Integrity protection of key identifier in lower layer message

Abstract

Embodiments include methods for a user equipment (UE). Such methods include receiving, from a first RAN node, a first message including layer-1 / layer-2 triggered mobility (LTM) candidate configuration(s) for respective LTM candidate cell(s) provided by a second RAN node. The first message is secured based on a first security key associated with a first key identifier. Such methods include obtaining a first token for integrity protection of key identifier(s) and receiving from the first RAN node a second message including: a command for an LTM cell switch from a serving cell to a target cell, a second key identifier associated with a second security key for the target cell, and a second token associated with the second key identifier. Such methods include, based on verifying the second token matches or corresponds to the first token, deriving the second security key based on a first parameter associated with the first key identifier.

Description

[0001] INTEGRITY PROTECTION OF KEY IDENTIFIER IN LOWER LAYER MESSAGE

[0002] TECHNICAL FIELD

[0003] The present disclosure relates generally to wireless networks, and more specifically to techniques for improving security of user equipment (UEs) operating in a radio access network (RAN), specifically in relation to secure provisioning of key identifiers in messages of a protocol layer that may lack other security mechanisms.

[0004] BACKGROUND

[0005] The fifth generation (5G) of cellular systems has been standardized within the Third- Generation Partnership Project (3GPP). 5G was 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. 5G was initially specified in 3GPP Release 15 (Rel-15) and continues to evolve through subsequent releases.

[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 may 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 may 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).

[0011] As described in 3GPP TS 38.501 (vl8.4.0), whenever an initial AS security context needs to be established between a UE and a gNB, both the AMF and the UE derive the KSNB key of the gNB and a Next Hop parameter (NH), based on the KAMF key provided by the AMF. The UE and the gNB use KSNB to secure the communication between each other, including derivation of the four AS security keys mentioned above. A NH Chaining Counter (NCC) is associated with each KgNB and NH parameter, and each KSNB is associated with the NCC corresponding to the NH value from which the KSNB was derived.

[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 PSCell (e.g., when DC is configured) and to release / add SCells. L3 serving cell change - also referred to as handover (HO) - also involves layer 1 (LI) and layer 2 (L2) resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching.

[0014] Certain L3 mobility operations may also involve changes to integrity protection and ciphering algorithms as well the AS keys KSNB, KRRCint, KRRCenc, Kupint and Kupenc. For example, during handovers, the basis for KSNB to be used between the UE and the target 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.” To facilitate horizontal key derivation, the UE must receive an indication of the NH value currently being used by the source RAN node (e.g., gNB). This indication is an NCC value (e.g., eight bits) that points to an NH value, and is secured by the source RAN node using one or more of the existing AS keys.

[0015] 3GPP Rel-18 includes new mobility procedures that facilitate serving cell changes via L1 / L2 signaling to address these problems and / or difficulties. These new procedures are often referred to as L1 / L2 based inter-cell mobility or L1 / L2 triggered mobility (LTM). In LTM, a UE is pre-configured by its serving RAN node with one RRC configuration per LTM candidate cell, sometimes referred to as an “LTM candidate cell configuration.” The UE performs lower- layer measurements on its 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, which is a lower-layer (e.g., LI or L2) message.

[0016] Rel-18 LTM also supports 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 associated DUs.

[0017] SUMMARY

[0018] According to 3 GPP agreements, Rel-19 will support inter-CU LTM between source and candidate cells associated with different CUs. It is desirable to use horizontal key derivation even for inter-CU LTM because it is more efficient and / or timely than vertical key derivation, which requires input from AMF in the 5GC. Even so, UE horizontal key derivation requires the NCC value that points to the NH being used by the source RAN node. In L3 handovers, NCC is provided to the UE via RRC with integrity protection based on the current KRRCint. However, this is not possible during LTM execution, which is controlled by lower layers and does not involve RRC.

[0019] An object of embodiments of the present disclosure is a secure technique for providing information (e.g., NCC) needed for horizontal key derivation in conjunction with LTM, such as by enabling and / or facilitating solutions to exemplary problems summarized above and described in more detail below. 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).

[0020] These exemplary methods include receiving, from a first RAN node via a serving cell, a first message including one or more LTM candidate configurations for respective one or LTM candidate cells provided by a second RAN node. The first message is secured based on a first security key associated with a first key identifier. These exemplary methods also include obtaining a first token for integrity protection of at least one key identifier. These exemplary methods also include receiving, from the first RAN node, a second message that includes the following:

[0021] • a command for an LTM cell switch from the serving cell to target cell, which is a first one of the LTM candidate cells provided by the second RAN node;

[0022] • a second key identifier associated with a second security key to be used in the target cell; and

[0023] • a second token associated with the second key identifier.

[0024] These exemplary methods also include, based on verifying that the second token matches or corresponds to the first token, deriving the second security key based on a first parameter associated with the first key identifier.

[0025] In some embodiments, these exemplary methods also include deriving the first parameter based on the received second key identifier, e.g., in response to verifying the token match or correspondence.

[0026] In some embodiments, these exemplary methods also include performing an LTM cell switch to the target cell and transmitting a third message to the second RAN node via the target cell. The third message is secured based on the derived second security key.

[0027] Other embodiments include exemplary methods (e.g., procedures) for a first RAN node configured to provide a serving cell for a UE. In general, these exemplary methods may be complementary to the exemplary methods for a UE summarized above.

[0028] These exemplary methods include sending, to a UE via the serving cell, a first message including one or more LTM candidate configurations for respective one or LTM candidate cells provided by a second RAN node. The first message is secured based on a first security key associated with a first key identifier. These exemplary methods also include obtaining a second token for integrity protection of at least one key identifier. These exemplary methods also include sending to the UE a second message that includes the following:

[0029] • a command for an LTM cell switch from the serving cell to target cell, which is a first one of the LTM candidate cells provided by the second RAN node;

[0030] • a second key identifier associated with a second security key to be used in the target cell; and

[0031] • the second token.

[0032] In some embodiments, these exemplary methods also include, prior to sending the second message, sending to the UE a first token for verification of the second token.

[0033] In some embodiments, these exemplary methods also include receiving the one or more LTM candidate configurations from the second RAN node. In some of these embodiments, the second token is obtained by being received from the second RAN node together with the one or more LTM candidate configurations.

[0034] In some embodiments, these exemplary methods also include, after sending the second message sending to the second RAN node a notification of the LTM cell switch by the UE.

[0035] Other embodiments include exemplary methods (e.g., procedures) for a second RAN node configured to provide a target cell for a UE. In general, these exemplary methods may be complementary to the exemplary methods for a UE and a first RAN node, summarized above.

[0036] These exemplary methods include determining that a UE has executed an LTM cell switch from a serving cell provided by a first RAN node to a first LTM candidate cell provided by the second RAN node. These exemplary methods also include obtaining a security context for the UE from a core network node or function (NNF). The security context includes a first key identifier associated with a first security key and a first parameter from which a second security key may be derived. These exemplary methods also include deriving a second token for integrity protection of at least one key identifier. These exemplary methods also include sending to the UE a lower layer message that includes the first key identifier and the second token.

[0037] In some embodiments, determining that the UE has executed the LTM cell switch is based on (or includes) receiving one or more of the following: a notification from the first RAN node, a notification from a DU of the second RAN node, or a message from the UE via the first LTM candidate cell. Some specific examples were discussed above.

[0038] In some embodiments, the second RAN node comprises a CU and a DU. The CU determines that the UE has executed the LTM cell switch and obtains the security context from the core NNF, while the DU sends the lower layer message to the UE.

[0039] In various UE and RAN node embodiments summarized above, the first and second RAN nodes may be different CUs or different gNBs, such that the LTM cell switch is inter-CU or inter- gNB. In various UE and RAN node embodiments summarized above, the first message is an RRC message and the lower layer message is a MAC CE.

[0040] In various embodiments summarized above, the first token may be derived by the UE and / or the second token may be derived by the first or second RAN node based on one or more of the following: • an identifier of the first LTM candidate cell;

[0041] • an identifier of the LTM candidate configuration for the first LTM candidate cells;

[0042] • an initialization token provided to the UE by the first or second RAN node;

[0043] • a freshness input that randomizes the derivation result; and

[0044] • a shared secret between the UE and the first or second RAN node.

[0045] In various embodiments summarized above, the shared secret may be derived by the UE and / or by the first RAN node based on one or more of the following:

[0046] • a user’s international mobile subscriber identity (IMSI);

[0047] • the UE’s international mobile equipment identity (IMEI);

[0048] • a hash value derived from a previous message between the UE and the first RAN node;

[0049] • a counter value or sequence number for a protocol between the UE and the first RAN node; and

[0050] • a first radio network temporary identifier (RNTI) assigned to the UE by the first RAN node.

[0051] In various embodiments summarized above, the second shared secret may be derived by the UE and / or by the second RAN node based on one or more of the following: a user’s IMSI, the UE’s IMEI, a hash value derived from a previous message between the UE and the second RAN node, a counter value or sequence number for a protocol between the UE and the second RAN node, and a second RNTI assigned to the UE by the second RAN node.

[0052] In various embodiments summarized above, one or more of the following applies:

[0053] • the first message is an RRC message and the second message is a MAC CE;

[0054] • the first parameter is a NH parameter;

[0055] • the first and second key identifiers are different NCC values; and

[0056] • the second key identifier indicates that the second security key should be derived vertically based on the first parameter.

[0057] 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.

[0058] These and other embodiments described herein can provide various advantages, benefits, and / or solutions to problems. For example, by providing integrity protection for lower layer signaling, embodiments may facilitate UE security key update for LTM without higher layer (e.g., RRC) signaling, which supports LTM advantages such as reduced latency, signaling, and / or interruptions. As such, embodiments may facilitate necessary updates for encryption and integrity protection as the UE moves between and within cells in the RAN, thereby increasing security of signaling and user data carried by radio bearers secured in this manner.

[0059] 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.

[0060] BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 shows an exemplary 5G network architecture.

[0062] Figure 2 shows an example NG-RAN node arranged in a split CU / DU architecture.

[0063] Figure 3 shows exemplary 5G user plane (UP) and control plane (CP) protocol stacks.

[0064] Figure 4 illustrates security key derivation for handover and other UE mobility procedures. Figure 5 shows a signaling diagram for an exemplary two-stage L3 handover for a UE.

[0065] Figure 6 shows an example system in which some embodiments of the present disclosure may be implemented.

[0066] Figure 7 shows a flow diagram of an exemplary method for a UE (e.g., wireless device), according to various embodiments of the present disclosure.

[0067] Figure 8 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.

[0068] Figure 9 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.

[0069] Figure 10 shows a communication system according to various embodiments of the present disclosure.

[0070] Figure 11 shows a UE according to various embodiments of the present disclosure.

[0071] Figure 12 shows a network node according to various embodiments of the present disclosure.

[0072] Figure 13 is a block diagram of a virtualization environment in which functions implemented by some embodiments of the present disclosure may be virtualized.

[0073] DETAILED DESCRIPTION

[0074] 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.

[0075] 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.

[0076] Furthermore, the following terms are used throughout the description given below:

[0077] • 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.

[0078] • 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.

[0079] • 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”.

[0080] • Radio Node: As used herein, a “radio node” can be either a “radio access node” (or equivalent term) or a “wireless device.”

[0081] • 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.

[0082] • 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] Figure 3 shows exemplary 5G 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.

[0087] 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.

[0088] 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.

[0089] 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, A UE in RRC IDLE, state is not known to the gNB serving the cell where the LIE is camping. However, NR RRC includes an RRC IN ACTIVE 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.

[0090] 3 GPP Rel-10 introduced support for channel bandwidths larger than 20 MHz in LTE networks. To remain compatible with UEs from earlier releases (e.g., LTE Rel-8), a wideband LTE Rel-10 carrier appears as multiple component carriers (CCs), each having the same structure as an LTE Rel-8 carrier. A Rel-10 UE can receive the multiple CCs based on Carrier Aggregation (CA). The CCs can also be considered “cells,” such that a UE in CA has one primary cell (PCell) and one or more secondary cells (SCells) that are referred to collectively as a “cell group.”

[0091] 3 GPP Rel-12 introduced LTE dual connectivity (DC), whereby a UE can be connected to two network nodes simultaneously. 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 for MCG, PSCell for SCG) and may include one or more secondary cells (SCells). 5G 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 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.

[0092] 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.

[0093] 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. 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 change upon 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.

[0094] For each DRB or SRB an independent counter (COUNT) is maintained for each direction and is used in an initialization vector (IV) input to ciphering and integrity protection algorithms applied to each packet of data. Specifically, COUNT is used to ensure fresh IVs when other, semistable parameters remain the same. However, it is not allowed to use the same COUNT value more than once for a given security key. In order to avoid such re-use, the network may use different RB identities for RB establishments, change the AS security key, or cause the UE to perform an RRC CONNECTED to RRC IDLE / RRC INACTIVE transition and then return to RRC CONNECTED. The network ensures that DL IVs are fresh and that the UE can generate fresh UL IVs. For example, since the UE cannot change DRB IDs or initiate re-keying, the gNB must ensure that such actions are taken when the UL COUNT for a DRB approaches wrap-around.

[0095] 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.

[0096] As described in 3GPP TS 38.501 (vl8.4.0), whenever an initial AS security context needs to be established between a UE and a gNB, the AMF and the UE derive KSNB 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 the KSNB was derived. At initial setup, KSNB is derived directly from KA F, 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.

[0097] 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).

[0098] 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.

[0099] 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 when the serving and target RAN nodes are part of the NG-RAN. 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).

[0100] On UE handovers 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.” To facilitate horizontal key derivation, the UE must receive an indication of the NH value currently being used by the source RAN node. This indication is an NCC value (e.g., eight bits) that points to an NH value, and is secured by the source RAN node using one or more of the existing AS keys.

[0101] These horizontal and vertical key derivations are illustrated by Figure 4, 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.

[0102] In some cases, however, the RAN may use both horizontal and vertical key derivation to achieve desired “key separation” between source and target RAN nodes for a UE handover. Figure 5 shows a signaling diagram for an exemplary two-stage L3 handover for a UE from a source cell provided by a source RAN node (gNB#l) to a target cell provided by a target RAN node (gNB#2). The first and target RAN nodes are connected via an Xn interface and both are coupled to an AMF and a UPF in the 5GC.

[0103] Initially, the source RAN node triggers inter-cell, Xn-based L3 handover of the UE to the target cell provided by the target RAN node. The source RAN node includes a masterKeyUpdate information element (IE) in the handover command to trigger security key update by the UE. During this procedure, the UE and the target RAN node perform horizontal key derivation, which facilitates initial secure communication between UE and target RAN node in the target cell. Subsequently, the target RAN sends a path switch request to AMF / UPF and receives in response a new {NH, NCC} pair for the UE. The target RAN node informs the source RAN node to release its context for the UE and then triggers an intra-cell L3 handover for the UE, during which the UE and the target RAN node perform vertical key derivation based on the new {NH, NCC} pair.

[0104] 3GPP Rel-18 also includes an NR mobility enhancement known as L1 / L2 triggered mobility (LTM). Current L3 -based inter-cell mobility involves complete 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 reduces latency, signaling overhead, and interruptions.

[0105] 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,” which may be part of an “LTM configuration” with additional information. Each LTM candidate cell 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., 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.

[0106] 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 transmission configuration indicator (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.

[0107] Accordingly, before receiving an LTM cell switch command, the UE performs early UL and DL synchronization with its configured LTM candidate cells. For early UL synchronization, the UE transmits a contention-free random access (CFRA) preamble towards an LTM candidate cell in response to a physical DL control channel (PDCCH) order received via the source cell. In order to minimize the data interruption of the source cell due to CFRA towards the LTM candidate cell, the UE doesn’t receive a RA response (with TA) from the LTM candidate cell. Instead, the TA for the LTM candidate cell is indicated in a subsequent LTM cell switch command.

[0108] As mentioned above, the LTM cell switch command from the source RAN node (or DU) may be a MAC control element (CE) that includes an identifier (e.g., index) of the corresponding LTM candidate cell configuration previously provided to the UE. The MAC CE may also include an identifier of a beam by which the UE should access the target cell. For example, the beam indication is given as a TCI state identifier (ID) associated with the LTM candidate cell. 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 to be “activated” when performing the LTM cell switch. The UE also applies the LTM candidate cell configuration identified in the MAC CE. Note that since the UE previously performed early UL synchronization, it does not need to perform RA at LTM cell switch.

[0109] Note the MAC layer does not include security mechanisms similar to the AS security keys and algorithms used for RRC -layer signaling. Thus, the LTM cell switch command MAC CE transmitted by the source RAN node is neither encrypted nor integrity protected. Although other layers (e.g., PHY) may try to detect and recover bit and block errors, it is still possible that information in the MAC CE may be corrupted or changed without detection by the MAC layer.

[0110] The split CU / DU architecture shown in Figure 1 also supports Rel-18 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 primary cell (PCell). 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. 3GPP Rel-19 will support inter-CU LTM between source and candidate cells associated with different CUs. An inter-CU LTM cell switch may require additional UE operations compared to intra-CU LTM cell switch, such as security key update. As such, an inter-CU LTM configuration (or LTM candidate cell configuration) may include the same information as an intra-CU LTM configuration (or LTM candidate cell configuration) as well as one or more of the following:

[0111] • Information needed to perform security key update, e.g., MasterKeyUpdate IE or a RadioBearerConfig IE that includes SecurityConfig with SecurityAlgorithmConfig

[0112] • Indication to perform L2 / PDCP re-establishment; and

[0113] • Indication to perform a full configuration, e.g., RRC field fullConfig.

[0114] It is desirable to use horizontal key derivation for security key updates in inter-CU LTM because it is more efficient and / or timely than vertical key derivation, which requires input from AMF in the 5GC (e.g., as illustrated in Figure 5). Even so, horizontal key derivation by the UE requires the NCC value associated with the NH currently being used by the source RAN node. In L3 handover, NCC is provided to the UE via RRC with integrity protection based on the current KRRCint. Since LTM execution is controlled by lower layers and does not involve RRC, it is desirable to provide NCC to the UE via lower layers. However, there are no existing mechanisms to ensure integrity of a lower layer message (e.g., LTM cell switch command MAC CE) that includes NCC.

[0115] Accordingly, embodiments of the present disclosure address these problems and / or issues by flexible and efficient techniques for determining a shared secret for lower layer (e.g., MAC) signaling between a RAN node and a UE, and for applying the shared secret for integrity protection of the lower layer signaling. For example, embodiments can be used to ensure integrity of an LTM cell switch command MAC CE that includes NCC to be used for security key derivation by a UE.

[0116] Embodiments of the present disclosure may provide various advantages and / or benefits. For example, by providing integrity protection for lower layer signaling, embodiments may facilitate UE security key update for LTM without higher layer (e.g., RRC) signaling, which supports LTM advantages such as reduced latency, signaling, and / or interruptions. As such, embodiments may facilitate necessary updates for encryption and integrity protection as the UE moves between and within cells in the RAN, thereby increasing security of signaling and user data carried by radio bearers secured in this manner.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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).

[0121] 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.

[0122] 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”.

[0123] 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.

[0124] 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.

[0125] The term “LTM configuration” refers to a data structure that is used for or related to UE LTM operations, and may include one or more of the following elements (non-exclusive):

[0126] • an LTM candidate cell configuration, such as one or more of the following for an LTM candidate cell: o lower layer information, such as Ll / PHY configuration, L2 / MAC / RLC configuration, cell group configuration, serving cell configuration, etc.; and o higher layer information, such as L3 / RRC parameter and / or timer values, PDCP configuration, radio bearer configuration, L3 / RRC measurement configuration, etc.;

[0127] • a measurement configuration, e.g., measurements for LTM, measurement reports for LTM, CSI resource configuration for LTM, CSI report configuration for LTM, etc.;

[0128] • a configuration for early DL synchronization, e.g., for early TCI state activation;

[0129] • a configuration for early UL synchronization, e.g., for transmission of PDCCH ordered preamble transmission and reception of timing advance (TA);

[0130] • a configuration for execution of an LTM cell switch procedure according to a given LTM candidate cell configuration (e.g., whether to perform RA, REC reestablishment, MAC reset, PDCP recovery, etc.).;

[0131] • additional information needed for an intra-CU / gNB LTM cell switch procedure. The term “part of an LTM 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).

[0132] 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 secondary cell (SCell). In other words, an LTM candidate cell can be a candidate for the UE’s PCell, PSCell, or SCell.

[0133] 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.

[0134] 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). 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:

[0135] • a first identifier associated to a candidate cell;

[0136] • a second identifier associated to a source cell the UE is connected to when the mobility procedure is executed;

[0137] • a key set change indicator;

[0138] • a next hop chaining counter (NCC);

[0139] • a non-access stratum (NAS) container (NASC);

[0140] • an indication, indicating that mobility from current serving cell to a certain candidate target cell requires security key refresh;

[0141] • an indication, indicating that mobility towards a certain target cell requires security key refresh (or does not require security key refresh);

[0142] • an indication, indicating that security key refresh is required (or is not required) for subsequent mobility towards a certain cell; and

[0143] • an indication that security key refresh is to be performed without RA. An example security configuration is the masterKeyUpdate IE.

[0144] The term “security key refresh,” “security key update,” “AS key refresh” or 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:

[0145] • UE receives a masterKeyUpdate IE included in a mobility configuration, e.g., in the candidate cell configuration;

[0146] • 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;

[0147] • 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;

[0148] • 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;

[0149] • UE derives KRRCenc and Kupenc associated with a ciphering algorithm (e.g., cipheringAlgorithm indicated in securityAlgorithmConfig), as specified in 3 GPP TS 33.501;

[0150] • 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;

[0151] • UE receives a security algorithm configuration included in a mobility configuration, based on which the UE derives UP security keys (e.g., Kupenc and Kupint) and / or CP security keys (e.g., KRRCenc and KRRCint) for encryption and / or integrity protection;

[0152] • UE uses its current security algorithm configuration, based on which the UE derives UP security keys (e.g., Kupenc and Kupint) and / or CP security keys (e.g., KRRCenc and KRRCint) for encryption and / or integrity protection;

[0153] • UE applies the provided ciphering algorithm and associated security key during a PDCP entity re-establishment procedure; and

[0154] • UE applies the provided integrity protection algorithm and associated security key during a PDCP entity re-establishment procedure;

[0155] • UE derives the security key(s) when it receives a mobility configuration, which it may user for a subsequent mobility procedure. Figure 6 shows an example system in which some embodiments of the present disclosure may be implemented. UE (601) is a wireless terminal, such as a cellular smartphone. The UE may be connected to the first RAN node (602) over a first wireless interface (604) and, in some cases, to a second RAN node (603) over a second wireless interface (605).

[0156] The first RAN node provides a first cell (607), which may be referred to as the UE’s source cell in the context of mobility (e.g., LTM or L3 HO), or the UE’s serving cell, Special Cell (SpCell), PCell, or PSCell in the context of CA and / or DC. The second RAN node provides a second cell (608), 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.

[0157] The first RAN node and the second RAN node may be gNBs of an NG-RAN, and may be interconnected over an interface (606), which may be an Xn or Xn-C type of interface. However, the first RAN node and the second RAN node are not necessarily interconnected.

[0158] In the context of mobility, the first RAN node may be referred to as source RAN node since it provides the source cell for UE mobility. Likewise, the second RAN node 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, the first RAN node and the second RAN node may be a single RAN node.

[0159] In case of a distributed CU / DU RAN architecture, the first RAN node and / or the second RAN node 603 may be divided into a CU and one or more DUs. As shown in Figure 6, first RAN node includes a first CU (609) and a first DU (610), which may be referred to as serving CU / DU or source CU / DU for the UE. Likewise, the second RAN node includes a second CU (612) and a second DU (613), 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, first / source CU and second / target CU may be a single CU.

[0160] The first CU and the first DU are connected over an interface (611), which may be an Fl type of interface in case of NG-RAN. Correspondingly, the second CU and the second DU are connected over an interface (614), which may also be an Fl type of interface in case of NG-RAN.

[0161] The first RAN node and the second RAN node may be connected to a third network node (615 over respective interfaces (616 and 617). The third network node may be a core network node, such as a UPF or an AMF. In the latter case, the respective interfaces are both NG interfaces and / or N2 reference points. Sometimes the third network node may comprise two different network nodes, such as a source AMF connected with the first RAN node and a target AMF connected with the second RAN node. These two network nodes are inter-connected over an interface, such as an N14 reference point or an Namf type of service-based interface.

[0162] Some embodiments include methods for a UE configured for mobility between cells in a RAN. Initially, the UE is configured with a first security key (e.g., KSNB) associated with a first RAN node that provides the UE’s serving cell. The UE receives from the first RAN node one or more LTM candidate configurations for respective LTM candidate cells provided by a second RAN node. This information may be received, for example, in an RRCReconfiguration message, in broadcast SI, NAS signaling, etc. The first security key may be associated with a first key identifier (e.g., NCCi) and a first parameter (e.g., NH) from which a second security key (e.g., KSNB’) may be derived. For example, in vertical key derivation, the NH is bound to physical cell identity (PCI) and DL frequency (ARFCN-DL) before it is used for the second security key (e.g., KgNB’). The first parameter and / or the second security key may also be associated with a second key identifier (e.g., NCCi+i).

[0163] The UE then derives a security token (or token, for short) that provides integrity protection for at least one key identifier. For example, the token may be a message authentication code (MAC) or checksum. In some embodiments, the token may be derived based on the first security key (e.g., KSNB) or the associated first parameter (e.g., NH). In some embodiments, the token may be derived based on the first key identifier (e.g., NCCi) or the second key identifier (e.g., NCCi+i).

[0164] In some embodiments, the token may be derived by the UE based on an identifier of one of the LTM candidate configurations or one of the LTM candidate cells. In such case, the token may be associated with LTM candidate configuration (or cell) whose identifier was used in the derivation.

[0165] In other embodiments, the token is not associated with any specific LTM candidate configuration (or cell) but rather with all LTM candidate configurations (or cells) that the UE has received. In some variants, when the UE receives new (or different) LTM candidate configurations, the UE re-derives the token. In some variants, when the UE executes an LTM cell switch to one of the configured LTM candidate cells (i.e., as a target cell), the UE discards the association between the token and all other configured LTM candidate cells (i.e., other than the target cell).

[0166] In other embodiments, the UE receives from the serving RAN node an initialization token that is not associated with any LTM candidate cell configuration. In this case, the UE derives the token associated with a particular one (or all) of the LTM candidate configurations (or cells) based on the initialization token and the LTM candidate configuration(s) currently held by the UE.

[0167] In some embodiments, the token may be derived based on a “freshness input” that increases the likelihood of different results for each derivation (i.e., randomizes the derivation result). For example, the freshness input may be any of the following:

[0168] • current value of a protocol counter, such as a PDCP counter (COUNT);

[0169] • a nonce value, generated for one-time use;. • content of a previous message, e.g., the RRC message that contained the LTM candidate configurations.

[0170] The UE then receives from the first RAN node a message that includes a command for an LTM cell switch (i.e., an LTM cell switch command) from the UE’s serving cell to one of the LTM candidate cells previously configured (i.e., as mobility target cell). For example, the message may be a MAC CE. The message includes the second key identifier (e.g., NCCi+i) and a second token (e.g., checksum, MAC) for integrity protection of the second key identifier. For example, the second key identifier may indicate the UE should perform vertical key derivation of a second security key (e.g., KSNB’) to be used for communication with the second RAN node in the target cell.

[0171] The UE verifies the message by comparing the second token with the token derived previously according to any of the embodiments described above. In some embodiments, the UE and the first RAN node may derive the first and second tokens, respectively, further based on the LTM cell switch command. If the verification is successful, the UE derives the second security key (e.g., KSNB’) based on the first parameter (e.g., NH) associated with the second key identifier (e.g., NCCi+i) included in the message.

[0172] In some embodiments, the UE derives the first parameter (e.g., NH) based on a shared secret between the UE and the first RAN node. For example, the shared secret may derived by the UE and the first RAN node based on one or more of the following:

[0173] • a user’s unique international mobile subscriber identity (IMSI);

[0174] • the UE’s unique international mobile equipment identity (IMEI);

[0175] • a hash value derived from a previous RRC message;

[0176] • a sequence number of a protocol between UE and first RAN node, e.g., PDCP; and

[0177] • the UE’s radio network temporary identifier (RNTI) assigned by the first RAN node. Alternately, rather than deriving the shared secret itself, the UE receives the shared secret from the first RAN node via broadcast SI, unicast RRC signaling, MAC CE, or DCI.

[0178] In some embodiments, the UE derives the second security key (e.g., KSNB’) further based on a “freshness input”, in a similar manner as described above for token derivation

[0179] Subsequently, the UE executes the LTM cell switch to the target cell and uses the second security key for secure communication with the second RAN node in the target cell. For example, UE may derive one or more UP and / or CP security keys from the second security key (e.g., KSNB’), as described above, and use these security keys for encryption / decryption and integrity protection of UP and / or CP messages exchanged with the second RAN node. As a more specific example, the UE may use security keys KRRCenc and KRRCint for encryption and integrity protection of an RRCReconfigurationComplete message sent to the second RAN node, indicating or confirming successful execution of the LTM cell switch.

[0180] In some embodiments, after completing the LTM cell switch, the UE and the second RAN node derive a second shared secret, which can be used to derive a second parameter (e.g., NH’) from which a third security key (e.g., KgNB”) may be derived. The second parameter and / or the third security key may also be associated with a third key identifier (e.g., NCCi+2). For example, the second shared secret may derived by the UE and the first RAN node based on one or more of the following:

[0181] • the user’ s unique IMSI;

[0182] • the UE’s unique IMEI;

[0183] • a hash value derived from a previous RRC message;

[0184] • a sequence number of a protocol between UE and second RAN node, e.g., PDCP; and

[0185] • the UE’s radio network temporary identifier (RNTI) assigned by the second RAN node. Alternately, rather than deriving the second shared secret itself, the UE receives the second shared secret from the second RAN node via broadcast SI, unicast RRC signaling, MAC CE, or DCI.

[0186] Other embodiments include complementary methods for a first RAN node configured to provide a serving cell for a UE. Initially, the first RAN node is configured with a first security key (e.g., KSNB) used for secure communications with the UE in the serving cell. The first security key may be associated with a first key identifier (e.g., NCCi) and a first parameter (e.g., NH) from which a second security key (e.g., KSNB’) may be derived. The first parameter and / or the second security key may also be associated with a second key identifier (e.g., NCCi+i).

[0187] The first RAN node sends to the UE one or more LTM candidate configurations for respective LTM candidate cells provided by a second RAN node. This information may be sent, for example, in an RRCReconfiguration message, in broadcast SI, NAS signaling, etc. The first RAN node may have previously received the LTM candidate configurations from the second RAN node.

[0188] The first RAN node obtains a security token (or token, for short) that provides integrity protection for at least one key identifier. For example, the token may be a MAC or checksum. In some embodiments, the token may be derived based on the first security key (e.g., KSNB) or the associated first parameter (e.g., NH). In some embodiments, the token may be derived based on the first key identifier (e.g., NCCi) or the second key identifier (e.g., NCCi+i). The first RAN node may derive the token itself or receive it from the second RAN node, which derives the token.

[0189] In case the first RAN node derives the token, it does so in a similar manner as described above for UE embodiments. In some embodiments, the token may be derived by the first RAN node based on an identifier of one of the LTM candidate configurations or one of the LTM candidate cells. In such case, the token may be associated with LTM candidate configuration (or cell) whose identifier was used in the derivation.

[0190] In other embodiments, the token is not associated with any specific LTM candidate configuration (or cell) but rather with all LTM candidate configurations (or cells) that the first RAN node sent to the UE. In some variants, when the first RAN node sends the UE new (or different) LTM candidate configurations, the first RAN node re-derives the token.

[0191] In other embodiments, the first RAN node sends the UE an initialization token that is not associated with any LTM candidate cell configuration. In this case, the first RAN node also derives the token associated with a particular one (or all) of the LTM candidate configurations (or cells) based on the initialization token and the LTM candidate configuration s) currently held by the UE.

[0192] In some embodiments, the token may be derived based on a “freshness input” that increases the likelihood of different results for each derivation. For example, the freshness input may be any of the following:

[0193] • current value of a counter, such as a PDCP counter (COUNT);

[0194] • a nonce value, generated for one-time use;.

[0195] • content of a previous message, e.g., the RRC message that contained the LTM candidate configurations.

[0196] The first RAN node then sends the UE a message that includes a command for an LTM cell switch (i.e., an LTM cell switch command) from the UE’s serving cell to one of the LTM candidate cells previously configured (i.e., as target cell). For example, the message may be a MAC CE. The message includes the second key identifier (e.g., NCCi+i) and a second token (e.g., checksum, MAC) for integrity protection of the second key identifier. For example, the second key identifier may indicate the UE should perform vertical key derivation of a second security key (e.g., KSNB’) to be used for communication with the second RAN node in the target cell.

[0197] In some embodiments, the first RAN node also sends to the second RAN node a notification of the LTM cell switch by the UE, such as an XnAP LTM SWITCH NOTIFICATION message.

[0198] As mentioned above, in some embodiments, the UE derives the first parameter (e.g., NH) based on a shared secret between the UE and the first RAN node. For example, the shared secret may derived by the UE and the first RAN node based on one or more of the following:

[0199] • the user’ s unique IMSI;

[0200] • the UE’s unique IMEI;

[0201] • a hash value derived from a previous RRC message;

[0202] • a sequence number of a protocol between UE and first RAN node, e.g., PDCP; and

[0203] • the UE’ s RNTI assigned by the first RAN node.

[0204] Alternately, rather than the UE deriving the shared secret itself, the first RAN node sends the derived shared secret to the UE via broadcast SI, unicast RRC signaling, MAC CE, or DCI.

[0205] Other embodiments include complementary methods for a second RAN node configured to provide a target cell for a UE. The second RAN node sends, to a first RAN node that provides a serving cell for the UE, one or more LTM candidate configurations for respective LTM candidate cells provided by the second RAN node. This may done in response to a request from the first RAN node.

[0206] The second RAN node determines that the UE has executed an LTM cell switch to one of the LTM candidate cells previously configured (i.e., as target cell). In some embodiments, this determination is based on receiving from the first RAN node a notification of the LTM cell switch by the UE, such as an XnAP LTM SWITCH NOTIFICATION message. In some embodiments, this determination is based on receiving an UL message from the UE in the LTM candidate cell. For example, this UL message can be a RA preamble, a scheduling request (SR) for CP or UP data, an RRCReconfigurationComplete message, a HARQ acknowledgement of a DL message from the second RAN node, etc.

[0207] Based on this determination, the second RAN node sends to a core network node or function (NNF, e.g., AMF) a path switch request (e.g., NGAP PATH SWITCH REQUEST message) to initiate switch of the UE’s data path from the first RAN node to the second RAN node. The core NNF responds to the second RAN node with an acknowledgement (e.g., NGAP PATH SWITCH REQUEST ACKNOWLEDGE message) that includes new security context information for the UE.

[0208] For example, the security context information may be carried in a SecurityContext information element (IE) and may include a first key identifier (e.g., NCCi) associated with a first security key (e.g., KSNB) and a first parameter (e.g., NH) from which a second security key (e.g., KSNB’) may be derived vertically. The first parameter and / or the second security key may also be associated with a second key identifier (e.g., NCCi+i). In other words, the security context information may include a new {NH, NCC} pair for the UE.

[0209] Subsequently, the second RAN node sends the UE a message (e.g., MAC CE) that includes the first key identifier (e.g., NCCi) and a token (e.g., checksum, message authentication code, etc.) for integrity protection of the first key identifier. In some embodiments, the token may be based on a shared secret between the UE and the second RAN node. In some embodiments, the first key identifier may indicate the UE should perform vertical key derivation of a first security key (e.g., KSNB) to be used for communication with the second RAN node in the target cell.

[0210] In general, the second RAN node may derive the token in any of the ways described above for the UE and the first RAN node. In some embodiments, the token may be derived based on the first security key (e.g., KSNB), the first key identifier (e.g., NCCi), and / or the first parameter (e.g., NH). In some embodiments, the token may be derived by the second RAN node based on an identifier of the LTM candidate configuration or the LTM candidate cell to which the UE performed the LTM cell switch.

[0211] In other embodiments, the second RAN node sends the UE an initialization token that is not associated with any LTM candidate cell configuration. In this case, the second RAN node also derives the token sent in the message based on the initialization token and the LTM candidate configuration or the LTM candidate cell to which the UE performed the LTM cell switch.

[0212] In some embodiments, the token may be derived based on a “freshness input” that increases the likelihood of different results for each derivation. For example, the freshness input may be any of the following:

[0213] • current value of a counter, such as a PDCP counter (COUNT);

[0214] • a nonce value, generated for one-time use; and

[0215] • content of a previous message, e.g., the RRC message that contained the LTM candidate configurations.

[0216] As mentioned above, the token may be based on a shared secret between the UE and the second RAN node. For example, the shared secret may derived by the UE and the second RAN node based on one or more of the following:

[0217] • the user’ s unique IMSI;

[0218] • the UE’s unique IMEI;

[0219] • a hash value derived from a previous RRC message;

[0220] • a sequence number of a protocol between UE and second RAN node, e.g., PDCP; and

[0221] • the UE’s RNTI assigned by the second RAN node.

[0222] Alternately, rather than the UE deriving the shared secret itself, the second RAN node sends the derived shared secret to the UE via broadcast SI, unicast RRC signaling, MAC CE, or DCI.

[0223] In some embodiments, the second RAN node may utilize the split CU-DU architecture illustrated in Figures 1-2. In such case, the CU of the second RAN node handles communication with the core NNF. The CU also determines that the UE has executed an LTM cell switch to the target cell based on one of the following:

[0224] • a notification from the first RAN node, such as an XnAP LTM SWITCH NOTIFICATION message; or

[0225] • a notification from the DU, such as an F1AP ACCESS SUCCESS message.

[0226] However, there are several variants of how other relevant operations are distributed between the CU and the DU that provides the UE’s target cell.

[0227] In some variants, CU derives the token and the shared secret (if used). The CU sends the first key identifier (e.g., NCCi) and the token to the DU, which prepares a lower layer message (e.g., MAC CE) that includes the first key identifier and the token and sends the lower layer message to the UE.

[0228] In other variants, the DU derives the token and the shared secret (if used). Upon receiving the security context information from the core NNF, the CU sends the DU a request (e.g., an F1AP UE CONTEXT MODIFICATION REQUEST message) for the token (or shared secret with the UE), which may include the first key identifier. In response, the DU provides the requested information (e g., in an F1AP UE CONTEXT MODIFICATION RESPONSE message). The DU prepares a lower layer message (e.g., MAC CE) that includes the first key identifier and the token and sends the lower layer message to the UE.

[0229] Although the embodiments described above are based on the second RAN node (or DU) sending the UE a single message that includes the token and the first key identifier, the second RAN node (or DU) may also send these two items in two different messages, i.e., one item in each message.

[0230] Various features of the embodiments summarized above correspond to various operations illustrated in Figures 7-9, 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 7-9 can be used cooperatively to provide various benefits, advantages, and / or solutions to problems described herein. Although Figures 7-9 show specific blocks in particular orders, the operations of the exemplary methods can be performed in different orders than shown and can be combined and / or divided into blocks having different functionality than shown. Optional blocks or operations are indicated by dashed lines.

[0231] In particular, Figure 7 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.

[0232] The exemplary method includes the operations of block 710, where the UE receives, from a first RAN node via a serving cell, a first message including one or more LTM candidate configurations for respective one or LTM candidate cells provided by a second RAN node. The first message is secured based on a first security key associated with a first key identifier. The exemplary method also includes the operations of block 730, where the UE obtains a first token for integrity protection of at least one key identifier. The exemplary method also includes the operations of block 740, where the UE receives, from the first RAN node, a second message that includes the following:

[0233] • a command for an LTM cell switch from the serving cell to target cell, which is a first one of the LTM candidate cells provided by the second RAN node;

[0234] • a second key identifier associated with a second security key to be used in the target cell; and

[0235] • a second token associated with the second key identifier.

[0236] The exemplary method also includes the operations of blocks 750 and 770, where based on verifying that the second token matches or corresponds to the first token, the UE derives the second security key based on a first parameter associated with the first key identifier.

[0237] In some embodiments, the first and second RAN nodes may be different CUs or different gNBs, such that the LTM cell switch is inter-CU or inter-gNB. In some embodiments, the exemplary method also includes the operations of block 760, where the UE derives the first parameter based on the received second key identifier, e.g., in response to verifying the token match or correspondence.

[0238] In some embodiments, the first token is obtained in block 730 by being received from the first RAN node prior to receiving the second message in block 740. In other embodiments, the first token is obtained in block 730 by being derived by the UE based on one or more of the following:

[0239] • an identifier of the first LTM candidate cell;

[0240] • an identifier of the LTM candidate configuration for the first LTM candidate cells;

[0241] • an initialization token received from the first RAN node;

[0242] • a freshness input that randomizes the derivation result; and

[0243] • a shared secret between the UE and the first RAN node.

[0244] In some of these embodiments, the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the first RAN node, a nonce value, and content of a previous message between the UE and the first RAN node.

[0245] In some of these embodiments, the exemplary method also includes the operations of block 720, where the UE derives the shared secret or receives the shared secret from the first RAN node. In some variants of these embodiments, the shared secret is derived based on one or more of the following:

[0246] • a user’s international mobile subscriber identity (IMSI);

[0247] • the UE’s international mobile equipment identity (IMEI);

[0248] • a hash value derived from a previous message between the UE and the first RAN node;

[0249] • a counter value or sequence number for a protocol between the UE and the first RAN node; and • a first radio network temporary identifier (RNTI) assigned to the UE by the first RAN node.

[0250] In some embodiments, the exemplary method also includes the operations of block 780, where the UE performs an LTM cell switch to the target cell and transmits a third message to the second RAN node via the target cell. The third message is secured based on the derived second security key (e.g., from block 770). In some of these embodiments, the exemplary method also includes the operations of block 790, where after performing the LTM cell switch, the UE derives a second shared secret between the UE and the second RAN node. For example, the second shared secret may be derived based on one or more of the following:

[0251] • a user’s IMSI;

[0252] • the UE’s IMEI;

[0253] • a hash value derived from a previous message between the UE and the second RAN node;

[0254] • a counter value or sequence number for a protocol between the UE and the second RAN node; and

[0255] • a second RNTI assigned to the UE by the second RAN node.

[0256] In some embodiments, the first message is an RRC message and the second message is a MAC CE. In some embodiments, the first parameter is a NH parameter, and the first and second key identifiers are different NCC values. In some embodiments, the second key identifier indicates that the second security key should be derived vertically based on the first parameter.

[0257] In addition, Figure 8 shows an exemplary method (e.g., procedure) for a first RAN node configured to provide a serving cell for a UE, 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.

[0258] The exemplary method includes the operations of block 820, where the first RAN node sends, to a UE via the serving cell, a first message including one or more LTM candidate configurations for respective one or LTM candidate cells provided by a second RAN node. The first message is secured based on a first security key associated with a first key identifier. The exemplary method also includes the operations of block 850, where the first RAN node obtains a second token for integrity protection of at least one key identifier. The exemplary method also includes the operations of block 870, where the first RAN node sends to the UE a second message that includes the following:

[0259] • a command for an LTM cell switch from the serving cell to target cell, which is a first one of the LTM candidate cells provided by the second RAN node;

[0260] • a second key identifier associated with a second security key to be used in the target cell; and • the second token.

[0261] In some embodiments, the first and second RAN nodes may be different CUs or different gNBs, such that the LTM cell switch is inter-CU or inter-gNB.

[0262] In some embodiments, the exemplary method also includes the operations of block 860, where prior to sending the second message in block 870, the first RAN node sends a first token to the UE for verification of the second token (i.e., to be included in the second message). For example, the first token may match or correspond to the second token. As a more specific example, the first RAN node provides a copy of the derived token to the UE, so that the UE may use it to verify the token later received with the LTM cell switch command.

[0263] In some embodiments, the exemplary method also includes the operations of block 810, where the first RAN node receives the one or more LTM candidate configurations from the second RAN node. In some of these embodiments, the second token is obtained in block 850 by being received from the second RAN node together with the one or more LTM candidate configurations. In such case, the operations of block 850 may be part of the operations of block 810.

[0264] In other embodiments, the second token is obtained in block 850 by being derived by the first RAN node based on one or more of the following:

[0265] • an identifier of the first LTM candidate cell;

[0266] • an identifier of the LTM candidate configuration for the first LTM candidate cells;

[0267] • an initialization token received from the first RAN node;

[0268] • a freshness input that randomizes the derivation result; and

[0269] • a shared secret between the UE and the first RAN node.

[0270] In some of these embodiments, the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the first RAN node, a nonce value, and content of a previous message between the UE and the first RAN node.

[0271] In some of these embodiments, the exemplary method also includes the operations of block 830, where the first RAN node derives the shared secret based on one or more of the following:

[0272] • a user’s IMSI;

[0273] • the UE’s IMEI;

[0274] • a hash value derived from a previous message between the UE and the first RAN node;

[0275] • a counter value or sequence number for a protocol between the UE and the first RAN node; and

[0276] • a first RNTI assigned to the UE by the first RAN node.

[0277] In some variants of these embodiments, the exemplary method also includes the operations of block 840, where the first RAN node sends the derived shared secret to the UE. In some embodiments, the exemplary method also includes the operations of block 880, where after sending the second message in block 870, the first RAN node sends to the second RAN node a notification of the LTM cell switch by the UE.

[0278] In some embodiments, the first message is an RRC message and the second message is a MAC CE. In some embodiments, the first parameter is a NH parameter and the first and second key identifiers are different NCC values. In some embodiments, the second key identifier indicates that the second security key should be derived vertically based on the first parameter.

[0279] In addition, Figure 9 shows an exemplary method (e.g., procedure) for a second RAN node configured to provide a target cell for UE mobility, 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.

[0280] The exemplary method includes the operations of block 920, where the second RAN node determines that a UE has executed an LTM cell switch from a serving cell provided by a first RAN node to a first LTM candidate cell provided by the second RAN node. The exemplary method also includes the operations of block 930, where the second RAN node obtains a security context for the UE from a core network node or function (NNF, e.g., AMF). The security context includes a first key identifier associated with a first security key and a first parameter from which a second security key may be derived. The exemplary method also includes the operations of block 960, where the second RAN node derives a second token for integrity protection of at least one key identifier. The exemplary method also includes the operations of block 990, where the second RAN node sends to the UE a lower layer message that includes the first key identifier and the second token.

[0281] In some embodiments, the first and second RAN nodes may be different CUs or different gNBs, such that the LTM cell switch is inter-CU or inter-gNB.

[0282] In some embodiments, the exemplary method also includes the operations of block 910, where the second RAN node sends, to the first RAN node, one or more LTM candidate configurations for respective one or LTM candidate cells provided by the second RAN node. The LTM cell switch is based on a first one of the LTM candidate configurations for the first LTM candidate cell.

[0283] In some embodiments, determining that the UE has executed the LTM cell switch in block 920 is based on (or includes) the operations of sub-block 921, where the second RAN node receives one or more of the following: a notification from the first RAN node, a notification from a DU of the second RAN node, or a message from the UE via the first LTM candidate cell. Some specific examples were discussed above. In some embodiments, the second token is derived by the second RAN node in block 960 based on one or more of the following:

[0284] • an identifier of the first LTM candidate cell;

[0285] • an identifier of the LTM candidate configuration for the first LTM candidate cell;

[0286] • an initialization token;

[0287] • a freshness input that randomizes the derivation result; and

[0288] • a shared secret between the UE and the second RAN node.

[0289] In some of these embodiments, the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the second RAN node, a nonce value, and content of a previous message between the UE and the second RAN node.

[0290] In some of these embodiments, the exemplary method also includes the operations of block 940, where the second RAN node derives the shared secret based on one or more of the following:

[0291] • a user’s IMSI;

[0292] • the UE’s IMEI;

[0293] • a hash value derived from a previous message between the UE and the second RAN node;

[0294] • a counter value or sequence number for a protocol between the UE and the second RAN node; and

[0295] • a second RNTI assigned to the UE by the second RAN node.

[0296] In some variants of these embodiments, the exemplary method also includes the operations of block 950, where the second RAN node sends the derived shared secret to the UE.

[0297] In some embodiments, the second RAN node comprises a CU and a DU (e.g., as illustrated in Figures 1-2). The CU determines that the UE has executed the LTM cell switch in block 920 and obtains the security context from the core NNF in block 930, while the DU sends the lower layer message to the UE in block 990.

[0298] In some of these embodiments, the CU derives the second token in block 960 and the exemplary method also includes the operations of block 970, where the CU sends to the DU the second token and the first key identifier for inclusion in the lower layer message. In other of these embodiments, the DU receives the derives the second token in block 960 and the exemplary method also includes the operations of block 980, where the CU sends to the DU a token request and the first key identifier.

[0299] In some embodiments, the lower layer message is a MAC CE. In some embodiments, the first parameter is a NH parameter, and the first key identifier is an NCC value. In some embodiments, the first key identifier indicates that the second security key should be derived vertically based on the first parameter.

[0300] 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.

[0301] Figure 10 shows an example of a communication system 1000 in accordance with some embodiments. In this example, communication system 1000 includes a telecommunication network 1002 that includes an access network 1004 (e.g., RAN) and a core network 1006, which includes one or more core network nodes 1008. Access network 1004 includes one or more access network nodes, such as network nodes lOlOa-b (one or more of which may be generally referred to as network nodes 1010), or any other similar 3GPP 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 1002 includes one or more Open-RAN (ORAN) network nodes. An ORAN network node is a node in telecommunication network 1002 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 1002, including one or more network nodes 1010 and / or core network nodes 1008.

[0302] 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 1010 facilitate direct or indirect connection of UEs, such as by connecting UEs 1012a-d (one or more of which may be generally referred to as UEs 1012) to core network 1006 over one or more wireless connections.

[0303] 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 1000 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 1000 may include and / or interface with any type of communication, telecommunication, data, cellular, radio network, and / or other similar type of system.

[0304] UEs 1012 may be any of a wide variety of communication devices, including wireless devices arranged, configured, and / or operable to communicate wirelessly with network nodes 1010 and other communication devices. Similarly, network nodes 1010 are arranged, capable, configured, and / or operable to communicate directly or indirectly with UEs 1012 and / or with other network nodes or equipment in telecommunication network 1002 to enable and / or provide network access, such as wireless network access, and / or to perform other functions, such as administration in telecommunication network 1002.

[0305] In the depicted example, core network 1006 connects network nodes 1010 to one or more hosts, such as host 1016. 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 1006 includes one or more core network nodes (e.g., 1008) 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 1008. 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).

[0306] Host 1016 may be under the ownership or control of a service provider other than an operator or provider of access network 1004 and / or telecommunication network 1002, and may be operated by the service provider or on behalf of the service provider. Host 1016 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.

[0307] As a whole, communication system 1000 of Figure 10 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) 802.11 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.

[0308] In some examples, telecommunication network 1002 is a cellular network that implements 3 GPP standardized features. Accordingly, telecommunication network 1002 may support network slicing to provide different logical networks to different devices that are connected to telecommunication network 1002. For example, telecommunication network 1002 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.

[0309] In some examples, UEs 1012 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 1004 on a predetermined schedule, when triggered by an internal or external event, or in response to requests from access network 1004. 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).

[0310] In the example, hub 1014 communicates with access network 1004 to facilitate indirect communication between one or more UEs (e.g., 1012c and / or 1012d) and network nodes (e.g., 1010b). In some examples, hub 1014 may be a controller, router, content source and analytics, or any of the other communication devices described herein regarding UEs. For example, hub 1014 may be a broadband router enabling access to core network 1006 for the UEs. As another example, hub 1014 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 1010, or by executable code, script, process, or other instructions in hub 1014. As another example, hub 1014 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 1014 may be a content source. For example, for a UE that is a VR headset, display, loudspeaker or other media delivery device, hub 1014 may retrieve VR assets, video, audio, or other media or data related to sensory information via a network node, which hub 1014 then provides to the UE either directly, after performing local processing, and / or after adding additional local content. In still another example, hub 1014 acts as a proxy server or orchestrator for the UEs, in particular if one or more of the UEs are low energy loT devices.

[0311] Hub 1014 may have a constant / persistent or intermittent connection to network node 1010b. Hub 1014 may also allow for a different communication scheme and / or schedule between hub 1014 and UEs (e.g., 1012c and / or 1012d), and between hub 1014 and core network 1006. In other examples, hub 1014 is connected to core network 1006 and / or one or more UEs via a wired connection. Moreover, hub 1014 may be configured to connect to an M2M service provider over access network 1004 and / or to another UE over a direct connection. In some scenarios, UEs may establish a wireless connection with network nodes 1010 while still connected via hub 1014 via a wired or wireless connection. In some embodiments, hub 1014 may be a dedicated hub - that is, a hub whose primary function is to route communications to / from the UEs from / to network node 1010b. In other embodiments, hub 1014 may be a non-dedicated hub - that is, a device which is capable of operating to route communications between the UEs and network node 1010b, but which is additionally capable of operating as a communication start and / or end point for certain data channels.

[0312] In some embodiments, any of UEs 1012 may be configured to perform operations attributed to a UE in various methods or procedures described above, including the exemplary method shown in Figure 7. In some embodiments, any of network nodes 1010 may be configured to perform operations attributed to a RAN node in various methods or procedures described above, including the exemplary methods shown in Figures 8-9.

[0313] Figure 11 shows a UE 1100 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.

[0314] A UE may support device-to-device (D2D) communication, for example by implementing a 3GPP 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, a UE may not necessarily have a user in the sense of a human user who owns and / or operates the relevant device. Instead, a UE 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, a UE 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).

[0315] UE 1100 includes processing circuitry 1102 that is operatively coupled via a bus 1104 to an input / output interface 1106, a power source 1108, a memory 1110, a communication interface 1112, and / or any other component, or any combination thereof. Certain UEs may utilize all or a subset of the components shown in Figure 11. 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.

[0316] Processing circuitry 1102 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 1110. Processing circuitry 1102 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 1102 may include multiple central processing units (CPUs).

[0317] In the example, input / output interface 1106 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 1100. 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.

[0318] In some embodiments, power source 1108 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 1108 may further include power circuitry for delivering power from power source 1108 itself, and / or an external power source, to the various parts of UE 1100 via input circuitry or an interface such as an electrical power cable. Delivering power may be, for example, for charging power source 1108. Power circuitry may perform any formatting, converting, or other modification to the power from power source 1108 to make the power suitable for the respective components of UE 1100 to which power is supplied.

[0319] Memory 1110 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 1110 includes one or more application programs 1114, such as an operating system, web browser application, a widget, gadget engine, or other application, and corresponding data 1116. Memory 1110 may store, for use by UE 1100, any of a variety of various operating systems or combinations of operating systems.

[0320] Memory 1110 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 1110 may allow UE 1100 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 1110, which may be or comprise a device-readable storage medium. Processing circuitry 1102 may be configured to communicate with an access network or other network using communication interface 1112. Communication interface 1112 may comprise one or more communication subsystems and may include or be communicatively coupled to an antenna 1122. Communication interface 1112 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 1118 and / or a receiver 1120 appropriate to provide network communications (e.g., optical, electrical, frequency allocations, and so forth). Moreover, transmitter 1118 and receiver 1120 may be coupled to one or more antennas (e.g., antenna 1122) and may share circuit components, software, or firmware, or alternatively be implemented separately.

[0321] In the illustrated embodiment, communication functions of communication interface 1112 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.

[0322] Regardless of the type of sensor, a UE may provide an output of data captured by its sensors, through its communication interface 1112, via a wireless connection to a network node. Data captured by sensors of a UE 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).

[0323] As another example, a UE 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, the UE 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. A UE, 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 1100 shown in Figure 11.

[0324] As yet another specific example, in an loT scenario, a UE 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. The UE 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, the UE may implement the 3GPP NB-IoT standard. In other scenarios, a UE 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.

[0325] 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.

[0326] In some embodiments, UE 1100 may be configured to perform operations attributed to a UE in various methods or procedures described above, including the exemplary method shown in Figure 7. Figure 12 shows a network node 1200 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., 0-RU, 0-DU, O-CU).

[0327] 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 0-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).

[0328] 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).

[0329] Network node 1200 includes processing circuitry 1202, memory 1204, communication interface 1206, and power source 1208. Network node 1200 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 1200 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 1200 may be configured to support multiple radio access technologies (RATs). In such embodiments, some components may be duplicated (e.g., separate memory 1204 for different RATs) and some components may be reused (e.g., a same antenna 1210 may be shared by different RATs). Network node 1200 may also include multiple sets of the various illustrated components for different wireless technologies integrated into network node 1200, 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 1200.

[0330] Processing circuitry 1202 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 1200 components, such as memory 1204, to provide network node 1200 functionality.

[0331] In some embodiments, processing circuitry 1202 includes a system on a chip (SOC). In some embodiments, processing circuitry 1202 includes one or more of radio frequency (RF) transceiver circuitry 1212 and baseband processing circuitry 1214. In some embodiments, RF transceiver circuitry 1212 and baseband processing circuitry 1214 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 1212 and baseband processing circuitry 1214 may be on the same chip or set of chips, boards, or units.

[0332] Memory 1204 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 1202. Memory 1204 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 (collectively denoted computer program 1204a, which may be in the form of a computer program product) capable of being executed by processing circuitry 1202 and utilized by network node 1200. Memory 1204 may be used to store any calculations made by processing circuitry 1202 and / or any data received via communication interface 1206. In some embodiments, processing circuitry 1202 and memory 1204 is integrated.

[0333] Communication interface 1206 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 1206 comprises port(s) / terminal(s) 1216 to send and receive data, for example to and from a network over a wired connection. Communication interface 1206 also includes radio frontend circuitry 1218 that may be coupled to, or in certain embodiments a part of, antenna 1210. Radio front-end circuitry 1218 comprises filters 1220 and amplifiers 1222. Radio front-end circuitry 1218 may be connected to an antenna 1210 and processing circuitry 1202. The radio front-end circuitry may be configured to condition signals communicated between antenna 1210 and processing circuitry 1202. Radio front-end circuitry 1218 may receive digital data that is to be sent out to other network nodes or UEs via a wireless connection. Radio front-end circuitry 1218 may convert the digital data into a radio signal having the appropriate channel and bandwidth parameters using a combination of filters 1220 and / or amplifiers 1222. The radio signal may then be transmitted via antenna 1210. Similarly, when receiving data, antenna 1210 may collect radio signals which are then converted into digital data by radio front-end circuitry 1218. The digital data may be passed to processing circuitry 1202. In other embodiments, the communication interface may comprise different components and / or different combinations of components.

[0334] In certain alternative embodiments, network node 1200 does not include separate radio front-end circuitry 1218, instead, processing circuitry 1202 includes radio front-end circuitry and is connected to antenna 1210. Similarly, in some embodiments, all or some of RF transceiver circuitry 1212 is part of communication interface 1206. In still other embodiments, communication interface 1206 includes one or more ports or terminals 1216, radio front-end circuitry 1218, and RF transceiver circuitry 1212, as part of a radio unit (not shown), and communication interface 1206 communicates with baseband processing circuitry 1214, which is part of a digital unit (not shown).

[0335] Antenna 1210 may include one or more antennas, or antenna arrays, configured to send and / or receive wireless signals. Antenna 1210 may be coupled to radio front-end circuitry 1218 and may be any type of antenna capable of transmitting and receiving data and / or signals wirelessly. In certain embodiments, antenna 1210 is separate from network node 1200 and connectable to network node 1200 through an interface or port.

[0336] Antenna 1210, communication interface 1206, and / or processing circuitry 1202 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 1210, communication interface 1206, and / or processing circuitry 1202 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.

[0337] Power source 1208 provides power to the various components of network node 1200 in a form suitable for the respective components (e.g., at a voltage and current level needed for each respective component). Power source 1208 may further comprise, or be coupled to, power management circuitry to supply the components of network node 1200 with power for performing the functionality described herein. For example, network node 1200 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 1208. As a further example, power source 1208 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.

[0338] Embodiments of network node 1200 may include additional components beyond those shown in Figure 12 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 1200 may include user interface equipment to allow input of information into network node 1200 and to allow output of information from network node 1200. This may allow a user to perform diagnostic, maintenance, repair, and other administrative functions for network node 1200.

[0339] In some embodiments, network node 1200 may be configured to perform operations attributed to a RAN node in various methods or procedures described above, including the exemplary methods shown in Figures 8-9.

[0340] Figure 13 is a block diagram illustrating a virtualization environment 1300 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 1300 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 1300 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.

[0341] Applications 1302 (which may alternatively be called software instances, virtual appliances, network functions, virtual nodes, virtual network functions, etc.) are run in the virtualization environment 1300 to implement some of the features, functions, and / or benefits of some of the embodiments disclosed herein. In some embodiments, one or more virtual nodes 1302 may be configured to perform operations attributed to a RAN node in various methods or procedures described above, including the exemplary methods shown in Figures 8-9.

[0342] Hardware 1304 includes processing circuitry, memory that stores software and / or instructions (collectively denoted computer program 1304a, 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 1306 (also referred to as hypervisors or virtual machine monitors (VMMs)), provide VMs 1308a-1308b (one or more of which may be generally referred to as VMs 1308), and / or perform any of the functions, features and / or benefits described in relation with some embodiments described herein. Virtualization layer 1306 may present a virtual operating platform that appears like networking hardware to the VMs 1308.

[0343] VMs 1308 comprise virtual processing, virtual memory, virtual networking or interface and virtual storage, and may be run by a corresponding virtualization layer 1306. Different embodiments of the instance of a virtual appliance 1302 may be implemented on one or more of VMs 1308, 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.

[0344] In the context of NFV, each VM 1308 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 1308, and that part of hardware 1304 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 1308 on top of the hardware 1304 and corresponds to the application 1302.

[0345] Hardware 1304 may be implemented in a standalone network node with generic or specific components. Hardware 1304 may implement some functions via virtualization. Alternatively, hardware 1304 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 1310, which, among others, oversees lifecycle management of applications 1302. In some embodiments, hardware 1304 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 1312 which may alternatively be used for communication between hardware nodes and radio units.

[0346] 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.

[0347] 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.

[0348] 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.

[0349] 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.

[0350] 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.

[0351] 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 may be used synonymously herein, there may be instances when such terms are not intended to be used synonymously.

[0352] Embodiments of the techniques and apparatus described herein also include, but are not limited to, the following enumerated examples:

[0353] 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, a first message including one or more layerl / layer2 triggered mobility (LTM) candidate configurations for respective one or LTM candidate cells provided by a second RAN node, wherein the first message is secured based on a first security key associated with a first key identifier; obtaining a first token for integrity protection of at least one key identifier; receiving, from the first RAN node, a second message that includes the following: a command for an LTM cell switch from the serving cell to target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and a second token associated with the second key identifier; and based on verifying that the second token matches or corresponds to the first token, deriving the second security key based on a first parameter associated with the first key identifier.

[0354] A2. The method of embodiment Al, further comprising deriving the first parameter based on the received second key identifier.

[0355] A3. The method of any of embodiments A1-A2, wherein the first token is received from the first RAN node prior to receiving the second message.

[0356] A4. The method of any of embodiments A1-A2, wherein the first token is derived by the UE based on one or more of the following: an identifier of the first LTM candidate cell; an identifier of the LTM candidate configuration for the first LTM candidate cells; an initialization token received from the first RAN node; a freshness input that randomizes the derivation result; and a shared secret between the UE and the first RAN node.

[0357] A4a. The method of embodiment A4, wherein the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the first RAN node, a nonce value, and content of a previous message between the UE and the first RAN node.

[0358] A4b. The method of any of embodiments A4-A4a, further comprising deriving the shared secret or receiving the shared secret from the first RAN node.

[0359] A4c. The method of embodiment A4b, wherein the shared secret is derived based on one or more of the following: a user’s international mobile subscriber identity (IMSI); the UE’s international mobile equipment identity (IMEI); a hash value derived from a previous message between the UE and the first RAN node; a counter value or sequence number for a protocol between the UE and the first RAN node; and the UE’s radio network temporary identifier (RNTI) assigned by the first RAN node.

[0360] A5. The method of any of embodiments Al-A4c, further comprising performing an LTM cell switch to the target cell and transmitting a third message to the second RAN node via the target cell, wherein the third message is secured based on the derived second security key. A5a. The method of embodiment A5, further comprising, after performing the LTM cell switch, deriving a second shared secret between the UE and the second RAN node.

[0361] A5b. The method of embodiment A5a, wherein the second shared secret is derived based on one or more of the following: a user’s international mobile subscriber identity (IMSI); the UE’s international mobile equipment identity (IMEI); a hash value derived from a previous message between the UE and the second RAN node; a counter value or sequence number for a protocol between the UE and the second RAN node; and the UE’s radio network temporary identifier (RNTI) assigned by the second RAN node.

[0362] A6. The method of any of embodiments Al-A5b, wherein the first message is a radio resource control (RRC) message and the second message is medium access control (MAC) control element (CE).

[0363] A7. The method of any of embodiments A1-A6, wherein the first parameter is a next-hop (NH) parameter, and the first and second key identifiers are different NH chaining counter (NCC) values.

[0364] A8. The method of any of embodiments A1-A7, wherein the second key identifier indicates that the second security key should be derived vertically based on the first parameter.

[0365] Bl. A method for a first radio access network (RAN) node configured to provide a serving cell for user equipment (UEs), the method comprising: sending, to a UE via the serving cell, a first message including one or more layerl / layer2 triggered mobility (LTM) candidate configurations for respective one or LTM candidate cells provided by a second RAN node, wherein the first message is secured based on a first security key associated with a first key identifier; obtaining a second token for integrity protection of at least one key identifier; sending, to the UE, a second message that includes the following: a command for an LTM cell switch from the serving cell to target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and the second token.

[0366] B2. The method of embodiment Bl, further comprising, prior to sending the second message, sending a first token to the UE for verification of the second token in the second message, wherein the first token matches or corresponds to the second token.

[0367] B3. The method of any of embodiments B1-B2, further comprising receiving the one or more LTM candidate configurations from the second RAN node.

[0368] B3a. The method of embodiment B3, wherein the second token is received from the second RAN node together with the one or more LTM candidate configurations.

[0369] B4. The method of any of embodiments B1-B3, wherein the second token is derived by the first RAN node based on one or more of the following: an identifier of the first LTM candidate cell; an identifier of the LTM candidate configuration for the first LTM candidate cells; an initialization token; a freshness input that randomizes the derivation result; and a shared secret between the UE and the first RAN node.

[0370] B4a. The method of embodiment B4, wherein the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the first RAN node, a nonce value, and content of a previous message between the UE and the first RAN node.

[0371] B4b. The method of any of embodiments B4-B4a, further comprising deriving the shared secret based on one or more of the following: a user’s international mobile subscriber identity (IMSI); the UE’s international mobile equipment identity (IMEI); a hash value derived from a previous message between the UE and the first RAN node; a counter value or sequence number for a protocol between the UE and the first RAN node; and the UE’s radio network temporary identifier (RNTI) assigned by the first RAN node. B4c. The method of embodiment B4b, further comprising sending the derived shared secret to the UE.

[0372] B5. The method of any of embodiments Bl-B4c, further comprising, after sending the second message, sending to the second RAN node a notification of the LTM cell switch by the UE.

[0373] B6. The method of any of embodiments Bl-B5b, wherein the first message is a radio resource control (RRC) message and the second message is medium access control (MAC) control element (CE).

[0374] B7. The method of any of embodiments B1-B6, wherein the first parameter is a next-hop (NH) parameter, and the first and second key identifiers are different NH chaining counter (NCC) values.

[0375] B8. The method of any of embodiments B1-B7, wherein the second key identifier indicates that the second security key should be derived vertically based on the first parameter. a second key identifier

[0376] Cl . A method for a second radio access network (RAN) node configured to provide a target cell for user equipment (UE) mobility, the method comprising: determining that a UE has executed a layerl / layer2 triggered mobility (LTM) cell switch from a serving cell provided by a first RAN node to a first LTM candidate cell provided by the second RAN node; obtaining a security context for the UE from a core network node or function (NNF), wherein the security context includes the following: a first key identifier associated with a first security key, and a first parameter from which a second security key may be derived; deriving a second token for integrity protection of at least one key identifier; and sending to the UE a lower layer message that includes the first key identifier and the second token.

[0377] C2. The method of embodiment Cl, further comprising sending, to the first RAN node, one or more LTM candidate configurations for respective one or LTM candidate cells provided by the second RAN node, wherein the LTM cell switch is based on a first one of the LTM candidate configurations for the first LTM candidate cell.

[0378] C3. The method of any of embodiments C1-C2, wherein determining that the UE has executed the LTM cell switch is based on receiving one or more of the following: a notification from the first RAN node, a notification from a distributed unit (DU) of the second RAN node, or a message from the UE via the first LTM candidate cell.

[0379] C4. The method of any of embodiments C1-C3, wherein the second token is derived by the second RAN node based on one or more of the following: an identifier of the first LTM candidate cell; an identifier of the LTM candidate configuration for the first LTM candidate cells; an initialization token; a freshness input that randomizes the derivation result; and a shared secret between the UE and the second RAN node.

[0380] C4a. The method of embodiment C4, wherein the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the second RAN node, a nonce value, and content of a previous message between the UE and the second RAN node.

[0381] C4b. The method of any of embodiments C4-C4a, further comprising deriving the shared secret based on one or more of the following: a user’s international mobile subscriber identity (IMSI); the UE’s international mobile equipment identity (IMEI); a hash value derived from a previous message between the UE and the second RAN node; a counter value or sequence number for a protocol between the UE and the second RAN node; and the UE’s radio network temporary identifier (RNTI) assigned by the second RAN node.

[0382] C4c. The method of embodiment C4b, further comprising sending the derived shared secret to the UE.

[0383] C5. The method of any of embodiments Cl-C4c, wherein: the second RAN node comprises a centralized unit (CU) and a distributed unit (DU); the CU determines that the UE has executed the LTM cell switch and obtains the security context from the core NNF; and the DU sends the lower layer message to the UE.

[0384] C5a. The method of embodiment C5, wherein the CU derives the second token and the method further comprises the CU sending to the DU the second token and the first key identifier for inclusion in the lower layer message.

[0385] C5b. The method of embodiment C5, wherein the DU derives the second token and the method further comprises the CU sending to the DU a token request and the first key identifier.

[0386] C6. The method of any of embodiments Cl-C5b, wherein the lower layer message is medium access control (MAC) control element (CE).

[0387] C7. The method of any of embodiments C1-C6, wherein the first parameter is a next-hop (NH) parameter, and the first key identifier is a NH chaining counter (NCC) value.

[0388] C8. The method of any of embodiments C1-C7, wherein the first key identifier indicates that the second security key should be derived vertically based on the first parameter.

[0389] 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.

[0390] 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.

[0391] 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. 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.

[0392] El . A first radio access network (RAN) node configured to provide a serving cell for 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 B1-B8.

[0393] E2. A first radio access network (RAN) node configured to provide a serving cell for user equipment (UEs), the first RAN node being further configured to perform operations corresponding to the methods of any of embodiments B1-B8.

[0394] 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 provide a serving cell for user equipment (UEs), configure the first RAN node to perform operations corresponding to the methods of any of embodiments B1-B8.

[0395] 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 provide a serving cell for user equipment (UEs), configure the first RAN node to perform operations corresponding to the methods of any of embodiments B1-B8.

[0396] Fl. A second radio access network (RAN) node configured to provide a target cell for user equipment (UE) mobility, 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 C1-C8.

[0397] F2. A second radio access network (RAN) node configured to provide a target cell for user equipment (UE) mobility, the second RAN node being further configured to perform operations corresponding to the methods of any of embodiments C1-C8.

[0398] 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 provide a target cell for user equipment (UE) mobility, configure the second RAN node to perform operations corresponding to the methods of any of embodiments C1-C8.

[0399] 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 provide a target cell for user equipment (UE) mobility, configure the second RAN node to perform operations corresponding to the methods of any of embodiments C1-C8.

Claims

CLAIMS1. A method for a user equipment, UE, configured for mobility between cells of a radio access network, RAN, the method comprising: receiving (710), from a first RAN node via a serving cell, a first message including one or more layer-l / layer-2 triggered mobility, LTM, candidate configurations for respective one or more LTM candidate cells provided by a second RAN node, wherein the first message is secured based on a first security key associated with a first key identifier; obtaining (730) a first token for integrity protection of at least one key identifier; receiving (740), from the first RAN node, a second message that includes the following: a command for an LTM cell switch from the serving cell to a target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and a second token associated with the second key identifier; and based on verifying (750) that the second token matches or corresponds to the first token, deriving (770) the second security key based on a first parameter associated with the first key identifier.

2. The method of claim 1, further comprising deriving (760) the first parameter based on the received second key identifier.

3. The method of any of claims 1-2, wherein the first token is obtained by being received from the first RAN node prior to receiving the second message.

4. The method of any of claims 1-2, wherein the first token is obtained by being derived by the UE based on one or more of the following: an identifier of the first LTM candidate cell; an identifier of the LTM candidate configuration for the first LTM candidate cells; an initialization token received from the first RAN node; a freshness input that randomizes the derivation result; and a shared secret between the UE and the first RAN node.

5. The method of claim 4, wherein the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the first RAN node, a nonce value, or content of a previous message between the UE and the first RAN node.

6. The method of any of claims 4-5, further comprising deriving (720) the shared secret or receiving the shared secret from the first RAN node.

7. The method of claim 6, wherein the shared secret is derived based on one or more of the following: a user’s international mobile subscriber identity, IMSI; the UE’s international mobile equipment identity, IMEI; a hash value derived from a previous message between the UE and the first RAN node; a counter value or sequence number for a protocol between the UE and the first RAN node; and a first radio network temporary identifier, RNTI, assigned to the UE by the first RAN node.

8. The method of any of claims 1-7, further comprising performing (780) an LTM cell switch to the target cell and transmitting a third message to the second RAN node via the target cell, wherein the third message is secured based on the derived second security key.

9. The method of claim 8, further comprising, after performing (780) the LTM cell switch, deriving (790) a second shared secret between the UE and the second RAN node.

10. The method of claim 9, wherein the second shared secret is derived based on one or more of the following: a user’s international mobile subscriber identity, IMSI; the UE’s international mobile equipment identity, IMEI; a hash value derived from a previous message between the UE and the second RAN node; a counter value or sequence number for a protocol between the UE and the second RAN node; and a second radio network temporary identifier, RNTI, assigned to the UE by the second RAN node.

11. The method of any of claims 1-10, wherein the first message is a radio resource control, RRC, message and the second message is a medium access control, MAC, control element, CE.

12. The method of any of claims 1-11, wherein the first parameter is a next-hop, NH, parameter and one or more of the following applies: the first and second key identifiers are different NH chaining counter, NCC, values, and the second key identifier indicates that the second security key should be derived vertically based on the first parameter.

13. A method for a first radio access network, RAN, node configured to provide a serving cell for user equipment, UEs, the method comprising: sending (820), to a UE via the serving cell, a first message including one or more layer- l / layer-2 triggered mobility, LTM, candidate configurations for respective one or LTM candidate cells provided by a second RAN node, wherein the first message is secured based on a first security key associated with a first key identifier; obtaining (850) a second token for integrity protection of at least one key identifier; sending (870), to the UE, a second message that includes the following: a command for an LTM cell switch from the serving cell to a target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and the second token.

14. The method of claim 13, further comprising, prior to sending (870) the second message, sending (860) to the UE a first token for verification of the second token.

15. The method of any of claims 13-14, further comprising receiving (810) the one or more LTM candidate configurations from the second RAN node.

16. The method of claim 15, wherein the second token is obtained by being received from the second RAN node together with the one or more LTM candidate configurations.

17. The method of any of claims 13-14, wherein the second token is obtained by being derived by the first RAN node based on one or more of the following: an identifier of the first LTM candidate cell; an identifier of the LTM candidate configuration for the first LTM candidate cells;an initialization token; a freshness input that randomizes the derivation result; and a shared secret between the UE and the first RAN node.

18. The method of claim 17, wherein the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the first RAN node, a nonce value, and content of a previous message between the UE and the first RAN node.

19. The method of any of claims 17-18, further comprising deriving (830) the shared secret based on one or more of the following: a user’s international mobile subscriber identity, IMSI; the UE’s international mobile equipment identity, IMEI; a hash value derived from a previous message between the UE and the first RAN node; a counter value or sequence number for a protocol between the UE and the first RAN node; and a first radio network temporary identifier, RNTI, assigned to the UE by the first RAN node.

20. The method of claim 19, further comprising sending (840) the derived shared secret to the UE.

21. The method of any of claims 13-19, further comprising, after sending (870) the second message, sending (880) to the second RAN node a notification of the LTM cell switch by the UE.

22. The method of any of claims 13-21, wherein the first message is a radio resource control, RRC, message and the second message is a medium access control, MAC, control element, CE.

23. The method of any of claims 13-22, wherein the first parameter is a next-hop, NH, parameter and one or more of the following applies: the first and second key identifiers are different NH chaining counter, NCC, values, and the second key identifier indicates that the second security key should be derived vertically based on the first parameter.

24. A method for a second radio access network, RAN, node configured to provide a target cell for user equipment, UE, mobility, the method comprising:determining (920) that a UE has executed a layer-l / layer-2 triggered mobility, LTM, cell switch from a serving cell provided by a first RAN node to a first LTM candidate cell provided by the second RAN node; obtaining (930) a security context for the UE from a core network node or function, NNF, wherein the security context includes the following: a first key identifier associated with a first security key, and a first parameter from which a second security key may be derived; deriving (960) a second token for integrity protection of at least one key identifier; and sending (990) to the UE a lower layer message that includes the first key identifier and the second token.

25. The method of claim 24, further comprising sending (910), to the first RAN node, one or more LTM candidate configurations for respective one or more LTM candidate cells provided by the second RAN node, wherein the LTM cell switch is based on a first one of the LTM candidate configurations for the first LTM candidate cell.

26. The method of any of claims 24-25, wherein determining (920) that the UE has executed the LTM cell switch is based on receiving (921) one or more of the following: a notification from the first RAN node, a notification from a distributed unit, DU, of the second RAN node, or a message from the UE via the first LTM candidate cell.

27. The method of any of claims 24-26, wherein the second token is derived by the second RAN node based on one or more of the following: an identifier of the first LTM candidate cell; an identifier of the LTM candidate configuration for the first LTM candidate cells; an initialization token; a freshness input that randomizes the derivation result; and a shared secret between the UE and the second RAN node.

28. The method of claim 27, wherein the freshness input is one of the following: a counter value or sequence number for a protocol between the UE and the second RAN node, a nonce value, and content of a previous message between the UE and the second RAN node.

29. The method of any of claims 27-28, further comprising deriving (940) the shared secret based on one or more of the following:a user’s international mobile subscriber identity, IMSI; the UE’s international mobile equipment identity, IMEI; a hash value derived from a previous message between the UE and the second RAN node; a counter value or sequence number for a protocol between the UE and the second RAN node; and a second radio network temporary identifier, RNTI, assigned to the UE by the second RAN node.

30. The method of claim 29, further comprising sending (950) the derived shared secret to the UE.

31. The method of any of claims 24-30, wherein: the second RAN node comprises a centralized unit, CU, and a distributed unit, DU; the CU determines that the UE has executed the LTM cell switch and obtains the security context from the core NNF; and the DU sends the lower layer message to the UE.

32. The method of claim 31, wherein the CU derives the second token and the method further comprises the CU sending (970) to the DU the second token and the first key identifier for inclusion in the lower layer message.

33. The method of claim 31, wherein the DU derives the second token and the method further comprises the CU sending (980) to the DU a token request and the first key identifier.

34. The method of any of claims 24-33, wherein the lower layer message is a medium access control, MAC, control element, CE.

35. The method of any of claims 24-34, wherein the first parameter is a next-hop, NH, parameter and one or more of the following applies: the first key identifier is a NH chaining counter, NCC, value, and the first key identifier indicates that the second security key should be derived vertically based on the first parameter.

36. User equipment, UE (310, 601, 1012, 1100) configured for mobility between cells of a radio access network, RAN (199, 1004), the UE comprising:communication interface circuitry (1112) configured to communicate with RAN nodes (100, 150, 320, 602, 603, 1010, 1200, 1302); and processing circuitry (1102) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: receive, from a first RAN node via a serving cell, a first message including one or more layer-l / layer-2 triggered mobility, LTM, candidate configurations for respective one or more LTM candidate cells provided by a second RAN node, wherein the first message is secured based on a first security key associated with a first key identifier; obtain a first token for integrity protection of at least one key identifier; receive, from the first RAN node, a second message that includes the following: a command for an LTM cell switch from the serving cell to a target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and a second token associated with the second key identifier; and based on verifying that the second token matches or corresponds to the first token, derive the second security key based on a first parameter associated with the first key identifier.

37. The UE of claim 36, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 2-12.

38. User equipment, UE (310, 601, 1012, 1100) configured for mobility between cells of a radio access network, RAN (199, 1004), the UE being further configured to: receive, from a first RAN node (100, 320, 602, 1010, 1200, 1302) via a serving cell, a first message including one or more layer-l / layer-2 triggered mobility, LTM, candidate configurations for respective one or more LTM candidate cells provided by a second RAN node (150, 320, 603, 1010, 1200, 1302), wherein the first message is secured based on a first security key associated with a first key identifier; obtain a first token for integrity protection of at least one key identifier;receive, from the first RAN node, a second message that includes the following: a command for an LTM cell switch from the serving cell to a target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and a second token associated with the second key identifier; and based on verifying that the second token matches or corresponds to the first token, derive the second security key based on a first parameter associated with the first key identifier.

39. The UE of claim 38, being further configured to perform operations corresponding to the methods of any of claims 2-12.

40. Non-transitory, computer-readable medium (1110) storing computer-executable instructions that, when executed by processing circuitry (1102) of user equipment, UE (310, 601, 1012, 1100) configured for mobility between cells of a radio access network, RAN (199, 1004), configure the UE to perform operations corresponding to the methods of any of claims 1- 12.

41. Computer program product (1114) comprising computer-executable instructions that, when executed by processing circuitry (1102) of user equipment, UE (310, 601, 1012, 1100) configured for mobility between cells of a radio access network, RAN (199, 1004), configure the UE to perform operations corresponding to the methods of any of claims 1-12.

42. First radio access network, RAN, node (100, 320, 602, 1010, 1200, 1302) configured to provide a serving cell for user equipment, UEs (310, 601, 1012, 1100), the first RAN node comprising: communication interface circuitry (1206, 1304) configured to communicate with UEs and with other RAN nodes; and processing circuitry (1202, 1304) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: send, to a UE via the serving cell, a first message including one or more layer- l / layer-2 triggered mobility, LTM, candidate configurations for respective one or LTM candidate cells provided by a second RAN node, wherein thefirst message is secured based on a first security key associated with a first key identifier; obtain a second token for integrity protection of at least one key identifier; send, to the UE, a second message that includes the following: a command for an LTM cell switch from the serving cell to a target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and the second token.

43. The first RAN node of claim 42, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 14-23.

44. First radio access network, RAN, node (100, 320, 602, 1010, 1200, 1302) configured to provide a serving cell for user equipment, UEs (310, 601, 1012, 1100), the first RAN node being further configured to: send, to a UE via the serving cell, a first message including one or more layer-l / layer-2 triggered mobility, LTM, candidate configurations for respective one or LTM candidate cells provided by a second RAN node (150, 320, 603, 1010, 1200, 1302), wherein the first message is secured based on a first security key associated with a first key identifier; obtain a second token for integrity protection of at least one key identifier; send, to the UE, a second message that includes the following: a command for an LTM cell switch from the serving cell to a target cell, which is a first one of the LTM candidate cells provided by the second RAN node; a second key identifier associated with a second security key to be used in the target cell; and the second token.

45. The first RAN node of claim 44, being further configured to perform operations corresponding to the methods of any of claims 14-23.

46. Non-transitory, computer-readable medium (1204, 1304) storing computer-executable instructions that, when executed by processing circuitry (1202, 1304) of a first radio access network, RAN, node (100, 320, 602, 1010, 1200, 1302) configured to provide a serving cell for user equipment, UEs (310, 601, 1012, 1100), configure the first RAN node to perform operations corresponding to the methods of any of claims 13-23.

47. Computer program product (1204a, 1304a) comprising computer-executable instructions that, when executed by processing circuitry (1202, 1304) of a first radio access network, RAN, node (100, 320, 602, 1010, 1200, 1302) configured to provide a serving cell for user equipment, UEs (310, 601, 1012, 1100), configure the first RAN node to perform operations corresponding to the methods of any of claims 13-23.

48. Second radio access network, RAN, node (150, 320, 603, 1010, 1200, 1302) configured to provide a target cell for user equipment, UE (310, 601, 1012, 1100) mobility, the second RAN node comprising: communication interface circuitry (1206, 1304) configured to communicate with UEs and with other RAN nodes; and processing circuitry (1202, 1304) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: determine that a UE has executed a layer-l / layer-2 triggered mobility, LTM, cell switch from a serving cell provided by a first RAN node to a first LTM candidate cell provided by the second RAN node; obtain a security context for the UE from a core network node or function, NNF, wherein the security context includes the following: a first key identifier associated with a first security key, and a first parameter from which a second security key may be derived; derive a second token for integrity protection of at least one key identifier; and send to the UE a lower layer message that includes the first key identifier and the second token.

49. The second RAN node of claim 48, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 25-35.

50. Second radio access network, RAN, node (150, 320, 603, 1010, 1200, 1302) configured to provide a target cell for user equipment, UE (310, 601, 1012, 1100) mobility, the second RAN node being further configured to: determine that a UE has executed a layer-l / layer-2 triggered mobility, LTM, cell switch from a serving cell provided by a first RAN node (100, 320, 602, 1010, 1200, 1302) to a first LTM candidate cell provided by the second RAN node; obtain a security context for the UE from a core network node or function, NNF, wherein the security context includes the following: a first key identifier associated with a first security key, and a first parameter from which a second security key may be derived; derive a second token for integrity protection of at least one key identifier; and send to the UE a lower layer message that includes the first key identifier and the second token.

51. The second RAN node of claim 50, being further configured to perform operations corresponding to the methods of any of claims 25-35.

52. Non-transitory, computer-readable medium (1204, 1304) storing computer-executable instructions that, when executed by processing circuitry (1202, 1304) of a second radio access network, RAN, node (150, 320, 603, 1010, 1200, 1302) configured to provide a target cell for user equipment, UE (310, 601, 1012, 1100) mobility, configure the second RAN node to perform operations corresponding to the methods of any of claims 24-35.

53. Computer program product (1204a, 1304a) comprising computer-executable instructions that, when executed by processing circuitry (1202, 1304) of a radio access network, RAN, node (150, 320, 603, 1010, 1200, 1302) configured to provide a target cell for user equipment, UE(310, 601, 1012, 1100) mobility, configure the second RAN node to perform operations corresponding to the methods of any of claims 24-35.

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