WO2025207010A1 - Network control of user equipment security key updates for mobility - Google Patents

Abstract

Embodiments include methods for user equipment (UE) configured for mobility between cells of a radio access network (RAN). Such methods include receiving, from a first RAN node via a serving cell, a mobility configuration comprising candidate configuration(s) for respective mobility candidate cell(s). Each candidate configuration includes a security cell set identifier for the mobility candidate cell. Such methods include receiving from the first RAN node a mobility command for execution of a mobility procedure to one of the mobility candidate cells as a target cell. Such methods include, during execution of the mobility procedure according to the mobility command, selectively updating access stratum (AS) security keys based on security cell set identifiers for the target cell and for the serving cell. Such methods include transmitting, to a second RAN node, a message indicating that the mobility procedure is complete. The message is secured using the selectively updated AS security keys.

Description

NETWORK CONTROL OF USER EQUIPMENT SECURITY KEY UPDATES FOR MOBILITY

TECHNICAL FIELD

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

BACKGROUND

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.

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).

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

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

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

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

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.

Conventionally, serving cell change is triggered by layer 3 (L3, e.g., RRC) measurements and involves RRC signaling to change PCell and PSCell (e.g., when DC is configured) and to release/add SCells. L3 serving cell change - also referred to as handover (HO) - also involves LI and L2 resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching. Certain L3 mobility operations may also involve changes to integrity protection and ciphering algorithms as well the AS keys K_§NB, KRRCint, KRRCenc, Kupint and KuPenc-

Even so, HO and other L3 mobility operations can have various robustness problems. For example, a HO command is normally sent when UE’s connection is degraded, such as at or near cell borders. As such, the HO command may need to be segmented (e.g., to allow for redundancy to protect against errors) and/or retransmitted one or more times before it reaches the UE. The HO command may not reach the UE in time (or at all) before the degraded connection is dropped. Failure of HO to a target cell may lead to the UE declaring radio link failure (RLF) in the serving cell and reestablishing its connection in another cell. 3GPP Rel-16 and Rel-17 support conditional HO (CHO) and other conditional mobility procedures in which transmission and execution of a mobility (e.g., HO) command are separated. This allows the mobility command to be sent to UE when the radio conditions are still good, thus increasing the likelihood of successful reception. The UE executes the mobility command later based on an associated execution condition. These conditional mobility procedures are facilitated by a conditional reconfiguration framework in which the network provides a UE with one or more reconfigurations, each with associated execution condition(s). 3GPP Rel-18 also supports subsequent L3 mobility, in which a UE may perform multiple L3 mobility procedures without intermediate reconfiguration by the RAN.

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

In LTM, a UE is pre-configured by its serving RAN node with one radio resource control (RRC) configuration per LTM candidate cell, sometimes referred to as an “LTM candidate 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.

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

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

SUMMARY

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

For example, security key updated is needed whenever the UE’s CU changes but may also be initiated by the RAN even for serving cell changes in a single CU. Since the UE cannot tell whether a LTM cell switch involves a change of CU, this creates an ambiguous situation for the UE regarding update of security keys at LTM cell switch.

As another example, the UE may have received an LTM candidate configuration for an LTM candidate cell that is provided by the same CU as the UEs current serving cell, and thus did not require security key update at LTM cell switch. After one or more LTM cell switches, the UE’s serving cell may be provided by a different CU than provided the LTM candidate configuration. In such case, performing an LTM cell switch to the LTM candidate cell would require a security key update, of which the UE is unaware.

Similar problems and/or difficulties may exist for subsequent CHO and conditional LTM.

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

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

These exemplary methods include receiving, from a first RAN node via a serving cell, a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells. Each candidate configuration includes a security cell set identifier for the mobility candidate cell. These exemplary methods also include receiving from the first RAN node a mobility command for execution of a mobility procedure from the serving cell. These exemplary methods also include, during execution of the mobility procedure to the target cell in accordance with the mobility command, selectively updating a plurality of access stratum (AS) security keys based on the security cell set identifier for the target cell and a security cell set identifier for the serving cell. These exemplary methods also include transmitting, to a second RAN node via the target cell, a message indicating that the mobility procedure is complete, wherein the message is secured using at least one of the selectively updated AS security keys.

In some embodiments, the mobility configuration also includes the security cell set identifier for the serving cell. In some embodiments, selectively updating the plurality of AS security keys based on the security cell set identifier for the target cell and the security cell set identifier for the serving cell includes the following operations:

• determining whether there is a match between the security cell set identifier for the target cell and the security cell set identifier for the serving cell;

• when no match is determined, performing an update of the plurality of AS security keys based on a first security configuration received from the first RAN node; and

• when a match is determined, refraining from updating the plurality of AS security keys.

In some embodiments, selectively updating the plurality of AS security keys based on the security cell set identifier for the target cell and on the security cell set identifier for the serving cell includes the following operations:

• when no match is determined, performing one or more link layer reset operations such that the updated plurality of AS security keys are used for link layer protocol data units, PDUs, sent or received after the update; and

• when a match is determined, refraining from performing any link layer reset operations. In some of these embodiments, the one or more link layer link reset operations include one or more of the following: packet data convergence protocol (PDCP) layer re-establishment; radio link control (RLC) layer re-establishment; and medium access control (MAC) layer reset.

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

These exemplary methods include sending, to a UE via a serving cell, a mobility configuration comprising: a security cell set identifier for the serving cell, and one or more candidate configurations for respective one or more mobility candidate cells. Each candidate configuration includes a security cell set identifier for the mobility candidate cell. These exemplary methods also include sending to the UE a mobility command for execution of a mobility procedure from the serving cell. The mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure. The security cell set identifier for the target cell and the security cell set identifier for the serving cell indicate whether a plurality of access stratum (AS) security keys should be updated by the UE during execution of the mobility procedure to the target cell. In some embodiments, no match between the security cell set identifiers for the target cell and the serving cell indicates that the plurality of AS security keys should be updated by the UE based on a first security configuration provided by the first RAN node. Also, a match between the security cell set identifiers for the target cell and the serving cell indicates that the plurality of AS security keys should not be updated by the UE during execution of the mobility procedure.

In various embodiments summarized above, the AS security keys include a master MN security key (e.g., K_§NB) and the following security keys derived from the master MN security key: a first key for integrity protection of signaling (e.g., KRRCint), a second key for ciphering of signaling (e.g., KRRCCIIC), a third key for integrity protection of user data (e.g., Kupint), and a fourth key for ciphering of user data (e.g., Kup_enc).

In various embodiments summarized above, the security configuration provided by the first RAN node and used by the UE may include one of more of the following parameters:

• an indication of whether a keyset change is needed;

• an indication whether horizontal or vertical key derivation should be used;

• a next-hop chaining counter (NCC) usable for vertical key derivation; and

• a container of non-AS (NAS) information.

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

These and other embodiments described herein may provide various advantages, benefits, and/or solutions to problems. For example, embodiments may facilitate UE mobility between a set of preconfigured candidate cells provided by different CUs, including security key update, without need for changing candidate cell configurations previously provided to the UE. As such, embodiments may facilitate necessary updates for encryption and integrity protection as the UE moves between cells in the RAN, thereby increasing security of signaling and user data carried by radio bearers secured in this manner

These and other objects, features, and advantages of embodiments of the present disclosure will become apparent upon reading the following Detailed Description in view of the Drawings briefly described below. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

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

Figure 7 illustrates some problems addressed by embodiments of the present disclosure.

Figure 8 illustrates a system structure in which some embodiments of the present disclosure may be implemented.

Figure 9 shows a signaling diagram for an exemplary inter-CU LTM procedure, according to some embodiments of the present disclosure.

Figure 10 shows a signaling diagram for an exemplary inter-CU conditional LTM procedure, according to some embodiments of the present disclosure.

Figure 11 shows a flowchart of an exemplary security procedure for a UE, according to some embodiments of the present disclosure.

Figure 12 shows a flowchart of an exemplary LTM procedure for a UE, according to other embodiments of the present disclosure.

Figures 13-15 show various ASN.l data structures that may be used for implementation of embodiments of the present disclosure.

Figure 16 shows a flow diagram of an exemplary method for a UE (e.g., wireless device), according to various embodiments of the present disclosure.

Figure 17 shows a flow diagram of an exemplary method for a first RAN node (e.g., base station, eNB, gNB, DU, etc.), according to various embodiments of the present disclosure.

Figure 18 shows a communication system according to various embodiments of the present disclosure.

Figure 19 shows a UE according to various embodiments of the present disclosure.

Figure 20 shows a network node according to various embodiments of the present disclosure.

Figure 21 is a block diagram of a virtualization environment in which various embodiments of the present disclosure may be virtualized. DETAILED DESCRIPTION

Some of the embodiments contemplated herein will now be described more fully with reference to the accompanying drawings. Other embodiments, however, are contained within the scope of the subject matter disclosed herein, the disclosed subject matter should not be construed as limited to only the embodiments set forth herein; rather, these embodiments are provided as examples to convey the scope of the subject matter to those skilled in the art.

In general, all terms used herein are to be interpreted according to their ordinary meaning to a person of ordinary skill in the relevant technical field, unless a different meaning is expressly defined and/or implied from the context of use. All references to a/an/the element, apparatus, component, means, step, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise or clearly implied from the context of use. The operations of any methods and/or procedures disclosed herein do not have to be performed in the exact order disclosed, unless an operation is explicitly described as following or preceding another operation and/or where it is implicit that an operation must follow or precede another operation. Any feature of any embodiment disclosed herein can apply to any other disclosed embodiment, as appropriate. Likewise, any advantage of any embodiment described herein can apply to any other disclosed embodiment, as appropriate.

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

• 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 3GPP 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 (c.g. micro, pi co, 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.

• 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), aPDN 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. • 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”.

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

• 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.

• 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.

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.

Note that the description given herein focuses on a 3GPP 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.

Figure 2 shows a logical architecture for an NG-RAN node (e.g., gNB or ng-eNB) arranged in the split CU/DU architecture, such as gNB 100 in Figure 1. This logical architecture separates the CU into control plane (CP) and user plane (UP) functionality, called CU-C (or CU-CP) and CU-U (or CU-UP) respectively. Furthermore, each of the NG, Xn, and Fl interfaces is split into a CP interface (e.g., NG-C) and a UP interface (e.g., NG-U). Moreover, the CU-U and CU-C can communicate via an El interface. Each DU may be connected to only one CU-C, and each CU-U may be connected to only one CU-C. However, a single DU may be connected to multiple CU- Us under the control of the same CU-C, or a single CU-U may be connected to multiple DUs under the control of the same CU-C. Note that the terms “Central Entity” and “Distributed Entity” in Figure 2 refer to physical network nodes.

Figure 3 shows an exemplary configuration of NR UP and CP protocol stacks between a UE (310), a gNB (320), and an AMF (330). Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Convergence Protocol (PDCP) layers between UE and gNB are common to UP and CP. PDCP provides ciphering/deciphering, integrity protection, sequence numbering, reordering, and duplicate detection for both CP and UP, as well as header compression and retransmission for UP data.

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

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.

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 RRCJDLE 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.

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

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

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

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

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

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

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

As described in 3GPP TS 38.501 (vl 8.4.0) section 6.9.2.1.1, whenever an initial AS security context needs to be established between a UE and a gNB, the AMF and the UE derive KgNB and a Next Hop parameter (NH) from KAMF provided by the AMF. A NH Chaining Counter (NCC) is associated with each KgNB and NH parameter, and each KgNB is associated with the NCC corresponding to the NH value from which the KgNB was derived. At initial setup, KgNB is derived directly from KAMF, and is then considered to be associated with a virtual NH parameter with NCC value equal to zero. At initial setup, the derived NH value is associated with the NCC value one. The UE and the gNB use KgNB to secure the communication between each other, including derivation of the four keys mentioned above.

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).

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 RRCReconflguration message with a reconflgurationWithSync field. The procedure to perform a handover is sometimes also referred to as “L3 mobility”, as it is controlled by layer 3 (L3, i.e., RRC) and the messages exchanged are part of L3.

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

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).

In general, UE nobility in RRC CONNECTED state is network-based since the network has the most information about conditions such as cell loading (UEs and/or traffic), available network node resources (e.g., processing), available frequencies, etc. However, there will be scenarios when the network fails to handover the UE to the “correct” neighbor cell in time, which can cause the UE will declare radio link failure (RLF) or handover failure (HOF). As briefly mentioned above, conditional handover (CHO) was introduced in 3GPP Rel- 16 to improve robustness of UE handover. The key idea in CHO is separation of transmission and execution of the handover command. This allows the handover command to be sent to a UE earlier when the radio conditions are still good, thus increasing the likelihood that the message is successfully transferred. The execution of the handover command is done later in time based on an associated execution condition.

The execution condition is typically based on a threshold. For example, a signal strength of candidate target cell becomes X dB better than the serving cell (so called “A3 event ”). A preceding measurement reporting event could use a threshold Y that is selected to be lower than X used as the handover execution condition. This allows the serving cell to prepare the handover upon reception of an early measurement report and to provide the RRCConnectionReconflguration with mobilityControlInfo (for LTE), or a RRCReconflguration with either a reconflgurationWithSync or a CellGroupConfig (for NR) at a time when the radio link between the source cell and the UE is still relatively stable.

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

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

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

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

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

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

When a UE successfully connects (e.g., completes RA) to a target cell during a CHO or a conventional handover, it releases all the conditional reconfigurations that it has stored. The RAN node serving the target cell may then provide the UE with new conditional reconfigurations if desired. On handovers (including CHO) and transitions from RRC INACTIVE to RRC_CONNECTED, the basis for K_§NB used between the UE and the target NG-RAN node (e.g., gNB) - called KNG-RAN* - is derived from either the currently active K_§NB or the NH parameter. Deriving KNG-RAN* from the currently active K_§NB is referred to as a “horizontal key derivation” while deriving KNG-RAN* from the NH parameter is referred to as “vertical key derivation.”

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

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

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

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

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

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

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

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

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

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

3GPP Rel-18 also includes an NR mobility enhancement known as L1/L2 based intercell mobility or L1/L2 triggered mobility (LTM). Current L3-based inter-cell mobility procedures involve LI and L2 resets, leading to longer latency, increased signaling overhead, and longer interruptions than for intra-cell beam switching. Thus, Rel-18 LTM is intended to facilitate serving cell changes via L1/L2 signaling that reduce latency, signaling overhead, and interruptions. 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 RRCReconflguration message or a portion thereof, such as one or more lEs/fields/parameters (e.g., CellGroupConflg 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.

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

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

In operation 1, the UE (610) sends a. Measure ent Re port message to the gNB (620). Based on this message, the gNB decides to configure LTM for the UE and initiates preparation of one or more LTM candidate cells. In operation 2, the gNB sends an RRCReconflguration 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.

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

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.

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.

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

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.

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

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

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

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

However, inter-CU LTM, conditional LTM, subsequent LTM, and subsequent CHO require the support of new security-related functionality that is not part of Rel-18. In particular, security key change is needed whenever the UE’s CU changes but may also be initiated by the RAN even for serving cell changes in a single CU. Since the UE cannot tell whether certain mobility operations involves a change of CU, this creates an ambiguous situation for the UE regarding change of security key in relation to these operations.

For example, inter-CU LTM requires support for change of security key (and possibly also encryption and integrity protection algorithms) as part of LTM execution triggered by an LTM Cell Switch MAC CE. However, security key change is not needed for intra-CU LTM but it may be triggered at the discretion of the CU. Since the UE cannot tell whether a LTM cell switch is intra- or inter-CU, some solution is needed to indicate to the UE whether security key/algorithm changes are needed in relation to an LTM cell switch.

Consider another example in the context of subsequent LTM. The UE may have received an LTM candidate configuration for an LTM candidate cell that is provided by the same CU as the UEs current serving cell, and thus did not require security key change at LTM cell switch. After one or more LTM cell switches triggered by MAC CEs without RRC intervention, the UE’s serving cell may be provided by a different CU than provided the LTM candidate configuration. In such case, performing an LTM cell switch to the LTM candidate cell would require a security key change, which was not the case when the UE received the candidate configuration for this LTM candidate cell. However, the UE is unaware of this situation.

Figure 7 illustrates this problem related to subsequent LTM. The UE shown in Figure 7 receives LTM candidate cell configurations for cells IB provided by gNBl and cells 2A/2B provided by gNB2, while being served by source cell 1 A provided by gNBl . Each LTM candidate cell configuration includes an indication of need for security key change. For example, the configurations for LTM candidate cells 2A/2B indicate a need for security key change since they are provided by gNB2 while the UE’s serving cell 1A is provided by gNBl. However, the configuration for LTM candidate cell IB indicates no need for security key change since cell IB and 1A are both provided by gNBl. These indications are correct for LTM cell switches in which cell 1A is the source cell.

However, these indications may be incorrect after one or more LTM cell switches. Assuming the UE moves first from cell 1A to celllB, the UE will not change the security key since target cell IB is served by the same CU/gNB. A subsequent LTM cell switch is triggered for the UE from cell IB to cell 2 A, for which the UE received an LTM candidate cell configuration previously. However, the indication of no security key change in this configuration is no longer correct since cell 2A is served by a different CU/gNB than source cell IB.

Another subsequent LTM cell switch is triggered from the UE from source cell 2A to cell 2B, for which the UE received an LTM candidate cell configuration previously. However, the indication of security key change in this configuration is no longer correct since cell 2B is served by the same CU/gNB as source cell 2A. Even more problematic is the UE’s next subsequent LTM cell switch from cell 2B back to cell IB. The LTM candidate configuration for cell IB indicates no security key change, which is incorrect since source cell 2B and target cell IB are served by different CUs/gNBs.

While the example in Figure 7 is for non-conditional LTM, the identified problems and/or issues would apply equally to inter-CU conditional LTM where execution is based on measurements meeting execution conditions rather than explicit LTM Cell Switch MAC CEs.

In addition to whether to perform security key update, the UE also needs to know how to perform the security key update, including input parameters for the security key generation. For the MCG, these parameters are currently carried in the field masterKeyUpdate of the RRCReconflguration message sent to UE to trigger a reconfiguration with sync is triggered, such as for L3 handover. However, the presence of this field is the indication for the UE to execute the security key update, which means that it cannot be used to control security key change for subsequent LTM.

Similar problems and/or issues exist for subsequent CP AC, in which a mechanism is specified to control the key change at inter-SN mobility. Even so, this solution cannot be used for inter-CU LTM or conditional LTM, because the security key derivation for SCG uses different input parameters than the security key derivation for MCG. Rel-19 may also support subsequent execution for CHO, in a similar manner as subsequent CP AC. Currently, the UE deletes other CHO candidate configurations after execution of a CHO. If subsequent CHO is support, the UE would need to retain these CHO candidate configurations, leading to the same problems and/or issues related to security key change as illustrated by Figure 7 for subsequent LTM.

To summarize, these problems and/or issues occur when a UE receives candidate configuration to apply while the UE is connected to a first cell, but after one or more mobility procedures (e.g., LTM Cell Switches), the UE needs to apply one of the candidate configurations when the UE is in a second cell provided by a different CU/gNB than the first cell.

Accordingly, embodiments of the present disclosure address these problems and/or issues by flexible and efficient security key change techniques for a UE, a source RAN node, and a target RAN node during execution of a mobility procedure by the UE from a source cell provided by the source RAN node to a target cell provided by the target RAN node. For example, the mobility procedure may be inter-CU LTM. In general, these techniques control security key change during the mobility procedure based on new fields in a mobility configuration, such as an LTM candidate configuration.

At a high level, the UE determines whether to perform security key changes (including updates and/or refresh) based on a security configuration associated with an source cell in which the UE receives a mobility command (e.g., LTM Cell Switch command) and a security configuration associated with a target cell of the mobility procedure This can be facilitated by configuring the UE with a candidate cell configuration (e.g., LTM candidate configuration) that includes fields to control the security key change during a mobility procedure, including subsequent LTM, inter-CU LTM, inter-CU conditional LTM, and inter-CU subsequent CHO. In some embodiments, the fields in each candidate cell configuration include a security cell identifier (e.g., securityCellSetld). The UE is also configured with a security set identifier for its current serving cell (e.g., servingSecurityCellSetld). In some embodiments, the fields in each candidate cell configuration include parameters to be used for security key change (e.g., master Key Update), In various embodiments, the UE performs security key update (including updates and/or refresh) during execution of the mobility procedure based on these fields. When the UE executes the mobility procedure from its serving/source cell to a target cell, based on a candidate cell configuration for the target cell, the UE determines whether the security set identifier associated with the target cell is the same as the security set identifier associated with the source cell. When the two security set identifiers are the same, the UE does not change the security key. When the two security set identifiers are not the same, the UE uses the parameters for security key change in the candidate configuration for the target cell to refresh the security key.

Other embodiments include methods for a first RAN node that provides a serving/source cell for a UE. The first RAN nodes configures the UE with a candidate cell configuration (e.g., LTM candidate configuration) that includes fields to control the security key change during a mobility procedure, including subsequent LTM, inter-CU LTM, inter-CU conditional LTM, and inter-CU subsequent CHO. For example, these fields may correspond to the fields summarized above in relation to UE embodiments. In some embodiments, the first RAN node may be a source CU that configures the UE to perform inter-CU LTM to one or more LTM candidate cells served by a candidate DU that is associated with a different CU.

Embodiments of the present disclosure can provide various advantages and/or benefits. For example, embodiments may facilitate UE mobility between a set of preconfigured candidate cells provided by different CUs, including security key refresh, without need for changing candidate cell configurations previously provided to the UE. As such, embodiments may facilitate necessary updates for encryption and integrity protection as the UE moves between cells in the RAN, thereby increasing security of signaling and user data carried by radio bearers secured in this manner.

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.

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.

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).

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 RRCReconflguration 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.

An LTM candidate cell configuration may be included in an RRC IE such as CellGroupConfig, SpCellConfig, or SCellConfig and/or an embedded RRCReconflguration 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”.

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

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.

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

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

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

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

• configuration of measurements for future candidate cells;

• measurement reporting configuration;

• CSI resource configuration;

• CSI reporting configuration;

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

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

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

• information related to security key refresh, e.g., RRC MasterKeyUpdate IE and/or RRC RadioBearerConflg IE that includes SecurityConflg field with SecurityAlgorithmConfig. The term “part of a mobility configuration” may refer to a subset of the elements in the above list, and/or a subset of items comprising any of the elements present (e.g., subset of configurations for DL pre-sync).

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

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

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

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. The terms “CHO,” “CHO execution,” and “CHO execution procedure” refer to the process of a UE evaluating certain conditions configured by the RAN and, upon the fulfilling of such criteria, switching (or changing) from a source cell to a CHO candidate cell (which becomes a target cell) without further involvement of the source cell (e.g., signaling). In switching to the CHO candidate cell, the UE applies an CHO candidate configuration such that the CHO candidate cell becomes the UE’s new special cell (SpCell, e.g., PCell for LTM in MCG or PSCell for LTM in SCG) or its new SCell. In other words, a CHO candidate cell can be a candidate for the UE’s PCell, PSCell, or SCell. Further, when the CHO candidate cell is the PSCell, CHO may also be referred to as CPA, CPC, CP AC, or subsequent CP AC.

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

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

• a first identifier associated to a candidate cell;

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

• a key set change indicator;

• a next hop chaining counter (NCC); and

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

An example security configuration is the masterKeyUpdate IE.

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

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

• When a NAS indication (e.g., NASC) is received masterKeyUpdate IE, the UE forwards the NAS indication to UE NAS layer and updates its NAS security context according to 3GPP TS 33.501 clause 6.9.2.3.4; • When a key set change indication (e.g., keySetChangelndicator) is received and/or is set to ‘true’ (e.g., within mas ter KeyUpdate IE), the UE derives or updates K_§NB based on KAMF, as specified in 3GPP TS 33.501;

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

• UE derives KRRCCIIC and Kup_enc associated with a ciphering algorithm (e.g., cipheringAlgorithm indicated in securityAlgorithmConflg), as specified in 3GPP TS 33.501;

• UE derives KRRCUU and Kupint associated with an integrity protection algorithm (e.g., integrityProtAlgorithm indicated in securityAlgorithmConflg), as specified in 3GPP TS 33.501;

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

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

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

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

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

Figure 8 illustrates a system structure in which some embodiments of the present disclosure may be implemented. The User Equipment (UE) 801 is a wireless terminal, such as a cellular smartphone, sometimes connected to the first RAN node 802 over a wireless interface 804 and sometimes connected to a second RAN node 803, to which the UE 801 is connected over a wireless interface 805.

First RAN node 802 provides a first cell 807, which may be referred to as the UE’s source cell in the context of mobility (e.g., LTM or L3 HO), or the UE’s serving cell, Special Cell, SpCell, PCell, or PSCell in the context of CA and/or DC. The second RAN node 803 provides a second cell 808, which may be referred to as neighbor cell to the serving cell or, in the context of mobility, as target cell, candidate cell, LTM candidate cell, or inter -CU LTM candidate cell for the UE. First RAN node 802 and second RAN node 803 may be gNBs of an NG-RAN, and may be interconnected over an interface 806, which may be an Xn or Xn-C type of interface. However, the first RAN node and the second RAN node are not necessarily interconnected.

In the context of mobility, first RAN node 802 may be referred to as source RAN node since it provides the source cell for UE mobility. Likewise, second RAN node 803 may be referred to as target or candidate RAN node since it provides the target or candidate cell for UE mobility. In some cases, such as during intra-gNB or intra-CU mobility, the first RAN node and the second RAN node may be a single RAN node.

In case of a distributed CU/DU RAN architecture, each of the first RAN node 802 and/or the second RAN node 803 may be divided into a CU and one or more DUs. As shown in Figure 8, first RAN node 802 includes CU 809 and DU 810, which may be referred to as serving CU/DU or source CU/DU for the UE. Likewise, second RAN node 803 includes CU 812 and DU 810, which may be referred to as target CU/DU or candidate CU/DU for the UE. In some cases, such as during intra-gNB or intra-CU mobility, source CU 809 and target CU 812 may be a single CU.

CU 809 and DU 810 are connected over an interface 811, which may be an Fl type of interface in case of NG-RAN. Correspondingly CU 812 and DU 813 are connected over an interface 814, which may be an Fl type of interface in case of NG-RAN.

First RAN node 802 and second RAN node 803 may be connected to a third network node 815 over interfaces 816 and 817, respectively. Third network node 815 may be a core network node, such as a UPF or an AMF. In the latter case, interfaces 816 and 817 are both an NG type of interface or an N2 reference point. Sometimes the third network node may comprise two different network nodes, such as a source AMF connected with the first RAN node and a target AMF connected with the second RAN node. These two network nodes are inter-connected over an interface, such as an N14 reference point or an Namf type of service-based interface.

Some embodiments include a security set identifier (referred to in ASN.l terminology as securityCellSetld) introduced per candidate configuration, e.g., for an LTM candidate configuration. The identifier is used to determine whether a security key change is to be performed at execution of a mobility procedure (e.g., LTM cell switch) to the candidate cell based on the corresponding candidate configuration.

In addition to the security set identifier in the candidate configuration, the UE may also be configured with a corresponding identifier (referred to in ASN. l terminology as servingSecurityCellSetld) associated with its current serving cell. This identifier can be included in the RRCReconfiguration message used to convey a mobility configuration, such as an LTM configuration that includes respective LTM candidate configurations. The principle behind the use of the securityCellSetld and servingSecurityCellSetld is that the UE will need perform security key change in conjunction with a mobility procedure only when servingSecurityCellSetld (for current source cell) and securityCellSetld (for candidate cell) are not the same, do not match, etc. This indicates that the source and candidate cells are associated with different sets or groups of cells, which may be provided by different CUs, gNBs, etc. When securityCellSetld and servingSecurityCellSetld avQ the same value, this indicates the source and candidate cells are associated with a single set or group of cells, which may be provided by a single CUs, a single gNB, etc. In another option, when the source and candidate cells belong to different groups this does not necessarily mean that these cells belong to two different CUs; instead, the RAN may have initiated a security key change for source and target cells that belong to the same CU.

In some embodiments, the UE also uses an internal variable to store the security set identifier associated with its current serving cell. In some variants, the variable VarServingSecurityCellSetID used for subsequent CP AC can be reused for this purpose. In some variants, a new variable (e.g., VarLTM-ServingSecurityCellSetID') can be introduced for this purpose.

In some embodiments, when the UE receives from its source RAN node a mobility command that triggers execution of a mobility procedure to a candidate cell, the UE compares the stored servingSecurityCellSetld with the securityCellSetld of the candidate configuration for the selected candidate cell (indicated by a reference or pointer in the command, such as LTM candidate cell ID in the LTM cell switch command). If the values are the same, the UE does not change the security key. If the values are different, the UE uses the parameters for security key change provided in the previously received mobility configuration to refresh the security key. From this point the security key change follows legacy procedure described in 3GPP TS 38.331 (v!8.0.0) section 5.3.5.7.

In addition, when the comparison indicates need for security key change, the method comprises the UE performing one or more link layer reset operations, to avoid PDUs encrypted with the old security key being sent/received after security key change. Link layer link reset operations may include PDCP re-establishment, RLC re-establishment, and/or MAC reset. For example, an LTM candidate configuration received by the UE may include one or more indications (e.g., fields) for PDCP re-establishment, RLC re-establishment, and/or MAC reset, but in these are applied by the UE only when a security key change is required, based on the comparison described above. In other words, the fields may be present, but the UE performs link layer reset operations according to the included fields only when it has determined that security key refresh is needed and has been performed; otherwise, the UE ignores these fields. As an alternative, such fields are not included, and the UE anyways performs link layer reset operations in response to a security key refresh.

In other embodiments, rather than using security set IDs, the UE may be configured with candidate configurations that are associated with one or more sets or groups. For example, the UE may be configured with two sets or groups of candidate cells. When the UE executes a mobility procedure between two cells of the same group or set, the UE does not perform security key change. When the UE executes a mobility procedure between two cells of different groups or sets, the UE performs security key change. These embodiments are based on the same principle that the UE determines whether to perform key refresh or not based on mobility candidate and the UE’s current source cell, which is not necessarily the source cell in which the UE received the candidate configuration being applied during the execution of the mobility procedure.

Once the mobility procedure has been executed, there is also a need to update the stored identifier corresponding to the serving cell servingSecurityCellSetld) with the value of the identifier for the selected candidate cell (secur ityCellSetld). This can be done by overwriting the value of servingSecurityCellSetld. with the value of securityCellSetld for the selected candidate, which ensures the correct serving cell identifier to be used for comparison in relation the next mobility procedure.

In some embodiments, the UE is configured with additional information for input to security key derivation, which may control whether horizontal or vertical security key generation is to be used. For this purpose, a new field with this additional information can be added to the mobility configuration (e.g., LTM configuration). Alternatively, an existing field in a candidate configuration (e.g., LTM candidate configuration) can convey this information. In either case, the information carried in this field can be the same as or similar to the information currently included in masterKeyUpdate field, including but not limited to the following:

• An indication of whether the UE shall derive a new master MN security key (e.g., K_§NB) from a key received from the core network (e.g., KAMF). This information can be carried in the Boolean type field, such as keySetChangelndicator .

• When key update is required, an indication of whether vertical or horizontal key derivation should be used;

• Information needed for vertical key derivation, e.g., next hop chaining counter (NCC).

• Information related to NAS, carried in an RRC transparent container and used by the UE at inter-system handover to NR.

• Security algorithms to be used for integrity protection and ciphering on the radio bearers. However, these algorithms do not need to be indicated together with the above items, but instead may be included in a separate field associated with radio bearer configuration. In some embodiments, the additional information for input to security key derivation can be single-use, such that UE can use it during the next execution of a mobility procedure only and afterward discards it. In other embodiments, the additional information for input to security key derivation can be a list, in which each element of the list includes input to security key derivation to be used once and then discarded. In other embodiments, the additional information for input to security key derivation can be multi-use, i.e., the same information used each time a security key change is triggered.

As an example of the multi-element list embodiments, the UE receive the following list of security indications:

• Security indication 1 [new key from old key, horizontal]

• Security indication 2 [new key from info from CN, vertical]

When the UE executes a first mobility procedure (e.g., LTM cell switch), the UE selects and applies security indication 1 and, when complete, the UE discards this element by deletion from memory or by marking it as no longer invalid. When the UE executes a second mobility procedure, e.g., LTM cell switch, is performed by the UE, the UE selects and applies security indication 1 and, when complete, the UE discards this element by deletion from memory or by marking it as no longer invalid. After these two executions, the UE will have no more security indications to use so a third mobility procedure with key update will result in a failure.

In some embodiments, the UE can receive the additional information or an indication thereof in a mobility command (e.g., LTM cell switch command) that triggers execution of a mobility procedure. If the additional information is explicitly included with the command, the UE applies it directly for security key update when performing the mobility procedure. If the additional information is implicit or referential, the UE identifies a previously received additional information that it references, and applies that identified additional information during security key update when performing the mobility procedure.

In the context of the most recent example above, the mobility command that triggers execution of the mobility procedure includes a pointer to security indication 2 in the previously received list, which indicates to the UE that configuration should be used for security key update during execution of the mobility procedure. After completion, the UE discards this element of the list since it is intended to be single-use.

In some variants, the indication received in the mobility command may partially override the security configuration received in advance. For instance, the UE may receive the following information in the mobility command: {ptr^Security indication 2, Key derivation = horizontal}. In this case, the UE will use the previously received security indication 2, where it will derive new key according to security information from the core network, but will use horizontal key derivation instead of the vertical key derivation explicitly indicated by security indication 2..

In some variants, the UE receives an indication of security key change dynamically within the mobility command that triggers execution of the mobility procedure. In one sub-option, the mobility command is integrity protected and/or encrypted based on security keys the UE uses with the serving cell. In addition to the indication, the mobility command may include one or more parameters (e.g., counters) that may be used as input to the security key derivation.

In other embodiments, the UE determines whether to perform a security key update during a mobility operation from a source cell to a candidate/target cell based on security key configurations of the source cell and the candidate/target cell, wherein the candidate/target cell was selected by the UE for a fast recovery procedure. For example, fast recovery is triggered when the UE detects a radio-related failure (e.g., RLF in source cell, mobility procedure failure, etc.), initiates a re-establishment procedure, starts timer T311, and selects a cell. If the UE is configured to perform fast recovery and the selected cell is configured mobility candidate cell (e.g., LTM and/or CHO candidate cell), the UE applies the candidate configuration and selectively performs key refresh depending on based on security key configurations of the source cell and the candidate/target cell.

In the example discussed above, after executing two mobility procedures with security key updates that utilize both single-use elements of the received list, the UE has no more security indications to be used for security key update and thus is unable to update security during execution of subsequent mobility procedure. When security key update is required, the UE may send its serving RAN node an indication that new security information is needed, or a request for such information. In some variants, this indication/request may be a simple one-bit indication. In other variants, the UE may also indicate one or more of the following:

• one or more identifiers of candidate configurations or candidate cells that are currently configured;

• one or more security indications previously received and used by the UE (or identifier thereof);

• a number of security indications available to be used (i.e., received and not discarded);

• a request to provided security-related information only within subsequent mobility commands that trigger execution of mobility procedure.

Although the above description of various embodiments is focused on non-conditional mobility procedures, similar principles may be applied to conditional mobility procedures such as conditional handover (CHO) or conditional LTM, in which a UE applies a candidate configuration that is part of a previously received mobility configuration. When a condition has been fulfilled for a candidate cell in conditional mobility, the UE applies the associated candidate cell configuration (e.g., RRCReconflguration message).

In some embodiments, the UE compares a security cell set identifier of a candidate configuration for a mobility candidate cell with the security cell set identifier of the UE’s current serving cell (e.g., stored servingSecurityCellSetld) in which the execution condition was fulfilled. If there is a match, the UE does not change the security key. If there is no match, the UE uses the parameters for security key change received with the candidate configuration in the mobility configuration to refresh the security keys. It is appreciated that from this point the UE can use the same methods during execution of mobility as described above for the network-triggered case.

Various alternatives may be used for handling of security keys on the network side, as discussed below.

In some embodiments, security keys may be distributed during configuration of a mobility procedure, e.g., in a mobility configuration. When the UE’s serving/source RAN node decides to request a mobility configuration or candidate cell configuration from a candidate RAN node, the source RAN node derives the next security key and includes it in or with the request, such as a HO request or an LTM configuration request. In this way, the next security key to be used is delivered to all RAN nodes that provide candidate cells for the UE’s mobility and is readily available for each RAN node to use during a subsequent mobility procedure of the UE to a candidate cell, such as to decipher an RRCReconfigurationComplete message sent by the UE upon completion of the mobility procedure.

For subsequent mobility after completion of the UE’s initial mobility procedure, the candidate RAN node becomes the UE’s serving RAN node and must perform the operations described above, including deriving a next security key and distribute to all RAN nodes that provide candidate cells for the UE’s subsequent mobility. For this purpose, the same message to request a mobility configuration or candidate cell configuration can be used, including an indication to store a next security key for subsequent mobility. Alternatively, a special purpose Xn-AP message for security key distribution to configured candidates can be used.

Also for these embodiments, there is a need after mobility execution to remove unused keys from RAN nodes that provide unselected candidate cells. This can be triggered by the source RAN node or the candidate/target RAN node during execution of the mobility procedure. For example, the source RAN node sends a discard indication to all RAN nodes that provided configured candidate cells, except the target RAN node selected for execution of the mobility procedure. As another example, the target RAN node sends a discard indication to all RAN nodes that provided configured candidate cells. This can be an explicit indication or performed implicitly together with the message informing the candidate RAN nodes of the next security key to be used. In other embodiments, security keys may be distributed during execution of the mobility procedure. For example, security keys may be sent from the source RAN node to the target RAN node as part of a message to trigger execution of the mobility procedure, such as an LTM cell switch indication message. In these embodiments, security keys for a UE are only sent to the target RAN node that is selected for execution of the mobility procedure rather than all RAN nodes that provide mobility candidate cells for the UE. Similarly, there is no need after execution to remove unused keys from RAN nodes that provide unselected candidate cells. However, since the target RAN node needs security keys for deciphering an RRCReconfigurationComplete message sent by the UE upon completion of the mobility procedure, the source RAN node may need to confirm that the target RAN node received the security key before the UE transmits the RRCReconfigurationComplete message. An acknowledgement message via the Xn interface can be used for this purpose.

In addition to security key distribution, there is also a need to reconfigure the UE with candidate configuration that include new security parameters to support vertical key derivation or master key set change. For example, vertical security key derivation requires the UE to be provided with a Next Chaining Counter (NCC), which included in the master KeyUpdate field. If the field is not present, the UE infers that horizontal key derivation should be performed based on the current key. Embodiments for UE reconfiguration described below should be performed before mobility execution, but may be performed before or after key distribution discussed above.

In some embodiments, to trigger vertical security key generation, the source RAN node sends the UE a mobility configuration that includes an NCC. In case of LTM, the mobility configuration could be an LTM configuration that includes the masterKeyUpdate field, which includes the NCC field. Alternatively, the masterKeyUpdate field with NCC can be included in a candidate configuration for an LTM candidate cell. After execution of the mobility procedure with vertical key derivation, in order to configure the UE back to horizontal key derivation, the target (new serving) RAN node sends the UE the mobility configuration which excludes NCC.

In other embodiments, to trigger derivation of a new security key from key material received from the core network (i.e., key set change), the source RAN node sends the UE a mobility configuration with an indication for key set change. In case of LTM, the mobility configuration could be an LTM configuration that includes the masterKeyUpdate field with sub- field keySetChangelndicator set to “true”. Alternatively, this masterKeyUpdate field can be included in a candidate configuration for an LTM candidate cell. After execution of the mobility procedure with key set change, in order to configure the UE back to horizontal key derivation, the target (new serving) RAN node sends the UE the mobility configuration with sub-field keySetChangelndicator set to “false”. Figure 9 shows a signaling diagram for an exemplary inter-CU LTM procedure, according to some embodiments of the present disclosure. The procedure involves a UE (910), a source RAN node (920), and a target RAN node (930). Although the operations shown in Figure 9 are given numerical labels, this is done to facilitate the following explanation rather than to require or imply any particular operational order, unless expressly stated otherwise.

In operation 1, the source RAN node decides to perform the configuration of LTM for the UE, including LTM candidate cell(s) controlled by the target RAN node. The source RAN node transmits, to the target RAN node, a request message to perform LTM configuration of at least one LTM candidate cell. In this example, the message is an LTM CONFIGURATION REQUEST message. The message in a first alternative includes target security key(s) for each requested LTM candidate cell.

In operation 2, the target RAN node generates an LTM candidate configuration for each accepted LTM candidate cell. The target RAN node transmits to the source RAN node a response message including the LTM candidate configuration(s). In this example, the message is a LTM CONFIGURATION REQUEST ACKNOWLEDGE message. In operation 3, the source RAN node sends the UE a reconfiguration message (e.g., RRCReconflguratiori) that includes an LTM configuration, which in turn includes at least one LTM candidate configuration for respective at least one LTM candidate cell. Each LTM candidate configuration includes a security cell identifier (e.g., securityCellSetld) associated with the LTM candidate cell.

In operation 4, the UE stores the received LTM configuration and transmits, to the source RAN node, a response message, such as an RRCReconflgurationComplete message to confirm that the LTM configuration has been received. In operation 5, the UE performs LI measurements on LTM candidate cell(s) and/or serving cell(s), according to the received LTM configuration and transmits lower-layer measurement reports to the source RAN node.

In operation 6, the source RAN node decides to trigger an LTM cell switch procedure for the UE to an LTM candidate cell (referred to as the target cell) controlled by the target RAN node. In operation 7, the source RAN node transmits an indication (e.g., LTM CELL SWITCH INDICATION) about the execution of an LTM cell switch procedure to the target RAN node. In one alternative, the indication includes security key(s) to be used in the target cell; in which case, the target RAN node transmits an acknowledgement message (e.g., LTM CELL SWITCH INDICATION ACK) to the source RAN node in operation 8. This acknowledgement message ensures that the target RAN node has received the security key(s) before the UE is triggered to perform the mobility procedure, which is an LTM cell switch in this example.

In operation 9, the source RAN node transmits an LTM cell switch command to the UE to trigger the LTM cell switch procedure. The LTM cell switch command contains an indication of LTM candidate configuration for the target cell. In operation 10, the UE executes the LTM cell switch including applying the indicated LTM candidate configuration for the target cell. The UE determines the need for security key change. In this example, the UE compares the stored servingSecurityCellSetld. for the current serving cell with the securityCellSetld of the indicated LTM candidate cell. If they are different, the UE derives new security keys based on the information in masterKeyUpdate and performs security key refresh.

In operation 11, the UE transmits an RRCReconflgurationComplete message in the target cell to the target RAN node according to the applied LTM candidate configuration, possibly after performing random access to the target cell. If the UE performed security key refresh in the operation 10, the message is ciphered and/or integrity protected based on the newly derived security keys. The message also indicates that the UE has successfully performed security key refresh.

Figure 10 shows a signaling diagram for an exemplary inter-CU conditional LTM procedure, according to some embodiments of the present disclosure. The procedure involves a UE (1010), a source RAN node (1020), and a target RAN node (1030). Although the operations shown in Figure 10 are given numerical labels, this is done to facilitate the following explanation rather than to require or imply any particular operational order, unless expressly stated otherwise.

In operation 1, the source RAN node decides to perform configuration of conditional LTM for the UE, including for one or more LTM candidate cells provided by the target RAN node. The source RAN node transmits to the target RAN node a request to perform configuration of one or more LTM candidate cells for the UE. In this example, the request is an LTM CONFIGURATION REQUEST message. In some embodiments, the request includes target security keys for each of the one or more LTM candidate cells requested to be configured. In operation 2, the target RAN node generates an LTM candidate configuration for each accepted LTM candidate cell and sends the source RAN node a response including the generated LTM candidate configuration(s). In this example, the response is a LTM CONFIGURATION REQUEST ACKNOWLEDGE) message.

In operation 3, the source RAN node transmits to the UE a reconfiguration message (e.g., RRCReconflguratiori) that includes an LTM configuration, which in turn includes at least one LTM candidate configuration for respective at least one LTM candidate cell. Each LTM candidate configuration includes a security cell identifier (e.g., securityCellSetld) associated with the LTM candidate cell. The reconfiguration message also includes LTM execution conditions.

In operation 4, the UE stores the received LTM configuration and sends the source RAN node a response (e.g., RRCReconflgurationComplete message) to confirm that the LTM configuration has been received. In operation 5, the UE performs LI measurements on the configured LTM candidate cells and/or the UE’s serving cells, according to the received LTM configuration, and evaluates the measurements against the LTM execution conditions. When an execution condition is fulfilled by an LTM candidate cell, the UE executes the LTM cell switch including applying the LTM candidate configuration for the LTM candidate cell for which the condition was fulfilled.

In operation 6, the UE determines the need for security key change. If security key change is needed, the UE derives new security keys based on the information in masterKeyUpdate and performs security key refresh. In operation 7, the UE transmits an RRCReconfigurationComplete message in the target cell to the target RAN node according to the applied LTM candidate configuration, possibly after performing random access to the target cell. If the UE performed security key refresh in the operation 6, the message is ciphered and/or integrity protected based on the newly derived security keys. The message also indicates that the UE has successfully performed security key refresh.

Figure 11 shows a flowchart of an exemplary security procedure for a UE, according to some embodiments of the present disclosure. In this procedure, the UE determines whether to perform security key change in relation to execution of a mobility procedure (e.g., LTM), based on a received security configuration. Although the operations shown in Figure 11 are given numerical labels, this is done to facilitate the following explanation rather than to require or imply any particular operational order, unless expressly stated otherwise.

In operation 5000, the UE receives a mobility configuration, which includes a security configuration. In operation 5001, the UE executes a mobility procedure. In operation 5002, the UE determines whether a security key change is needed based on the received security configuration. In operation 5003, if the UE determines a security key change is needed in operation 5002, the UE proceeds to operation 5004 where the UE performs the needed security key change. Otherwise the UE proceeds to operation 5005 where the UE refrains from performing a security key change.

Figure 12 shows a flowchart of an exemplary LTM procedure for a UE, according to other embodiments of the present disclosure. Although the operations shown in Figure 12 are given numerical labels, this is done to facilitate the following explanation rather than to require or imply any particular operational order, unless expressly stated otherwise.

In operation 6000, the UE receives an LTM configuration, which includes one or more LTM candidate cell configurations. Each LTM candidate cell configuration includes a security cell set identifier securityCellSetID). In addition, the LTM configuration includes a security cell set identifier for the UE’s serving cell servingSecurityCellSetID) as well as information needed for security key derivation masterKeyUpdate). In operation 6002, the UE receives an LTM cell switch command, which includes an identifier corresponding to one of the received LTM candidate cell configurations. In operation 6004, the UE determines the values of the security cell set identifier of the current serving cell and the security cell set identifier of the LTM candidate cell configuration identified in operation 6002.

In operation 6006, the UE compares the two values obtained in operation 6004. If the values are the same, the UE proceeds to operation 6012, where the UE concludes the LTM cell switch procedure by transmitting an LTM cell switch complete message, typically a RRCReconflgurationComplete message, encrypted using the security keys.

If the values compared in operation 6006 are different, this indicates a need to derive new security keys so the UE proceeds instead to operation 6008, where the UE derives new security keys using the key derivation parameters included in masterCellUpdate . In operation 6010, the UE re-establishes its link layer with the RAN, including the PDCP and RLC protocols. This is necessary to preventing PDUs encrypted with the security keys used before the LTM cell switch from being received after the LTM cell switch, which will result in deciphering failure. The UE then proceeds to operation 6012.

Some embodiments of the present disclosure may be realized as procedural text in a 3GPP specification, such as 3GPP TS 38.331 (vl8.1.0)NRRRC specification. In the examples are given below, underline indicates text added to existing 3GPP TS 38.331, while ellipses indicate existing text omitted for conciseness. In this example, the added procedural text is used to trigger reestablishment of PDCP and RLC protocols. An alternative approach would be to use existing fields in RadioBearerConflg and CellGroupConflg IES to trigger these operations.

*** Begin exemplary 3GPP TS 38.331 text ***

5.3.5.7 AS Security key update

The UE shall: l>if UE is connected to E-UTRA/EPC or E-UTRA/5GC:

2>upon reception of sk-Counter as specified in TS 36.331 [10]:

3> update the S-K_§NB key based on the K_£NB key and using the received sk-Counter value, as specified in TS 33.401 [30] for EN-DC, or TS 33.501 [11] for NGEN-DC;

3>derive the KRRCCIIC and Kup_enc keys as specified in TS 33.401 [30] for EN-DC, or TS 33.501 [11] for NGEN-DC;

3>derive the KRRCint and Kupint keys as specified in TS 33.401 [30] for EN-DC or TS 33.501 [11] for NGEN-DC. l>else if this procedure was initiated due to reception of the masterKeyUpdate or by LTM cell switch execution:

2> if the nas-Container is included in the received masterKeyUpdate'. 3> forward the nas-Container to the upper layers;

2> if the keySetChangelndicator is set to true'.

3>derive or update the K_§NB key based on the KAMF key, as specified in TS 33.501 [11]; 2> else:

3> derive or update the K_§NB key based on the current K_§NB key or the NH, using the nextHopChainingCount value indicated in the received master KeyUpdate, as specified in TS 33.501 [11];

2> store the nextHopChainingCount value;

2> derive the keys associated with the K_§NB key as follows:

3>if the securityAlgorithmConflg is included in SecurityConflg'.

4> derive the KRRCenc and Kup_enc keys associated with the cipheringAlgorithm indicated in the securityAlgorithmConflg, as specified in TS 33.501 [11];

4> derive the KRRCint and Kupint keys associated with the integrityProtAlgorithm indicated in the securityAlgorithmConflg, as specified in TS 33.501 [11];

3>else:

4> derive the KRRCenc and Kup_enc keys associated with the current cipheringAlgorithm, as specified in TS 33.501 [11];

4> derive the KRRCint and Kupint keys associated with the current integrityProtAlgorithm, as specified in TS 33.501 [11],

NOTE 1 : Ciphering and integrity protection are optional to configure for the DRBs.

[... ]

5.3.5.18.6 LTM cell switch execution

Upon the indication by lower layers that an LTM cell switch procedure is triggered, or upon performing LTM cell switch following cell selection performed while timer T311 was running, as specified in 5.3.7.3, the UE shall:

1> release/ clear all current dedicated radio configuration associated with the cell group for which the LTM cell switch procedure is triggered except for the following:

- the logicalChannelldentity and logicalChannelldentityExt of RLC bearers configured in RLC-BearerConfig and the associated RLC entities, their state variables, buffers, and timers;

- the UE variables VarLTM-Config, VarLTM-ServingCellNoResetID, and VarLTM- ServingCellUE-MeasuredTA-ID .

[... ]

1> if the value of field securityCellSetld contained within the LTM-Candidate IE in VarLTM- Config indicated by lower layers or for the selected cell in accordance with 5, 3, 7, 3 is not equal to the value of Itm-ServingSecurityCellSetID within VarLTM-

ServingSecurityCellSetlD'.

2> re-establish RLC and PDCP entities of all configured SRBs and DRBs;

2> perform security key update procedure as specified in 5, 3, 5, 7. using the masterKeyUpdate field in LTM-Conflg,'

2>ifthe current VarSerwngSecurityCellSetID includes servingSecurityCellSetld;

3>replace the value of servingSecurityCellSetld within VarServingSecurityCellSetlD with the value of securityCellSetld associated with the selected cell;

2> else

3> store the servingSecurityCellSetld within VarServingSecurityCellSetlD with the value of securityCellSetld associated with the selected cell;

[ . ] l>release the radio bearer(s) and the logical channel(s) that are part of the current UE configuration but not part of the LTM candidate configuration either indicated by lower layers or for the selected cell in accordance with 5.3.7.3, or the LTM reference configuration (in case the LTM candidate configuration does not include Itm- ConfigComplete).

NOTE 2: When Itm-ConflgComplete is not included for an LTM candidate configuration, before an LTM cell switch is triggered a UE implementation may generate and store an RRC reconfiguration message by applying the received LTM candidate configuration on top of the LTM reference configuration, and the stored RRC reconfiguration message is applied when the LTM cell switch is triggered.

*** End exemplary 3GPP TS 38.331 text ***

Figure 13 shows an ASN.l data structure for an exemplary RRC LTM-Candidate IE which conveys an LTM candidate configuration to be added or modified by a UE. Of particular interest to the present disclosure is the securityCellSetld field, which the UE uses to determine whether to perform a security update upon applying this LTM candidate configuration when executing LTM cell switch.

Figure 14 shows an ASN.1 data structure for an exemplary RRC LTM-Conflg IE, which is used to indicate LTM candidate configurations to add, modify, or release, as well as various other LTM related information. This IE includes a servingSecurityCellSetld field that identifies the security cell set for the UE’s serving PCell, as well as a masterKeyUpdate IE that can include similar contents as discussed above (e.g., keySetChangelndicator , nextHopeChainingCounter , etc.).

Figure 15 shows an ASN.l data structure for an exemplary VarLTMServingSecurityCellSetID internal UE variable, which is used to store the security cell set ID of the UE’s serving PCell in some embodiments.

Various features of the embodiments summarized above correspond to various operations illustrated in Figures 16-17, 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 16-17 may be used cooperatively to provide various benefits, advantages, and/or solutions to problems described herein. Although Figures 16-17 show specific blocks in particular orders, the operations of the exemplary methods may be performed in different orders than shown and may be combined and/or divided into blocks having different functionality than shown. Optional blocks or operations are indicated by dashed lines.

In particular, Figure 16 shows an exemplary method (e.g., procedure) for a UE configured for mobility between cells of a RAN, according to various embodiments of the present disclosure. The exemplary method can be performed by a UE (e.g, wireless device) such as described elsewhere herein.

The exemplary method includes the operations of block 1610, where the UE receives, from a first RAN node via a serving cell, a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells. Each candidate configuration includes a security cell set identifier for the mobility candidate cell (e.g., securityCellSetld). The exemplary method also includes the operations of block 1620, where the UE receives from the first RAN node a mobility command for execution of a mobility procedure from the serving cell. The mobility command identifies one of the mobility candidate cells as a target cell for the mobility procedure. The exemplary method also includes the operations of block 1640, where during execution of the mobility procedure to the target cell in accordance with the mobility command, the UE selectively updates a plurality of access stratum (AS) security keys based on the security cell set identifier for the target cell and on a security cell set identifier for the serving cell (e.g., servingSecurityCellSetld). The exemplary method also includes the operations of block 1650, where the UE transmits, to a second RAN node that provides the target cell, a message indicating that the mobility procedure is complete. The message is secured using at least one of the selectively updated AS security keys.

In some embodiments, the mobility configuration also includes the security cell set identifier for the serving cell. In some embodiments, the AS security keys include a master MN security key (e.g., K_§NB) and the following security keys derived from the master MN security key: a first key for integrity protection of signaling (e.g., KRRCint), a second key for ciphering of signaling (e.g., KRRCenc), a third key for integrity protection of user data (e.g., Kupint), and a fourth key for ciphering of user data (e.g., Kup_enc). In some of these embodiments, the message indicating that the mobility procedure is complete is an RRCReconflgurationComplete message and is secured using at least one of the first and second keys.

In some embodiments, selectively updating the plurality of AS security keys based on the security cell set identifier for the target cell and on the security cell set identifier for the serving cell in block 1640 includes the following operations labelled with corresponding sub-block numbers:

• (1641) determining whether there is a match between the security cell set identifier for the target cell and the security cell set identifier for the serving cell;

• (1643) when no match is determined, performing an update of the plurality of AS security keys based on a first security configuration received from the first RAN node; and

• (1645) when a match is determined, refraining from updating the plurality of AS security keys.

In some of these embodiments, the first security configuration (e.g., a masterKeyUpdate IE) includes one of more of the following parameters:

• an indication of whether a keyset change is needed;

• an indication whether horizontal or vertical key derivation should be used;

• a next-hop chaining counter (NCC) usable for vertical key derivation; and

• a container of non-AS (NAS) information.

In some variants of these embodiments, the mobility command includes one or more parameters that override corresponding parameters of the first security configuration. In such case, the update of the plurality of AS security keys is performed based on the first security configuration updated according to the one or more parameters included with the command.

In some variants of these embodiments, selectively updating the plurality of AS security keys in block 1640 also includes the operations of sub-block 1644, where after performing the update in sub-block 1643, the UE selectively discards or retains the first security configuration based on whether the first security configuration is single-use or multi-use.

In some variants of these embodiments, the first security configuration is received in the mobility configuration separate from the one or more candidate configurations, and is associated with all of the one or more mobility candidate cells. In other variants of these embodiments, the first security configuration is received as part of the candidate configuration for the target cell, and is associated only with the target cell.

In some variants of these embodiments, the mobility configuration includes a plurality of security configurations and selectively updating the plurality of AS security keys in block 1640 also includes the operations of sub-block 1642, where when no match is determined, the UE selects the first security configuration from the plurality of security configurations based on one of the following:

• an identifier of the first security configuration, received with the command; or

• a predetermined order of the plurality (e.g., an ordered list), which indicates the first security configuration as next available.

In other variants of these embodiments, performing the update of the plurality of AS security keys based on the security configuration received from the first RAN node in sub-block 1643 includes the following operations:

• determining that no security configurations are available to use for update of the plurality of AS security keys; and

• sending to the second RAN a request for one or more security configuration,

In such case, the first security configuration is received from the RAN node in response to the request. In some further variants, the request includes one or more of the following:

• identifiers of the one or more candidate configurations;

• identifiers of the one or more or mobility candidate cells;

• one or more security configurations previously received and discarded by the UE, or identifiers thereof; and

• a request to provided security configurations only within subsequent mobility commands that trigger execution of mobility procedures.

In some embodiments, selectively updating the plurality of AS security keys based on the security cell set identifier for the target cell and on the security cell set identifier for the serving cell in block 1640 includes the following operations, labelled with corresponding sub-block numbers:

• (1646) when no match is determined, performing one or more link layer reset operations such that the updated plurality of AS security keys are used for link layer protocol data units, PDUs, sent or received after the update; and

• (1647) when a match is determined, refraining from performing any link layer reset operations.

In some of these embodiments, the one or more link layer link reset operations include one or more of the following: packet data convergence protocol (PDCP) layer re-establishment; radio link control (RLC) layer re-establishment; and medium access control (MAC) layer reset.

In some embodiments, the mobility command includes an update indication of whether a security key update is needed in conjunction with the mobility procedure, and selectively updating the plurality of AS security keys in block 1640 is further based on the update indication. In some embodiments, the security cell set identifier for the serving cell is stored in a UE variable and the exemplary method also includes the operations of block 1660, where the UE updates the UE variable to contain the security cell set identifier for the target cell after executing the mobility procedure.

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

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

In other embodiments, the mobility procedure is a conditional mobility procedure, each candidate configuration includes an associated execution condition, and the exemplary method also includes the operations of block 1630, where after receiving the command, the UE determines that the execution condition associated with the target cell is fulfilled. In such case, execution of the mobility procedure is responsive to determining that the execution condition associated with target cell is fulfilled in block 1630. In some of these embodiments, the mobility procedure is one of the following: an initial layer-3 (L3) conditional handover (CHO), a subsequent L3 CHO, an initial conditional layer-l/layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent conditional LTM cell switch.

In addition, Figure 17 shows an exemplary method (e.g., procedure) for a first RAN node configured to facilitate mobility between cells by UEs, according to various embodiments of the present disclosure. The exemplary method can be performed by a RAN node (e.g, base station, eNB, gNB, ng-eNB, DU, etc.) such as described elsewhere herein.

The exemplary method includes the operations of block 1730, where the first RAN node sends, to a UE via a serving cell, a mobility configuration comprising: a security cell set identifier for the serving cell (e.g., servingSecurityCellSetld), and one or more candidate configurations for respective one or more mobility candidate cells. Each candidate configuration includes a security cell set identifier for the mobility candidate cell (e.g., secur ityCellSetld). The exemplary method also includes the operations of block 1760, where the first RAN node sends to the UE a mobility command for execution of a mobility procedure from the serving cell. The mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure. The security cell set identifier for the target cell and the security cell set identifier for the serving cell indicate whether a plurality of AS security keys should be updated by the UE during execution of the mobility procedure to the target cell. In some embodiments, the AS security keys include a master MN security key (e.g., K_§NB) and the following security keys derived from the master MN security key: a first key for integrity protection of signaling (e.g., KRRCint), a second key for ciphering of signaling (e.g., KRRCenc), a third key for integrity protection of user data (e.g., Kupint), and a fourth key for ciphering of user data (e.g., KUP enc)-

In some embodiments, no match between the security cell set identifiers for the target cell and the serving cell indicates that the plurality of AS security keys should be updated by the UE based on a first security configuration provided by the first RAN node. Also, a match between the security cell set identifiers for the target cell and the serving cell indicates that the plurality of AS security keys should not be updated by the UE during execution of the mobility procedure. In some of these embodiments, the security configuration (e.g., master KeyUpdate IE) includes one of more of the following parameters:

• an indication of whether a keyset change is needed;

• an indication whether horizontal or vertical key derivation should be used;

• a next-hop chaining counter (NCC) usable for vertical key derivation; and

• a container of non-AS (NAS) information.

In some variants of these embodiments, the mobility command includes one or more parameters that override corresponding parameters of the first security configuration.

In some variants of these embodiments, the first security configuration is sent in the mobility configuration separate from the one or more candidate configurations, and is associated with all of the one or more mobility candidate cells. In other variants of these embodiments, the first security configuration is sent as part of the candidate configuration for the target cell, and is associated only with the target cell.

In some variants of these embodiments, the mobility configuration includes a plurality of security configurations, and one of the following indicates that the first security configuration should be selected, from the plurality of security configurations, to be used for updating the plurality of AS security keys:

• an identifier of the first security configuration, sent with the command; or

• a predetermined order of the plurality, which indicates the first security configuration as next available.

In other variants of these embodiments, the exemplary method also includes the operations of block 1740, where the first RAN node receives from the UE a request for one or more security configurations. The first security configuration is sent to the UE in response to the request. In some further variants, the request includes one or more of the following:

• identifiers of the one or more candidate configurations; • identifiers of the one or more or mobility candidate cells;

• one or more security configurations previously received and discarded by the UE, or identifiers thereof; and

• a request to provide security configurations only within subsequent mobility commands that trigger execution of mobility procedures.

In some embodiments, the mobility command includes an update indication of whether a security key update is needed in conjunction with the mobility procedure. In some embodiments, the exemplary method also includes the following operations, labelled with corresponding block numbers:

• (1710) sending to the second RAN node a request for a candidate configurations for the one or more mobility candidate cells, wherein the request includes one or more AS security keys to be used by the UE after a next security key update; and

• (1720) receiving from the second RAN node a response including the one or more candidate configurations for the respective mobility candidate cells.

In other embodiments, the exemplary method also includes the operations of block 1750, where the first RAN node sends to the second RAN node an indication of the mobility procedure for the UE from the serving cell to the target cell. The request includes one or more AS security keys to be used by the UE after a next security key update.

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

In some embodiments, the mobility procedure is anon-conditional mobility procedure and the mobility command causes the UE to execute the mobility procedure. In some of these embodiments, the mobility procedure is one of the following: an initial L3 HO, a subsequent L3 HO, an initial LTM cell switch, or a subsequent LTM cell switch.

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

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.

Figure 18 shows an example of a communication system 1800 in accordance with some embodiments. In this example, communication system 1800 includes a telecommunication network 1802 that includes an access network 1804 (e.g., RAN) and a core network 1806, which includes one or more core network nodes 1808. Access network 1804 includes one or more access network nodes, such as network nodes 1810a-b (one or more of which may be referred to as network nodes 1810), or any other similar 3GPP access nodes or non-3GPP access points. Moreover, as will be appreciated by those of skill in the art, a network node is not necessarily limited to an implementation in which a radio portion and a baseband portion are supplied and integrated by a single vendor. Thus, it will be understood that network nodes include disaggregated implementations or portions thereof. For example, in some embodiments, telecommunication network 1802 includes one or more Open-RAN (ORAN) network nodes. An ORAN network node is a node in telecommunication network 1802 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 1802, including one or more network nodes 1810 and/or core network nodes 1808.

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. , r App), or any combination thereof (the adj ective “open” designating support of an ORAN specification). The network node may support a specification by, for example, supporting an interface defined by the ORAN specification, such as an Al, Fl, Wl, El, E2, X2, Xn interface, an open fronthaul user plane interface, or an open fronthaul management plane interface. Moreover, an ORAN access node may be a logical node in a physical node. Furthermore, an ORAN network node may be implemented in a virtualization environment (described further below) in which one or more network functions are virtualized. For example, the virtualization environment may include an O-Cloud computing platform orchestrated by a Service Management and Orchestration Framework via an O-2 interface defined by the O-RAN Alliance or comparable technologies. Network nodes 1810 facilitate direct or indirect connection of UEs, such as by connecting UEs 1812a-d (one or more of which may be referred to as UEs 1812) to core network 1806 over one or more wireless connections.

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 1800 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 1800 may include and/or interface with any type of communication, telecommunication, data, cellular, radio network, and/or other similar type of system.

UEs 1812 may be any of a wide variety of communication devices, including wireless devices arranged, configured, and/or operable to communicate wirelessly with network nodes 1810 and other communication devices. Similarly, network nodes 1810 are arranged, capable, configured, and/or operable to communicate directly or indirectly with UEs 1812 and/or with other network nodes or equipment in telecommunication network 1802 to enable and/or provide network access, such as wireless network access, and/or to perform other functions, such as administration in telecommunication network 1802.

In the depicted example, core network 1806 connects network nodes 1810 to one or more hosts, such as host 1816. 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 1806 includes one or more core network nodes (e.g., 1808) 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 also applicable to the corresponding components of core network node 1808. 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).

Host 1816 may be under the ownership or control of a service provider other than an operator or provider of access network 1804 and/or telecommunication network 1802, and may be operated by the service provider or on behalf of the service provider. Host 1816 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. As a whole, communication system 1800 of Figure 18 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) 802.18 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 1802 is a cellular network that implements 3GPP standardized features. Accordingly, telecommunication network 1802 may support network slicing to provide different logical networks to different devices that are connected to telecommunication network 1802. For example, telecommunication network 1802 may provide Ultra Reliable Low Latency Communication (URLLC) services to some UEs, while providing Enhanced Mobile Broadband (eMBB) services to other UEs, and/or Massive Machine Type Communication (mMTC)/Massive loT services to yet further UEs.

In some examples, UEs 1812 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 1804 on a predetermined schedule, when triggered by an internal or external event, or in response to requests from access network 1804. 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).

In the example, hub 1814 communicates with access network 1804 to facilitate indirect communication between one or more UEs (e.g., 1812c and/or 1812d) and network nodes (e.g., 1810b). In some examples, hub 1814 may be a controller, router, content source and analytics, or any of the other communication devices described herein regarding UEs. For example, hub 1814 may be a broadband router enabling access to core network 1806 for the UEs. As another example, hub 1814 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 1810, or by executable code, script, process, or other instructions in hub 1814. As another example, hub 1814 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 1814 may be a content source. For example, for a UE that is a VR headset, display, loudspeaker or other media delivery device, hub 1814 may retrieve VR assets, video, audio, or other media or data related to sensory information via a network node, which hub 1814 then provides to the UE either directly, after performing local processing, and/or after adding additional local content. In still another example, hub 1814 acts as a proxy server or orchestrator for the UEs, in particular if one or more of the UEs are low energy loT devices.

Hub 1814 may have a constant/persistent or intermittent connection to network node 1810b. Hub 1814 may also allow for a different communication scheme and/or schedule between hub 1814 and UEs (e.g., 1812c and/or 1812d), and between hub 1814 and core network 1806. In other examples, hub 1814 is connected to core network 1806 and/or one or more UEs via a wired connection. Moreover, hub 1814 may be configured to connect to an M2M service provider over access network 1804 and/or to another UE over a direct connection. In some scenarios, UEs may establish a wireless connection with network nodes 1810 while still connected via hub 1814 via a wired or wireless connection. In some embodiments, hub 1814 may be a dedicated hub - that is, a hub whose primary function is to route communications to/from the UEs from/to network node 1810b. In other embodiments, hub 1814 may be a non-dedicated hub - that is, a device which is capable of operating to route communications between the UEs and network node 1810b, but which is additionally capable of operating as a communication start and/or end point for certain data channels.

In some embodiments, any of network nodes 1810 may be configured to perform operations attributed to a RAN node in various embodiments described above, including the exemplary method shown in Figure 17. In some embodiments, any of UEs 1812 may be configured to perform operations attributed to a UE in various embodiments described above, including the exemplary method shown in Figure 16.

Figure 19 shows a UE 1900 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 3GPP, including a narrow band internet of things (NB-IoT) UE, a machine type communication (MTC) UE, and/or an enhanced MTC (eMTC) UE. 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).

UE 1900 includes processing circuitry 1902 that is operatively coupled via a bus 1904 to an input/output interface 1906, a power source 1908, a memory 1910, a communication interface 1912, and/or any other component, or any combination thereof. Certain UEs may utilize all or a subset of the components shown in Figure 19. 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.

Processing circuitry 1902 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 1910. Processing circuitry 1902 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 1902 may include multiple central processing units (CPUs).

In the example, input/output interface 1906 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 1900. Examples of an input device include a touch-sensitive or presence-sensitive display, a camera (e.g., a digital camera, a digital video camera, a web camera, etc.), a microphone, a sensor, a mouse, a trackball, a directional pad, a trackpad, a scroll wheel, a smartcard, and the like. The presence-sensitive display may include a capacitive or resistive touch sensor to sense input from a user. A sensor may be, for instance, an accelerometer, a gyroscope, a tilt sensor, a force sensor, a magnetometer, an optical sensor, a proximity sensor, a biometric sensor, etc., or any combination thereof. An output device may use the same type of interface port as an input device. For example, a Universal Serial Bus (USB) port may be used to provide an input device and an output device.

In some embodiments, power source 1908 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 1908 may further include power circuitry for delivering power from power source 1908 itself, and/or an external power source, to the various parts of UE 1900 via input circuitry or an interface such as an electrical power cable. Delivering power may be, for example, for charging of power source 1908. Power circuitry may perform any formatting, converting, or other modification to the power from power source 1908 to make the power suitable for the respective components of UE 1900 to which power is supplied.

Memory 1910 may be or be configured to include memory such as random access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), magnetic disks, optical disks, hard disks, removable cartridges, flash drives, and so forth. In one example, memory 1910 includes one or more application programs 1914, such as an operating system, web browser application, a widget, gadget engine, or other application, and corresponding data 1916. Memory 1910 may store, for use by UE 1900, any of a variety of various operating systems or combinations of operating systems.

Memory 1910 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 1910 may allow UE 1900 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 1910, which may be or comprise a device-readable storage medium.

Processing circuitry 1902 may be configured to communicate with an access network or other network using communication interface 1912. Communication interface 1912 may comprise one or more communication subsystems and may include or be communicatively coupled to an antenna 1922. Communication interface 1912 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 1918 and/or a receiver 1920 appropriate to provide network communications (e.g., optical, electrical, frequency allocations, and so forth). Moreover, transmitter 1918 and receiver 1920 may be coupled to one or more antennas (e.g., antenna 1922) and may share circuit components, software, or firmware, or alternatively be implemented separately.

In the illustrated embodiment, communication functions of communication interface 1912 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/intemet protocol (TCP/IP), synchronous optical networking (SONET), Asynchronous Transfer Mode (ATM), QUIC, Hypertext Transfer Protocol (HTTP), and so forth.

Regardless of the type of sensor, a UE may provide an output of data captured by its sensors, through its communication interface 1912, 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).

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

A UE, when in the form of an Internet of Things (loT) device, may be a device for use in one or more application domains, these domains comprising, but not limited to, city wearable technology, extended industrial application and healthcare. Non-limiting examples of such an loT device are a device which is or which is embedded in: a connected refrigerator or freezer, a TV, a connected lighting device, an electricity meter, a robot vacuum cleaner, a voice controlled smart speaker, a home security camera, a motion detector, a thermostat, a smoke detector, a door/window sensor, a flood/moisture sensor, an electrical door lock, a connected doorbell, an air conditioning system like a heat pump, an autonomous vehicle, a surveillance system, a weather monitoring device, a vehicle parking monitoring device, an electric vehicle charging station, a smart watch, a fitness tracker, a head-mounted display for Augmented Reality (AR) or Virtual Reality (VR), a wearable for tactile augmentation or sensory enhancement, a water sprinkler, an animal- or item-tracking device, a sensor for monitoring a plant or animal, an industrial robot, an Unmanned Aerial Vehicle (UAV), and any kind of medical device, like a heart rate monitor or a remote controlled surgical robot. A UE in the form of an loT device comprises circuitry and/or software in dependence of the intended application of the loT device in addition to other components as described in relation to UE 1900 shown in Figure 19.

As yet another specific example, in an loT scenario, a UE may represent a machine or other device that performs monitoring and/or measurements, and transmits the results of such monitoring and/or measurements to another UE and/or a network node. The UE may in this case be an M2M device, which may in a 3GPP 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.

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.

In some embodiments, UE 1900 may be configured to perform operations attributed to a UE in various embodiments described above, such as the exemplary method shown in Figure 16.

Figure 20 shows a network node 2000 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 O-RAN nodes or components of an O-RAN node (e.g., O-RU, O-DU, O-CU). Base stations may be categorized based on the amount of coverage they provide (or, stated differently, their transmit power level) and so, depending on the provided amount of coverage, may be referred to as femto base stations, pico base stations, micro base stations, or macro base stations. A base station may be a relay node or a relay donor node controlling a relay. A network node may also include one or more (or all) parts of a distributed radio base station such as centralized digital units, distributed units (e.g., in an O-RAN access node) and/or remote radio units (RRUs), sometimes referred to as Remote Radio Heads (RRHs). Such remote radio units may or may not be integrated with an antenna as an antenna integrated radio. Parts of a distributed radio base station may also be referred to as nodes in a distributed antenna system (DAS).

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).

Network node 2000 includes processing circuitry 2002, memory 2004, communication interface 2006, and power source 2008. Network node 2000 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 2000 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 2000 may be configured to support multiple radio access technologies (RATs). In such embodiments, some components may be duplicated (e.g., separate memory 2004 for different RATs) and some components may be reused (e.g., a same antenna 2010 may be shared by different RATs). Network node 2000 may also include multiple sets of the various illustrated components for different wireless technologies integrated into network node 2000, 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 2000.

Processing circuitry 2002 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 2000 components, such as memory 2004, to provide network node 2000 functionality.

In some embodiments, processing circuitry 2002 includes a system on a chip (SOC). In some embodiments, processing circuitry 2002 includes one or more of radio frequency (RF) transceiver circuitry 2012 and baseband processing circuitry 2014. In some embodiments, RF transceiver circuitry 2012 and baseband processing circuitry 2014 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 2012 and baseband processing circuitry 2014 may be on the same chip or set of chips, boards, or units.

Memory 2004 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 2002. Memory 2004 may store any suitable instructions, data, or information, including a computer program, software, an application including one or more of logic, rules, code, tables, and/or other instructions (collected denoted computer program 2004a, which may be in the form of a computer program product) capable of being executed by processing circuitry 2002 and utilized by network node 2000. Memory 2004 may be used to store any calculations made by processing circuitry 2002 and/or any data received via communication interface 2006. In some embodiments, processing circuitry 2002 and memory 2004 is integrated.

Communication interface 2006 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 2006 comprises port(s)/terminal(s) 2016 to send and receive data, for example to and from a network over a wired connection. Communication interface 2006 also includes radio frontend circuitry 2018 that may be coupled to, or in certain embodiments a part of, antenna 2010. Radio front-end circuitry 2018 comprises filters 2020 and amplifiers 2022. Radio front-end circuitry 2018 may be connected to an antenna 2010 and processing circuitry 2002. The radio front-end circuitry may be configured to condition signals communicated between antenna 2010 and processing circuitry 2002. Radio front-end circuitry 2018 may receive digital data that is to be sent out to other network nodes or UEs via a wireless connection. Radio front-end circuitry 2018 may convert the digital data into a radio signal having the appropriate channel and bandwidth parameters using a combination of filters 2020 and/or amplifiers 2022. The radio signal may then be transmitted via antenna 2010. Similarly, when receiving data, antenna 2010 may collect radio signals which are then converted into digital data by radio front-end circuitry 2018. The digital data may be passed to processing circuitry 2002. In other embodiments, the communication interface may comprise different components and/or different combinations of components.

In certain alternative embodiments, network node 2000 does not include separate radio front-end circuitry 2018, instead, processing circuitry 2002 includes radio front-end circuitry and is connected to antenna 2010. Similarly, in some embodiments, all or some of RF transceiver circuitry 2012 is part of communication interface 2006. In still other embodiments, communication interface 2006 includes one or more ports or terminals 2016, radio front-end circuitry 2018, and RF transceiver circuitry 2012, as part of a radio unit (not shown), and communication interface 2006 communicates with baseband processing circuitry 2014, which is part of a digital unit (not shown).

Antenna 2010 may include one or more antennas, or antenna arrays, configured to send and/or receive wireless signals. Antenna 2010 may be coupled to radio front-end circuitry 2018 and may be any type of antenna capable of transmitting and receiving data and/or signals wirelessly. In certain embodiments, antenna 2010 is separate from network node 2000 and connectable to network node 2000 through an interface or port.

Antenna 2010, communication interface 2006, and/or processing circuitry 2002 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 2010, communication interface 2006, and/or processing circuitry 2002 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.

Power source 2008 provides power to the various components of network node 2000 in a form suitable for the respective components (e.g., at a voltage and current level needed for each respective component). Power source 2008 may further comprise, or be coupled to, power management circuitry to supply the components of network node 2000 with power for performing the functionality described herein. For example, network node 2000 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 2008. As a further example, power source 2008 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.

Embodiments of network node 2000 may include additional components beyond those shown in Figure 20 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 2000 may include user interface equipment to allow input of information into network node 2000 and to allow output of information from network node 2000. This may allow a user to perform diagnostic, maintenance, repair, and other administrative functions for network node 2000.

In some embodiments, network node 2000 may be configured to perform operations attributed to a RAN node in various embodiments described above, including the exemplary method shown in Figure 17.

Figure 21 is a block diagram illustrating a virtualization environment 2100 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 2100 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 2100 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.

Applications 2102 (which may alternatively be called software instances, virtual appliances, network functions, virtual nodes, virtual network functions, etc.) are run in the virtualization environment 2100 to implement some of the features, functions, and/or benefits of some of the embodiments disclosed herein. For example, one or more virtual nodes 2100 may be configured to perform operations attributed to a RAN node in various embodiments described above, including the exemplary method shown in Figure 17.

Hardware 2104 includes processing circuitry, memory that stores software and/or instructions (collected denoted computer program 2104a, 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 2106 (also referred to as hypervisors or virtual machine monitors (VMMs)), provide VMs 2108a and 2108b (one or more of which may be referred to as VMs 2108), and/or perform any of the functions, features and/or benefits described in relation with some embodiments described herein. Virtualization layer 2106 may present a virtual operating platform that appears like networking hardware to the VMs 2108.

VMs 2108 comprise virtual processing, virtual memory, virtual networking or interface and virtual storage, and may be run by a corresponding virtualization layer 2106. Different embodiments of the instance of a virtual appliance 2102 may be implemented on one or more of VMs 2108, and the implementations may be made in different ways. Virtualization of the hardware is in some contexts referred to as network function virtualization (NFV). NFV may be used to consolidate many network equipment types onto industry standard high volume server hardware, physical switches, and physical storage, which can be located in data centers, and customer premise equipment.

In the context of NFV, each VM 2108 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 2108, and that part of hardware 2104 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 2108 on top of the hardware 2104 and corresponds to the application 2102.

Hardware 2104 may be implemented in a standalone network node with generic or specific components. Hardware 2104 may implement some functions via virtualization. Alternatively, hardware 2104 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 2110, which, among others, oversees lifecycle management of applications 2102. In some embodiments, hardware 2104 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 2112 which may alternatively be used for communication between hardware nodes and radio units. The foregoing merely illustrates the principles of the disclosure. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements, and procedures that, although not explicitly shown or described herein, embody the principles of the disclosure and can be thus within the spirit and scope of the disclosure. Various embodiments can be used together with one another, as well as interchangeably therewith, as should be understood by those having ordinary skill in the art.

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.

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.

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.

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

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

Al. A method for a user equipment (UE) configured for mobility between cells of a radio access network (RAN), the method comprising: receiving, from a first RAN node via a serving cell, a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; receiving from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells as a target cell for the mobility procedure; during execution of the mobility procedure to the target cell in accordance with the mobility command, selectively updating a plurality of access stratum (AS) security keys based on the security cell set identifier for the target cell and a security cell set identifier for the serving cell; and transmitting, to a second RAN node that provides the target cell, a message indicating that the mobility procedure is complete, wherein the message is secured using at least one of the selectively updated AS security keys.

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

Alb. The method of embodiment Ala, wherein the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one of the first and second keys.

A2. The method of any of embodiments Al-Alb, wherein selectively updating the plurality of AS security keys based on the security cell set identifier for the target cell and the security cell set identifier for the serving cell comprises: determining whether there is a match between the security cell set identifier for the target cell and the security cell set identifier for the serving cell; when no match is determined, performing an update of the plurality of AS security keys based on a security configuration received from the first RAN node; and when a match is determined, refraining from updating the plurality of AS security keys.

A2a. The method of embodiment A2, wherein the security configuration includes one of more of the following parameters: an indication of whether a keyset change is needed; an indication whether horizontal or vertical key derivation should be used; a next-hop chaining counter (NCC) usable for vertical key derivation; and a container of non-AS (NAS) information.

A2b. The method of embodiment A2a, wherein the mobility command includes one or more parameters that override corresponding parameters of the security configuration, the update of the plurality of AS security keys is performed based on the security configuration updated based on the one or more parameters included with the command.

A2c. The method of any of embodiments A2-A2b, wherein selectively updating the plurality of AS security keys further comprises, after performing the update, selectively discarding or retaining the security configuration based on whether the security configuration is single-use or multi-use.

A2d. The method of any of embodiments A2-A2c, wherein one of the follow applies: the security configuration is received in the mobility configuration separate from the one or more candidate configurations, and is associated with all of the one or more mobility candidate cells; or the security configuration is received as part of the candidate configuration for the target cell, and is associated only with the target cell.

A2e. The method of any of embodiments A2-A2d, wherein the mobility configuration includes a plurality of security configurations, and selectively updating the plurality of AS security keys further comprises, when no match is determined, selecting the security configuration from the plurality of security configurations based on one of the following: an identifier of the security configuration, received with the command; or a predetermined order of the plurality, which indicates the security configuration as next available.

A2f. The method of any of embodiments A2-A2a, wherein performing the update of the plurality of AS security keys based on the security configuration received from the first RAN node comprises: determining that no security configurations are available to use for update of the plurality of AS security keys; and sending to the second RAN a request for one or more security configuration, wherein the security configuration used for the update is received from the RAN node in response to the request.

A2g. The method of embodiment A2f, wherein the request includes one or more of the following: identifiers of the one or more candidate configurations; identifiers of the one or more or mobility candidate cells; one or more security configurations previously received and discarded by the UE, or identifiers thereof; and a request to provided security configurations only within subsequent mobility commands that trigger execution of mobility procedures.

A3. The method of any of embodiments Al-A2g, wherein the mobility command includes an update indication of whether a security key update is needed in conjunction with the mobility procedure, and selectively updating the plurality of AS security keys is further based on the update indication. A4. The method of any of embodiments Al -A3, wherein the security cell set identifier for the serving cell is stored in a UE variable, and the method further comprises updating the UE variable to contain the security cell set identifier for the target cell after executing the mobility procedure.

A5. The method of any of embodiments A1-A4, wherein the first and second RAN nodes are one of the following: different centralized units (CU) of a single RAN node, or different RAN nodes.

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

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

A7. The method of any of embodiments A1-A4, wherein: the mobility procedure is a conditional mobility procedure, each candidate configuration includes an associated execution condition, the method further comprises, after receiving the command, determining that the execution condition associated with the target cell is fulfilled, and execution of the mobility procedure is responsive to determining that the execution condition associated with target cell is fulfilled.

A7a. The method of embodiment A7, wherein the mobility procedure is one of the following: an initial layer-3 (L3) conditional handover (CHO), a subsequent L3 CHO, an initial conditional layer-l/layer-2 triggered inter-cell mobility (LTM) cell switch, or a subsequent conditional LTM cell switch.

A8. The method of any of embodiments Al-A7a, wherein the mobility configuration also includes the security cell set identifier for the serving cell.

BL A method for a first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the method comprising: sending, to a UE via a serving cell, a mobility configuration comprising: a security cell set identifier for the serving cell, and one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; and sending to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure, and the security cell set identifier for the target cell and the security cell set identifier for the serving cell indicate whether a plurality of access stratum (AS) security keys should be updated by the UE during execution of the mobility procedure to the target cell.

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

B2. The method of any of embodiments Bl -Bl a, wherein: no match between the security cell set identifiers for the target cell and the serving cell indicates the plurality of AS security keys should be updated based on a security configuration provided by the first RAN node; and a match between the security cell set identifiers for the target cell and the serving cell should not be updated by the UE during execution of the mobility procedure.

B2a. The method of embodiment B2, wherein the security configuration includes one of more of the following parameters: an indication of whether a keyset change is needed; an indication whether horizontal or vertical key derivation should be used; a next-hop chaining counter (NCC) usable for vertical key derivation; and a container of non-AS (NAS) information.

B2b. The method of embodiment B2a, wherein the mobility command includes one or more parameters that override corresponding parameters of the security configuration. B2c. The method of any of embodiments B2-B2b, wherein one of the follow applies: the security configuration is sent in the mobility configuration separate from the one or more candidate configurations, and is associated with all of the one or more mobility candidate cells; or the security configuration is sent as part of the candidate configuration for the target cell, and is associated only with the target cell.

B2d. The method of any of embodiments B2-B2c, wherein the mobility configuration includes a plurality of security configurations, and one of the following indicates that the security configuration should be selected, from the plurality of security configurations, to be used for updating the plurality of AS security keys: an identifier of the security configuration, sent with the command; or a predetermined order of the plurality, which indicates the security configuration as next available.

B2e. The method of any of embodiments B2-B2a, further comprising receiving from the UE a request for one or more security configurations, wherein the security configuration is sent to the UE in response to the request.

B2f The method of embodiment B2e, wherein the request includes one or more of the following: identifiers of the one or more candidate configurations; identifiers of the one or more or mobility candidate cells; one or more security configurations previously received and discarded by the UE, or identifiers thereof; and a request to provided security configurations only within subsequent mobility commands that trigger execution of mobility procedures.

B3. The method of any of embodiments Bl-B2f, wherein the mobility command includes an update indication of whether a security key update is needed in conjunction with the mobility procedure.

B4. The method of any of embodiments B1-B3, further comprising: sending to the second RAN node a request for a candidate configurations for the one or more mobility candidate cells, wherein the request includes one or more AS security keys to be used by the UE after a next security key update; and receiving from the second RAN node a response including the one or more candidate configurations for the respective mobility candidate cells.

B5. The method of any of embodiments B1-B3, further comprising sending to the second RAN node an indication of the mobility procedure for the UE from the serving cell to the target cell, wherein the request includes one or more AS security keys to be used by the UE after a next security key update.

B6. The method of any of embodiments B1-B5, wherein the first and second RAN nodes are one of the following: different centralized units (CU) of a single RAN node, or different RAN nodes.

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

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

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

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

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

C2. User equipment (UE) configured for mobility between cells of a radio access network (RAN), the UE being further configured to perform operations corresponding to the methods of any of embodiments A1-A8.

C3. A non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry of user equipment (UE) configured for mobility between cells of a radio access network (RAN), configure the UE to perform operations corresponding to the methods of any of embodiments A1-A8.

C4. A computer program product comprising computer-executable instructions that, when executed by processing circuitry of user equipment (UE) configured for mobility between cells of a radio access network (RAN), configure the UE to perform operations corresponding to the methods of any of embodiments A1-A8.

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

D2. A first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), the first RAN node being further configured to perform operations corresponding to the methods of any of embodiments Bl-B8a.

D3. A non-transitory, computer-readable medium storing computer-executable instructions that, when executed by processing circuitry of a first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), configure the first RAN node to perform operations corresponding to the methods of any of embodiments Bl-B8a.

D4. A computer program product comprising computer-executable instructions that, when executed by processing circuitry of a first radio access network (RAN) node configured to facilitate mobility between cells by user equipment (UEs), configure the first RAN node to perform operations corresponding to the methods of any of embodiments Bl-B8a.

Claims

1. A method for a user equipment, UE, configured for mobility between cells of a radio access network, RAN, the method comprising: receiving (1610), from a first RAN node via a serving cell, a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; receiving (1620) from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells as a target cell for the mobility procedure; during execution of the mobility procedure to the target cell in accordance with the mobility command, selectively updating (1640) a plurality of access stratum, AS, security keys based on the security cell set identifier for the target cell and on a security cell set identifier for the serving cell; and transmitting (1650), to a second RAN node that provides the target cell, a message indicating that the mobility procedure is complete, wherein the message is secured using at least one of the selectively updated AS security keys.

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

3. The method of claim 2, wherein the message indicating that the mobility procedure is complete is an RRCReconfigurationComplete message and is secured using at least one of the first and second keys.

4. The method of any of claims 1-3, wherein selectively updating (1640) the plurality of AS security keys based on the security cell set identifier for the target cell and on the security cell set identifier for the serving cell comprises: determining (1641) whether there is a match between the security cell set identifier for the target cell and the security cell set identifier for the serving cell; when no match is determined, performing (1643) an update of the plurality of AS security keys based on a first security configuration received from the first RAN node; and when a match is determined, refraining (1645) from updating the plurality of AS security keys.

5. The method of claim 4, wherein the first security configuration includes one of more of the following parameters: an indication of whether a keyset change is needed; an indication whether horizontal or vertical key derivation should be used; a next-hop chaining counter, NCC, usable for vertical key derivation; and a container of non-AS information.

6. The method of claim 5, wherein the mobility command includes one or more parameters that override corresponding parameters of the first security configuration, the update of the plurality of AS security keys is performed based on the first security configuration updated according to the one or more parameters included with the command.

7. The method of any of claims 4-6, wherein selectively updating (1640) the plurality of AS security keys further comprises, after performing the update, selectively discarding or retaining (1644) the first security configuration based on whether the first security configuration is singleuse or multi-use.

8. The method of any of claims 4-7, wherein one of the follow applies: the first security configuration is received in the mobility configuration separate from the one or more candidate configurations, and is associated with all of the one or more mobility candidate cells; or the first security configuration is received as part of the candidate configuration for the target cell, and is associated only with the target cell.

9. The method of any of claims 4-8, wherein the mobility configuration includes a plurality of security configurations, and selectively updating (1640) the plurality of AS security keys further comprises, when no match is determined, selecting (1642) the first security configuration from the plurality of security configurations based on one of the following: an identifier of the first security configuration, received with the command; or a predetermined order of the plurality, which indicates the first security configuration as next available.

10. The method of any of claims 4-5, wherein performing (1643) the update of the plurality of AS security keys based on the first security configuration received from the first RAN node comprises: determining that no security configurations are available to use for update of the plurality of AS security keys; and sending to the second RAN a request for one or more security configuration, wherein the first security configuration is received from the RAN node in response to the request.

11. The method of claim 10, wherein the request includes one or more of the following: identifiers of the one or more candidate configurations; identifiers of the one or more or mobility candidate cells; one or more security configurations previously received and discarded by the UE, or identifiers thereof; and a request to provided security configurations only within subsequent mobility commands that trigger execution of mobility procedures.

12. The method of any of claims 4-11, wherein selectively updating (1640) the plurality of AS security keys based on the security cell set identifier for the target cell and on the security cell set identifier for the serving cell further comprises: when no match is determined, performing (1646) one or more link layer reset operations such that the updated plurality of AS security keys are used for link layer protocol data units, PDUs, sent or received after the update; and when a match is determined, refraining (1647) from performing any link layer reset operations.

13. The method of claim 12, wherein the one or more link layer link reset operations include one or more of the following: packet data convergence protocol, PDCP, layer re-establishment; radio link control, RLC, layer re-establishment; and medium access control, MAC, layer reset.

14. The method of any of claims 1-13, wherein the mobility command includes an update indication of whether a security key update is needed in conjunction with the mobility procedure, and selectively updating (1640) the plurality of AS security keys is further based on the update indication.

15. The method of any of claims 1-14, wherein the security cell set identifier for the serving cell is stored in a UE variable, and the method further comprises updating the UE variable to contain the security cell set identifier for the target cell after executing the mobility procedure.

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

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

18. The method of any of claims 1-16, wherein: the mobility procedure is one of the following: an initial layer-3, L3, conditional handover, CHO; a subsequent L3 CHO; an initial conditional layer-l/layer-2 triggered inter-cell mobility, LTM, cell switch; or a subsequent conditional LTM cell switch; each candidate configuration includes an associated execution condition; the method further comprises, after receiving the command, determining (1630) that the execution condition associated with the target cell is fulfilled; and execution of the mobility procedure is responsive to determining (1630) that the execution condition associated with target cell is fulfilled.

19. The method of any of claims 1-18, wherein the mobility configuration also includes the security cell set identifier for the serving cell.

20. A method for a first radio access network, RAN, node configured to facilitate mobility between cells by user equipment, UEs,, the method comprising: sending (1730), to a UE via a serving cell, a mobility configuration comprising: a security cell set identifier for the serving cell; and one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; and sending (1760) to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure; and the security cell set identifier for the target cell and the security cell set identifier for the serving cell indicate whether a plurality of access stratum, AS, security keys should be updated by the UE during execution of the mobility procedure to the target cell.

21. The method of claim 20, wherein the AS security keys include a master MN security key and the following security keys derived from the master MN security key: a first key for integrity protection of signaling, a second key for ciphering of signaling, a third key for integrity protection of user data, and a fourth key for ciphering of user data.

22. The method of any of claims 20-21, wherein: no match between the security cell set identifiers for the target cell and the serving cell indicates that the plurality of AS security keys should be updated by the UE based on a security configuration provided by the first RAN node; and a match between the security cell set identifiers for the target cell and the serving cell indicates that the plurality of AS security keys should not be updated by the UE during execution of the mobility procedure.

23. The method of claim 22, wherein the first security configuration includes one of more of the following parameters: an indication of whether a keyset change is needed; an indication whether horizontal or vertical key derivation should be used; a next-hop chaining counter, NCC, usable for vertical key derivation; and a container of non-AS information.

24. The method of claim 23, wherein the mobility command includes one or more parameters that override corresponding parameters of the first security configuration.

25. The method of any of claims 22-24, wherein one of the follow applies: the first security configuration is sent in the mobility configuration separate from the one or more candidate configurations, and is associated with all of the one or more mobility candidate cells; or the first security configuration is sent as part of the candidate configuration for the target cell, and is associated only with the target cell.

26. The method of any of claims 22-25, wherein the mobility configuration includes a plurality of security configurations, and one of the following indicates that the first security configuration should be selected, from the plurality of security configurations, to be used for updating the plurality of AS security keys: an identifier of the first security configuration, sent with the command; or a predetermined order of the plurality, which indicates the first security configuration as next available.

27. The method of any of claims 22-23, further comprising receiving (1740) from the UE a request for one or more security configurations, wherein the first security configuration is sent to the UE in response to the request.

28. The method of claim 27, wherein the request includes one or more of the following: identifiers of the one or more candidate configurations; identifiers of the one or more or mobility candidate cells; one or more security configurations previously received and discarded by the UE, or identifiers thereof; and a request to provided security configurations only within subsequent mobility commands that trigger execution of mobility procedures.

29. The method of any of claims 20-28, wherein the mobility command includes an update indication of whether a security key update is needed in conjunction with the mobility procedure.

30. The method of any of claims 20-29, further comprising: sending (1710) to the second RAN node a request for a candidate configurations for the one or more mobility candidate cells, wherein the request includes one or more AS security keys to be used by the UE after a next security key update; and receiving (1720) from the second RAN node a response including the one or more candidate configurations for the respective mobility candidate cells.

31. The method of any of claims 20-29, further comprising sending (1750) to the second RAN node an indication of the mobility procedure for the UE from the serving cell to the target cell, wherein the request includes one or more AS security keys to be used by the UE after a next security key update.

32. The method of any of claims 20-31, wherein the first and second RAN nodes are one of the following: different centralized units, CUs, of a single RAN node; or different RAN nodes.

33. The method of any of claims 20-32, wherein the mobility command causes execution of the mobility procedure by the UE, with the mobility procedure being one of the following: an initial layer-3, L3, handover, HO; a subsequent L3 HO; an initial layer- l/layer-2 triggered intercell mobility, LTM, cell switch; or a subsequent LTM cell switch.

34. The method of any of claims 20-32, wherein: the mobility procedure is one of the following: an initial layer-3, L3, conditional handover, CHO; a subsequent L3 CHO; an initial conditional layer-l/layer-2 triggered inter-cell mobility, LTM, cell switch; or a subsequent conditional LTM cell switch; each candidate configuration includes an associated execution condition; and fulfillment of the execution condition associated with target cell, after sending the mobility command, causes execution of the mobility procedure by the UE.

35. The method of any of claims 20-34, wherein the mobility configuration also includes the security cell set identifier for the serving cell.

36. User equipment, UE (310, 410, 610, 801, 910, 1010, 1812, 1900) configured for mobility between cells of a radio access network, RAN (199, 1804), the UE comprising: communication interface circuitry (1912) configured to communicate with RAN nodes (100, 150, 320, 420, 430, 620, 802, 803, 920, 930, 1020, 1030, 1810, 2000, 2102); and processing circuitry (1902) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: receive, from a first RAN node via a serving cell, a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; receive from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells as a target cell for the mobility procedure; during execution of the mobility procedure to the target cell in accordance with the mobility command, selectively update a plurality of access stratum, AS, security keys based on the security cell set identifier for the target cell and on a security cell set identifier for the serving cell; and transmit, to a second RAN node that provides the target cell, a message indicating that the mobility procedure is complete, wherein the message is secured using at least one of the selectively updated AS security keys.

37. The UE of claim 36, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 2-19.

38. User equipment, UE (310, 410, 610, 801, 910, 1010, 1812, 1900) configured for mobility between cells of a radio access network, RAN (199, 1804), the UE being further configured to: receive, from a first RAN node (100, 150, 320, 420, 620, 802, 920, 1020, 1810, 2000, 2102) via a serving cell, a mobility configuration comprising one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; receive from the first RAN node a mobility command for execution of a mobility procedure from the serving cell, wherein the mobility command identifies one of the mobility candidate cells as a target cell for the mobility procedure; during execution of the mobility procedure to the target cell in accordance with the mobility command, selectively update a plurality of access stratum, AS, security keys based on the security cell set identifier for the target cell and on a security cell set identifier for the serving cell; and transmit, to a second RAN node that provides the target cell, a message indicating that the mobility procedure is complete, wherein the message is secured using at least one of the selectively updated AS security keys.

39. The UE of claim C3, being further configured to perform operations corresponding to the methods of any of claims 2-19.

40. Non-transitory, computer-readable medium (1910) storing computer-executable instructions that, when executed by processing circuitry (1902) of user equipment, UE (310, 410, 610, 801, 910, 1010, 1812, 1900) configured for mobility between cells of a radio access network, RAN (199, 1804), configure the UE to perform operations corresponding to the methods of any of claims 1-19.

41. Computer program product (! 914) comprising computer-executable instructions that, when executed by processing circuitry (1902) of user equipment, UE (310, 410, 610, 801, 910, 1010, 1812, 1900) configured for mobility between cells of a radio access network, RAN (199, 1804), configure the UE to perform operations corresponding to the methods of any of claims 1- 19.

42. First radio access network, RAN, node (100, 150, 320, 420, 620, 802, 920, 1020, 1810, 2000, 2102) configured to facilitate mobility between cells by user equipment UEs (310, 410, 610, 801, 910, 1010, 1812, 1900), the first RAN node comprising: communication interface circuitry (2006, 2104) configured to communicate with UEs and with other RAN nodes (100, 150, 320, 430, 803, 930, 1030, 1810, 2000, 2102); and processing circuitry (2002, 2104) operatively coupled to the communication interface circuitry, wherein the processing circuitry and the communication interface circuitry are configured to: send, to a UE via a serving cell, a mobility configuration comprising: a security cell set identifier for the serving cell; and one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; and send to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node as a target cell for the mobility procedure; and the security cell set identifier for the target cell and the security cell set identifier for the serving cell indicate whether a plurality of access stratum, AS, security keys should be updated by the UE during execution of the mobility procedure to the target cell.

43. The first RAN node of claim 42, wherein the processing circuitry and the communication interface circuitry are further configured to perform operations corresponding to the methods of any of claims 21-35.

44. First radio access network, RAN, node (100, 150, 320, 420, 620, 802, 920, 1020, 1810, 2000, 2102) configured to facilitate mobility between cells by user equipment, UEs (310, 410, 610, 801, 910, 1010, 1812, 1900), the first RAN node being further configured to: send, to a UE via a serving cell, a mobility configuration comprising: a security cell set identifier for the serving cell; and one or more candidate configurations for respective one or more mobility candidate cells, wherein each candidate configuration includes a security cell set identifier for the mobility candidate cell; and send to the UE a mobility command for execution of a mobility procedure from the serving cell, wherein: the mobility command identifies one of the mobility candidate cells provided by a second RAN node (100, 150, 320, 430, 803, 930, 1030, 1810, 2000, 2102) as a target cell for the mobility procedure; and the security cell set identifier for the target cell and the security cell set identifier for the serving cell indicate whether a plurality of access stratum, AS, security keys should be updated by the UE during execution of the mobility procedure to the target cell.

45. The first RAN node of claim 44, being further configured to perform operations corresponding to the methods of any of claims 21-35.

46. Non-transitory, computer-readable medium (2004, 2104) storing computer-executable instructions that, when executed by processing circuitry (2002, 2104) of a first radio access network, RAN, node (100, 150, 320, 420, 620, 802, 920, 1020, 1810, 2000, 2102) configured to facilitate mobility between cells by user equipment UEs (310, 410, 610, 801, 910, 1010, 1812, 1900), configure the first RAN node to perform operations corresponding to the methods of any of claims 20-35.

47. Computer program product (2004a, 2104a) comprising computer-executable instructions that, when executed by processing circuitry (2002, 2104) of a first radio access network, RAN, node (100, 150, 320, 420, 620, 802, 920, 1020, 1810, 2000, 2102) configured to facilitate mobility between cells by user equipment UEs (310, 410, 610, 801, 910, 1010, 1812, 1900), configure the first RAN node to perform operations corresponding to the methods of any of claims 20-35.