WO2025209888A1

WO2025209888A1 - Apparatus, method, and computer program for a cell switch

Info

Publication number: WO2025209888A1
Application number: PCT/EP2025/058153
Authority: WO
Inventors: Prasna Kumar SAHU; Panagiotis SPAPIS; Endrit DOSTI; Umur KARABULUT; Anastasios KAKKAVAS
Original assignee: Nokia Technologies Oy
Current assignee: Nokia Technologies Oy
Priority date: 2024-04-05
Filing date: 2025-03-25
Publication date: 2025-10-09
Anticipated expiration: 2026-10-05
Also published as: GB2640213A

Abstract

The disclosure relates to a method comprising: receiving, by a user equipment from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; determining, by the user equipment, that the subset of candidate target cells comprises a first candidate target cell; attempting, by the user equipment, a cell switch from the source cell to the first candidate target cell; determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell is unsuccessful; determining, by the user equipment, that the subset of candidate target cells comprises a second candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells

Description

APPARATUS, METHOD, AND COMPUTER PROGRAM FOR A CELL SWITCH

Field of the disclosure

The present disclosure relates to a method, an apparatus and a computer program for managing mobility in a communication system.

Background

A communication system can be seen as a facility that enables communication sessions between two or more entities such as communication devices, base stations and/or other nodes by providing carriers between the various entities involved in the communications path.

The communication system and associated entities typically operate in accordance with a given standard or specification which sets out what the associated entities are permitted to do and how that should be achieved. Communication protocols and/or parameters which shall be used for the connection are also typically defined. Examples of standard are the so-called 5G standards.

Summary

According to an aspect there is provided a method comprising: receiving, by a user equipment from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; determining, by the user equipment, that the subset of candidate target cells comprises a first candidate target cell; attempting, by the user equipment, a cell switch from the source cell to the first candidate target cell; determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell is unsuccessful; determining, by the user equipment, that the subset of candidate target cells comprises a second candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells.

The expressions “cell switch”, “cell handover” or “mobility” may be used interchangeably. Receiving, by the user equipment from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell may comprise: receiving, by the user equipment from a source distributed unit of a base station providing the source cell, a command to trigger the cell switch from the source cell to the candidate target cell.

The subset of candidate target cells may comprise multiple candidate target cells in a priority order; the subset of candidate target cell may comprise a preferred candidate target cell and multiple candidate target cells in no priority order; or the subset of candidate target cell may comprise a preferred candidate target cell and multiple candidate target cells in a priority order.

The first candidate target cell may have a first priority and the second candidate target cell.

The first candidate target cell may be the preferred candidate target cell and the second candidate target cell may be selected amongst the multiple candidate target cells based on measurements of the multiple candidate target cells.

Attempting, by the user equipment, a cell switch from the source cell to the first candidate target cell or the second candidate target cell may comprise: attempting a random access channel less cell switch from the source cell to the first candidate target cell or the second candidate target cell.

The method may comprise: determining, by the user equipment, that the cell switch from the source cell to the second target cell is successful.

Determining, by the user equipment, that the cell switch from the source cell to the second candidate target cell is successful may comprise: determining, by the user equipment, completion of the cell switch from the source cell to the second candidate target cell prior to expiry of a timer for the second candidate target cell.

The method may comprise: determining, by the user equipment, that the cell switch from the source cell to the second candidate target cell is unsuccessful; determining, by the user equipment, that the subset of candidate target cells comprises a third candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the third candidate target cell. The method may comprise: determining, by the user equipment, that the cell switch from the source cell to the second target cell is unsuccessful; determining, by the user equipment, that the subset of candidate target cells does not comprise a third candidate target cell; and attempting, by the user equipment, a cell switch recovery.

Determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell or the second candidate target cell is unsuccessful may comprise: determining, by the user equipment, expiry of a timer for the second candidate target cell prior to completion of the cell switch from the source cell to the second candidate target cell.

Attempting, by the user equipment, a cell switch recovery may comprise: selecting, by the user equipment, a recovery cell; determining, by the user equipment, that the recovery cell is amongst the set of configured candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the recovery cell.

Attempting, by the user equipment, a cell switch from the source cell to the recovery cell may comprise attempting a random access channel based cell switch from the source cell to the recovery cell.

Attempting, by the user equipment, a cell switch recovery may comprise: selecting, by the user equipment, a recovery cell; determining, by the user equipment, that the recovery cell is not amongst the set of configured candidate target cells; and transmitting, by the user equipment to a base station providing the recovery cell, a radio resource control reestablishment request.

Selecting, by the user equipment, a recovery cell may comprise: detecting, by the user equipment, at least one neighbouring cell; and selecting, by the user equipment, the recovery cell amongst the at least one neighbouring cell.

The recovery cell may or may not be selected amongst the subset of candidate target cells.

The command to trigger a cell switch from the source cell to a candidate target cell may include information indicating at least one of: a subset of timing advance values for the subset of candidate target cells; a subset of timer values for the subset of candidate target cells; or a subset of timer identifiers or pointers identifying or pointing to timers for the subset of candidate target cells.

The subset of timer values may comprise a subset of T304 values.

Determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell or the second candidate target cell is unsuccessful may comprise: detecting, by the user equipment, a radio link failure with the first candidate target cell before expiry of a timer for the first candidate target cell or the second candidate target cell; or detecting expiry of a timer for the first candidate target cell or the second candidate target cell or radio link failure prior to expiry of a timer for the first candidate target cell or the second candidate target cell.

The command to trigger a cell switch from the source cell to the candidate target cell may be received in a medium access control control element.

The method may comprise: transmitting, by the user equipment to the base station, measurements of the set of configured candidate target cells; and the subset of candidate target cells may be selected amongst the set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

The measurements of the set of configured candidate target cells may comprise layer 1 measurements.

The measurements of the set of configured candidate target cells may comprise at least one of synchronization signal block measurements or channel state information reference signal measurements.

The measurements of the set of configured candidate target cells may comprise at least one of strength measurements or quality measurements.

The selection criterion may comprise at least one of a strength threshold or a quality threshold. The method may comprise: receiving, by the user equipment from the base station, a set of radio resource control configurations for the set of configured candidate target cells; and decoding, by the user equipment, the set of radio resource control configurations for the set of configured candidate target cells.

Each radio resource control configuration for a configured candidate target cell may include at least one of first configuration information for performing measurements for the configured candidate target cell or second configuration information for attempting a cell switch from the source cell to the configured candidate target cell.

The user equipment may perform downlink synchronization for the set of configured candidate target cells.

Performing downlink synchronization for the subset of candidate target cells selected may comprise decoding a subset of synchronization signal blocks for the set of configured candidate target cells.

The user equipment may acquire a subset of timing advance values for the set of configured candidate target cells.

Acquiring a subset of timing advance values for the set of configured candidate target cells may comprise performing random access procedure for the set of configured candidate target cells.

According to an aspect there is provided a user equipment comprising: means for receiving, from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; means for determining that the subset of candidate target cells comprises a first candidate target cell; means for attempting a cell switch from the source cell to the first candidate target cell; means for determining that the cell switch from the source cell to the first candidate target cell is unsuccessful; means for determining that the subset of candidate target cells comprises a second candidate target cell; and means for attempting a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells. The expressions “cell switch”, “cell handover” or “mobility” may be used interchangeably.

Means for receiving, from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell may comprise: means for receiving, from a source distributed unit of a base station providing the source cell, a command to trigger the cell switch from the source cell to the candidate target cell.

Means for attempting a cell switch from the source cell to the first candidate target cell or the second candidate target cell may comprise: means for attempting a random access channel less cell switch from the source cell to the first candidate target cell or the second candidate target cell.

The user equipment may comprise: means for determining that the cell switch from the source cell to the second target cell is successful.

Means for determining that the cell switch from the source cell to the second candidate target cell is successful may comprise: means for determining, by the user equipment, completion of the cell switch from the source cell to the second candidate target cell prior to expiry of a timer for the second candidate target cell.

The user equipment may comprise: means for determining that the cell switch from the source cell to the second candidate target cell is unsuccessful; means for determining that the subset of candidate target cells comprises a third candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the third candidate target cell. The user equipment may comprise: means for determining that the cell switch from the source cell to the second target cell is unsuccessful; means for determining that the subset of candidate target cells does not comprise a third candidate target cell; and attempting, by the user equipment, a cell switch recovery.

Means for determining that the cell switch from the source cell to the first candidate target cell or the second candidate target cell is unsuccessful may comprise: means for determining expiry of a timer for the second candidate target cell prior to completion of the cell switch from the source cell to the second candidate target cell.

Means for attempting a cell switch recovery may comprise: means for selecting a recovery cell; means for determining that the recovery cell is amongst the set of configured candidate target cell; and means for attempting a cell switch from the source cell to the recovery cell.

Means for attempting a cell switch from the source cell to the recovery cell may comprise means for attempting a random access channel based cell switch from the source cell to the recovery cell.

Means for attempting a cell switch recovery may comprise: means for selecting a recovery cell; means for determining that the recovery cell is not amongst the set of configured candidate target cells; and means for transmitting, to a base station providing the recovery cell, a radio resource control re-establishment request.

Means for selecting a recovery cell may comprise: means for detecting at least one neighbouring cell; and means for selecting the recovery cell amongst the at least one neighbouring cell.

The command to trigger a cell switch from the source cell to a candidate target cell may include information indicating at least one of: a subset of timing advance values for the subset of candidate target cells; a subset of timer values for the subset of candidate target cells; or a subset of timer identifiers or pointers identifying or pointing to timers for the subset of candidate target cells. The subset of timer values may comprise a subset of T304 values.

Means for determining that the cell switch from the source cell to the first candidate target cell or the second candidate target cell is unsuccessful may comprise: means for detecting a radio link failure with the first candidate target cell before expiry of a timer for the first candidate target cell or the second candidate target cell; or means for detecting expiry of a timer for the first candidate target cell or the second candidate target cell or radio link failure prior to expiry of a timer for the first candidate target cell or the second candidate target cell.

The user equipment may comprise: means for transmitting, to the base station, measurements of the set of configured candidate target cells; and the subset of candidate target cells may be selected amongst the set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

The selection criterion may comprise at least one of a strength threshold or a quality threshold.

The user equipment may comprise: means for receiving, from the base station, a set of radio resource control configurations for the set of configured candidate target cells; and means for decoding the set of radio resource control configurations for the set of configured candidate target cells. Each radio resource control configuration for a configured candidate target cell may include at least one of first configuration information for performing measurements for the configured candidate target cell or second configuration information for attempting a cell switch from the source cell to the configured candidate target cell.

According to an aspect there is provided a user equipment comprising circuitry configured to perform: receiving, from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; determining that the subset of candidate target cells comprises a first candidate target cell; attempting a cell switch from the source cell to the first candidate target cell; determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell is unsuccessful; determining that the subset of candidate target cells comprises a second candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells.

According to an aspect there is provided a user equipment comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to perform: receiving, from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; determining that the subset of candidate target cells comprises a first candidate target cell; attempting a cell switch from the source cell to the first candidate target cell; determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell is unsuccessful; determining that the subset of candidate target cells comprises a second candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells.

According to an aspect there is provided a computer program comprising computer executable code which when run on at least one processor is configured to perform: receiving, from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; determining that the subset of candidate target cells comprises a first candidate target cell; attempting a cell switch from the source cell to the first candidate target cell; determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell is unsuccessful; determining that the subset of candidate target cells comprises a second candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells.

According to an aspect there is provided a method comprising: providing, by a base station, a source cell; selecting, by the base station, a subset of candidate target cells amongst a set of configured candidate target cells; and transmitting, by the base station to a user equipment, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating the subset of candidate target cells.

Providing, by a base station, a source cell may comprise: providing, by a source distributed unit of the base station, a source cell; selecting, by the base station, the subset of candidate target cells amongst the set of configured candidate target cells may comprise: selecting, by the source distributed unit of the base station, the subset of candidate target cells amongst the set of configured candidate target cells; and transmitting, by the base station to the user equipment, a command to trigger the cell switch from the source cell to the candidate target cell may comprise: transmitting, by the source distributed unit of the base station to the user equipment, a command to trigger the cell switch from the source cell to the candidate target cell.

The method may comprise: receiving, by a base station from the user equipment, measurements of the set of configured candidate target cells; and selecting, by the base station, a subset of candidate target cells amongst a set of configured candidate target cells comprises selecting a subset of candidate target cells amongst a set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

Receiving, by the base station from the user equipment, measurements of the set of configured candidate target cells may comprise: receiving, by the source distributed unit of the base station from the user equipment, measurements of the set of configured candidate target cells; and selecting, by the base station, the subset of candidate target cells amongst the set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion may comprise: selecting, by the source distributed unit of the base station, the subset of candidate target cells amongst the set of configured candidate target cells based on the measurements and at least one selection criterion.

The method may comprise: configuring, by a central unit of the base station, data forwarding to the subset of candidate target cells.

The method may comprise: receiving, by the central unit of the base station from a target distributed unit of the base station providing a candidate target cell amongst the subset of candidate target cells, information indicating that a cell switch from the source cell to the candidate target cell amongst the subset of candidate target cells is successful; maintaining, by the central unit of the base station, data forwarding to the candidate target cell amongst the subset of candidate target cells; and releasing, by the central unit of the base station, data forwarding to other candidate target cells amongst the subset of candidate target cells.

According to an aspect there is provided a base station comprising: means for providing a source cell; means for selecting a subset of candidate target cells amongst a set of configured candidate target cells; and means for transmitting, to a user equipment, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating the subset of candidate target cells. Means for providing a source cell may comprise: means for providing, by a source distributed unit of the base station, a source cell; means for selecting the subset of candidate target cells amongst the set of configured candidate target cells may comprise: means for selecting, by the source distributed unit of the base station, the subset of candidate target cells amongst the set of configured candidate target cells; and means for transmitting, to the user equipment, a command to trigger the cell switch from the source cell to the candidate target cell may comprise: means for transmitting, by the source distributed unit of the base station to the user equipment, a command to trigger the cell switch from the source cell to the candidate target cell.

The base station may comprise: means for receiving, from the user equipment, measurements of the set of configured candidate target cells; and means for selecting a subset of candidate target cells amongst a set of configured candidate target cells comprises selecting a subset of candidate target cells amongst a set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

Means for receiving, from the user equipment, measurements of the set of configured candidate target cells may comprise: means for receiving, by the source distributed unit of the base station from the user equipment, measurements of the set of configured candidate target cells; and means for selecting the subset of candidate target cells amongst the set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion may comprise: means for selecting, by the source distributed unit of the base station, the subset of candidate target cells amongst the set of configured candidate target cells based on the measurements and at least one selection criterion.

The base station may comprise: means for configuring, by a central unit of the base station, data forwarding to the subset of candidate target cells.

The base station may comprise: means for receiving, by the central unit of the base station from a target distributed unit of the base station providing a candidate target cell amongst the subset of candidate target cells, information indicating that a cell switch from the source cell to the candidate target cell amongst the subset of candidate target cells is successful; maintaining, by the central unit of the base station, data forwarding to the candidate target cell amongst the subset of candidate target cells; and releasing, by the central unit of the base station, data forwarding to other candidate target cells amongst the subset of candidate target cells.

According to an aspect there is provided a base station comprising circuitry configured to perform: receiving, from the user equipment, measurements of the set of configured candidate target cells; and selecting a subset of candidate target cells amongst a set of configured candidate target cells comprises selecting a subset of candidate target cells amongst a set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

According to an aspect there is provided a base station comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to perform: receiving, from the user equipment, measurements of the set of configured candidate target cells; and selecting a subset of candidate target cells amongst a set of configured candidate target cells comprises selecting a subset of candidate target cells amongst a set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

According to an aspect there is provided a computer program comprising computer executable code which when run on at least one processor is configured to perform: receiving, from the user equipment, measurements of the set of configured candidate target cells; and selecting a subset of candidate target cells amongst a set of configured candidate target cells comprises selecting a subset of candidate target cells amongst a set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

According to an aspect, there is provided a computer readable medium comprising program instructions stored thereon for performing at least one of the above methods.

According to an aspect, there is provided a non-transitory computer readable medium comprising program instructions stored thereon for performing at least one of the above methods. According to an aspect, there is provided a non-volatile tangible memory medium comprising program instructions stored thereon for performing at least one of the above methods.

In the above, many different aspects have been described. It should be appreciated that further aspects may be provided by the combination of any two or more of the aspects described above.

Various other aspects are also described in the following detailed description and in the attached claims.

List of abbreviations

AF: Application Function

AMF: Access and Mobility Management Function

ALISF: Authentication Server Function

BS: Base Station

CSI: Channel State Information

CU: Centralized Unit

DL: Downlink

DN: Data Network

DU: Distributed Unit gNB: gNodeB

LTM: Lower layer Triggered Mobility

MAC CE : Medium Access Control Control Element

MS: Mobile Station

NEF: Network Exposure Function

NR: New radio

PDCCH: Physical Downlink Control Channel

PDSCH: Physical Downlink Control Channel

RAM: Random Access Memory

RAN: Radio Access Network

ROM: Read Only Memory

RRC: Radio Resource Control

RS: Reference Signal

RSRQ: Reference Signal Received Quality RSRP: Reference Signal Received Power

SMF: Session Management Function

SSB: Synchronisation Signal Block

UDM: Unified Data Management

UE: User Equipment

UL: Uplink

UPF: User Plane Function

5G: 5^th Generation

5GC: 5G Core Network

5GS: 5G System

Brief Description of the Figures

Embodiments will now be described, by way of example only, with reference to the accompanying Figures in which:

Fig. 1 shows a schematic representation of a communication system;

Fig. 2 shows a schematic representation of a control apparatus;

Fig. 3 shows a schematic representation of a user equipment;

Fig. 4a, Fig. 4b and Fig. 4c show a signalling diagram of a process for managing mobility in a communication system;

Fig. 5 shows a block diagram of a method for managing mobility in a communication system;

Fig. 6 shows a block diagram of another method for managing mobility in a communication system; and

Fig. 7 shows a schematic representation of a non-volatile memory medium storing instructions which when executed by a processor allow a processor to perform one or more of the steps of the methods of Fig. 5 and Fig. 6.

Detailed Description of the Figures Before explaining in detail the exemplifying embodiments, certain general principles of a wireless communication system, access systems thereof, and communication devices are briefly explained with reference to Fig. 1 , Fig. 2 and Fig. 3 to assist in understanding the technology underlying the described examples. In the following description, the terms communication device and user equipment may be interchanged.

FIG. 1 shows a schematic representation of a communication system. The communication system may comprise a user equipment, a 5G system (5GS), an application function (AF) and a data network (DN).

The 5GS may comprise multiple public land mobile networks (PLMNs). Each PLMN may comprise a 5G radio access network (5G RAN) and a 5G core network (5GC).

The 5G RAN may comprise one or more base stations. A base station may be a gNodeB (gNB). A gNB may comprise gNB distributed units (gNB-DUs) connected to one or more gNB centralized units (gNB-CUs).

The 5GC may comprise one or more of an authentication server function (ALISF), a user data management (UDM), a network exposure function (NEF), an access and mobility management function (AMF), a session management function (SMF) and a user plane function (UPF).

Fig. 2 illustrates an example of a control apparatus 200 for controlling a function of the RAN illustrated in Fig. 1. The control apparatus 200 may comprise at least one random access memory (RAM) 211a, at least on read only memory (ROM) 211 b, at least one processor 212, 213 and an input/output interface 214. The at least one processor 212, 213 may be coupled to the RAM 211a and the ROM 211b. The at least one processor 212, 213 may be configured to execute an appropriate software code 215. The software code 215 may for example allow to perform one or more steps to perform one or more of the present aspects. The software code 215 may be stored in the ROM 211b. The control apparatus 200 may be interconnected with another control apparatus 200 controlling another function of the 5G RAN or the 5GC. In some embodiments, each RAN comprises a control apparatus 200. In alternative embodiments, two or more RANs may share a control apparatus.

Fig. 3 illustrates an example of a UE 300, such as the UE illustrated on Fig. 1. The UE 300 is a communication device that is capable of (or configured for) sending and receiving radio signals. Non-limiting examples comprise a mobile station (MS) or mobile device such as a mobile phone or what is known as a ’smart phone’, a computer provided with a wireless interface card or other wireless interface facility (e.g., USB dongle), a personal data assistant (PDA) or a tablet provided with wireless communication capabilities, a machinetype communications (MTC) device, a Cellular Internet of things (CloT) device or any combinations of these or the like. The UE 300 may provide, for example, communication of data for carrying communications. The communications may be one or more of voice, electronic mail (email), text message, multimedia, data, machine data and so on.

The UE 300 may receive signals over an air or radio interface 307 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Fig. 3 transceiver apparatus is designated schematically by block 306. The transceiver apparatus 306 may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device.

The UE 300 may be provided with at least one processor 301 , at least one memory ROM 302a, at least one RAM 302b and other possible components 303 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The at least one processor 301 is coupled to the RAM 302b and the ROM 302a. The at least one processor 301 may be configured to execute an appropriate software code 308. The software code 308 may for example allow to perform one or more of the present aspects. The software code 308 may be stored in the ROM 302a.

The processor, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 304. The device may optionally have a user interface such as keypad 305, touch sensitive screen or pad, combinations thereof or the like. Optionally one or more of a display, a speaker and a microphone may be provided depending on the type of the device.

One or more aspects of this disclosure relates to managing mobility in a communication system. It will be understood that, although one or more aspects of this disclosure relates to a communication system comprising a 5GS, one or more aspects of this disclosure may apply to a communication system comprising other cellular systems.

It will be understood that, although one or more aspects of this disclosure relates to lower layer (e.g., layer 1 or layer 2) triggered mobility, one or more aspects of this disclosure may apply to higher layer (e.g., layer 3) triggered mobility.

It will be understood that, although one or more aspects of this disclosure relates to a split architecture (e.g., distributed unit and central unit of a base station split in different apparatus), one or more aspects of this disclosure may apply to non-split architecture (e.g., distributed unit and central unit of a base station collocated in a same apparatus).

As a UE changes location in a communication system, a radio link between the UE and a serving cell provided by a gNB-Dll may experience significant fluctuations. In lower layer (e.g., layer 1 or layer 2) triggered mobility (LTM), a gNB-Cll may receive, from the UE via the gNB-DU, a measurement report comprising layer 1 measurements. The gNB-CU may determine that the UE should perform a cell switch. The gNB-CU may transmit, to the UE via the gNB-DU, a command to trigger a cell switch from a source cell to a candidate target cell. The command may be transmitted in a medium access control control element (MAC CE). The command may comprise information indicating the candidate target cell. The UE may use radio resource control configuration (RRC) information of the candidate target cell previously prepared by the gNB-CU and received, from the gNB-CU via the gNB-DU, through RRC signaling to attempt the cell switch from the source cell to the candidate target cell.

One or more aspect of this disclosure provides techniques for reducing the delay to complete a cell switch.

One or more aspect of this disclosure provides techniques for reducing service interruption during a cell switch.

One or more aspect of this disclosure provides techniques for reducing occurrences of a cell switch recovery. It will be understood that in this disclosure the expressions “cell switch”, “cell handover” or “mobility” may be used interchangeably.

Fig. 4a, Fig. 4b and Fig. 4c show a signalling diagram of a process for managing mobility in a communication system.

At step 1 , the UE may transmit to a source gNB-Dll (i.e. , gNB DU providing a source cell), a measurement report. The measurement report may comprise measurements of a source cell and neighbouring cells. The measurements may comprise layer 3 measurements. The layer 3 measurements may be obtained by filtering layer 1 measurements. The layer 1 measurements may comprise at least one of synchronization signal block (SSB) measurements or channel state information (CSI) reference signal (RS) measurements. The layer 1 measurements may comprise at least one of strength measurements or quality measurements. The strength measurements may comprise reference signal received power (RSRP) measurements. The quality measurements may comprise reference signal received quality (RSRQ) measurements.

At step 2, the source gNB-DU may transmit, to a gNB-CU, the measurement report. The measurement report may be transmitted in an uplink (UL) RRC message transfer message.

At step 3, the gNB-CU may select a set of configured candidate target cells based on the measurements and a selection criterion. The selection criterion may comprise at least one of a strength threshold or a quality threshold.

At step 4, the gNB-CU may transmit, to a target gNB-DU (i.e., gNB-DU providing the configured candidate target cells), a UE context setup request.

At step 5, the gNB-CU may receive, from the a target gNB-DU, a UE context setup response.

At step 6, the gNB-CU may transmit, to the gNB-DU, a UE context modification request.

At step 7, the gNB-CU may receive, from the gNB-DU, a UE context modification response.

At step 8, the gNB-CU may generate a set of RRC configurations for the set of configured candidate target cells. Each radio resource control configuration for a configured candidate target cell may include at least one of first configuration information for performing measurements for the configured candidate target cell or second configuration information for attempting a cell switch from the source cell to the configured candidate target cell.

At step 9, the gNB-Cll may transmit, to the source gNB-Dll, the set of RRC configurations for the set of configured candidate target cells. The set of RRC configurations for the set of configured candidate target cells may be transmitted in a downlink (DL) RRC message transfer message.

At step 10, the source gNB-Dll may transmit, to the UE, the set of RRC configurations for the set of configured candidate target cells.

At step 11 , the UE may decode the set of RRC configurations for the set of configured candidate target cells. The UE may transmit, to the source DU, a RRC reconfiguration complete message.

At step 12, the source DU may transmit, to the gNB-CU, an UL RRC message transfer message.

At step 13, the UE may perform DL synchronization for the set of configured candidate target cells. The UE may decode a subset of SSBs for the subset of configured candidate target cells and may perform DL synchronization for the set of configured candidate target cells based on the subset of SSBs for the subset of configured candidate target cells.

At step 14, the UE may acquire a subset of timing advance (TA) values for the set of configured candidate target cells. The UE may perform a random access (RACH) procedure for the set of configured candidate target cells.

At step 15, the UE may transmit, to the gNB-DU, a measurement report. The measurement report may comprise measurements of the set of configured candidate target cells.

The measurements may comprise layer 1 measurements. The layer 1 measurements may comprise at least one of SSB measurements or CSI RS measurements. The layer 1 measurements may comprise at least one of strength measurements or quality measurements. The strength measurements may comprise RSRP measurements. The quality measurements may comprise RSRQ measurements. At step 16, the source DU may select a subset of candidate target cells selected amongst the set of configured candidate target cells based on the measurements and at least one selection criterion. The at least one criterion may comprise at least one of a strength threshold or a quality threshold.

Here, the source DU may select cell 1 , cell 2 and cell 3.

At step 17, the source DU may transmit, to the UE, a command to trigger a cell switch from the source cell to a candidate target cell. The command may include the subset of candidate target cells. The command may include a subset of TA values for the subset of candidate target cells. The command may include a subset of timer identifiers identifying or pointing to timers for the subset of candidate target cells (e.g., subset of T304 or guarding timer identifiers).

In an implementation, the subset of candidate target cells may comprise multiple candidate target cells in a priority order. The UE may select one of the multiple candidate target cells based on the priority order to perform a cell switch. If the cell switch fail, the UE may select another one of the multiple candidate target cells to perform a cell switch

In another implementation, the subset of candidate target cells may comprise a preferred candidate target cell and multiple candidate target cells in no priority order. The UE may select the preferred candidate target cell to perform a cell switch. If the cell switch fail, the UE may select one of the multiple candidate target cells based on measurements of the multiple candidate target cells to perform a cell switch.

In another implementation, the subset of candidate target cells may comprise a preferred candidate target cell and multiple candidate target cells in a priority order. The UE may select the preferred candidate target cell to perform a cell switch. If the cell switch fail, the UE may select one of the multiple candidate target cells based on the priority order to perform a cell switch.

Here, the subset of candidate target cells may comprise cell 1 , cell 2 and cell 3. Cell 1 may have the highest priority and cell 3 may have the lower priority. In an implementation, the command may include a subset of TA values for the subset of candidate target cells. The command may include a subset of timer values for the subset of candidate target cells (e.g., subset of T304 or guarding timer values).

Here, the command may include a subset of TA values TA1 , TA2 and TA3 and a subset of timer values T304_1 , T304_2 and T304_3 for cell 1 , cell 2 and cell 3.

In another implementation, the command may include a subset of TA values for the subset of candidate target cells. The command may include a subset of timer identifiers or pointers identifying or pointing to timers for the subset of candidate target cells (e.g., subset of T304 or guarding timer values). A set of timer values for the set of configured candidate target cells (e.g., set of T304 or guarding timer values), a set of timer values for the set of configured candidate target cells (e.g., set of T304 or guarding timer values) and a set of timer identifiers or pointers for the set of configured candidate target cells (e.g., set of T304 or guarding timer identifiers or pointers) may already have received as part of step 10.

The UE may configure a subset of timers for the subset of candidate target cells based on the subset of timer values for the subset of candidate target cells.

At step 18, the source DU may transmit, to the gNB-CU, the subset of candidate target cells. The source DU may transmit, to the gNB-CU, the subset of TA values for the subset of candidate target cells. The source DU may transmit, to the gNB-CU, the subset of timer identifiers or pointers for the subset of candidate target cells (e.g., subset of T304 or guarding timer identifiers or pointers).

At step 19, the gNB-CU may configure data forwarding to the source cell. The gNB-CU may configure data forwarding to the subset of candidate target cells.

At step 20, the UE may determine that the subset of candidate target cells comprises a first candidate target cell (e.g., cell 1). The UE may start the timer associated with the first candidate target cell. The UE may attempt a cell switch from the source cell to the first candidate target cell (i.e. , the UE may attempt access to the first candidate target cell). The cell switch from the source cell to the first candidate target cell may comprise a RACH less switch from the source cell to the first candidate target cell (i.e., cell switch from the source cell to the first candidate target cell without RACH procedure). The UE may determine that the cell switch from the source cell to the first candidate target cell is unsuccessful. The UE may determine expiry of the timer associated with the first candidate target cell prior to completion of the cell switch from the source cell to the first candidate target cell or radio link failure prior to expiry of the timer associated with the first candidate target cell prior to completion of the cell switch from the source cell to the first candidate target cell. Completion of the cell switch from the source cell to the first candidate target cell may comprise the UE receiving, from the first candidate target cell, a DL communication on a physical downlink control channel (PDCCH) or a physical downlink shared channel (PDSCH). The DL communication cell may be an UL scheduling grant, a DL assignment or an acknowledgement.

The cell switch from the source cell to the first candidate target cell may be unsuccessful for various reasons, such as radio conditions, TA value usage failure, access failure or expiry of a configured grant. Access failure to a target candidate cell may refer to failure to receive DL data until T304 expires, scheduling grant until T304 expires on PDCCH, etc.

At step 21 , the UE may determine that the subset of candidate target cells comprises a second candidate target cell (e.g., cell 2). The UE may start the timer associated with the second candidate target cell. The UE may attempt a cell switch from the source cell to the second candidate target cell (i.e., the UE may attempt access to the second candidate target cell).

The cell switch from the source cell to the second candidate target cell may comprise a RACH less switch from the source cell to the second candidate target cell (i.e., cell switch from the source cell to the second candidate target cell without RACH procedure).

The UE may determine that the cell switch from the source cell to the second candidate target cell is successful. The UE may determine completion of the cell switch from the source cell to the second candidate target cell prior to expiry of the timer associated with the second candidate target cell. Completion of the cell switch from the source cell to the second candidate target cell may comprise the UE receiving a DL communication from the second candidate target cell on a PDCCH or a PDSCH. The DL communication may be an UL scheduling grant, a DL assignment or an acknowledgement.

At step 22, the UE may transmit, to the target DU, a RRC reconfiguration complete message. At step 23, the target DU may transmit, to the gNB-CU, an UL RRC message transfer message.

At step 24, the gNB-CU may transmit, to the source DU, a UE context release command message.

At step 25, the gNB-CU may receive, from the source DU, a UE context release complete message.

At step 26, the gNB-CU may maintain data forwarding to the second candidate target cell amongst the subset of candidate target cells. The gNB-CU may release data forwarding to other candidate target cells amongst the subset of candidate target cells. The gNB-CU may release data forwarding to the source cell.

Alternatively, at step 21 , the UE may determine that the subset of candidate target cells comprises a second candidate target cell (e.g., cell 2). The UE may start the timer associated with the second candidate target cell. The UE may attempt a cell switch from the source cell to the second candidate target cell. The UE may determine that the cell switch from the source cell to the second candidate target cell is unsuccessful.

The UE may determine that the subset of candidate target cells comprises a third candidate target cell (e.g., cell 3). The UE may start the timer associated with the third candidate target cell. The UE may attempt a cell switch from the source cell to the third candidate target cell. The UE may determine that the cell switch from the source cell to the third candidate target cell is successful. The method proceeds with steps similar to steps 22 to 26.

The UE may determine that the subset of candidate target cells comprises a third candidate target cell (e.g., cell 3). The UE may start the timer associated with the third candidate target cell. The UE may attempt a cell switch from the source cell to the third candidate target cell. The UE may determine that the cell switch from the source cell to the third candidate target cell is unsuccessful.

The UE may determine that the subset of candidate target cells does not comprise a fourth candidate target cell. The UE may attempt a cell switch recovery. The UE may select a recovery cell. For example, the UE may detect at least one neighbouring cell. The UE may perform measurements of the least one neighbouring cell. The UE may select the recovery cell amongst the at least one neighbouring cell based on the measurements and at least one selection criterion. The measurements may comprise layer 1 measurements. The layer 1 measurements may comprise at least one of SSB measurements or CSI RS measurements. The layer 1 measurements may comprise at least one of strength measurements or quality measurements. The strength measurements may comprise RSRP measurements. The quality measurements may comprise RSRQ measurements. The selection criterion may comprise at least one of a highest strength or a highest quality.

The recovery cell may or may not be a configured candidate target cell amongst the set of configured candidate target cells.

The recovery cell may or may not be a candidate target cell amongst the subset of candidate target cells.

The UE may determine whether the recovery cell is amongst the set of configured candidate target cells.

If the UE determines that the recovery cell is amongst the set of configured candidate target cells, the UE may attempt a cell switch from the source cell to the recovery cell. The cell switch from the source cell to the recovery cell may be a RACH based cell switch from the source cell to the recovery cell (i.e. , the cell switch from the source cell to the recovery cell includes a RACH procedure with the recovery cell).

If the UE determines that the recovery cell is not amongst the set of configured candidate target cells, the UE may transmit to a gNB-DU providing the recovery cell, a RRC reestablishment request.

One or more of the aspects of this disclosure may be advantageous in that it may reduce the delay to complete a cell switch. The UE may attempt multiple cell switches from the source cell to multiple candidate target cell. The UE may attempt the cell switch recovery procedure from the source cell to a recovery cell when the multiple cell switches from the source cell to the multiple candidate target cells are unsuccessful. One or more of the aspects of this disclosure may be advantageous in that it may reduce the service interruption during a cell switch. The gNB-Cll may forward data to the candidate target cell prior to the UE attempting multiple cell switches from the source cell to multiple candidate target cells.

One or more of the aspects of this disclosure may be advantageous in that it may reduce occurrences of a cell switch recovery.

Fig. 5 shows a block diagram of a method for managing mobility in a communication system.

At 500, a UE may receive, from a BS providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells.

At 502, the UE may determine that the subset of candidate target cells comprises a first candidate target cell.

At 504, the UE may attempt a cell switch from the source cell to the first candidate target cell.

At 506, the UE may determine that the cell switch from the source cell to the first candidate target cell is unsuccessful.

At 508, the UE may determine that the subset of candidate target cells comprises a second candidate target cell.

At 510, the UE may attempt a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells.

Fig. 6 shows a block diagram of method for managing mobility in a communication system.

At 600, a BS may provide a source cell.

At 602, the BS may select a subset of candidate target cells amongst a set of configured candidate target cells. At 604, the BS may transmit, to a UE, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating the subset of candidate target cells.

Fig. 7 shows a schematic representation of non-volatile memory media 1100 storing instructions which when executed by a processor allow the processor to perform one or more of the steps of the methods of Fig. 5 and Fig. 6.

It is noted that while the above describes example embodiments, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention.

The embodiments may thus vary within the scope of the attached claims. In general, some embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although embodiments are not limited thereto. While various embodiments may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The embodiments may be implemented by computer software stored in a memory and executable by at least one data processor of the involved entities or by hardware, or by a combination of software and hardware. Further in this regard it should be noted that any procedures, e.g., as in Fig. 5 and Fig. 6, may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), gate level circuits and processors based on multi-core processor architecture, as non-limiting examples.

Alternatively or additionally some embodiments may be implemented using circuitry. The circuitry may be configured to perform one or more of the functions and/or method steps previously described. The circuitry may be provided in a base station and/or in a user equipment.

As used in this application, the term “circuitry” may refer to one or more or all of the following:

(a) hardware-only circuits (such as only analogue and/or digital circuits);

(b) combinations of hardware circuits and software, such as:

(i) a combination of analogue and/or digital hardware circuit(s) with software/firmware and

(ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as the communications device or base station to perform the various functions previously described; and

(c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

This definition of circuitry applies to all uses of this term “means” in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example integrated device.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of some embodiments However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings will still fall within the scope as defined in the appended claims.

Claims

1. Method comprising: receiving, by a user equipment from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; determining, by the user equipment, that the subset of candidate target cells comprises a first candidate target cell; attempting, by the user equipment, a cell switch from the source cell to the first candidate target cell; determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell is unsuccessful; determining, by the user equipment, that the subset of candidate target cells comprises a second candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells.

2. The method of claim 1 , wherein receiving, by the user equipment from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell comprises: receiving, by the user equipment from a source distributed unit of a base station providing the source cell, a command to trigger the cell switch from the source cell to the candidate target cell.

3. The method of claim 1 or claim 2, wherein the subset of candidate target cells comprises multiple candidate target cells in a priority order; wherein the subset of candidate target cell comprises a preferred candidate target cell and multiple candidate target cells in no priority order; or wherein the subset of candidate target cell comprises a preferred candidate target cell and multiple candidate target cells in a priority order.

4. The method of any of claims 1 to 3, wherein attempting, by the user equipment, a cell switch from the source cell to the first candidate target cell or the second candidate target cell comprises: attempting a random access channel less cell switch from the source cell to the first candidate target cell or the second candidate target cell.

5. The method of any of claims 1 to 4, comprising: determining, by the user equipment, that the cell switch from the source cell to the second target cell is successful.

6. The method of any of claims 1 to 4, comprising: determining, by the user equipment, that the cell switch from the source cell to the second candidate target cell is unsuccessful; determining, by the user equipment, that the subset of candidate target cells comprises a third candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the third candidate target cell.

7. The method of any of claims 1 to 4, comprising: determining, by the user equipment, that the cell switch from the source cell to the second target cell is unsuccessful; determining, by the user equipment, that the subset of candidate target cells does not comprise a third candidate target cell; and attempting, by the user equipment, a cell switch recovery.

8. The method of claim 7, wherein attempting, by the user equipment, a cell switch recovery comprises: selecting, by the user equipment, a recovery cell; determining, by the user equipment, that the recovery cell is amongst the set of configured candidate target cell; and attempting, by the user equipment, a cell switch from the source cell to the recovery cell.

9. The method of claim 8, wherein attempting, by the user equipment, a cell switch from the source cell to the recovery cell comprises attempting a random access channel based cell switch from the source cell to the recovery cell.

10. The method of claim 7, wherein attempting, by the user equipment, a cell switch recovery comprises: selecting, by the user equipment, a recovery cell; determining, by the user equipment, that the recovery cell is not amongst the set of configured candidate target cells; and transmitting, by the user equipment to a base station providing the recovery cell, a radio resource control re-establishment request.

11. The method of any of claims 8 to 10, wherein selecting, by the user equipment, a recovery cell comprises: detecting, by the user equipment, at least one neighbouring cell; and selecting, by the user equipment, the recovery cell amongst the at least one neighbouring cell.

12. The method of any of claims 1 to 11 , wherein the command to trigger a cell switch from the source cell to a candidate target cell includes information indicating at least one of: a subset of timing advance values for the subset of candidate target cells; a subset of timer values for the subset of candidate target cells; or a subset of timer identifiers or pointers identifying or pointing to timers for the subset of candidate target cells.

13. The method of any of claims 1 to 12, wherein determining, by the user equipment, that the cell switch from the source cell to the first candidate target cell or the second candidate target cell is unsuccessful comprises: detecting, by the user equipment, a radio link failure with the first candidate target cell before expiry of a timer for the first candidate target cell or the second candidate target cell; or detecting expiry of a timer for the first candidate target cell or the second candidate target cell or radio link failure prior to expiry of a timer for the first candidate target cell or the second candidate target cell.

14. The method of any of claims 1 to 13, wherein the command to trigger a cell switch from the source cell to the candidate target cell is received in a medium access control control element.

15. The method of any of claims 1 to 14, comprising: transmitting, by the user equipment to the base station, measurements of the set of configured candidate target cells; and wherein the subset of candidate target cells is selected amongst the set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

16. The method of any of claims 1 to 15, comprising: receiving, by the user equipment from the base station, a set of radio resource control configurations for the set of configured candidate target cells; and decoding, by the user equipment, the set of radio resource control configurations for the set of configured candidate target cells.

17. Method comprising: providing, by a base station, a source cell; selecting, by the base station, a subset of candidate target cells amongst a set of configured candidate target cells; and transmitting, by the base station to a user equipment, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating the subset of candidate target cells.

18. The method of claim 17, wherein providing, by a base station, a source cell comprises: providing, by a source distributed unit of the base station, a source cell; wherein selecting, by the base station, the subset of candidate target cells amongst the set of configured candidate target cells comprises: selecting, by the source distributed unit of the base station, the subset of candidate target cells amongst the set of configured candidate target cells; and transmitting, by the base station to the user equipment, a command to trigger the cell switch from the source cell to the candidate target cell comprises: transmitting, by the source distributed unit of the base station to the user equipment, a command to trigger the cell switch from the source cell to the candidate target cell.

19. The method of claim 17 or claim 18, comprising: receiving, by a base station from the user equipment, measurements of the set of configured candidate target cells; and selecting, by the base station, a subset of candidate target cells amongst a set of configured candidate target cells comprises selecting a subset of candidate target cells amongst a set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion.

20. The method of claim 19, wherein receiving, by the base station from the user equipment, measurements of the set of configured candidate target cells comprises: receiving, by the source distributed unit of the base station from the user equipment, measurements of the set of configured candidate target cells; and wherein selecting, by the base station, the subset of candidate target cells amongst the set of configured candidate target cells based on the measurements of the set of configured candidate target cells and at least one selection criterion comprises: selecting, by the source distributed unit of the base station, the subset of candidate target cells amongst the set of configured candidate target cells based on the measurements and at least one selection criterion.

21 . The method of any of claims 18 to 20, comprising: configuring, by a central unit of the base station, data forwarding to the subset of candidate target cells.

22. The method of claim 21 , comprising: receiving, by the central unit of the base station from a target distributed unit of the base station providing a candidate target cell amongst the subset of candidate target cells, information indicating that a cell switch from the source cell to the candidate target cell amongst the subset of candidate target cells is successful; maintaining, by the central unit of the base station, data forwarding to the candidate target cell amongst the subset of candidate target cells; and releasing, by the central unit of the base station, data forwarding to other candidate target cells amongst the subset of candidate target cells.

23. A user equipment comprising: means for receiving, from a base station providing a source cell, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating a subset of candidate target cells selected amongst a set of configured candidate target cells; means for determining that the subset of candidate target cells comprises a first candidate target cell; means for attempting a cell switch from the source cell to the first candidate target cell; means for determining that the cell switch from the source cell to the first candidate target cell is unsuccessful; means for determining that the subset of candidate target cells comprises a second candidate target cell; and means for attempting a cell switch from the source cell to the second candidate target cell amongst the subset of candidate target cells.

24. A base station comprising: means for providing a source cell; means for selecting a subset of candidate target cells amongst a set of configured candidate target cells; and means for transmitting, to a user equipment, a command to trigger a cell switch from the source cell to a candidate target cell, wherein the command includes information indicating the subset of candidate target cells.

25. Computer program comprising computer executable instructions which when run on one or more processors perform the steps of the method of any of claims 1 to 22.