Figure 10 shows MN1 sending an LTM configuration with additional sk-counter values, it is also possible that MN2 sends the UE an LTM configuration with additional sk-counter values prior to the LTM cell switch back to the target cell served by MN1.

[0260] Figure 11 shows a signaling diagram of an LTM cell switch procedure that utilizes embodiments of the fourth option summarized above. In particular, the procedure involves a UE (1110), the UE’s SN (1120), the source MN (MN1, 1130) for the procedure, and the target MN (MN2, 1140) for the procedure.

[0261] The UE receives an LTM configuration from MN 1 that provides the UE’ s MCG. The LTM configuration includes LTM candidate configurations for respective one or more LTM candidate cells, at least one of which is provided by MN2. The LTM configuration also includes one or more security configurations, and may also include other typical content of an LTM configuration discussed above. Each security configuration includes information needed by the UE to perform a security key update in conjunction with an LTM cell switch procedure to one of the configured LTM candidate cells. For example, the security configuration includes a list of sk-counters that the UE may use an SN security key update in conjunction with an LTM cell switch for the MN, regardless of whether the current SCG is kept or changed by the new MN.

[0262] In some embodiments, the security configuration can be separate from the one or more LTM candidate configurations, but possibly in the same message (e.g., RRCReconfiguratiori). In other embodiments, each LTM candidate configuration may include a security configuration, including a list of sk-counters applicable to the corresponding LTM candidate cell.

[0263] For example, MN1 provides the UE with the following ordered list of sk-counters in the LTM configuration:

[0264] • Sk-counterl

[0265] • Sk-counter2

[0266] • Sk-counter3

[0267] The UE performs LI measurement reporting to MN1 according to the LTM configuration. Based on the measurement reporting, MN1 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provide by MN2 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN.

[0268] In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN2, and selects the first entry in the list (Sk-counterl) and uses it to derive the SN security keys from the MN security keys for MN2. The UE may use these derived keys to secure communication with SN via the SCG. The UE then discards the used value Sk-counterl such that the ordered list can be represented as follows:

[0269] • — Sk-counterl

[0270] • Sk-counter2

[0271] • Sk-counter3

[0272] Upon transmitting the LTM cell switch command to the UE, MN1 notifies MN2 of the UE’s LTM cell switch to the target cell served by MN2 with no SN change. Additionally, to complete the LTM cell switch, the UE sends an RRCReconfigurationComplete message to MN2 via the new MCG. In some embodiments, the UE includes status of the UE’s sk-counters, such as discussed above in relation to other embodiments. MN2 then sends the UE another LTM configuration that includes a security configuration with a second ordered list of three sk-counter values available for use. The UE replaces its existing ordered list with the second ordered list, as follows:

[0273] • Sk-counter4;

[0274] • Sk-counter5;

[0275] • Sk-counter6.

[0276] The UE performs LI measurement reporting to MN2 according to the LTM configuration. Based on the measurement reporting, MN2 sends the UE an LTM cell switch command (e.g., MAC CE) that identifies one of the configured LTM candidate cells provide by MN1 as the target cell. The LTM cell switch command also instructs the UE to maintain its SCG provided by the SN.

[0277] In response, the UE applies the LTM candidate configuration for the indicated target cell, derives MN security keys for communication with target MN2, and selects the first entry in the list (Sk-counter4) and uses it to derive the SN security keys from the MN security keys for MN1. The UE may use these derived keys to secure communication with SN via the SCG. The UE then discards the used value Sk-counter4 such that the ordered list can be represented as follows:

[0278] • — Sk-counter4;

[0279] • Sk-counter5;

[0280] • Sk-counter6.

[0281] Upon transmitting the LTM cell switch command to the UE, MN2 notifies MN1 of the UE’s LTM cell switch to the target cell served by MN1 with no SN change. Additionally, to complete the LTM cell switch, the UE sends an RRCReconfigurationComplete message to MN 1 via the new MCG. In some embodiments, the UE includes status of the UE’s sk-counters, such as discussed above in relation to other embodiments.

[0282] MN1 then sends the UE another LTM configuration that includes a security configuration with a third ordered list of three sk-counter values available for use. The UE replaces its existing ordered list with the third ordered list, as follows:

[0283] • Sk-counter7;

[0284] • Sk-counter8;

[0285] • Sk-counter9.

[0286] In various embodiments described above, an MN (e.g., MN1 or MN2) sends the UE a further list of sk-counter values. This may be based on the MN determining that the UE has no, or an insufficient number of, unused sk-counter values available. The MN may determine this based on one or more of the following criteria:

[0287] • UE indicates that no sk-counter are available; • UE indicates that “ Y” sk-counter are left to be used;

[0288] • UE indicates the need for more sk-counters;

[0289] • UE indicates the need for “X” more sk-counters;

[0290] • A new LTM candidate cell configuration has been added / modified / released at the UE;

[0291] • An indication from a second MCG or a third MCG, or another network node (e.g., AMF);

[0292] • After triggering “N” LTM cell switch procedures at the UE;

[0293] • After sending “Z” LTM cell switch commands to the UE;

[0294] • After receiving “T” RRCReconfigurationComplete messages from the UE to acknowledge a successful completion of an LTM cell switch procedure;

[0295] • Upon adding / releasing / changing an SCG; and

[0296] • Upon receiving new security information from a network node (e.g., AMF).

[0297] Various features of the embodiments summarized above correspond to various operations illustrated in Figures 12-14, which show exemplary methods (e.g., procedures) for a UE, a first RAN node, and a second RAN node, respectively. In other words, various features of the operations described below correspond to various embodiments described above. Furthermore, the exemplary methods shown in Figures 12-14 may be used cooperatively to provide various benefits, advantages, and / or solutions to problems described herein. Although Figures 12-14 show specific blocks in particular orders, the operations of these exemplary methods may be performed in different orders than shown and may be combined and / or divided into blocks having different functionality than shown. Optional blocks or operations are indicated by dashed lines.

[0298] In particular, Figure 12 shows an exemplary method (e.g., procedure) for a UE configured for mobility between cells of a RAN, according to various embodiments of the present disclosure. The exemplary method can be performed by a UE (e.g., wireless device) such as described elsewhere herein.

[0299] The exemplary method includes the operations of block 1210, where the UE receives, from a first RAN node via a serving cell, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells; and one or more counter lists, with each counter list including one or more counter values usable for deriving a plurality of secondary node (SN) security keys for communication between the UE and a third RAN node that provides a secondary cell group (SCG) for the UE.

[0300] The exemplary method also includes the operations of block 1220, where the UE receives from the first RAN node a mobility command for execution of a mobility procedure from the serving cell. The mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure. The exemplary method also includes the operations of block 1240, where in response to the mobility command, the UE selects a first counter value from one of the counter lists and, based on the first counter value, derives the plurality of SN security keys for use after execution of the mobility procedure.

[0301] In some embodiments, the plurality of SN security keys include a master SN security key (e.g., KSN or S-KSNB) and the following derived from the master SN security key: a first SN security key for integrity protection of signaling, a second SN security key for ciphering of signaling, a third SN security key for integrity protection of user data, and a fourth SN security key for ciphering of user data. In some embodiments, the mobility command also includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0302] In some embodiments, the exemplary method also includes the operations of block 1250, where the UE transmits, to the second RAN node via the target cell, a message indicating that the mobility procedure is complete. In some of these embodiments, the exemplary method also includes the operations of block 1230, where in response to the mobility command, the UE derives a plurality of MN security keys usable to secure communication with the second RAN node after execution of the mobility procedure.

[0303] In some variants of these embodiments, the plurality of MN security keys include a master MN security key (e.g., KSNB) and the following derived from the master MN security key: a first MN security key for integrity protection of signaling (e.g., KRRCint), a second MN security key for ciphering of signaling (e.g., KRRCenc), a third MN security key for integrity protection of user data (e.g., Kupint), and a fourth MN security key for ciphering of user data (e.g., KuPenc).

[0304] In some further variants, deriving the plurality of SN security keys in block 1240 is further based on the master MN security key. In some further variants, the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one of the first and second MN security keys.

[0305] In some of these embodiments, the message sent to the second RAN node includes one or more of the following status information:

[0306] • the first counter value, or an identifier thereof;

[0307] • a list of counter values used and discarded by the UE, or identifiers thereof;

[0308] • a number of unused counter values available to the UE;

[0309] • a list of unused counter values available to the UE, or identifiers thereof;

[0310] • an indication that no unused counter values are available to the UE;

[0311] • a request for additional counter values; and

[0312] • a request for a specific number of additional counter values.

[0313] In some embodiments, the exemplary method also includes the operations of blocks 1215 and 1245, where the UE stores the received one or more counter lists and, after deriving the SN security keys in block 1240, discards the first counter value from the stored counter list from which it was selected. In some of these embodiments, the one or most received counter lists include a single counter list usable for all MNs for the UE, and the first counter value is selected in block 1240 from the single counter list.

[0314] In other of these embodiments, the one or more counter lists include a first counter list usable for the first RAN node (i.e., while the first RAN node is MN) and a second counter list usable for the second RAN node (i.e., while the second RAN node is MN), and the first counter value is selected in block 1240 from the second counter list. In some variants of these embodiments, the mobility configuration includes the following: a first subset of the one or more candidate configurations, the first counter list, and a nested second mobility configuration that includes the second counter list and a second subset of the one or more candidate configurations.

[0315] In some embodiments, the exemplary method also includes the following operations, labelled with corresponding block numbers:

[0316] • (1260) after execution of the mobility procedure to the target cell, receiving from the second RAN node a second mobility configuration comprising a further counter list that includes one or more further counter values, with each further counter value being usable for deriving the plurality of SN security keys; and

[0317] • (1270) updating a stored counter list based on the further counter list.

[0318] In some of these embodiments, updating the stored counter list in block 1270 comprises one of sub-blocks 1271-1272, where the UE replaces the stored counter list with the further counter list, or appends the further counter list to the end of the stored counter list.

[0319] In some embodiments, after execution of the mobility procedure, the target cell becomes the serving cell and the exemplary method also includes the following operations labelled with corresponding block numbers:

[0320] • (1280) receiving from the second RAN node a second mobility command for execution of a second mobility procedure from the serving cell, wherein the second mobility command identifies a second one of the mobility candidate cells provided by the first RAN node as a target cell for the second mobility procedure; and

[0321] • (1290) in response to the second mobility command, selecting a second counter value from one of the counter lists and, based on the second counter value, deriving the plurality of SN security keys for use after execution of the second mobility procedure.

[0322] In some of these embodiments, the second counter value is a next available value in one of the following: a single counter list usable for all MNs for the UE, or a first counter list usable only for the first RAN node. In some embodiments, the first and second RAN nodes are different CUs of a single RAN node. In other embodiments, the first and second RAN nodes are different RAN nodes.

[0323] In some embodiments, the mobility procedure is a non-conditional mobility procedure and execution of the mobility procedure is responsive to the mobility command. In some of these embodiments, the mobility procedure is one of the following: an initial layer-3 (L3) handover (HO), a subsequent L3 HO, an initial layer-l / layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent LTM cell switch.

[0324] In other embodiments, the mobility procedure is a conditional mobility procedure, each candidate configuration includes an associated execution condition, and the exemplary method also includes the operations of block 1225, where after receiving the mobility command, the UE determines that the execution condition associated with the target cell is fulfilled. In such case, execution of the mobility procedure is responsive to determining that the execution condition associated with target cell is fulfilled in block 1230. In some of these embodiments, the mobility procedure is one of the following: an initial L3 conditional handover (CHO), a subsequent L3 CHO, an initial conditional LTM cell switch, or a subsequent conditional LTM cell switch.

[0325] In addition, Figure 13 shows an exemplary method (e.g., procedure) for a first RAN node configured to facilitate mobility between cells by UEs, according to various embodiments of the present disclosure. The exemplary method can be performed by a RAN node (e.g., base station, eNB, gNB, ng-eNB, DU, etc.) such as described elsewhere herein.

[0326] The exemplary method includes the operations of block 1320, where the first RAN node sends, to a UE via a serving cell, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists. Each counter list includes one or more counter values usable for deriving a plurality of SN security keys for communication between the UE and a third RAN node that provides an SCG for the UE. The exemplary method also includes the operations of block 1330, where the first RAN node sends to the UE a mobility command for execution of a mobility procedure from the serving cell. The mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure. The one or more counter lists facilitate UE derivation of the plurality of SN security keys for use after execution of the mobility procedure to the target cell.

[0327] In some embodiments, the plurality of SN security keys include a master SN security key (e.g., KSN or S-KSNB) and the following security keys derived from the master SN security key: a first SN security key for integrity protection of signaling, a second SN security key for ciphering of signaling, a third SN security key for integrity protection of user data, and a fourth SN security key for ciphering of user data. In some embodiments, the mobility command also includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0328] In some embodiments, the one or more counter lists include a single counter list usable for all MNs for the UE. In other embodiments, the one or most counter lists include a first counter list usable for the first RAN node (i.e., while the first RAN node is the UE’s MN) and a second counter list usable for the second RAN node (i.e., while the first RAN node is the UE’s MN). In some of these embodiments, the mobility configuration includes the following: a first subset of the one or more candidate configurations, the first counter list, and a nested second mobility configuration that includes the second counter list and a second subset of the one or more candidate configurations. In some variants of these embodiments, the exemplary method also includes the operations of block 1310, where the first RAN node receives the second mobility configuration from the second RAN node.

[0329] In some embodiments, the exemplary method also includes the operations of block 1340, where the first RAN node sends to the second RAN node a notification of the mobility procedure for the UE from the serving cell to the target cell provided by the second RAN node. The notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0330] In some embodiments, the exemplary method also includes the operations of block 1350, where the first RAN node receives from the second RAN node a notification of a second mobility procedure for the UE from a serving cell provided by the second RAN node to a target cell provided by the first RAN node. The notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the second mobility procedure to the target cell.

[0331] In some of these embodiments, the target cell for the second mobility procedure is a second one of the mobility candidate cells. In some of these embodiments, the exemplary method also includes the operations of block 1360, where after UE execution of the second mobility procedure to the target cell, the first RAN node sends to the UE a second mobility configuration comprising a further counter list that includes one or more further counter values, with each further counter value being usable for deriving the plurality of SN security keys.

[0332] In some embodiments, the first and second RAN nodes are different CUs of a single RAN node. In other embodiments, the first and second RAN nodes are different RAN nodes.

[0333] In some embodiments, the mobility procedure is a non-conditional mobility procedure and the mobility command causes execution of the mobility procedure by the UE. In some of these embodiments, the mobility procedure is one of the following: an initial L3 HO, a subsequent L3 HO, an initial LTM cell switch, or a subsequent LTM cell switch. In other embodiments, the mobility procedure is a conditional mobility procedure, each candidate configuration includes an associated execution condition, and fulfillment of the execution condition associated with target cell (i.e., after the mobility command) causes execution of the mobility procedure by the UE. In some of these embodiments, the mobility procedure is one of the following: an initial L3 CHO, a subsequent L3 CHO, an initial conditional LTM cell switch, or a subsequent conditional LTM cell switch.

[0334] In addition, Figure 14 shows an exemplary method (e.g., procedure) for a second RAN node configured to facilitate mobility between cells by UEs, according to various embodiments of the present disclosure. The exemplary method can be performed by a RAN node (e.g., base station, eNB, gNB, ng-eNB, DU, etc. such as described elsewhere herein.

[0335] The exemplary method includes the operations of block 1410, where the second RAN node sends to a first RAN node a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells provided by the second RAN node. The exemplary method also includes the operations of block 1420, where the second RAN node receives from the first RAN node a notification of a mobility procedure for a UE from a serving cell provided by the first RAN node to one of the mobility candidate cells as a target cell. The exemplary method also includes the operations of block 1430, where the second RAN node receives from the UE a message indicating that the mobility procedure is complete. The message includes status information for counter values usable by the UE for deriving a plurality of SN security keys for communication between the UE and a third RAN node that provides an SCG for the UE.

[0336] In some embodiments, the status information includes one or more of the following::

[0337] • a first counter value used and discarded by the UE in conjunction with the mobility procedure, or an identifier thereof;

[0338] • a list of counter values used and discarded by the UE, or identifiers thereof;

[0339] • a number of unused counter values available to the UE;

[0340] • a list of unused counter values available to the UE, or identifiers thereof;

[0341] • an indication that no unused counter values are available to the UE;

[0342] • a request for additional counter values; and

[0343] • a request for a specific number of additional counter values.

[0344] In some embodiments, the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one of the first and second keys. In some embodiments, the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0345] In some embodiments, the mobility configuration includes a second counter list including one or more counter values and each counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node, but only while being served by the second RAN node.

[0346] In some embodiments, the exemplary method also includes the operations of block 1440, where the second RAN node sends to the UE a second mobility configuration comprising a further counter list that includes one or more further counter values. Each further counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node. In some of these embodiments, the further counter list facilitates UE update of a counter list provided by the first RAN node and stored by the UE.

[0347] In some embodiments, after UE execution of the mobility procedure the target cell becomes the serving cell and the exemplary method also includes the operations of block 1450, where the second RAN node sends to the UE a second mobility command for execution of a second mobility procedure from the serving cell to a target cell provided by the first RAN node. The second mobility command includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0348] In some embodiments, the first and second RAN nodes are different centralized units (CU) of a single RAN node. In other embodiments, the first and second RAN nodes are different RAN nodes.

[0349] In some embodiments, the mobility procedure is a non-conditional mobility procedure and the mobility command causes execution of the mobility procedure by the UE. In some of these embodiments, the mobility procedure is one of the following: an initial L3 HO, a subsequent L3 HO, an initial LTM cell switch, or a subsequent LTM cell switch.

[0350] In other embodiments, the mobility procedure is a conditional mobility procedure, each candidate configuration includes an associated execution condition, and fulfillment of the execution condition associated with target cell (i.e., after the mobility command) causes execution of the mobility procedure by the UE. In some of these embodiments, the mobility procedure is one of the following: an initial L3 CHO, a subsequent L3 CHO, an initial conditional LTM cell switch, or a subsequent conditional LTM cell switch.

[0351] Although various embodiments are described above in terms of methods, techniques, and / or procedures, the person of ordinary skill will readily comprehend that such methods, techniques, and / or procedures can be embodied by various combinations of hardware and software in various systems, communication devices, computing devices, control devices, apparatuses, non-transitory computer-readable media, computer program products, etc.

[0352] Figure 15 shows an example of a communication system 1500 in accordance with some embodiments. In this example, communication system 1500 includes a telecommunication network 1502 that includes an access network 1504 (e.g., RAN) and a core network 1506, which includes one or more core network nodes 1508. Access network 1504 includes one or more access network nodes, such as network nodes 1510a-b (one or more of which may be generally referred to as network nodes 1510), or any other similar 3 GPP access nodes or non-3GPP access points. Moreover, as will be appreciated by those of skill in the art, a network node is not necessarily limited to an implementation in which a radio portion and a baseband portion are supplied and integrated by a single vendor. Thus, it will be understood that network nodes include disaggregated implementations or portions thereof. For example, in some embodiments, telecommunication network 1502 includes one or more Open-RAN (ORAN) network nodes. An ORAN network node is a node in telecommunication network 1502 that supports an ORAN specification (e.g., a specification published by the O-RAN Alliance, or any similar organization) and may operate alone or together with other nodes to implement one or more functionalities of any node in telecommunication network 1502, including one or more network nodes 1510 and / or core network nodes 1508.

[0353] Examples of an ORAN network node include an open radio unit (O-RU), an open distributed unit (O-DU), an open central unit (O-CU), including an O-CU control plane (O-CU- CP) or an O-CU user plane (O-CU-UP), a RAN intelligent controller (near-real time or non-real time) hosting software or software plug-ins, such as a near-real time control application (e.g., xApp) or a non-real time control application (e.g., rApp), or any combination thereof (the adjective “open” designating support of an ORAN specification). The network node may support a specification by, for example, supporting an interface defined by the ORAN specification, such as an Al, Fl, Wl, El, E2, X2, Xn interface, an open fronthaul user plane interface, or an open fronthaul management plane interface. Moreover, an ORAN access node may be a logical node in a physical node. Furthermore, an ORAN network node may be implemented in a virtualization environment (described further below) in which one or more network functions are virtualized. For example, the virtualization environment may include an O-Cloud computing platform orchestrated by a Service Management and Orchestration Framework via an O-2 interface defined by the O-RAN Alliance or comparable technologies. Network nodes 1510 facilitate direct or indirect connection of UEs, such as by connecting UEs 1512a-d (one or more of which may be generally referred to as UEs 1512) to core network 1506 over one or more wireless connections.

[0354] Example wireless communications over a wireless connection include transmitting and / or receiving wireless signals using electromagnetic waves, radio waves, infrared waves, and / or other types of signals suitable for conveying information without the use of wires, cables, or other material conductors. Moreover, in different embodiments, communication system 1500 may include any number of wired or wireless networks, network nodes, UEs, and / or any other components or systems that may facilitate or participate in the communication of data and / or signals whether via wired or wireless connections. Communication system 1500 may include and / or interface with any type of communication, telecommunication, data, cellular, radio network, and / or other similar type of system.

[0355] UEs 1512 may be any of a wide variety of communication devices, including wireless devices arranged, configured, and / or operable to communicate wirelessly with network nodes 1510 and other communication devices. Similarly, network nodes 1510 are arranged, capable, configured, and / or operable to communicate directly or indirectly with UEs 1512 and / or with other network nodes or equipment in telecommunication network 1502 to enable and / or provide network access, such as wireless network access, and / or to perform other functions, such as administration in telecommunication network 1502.

[0356] In the depicted example, core network 1506 connects network nodes 1510 to one or more hosts, such as host 1516. These connections may be direct or indirect via one or more intermediary networks or devices. In other examples, network nodes may be directly coupled to hosts. Core network 1506 includes one or more core network nodes (e.g., 1508) that are structured with hardware and software components. Features of these components may be substantially similar to those described with respect to the UEs, network nodes, and / or hosts, such that the descriptions thereof are generally applicable to the corresponding components of core network node 1508. Example core network nodes include functions of one or more of a Mobile Switching Center (MSC), Mobility Management Entity (MME), Home Subscriber Server (HSS), Access and Mobility Management Function (AMF), Session Management Function (SMF), Authentication Server Function (AUSF), Subscription Identifier De-concealing function (SIDF), Unified Data Management (UDM), Security Edge Protection Proxy (SEPP), Network Exposure Function (NEF), and / or a User Plane Function (UPF).

[0357] Host 1516 may be under the ownership or control of a service provider other than an operator or provider of access network 1504 and / or telecommunication network 1502, and may be operated by the service provider or on behalf of the service provider. Host 1516 may host a variety of applications to provide one or more service. Examples of such applications include live and pre-recorded audio / video content, data collection services such as retrieving and compiling data on various ambient conditions detected by a plurality of UEs, analytics functionality, social media, functions for controlling or otherwise interacting with remote devices, functions for an alarm and surveillance center, or any other such function performed by a server.

[0358] As a whole, communication system 1500 of Figure 15 enables connectivity between the UEs, network nodes, and hosts. In that sense, the communication system may be configured to operate according to predefined rules or procedures, such as specific standards that include, but are not limited to: Global System for Mobile Communications (GSM); Universal Mobile Telecommunications System (UMTS); Long Term Evolution (LTE), and / or other suitable 2G, 3G, 4G, 5G standards, or any applicable future generation standard (e.g., 6G); wireless local area network (WLAN) standards, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.15 standards (WiFi); and / or any other appropriate wireless communication standard, such as the Worldwide Interoperability for Microwave Access (WiMax), Bluetooth, Z-Wave, Near Field Communication (NFC) ZigBee, LiFi, and / or any low-power wide-area network (LPWAN) standards such as LoRa and Sigfox.

[0359] In some examples, telecommunication network 1502 is a cellular network that implements 3GPP standardized features. Accordingly, telecommunication network 1502 may support network slicing to provide different logical networks to different devices that are connected to telecommunication network 1502. For example, telecommunication network 1502 may provide Ultra Reliable Low Latency Communication (URLLC) services to some UEs, while providing Enhanced Mobile Broadband (eMBB) services to other UEs, and / or Massive Machine Type Communication (mMTC) / Massive loT services to yet further UEs.

[0360] In some examples, UEs 1512 are configured to transmit and / or receive information without direct human interaction. For instance, a UE may be designed to transmit information to access network 1504 on a predetermined schedule, when triggered by an internal or external event, or in response to requests from access network 1504. Additionally, a UE may be configured for operating in single- or multi-RAT or multi-standard mode. For example, a UE may operate with any one or combination of Wi-Fi, NR (New Radio) and LTE, i.e. being configured for multi-radio dual connectivity (MR-DC), such as E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) New Radio - Dual Connectivity (EN-DC).

[0361] In the example, hub 1514 communicates with access network 1504 to facilitate indirect communication between one or more UEs (e.g., 1512c and / or 1512d) and network nodes (e.g., 1510b). In some examples, hub 1514 may be a controller, router, content source and analytics, or any of the other communication devices described herein regarding UEs. For example, hub 1514 may be a broadband router enabling access to core network 1506 for the UEs. As another example, hub 1514 may be a controller that sends commands or instructions to one or more actuators in the UEs. Commands or instructions may be received from the UEs, network nodes 1510, or by executable code, script, process, or other instructions in hub 1514. As another example, hub 1514 may be a data collector that acts as temporary storage for UE data and, in some embodiments, may perform analysis or other processing of the data. As another example, hub 1514 may be a content source. For example, for a UE that is a VR headset, display, loudspeaker or other media delivery device, hub 1514 may retrieve VR assets, video, audio, or other media or data related to sensory information via a network node, which hub 1514 then provides to the UE either directly, after performing local processing, and / or after adding additional local content. In still another example, hub 1514 acts as a proxy server or orchestrator for the UEs, in particular if one or more of the UEs are low energy loT devices.

[0362] Hub 1514 may have a constant / persistent or intermittent connection to network node 1510b. Hub 1514 may also allow for a different communication scheme and / or schedule between hub 1514 and UEs (e.g., 1512c and / or 1512d), and between hub 1514 and core network 1506. In other examples, hub 1514 is connected to core network 1506 and / or one or more UEs via a wired connection. Moreover, hub 1514 may be configured to connect to an M2M service provider over access network 1504 and / or to another UE over a direct connection. In some scenarios, UEs may establish a wireless connection with network nodes 1510 while still connected via hub 1514 via a wired or wireless connection. In some embodiments, hub 1514 may be a dedicated hub - that is, a hub whose primary function is to route communications to / from the UEs from / to network node 1510b. In other embodiments, hub 1514 may be a non-dedicated hub - that is, a device which is capable of operating to route communications between the UEs and network node 1510b, but which is additionally capable of operating as a communication start and / or end point for certain data channels.

[0363] In some embodiments, any of network nodes 1510 may be configured to perform operations attributed to a RAN node in various embodiments described above, including the exemplary methods shown in Figures 8-11 and 13-14. In some embodiments, any of UEs 1512 may be configured to perform operations attributed to a UE in various embodiments described above, including the exemplary method shown in Figures 8-12.

[0364] Figure 16 shows a UE 1600 in accordance with some embodiments. Examples of a UE include, but are not limited to, a smart phone, mobile phone, cell phone, voice over IP (VoIP) phone, wireless local loop phone, desktop computer, personal digital assistant (PDA), wireless cameras, gaming console or device, music storage device, playback appliance, wearable terminal device, wireless endpoint, mobile station, tablet, laptop, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), smart device, wireless customer-premise equipment (CPE), vehicle, vehicle-mounted or vehicle embedded / integrated wireless device, etc. Other examples include any UE identified by 3 GPP, including a narrow band internet of things (NB-IoT) UE, a machine type communication (MTC) UE, and / or an enhanced MTC (eMTC) UE.

[0365] UE 1600 may support device-to-device (D2D) communication, for example by implementing a 3 GPP standard for sidelink communication, Dedicated Short-Range Communication (DSRC), vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), or vehicle-to- everything (V2X). In other examples, UE 1600 may not necessarily have a user in the sense of a human user who owns and / or operates the relevant device. Instead, UE 1600 may represent a device that is intended for sale to, or operation by, a human user but which may not, or which may not initially, be associated with a specific human user (e.g., a smart sprinkler controller). Alternatively, UE 1600 may represent a device that is not intended for sale to, or operation by, an end user but which may be associated with or operated for the benefit of a user (e.g., a smart power meter).

[0366] UE 1600 includes processing circuitry 1602 that is operatively coupled via a bus 1604 to an input / output interface 1606, a power source 1608, a memory 1610, a communication interface 1612, and / or any other component, or any combination thereof. Certain UEs may utilize all or a subset of the components shown in Figure 16. The level of integration between the components may vary from one UE to another UE. Further, certain UEs may contain multiple instances of a component, such as multiple processors, memories, transceivers, transmitters, receivers, etc.

[0367] Processing circuitry 1602 is configured to process instructions and data and may be configured to implement any sequential state machine operative to execute instructions stored as machine-readable computer programs in memory 1610. Processing circuitry 1602 may be implemented as one or more hardware-implemented state machines (e.g., in discrete logic, field- programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), etc.); programmable logic together with appropriate firmware; one or more stored computer programs, general -purpose processors, such as a microprocessor or digital signal processor (DSP), together with appropriate software; or any combination of the above. For example, processing circuitry 1602 may include multiple central processing units (CPUs).

[0368] In the example, input / output interface 1606 may be configured to provide an interface or interfaces to an input device, output device, or one or more input and / or output devices. Examples of an output device include a speaker, a sound card, a video card, a display, a monitor, a printer, an actuator, an emitter, a smartcard, another output device, or any combination thereof. An input device may allow a user to capture information into UE 1600. Examples of an input device include a touch-sensitive or presence-sensitive display, a camera (e.g., a digital camera, a digital video camera, a web camera, etc.), a microphone, a sensor, a mouse, a trackball, a directional pad, a trackpad, a scroll wheel, a smartcard, and the like. The presence-sensitive display may include a capacitive or resistive touch sensor to sense input from a user. A sensor may be, for instance, an accelerometer, a gyroscope, a tilt sensor, a force sensor, a magnetometer, an optical sensor, a proximity sensor, a biometric sensor, etc., or any combination thereof. An output device may use the same type of interface port as an input device. For example, a Universal Serial Bus (USB) port may be used to provide an input device and an output device. In some embodiments, power source 1608 is structured as a battery or battery pack. Other types of power sources, such as an external power source (e.g., an electricity outlet), photovoltaic device, or power cell, may be used. Power source 1608 may further include power circuitry for delivering power from power source 1608 itself, and / or an external power source, to the various parts of UE 1600 via input circuitry or an interface such as an electrical power cable. Delivering power may be, for example, for charging of power source 1608. Power circuitry may perform any formatting, converting, or other modification to the power from power source 1608 to make the power suitable for the respective components of UE 1600 to which power is supplied.

[0369] Memory 1610 may be or be configured to include memory such as random access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), magnetic disks, optical disks, hard disks, removable cartridges, flash drives, and so forth. In one example, memory 1610 includes one or more application programs 1614, such as an operating system, web browser application, a widget, gadget engine, or other application, and corresponding data 1616. Memory 1610 may store, for use by UE 1600, any of a variety of various operating systems or combinations of operating systems.

[0370] Memory 1610 may be configured to include a number of physical drive units, such as redundant array of independent disks (RAID), flash memory, USB flash drive, external hard disk drive, thumb drive, pen drive, key drive, high-density digital versatile disc (HD-DVD) optical disc drive, internal hard disk drive, Blu-Ray optical disc drive, holographic digital data storage (HDDS) optical disc drive, external mini-dual in-line memory module (DIMM), synchronous dynamic random access memory (SDRAM), external micro-DIMM SDRAM, smartcard memory such as tamper resistant module in the form of a universal integrated circuit card (UICC) including one or more subscriber identity modules (SIMs), such as a USIM and / or ISIM, other memory, or any combination thereof. The UICC may for example be an embedded UICC (eUICC), integrated UICC (iUICC) or a removable UICC commonly known as ‘SIM card.’ Memory 1610 may allow UE 1600 to access instructions, application programs and the like, stored on transitory or non- transitory memory media, to off-load data, or to upload data. An article of manufacture, such as one utilizing a communication system may be tangibly embodied as or in memory 1610, which may be or comprise a device-readable storage medium.

[0371] Processing circuitry 1602 may be configured to communicate with an access network or other network using communication interface 1612. Communication interface 1612 may comprise one or more communication subsystems and may include or be communicatively coupled to an antenna 1622. Communication interface 1612 may include one or more transceivers used to communicate, such as by communicating with one or more remote transceivers of another device capable of wireless communication (e.g., another UE or a network node in an access network). Each transceiver may include a transmitter 1618 and / or a receiver 1620 appropriate to provide network communications (e.g., optical, electrical, frequency allocations, and so forth). Moreover, transmitter 1618 and receiver 1620 may be coupled to one or more antennas (e.g., antenna 1622) and may share circuit components, software, or firmware, or alternatively be implemented separately.

[0372] In the illustrated embodiment, communication functions of communication interface 1612 may include cellular communication, Wi-Fi communication, LPWAN communication, data communication, voice communication, multimedia communication, short-range communications such as Bluetooth, near-field communication, location-based communication such as the use of the global positioning system (GPS) to determine a location, another like communication function, or any combination thereof. Communications may be implemented in according to one or more communication protocols and / or standards, such as IEEE 802.11, Code Division Multiplexing Access (CDMA), Wideband Code Division Multiple Access (WCDMA), GSM, LTE, New Radio (NR), UMTS, WiMax, Ethernet, transmission control protocol / internet protocol (TCP / IP), synchronous optical networking (SONET), Asynchronous Transfer Mode (ATM), QUIC, Hypertext Transfer Protocol (HTTP), and so forth.

[0373] Regardless of the type of sensor, UE 1600 may provide an output of data captured by its sensors, through its communication interface 1612, via a wireless connection to a network node. Data captured by sensors of UE 1600 can be communicated through a wireless connection to a network node via another UE. The output may be periodic (e.g., once every 15 minutes if it reports the sensed temperature), random (e.g., to even out the load from reporting from several sensors), in response to a triggering event (e.g., when moisture is detected an alert is sent), in response to a request (e.g., a user initiated request), or a continuous stream (e.g., a live video feed of a patient).

[0374] As another example, UE 1600 comprises an actuator, a motor, or a switch, related to a communication interface configured to receive wireless input from a network node via a wireless connection. In response to the received wireless input the states of the actuator, the motor, or the switch may change. For example, UE 1600 may comprise a motor that adjusts the control surfaces or rotors of a drone in flight according to the received input or to a robotic arm performing a medical procedure according to the received input.

[0375] UE 1600, when in the form of an Internet of Things (loT) device, may be a device for use in one or more application domains, these domains comprising, but not limited to, city wearable technology, extended industrial application and healthcare. Non-limiting examples of such an loT device are a device which is or which is embedded in: a connected refrigerator or freezer, a TV, a connected lighting device, an electricity meter, a robot vacuum cleaner, a voice controlled smart speaker, a home security camera, a motion detector, a thermostat, a smoke detector, a door / window sensor, a flood / moisture sensor, an electrical door lock, a connected doorbell, an air conditioning system like a heat pump, an autonomous vehicle, a surveillance system, a weather monitoring device, a vehicle parking monitoring device, an electric vehicle charging station, a smart watch, a fitness tracker, a head-mounted display for Augmented Reality (AR) or Virtual Reality (VR), a wearable for tactile augmentation or sensory enhancement, a water sprinkler, an animal- or item-tracking device, a sensor for monitoring a plant or animal, an industrial robot, an Unmanned Aerial Vehicle (UAV), and any kind of medical device, like a heart rate monitor or a remote controlled surgical robot. A UE in the form of an loT device comprises circuitry and / or software in dependence of the intended application of the loT device in addition to other components as described in relation to UE 1600 shown in Figure 16.

[0376] As yet another specific example, in an loT scenario, UE 1600 may represent a machine or other device that performs monitoring and / or measurements, and transmits the results of such monitoring and / or measurements to another UE and / or a network node. UE 1600 may in this case be an M2M device, which may in a 3 GPP context be referred to as an MTC device. As one particular example, UE 1600 may implement the 3 GPP NB-IoT standard. In other scenarios, UE 1600 may represent a vehicle, such as a car, a bus, a truck, a ship and an airplane, or other equipment that is capable of monitoring and / or reporting on its operational status or other functions associated with its operation.

[0377] In practice, any number of UEs may be used together with respect to a single use case. For example, a first UE might be or be integrated in a drone and provide the drone’s speed information (obtained through a speed sensor) to a second UE that is a remote controller operating the drone. When the user makes changes from the remote controller, the first UE may adjust the throttle on the drone (e.g. by controlling an actuator) to increase or decrease the drone’s speed. The first and / or the second UE can also include more than one of the functionalities described above. For example, a UE might comprise the sensor and the actuator, and handle communication of data for both the speed sensor and the actuators.

[0378] In some embodiments, UE 1600 may be configured to perform operations attributed to a UE in various embodiments described above, including the exemplary method shown in Figures 8-12.

[0379] Figure 17 shows a network node 1700 in accordance with some embodiments. Examples of network nodes include, but are not limited to, access points (APs) (e.g., radio access points), base stations (e.g., radio base stations, Node Bs, eNBs, gNBs), and 0-RAN nodes or components of an 0-RAN node (e g., O-RU, O-DU, O-CU). Base stations may be categorized based on the amount of coverage they provide (or, stated differently, their transmit power level) and so, depending on the provided amount of coverage, may be referred to as femto base stations, pico base stations, micro base stations, or macro base stations. A base station may be a relay node or a relay donor node controlling a relay. A network node may also include one or more (or all) parts of a distributed radio base station such as centralized digital units, distributed units (e.g., in an O-RAN access node) and / or remote radio units (RRUs), sometimes referred to as Remote Radio Heads (RRHs). Such remote radio units may or may not be integrated with an antenna as an antenna integrated radio. Parts of a distributed radio base station may also be referred to as nodes in a distributed antenna system (DAS).

[0380] Other examples of network nodes include multiple transmission point (multi-TRP) 5G access nodes, multi-standard radio (MSR) equipment such as MSR BSs, network controllers such as radio network controllers (RNCs) or base station controllers (BSCs), base transceiver stations (BTSs), transmission points, transmission nodes, multi-cell / multicast coordination entities (MCEs), Operation and Maintenance (O&M) nodes, Operations Support System (OSS) nodes, Self-Organizing Network (SON) nodes, positioning nodes (e.g., Evolved Serving Mobile Location Centers (E-SMLCs)), and / or Minimization of Drive Tests (MDTs).

[0381] Network node 1700 includes processing circuitry 1702, memory 1704, communication interface 1706, and power source 1708. Network node 1700 may be composed of multiple physically separate components (e.g., a NodeB component and a RNC component, or a BTS component and a BSC component, etc.), which may each have their own respective components. In certain scenarios in which network node 1700 comprises multiple separate components (e.g., BTS and BSC components), one or more of the separate components may be shared among several network nodes. For example, a single RNC may control multiple NodeBs. In such a scenario, each unique NodeB and RNC pair, may in some instances be considered a single separate network node. In some embodiments, network node 1700 may be configured to support multiple radio access technologies (RATs). In such embodiments, some components may be duplicated (e.g., separate memory 1704 for different RATs) and some components may be reused (e.g., a same antenna 1710 may be shared by different RATs). Network node 1700 may also include multiple sets of the various illustrated components for different wireless technologies integrated into network node 1700, for example GSM, WCDMA, LTE, NR, WiFi, Zigbee, Z-wave, LoRaWAN, Radio Frequency Identification (RFID) or Bluetooth wireless technologies. These wireless technologies may be integrated into the same or different chip or set of chips and other components within network node 1700.

[0382] Processing circuitry 1702 may comprise a combination of one or more of a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application-specific integrated circuit, field programmable gate array, or any other suitable computing device, resource, or combination of hardware, software and / or encoded logic operable to provide, either alone or in conjunction with other network node 1700 components, such as memory 1704, to provide network node 1700 functionality.

[0383] In some embodiments, processing circuitry 1702 includes a system on a chip (SOC). In some embodiments, processing circuitry 1702 includes one or more of radio frequency (RF) transceiver circuitry 1712 and baseband processing circuitry 1714. In some embodiments, RF transceiver circuitry 1712 and baseband processing circuitry 1714 may be on separate chips (or sets of chips), boards, or units, such as radio units and digital units. In alternative embodiments, part or all of RF transceiver circuitry 1712 and baseband processing circuitry 1714 may be on the same chip or set of chips, boards, or units.

[0384] Memory 1704 may comprise any form of volatile or non-volatile computer-readable memory including, without limitation, persistent storage, solid-state memory, remotely mounted memory, magnetic media, optical media, random access memory (RAM), read-only memory (ROM), mass storage media (for example, a hard disk), removable storage media (for example, a flash drive, a Compact Disk (CD) or a Digital Video Disk (DVD)), and / or any other volatile or non-volatile, non-transitory device-readable and / or computer-executable memory devices that store information, data, and / or instructions that may be used by processing circuitry 1702. Memory 1704 may store any suitable instructions, data, or information, including a computer program, software, an application including one or more of logic, rules, code, tables, and / or other instructions (collected denoted computer program 1704a, which may be in the form of a computer program product) capable of being executed by processing circuitry 1702 and utilized by network node 1700. Memory 1704 may be used to store any calculations made by processing circuitry 1702 and / or any data received via communication interface 1706. In some embodiments, processing circuitry 1702 and memory 1704 is integrated.

[0385] Communication interface 1706 is used in wired or wireless communication of signaling and / or data between a network node, access network, and / or UE. As illustrated, communication interface 1706 comprises port(s) / terminal(s) 1716 to send and receive data, for example to and from a network over a wired connection. Communication interface 1706 also includes radio frontend circuitry 1718 that may be coupled to, or in certain embodiments a part of, antenna 1710. Radio front-end circuitry 1718 comprises filters 1720 and amplifiers 1722. Radio front-end circuitry 1718 may be connected to an antenna 1710 and processing circuitry 1702. The radio front-end circuitry may be configured to condition signals communicated between antenna 1710 and processing circuitry 1702. Radio front-end circuitry 1718 may receive digital data that is to be sent out to other network nodes or UEs via a wireless connection. Radio front-end circuitry 1718 may convert the digital data into a radio signal having the appropriate channel and bandwidth parameters using a combination of filters 1720 and / or amplifiers 1722. The radio signal may then be transmitted via antenna 1710. Similarly, when receiving data, antenna 1710 may collect radio signals which are then converted into digital data by radio front-end circuitry 1718. The digital data may be passed to processing circuitry 1702. In other embodiments, the communication interface may comprise different components and / or different combinations of components.

[0386] In certain alternative embodiments, network node 1700 does not include separate radio front-end circuitry 1718, instead, processing circuitry 1702 includes radio front-end circuitry and is connected to antenna 1710. Similarly, in some embodiments, all or some of RF transceiver circuitry 1712 is part of communication interface 1706. In still other embodiments, communication interface 1706 includes one or more ports or terminals 1716, radio front-end circuitry 1718, and RF transceiver circuitry 1712, as part of a radio unit (not shown), and communication interface 1706 communicates with baseband processing circuitry 1714, which is part of a digital unit (not shown).

[0387] Antenna 1710 may include one or more antennas, or antenna arrays, configured to send and / or receive wireless signals. Antenna 1710 may be coupled to radio front-end circuitry 1718 and may be any type of antenna capable of transmitting and receiving data and / or signals wirelessly. In certain embodiments, antenna 1710 is separate from network node 1700 and connectable to network node 1700 through an interface or port.

[0388] Antenna 1710, communication interface 1706, and / or processing circuitry 1702 may be configured to perform any receiving operations and / or certain obtaining operations described herein as being performed by the network node. Any information, data and / or signals may be received from a UE, another network node and / or any other network equipment. Similarly, antenna 1710, communication interface 1706, and / or processing circuitry 1702 may be configured to perform any transmitting operations described herein as being performed by the network node. Any information, data and / or signals may be transmitted to a UE, another network node and / or any other network equipment.

[0389] Power source 1708 provides power to the various components of network node 1700 in a form suitable for the respective components (e.g., at a voltage and current level needed for each respective component). Power source 1708 may further comprise, or be coupled to, power management circuitry to supply the components of network node 1700 with power for performing the functionality described herein. For example, network node 1700 may be connectable to an external power source (e.g., the power grid, an electricity outlet) via an input circuitry or interface such as an electrical cable, whereby the external power source supplies power to power circuitry of power source 1708. As a further example, power source 1708 may comprise a source of power in the form of a battery or battery pack which is connected to, or integrated in, power circuitry. The battery may provide backup power should the external power source fail.

[0390] Embodiments of network node 1700 may include additional components beyond those shown in Figure 17 for providing certain aspects of the network node’s functionality, including any of the functionality described herein and / or any functionality necessary to support the subject matter described herein. For example, network node 1700 may include user interface equipment to allow input of information into network node 1700 and to allow output of information from network node 1700. This may allow a user to perform diagnostic, maintenance, repair, and other administrative functions for network node 1700.

[0391] In some embodiments, network node 1700 may be configured to perform operations attributed to a RAN node in various embodiments described above, including the exemplary methods shown in Figures 8-11 and 13-14.

[0392] Figure 18 is a block diagram illustrating a virtualization environment 1800 in which functions implemented by some embodiments may be virtualized. In the present context, virtualizing means creating virtual versions of apparatuses or devices which may include virtualizing hardware platforms, storage devices and networking resources. As used herein, virtualization can be applied to any device described herein, or components thereof, and relates to an implementation in which at least a portion of the functionality is implemented as one or more virtual components. Some or all of the functions described herein may be implemented as virtual components executed by one or more virtual machines (VMs) implemented in one or more virtual environments 1800 hosted by one or more of hardware nodes, such as a hardware computing device that operates as a network node, UE, core network node, or host. Further, in embodiments in which the virtual node does not require radio connectivity (e.g., a core network node or host), then the node may be entirely virtualized. In some embodiments, the virtualization environment 1800 includes components defined by the O-RAN Alliance, such as an O-Cloud environment orchestrated by a Service Management and Orchestration Framework via an O-2 interface.

[0393] Applications 1802 (which may alternatively be called software instances, virtual appliances, network functions, virtual nodes, virtual network functions, etc.) are run in the virtualization environment 1800 to implement some of the features, functions, and / or benefits of some of the embodiments disclosed herein. For example, one or more virtual nodes 1802 may be configured to perform operations attributed to a RAN node in various embodiments described above, including the exemplary methods shown in Figures 8-11 and 13-14.

[0394] Hardware 1804 includes processing circuitry, memory that stores software and / or instructions (collected denoted computer program 1804a, which may be in the form of a computer program product) executable by hardware processing circuitry, and / or other hardware devices as described herein, such as a network interface, input / output interface, and so forth. Software may be executed by the processing circuitry to instantiate one or more virtualization layers 1806 (also referred to as hypervisors or virtual machine monitors (VMMs)), provide VMs 1808a and 1808b (one or more of which may be generally referred to as VMs 1808), and / or perform any of the functions, features and / or benefits described in relation with some embodiments described herein. Virtualization layer 1806 may present a virtual operating platform that appears like networking hardware to the VMs 1808.

[0395] VMs 1808 comprise virtual processing, virtual memory, virtual networking or interface and virtual storage, and may be run by a corresponding virtualization layer 1806. Different embodiments of the instance of a virtual appliance 1802 may be implemented on one or more of VMs 1808, and the implementations may be made in different ways. Virtualization of the hardware is in some contexts referred to as network function virtualization (NFV). NFV may be used to consolidate many network equipment types onto industry standard high volume server hardware, physical switches, and physical storage, which can be located in data centers, and customer premise equipment.

[0396] In the context of NFV, each VM 1808 may be a software implementation of a physical machine that runs programs as if they were executing on a physical, non-virtualized machine. Each VM 1808, and that part of hardware 1804 that executes that VM, be it hardware dedicated to that VM and / or hardware shared by that VM with others of the VMs, forms separate virtual network elements. Still in the context of NFV, a virtual network function is responsible for handling specific network functions that run in one or more VMs 1808 on top of the hardware 1804 and corresponds to the application 1802.

[0397] Hardware 1804 may be implemented in a standalone network node with generic or specific components. Hardware 1804 may implement some functions via virtualization. Alternatively, hardware 1804 may be part of a larger cluster of hardware (e.g. such as in a data center or CPE) where many hardware nodes work together and are managed via management and orchestration function 1810, which, among others, oversees lifecycle management of applications 1802. In some embodiments, hardware 1804 is coupled to one or more radio units that each include one or more transmitters and one or more receivers that may be coupled to one or more antennas. Radio units may communicate directly with other hardware nodes via one or more appropriate network interfaces and may be used in combination with the virtual components to provide a virtual node with radio capabilities, such as a radio access node or a base station. In some embodiments, some signaling can be provided with the use of a control system 1812 which may alternatively be used for communication between hardware nodes and radio units. The foregoing merely illustrates the principles of the disclosure. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements, and procedures that, although not explicitly shown or described herein, embody the principles of the disclosure and can be thus within the spirit and scope of the disclosure. Various embodiments can be used together with one another, as well as interchangeably therewith, as should be understood by those having ordinary skill in the art.

[0398] The term unit, as used herein, can have conventional meaning in the field of electronics, electrical devices and / or electronic devices and can include, for example, electrical and / or electronic circuitry, devices, modules, processors, memories, logic solid state and / or discrete devices, computer programs or instructions for carrying out respective tasks, procedures, computations, outputs, and / or displaying functions, and so on, as such as those that are described herein.

[0399] Any appropriate steps, methods, features, functions, or benefits disclosed herein may be performed through one or more functional units or modules of one or more virtual apparatuses. Each virtual apparatus may comprise a number of these functional units. These functional units may be implemented via processing circuitry, which may include one or more microprocessor or microcontrollers, as well as other digital hardware, which may include Digital Signal Processor (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as Read Only Memory (ROM), Random Access Memory (RAM), cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory includes program instructions for executing one or more telecommunications and / or data communications protocols as well as instructions for carrying out one or more of the techniques described herein. In some implementations, the processing circuitry may be used to cause the respective functional unit to perform corresponding functions according to one or more embodiments of the present disclosure.

[0400] As described herein, device and / or apparatus can be represented by a semiconductor chip, a chipset, or a (hardware) module comprising such chip or chipset; this, however, does not exclude the possibility that a functionality of a device or apparatus, instead of being hardware implemented, be implemented as a software module such as a computer program or a computer program product comprising executable software code portions for execution or being run on a processor. Furthermore, functionality of a device or apparatus can be implemented by any combination of hardware and software. A device or apparatus can also be regarded as an assembly of multiple devices and / or apparatuses, whether functionally in cooperation with or independently of each other. Moreover, devices and apparatuses can be implemented in a distributed fashion throughout a system, so long as the functionality of the device or apparatus is preserved. Such and similar principles are considered as known to a skilled person.

[0401] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0402] In addition, certain terms used in the present disclosure, including the specification and drawings, can be used synonymously in certain instances (e.g., “data” and “information”). It should be understood that although such terms can be used synonymously herein, there can be instances when such terms are not intended to be used synonymously.

[0403] Embodiments of the techniques and apparatus described herein also include, but are not limited to, the following enumerated examples:

[0404] Al . A method for a user equipment (UE) configured for mobility between cells of a radio access network (RAN), the method comprising: receiving, from a first RAN node via a serving cell of a first master cell group (MCG), a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, each counter list including one or more counter values usable for deriving a plurality of secondary node (SN) security keys for communication between the UE and a third RAN node that provides a secondary cell group (SCG) for the UE; receiving from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure; and in response to the mobility command, selecting a first counter value from one of the counter lists and, based on the first counter value, deriving the plurality of SN security keys usable to secure communication with the third RAN node after execution of the mobility procedure.

[0405] Ala. The method of embodiment Al, wherein the plurality of SN security keys include a master SN security key and the following security keys derived from the master SN security key: a first key for integrity protection of signaling, a second key for ciphering of signaling, a third key for integrity protection of user data, and a fourth key for ciphering of user data.

[0406] Alb. The method of any of embodiments Al-Ala, wherein the mobility command also includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0407] A2. The method of any of embodiments Al -Alb, further comprising transmitting, to the second RAN node via the target cell, a message indicating that the mobility procedure is complete.

[0408] A2a. The method of embodiment A2, further comprising, in response to the mobility command, deriving a plurality of master node (MN) security keys usable to secure communication with the second RAN node after execution of the mobility procedure.

[0409] A2b. The method of embodiment A2a, wherein the plurality of MN security keys include a master MN security key and the following security keys derived from the master MN security key: a first key for integrity protection of signaling, a second key for ciphering of signaling, a third key for integrity protection of user data, and a fourth key for ciphering of user data.

[0410] A2c. The method of embodiment A2b, wherein deriving the plurality of SN security keys is further based on the master MN security key.

[0411] A2d. The method of any of embodiments A2b-A2c, wherein the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one of the first and second keys.

[0412] A2e. The method of any of embodiments A2-A2d, wherein the message sent to the second RAN node includes one or more of the following status information: the first counter value, or an identifier thereof; a list of counter values used and discarded by the UE, or identifiers thereof; a number of unused counter values available to the UE; a list of unused counter values available to the UE, or identifiers thereof; an indication that no unused counter values are available to the UE; a request for additional counter values; and a request for a specific number of additional counter values.

[0413] A3. The method of any of embodiments Al-A2e, further comprising: storing the received one or more counter lists; and after deriving the SN security keys, discarding the first counter value from the stored counter list from which it was selected.

[0414] A3a. The method of embodiment A3, wherein the one or most received counter lists include a single counter list usable for all MNs for the UE, and the first counter value is selected from the single counter list.

[0415] A3b. The method of embodiment A3, wherein the one or most received counter lists include a first counter list usable for the first RAN node and a second counter list usable for the second RAN node, and the first counter value is selected from the second counter list.

[0416] A3c. The method of embodiment A3b, wherein the mobility configuration includes the following: a first subset of the one or more candidate configurations, the first counter list, and a nested second mobility configuration that includes the second counter list and a second subset of the one or more candidate configurations.

[0417] A3d. The method of any of embodiments A3-A3c, further comprising: after execution of the mobility procedure to the target cell, receiving from the second RAN node a second mobility configuration comprising a further counter list that includes one or more further counter values, each further counter value being usable for deriving the plurality of SN security keys; and updating a stored counter list based on the further counter list.

[0418] A3e. The method of embodiment A3d, wherein updating the stored counter list comprises one of the following: replacing the stored counter list with the further counter list, or appending the further counter list to the end of the stored counter list.

[0419] A4. The method of any of embodiments Al-A3e, wherein after execution of the mobility procedure, the target cell becomes the serving cell and the method further comprises: receiving from the second RAN node a second mobility command for execution of a second mobility procedure from the serving cell, wherein the second mobility command identifies a second one of the mobility candidate cells provided by the first RAN node as a target cell for the second mobility procedure; and in response to the second mobility command, selecting a second counter value from one of the counter lists and, based on the second counter value, deriving a second plurality of SN security keys usable to secure communication with the third RAN node after execution of the second mobility procedure.

[0420] A4a. The method of embodiment A4, wherein the second counter value is a next available value in one of the following: a single counter list usable for all MNs for the UE, or a first counter list usable only for the first RAN node.

[0421] A5. The method of any of embodiments Al-A4a, wherein the first and second RAN nodes are one of the following: different centralized units (CU) of a single RAN node, or different RAN nodes.

[0422] A6. The method of any of embodiments A1-A5, wherein the mobility procedure is a nonconditional mobility procedure and execution of the mobility procedure is responsive to the mobility command.

[0423] A6a. The method of embodiment A6, wherein mobility procedure is one of the following: an initial layer-3 (L3) handover (HO), a subsequent L3 HO, an initial layer-l / layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent LTM cell switch.

[0424] A7. The method of any of embodiments Al-A4a, wherein: the mobility procedure is a conditional mobility procedure, each candidate configuration includes an associated execution condition, the method further comprises, after receiving the command, determining that the execution condition associated with the target cell is fulfilled, and execution of the mobility procedure is responsive to determining that the execution condition associated with target cell is fulfilled.

[0425] A8. The method of embodiment A7, wherein the mobility procedure is one of the following: an initial layer-3 (L3) conditional handover (CHO), a subsequent L3 CHO, an initial conditional layer-l / layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent conditional LTM cell switch.

[0426] Bl. A method for a first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the method comprising: sending, to a UE via a serving cell of a first master cell group (MCG), a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, each counter list including one or more counter values, each counter value being usable for deriving a plurality of secondary node (SN) security keys for communication between the UE and a third RAN node that provides a secondary cell group (SCG) for the UE; and sending to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure, and the one or more counter lists facilitate UE derivation of a plurality of secondary node (SN) security keys usable to secure communication with the third RAN node after execution of the mobility procedure to the target cell.

[0427] Bia. The method of embodiment Bl, wherein the plurality of SN security keys include a master SN security key and the following security keys derived from the master SN security key: a first key for integrity protection of signaling, a second key for ciphering of signaling, a third key for integrity protection of user data, and a fourth key for ciphering of user data.

[0428] Bib. The method of any of embodiments Bl-Bla, wherein the mobility command also includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0429] B2. The method of any of embodiments Bl -Bib, wherein the one or more counter lists include a single counter list usable for all master nodes (MNs) for the UE.

[0430] B3. The method of any of embodiments Bl -Bib, wherein the one or most counter lists include a first counter list usable for the first RAN node and a second counter list usable for the second RAN node, and the first counter value is selected from the second counter list. B3a. The method of embodiment B3, wherein the mobility configuration includes the following: a first subset of the one or more candidate configurations, the first counter list, and a nested second mobility configuration that includes the second counter list and a second subset of the one or more candidate configurations.

[0431] B3b. The method of embodiment B3a, further comprising receiving the second mobility configuration from the second RAN node.

[0432] B4. The method of any of embodiments Bl-B3b, further comprising sending to the second RAN node a notification of the mobility procedure for the UE from the serving cell to the target cell provided by the second RAN node, wherein the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0433] B5. The method of any of embodiments B1-B4, further comprising receiving from the second RAN node a notification of a second mobility procedure for the UE from a serving cell provided by the second RAN node to a target cell provided by the first RAN node, wherein the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the second mobility procedure to the target cell.

[0434] B5a. The method of embodiment B5, wherein the target cell for the second mobility procedure is a second one of the mobility candidate cells.

[0435] B5b. The method of any of embodiments B5-B5a, further comprising, after UE execution of the second mobility procedure to the target cell, sending to the UE a second mobility configuration comprising a further counter list that includes one or more further counter values, each further counter value being usable for deriving the plurality of SN security keys.

[0436] B6. The method of any of embodiments Bl-B5b, wherein the first and second RAN nodes are one of the following: different centralized units (CU) of a single RAN node, or different RAN nodes.

[0437] B7. The method of any of embodiments B1-B6, wherein the mobility procedure is a nonconditional mobility procedure and the mobility command causes execution of the mobility procedure by the UE.

[0438] B7a. The method of embodiment B7, wherein mobility procedure is one of the following: an initial layer-3 (L3) handover (HO), a subsequent L3 HO, an initial layer-l / layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent LTM cell switch.

[0439] B8. The method of any of embodiments B1-B6, wherein the mobility procedure is a conditional mobility procedure; each candidate configuration includes an associated execution condition; and fulfillment of the execution condition associated with the target cell, after the mobility command, causes execution of the mobility procedure by the UE.

[0440] B8a. The method of embodiment B8, wherein the mobility procedure is one of the following: an initial layer-3 (L3) conditional handover (CHO), a subsequent L3 CHO, an initial conditional layer-l / layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent conditional LTM cell switch.

[0441] CL A method for a second radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the method comprising: sending to a first RAN node a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells provided by the second RAN node; receiving from the first RAN node a notification of a mobility procedure for a UE from a serving cell provided by the first RAN node to one of the mobility candidate cells as a target cell; and receiving from the UE a message indicating that the mobility procedure is complete, wherein the message includes status information for counter values usable by the UE for deriving a plurality of secondary node (SN) security keys for communication between the UE and a third RAN node that provides a secondary cell group (SCG) for the UE.

[0442] Cl a. The method of embodiment Cl, wherein the status information includes one or more of the following:: a first counter value used and discarded by the UE in conjunction with the mobility procedure, or an identifier thereof; a list of counter values used and discarded by the UE, or identifiers thereof; a number of unused counter values available to the UE; a list of unused counter values available to the UE, or identifiers thereof; an indication that no unused counter values are available to the UE; a request for additional counter values; and a request for a specific number of additional counter values.

[0443] Clb. The method of any of embodiments Cl-Cla, wherein the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one of the first and second keys.

[0444] Clc. The method of any of embodiments Cl -Clb, wherein the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0445] C2. The method of any of embodiments Cl -Clc, wherein: the mobility configuration includes a second counter list including one or more counter values; and each counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node, but only while being served by the second RAN node.

[0446] C3. The method of any of embodiments C1-C2, further comprising sending to the UE a second mobility configuration comprising a further counter list that includes one or more further counter values, wherein each further counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node.

[0447] C3a. The method of embodiment C3, wherein the further counter list facilitates UE update of a counter list provided by the first RAN node and stored by the UE.

[0448] C4. The method of any of embodiments Cl-C3a, wherein: after UE execution of the mobility procedure, the target cell becomes the serving cell; the method further comprises sending to the UE a mobility command for execution of a second mobility procedure from the serving cell to a target cell provided by the first RAN node; and the mobility command includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

[0449] C5. The method of any of embodiments C1-C4, wherein the first and second RAN nodes are one of the following: different centralized units (CU) of a single RAN node, or different RAN nodes.

[0450] C6. The method of any of embodiments C1-C5, wherein the mobility procedure is a nonconditional mobility procedure and the mobility command causes execution of the mobility procedure by the UE.

[0451] C6a. The method of embodiment C6, wherein mobility procedure is one of the following: an initial layer-3 (L3) handover (HO), a subsequent L3 HO, an initial layer-l / layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent LTM cell switch.

[0452] C7. The method of any of embodiments C1-C5, wherein the mobility procedure is a conditional mobility procedure; each candidate configuration includes an associated execution condition; and fulfillment of the execution condition associated with the target cell, after the mobility command, causes execution of the mobility procedure by the UE.

[0453] C7a. The method of embodiment C7, wherein the mobility procedure is one of the following: an initial layer-3 (L3) conditional handover (CHO), a subsequent L3 CHO, an initial conditional layer-l / layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent conditional LTM cell switch.

[0454] DI. User equipment (UE) configured for mobility between cells of a radio access network (RAN), the UE comprising: communication interface circuitry configured to communicate with RAN nodes; and processing circuitry operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to perform operations corresponding to the methods of any of embodiments A1-A8.

[0455] D2. User equipment (UE) configured for mobility between cells of a radio access network (RAN), the UE being further configured to perform operations corresponding to the methods of any of embodiments A1-A8. D3. A non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry of user equipment (UE) configured for mobility between cells of a radio access network (RAN), configure the UE to perform operations corresponding to the methods of any of embodiments A1-A8.

[0456] D4. A computer program product comprising computer-executable instructions that, when executed by processing circuitry of user equipment (UE) configured for mobility between cells of a radio access network (RAN), configure the UE to perform operations corresponding to the methods of any of embodiments A1-A8.

[0457] El . A first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the first RAN node comprising: communication interface circuitry configured to communicate with UEs and with other RAN nodes; and processing circuitry operatively coupled to the communication interface circuitry, whereby the processing circuitry and the communication interface circuitry are configured to perform operations corresponding to the methods of any of embodiments Bl-B8a.

[0458] E2. A first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the first RAN node being further configured to perform operations corresponding to the methods of any of embodiments Bl-B8a.

[0459] E3. A non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry of a first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), configure the first RAN node to perform operations corresponding to the methods of any of embodiments Bl-B8a.

[0460] E4. A computer program product comprising computer-executable instructions that, when executed by processing circuitry of a first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), configure the first RAN node to perform operations corresponding to the methods of any of embodiments Bl-B8a.

[0461] Fl. A second radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the second RAN node comprising: communication interface circuitry configured to communicate with UEs and with other RAN nodes; and processing circuitry operatively coupled to the communication interface circuitry, whereby the processing circuitry and the communication interface circuitry are configured to perform operations corresponding to the methods of any of embodiments Cl-C7a.

[0462] F2. A second radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the second RAN node being further configured to perform operations corresponding to the methods of any of embodiments Cl-C7a.

[0463] F3. A non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry of a second radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), configure the second RAN node to perform operations corresponding to the methods of any of embodiments Cl-C7a.

[0464] F4. A computer program product comprising computer-executable instructions that, when executed by processing circuitry of a second radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), configure the second RAN node to perform operations corresponding to the methods of any of embodiments Cl-C7a.

Claims

CLAIMS1. A method for a user equipment, UE, configured for mobility between cells of a radio access network, RAN, the method comprising: receiving (1210), from a first RAN node via a serving cell of a first master cell group, MCG, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, wherein each counter list includes one or more counter values usable for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node that provides a secondary cell group, SCG, for the UE; receiving (1220) from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure; and in response to the mobility command, selecting (1240) a first counter value from one of the counter lists and, based on the first counter value, deriving the plurality of SN security keys for use after execution of the mobility procedure.

2. The method of claim 1, wherein the plurality of SN security keys include a master SN security key and the following derived from the master SN security key: a first SN security key for integrity protection of signaling, a second SN security key for ciphering of signaling, a third SN security key for integrity protection of user data, and a fourth SN security key for ciphering of user data.

3. The method of any of claims 1-2, wherein the mobility command also includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

4. The method of any of claims 1-3, further comprising transmitting (1250), to the second RAN node via the target cell, a message indicating that the mobility procedure is complete.

5. The method of claim 4, further comprising, in response to the mobility command, deriving (1230) a plurality of master node, MN, security keys usable to secure communication with the second RAN node after execution of the mobility procedure.

6. The method of claim 5, wherein the plurality of MN security keys include a master MN security key and the following derived from the master MN security key: a first MN security key for integrity protection of signaling, a second MN security key for ciphering of signaling, a third MN security key for integrity protection of user data, and a fourth MN security key for ciphering of user data.

7. The method of claim 6, wherein one or more of the following applies: deriving (1230) the plurality of SN security keys is further based on the master MN security key; and the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one of the first and second MN security keys.

8. The method of any of claims 4-7, wherein the message sent to the second RAN node includes one or more of the following status information: the first counter value, or an identifier thereof; a list of counter values used and discarded by the UE, or identifiers thereof; a number of unused counter values available to the UE; a list of unused counter values available to the UE, or identifiers thereof; an indication that no unused counter values are available to the UE; a request for additional counter values; and a request for a specific number of additional counter values.

9. The method of any of claims 1-8, further comprising: storing (1215) the one or more counter lists received in the mobility configuration; and after deriving (1230) the plurality of SN security keys, discarding (1245) the first counter value from the stored counter list from which it was selected.

10. The method of claim 9, wherein the one or more counter lists include one of the following:a single counter list usable in conjunction with any MN for the UE, wherein the first counter value is selected from the single counter list; or a first counter list usable while being served by the first RAN node and a second counter list usable while being served by the second RAN node, wherein the first counter value is selected from the second counter list.

11. The method of claim 10, wherein the mobility configuration includes the following: a first subset of the one or more candidate configurations, the first counter list, and a nested second mobility configuration that includes the second counter list and a second subset of the one or more candidate configurations.

12. The method of any of claims 9-11, further comprising: after execution of the mobility procedure to the target cell, receiving (1260) from the second RAN node a second mobility configuration comprising a further counter list that includes one or more further counter values, each further counter value being usable for deriving the plurality of SN security keys; and updating (1270) a stored counter list based on the further counter list, according to one of the following: replacing (1271) the stored counter list with the further counter list, or appending (1272) the further counter list to the end of the stored counter list.

13. The method of any of claims 1-12, wherein after execution of the mobility procedure, the target cell becomes the serving cell and the method further comprises: receiving (1280) from the second RAN node a second mobility command for execution of a second mobility procedure from the serving cell, wherein the second mobility command identifies a second one of the mobility candidate cells provided by the first RAN node as a target cell for the second mobility procedure; and in response to the second mobility command, selecting (1290) a second counter value from one of the counter lists and, based on the second counter value, deriving the plurality of SN security keys for use after execution of the second mobility procedure.

14. The method of claim 13, wherein the second counter value is a next available value in one of the following: a single counter list usable in conjunction with any MN for the UE, or a first counter list usable only for the first RAN node.

15. The method of any of claims 1-14, wherein the first and second RAN nodes are one of the following: different centralized units, CUs of a single RAN node; or different RAN nodes.

16. The method of any of claims 1-15, wherein: the mobility procedure is one of the following: an initial layer-3, L3, handover, HO; a subsequent L3 HO, an initial layer-l / layer-2 triggered inter-cell mobility, LTM, cell switch; or a subsequent LTM cell switch; and execution of the mobility procedure is responsive to the mobility command.

17. The method of any of claims 1-15, wherein: the mobility procedure is the mobility procedure is one of the following: an initial layer- 3, L3, conditional handover, CHO; a subsequent L3 CHO; an initial conditional layer-l / layer-2 triggered inter-cell mobility, LTM, cell switch; or a subsequent conditional LTM cell switch; each candidate configuration includes an associated execution condition, the method further comprises, after receiving (1220) the mobility command, determining (1225) that the execution condition associated with the target cell is fulfilled, and execution of the mobility procedure is responsive to determining (1225) that the execution condition associated with target cell is fulfilled.

18. A method for a first radio access network, RAN, node configured to facilitate mobility between cells by user equipment, UEs, the method comprising: sending (1320), to a UE via a serving cell of a first master cell group, MCG, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, wherein each counter list includes one or more counter values usable for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node that provides a secondary cell group, SCG, for the UE; andsending (1330) to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure, and the one or more counter lists facilitate UE derivation of the plurality of SN security keys for use after execution of the mobility procedure to the target cell.

19. The method of claim 18, wherein the plurality of SN security keys include a master SN security key and the following derived from the master SN security key: a first SN security key for integrity protection of signaling, a second SN security key for ciphering of signaling, a third SN security key for integrity protection of user data, and a fourth SN security key for ciphering of user data.

20. The method of any of claims 18-19, wherein the mobility command also includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

21. The method of any of claims 18-20, wherein the one or more counter lists include one of the following: a single counter list usable in conjunction with any master node, MN, for the UE; or a first counter list usable while being served by the first RAN node and a second counter list usable while being served by the second RAN node.

22. The method of claim 21, wherein the mobility configuration includes the following: a first subset of the one or more candidate configurations, the first counter list, and a nested second mobility configuration that includes the second counter list and a second subset of the one or more candidate configurations.

23. The method of claim 22, further comprising receiving (1310) the second mobility configuration from the second RAN node.

24. The method of any of claims 18-23, further comprising sending (1340) to the second RAN node a notification of the mobility procedure for the UE from the serving cell to the targetcell provided by the second RAN node, wherein the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

25. The method of any of claims 18-24, further comprising receiving (1350) from the second RAN node a notification of a second mobility procedure for the UE from a serving cell provided by the second RAN node to a target cell provided by the first RAN node, wherein one or more of the following applies: the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the second mobility procedure to the target cell; and the target cell for the second mobility procedure is a second one of the mobility candidate cells.

26. The method of claim 25, further comprising, after UE execution of the second mobility procedure to the target cell, sending (1360) to the UE a second mobility configuration comprising a further counter list that includes one or more further counter values, each further counter value being usable for deriving the plurality of SN security keys.

27. The method of any of claims 18-26, wherein the first and second RAN nodes are one of the following: different centralized units, CUs, of a single RAN node; or different RAN nodes.

28. The method of any of claims 18-27, wherein: the mobility procedure is one of the following: an initial layer-3, L3, handover, HO; a subsequent L3 HO; an initial layer-l / layer-2 triggered inter-cell mobility, LTM, cell switch; or a subsequent LTM cell switch; and the mobility command causes execution of the mobility procedure by the UE.

29. The method of any of claims 18-27, wherein: the mobility procedure is one of the following: an initial layer-3, L3, conditional handover, CHO; a subsequent L3 CHO; an initial conditional layer-l / layer-2 triggered inter-cell mobility, LTM, cell switch; or a subsequent conditional LTM cell switch; each candidate configuration includes an associated execution condition; andfulfillment of the execution condition associated with the target cell, after the mobility command, causes execution of the mobility procedure by the UE.

30. A method for a second radio access network, RAN, node configured to facilitate mobility between cells by user equipment, UEs, the method comprising: sending (1410) to a first RAN node a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells provided by the second RAN node; receiving (1420) from the first RAN node a notification of a mobility procedure for a UE from a serving cell provided by the first RAN node to one of the mobility candidate cells as a target cell; and receiving (1430) from the UE a message indicating that the mobility procedure is complete, wherein the message includes status information for counter values usable by the UE for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node that provides a secondary cell group, SCG, for the UE.

31. The method of claim 30, wherein the status information includes one or more of the following:: a first counter value used and discarded by the UE in conjunction with the mobility procedure, or an identifier thereof; a list of counter values used and discarded by the UE, or identifiers thereof; a number of unused counter values available to the UE; a list of unused counter values available to the UE, or identifiers thereof; an indication that no unused counter values are available to the UE; a request for additional counter values; and a request for a specific number of additional counter values.

32. The method of any of claims 30-31, wherein one or more of the following applies: the plurality of SN security keys are derived by the UE based on an master MN security key; and the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one other MN security key derived by the UE from the master MN security key.

33. The method of any of claims 30-32, wherein the notification includes an indication that the UE will maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

34. The method of any of claims 30-33, wherein: the mobility configuration includes a second counter list including one or more counter values; and each counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node, but only while being served by the second RAN node.

35. The method of any of claims 30-34, further comprising, after receiving (1430) the message from the UE, sending (1440) to the UE a second mobility configuration comprising a further counter list that includes one or more further counter values, wherein one or more of the following applies: each further counter value is usable by the UE for deriving the plurality of SN security keys for communication between the UE and the third RAN node; and the further counter list facilitates UE update of a counter list provided by the first RAN node and stored by the UE.

36. The method of any of claims 30-35, wherein: after the mobility procedure is complete, the target cell becomes the serving cell; the method further comprises, after receiving (1430) the message from the UE, sending (1450) to the UE a second mobility command for execution of a second mobility procedure from the serving cell to a target cell provided by the first RAN node; and the second mobility command includes an indication for the UE to maintain the SCG provided by the third RAN node after execution of the mobility procedure to the target cell.

37. The method of any of claims 30-36, wherein the first and second RAN nodes are one of the following: different centralized units, CUs, of a single RAN node; or different RAN nodes.

38. The method of any of claims 30-37, wherein:the mobility procedure is one of the following: initial layer-3, L3, handover, HO; subsequent L3 HO; initial layer-l / layer-2 triggered inter-cell mobility, LTM, cell switch; or subsequent LTM cell switch; and the mobility command causes execution of the mobility procedure by the UE.

39. The method of any of claims 30-37, wherein: the mobility procedure is one of the following: initial layer-3, L3, conditional handover, CHO; subsequent L3 CHO; initial conditional layer-l / layer-2 triggered inter-cell mobility, LTM, cell switch; or subsequent conditional LTM cell switch; each candidate configuration includes an associated execution condition; and fulfillment of the execution condition associated with the target cell, after the mobility command, causes execution of the mobility procedure by the UE.

40. User equipment, UE (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600) configured for mobility between cells of a radio access network, RAN (199, 1504), the UE comprising: communication interface circuitry (1612) configured to communicate with RAN nodes; and processing circuitry (1602) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: receiving, from a first RAN node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802) via a serving cell of a first master cell group, MCG, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, wherein each counter list includes one or more counter values usable for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node (100, 150, 320, 715, 820, 920, 1020, 1120, 1510, 1700, 1802) that provides a secondary cell group, SCG, for the UE; receiving from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells provided by a second RAN node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) as a target cell for the mobility procedure; andin response to the mobility command, selecting a first counter value from one of the counter lists and, based on the first counter value, deriving the plurality of SN security keys usable to secure communication with the third RAN node after execution of the mobility procedure.

41. The UE of claim 40, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 2-17.

42. User equipment, UE (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600) configured for mobility between cells of a radio access network, RAN (199, 1504), the UE being further configured to: receiving, from a first RAN node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802) via a serving cell of a first master cell group, MCG, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, wherein each counter list includes one or more counter values usable for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node (100, 150, 320, 715, 820, 920, 1020, 1120, 1510, 1700, 1802) that provides a secondary cell group, SCG, for the UE; receiving from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells provided by a second RAN node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) as a target cell for the mobility procedure; and in response to the mobility command, selecting a first counter value from one of the counter lists and, based on the first counter value, deriving the plurality of SN security keys usable to secure communication with the third RAN node after execution of the mobility procedure.

43. The UE of claim 42, being further configured to perform operations corresponding to the methods of any of claims 2-17.

44. Non-transitory, computer-readable medium (1610) storing computer-executable instructions that, when executed by processing circuitry (1602) of user equipment, UE (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600) configured for mobility between cells of a radio access network, RAN (199, 1504), configure the UE to perform operations corresponding to the methods of any of claims 1-17.

45. Computer program product (1614) comprising computer-executable instructions that, when executed by processing circuitry (1602) of user equipment, UE (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600) configured for mobility between cells of a radio access network, RAN (199, 1504), configure the UE to perform operations corresponding to the methods of any of claims 1-17.

46. First radio access network, RAN, node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802) configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), the first RAN node comprising: communication interface circuitry (1706, 1804) configured to communicate with UEs and with other RAN nodes; and processing circuitry (1702, 1804) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: send, to a UE via a serving cell of a first master cell group, MCG, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, wherein each counter list includes one or more counter values usable for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node (100, 150, 320, 715, 820, 920, 1020, 1120, 1510, 1700, 1802) that provides a secondary cell group, SCG, for the UE; and send to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) as a target cell for the mobility procedure, andthe one or more counter lists facilitate UE derivation of the plurality of SN security keys after execution of the mobility procedure to the target cell.

47. The first RAN node of claim 46, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 19-29.

48. First radio access network, RAN, node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802) configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), the first RAN node being further configured to: send, to a UE via a serving cell of a first master cell group, MCG, a mobility configuration comprising: one or more candidate configurations for respective one or more mobility candidate cells, and one or more counter lists, wherein each counter list includes one or more counter values usable for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node (100, 150, 320, 715, 820, 920, 1020, 1120, 1510, 1700, 1802) that provides a secondary cell group, SCG, for the UE; and send to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) as a target cell for the mobility procedure, and the one or more counter lists facilitate UE derivation of the plurality of SN security keys after execution of the mobility procedure to the target cell.

49. The first RAN node of claim 48, being further configured to perform operations corresponding to the methods of any of claims 19-29.

50. Non-transitory, computer-readable medium (1704, 1804) storing computer-executable instructions that, when executed by processing circuitry (1702, 1804) of a first radio access network, RAN, node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802)configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), configure the first RAN node to perform operations corresponding to the methods of any of claims 18-29.

51. Computer program product (1704a, 1804a) comprising computer-executable instructions that, when executed by processing circuitry (1702, 1804) of a first radio access network, RAN, node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802) configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), configure the first RAN node to perform operations corresponding to the methods of any of claims 18-29.

52. Second radio access network, RAN, node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), the second RAN node comprising: communication interface circuitry (1706, 1804) configured to communicate with UEs and with other RAN nodes; and processing circuitry (1702, 1804) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: send to a first RAN node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802) a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells provided by the second RAN node; receive from the first RAN node a notification of a mobility procedure for a UE from a serving cell provided by the first RAN node to one of the mobility candidate cells as a target cell; and receive from the UE a message indicating that the mobility procedure is complete, wherein the message includes status information for counter values usable by the UE for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node (100, 150, 320, 715, 820, 920, 1020, 1120, 1510, 1700, 1802) that provides a secondary cell group, SCG, for the UE.

53. The second RAN node of claim 52, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 31-39.

54. Second radio access network, RAN, node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), the second RAN node being further configured to: send to a first RAN node (100, 150, 320, 420, 620, 702, 830, 930, 1030, 1130, 1510, 1700, 1802) a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells provided by the second RAN node; receive from the first RAN node a notification of a mobility procedure for a UE from a serving cell provided by the first RAN node to one of the mobility candidate cells as a target cell; and receive from the UE a message indicating that the mobility procedure is complete, wherein the message includes status information for counter values usable by the UE for deriving a plurality of secondary node, SN, security keys for communication between the UE and a third RAN node (100, 150, 320, 715, 820, 920, 1020, 1120, 1510, 1700, 1802) that provides a secondary cell group, SCG, for the UE.

55. The second RAN node of claim 54, being further configured to perform operations corresponding to the methods of any of claims 31-39.

56. Non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry (1702, 1804) of a second radio access network, RAN, node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), configure the second RAN node to perform operations corresponding to the methods of any of claims 30-39.

57. Computer program product comprising computer-executable instructions that, when executed by processing circuitry (1702, 1804) of a second radio access network, RAN, node (100, 150, 320, 430, 620, 702, 840, 940, 1040, 1140, 1510, 1700, 1802) configured to facilitatemobility between cells by user equipment, UEs (310, 410, 610, 701, 810, 910, 1010, 1110, 1512, 1600), configure the second RAN node to perform operations corresponding to the methods of any of claims 30-39.

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