Early uplink synchronization based on relative time difference

Abstract

The disclosure relates to a method for a user equipment, UE, (800) configured for conditional layer-1 / layer-2 triggered inter-cell mobility, LTM, in a radio access network, RAN. The method comprises receiving (510) first timing advance, TA, values for the UE in one or more LTM candidate cells, and one or more validity durations associated with the each of the first TA values; initiating (530) a validity timer for the first TA value, performing (540) relative time difference, RTD, measurements between a radio interface event in the serving cell and a radio interface event in the LTM candidate cell; and for each LTM candidate cell, selectively performing (570) updating (571) the first TA value to a second TA value; or determining (572) whether to update validity duration for the first TA value, or a validity duration for the second TA value; and reinitiating or pausing (573) the validity timer.

Description

[0001] EARLY UPLINK SYNCHRONIZATION BASED ON RELATIVE TIME DIFFERENCE TECHNICAL FIELD

[0002] The present disclosure relates generally to wireless networks, and more specifically to techniques for improving conditional layer-l / layer-2 (L1 / L2) triggered mobility (LTM) of user equipment (UEs) across multiple cells in a radio access network (RAN), even more specifically in relation to UEs maintaining early uplink (UL) synchronization with LTM candidate cells.

[0003] BACKGROUND

[0004] Currently the fifth generation (5G) of cellular systems is being standardized within the Third-Generation Partnership Project (3GPP). 5G is developed for maximum flexibility to support many different use cases including enhanced mobile broadband (eMBB), machine type communications (MTC), ultra-reliable low latency communications (URLLC), side-link device-to-device (D2D), and several other use cases.

[0005] Figure 1 illustrates a high-level view of an exemplary 5G network architecture, consisting of a Next Generation Radio Access Network (NG-RAN, 199) and a 5G Core (5GC, 198). The NG-RAN can include one or more gNodeB’s (gNBs) connected to the 5GC via one or more NG interfaces, such as gNBs (100, 150) connected via respective interfaces (102, 152). More specifically, the gNBs can be connected to one or more Access and Mobility Management Functions (AMFs) in the 5GC via respective NG-C interfaces and to one or more User Plane Functions (UPFs) in 5GC via respective NG-U interfaces. The 5GC can include various other network functions (NFs), such as Session Management Function(s) (SMF).

[0006] 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 downlink (DL) “beam” is a coverage area of a network-transmitted reference signal (RS) that may be measured or monitored by a UE.

[0007] 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 DUcan include various circuitry needed to perform their respective functions, including processing circuitry, communication interface circuitry (e.g., transceivers), and power supply circuitry. A gNB-CU connects to one or more gNB-DUs over respective F1 logical interfaces (e.g., 122 and 132 shown in Figure 1).

[0008] 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.

[0009] Even so, handover and other mobility procedures can have various problems related to robustness. For example, a HO command is normally sent when the radio conditions for the UE are already quite bad, such as at or near cell borders. As such, the HO command may need to be segmented (e.g., to allow for redundancy to protect against errors) and / or retransmitted one or more times before it reaches the UE. In such case, the HO command may not reach the UE in time (or at all) before the degraded connection with the source node (e.g., the node hosting the UE’s current serving cell) is dropped. Failure of handover to a target cell may lead to the UE declaring radio link failure (RLF) in the source cell.

[0010] To address various difficulties with handovers and other mobility procedures, 3 GPP Rel-16 includes support for conditional handover (CHO) and SN-initiated intra-SN conditional PSCell change (CPC) procedures while Rel-17 includes support for various other conditional mobility procedures. A main principle of conditional mobility is separation of transmission and execution of a mobility (e.g., handover) command. This allows the mobility command to be sent earlier to UE when the radio conditions are still good, thus increasing the likelihood that it is successfully transferred. The execution of the mobility command is done at later point in time based on an associated execution condition.

[0011] Conditional mobility procedures are based on a conditional reconfiguration framework in which the network provides a UE with one or more reconfigurations, each with associated execution condition(s). Each reconfiguration can be provided in an RRCReconfiguration message (in NR) or an RRCConnectionReconfiguration message (in LTE). When the UE later detects the execution condition(s) associated with one of the earlier-received reconfigurations, the UE executes the associated reconfiguration to perform the mobility procedure (e.g., HO, PSCell change, PSCell addition, etc.).

[0012] Even so, conditional (e.g., CHO) and non-conditional (e.g., HO) mobility operations are triggered by layer 3 (L3) measurements and involves radio resource control (RRC) signaling to change primary cells as well as to release / add secondary cells as needed. Moreover,conventional inter-cell mobility involves complete layer 2 (L2) and layer 1 (L1) resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching.

[0013] 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.”

[0014] Each LTM candidate cell configuration may include a configuration for early uplink (UL) synchronization (or “sync’) in the cell. Using the early UL sync configuration for an LTM candidate cell, the UE transmits on a random access channel (RACH) in the cell prior to receiving an LTM cell switch command for the cell, which may include a timing advance (TA) determined based on the UE’s earlier RACH transmission. In this manner, the UE can become UL synchronized with LTM candidate cells that are different from the UE’s current serving cell.

[0015] The UE performs measurements on configured LTM candidate cells and reports these measurements to its serving RAN node, based on which the RAN node triggers execution of an LTM cell switch procedure by the UE to one of the configured LTM candidate cells. The RAN node may trigger LTM cell switch by sending the UE an LTM cell switch command.

[0016] There are some notable differences between Rel-18 LTM and conditional L3 mobility. For example, unlike conditional L3 mobility in which the UE initiates execution, LTM is initiated by a UE’s serving RAN node based on measurements reported by the UE. This introduces some amount of delay in an LTM cell switch. To capture advantages of both LTM and conditional L3 mobility, 3GPP Rel-19 will introduce conditional LTM (CLTM). Much like conditional L3 mobility, when a UE detects the execution condition(s) associated with an earlier-received LTM candidate cell configuration, the UE executes an LTM cell switch to the candidate cell.

[0017] SUMMARY

[0018] As briefly mentioned above, in Rel-18 (non-conditional) LTM, the UE’s serving RAN node sends the UE a command for LTM cell switch to one of the UE’s configured LTM candidate cells, and the command may include a TA value determined based on the UE’s earlier RACH transmission towards the LTM candidate cell. Since there is no LTM cell switch command in Rel-19 CLTM, however, other solutions may be used to facilitate UE early UL synchronization with a CLTM candidate cell. One such solution is to provide the TA value for the CLTM candidate cell to the UE in an earlier message. Even so, there may be some time betweenwhen the UE receives this TA value and when the UE is ready to use it based on the LTM conditions for that candidate cell being fulfilled. During this time, the received TA value may become inaccurate (e.g., due to UE movement) such that the UE is not actually UL synchronized with the CLTM candidate cell upon LTM execution. It is unclear how the UE and / or the network should maintain and / or update the UE’s TA values for CLTM candidate cells.

[0019] An object of embodiments of the present disclosure is to improve early UL synchronization for UEs in CLTM, such as by providing, enabling, and / or facilitating solutions to overcome exemplary problems summarized above and described in more detail below.

[0020] Embodiments include methods e.g., procedures) for a UE configured for CLTM in a radio access network (RAN, e.g., E-UTRAN, NG-RAN).

[0021] These exemplary methods include receiving the following from a RAN node that provides a serving cell for the UE: first timing advance (TA) values for the UE in one or more LTM candidate cells, and one or more validity durations associated with the each of the first TA values. These exemplary methods also include, for each LTM candidate cell, initiating a validity timer for the first TA value for the UE in the LTM candidate cell, based on an associated validity duration. These exemplary methods also include, for each LTM candidate cell, performing RTD measurements between a radio interface event in the serving cell and a radio interface event in the LTM candidate cell. These exemplary methods also include, for each LTM candidate cell, selectively performing one or more of the following operations based on results of the RTD measurements performed for the LTM candidate cell:

[0022] • updating the first TA value to a second TA value;

[0023] • determining one of the following: an updated validity duration for the first TA value, or a validity duration for the second TA value; and

[0024] • reinitiating or pausing the validity timer based on the updated validity duration for the first TA value or the validity duration for the second TA value.

[0025] In some embodiments, these exemplary methods also include receiving an RTD measurement configuration from the RAN node. The RTD measurements are performed in accordance with the RTD measurement configuration. In some of these embodiments, these exemplary methods also include sending the results of the RTD measurements to the RAN node. In some variants of these embodiments, the RTD measurement configuration includes an RTD measurement reporting configuration, and the results of the RTD measurements are sent to the RAN node in accordance with the RTD measurement reporting configuration. In some further variants, the RTD measurement reporting configuration indicates one of the following types of RTD measurement reporting: aperiodic, event-triggered, semi-persistent, or periodic.

[0026] In some variants of these embodiments, these exemplary methods also include, in responseto sending the results of the RTD measurements to the RAN node, receiving from the RAN node at least one of the following for each LTM candidate cell: the second TA value, the validity duration for the second TA value, and the updated validity duration for the first TA value.

[0027] In some embodiments, these exemplary methods also include, after initiating the validity timer, determining whether the UE is stationary. Selectively performing the one or more operations for each LTM candidate cell is further based on whether the UE is stationary. Various examples of these operations are disclosed herein.

[0028] In some embodiments, these exemplary methods also include receiving from the RAN node synchronization assistance information. In some of these embodiments, for each LTM candidate cell, the one or more operations are selectively performed further based on the received synchronization assistance information. Various examples of synchronization assistance information are disclosed herein.

[0029] Other embodiments include exemplary methods (e.g., procedures) for a RAN node configured to facilitate CLTM by UEs. In general, these exemplary methods may be complementary to the exemplary methods for a UE summarized above.

[0030] These exemplary methods include receiving the following from a second RAN node that provides one or more LTM candidate cells for a UE: first TA values for the UE in the one or more LTM candidate cells, and one or more validity durations associated with each of the first TA values. These exemplary methods also include sending the following to the UE via a serving cell: the first TA values, the one or more validity durations associated with each of the first TA values, and an RTD measurement configuration for the one or more LTM candidate cells. These exemplary methods also include receiving from the UE results of UE RTD measurements between a radio interface event in the serving cell and respective radio interface events in the one or more LTM candidate cells. These exemplary methods also include, for each LTM candidate cell, the RAN node selectively performs one or more of the following operations based on the results of the RTD measurements:

[0031] • updating the first TA value to a second TA value, and

[0032] • determining one of the following: an updated validity duration for the first TA value, or a validity duration for the second TA value.

[0033] In some embodiments, these exemplary methods also include sending to the UE at least one of the following for each LTM candidate cell: the second TA value, the validity duration for the second TA value, and the updated validity duration for the first TA value.

[0034] In some embodiments, the RTD measurement configuration includes an RTD measurement reporting configuration, and the results of the RTD measurements are received from the UE in accordance with the RTD measurement reporting configuration. In some of theseembodiments, the RTD measurement reporting configuration indicates one of the following types ofRTD measurement reporting: aperiodic, event-triggered, semi-persistent, or periodic.

[0035] In some embodiments, these exemplary methods also include determining whether the UE is stationary. Selectively performing the one or more operations for each LTM candidate is further based on whether the UE is stationary. In some of these embodiments, determining whether the UE is stationary is based on one or more of the following: the results of the RTD measurements, and positioning measurements of the UE performed by the RAN node.

[0036] In some embodiments, these exemplary methods also include sending to the UE synchronization assistance information. Various examples of synchronization assistance information are disclosed herein. In some of these embodiments, for each LTM candidate cell, the one or more operations are selectively performed further based on the synchronization assistance information. In some of these embodiments, these exemplary methods also include receiving various synchronization status information from the second RAN node. The synchronization assistance information sent to the UE is determined based on the synchronization status information received from the second RAN node.

[0037] In some variants of these embodiments, these exemplary methods also include sending to the second RAN node one or more of the following information:

[0038] • a request for synchronization status; and

[0039] • one or more of the following synchronization status information for the RAN node:

[0040] maximum synchronization error of the RAN node with respect to the global time reference, and maximum clock drift of the RAN node.

[0041] Other embodiments and variants of the exemplary methods summarized above are described herein.

[0042] 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.

[0043] These and other embodiments described herein may provide various advantages and / or benefits. For example, by enabling a UE to maintain an LTM candidate cell TA value received from its serving RAN node, embodiments may facilitate timely UE conditional LTM cell switch while reducing TA-related signaling with the serving RAN node. As another example, by using a time alignment timer and RTD measurements as a basis for maintaining LTM candidate cell TA values, embodiments may enable UEs and / or RAN nodes to easily determine UE TA validity foran LTM candidate cell in a timely and efficient manner. At a high level, embodiments may facilitate conditional LTM cell switch without connection interruption, excess signaling overhead, and excess UE energy consumption.

[0044] 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.

[0045] BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 shows a high-level view of an exemplary 5G / NR network architecture.

[0047] Figure 2 shows exemplary NR user plane (UP) and control plane (CP) protocol stacks. Figure 3 shows an exemplary procedure for a UE to obtain timing advance (TA) for a serving cell.

[0048] Figure 4 shows a signaling diagram for an exemplary LTM cell switch procedure.

[0049] Figure 5 shows a flow diagram of an exemplary method for a UE (e.g., base station, eNB, gNB, DU, etc.), according to various embodiments of the present disclosure.

[0050] Figure 6 shows a flow diagram of an exemplary method for a RAN node (e.g., base station, eNB, gNB, DU, etc.), according to various embodiments of the present disclosure.

[0051] Figure 7 shows a communication system according to various embodiments of the present disclosure.

[0052] Figure 8 shows a UE according to various embodiments of the present disclosure.

[0053] Figure 9 shows a network node according to various embodiments of the present disclosure.

[0054] Figure 10 is a block diagram of a virtualization environment in which some embodiments of the present disclosure may be virtualized.

[0055] DETAILED DESCRIPTION

[0056] 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.

[0057] 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 clearlyimplied 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.

[0058] Furthermore, the following terms are used throughout the description given below:

[0059] • 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.

[0060] • 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.

[0061] • 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”.

[0062] • Radio Node: As used herein, a “radio node” can be either a “radio access node” (or equivalent term) or a “wireless device.”

[0063] • 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.

[0064] • 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.

[0065] 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.

[0066] 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.

[0067] Figure 2 shows an exemplary configuration of NR user plane (UP) and control plane (CP) protocol stacks between a UE (210), a gNB (220), and an AMF (230). 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.

[0068] 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 providestransport channel services to MAC and handles transfer over the NR radio interface, e.g., via modulation, coding, antenna mapping, and beam forming.

[0069] 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.

[0070] After a UE is powered ON it will be in the RRC IDLE state until an RRC connection is established with the network, at which time the UE will transition to RRC CONNECTED state (e.g., where data transfer can occur). The UE returns to RRC IDLE after the connection with the network is released. In RRC IDLE state, the UE’s radio is active on a discontinuous reception (DRX) schedule configured by upper layers. During DRX active periods (also referred to as “DRX On durations”), an RRC IDLE UE receives SI broadcast in the cell where the UE is camping, performs measurements of neighbor cells to support cell reselection, and monitors a paging channel on PDCCH for pages from 5GC via gNB. An NR UE in RRC IDLE state is not known to the gNB serving the cell where the UE is camping. However, NR RRC includes an RRC_INACTIVE state in which a UE is known (e.g., via UE context) by the serving gNB. RRC INACTIVE has some properties similar to a “suspended” condition used in LTE.

[0071] 3 GPP Rel-10 introduced support for channel bandwidths larger than 20 MHz in Fourthgeneration (4G) Long-Term Evolution (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.”

[0072] LTE Rel-12 introduced dual connectivity (DC) whereby a UE can be connected to two network nodes simultaneously, thereby improving connection robustness and / or capacity. 5G / NR also supports various DC (or more generally, multi-connectivity) configurations for UEs. 3 GPP TR 38.804 (vl4.0.0) describes various exemplary DC scenarios or configurations in which the MN and SN can apply NR, LTE, or both. In particular, a UE is configured with a Master Cell Group (MCG) provided by a master node (MN) and a Secondary Cell Group (SCG) provided by a secondary node (SN). Each cell group includes one MAC entity, a set of logical channels withassociated RLC entities, a primary cell (i.e., PCell for MCG, PSCell for SCG), and optionally one or more SCells.

[0073] While in RRC CONNECTED state, the UE maintains a timing advance (TA) value for its UL transmissions to the serving gNB. The TA value indicates how much earlier in time the UE should initiate its UL transmissions to compensate for the propagation delay between the UE and the gNB antenna / receiver. The UEs in a cell apply individualized TAs to ensure that their respective UL transmissions arrive to the gNB receiver in the appropriate (e.g., scheduled) time window.

[0074] Figure 3 shows an exemplary procedure for a UE (310) to obtain TA for a serving cell provided by a RAN node (320, e.g., gNB). As a pre-condition, the UE is in RRC_CONNECTED state with the RAN node. The UE obtains an initial TA value when it performs random access (RA) to a cell. Specifically, in operation 1, the UE transmits a RA preamble (also called “Msgl”) in the serving cell. In operation 2, the RAN node determines the TA value for the UE based on the received RA preamble. In operation 3, the RAN node provides the TA value to the UE in a RA response (RAR, also called “msg2”). While in RRC CONNECTED state in the cell, the UE maintains and updates TA based on continuous monitoring of the alignment of the DL signal from the RAN node with the expected arrival window. Also, if the RAN node notices the UE’s UL transmissions falling out of synchronization, it can send the UE a PDCCH order that causes the UE to send another RA preamble, based on which the RAN node can determine and provide a new TA value.

[0075] In general, the UE has no TA value while in RRC IDLE and RRC_ INACTIVE states since it does not perform UL transmissions in these states. There is one exception for RRC INACTIVE state, specifically when the UE has been assigned a configured grant (CG) for small data transmission (SDT) when moving from RRC CONNECTED state to RRC INACTIVE state. This CG enables the UE to immediately transmit any UL data that has arrived at its buffer while in RRC INACTIVE state. This transmission requires TA alignment and since the UE does not continuously adjust TA while in RRC INACTIVE, another technique was introduced for CG-SDT in 3 GPP Rel-17 and Rel-18.

[0076] Initially, the UE is in RRC CONNECTED state with a RAN node and has maintained TA in its serving cell. The RAN node determines to send the UE to RRC INACTIVE and sends an RRCRelease message that includes a CG for SDT, i.e., an sdt-Config information element (IE). The CG may include a plurality of periodic grants of UL resources, which the UE uses as needed for arrival of UL data. Upon entering RRC INACTIVE state, the UE stops TA maintenance and measures DL reference signal received power (RSRP), which it stores for later use. When UL data arrives prior to one of periodic grants, the UE again measures DL RSRP and compares against thestored DL RSRP. If the difference is less than a threshold, the UE assumes that the most recent TA value is still valid and uses it for transmitting the UL data in the granted resources.

[0077] The UE’s various serving cells (e.g., PCell, PSCell, SCells) may be divided into one or more timing alignment groups (TAGs). The UE also uses various timers during TA maintenance. According to 3GPP TS 38.321 (v) section 5.2, the gNB configures (i.e., via RRC) the following in the UE for maintenance of UL time alignment:

[0078] • timeAlignmentTimer (per TAG), which controls how long the UE MAC entity considers the serving cells of a TAG to be UL time aligned;

[0079] • inactivePosSRS-TimeAlignmentTimer, which controls how long the UE MAC entity considers the Positioning SRS transmission in RRC INACTIVE (3GPP TS 38.321 section 5.26) to be UL time aligned;

[0080] • cg-SDT-TimeAlignmentTimer, which controls how long the UE MAC entity considers the UL transmission for CG-SDT to be uplink time aligned; and

[0081] • inactivePosSRS-ValidityAreaTAT which controls how long the UE MAC entity considers Positioning SRS transmission in RRC INACTIVE (3GPP TS 38.321 section 5.26) to be UL time aligned when SRS positioning validity area is configured.

[0082] According to 3GPP TS 38.321 section 5.2, when a UE receives a Timing Advance Command in a RAR message for a serving cell configured with two TAGs, and if the RA preamble was not selected by the UE’s MAC entity among the contention-based Random Access Preamble, the UE shall apply the Timing Advance Command for the TAG indicated in the received RAR message. The UE shall also start or restart the timeAlignmentTimer associated with TAG indicated in the received RAR message.

[0083] 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).

[0084] 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 L3 and the RRC messages exchanged are part of L3.These reconfigurations are prepared in advance by a target RAN node serving the target cell, upon a request from the UE’s serving RAN node. This request is transmitted over the Xn interface in case the serving and target RAN nodes are part of the NG-RAN. The reconfiguration in the handover command is based on the UE’s existing RRC configuration in its current serving cell (also referred to as “source cell”), which are provided in the inter-node request. In some cases, the reconfiguration can be provided as a “delta” to the UE’s existing configuration in the source cell, which reduces the size of the handover command.

[0085] 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).

[0086] In general, UE mobility in RRC_CONNECTED state is network-based since the network has the most information about conditions such as cell loading (UEs and / or traffic), available node resources (e.g., processing), available frequencies, etc. Seamless handovers ensure that the UE moves around in the coverage area of different cells without excessive interruption to data transmission. However, there will be scenarios when the network fails to handover the UE to the “correct” neighbor cell in time, which can cause the UE will declare radio link failure (RLF) or handover failure (HOF).

[0087] To address various difficulties with handovers and other mobility procedures, 3 GPP Rel-16 includes support for conditional handover (CHO) and SN-initiated intra-SN conditional PSCell change (CPC) procedures while Rel-17 includes support for various other conditional mobility procedures. A main principle of conditional mobility is separation of transmission and execution of a mobility (e.g., handover) command. This allows the mobility command to be sent earlier to UE when the radio conditions are still good, thus increasing the likelihood that it is successfully transferred. The execution of the mobility command is done at later point in time based on an associated execution condition.

[0088] Conditional mobility procedures are based on a conditional reconfiguration framework in which the network provides a UE with one or more reconfigurations, each with associated execution condition(s). Each reconfiguration can be provided in an RRCReconfiguration message (in NR) or an RRCConnectionReconfiguration message (in LTE). When the UE later detects the execution condition(s) associated with one of the earlier-received reconfigurations, the UE executes the associated reconfiguration to perform the mobility procedure (e.g., HO, PSCell change, PSCell addition, etc.).Even so, conditional (e.g., CHO) and non-conditional (e.g., HO) are triggered by layer 3 (L3) measurements and involves radio resource control (RRC) signaling to change PCell and PSCell (e.g., when DC is configured), as well as release / add SCells as needed. Moreover, conventional inter-cell mobility involves complete layer 2 (L2) and layer 1 (LI, i.e., PHY) resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching.

[0089] 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 measurements on configured LTM candidate cells and transmits corresponding measurement reports to the RAN node, based on which the RAN node triggers execution of a LTM cell switch procedure by the UE to one of the configured LTM candidate cells. The RAN node may trigger the LTM cell switch procedure by sending the UE an LTM cell switch command.

[0090] In LTM, a UE is pre-configured by its serving RAN with one RRC configuration per LTM candidate cell, sometimes referred to as an “LTM candidate cell configuration”. This configuration may be an RRCReconfiguration message or a portion thereof, such as one or more lEs / fields / parameters (e.g., CellGroupConfig IE). The UE performs measurements on configured LTM candidate cells and transmits corresponding measurement reports to the RAN, which triggers the execution of a LTM cell switch procedure by the UE to one of the configured LTM candidate cells. This triggering is done by transmitting an LTM cell switch command to the UE in lower layer signaling (e.g., DCI or MAC CE). Based on this command, the UE connects to the associated LTM candidate cell and uses the previously received RRC configuration for this cell.

[0091] Regarding terminology, a candidate cell configured for a UE mobility procedure (e.g., LTM or L3) becomes a target cell when the UE performs the mobility procedure, either due to a command from the UE’s current serving RAN node or due to execution conditions being met at the UE. As such, in the context of conditions. As such, the terms “candidate,” “target,” and “candidate (target)” may be used interchangeably when referring to that cell or to the RAN node serving that cell. Likewise, the UE’s serving cell becomes when the UE performs the mobility procedure, and so the terms “source,” “serving,” and “serving (source)” may be used interchangeably when referring to that cell or to the RAN node serving that cell.

[0092] Figure 5 shows a signaling diagram for an exemplary LTM cell switch procedure. Although the operations are shown with numerical labels, this is done to facilitate explanation rather than to require or imply any specific operational order, unless expressly stated otherwise.In operation 1, the UE (410) sends MeasurementReport message to the gNB (420). Based on this message, the gNB decides to configure LTM for the UE and initiates preparation of one or more LTM candidate cells. In operation 2, the gNB sends an RRCReconfiguration message to the UE including LTM candidate cell configurations of one or more candidate cells. In operation 3, the UE stores the received LTM candidate cell configurations and transmits an RRCReconfigurationComplete message to the gNB.

[0093] Since a goal of LTM is to reduce interruption time for UE data transmissions, the UE needs to be ready to communicate with an LTM candidate cell upon (or shortly after) receiving the L1 / L2 signaling for mobility execution from the source cell. For example, the UE must be able to transmit UL data or a scheduling request (SR) to the LTM candidate cell and / or monitor a DL control channel (e.g., PDCCH) from the LTM candidate cell. In other words, UE needs to know the cell that it is moving to so it can apply the corresponding configuration, including the correct timing alignment and / or TCI state for the cell. Likewise, in the case of inter-DU LTM, when the source DU transmits the L1 / L2 signaling for mobility execution, the target DU needs to be prepared for scheduling UL and DL transmissions for the UE in the target cell, and for receiving SR from the UE.

[0094] The UE performs operations 4a-b before receiving any LTM cell switch command. In operation 4a, the UE performs early DL synchronization with the configured LTM candidate cells. Each LTM candidate cell configuration may include a configuration for early DL synchronization, which may also be referred to as an “LTM candidate TCI state configuration” or more simply as a “TCI state configuration.” For example, the TCI state configuration may include an RRC CandidateTCI-State IE and / or an RRC CandidateTCI-UL-State IE. Once configured in this manner, early TCI state activation in an LTM candidate cell may be triggered by a MAC CE from the serving RAN node.

[0095] In operation 4b, when UE-based TA measurement is configured, UE acquires the TA value(s) of the candidate cell(s) by measurement. Otherwise, in operation 4b, the UE performs early TA acquisition with the candidate cell(s) as requested by the network. This is done via contention-free random access (CFRA) triggered by a physical DL control channel (PDCCH) order from the source cell, following which the UE sends a RA preamble towards the indicated LTM candidate cell. In order to minimize the data interruption of the source cell due to CFRA towards the LTM candidate cell(s), the UE doesn’t receive RA response (with TA) from the LTM candidate cell; instead, a TA value for the LTM candidate cell is indicated in a subsequent LTM cell switch command (e.g., operation 6 below). Similarly, the UE doesn’t maintain a TA timer for the LTM candidate cell but relies on the RAN to guarantee the TA validity.In operation 5, the UE performs LI measurements on the configured LTM candidate cells and transmits LI measurement reports to the gNB. The UE performs such LI measurement as long as the LTM candidate cell configurations received in operation 2 remain applicable.

[0096] In operation 6, the gNB decides to trigger an LTM cell switch for the UE to one of the configured LTM candidate cells ( “target cell”) and transmits an LTM cell switch command, which is a MAC CE that includes an identifier (e.g., index) of the corresponding LTM candidate cell configurations provided to the UE in operation 2. The MAC CE may also include an identifier of a beam (e.g., a TCI State ID) by which the UE should access the target cell, as well as a TA value for the UE to use when transmitting to the target cell. The gNB selects the identified beam based on the LI measurements reported by the UE.

[0097] Upon receiving the LTM cell switch command, the UE monitors PDCCH on the indicated beam of the target cell. In other words, the UE considers the TCI state for the indicated beam / TCI state ID to be “activated” when performing the LTM cell switch. The UE also applies the configuration identified in the MAC CE.

[0098] In operation 7, if UE did not receive a TA value for the target cell, the UE performs a RA procedure towards the target cell. The UE performs CFRA if the LTM cell switch command contains the necessary information, as specified in clause 6.1.3.xy of 3GPP TS 38.321 (v17.7.0). In operation 8, the UE completes the LTM cell switch procedure by transmitting an RRCReconfigurationComplete message to the gNB via the target cell, using the TA value obtained in operation 6 or 7. If the UE performed a RA procedure in operation 7, the UE considers LTM cell switch execution successfully completed when the RA procedure is successfully completed. For RACH-less LTM, the UE considers LTM cell switch execution successfully completed when the UE determines that the gNB has successfully received its first UL data. The UE determines successful reception of its first UL data by receiving a PDCCH addressing the UE’s C-RNTI in the target cell, which schedules a new transmission following the first UL data. The PDCCH carries either a DL assignment or an UL grant addressing the same HARQ process as the first UL data.

[0099] The split CU / DU architecture shown in Figure 1 also supports LTM, including for intra-DU and inter-DU / intra-CU cell changes. In the inter-DU / intra-CU scenario, the candidate cell for LTM is a cell served by a neighbor DU to the source DU that currently provides the UE’s PCell (or PSCell, for SCG change in DC). In the intra-DU scenario, the candidate cell for LTM is a cell served by the same DU that currently provides the UE’s PCell (or PSCell, for SCG change in DC). Since the procedure shown in Figure 4 involves a single gNB, it can also be considered an intra-CU LTM cell switch. In contrast, an inter-CU (or inter-gNB) LTMprocedure involves a cell switch from a source cell served by a first CU / gNB to a candidate (target) cell served by a second CU / gNB.

[0100] As discussed above, in Rel-18 (non-conditional) LTM, the UE’s serving RAN node sends the UE a command for LTM cell switch to one of the UE’s configured LTM candidate cells, and the command may include a TA value determined based on the UE’s earlier RA transmission towards the LTM candidate cell. In case of intra-DU / intra-gNB LTM, the RAN node serving the LTM candidate cell may determine the TA value for the UE and provide it to the UE’s current serving RAN node.

[0101] Since there is no LTM cell switch command in Rel-19 CLTM, however, the serving RAN node is unable to provide a TA value for an LTM candidate cell at the time of the LTM cell switch. One solution is to provide the TA value to the UE in an earlier message.

[0102] According to some current agreements within 3 GPP RAN 2 working group, PDCCH-ordered early TA acquisition will be supported for CLTM. In other words, the UE’s current serving RAN node will send the UE a PDCCH order to transmit a RA preamble towards the UE’s CLTM candidate cell. If the CLTM candidate cell is provided by another RAN node, the serving RAN node receives the TA value determined based on receiving the RA preamble. In any case, the serving RAN node sends the TA value for the CLTM candidate cell to the UE.

[0103] Even so, there may be some time between when the UE receives this TA value and when the UE is ready to use it based on the LTM conditions for that candidate cell being fulfilled. During this time, the received TA value may become inaccurate (e.g., due to UE movement) such that the UE is not actually UL synchronized with the CLTM candidate cell upon LTM execution. If the UE transmits an initial message in the LTM candidate (now target) cell using an inaccurate TA, the target RAN node may not receive this message correctly, causing it to trigger a RA procedure by the UE to re-acquire UL synchronization with the target cell. This is undesirable since it increases LTM cell switch latency and interruptions, which is contrary to the main goals of LTM.

[0104] To address these issues, the UE may use a timer similar to the current timeAlignmentTimer used for the UE’s serving cell. The UE initiates the timer when it receives the TA value for the CLTM candidate cell. If the timer is still running at the time of the LTM cell switch, the UE assumes that the previous TA value is still valid and may be used for the initial transmission in the CLTM candidate cell. If the timer has expired at the time of LTM cell switch, the UE assumes that the previous TA value is no longer valid and may not be used for the initial transmission. In such case, the UE must perform RA to the target cell to obtain a new TA value used for its initial transmission.

[0105] Another possible solution is the network (e.g., serving RAN node) determining whether the TA value previously provided to the UE is still valid. This may be based on a timer similar tothe UE timer discussed above, or on other techniques. When the TA value is no longer valid, the network may send another PDCCH order to the UE, based on which the UE transmits a RA preamble and receives a new TA value accordingly.

[0106] Even so, both of these solutions have drawbacks. For example, whether the timer is still running may not accurately represent validity of the TA value previously provided to the UE. If the UE position significantly changes while the timer is running, the previous TA value may become invalid well before the timer expires. Likewise, if the UE is relatively stationary while the timer is running, the previous TA value may remain valid well after the timer has expired.

[0107] Accordingly, embodiments of the present disclosure address these and related problems and / or issues by various techniques by which the validity of a previously-obtained TA value for an LTM candidate cell can be extended if the UE is deemed to be relatively stationary, and / or the previous TA value can be updated (with extended validity) based on relative time difference (RTD) measurements of the LTM candidate cell. In different embodiments, updating the TA value and extending its TA validity can be controlled by the RAN or performed by the UE autonomously.

[0108] The RAN-controlled techniques may be based on UE reports of RTD (or other) measurements that enable the RAN to reinitiate TA acquisition (e.g., triggering RACH), determine an updated TA value, and / or extend TA validity time. In some embodiments, the RAN then sends the UE an updated TA value and an indication of a validity duration for this updated TA value. In other embodiments, the RAN sends the UE a command (e.g., PDCCH order) to perform RA preamble transmission towards a CLTM candidate cell.

[0109] The UE-autonomous techniques may involve the UE updating expired / stale TA values and restarting an associated validity timer. In some embodiments, both the UE and the RAN may keep track of the validity of a TA value for a CLTM candidate cell.

[0110] In some embodiments, the RAN may configure the UE with RTD measurements for an LTM candidate cell, based on which the UE performs RTD measurements and reports these measurements to the RAN, possibly together with other relevant information. This reported information enables the RAN to compute TA values without requiring the UE to perform RA preamble transmission toward the LTM candidate cell.

[0111] In some embodiments, the serving RAN node may send the UE a TA value and a TA validity duration associated with an LTM candidate cell. The UE uses these to start / restart / maintain a validity timer, such as discussed above. In some variants, the UE may also start / restart / maintain a validity timer based on UE RTD measurements of the LTM candidate cell and / or a determination that the UE is stationary.In some embodiments, the UE, the RAN, or both may track changes in UE RTD measurements of the LTM candidate cell over time. In some embodiments, the RAN may provide Embodiments of the present disclosure may provide various advantages and / or benefits. For example, by enabling a UE to maintain an LTM candidate cell TA value received from its serving RAN node, embodiments may facilitate timely UE conditional LTM cell switch while reducing TA-related signaling with the serving RAN node. As another example, by using a time alignment timer and RTD measurements as a basis for maintaining LTM candidate cell TA values, embodiments may enable UEs and / or RAN nodes to easily determine UE TA validity for an LTM candidate cell in a timely and efficient manner. At a high level, embodiments may facilitate conditional LTM cell switch without connection interruption, excess signaling overhead, and excess UE energy consumption.

[0112] 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.

[0113] 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.

[0114] 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.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).

[0115] 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.

[0116] 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”.

[0117] A UE may receive an LTM candidate cell configuration in 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.

[0118] 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.

[0119] 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):

[0120] • 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

[0121] o higher layer information, such as L3 / RRC parameter and / or timer values, PDCP configuration, radio bearer configuration, L3 / RRC measurement configuration, etc.;

[0122] • a measurement configuration, e.g., measurements for LTM, measurement reports for LTM, CSI resource configuration for LTM, CSI report configuration for LTM, etc.; • a configuration for early DL synchronization, e.g., for early TCI state activation;

[0123] • a configuration for early UL synchronization, e.g., for transmission of PDCCH ordered preamble transmission and reception of timing advance (TA);

[0124] • a configuration for execution of an LTM cell switch procedure according to a given LTM candidate cell configuration (e.g., whether to perform RA, RLC reestablishment, MAC reset, PDCP recovery, etc.).;

[0125] • additional information needed for an intra-CU / gNB LTM cell switch procedure.

[0126] 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).

[0127] The term “conditional LTM configuration” may refer to an LTM configuration that also includes one or more execution conditions whose fulfillment triggers execution of an LTM cell switch. The CLTM execution condition(s) may also include identified s), offset(s), threshold(s), reference signal (RS) type, measurement quantity (e.g., RSRP, RSRQ, SINR), time-to-trigger (TTT), etc. Evaluation of the CLTM execution condition(s) may be based on lower layer measurements, e.g., Layer 1 reference signal received power (Ll-RSRP) and / or SS-RSRP measurements of SSB and / or CSLRS transmitted by the UE’s source cell and / or the LTM candidate cell. Such lower layer measurements may also be used in lower layer procedures like beam management, candidate cell TCI state activation / deactivation, early timing advance (TA) acquisition, and link adaptation. In contrast to Layer 3 (L3) measurements, lower layer measurements are not filtered based on Layer 3 (L3) parameters, although there may be some filtering of these measurements based on the other lower layer parameters.

[0128] 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 MCGor PSCell for LTM in SCG) or its new SCell. In other words, an LTM candidate cell can be a candidate for the UE’s PCell, PSCell, or SCell.

[0129] 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. More generally, embodiments are not limited to cells but are applicable to any UE switch from a first (or source) set of radio resources to a second (or target) set of radio resources.

[0130] In some cases, multiple LTM candidate cells may have similar UL synchronization characteristics, e.g., based on belonging to the same UL synchronization group that is assigned a particular identifier (ID). In other words, each LTM candidate cell may be assigned a group ID, and a UE may determine that multiple LTM candidate cells may have similar UL synchronization characteristics based on the cells all being assigned the same group ID. Moreover, these LTM candidate cells may have an identical TA value and may use the same validity timer. This relationship may be analogous to as Timing Advance Group (TAG) of the UE’s serving cells, i.e., serving cells that have the same timing reference cell and the same TA value. This may occur when multiple cells are very close to each other with a relatively small coverage or when multiple cells overlap in coverage even if deployed on different frequencies.

[0131] Embodiments will now be described in more detail. In the following description, “relative time difference” (or RTD) refers to a UE measurement of the DL timing of a first cell (e.g., LTM candidate cell) relative to the DL timing of a second cell (e.g., serving / source cell). “DL timing” may refer to UE reception timing of certain timing events on the DL radio interface of the cell measured. Assuming each of the cells is served by a corresponding TRP, RTD may also be viewed as a UE measurement of DL timing of a first TRP (e.g., that provides the LTM candidate cell) relative to the DL timing of a second TRP (e.g., that provides the serving / source cell).

[0132] Furthermore, the following mathematical notation will be used in the description of various embodiments:

[0133] • Range from UE to TRPi: rt(range of the shortest path, not necessarily line-of-sight). • RTD between T

[0134]

[0135] RPi and TRPj:

[0136] • DL time of arrival measured by the UE, for TRPi: T;

[0137] • Clock error at TRPi: <5;

[0138] • Clock error at UE: y

[0139] • TA with respect to TRPi: Tt

[0140] • Speed of light: cTA corresponds to the round-trip delay according to:

[0141] T

[0142]

[0143] i = i = U (1) Relative time difference is the difference in time of arrival of DLs from the two TRPs, i.e.:

[0144] R

[0145]

[0146] ij = Ti - Tj, i, (2) where time of arrival depends on the propagation delay and clock-errors at the TRP and the UE,

[0147] E = ^ + ^ + y. (3) Combing equations (2) and (3) results in the following relation:

[0148] Rij= -r^ + S. - Sj, (4)

[0149]

[0150] Assigning i=l and j=2 and rewriting equation (4) results in the following relation:

[0151] 2 / ?i2 = - T2+ 2^ - 262^ T2= T + 26 - 262- 2R12. (5) If TRP 1 and TRP2 are assumed to be synchronized, then = 62and T2= Ti — 2R12

[0152]

[0153] (6) In other cases, TRP1 and TRP2 are not synchronized but their respective clock errors S1:62are stable overtime. In other words, if

[0154]

[0155] t(2) are two time-instances,

[0156]

[0157] < t(2), then<51(t(1)) = <5i(t(2)) and 62(

[0158]

[0159] t<i)) = 82(t(2)\ In such case, differentials of various above values at these two time instances may be written as follows:

[0160] ARij = / ?jy(t(-2))—

[0161] A?1 = 71(t(2)) —

[0162] AT2= T2(t^2y) — T2(t(i)),

[0163] A<5i = <5i(t(2))—£i(t(i)) = 0,

[0164]

[0165] A<52= <^2(^(2))—^2(^(1))=0

[0166] Similarly, the differential of equation (5) may be written as:

[0167] AT2= ATj + 2A<5! - 2A<52- 2A / ?12= A^ - 2A / ?12(7) In the context of LTM and early UL synchronization, assume TRP1 provides a UE’s serving cell and TRP2 provides the UE’s LTM Candidate. The UE maintains UL synchronization with the serving cell, such that 7 is known by the UE. If the UE also measures the RTD between the serving cell and the LTM candidate cell, then the UE can compute the UL synchronization factor T2autonomously using equation (5) without RA preamble transmission to the LTM candidate cell.

[0168] If TRP1 and TRP2 are assumed to be unsynchronized but have stable clock errors, then the UE may use equation (7) instead. The UE maintains UL synchronization with the serving cell such that AT s known by the UE. Similarly, the UE can measure RTD for the LTM candidate cell at various time instances, such that A / ?12between any two time is known to the UE. In such case,the UE needs to acquire UL synchronization with the LTM candidate cell only once at tl, so the UE is aware of E2(t(i)). Based on these known values, the UE may compute T2(t(2)) using (7).

[0169] If the UE is assumed to be stationary and TRP1 and TRP2 are assumed to be unsynchronized but have stable clock errors, then

[0170]

[0171] = u(t(2)) and r2(t^y) = r2(t(2)), which gives the following result:

[0172] A / ?12= A^ - A<52= 0. (8) Thus, for a stationary UE and stable TRP clock errors, the UE only needs to measure T2and R12once in order to maintain UL synchronization with the LTM candidate cell.

[0173] Some embodiments may include methods performed by a UE configured with one or more LTM candidate cells. The UE receives, from a RAN node that provides the UE’s serving cell, respective TA values for the one or more LTM candidate cells and validity durations for the respective TA values (e.g., one validity duration for all TA values, a validity duration of each TA value, etc.). The UE also receives from the serving RAN node one or more RTD measurement configurations. Each RTD measurement configuration may be associated with a particular LTM candidate cell, a single RTD measurement configuration may be associated with all of the configured LTM candidate cells, etc.

[0174] The UE initiates one or more validity timers (e.g., a time alignment timer) corresponding to the validity durations for the respective TA values. While the one or more validity timers are running, the UE maintains them based on various information, including updated TA values for the LTM candidate cells (which may be based on RTD measurements by the UE), determinations of whether the UE is stationary or moving, synchronization assistance information from the serving RAN node, etc.

[0175] In some variants, each received TA value is associated with one or more LTM candidate cells and with one of the received validity durations. In other variants, each received TA value is associated with one or more LTM candidate cells and with multiple received validity durations. For example, each received validity duration may be applicable under specific conditions, e.g., a first validity duration applicable when the UE is moving and a second validity duration applicable when the UE is stationary.

[0176] In an alternative solution, rather than providing the validity durations with the associated TA values (e.g., in the same message), the validity durations may be preconfigured in the UE before the UE receives any of the TA values.

[0177] In some embodiments, each RTD measurement configuration may include an RTD resource configuration and an RTD measurement report configuration. In such case, the UE sends the RAN node RTD measurements of its configured LTM candidate cells in accordance with the associated RTD measurement report configuration. The RTD measurement report configurationmay indicate aperiodic, event-triggered, semi-persistent, or periodic RTD measurement reporting for the associated LTM candidate cell(s). For example, an aperiodic RTD measurement report configuration may indicate a trigger from the UE’s serving cell that causes the UE to send an RTD measurement report to the serving RAN node (e.g., using LI or L2 signaling).

[0178] As another example, a semi-persistent RTD measurement report configuration may indicate ON / OFF activation commands from the UE’s serving cell that cause the UE to start / stop sending RTD measurement reports to the serving RAN node. As another example, an event-triggered RTD measurement report configuration may indicate event-related parameters (e.g., offset, threshold, hysteresis, etc.) based on which the UE monitors for fulfillment of a condition that causes the UE to send an RTD measurement report to the serving RAN node. As a more specific example, the condition may be a certain change in an RTD measurement for an LTM candidate cell, e.g., an increase of at least some threshold amount.

[0179] In some embodiments, the RTD measurement configuration may be received in RRC or L2 (e.g., MAC) signaling. In some embodiments, the RTD resource configuration may indicate a reference cell for RTD measurements. For example, the reference cell may be one of the UE’s serving cells or one of the UE’s LTM candidate cells. In some embodiments, the RTD resource configuration may indicate a particular cell timing event on which the RTD measurement is based (e.g., frame boundary, timeslot boundary, symbol, etc.). Alternately, the cell timing event on which the RTD measurement is based may be implicit and / or specified.

[0180] In some embodiments, if the UE previously received a TA value for an LTM candidate cell and has a running validity timer for that cell, and the UE receives a new TA value for that LTM candidate cell, then the UE reinitializes the validity timer to the applicable validity duration. In the case the UE has received multiple validity durations that are associated with different conditions (e.g., UE moving, UE stationary), the serving RAN node may indicate which of the multiple validity durations is currently applicable. Alternately, the UE may determine which of the multiple validity durations is currently applicable. As yet another alternative, the new TA value may be received with an applicable validity duration. In any case, the UE uses the applicable validity duration to initialize the validity timer.

[0181] In some embodiments, the UE may receive from synchronization assistance information from the serving RAN node. This information may indicate a degree of synchronization between the serving RAN node (or a serving cell provided by the RAN node) and a second RAN node that provides one or more of the configured LTM candidate cells for the UE. In different variants, the degree of synchronization may indicate one or more of the following:

[0182] • a maximum synchronization error between the serving RAN node and the second RAN node;• maximum synchronization errors of each of the serving and second RAN nodes with respect to a global time reference (e.g., universal time coordinate, UTC);

[0183] • maximum clock drift of each of the serving and second RAN nodes.

[0184] • which configured LTM candidate cells (or RAN nodes that provide such cells) have the same maximum synchronization error as the serving cell;

[0185] • whether degree of synchronization between serving RAN node and second RAN node is sufficient to maintain LTM candidate cell UL synchronization based on RTD;

[0186] • whether degree of synchronization of serving RAN node and second RAN node to a global time reference is sufficient to maintain LTM candidate cell UL synchronization based on RTD; and

[0187] • which of the configured LTM candidate cells the UE may maintain UL synchronization based on RTD.

[0188] In some embodiments, after the UE has received a TA value and an associated validity duration for a particular LTM candidate cell, and has initiated a validity timer based on the validity duration, the UE may extend the validity timer (e.g., past the validity duration) or reset the validity timer (i.e., to the validity duration) based on a determination that the UE is stationary. To determine the stationary condition, the UE may use RTD measurements or positioning measurements (e.g., GNSS measurements). Alternately, the UE may receive an indication that the UE is stationary from the serving RAN node, which may be based on network-based positioning measurements made by the serving RAN node and possibly other RAN nodes.

[0189] For example, the UE may pause the validity timer for the duration that the UE is determined to be stationary. Alternately, how long the TA validity duration is extended may be pre-configured or configured by the serving RAN node. In case the UE has received multiple TA validity durations associated with the same LTM candidate cell, then the UE may apply one of these that is most consistent with the UE’s current conditions (e.g., moving or stationary).

[0190] In some embodiments, after the UE has received a TA value and an associated validity duration for a particular LTM candidate cell, and has initiated a validity timer based on the validity duration, the UE may autonomously updates the TA value based on RTD measurements it performs for the LTM candidate cell. In such case, UE may extend the validity timer (e.g., past the validity duration) or reset the validity timer (i.e., to the validity duration) based on updating the TA value using the RTD measurements. Alternately, if the UE has received synchronization assistance information (discussed above), the UE may use this information when computing the new TA value and extending its validity duration.

[0191] As mentioned above, the RTD measurement reporting configuration received by the UE may indicate for the UE to report RTD measurements and / or RTD change indications to theserving RAN node. Below are some illustrative examples of what the UE may be configured to report:

[0192] • complete RTD measurements;

[0193] • an indication of whether the measured RTD has increased, decreased, or has not changed;

[0194] • an indication that the measured RTD has changed more than a threshold, which may also be configured in the RTD measurement reporting configuration;.

[0195] In some variants, the UE reports this information using layer-1 uplink control information (UCI). In other variants, the UE reports this information using a MAC CE (newly defined or existing).

[0196] In some embodiments, the UE may send a message to the serving RAN node about a change in RTD measured by the UE for one or more configured LTM candidate cells. For example, the UE may send the message as layer- 1 UCI or as part of a MAC CE (newly defined or existing). As a more specific example, the message may include the following three-bit bitfield:

[0197] • bit l: presence of RTD indications

[0198] • bit 2: RTD is the same / TA value has changed according to estimations

[0199] • bit 3: RTD has increased / decreased

[0200] Other embodiments include methods performed by a RAN node that provides a serving cell for a UE and has configured the UE with one or more LTM candidate cells. The RAN node sends the UE respective TA values for the one or more LTM candidate cells and validity durations for the respective TA values (e.g., one validity duration for all TA values, a validity duration of each TA value, etc.). The RAN node also sends the UE one or more RTD measurement configurations. Each RTD measurement configuration may be associated with a particular LTM candidate cell, a single RTD measurement configuration may be associated with all of the configured LTM candidate cells, etc.

[0201] In some variants, each TA value sent to the UE is associated with one or more LTM candidate cells and with one of the validity durations. In other variants, each TA value is associated with one or more LTM candidate cells and with multiple received validity durations. For example, each validity duration may be applicable under specific conditions, e.g., a first validity duration applicable when the UE is moving and a second validity duration applicable when the UE is stationary. If the RAN node provided multiple validity durations that are associated with different conditions (e.g., UE moving, UE stationary), the RAN node may indicate which of the multiple validity durations is currently applicable.

[0202] In an alternative solution, rather than including the validity durations with the associated TA values (e.g., in the same message), the validity durations may be preconfigured in the UE before the RAN node sends any of the TA values.In some embodiments, each RTD measurement configuration may include an RTD resource configuration and an RTD measurement report configuration. In such case, the RAN node receives from the UE RTD measurements of its configured LTM candidate cells in accordance with the associated RTD measurement report configuration. The RTD measurement report configuration may indicate aperiodic, event-triggered, semi-persistent, or periodic RTD measurement reporting for the associated LTM candidate cell(s), such as the various examples described above.

[0203] In some embodiments, the RTD measurement configuration may be sent in RRC or L2 (e.g., MAC) signaling. In some embodiments, the RTD resource configuration may indicate a reference cell for RTD measurements. For example, the reference cell may be one of the UE’s serving cells or one of the UE’s LTM candidate cells. In some embodiments, the RTD resource configuration may indicate a particular cell timing event on which the RTD measurement is based (e.g., frame boundary, timeslot boundary, symbol, etc.). Alternately, the cell timing event on which the RTD measurement is based may be implicit and / or specified.

[0204] As mentioned above, the RTD measurement reporting configuration sent to the UE may indicate for the UE to report RTD measurements and / or RTD change indications to the serving RAN node. Below are some illustrative examples of what the UE may be configured to report:

[0205] • complete RTD measurements;

[0206] • an indication of whether the measured RTD has increased, decreased, or has not changed;

[0207] • an indication that the measured RTD has changed more than a threshold, which may also be configured in the RTD measurement reporting configuration;.

[0208] In some variants, the RAN node receives this information from the UE via layer-1 UCI. In other variants, the RAN node receives this information from the UE via MAC CE (newly defined or existing).

[0209] In some embodiments, based on the RTD measurements and / or change indication(s) received from the UE in accordance with the RTD measurement reporting configuration, the RAN node may determine a new TA value for the UE in one or more configured LTM candidate cells. For example, the serving RAN node may determine a new TA value for the UE based on the TA value previously sent to the UE and UE-reported RTD measurements for the LTM candidate cell. The RAN node may provide this new TA value to the UE in a similar manner as described above, possibly together with an associated validity duration.

[0210] In some embodiments, based on the RTD measurements and / or change indication(s) received from the UE, the RAN node may determine that the UE’s current TA value (i.e., that the RAN node previously sent to the UE) for one or more LTM candidate cells is still valid and / or that the UE is stationary. In such case, the RAN node may send an indication of extended validityto the UE, such as an indication to pause a running validity timer, a new validity duration to use for the TA value, or an indication to use a previously provided validity duration.

[0211] In some embodiments, based on the RTD measurements and / or change indication(s) received from the UE in accordance with the RTD measurement reporting configuration, the RAN node may determine that the UE’s current TA value for a configured LTM candidate cell is no longer valid, e.g., has changed more than a threshold amount. Based on this determination, the RAN node sends the UE a PDCCH order requesting the UE to transmit a RA preamble towards the LTM candidate cell. Subsequently, the RAN node obtains a new TA value from a second RAN node serving the LTM candidate cell, and provides this new TA value to the UE in a similar manner as described above, possibly together with an associated validity duration.

[0212] In some embodiments, the serving RAN node may send the UE synchronization assistance information, which may indicate a degree of synchronization between the serving RAN node (or a serving cell provided by the RAN node) and a second RAN node that provides one or more of the configured LTM candidate cells for the UE. In different variants, the degree of synchronization may indicate any of the information listed above in the description of UE embodiments.

[0213] In some embodiments, the serving RAN node may determine whether a UE may use RTD measurements of an LTM candidate cell for UL synchronization maintenance when preparing an LTM configuration for the LTM candidate cell to send to the UE. For example, the RAN node may obtain from a second RAN node serving the LTM candidate cell some information about synchronization of the LTM candidate cell and / or the second RAN node. As a more specific example, the serving RAN node may request / obtain any of the following relevant information from the second RAN node:

[0214] • maximum synchronization error of the second RAN node with respect to a global time reference (e.g., universal time coordinate, UTC); and

[0215] • maximum clock drift of the second RAN nodes.

[0216] In some variants, the second RAN node may send the serving RAN node information about synchronization of the LTM candidate cell and / or the second RAN node as part of LTM preparation. For example, during a Handover Preparation procedure, the second network node sends a HANDOVER REQUEST ACKNOWLEDGE message that includes a NodeSyncInformationResponse information element (IE) that includes any of the above-listed synchronization information for the LTM candidate cell and / or the second RAN node. As another example, the second RAN node can send such information an LTM CONFIGURATION UPDATE ACKNOWLEDGE message as part of an LTM Configuration Update procedure.

[0217] Based on the synchronization information received from the second RAN node and the serving RAN node’s own synchronization status, the serving RAN node may determine thesynchronization assistance information discussed above. In some embodiments, the serving RAN node may determine the RTD measurement configuration based on the synchronization information received from the second RAN node. For example, as part of a Handover Preparation procedure, the serving RAN node sends a request (e.g., a NodeSyncInformationRequest IE in a HANDOVER REQUEST message) for maximum synchronization error of the second RAN node with respect to a global time reference. As another example, the serving RAN node sends this request as part of an LTM Configuration Update procedure, e.g., a NodeSyncInformationRequest IE in an LTM CONFIGURATION UPDATE REQUEST message.

[0218] In some embodiments, as part of LTM preparation, the serving RAN node sends to the second RAN node that serves one or more LTM candidate cells information about serving RAN node synchronization. The second RAN node may use the information - together with its own synchronization information - to determine whether the UE can be configured (or reconfigured) to use RTD for maintaining UL synchronization with the LTM candidate cell(s) that it serves.

[0219] In some embodiments, as part of the LTM preparation, the serving RAN node obtains from the second network node that serves one or more LTM candidate cell the RTD measurement configuration provided to the UE, including the RTD resource configuration and / or the RTD measurement report configuration. For example, as part of a Handover Preparation procedure, the second RAN node sends this information to the serving RAN in an RTDMeasurement IE in a HANDOVER REQUEST ACKNOWLEDGE message. As another example, as part of an LTM Configuration Update procedure, the second RAN node sends this information to the serving RAN in an RTDMeasurement IE in an LTM CONFIGURATION UPDATE ACKNOWLEDGE message.

[0220] Embodiments of the present disclosure are also applicable to the split-node architecture illustrated in Figure 1. For example, the UE’s serving RAN node may include a gNB-CU (also referred to as serving or source gNB-CU) and one or more gNB-DUs, one of which provides the UE’s serving cell (also referred to as serving or source gNB-DU). Also, a second RAN node that provides one or more of the UE’ s configured LTM candidate may include a gNB-CU (also referred to as candidate or target gNB-CU) and one or more gNB-DUs, one or more of which provides the configured LTM candidate cells (also referred to as candidate gNB-DUs). In some cases, the serving and candidate gNB-DUs may be the same (i.e., inter-DU LTM). In other cases, the serving and candidate gNB-DUs may be different but the serving and candidate gNB-CUs may be the same (i.e., inter-CU LTM). In this architecture, embodiments may also include operations between the CU and DU of the serving RAN node and second RAN node that provides one or more LTM candidate cells, methods can be executed between the logical functions of the first network node and / or between the logical functions of the second network node.In some embodiments, the serving CU sends the serving DU the RTD measurement configuration for the UE, including the RTD resource configuration and / or RTD measurement report configuration for one or more LTM candidate cells configured for the UE. For example, the serving CU may send this information in an F1AP message, such as a UE CONTEXT SETUP REQUEST message or a UE CONTEXT MODIFICATION REQUEST message.

[0221] In other embodiments, the serving CU receives from the serving DU the RTD measurement configuration for the UE, including the RTD resource configuration and / or RTD measurement report configuration for the UE’s serving cell. For example, the serving CU may receive this information in an F1AP message, such as a UE CONTEXT SETUP RESPONSE message or a UE CONTEXT MODIFICATION RESPONSE message.

[0222] In other embodiments (e.g., intra-CU LTM), the serving CU sends to the candidate DU the RTD measurement configuration for the UE, including the RTD resource configuration and / or RTD measurement report configuration for one or more LTM candidate cells provided by the candidate DU. For example, the serving CU may send this information in an F1AP message, such as a UE CONTEXT SETUP REQUEST message or a UE CONTEXT MODIFICATION REQUEST message. The serving CU may also include with this information the corresponding RTD measurement configuration of the UE’s current serving cell.

[0223] In other embodiments (e.g., intra-CU LTM), the serving CU receives from the candidate DU the RTD measurement configuration for the UE, including the RTD resource configuration and / or RTD measurement report configuration for one or more LTM candidate cells provided by the candidate DU. For example, the serving CU may receive this information in an F1AP message, such as a UE CONTEXT SETUP RESPONSE message or a UE CONTEXT MODIFICATION RESPONSE message.

[0224] In other embodiments (e.g., intra-CU LTM), the serving CU sends to the candidate DU a request for an RTD measurement configuration for the UE. Alternatively or additionally, the serving CU sends to the serving DU a request for an RTD measurement configuration for the UE. In both cases, the request may be part of an F1AP message, such as a UE CONTEXT SETUP REQUEST message or a UE CONTEXT MODIFICATION REQUEST message. The serving DU may receive the requested RTD measurement configuration in response, such as part of an F1AP message, e.g., UE CONTEXT SETUP REQUEST or UE CONTEXT MODIFICATION REQUEST.

[0225] In cases where the candidate DU is associated with a candidate CU that is different from the serving CU, the serving CU may send the request to the candidate CU via Xn signaling, and the candidate CU then sends it to the candidate DU via F1AP signaling. The candidate DU thensends the response to the candidate CU via F1AP signaling, and the candidate CU sends the response to the serving CU via Xn signaling.

[0226] Various features of the embodiments described above correspond to various operations illustrated in Figures 5-6, which show exemplary methods (e.g., procedures) for a UE and a first 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 5-6 can be used cooperatively to provide various benefits, advantages, and / or solutions to problems described herein. Although Figures 5-6 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.

[0227] In particular, Figure 5 shows an exemplary method (e.g., procedure) for a UE configured for conditional LTM in 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.

[0228] The exemplary method includes the operations of block 510, where the UE receives the following from a RAN node that provides a serving cell for the UE: first timing advance (TA) values for the UE in one or more LTM candidate cells, and one or more validity durations associated with the each of the first TA values. The exemplary method also includes the operations of block 530, where for each LTM candidate cell, the UE initiates a validity timer for the first TA value for the UE in the LTM candidate cell, based on an associated validity duration. The exemplary method also includes the operations of block 540, where for each LTM candidate cell, the UE performs RTD measurements between a radio interface event in the serving cell and a radio interface event in the LTM candidate cell. The exemplary method also includes the operations of block 570, where for each LTM candidate cell, the UE selectively performs one or more of the following operations (labelled with corresponding sub-block numbers) based on results of the RTD measurements performed for the LTM candidate cell:

[0229] • (571) updating the first TA value to a second TA value;

[0230] • (572) determining one of the following: an updated validity duration for the first TA value, or a validity duration for the second TA value; and

[0231] • (573) reinitiating or pausing the validity timer based on the updated validity duration for the first TA value or the validity duration for the second TA value.

[0232] In some embodiments, the exemplary method also includes the operations of block 520, where the UE receives an RTD measurement configuration from the RAN node. The RTD measurements are performed in block 540 in accordance with the RTD measurementconfiguration. In some of these embodiments, the exemplary method also includes the operations of block 560, where the UE sends the results of the RTD measurements to the RAN node. In some variants of these embodiments, the RTD measurement configuration includes an RTD measurement reporting configuration, and the results of the RTD measurements are sent to the RAN node in accordance with the RTD measurement reporting configuration.

[0233] In some further variants, the RTD measurement reporting configuration indicates one of the following types of RTD measurement reporting: aperiodic, event-triggered, semi-persistent, or periodic. For example, one of the following applies:

[0234] • the RTD measurement reporting configuration indicates semi-persistent reporting, and the results of the RTD measurements are sent to the RAN node in response to the operations of block 550, where the UE receives a reporting activation command from the RAN node; or

[0235] • the RTD measurement reporting configuration indicates event-triggered reporting, and the results of the RTD measurements are sent to the RAN node in response to the operations of block 555, where the UE determines that the RTD measurements fulfill a reporting condition also included in RTD measurement reporting configuration.

[0236] In some variants of these embodiments, the results of the RTD measurements include one of the following:

[0237] • an indication that results of RTD measurements are available;

[0238] • the measured RTD for each LTM candidate cell;

[0239] • for each LTM candidate cell, an indication of whether the measured RTD has increased, decreased, or has not changed from a previous measured RTD; or

[0240] • for each LTM candidate cell, an indication of whether the measured RTD has changed more than a threshold that is included in the RTD measurement reporting configuration. In some variants of these embodiments, the exemplary method also includes the operations of block 565, where in response to sending the results of the RTD measurements to the RAN node in block 560, the UE receives from the RAN node at least one of the following for each LTM candidate cell: the second TA value, the validity duration for the second TA value, and the updated validity duration for the first TA value.

[0241] In some of these embodiments, the RTD measurement configuration indicates one or more of the following: a reference cell for RTD measurements, and a radio interface event on which the RTD measurements are based.

[0242] In some embodiments, the exemplary method also includes the operations of block 545, where after initiating the validity timer in block 530, the UE determines whether it (i.e., the UE) is stationary. Selectively performing the one or more operations for each LTM candidate cell inblock 570 is further based on whether the UE is stationary.

[0243] In some of these embodiments, when it is determined that the UE is stationary, for each LTM candidate cell, the first TA value is not updated, the validity duration is updated for the first TA value, and the validity timer is reinitiated or paused based on the updated validity duration. In some of these embodiments, determining whether the UE is stationary is based on one or more of the following: the RTD measurements performed by the UE, positioning measurements performed by the UE, and a stationarity indication received from the RAN node.

[0244] In some of these embodiments, each of the TA values is associated with first and second validity durations, with the second duration being less than the first duration. When it is determined that the UE is stationary, for each LTM candidate cell, the validity timer is reinitiated or paused according to the first validity duration. When it is determined that the UE is not stationary, for each LTM candidate cell, the validity timer is reinitiated or paused according to the second validity duration.

[0245] In some embodiments, the exemplary method also includes the operations of block 525, where the UE receives from the RAN node synchronization assistance information that includes or indicates one or more of the following:

[0246] • a maximum synchronization error between the RAN node and a second RAN node that provides the one or more LTM candidate cells;

[0247] • maximum synchronization errors of the RAN node and the second RAN nodes with respect to a global time reference;

[0248] • maximum clock drifts of the RAN node and the second RAN node;

[0249] • which of the one or more LTM candidate cells have a same maximum synchronization error as the serving cell;

[0250] • which of the one or more LTM candidate cells the UE may maintain synchronization with based on RTD;

[0251] • whether degree of synchronization between the RAN node and the second RAN node is sufficient to maintain LTM candidate cell synchronization based on RTD; and

[0252] • whether degree of synchronization of the RAN node and the second RAN node to a global time reference is sufficient to maintain LTM candidate cell synchronization based on RTD. In some of these embodiments, for each LTM candidate cell, the one or more operations are selectively performed further based on the received synchronization assistance information.

[0253] In some embodiments, for each LTM candidate cell, the first TA value and the one or more validity durations associated with the first TA value are received as part of a conditional configuration for the LTM candidate cell. For example, the conditional configuration may include an execution condition. In some of these embodiments, the exemplary method also includes theoperations of block 580, where based on determining that the execution condition for an LTM candidate cell is fulfilled, the UE selectively performs an LTM cell switch to an LTM candidate cell based on whether the first or second TA value is valid. In other words, the UE uses its current TA value for the LTM candidate cell, whether first (if not updated) or second (if updated).

[0254] In some variants of these embodiments, selectively performing an LTM cell switch in block 580 includes the following operations, labelled with corresponding sub-block numbers:

[0255] • (581) when the first or second TA value is determined to be invalid, performing a random access (RA) to the LTM candidate cell, thereby obtaining an updated TA value; and • (582) when the first or second TA value is determined to be valid, transmitting a message to the LTM candidate cell based on the first or second TA value without performing a RA to the LTM candidate cell.

[0256] In addition, Figure 6 shows an exemplary method (e.g., procedure) for a RAN node configured to facilitate conditional LTM for UEs, according to various embodiments of the present disclosure. The exemplary method can be performed by a RAN node (e.g., base station, eNB, gNB, ng-eNB, DU, etc. such as described elsewhere herein.

[0257] The exemplary method includes the operations of block 610, where the RAN node receives the following from a second RAN node that provides one or more LTM candidate cells for a UE: first timing advance (TA) values for the UE in the one or more LTM candidate cells, and one or more validity durations associated with each of the first TA values. The exemplary method also includes the operations of block 650, where the RAN node sends the following to the UE via a serving cell: the first TA values, the one or more validity durations associated with each of the first TA values, and an RTD measurement configuration for the one or more LTM candidate cells. The exemplary method also includes the operations of block 660, where the RAN node receives from the UE results of UE RTD measurements between a radio interface event in the serving cell and respective radio interface events in the one or more LTM candidate cells. The exemplary method also includes the operations of block 680, where for each LTM candidate cell, the RAN node selectively performs one or more of the following operations (labelled with corresponding sub-block numbers) based on the results of the RTD measurements:

[0258] • (681) updating the first TA value to a second TA value, and

[0259] • (682) determining one of the following: an updated validity duration for the first TA value, or a validity duration for the second TA value.

[0260] In some embodiments, the exemplary method also includes the operations of block 690, where the RAN node sends to the UE at least one of the following for each LTM candidate cell: the second TA value, the validity duration for the second TA value, and the updated validity duration for the first TA value.In some embodiments, the RTD measurement configuration includes an RTD measurement reporting configuration, and the results of the RTD measurements are received from the UE in accordance with the RTD measurement reporting configuration. In some of these embodiments, the RTD measurement reporting configuration indicates one of the following types of RTD measurement reporting: aperiodic, event-triggered, semi-persistent, or periodic. In some variants of these embodiments, one of the following applies:

[0261] • the RTD measurement reporting configuration indicates semi-persistent reporting, and the results of the RTD measurements are received from the UE in response to the operations of block 655, where the RAN node sends a reporting activation command to the UE; or • the RTD measurement reporting configuration indicates event-triggered reporting, and the results of the RTD measurements are received from the UE in response to a UE determination that the RTD measurements fulfill a reporting condition also included in RTD measurement reporting configuration.

[0262] In some of these embodiments, the results of the RTD measurements include one of the following:

[0263] • an indication that results of RTD measurements are available;

[0264] • the measured RTD for each LTM candidate cell;

[0265] • for each LTM candidate cell, an indication of whether the measured RTD has increased, decreased, or has not changed from a previous measured RTD; or

[0266] • for each LTM candidate cell, an indication of whether the measured RTD has changed more than a threshold that is included in the RTD measurement reporting configuration. In some of these embodiments, the RTD measurement configuration indicates one or more of the following: a reference cell for RTD measurements, and a radio interface event on which the RTD measurements are based.

[0267] In some embodiments, the exemplary method also includes the operations of block 670, where the RAN node determines whether the UE is stationary. Selectively performing the one or more operations for each LTM candidate cell in block 680 is further based on whether the UE is stationary. In some of these embodiments, when it is determined that the UE is stationary, for each LTM candidate cell, the first TA value is not updated and the validity duration is updated for the first TA value.

[0268] In some of these embodiments, determining whether the UE is stationary in block 670 is based on one or more of the following: the results of the RTD measurements, and positioning measurements of the UE performed by the RAN node. In some of these embodiments, each of the first TA values is associated with first and second validity durations, with the second duration being less than the first duration. Also, when it is determined that the UE is stationary, for eachLTM candidate cell, the updated validity duration for the first TA value is determined based on the first validity duration. Likewise, when it is determined that the UE is not stationary, for each LTM candidate cell, the updated validity duration for the first TA value or the validity duration for the second TA value is determined based on the second validity duration.

[0269] In some embodiments, the exemplary method also includes the operations of block 640, where the RAN node sends to the UE synchronization assistance information that includes or indicates one or more of the following:

[0270] • a maximum synchronization error between the RAN node and a second RAN node that provides the one or more LTM candidate cells;

[0271] • maximum synchronization errors of the RAN node and the second RAN nodes with respect to a global time reference;

[0272] • maximum clock drifts of the RAN node and the second RAN node;

[0273] • which of the one or more LTM candidate cells have a same maximum synchronization error as the serving cell;

[0274] • which of the one or more LTM candidate cells the UE may maintain synchronization with based on RTD;

[0275] • whether degree of synchronization between the RAN node and the second RAN node is sufficient to maintain LTM candidate cell synchronization based on RTD; and

[0276] • whether degree of synchronization of the RAN node and the second RAN node to a global time reference is sufficient to maintain LTM candidate cell synchronization based on RTD. In some of these embodiments, for each LTM candidate cell, the one or more operations are selectively performed in block 680 further based on the synchronization assistance information. In some of these embodiments, the exemplary method also includes the operations of block 630, where the RAN node receives one or more of the following synchronization status information from the second RAN node:

[0277] • maximum synchronization error of the second RAN node with respect to a global time reference; and

[0278] • maximum clock drift of the second RAN node,

[0279] The synchronization assistance information sent to the UE is determined based on the synchronization status information received from the second RAN node.

[0280] In some variants of these embodiments, the exemplary method also includes the operations of block 620, where the RAN node sends to the second RAN node one or more of the following information:

[0281] • a request for synchronization status; and• one or more of the following synchronization status information for the RAN node: maximum synchronization error of the RAN node with respect to the global time reference, and maximum clock drift of the RAN node.

[0282] In some further variants, the synchronization status information is received from the second RAN node in block 630 in response to the information sent to the second RAN node in block 620. In other further variants, the information is sent to the second RAN node in block 620 in response to the synchronization status information received from the second RAN node in block 630.

[0283] In some embodiments, for each LTM candidate cell, the first TA value and the one or more validity durations associated with the first TA value are sent to the UE as part of a conditional configuration for the LTM candidate cell.

[0284] In some embodiments, the method is performed by a centralized unit (CU) of the RAN node and one or more of the following applies:

[0285] • the results of UE RTD measurements are received via a distributed unit (DU) of the RAN node that provides the serving cell for the UE; and

[0286] • the first TA values and the one or more validity durations associated with each of the first TA values are received from one of the following: a CU of the second RAN node, or a DU of the RAN node that provides the one or more LTM candidate cells. In the latter case, the DU of the RAN node can be considered the second RAN node.

[0287] 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.

[0288] Figure 7 shows an example of a communication system 700 in accordance with some embodiments. In this example, communication system 700 includes a telecommunication network 702 that includes an access network 704 (e.g., RAN) and a core network 706, which includes one or more core network nodes 708. Access network 704 includes one or more access network nodes, such as network nodes 710a-b (one or more of which may be generally referred to as network nodes 710), or any other similar 3 GPP access nodes or non-3GPP access points. Moreover, as will be appreciated by those of skill in the art, a network node is not necessarily limited to an implementation in which a radio portion and a baseband portion are supplied and integrated by a single vendor.

[0289] Thus, it will be understood that network nodes include disaggregated implementations or portions thereof. For example, in some embodiments, telecommunication network 702 includes one or more Open-RAN (ORAN) network nodes. An ORAN network node is a node intelecommunication network 702 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 702, including one or more network nodes 710 and / or core network nodes 708.

[0290] 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 0-2 interface defined by the O-RAN Alliance or comparable technologies. Network nodes 710 facilitate direct or indirect connection of UEs, such as by connecting UEs 712a-d (one or more of which may be generally referred to as UEs 712) to core network 706 over one or more wireless connections.

[0291] 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 700 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 700 may include and / or interface with any type of communication, telecommunication, data, cellular, radio network, and / or other similar type of system.

[0292] UEs 712 may be any of a wide variety of communication devices, including wireless devices arranged, configured, and / or operable to communicate wirelessly with network nodes 710 and other communication devices. Similarly, network nodes 710 are arranged, capable, configured, and / or operable to communicate directly or indirectly with UEs 712 and / or with other network nodes or equipment in telecommunication network 702 to enable and / or provide networkaccess, such as wireless network access, and / or to perform other functions, such as administration in telecommunication network 702.

[0293] In the depicted example, core network 706 connects network nodes 710 to one or more hosts, such as host 716. 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 706 includes one or more core network nodes (e.g., 708) 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 708. 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).

[0294] Host 716 may be under the ownership or control of a service provider other than an operator or provider of access network 704 and / or telecommunication network 702, and may be operated by the service provider or on behalf of the service provider. Host 716 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.

[0295] As a whole, communication system 700 of Figure 7 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.In some examples, telecommunication network 702 is a cellular network that implements 3GPP standardized features. Accordingly, telecommunication network 702 may support network slicing to provide different logical networks to different devices that are connected to telecommunication network 702. For example, telecommunication network 702 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 IoT services to yet further UEs.

[0296] In some examples, UEs 712 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 704 on a predetermined schedule, when triggered by an internal or external event, or in response to requests from access network 704. 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).

[0297] In the example, hub 714 communicates with access network 704 to facilitate indirect communication between one or more UEs (e.g., UE 712c and / or 712d) and network nodes (e.g., network node 710b). In some examples, hub 714 may be a controller, router, content source and analytics, or any of the other communication devices described herein regarding UEs. For example, hub 714 may be a broadband router enabling access to core network 706 for the UEs. As another example, hub 714 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 710, or by executable code, script, process, or other instructions in hub 714. As another example, hub 714 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 714 may be a content source. For example, for a UE that is a VR headset, display, loudspeaker or other media delivery device, hub 714 may retrieve VR assets, video, audio, or other media or data related to sensory information via a network node, which hub 714 then provides to the UE either directly, after performing local processing, and / or after adding additional local content. In still another example, hub 714 acts as a proxy server or orchestrator for the UEs, in particular if one or more of the UEs are low energy IoT devices.

[0298] Hub 714 may have a constant / persistent or intermittent connection to network node 710b. Hub 714 may also allow for a different communication scheme and / or schedule between hub 714 and UEs (e.g., UE 712c and / or 712d), and between hub 714 and core network 706. In other examples, hub 714 is connected to core network 706 and / or one or more UEs via a wiredconnection. Moreover, hub 714 may be configured to connect to an M2M service provider over access network 704 and / or to another UE over a direct connection. In some scenarios, UEs may establish a wireless connection with network nodes 710 while still connected via hub 714 via a wired or wireless connection. In some embodiments, hub 714 may be a dedicated hub - that is, a hub whose primary function is to route communications to / from the UEs from / to network node 710b. In other embodiments, hub 714 may be a non-dedicated hub - that is, a device which is capable of operating to route communications between the UEs and network node 710b, but which is additionally capable of operating as a communication start and / or end point for certain data channels.

[0299] In some embodiments, any of UEs 712 may be configured to perform operations attributed to a UE in various methods or procedures described above, including the exemplary method shown in Figure 5. In some embodiments, any of network nodes 710 may be configured to perform operations attributed to a RAN node in various methods or procedures described above, including the exemplary method shown in Figure 6.

[0300] Figure 8 shows a UE 800 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.

[0301] 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).

[0302] UE 800 includes processing circuitry 802 that is operatively coupled via a bus 804 to an input / output interface 806, a power source 808, a memory 810, a communication interface 812, and / or any other component, or any combination thereof. Certain UEs may utilize all or a subsetof the components shown in Figure 8. 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.

[0303] Processing circuitry 802 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 810. Processing circuitry 802 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 802 may include multiple central processing units (CPUs).

[0304] In the example, input / output interface 806 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 800. 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.

[0305] In some embodiments, power source 808 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 808 may further include power circuitry for delivering power from power source 808 itself, and / or an external power source, to the various parts of UE 800 via input circuitry or an interface such as an electrical power cable. Delivering power may be, for example, for charging power source 808. Power circuitry may perform any formatting, converting, or other modification to the power from power source 808 to make the power suitable for the respective components of UE 800 to which power is supplied.

[0306] Memory 810 may be or be configured to include memory such as random access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), erasableprogrammable 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 810 includes one or more application programs 814, such as an operating system, web browser application, a widget, gadget engine, or other application, and corresponding data 816. Memory 810 may store, for use by UE 800, any of a variety of various operating systems or combinations of operating systems.

[0307] Memory 810 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 810 may allow UE 800 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 810, which may be or comprise a device-readable storage medium.

[0308] Processing circuitry 802 may be configured to communicate with an access network or other network using communication interface 812. Communication interface 812 may comprise one or more communication subsystems and may include or be communicatively coupled to an antenna 822. Communication interface 812 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 818 and / or a receiver 820 appropriate to provide network communications (e.g., optical, electrical, frequency allocations, and so forth). Moreover, transmitter 818 and receiver 820 may be coupled to one or more antennas (e.g., antenna 822) and may share circuit components, software, or firmware, or alternatively be implemented separately.

[0309] In the illustrated embodiment, communication functions of communication interface 812 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.

[0310] Regardless of the type of sensor, a UE may provide an output of data captured by its sensors, through its communication interface 812, 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).

[0311] 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.

[0312] A UE, when in the form of an Internet of Things (IoT) 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 IoT 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 IoT device comprises circuitry and / or software in dependence of the intended application of the IoT device in addition to other components as described in relation to UE 800 shown in Figure 8.As yet another specific example, in an IoT 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.

[0313] 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.

[0314] In some embodiments, UE 800 may be configured to perform operations attributed to a UE in various methods or procedures described above, including the exemplary method shown in Figure 5.

[0315] Figure 9 shows a network node 900 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).

[0316] 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).

[0317] 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).

[0318] Network node 900 includes processing circuitry 902, memory 904, communication interface 906, and power source 908. Network node 900 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 900 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 900 may be configured to support multiple radio access technologies (RATs). In such embodiments, some components may be duplicated (e.g., separate memory 904 for different RATs) and some components may be reused (e.g., a same antenna 910 may be shared by different RATs). Network node 900 may also include multiple sets of the various illustrated components for different wireless technologies integrated into network node 900, 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 900.

[0319] Processing circuitry 902 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 900 components, such as memory 904, to provide network node 900 functionality.

[0320] In some embodiments, processing circuitry 902 includes a system on a chip (SOC). In some embodiments, processing circuitry 902 includes one or more of radio frequency (RF) transceiver circuitry 912 and baseband processing circuitry 914. In some embodiments, RF transceiver circuitry 912 and baseband processing circuitry 914 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 912 and baseband processing circuitry 914 may be on the same chip or set of chips, boards, or units.Memory 904 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 902. Memory 904 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 904a, which may be in the form of a computer program product) capable of being executed by processing circuitry 902 and utilized by network node 900. Memory 904 may be used to store any calculations made by processing circuitry 902 and / or any data received via communication interface 906. In some embodiments, processing circuitry 902 and memory 904 is integrated.

[0321] Communication interface 906 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 906 comprises port(s) / terminal(s) 916 to send and receive data, for example to and from a network over a wired connection. Communication interface 906 also includes radio front-end circuitry 918 that may be coupled to, or in certain embodiments a part of, antenna 910. Radio front-end circuitry 918 comprises filters 920 and amplifiers 922. Radio front-end circuitry 918 may be connected to an antenna 910 and processing circuitry 902. The radio front-end circuitry may be configured to condition signals communicated between antenna 910 and processing circuitry 902. Radio front-end circuitry 918 may receive digital data that is to be sent out to other network nodes or UEs via a wireless connection. Radio front-end circuitry 918 may convert the digital data into a radio signal having the appropriate channel and bandwidth parameters using a combination of filters 920 and / or amplifiers 922. The radio signal may then be transmitted via antenna 910. Similarly, when receiving data, antenna 910 may collect radio signals which are then converted into digital data by radio front-end circuitry 918. The digital data may be passed to processing circuitry 902. In other embodiments, the communication interface may comprise different components and / or different combinations of components.

[0322] In certain alternative embodiments, network node 900 does not include separate radio front-end circuitry 918, instead, processing circuitry 902 includes radio front-end circuitry and is connected to antenna 910. Similarly, in some embodiments, all or some of RF transceiver circuitry 912 is part of communication interface 906. In still other embodiments, communication interface 906 includes one or more ports or terminals 916, radio front-end circuitry 918, and RF transceivercircuitry 912, as part of a radio unit (not shown), and communication interface 906 communicates with baseband processing circuitry 914, which is part of a digital unit (not shown).

[0323] Antenna 910 may include one or more antennas, or antenna arrays, configured to send and / or receive wireless signals. Antenna 910 may be coupled to radio front-end circuitry 918 and may be any type of antenna capable of transmitting and receiving data and / or signals wirelessly. In certain embodiments, antenna 910 is separate from network node 900 and connectable to network node 900 through an interface or port.

[0324] Antenna 910, communication interface 906, and / or processing circuitry 902 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 910, communication interface 906, and / or processing circuitry 902 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.

[0325] Power source 908 provides power to the various components of network node 900 in a form suitable for the respective components (e.g., at a voltage and current level needed for each respective component). Power source 908 may further comprise, or be coupled to, power management circuitry to supply the components of network node 900 with power for performing the functionality described herein. For example, network node 900 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 908. As a further example, power source 908 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.

[0326] Embodiments of network node 900 may include additional components beyond those shown in Figure 9 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 900 may include user interface equipment to allow input of information into network node 900 and to allow output of information from network node 900. This may allow a user to perform diagnostic, maintenance, repair, and other administrative functions for network node 900.

[0327] In some embodiments, network node 900 may be configured to perform operations attributed to a RAN node in various methods or procedures described above, including the exemplary method shown in Figure 6.Figure 10 is a block diagram illustrating a virtualization environment 1000 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 1000 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 1000 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.

[0328] Applications 1002 (which may alternatively be called software instances, virtual appliances, network functions, virtual nodes, virtual network functions, etc.) are run in the virtualization environment 1000 to implement some of the features, functions, and / or benefits of some of the embodiments disclosed herein. For example, In some embodiments, one or more virtual nodes 1002 may be configured to perform operations attributed to a RAN node in various methods or procedures described above, including the exemplary method shown in Figure 6.

[0329] Hardware 1004 includes processing circuitry, memory that stores software and / or instructions (collectively denoted computer program 1004a, 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 1006 (also referred to as hypervisors or virtual machine monitors (VMMs)), provide VMs 1008a-1008b (one or more of which may be generally referred to as VMs 1008), and / or perform any of the functions, features and / or benefits described in relation with some embodiments described herein. Virtualization layer 1006 may present a virtual operating platform that appears like networking hardware to the VMs 1008.

[0330] VMs 1008 comprise virtual processing, virtual memory, virtual networking or interface and virtual storage, and may be run by a corresponding virtualization layer 1006. Different embodiments of the instance of a virtual appliance 1002 may be implemented on one or more of VMs 1008, 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 beused 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.

[0331] In the context of NFV, each VM 1008 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 1008, and that part of hardware 1004 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 1008 on top of the hardware 1004 and corresponds to the application 1002.

[0332] Hardware 1004 may be implemented in a standalone network node with generic or specific components. Hardware 1004 may implement some functions via virtualization. Alternatively, hardware 1004 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 1010, which, among others, oversees lifecycle management of applications 1002. In some embodiments, hardware 1004 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 1012 which may alternatively be used for communication between hardware nodes and radio units.

[0333] 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.

[0334] 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.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.

[0335] 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.

[0336] 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.

[0337] 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.

Claims

CLAIMS:

1. A method for a user equipment, UE,(800) configured for conditional layer-1 / layer-2 triggered inter-cell mobility, LTM, in a radio access network, RAN, the method comprising: receiving (510) the following from a RAN node that provides a serving cell for the UE:first timing advance, TA, values for the UE in one or more LTM candidate cells, andone or more validity durations associated with the each of the first TA values; for each LTM candidate cell, initiating (530) a validity timer for the first TA value for the UE in the LTM candidate cell, based on an associated validity duration; for each LTM candidate cell, performing (540) relative time difference, RTD, measurements between a radio interface event in the serving cell and a radio interface event in the LTM candidate cell; andfor each LTM candidate cell, selectively performing (570) one or more of the following operations based on results of the RTD measurements performed for the LTM candidate cell:updating (571) the first TA value to a second TA value;determining (572) one of the following: an updated validity duration for the first TA value, or a validity duration for the second TA value; and reinitiating or pausing (573) the validity timer based on the updated validity duration for the first TA value or the validity duration for the second TA value.

2. The method of claim 1, further comprising receiving (520) an RTD measurement configuration from the RAN node, wherein the RTD measurements are performed in accordance with the RTD measurement configuration.

3. The method of claim 2, further comprising sending (560) the results of the RTD measurements to the RAN node.

4. The method of claim 3, wherein the RTD measurement configuration includes an RTD measurement reporting configuration, and the results of the RTD measurements are sent to the RAN node in accordance with the RTD measurement reporting configuration.

5. The method of claim 4, wherein the RTD measurement reporting configuration indicates one of the following types of RTD measurement reporting: aperiodic, event-triggered, semi-persistent, or periodic.

6. The method of claim 5, wherein one of the following applies:the RTD measurement reporting configuration indicates semi-persistent reporting, and the results of the RTD measurements are sent to the RAN node in response to receiving (550) a reporting activation command from the RAN node; or the RTD measurement reporting configuration indicates event-triggered reporting, and the results of the RTD measurements are sent to the RAN node in response to determining (555) that the RTD measurements fulfill a reporting condition also included in RTD measurement reporting configuration.

7. The method of any of claims 3-6, wherein the results of the RTD measurements include one of the following:an indication that results of RTD measurements are available;the measured RTD for each LTM candidate cell;for each LTM candidate cell, an indication of whether the measured RTD has increased, decreased, or has not changed from a previous measured RTD; or for each LTM candidate cell, an indication of whether the measured RTD has changed more than a threshold that is included in the RTD measurement reporting configuration.

8. The method of any of claims 3-7, further comprising, in response to sending the results of the RTD measurements to the RAN node, receiving (565) from the RAN node at least one of the following for each LTM candidate cell: the second TA value, the validity duration for the second TA value, and the updated validity duration for the first TA value.

9. The method of any of claims 2-8, wherein the RTD measurement configuration indicates one or more of the following: a reference cell for RTD measurements, and a radio interface event on which the RTD measurements are based.

10. The method of any of claims 1-9, further comprising, after initiating the validity timer, determining (545) whether the UE is stationary, wherein selectively performing the one or more operations for each LTM candidate cell is further based on whether the UE is stationary.

11. The method of claim 10, wherein when it is determined that the UE is stationary, foreach LTM candidate cell, the first TA value is not updated, the validity duration is updated for the first TA value, and the validity timer is reinitiated or paused based on the updated validity duration.

12. The method of claims 10-11, wherein determining whether the UE is stationary is based on one or more of the following: the RTD measurements performed by the UE, positioning measurements performed by the UE, and a stationarity indication received from the RAN node.

13. The method of any of claims 10-12, wherein:each of the TA values is associated with first and second validity durations, with the second duration being less than the first duration;when it is determined that the UE is stationary, for each LTM candidate cell, the validity timer is reinitiated or paused according to the first validity duration; and when it is determined that the UE is not stationary, for each LTM candidate cell, the validity timer is reinitiated or paused according to the second validity duration.

14. The method of any of claims 1-13, further comprising receiving (525) from the RAN node synchronization assistance information that includes or indicates one or more of the following:a maximum synchronization error between the RAN node and a second RAN node that provides the one or more LTM candidate cells;maximum synchronization errors of the RAN node and the second RAN nodes with respect to a global time reference;maximum clock drifts of the RAN node and the second RAN node;which of the one or more LTM candidate cells have a same maximum synchronization error as the serving cell;which of the one or more LTM candidate cells the UE may maintain synchronization with based on RTD;whether degree of synchronization between the RAN node and the second RAN node is sufficient to maintain LTM candidate cell synchronization based on RTD; and whether degree of synchronization of the RAN node and the second RAN node to a global time reference is sufficient to maintain LTM candidate cell synchronization based on RTD.

15. The method of claim 14, wherein for each LTM candidate cell, the one or moreoperations are selectively performed further based on the received synchronization assistance information.

16. The method of any of claims 1-15, wherein for each LTM candidate cell, the first TA value and the one or more validity durations associated with the first TA value are received as part of a conditional configuration for the LTM candidate cell.

17. A method for a radio access network, RAN, node (900) configured to facilitate conditional layer-1 / layer-2 triggered inter-cell mobility, LTM, by user equipment, UEs (800), the method comprising:receiving (610) the following from a second RAN node that provides one or more LTM candidate cells for a UE:first timing advance, TA, values for the UE in the one or more LTM candidate cells, andone or more validity durations associated with each of the first TA values; sending (650) the following to the UE via a serving cell: the first TA values, the one or more validity durations associated with each of the first TA values, and a relative time difference, RTD, measurement configuration for the one or more LTM candidate cells;receiving (660), from the UE, results of UE RTD measurements between a radio interface event in the serving cell and respective radio interface events in the one or more LTM candidate cells; andfor each LTM candidate cell, selectively (680) performing one or more of the following operations based on the results of the RTD measurements:updating (681) the first TA value to a second TA value, anddetermining (682) one of the following: an updated validity duration for the first TA value, or a validity duration for the second TA value.

18. The method of claim 17, further comprising sending (690) to the UE at least one of the following for each LTM candidate cell: the second TA value, the validity duration for the second TA value, and the updated validity duration for the first TA value.

19. The method of any of claims 17-18, wherein the RTD measurement configuration includes an RTD measurement reporting configuration, and the results of the RTD measurements are received from the UE in accordance with the RTD measurement reportingconfiguration.

20. The method of claim 19, wherein the RTD measurement reporting configuration indicates one of the following types of RTD measurement reporting: aperiodic, event-triggered, semi-persistent, or periodic.

21. The method of claim 20, wherein one of the following applies:the RTD measurement reporting configuration indicates semi-persistent reporting, and the results of the RTD measurements are received from the UE in response to sending (655) a reporting activation command to the UE; orthe RTD measurement reporting configuration indicates event-triggered reporting, and the results of the RTD measurements are received from the UE in response to a UE determination that the RTD measurements fulfill a reporting condition also included in RTD measurement reporting configuration.

22. The method of any of claims 19-21, wherein the results of the RTD measurements include one of the following:an indication that results of RTD measurements are available;the measured RTD for each LTM candidate cell;for each LTM candidate cell, an indication of whether the measured RTD has increased, decreased, or has not changed from a previous measured RTD; or for each LTM candidate cell, an indication of whether the measured RTD has changed more than a threshold that is included in the RTD measurement reporting configuration.

23. The method of any of claims 19-22, wherein the RTD measurement configuration indicates one or more of the following: a reference cell for RTD measurements, and a radio interface event on which the RTD measurements are based.

24. The method of any of claims 17-23, further comprising determining (670) whether the UE is stationary, wherein selectively performing the one or more operations for each LTM candidate cell is further based on whether the UE is stationary.

25. The method of claim 24, wherein when it is determined that the UE is stationary, for each LTM candidate cell, the first TA value is not updated and the validity duration is updatedfor the first TA value.

26. The method of claims 24-25, wherein determining whether the UE is stationary is based on one or more of the following: the results of the RTD measurements, and positioning measurements of the UE performed by the RAN node.

27. The method of any of claims 24-26, wherein:each of the first TA values is associated with first and second validity durations, with the second duration being less than the first duration;when it is determined that the UE is stationary, for each LTM candidate cell, the updated validity duration for the first TA value is determined based on the first validity duration; andwhen it is determined that the UE is not stationary, for each LTM candidate cell, the updated validity duration for the first TA value or the validity duration for the second TA value is determined based on the second validity duration.

28. The method of any of claims 17-27, further comprising sending (640) to the UE synchronization assistance information that includes or indicates one or more of the following:a maximum synchronization error between the RAN node and a second RAN node that provides the one or more LTM candidate cells;maximum synchronization errors of the RAN node and the second RAN nodes with respect to a global time reference;maximum clock drifts of the RAN node and the second RAN node;which of the one or more LTM candidate cells have a same maximum synchronization error as the serving cell;which of the one or more LTM candidate cells the UE may maintain synchronization with based on RTD;whether degree of synchronization between the RAN node and the second RAN node is sufficient to maintain LTM candidate cell synchronization based on RTD; and whether degree of synchronization of the RAN node and the second RAN node to a global time reference is sufficient to maintain LTM candidate cell synchronization based on RTD.

29. The method of claim 28, wherein for each LTM candidate cell, the one or more operations are selectively performed further based on the synchronization assistanceinformation.

30. The method of any of claims 28-29, further comprising receiving (630) one or more of the following synchronization status information from the second RAN node:maximum synchronization error of the second RAN node with respect to a global time reference; andmaximum clock drift of the second RAN node,wherein the synchronization assistance information is determined based on the synchronization status information received from the second RAN node.

31. The method of claim 30, further comprising sending (620) to the second RAN node one or more of the following information:a request for synchronization status; andone or more of the following synchronization status information for the RAN node: maximum synchronization error of the RAN node with respect to the global time reference; andmaximum clock drift of the RAN node.

32. The method of claim 31, wherein one of the following applies:the synchronization status information is received from the second RAN node in response to the information sent to the second RAN node; orthe information is sent to the second RAN node in response to the synchronization status information received from the second RAN node.

33. The method of any of claims 17-32, wherein for each LTM candidate cell, the first TA value and the one or more validity durations associated with the first TA value are sent to the UE as part of a conditional configuration for the LTM candidate cell.

34. The method of any of claims 17-33, wherein the method is performed by a centralized unit, CU, of the RAN node and one or more of the following applies:the results of UE RTD measurements are received via a distributed unit, DU, of the RAN node that provides the serving cell for the UE; andthe first TA values and the one or more validity durations associated with each of the first TA values are received from one of the following: a CU of the second RAN node, or a DU of the RAN node that provides the one or more LTM candidatecells.

35. A user equipment, UE, (800) configured for conditional layer- l / layer-2 triggered intercell mobility. LTM, in a radio access network, RAN, the UE comprising:communication interface circuitry (812) configured to communicate with RAN nodes;andprocessing circuitry (802) operatively coupled to the communication interface circuitry (812), whereby the processing circuitry (802) and the communication interface circuitry (812) are configured to perform operations corresponding to the methods of any of claims 1-16.

36. A user equipment, UE, (800) configured for conditional layer-1 / layer-2 triggered inter-cell mobility, LTM, in a radio access network, RAN, the UE being further configured to perform operations corresponding to the methods of any of claims 1-16.

37. A non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry (802) of a user equipment, UE, (800) configured for conditional layer- l / layer-2 triggered inter-cell mobility, LTM, in a radio access network, RAN, configure the UE to perform operations corresponding to the methods of any of claims 1-16.

38. A computer program product comprising computer-executable instructions that, when executed by processing circuitry (802) of user equipment, UE, (800) configured for conditional layer- l / layer-2 triggered inter-cell mobility, LTM, in a radio access network, RAN, configure the UE to perform operations corresponding to the methods of any of claims 1-16.

39. A radio access network, RAN, node (900) configured to facilitate conditional layer-1 / layer-2 triggered inter-cell mobility, LTM, by user equipment, UEs, the RAN node comprising:communication interface circuitry (906) configured to communicate with UEs and with other RAN nodes; andprocessing circuitry (902) operatively coupled to the communication interface circuitry (906), whereby the processing circuitry (902) and the communication interface circuitry (906) are configured to perform operations corresponding to the methods of any of claims 17-34.

40. A radio access network, RAN node (900) configured to facilitate conditional layer-1 / layer-2 triggered inter-cell mobility, LTM by user equipment, UEs, the RAN node being further configured to perform operations corresponding to the methods of any of claims 17-34.

41. A non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry (902) of a radio access network, RAN node (900) configured to facilitate conditional layer-1 / layer-2 triggered inter-cell mobility, LTM by user equipment, UEs, configure the RAN node (900) to perform operations corresponding to the methods of any of claims 17-34.

42. A computer program product comprising computer-executable instructions that, when executed by processing circuitry of a radio access network, RAN node configured to facilitate conditional layer-1 / layer-2 triggered inter-cell mobility, LTM by user equipment, UEs, configure the RAN node to perform operations corresponding to the methods of any of claims 17-34.

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