WO2025214714A1

WO2025214714A1 - Beam cross-cell reference signal mapping in a terminal and associated measurements

Info

Publication number: WO2025214714A1
Application number: PCT/EP2025/057082
Authority: WO
Inventors: Diomidis Michalopoulos; Daniel Medina; Ali Karimidehkordi; Sanjay Goyal
Original assignee: Nokia Technologies Oy
Current assignee: Nokia Technologies Oy
Priority date: 2024-04-10
Filing date: 2025-03-14
Publication date: 2025-10-16
Anticipated expiration: 2026-10-10

Abstract

A technical solution is provided, which allows reducing unnecessary Reference Signal (RS) transmissions and configurations associated with each target or non-serving cell, resulting in a reduction in unnecessary RS measurements performed by UEs For this purpose, a mapping is used, which associates each RS of a serving cell with one or more RSs of each non-serving cell. The mapping may be created by a network node based on past RS measurements relating to previous handover events, or a UE may be configured by the network node to do so. By using the mapping, the UE determines which RS(s) of each non-serving cell is(are) associated with the strongest RS(s) of the serving cell, whereupon the UE starts performing measurements on each of the determined RSs of the non-serving cell. In one other alternative embodiment, the mapping created by the network node is provided to another network node rather than the UE, and said another network node configures and broadcasts only those RSs which are associated with the strongest RS(s) of the serving cell.

Description

BEAM CROSS-CELL REFERENCE SIGNAL MAPPING IN A TERMINAL AND ASSOCIATED MEASUREMENTS

TECHNICAL FIELD

The present disclosure relates generally to the field of wireless communications. In particular, the present disclosure relates to techniques for cross-cell Reference Signal (RS) mapping and measurement, e.g., in lower-level (i.e. Level 1 (LI) or Level 2 (L2)) mobility scenarios.

BACKGROUND

Ll/2 inter-cell mobility was one of the objectives for mobility enhancement in Rel.18, and further enhancements are under discussion in Rel.19. In contrast to Level 3 (L3) mobility procedures where handover between two cells is decided by a Radio Resource Control (RRC) layer, Ll/2 inter-cell mobility is performed by a Medium Access Control (MAC) layer terminated, e.g., in a Distributed Unit (DU) in the split or disaggregated architecture of a Radio Access Network (RAN) node (e.g., gNB).

In case of Channel State Information RS (CSI-RS) configuration and transmission in Ll/2 intercell mobility scenarios, a target DU (i.e., the one to which a User Equipment (UE) should be handed over from a serving DU) typically configures all CSI-RSs for the UE to ensure that all of them are measured by the UE. However, this leads to the following drawbacks: (1) the serving DU must transmit all CSI-RS indices even in the case where they are not needed (e.g., if their parent SSB-index is not detectable by the UE); and (2) the UE needs to perform measurements on all configured CSI-RS indices of a prepared target cell which can be a limiting factor given that multiple cells can be prepared in Ll/L2-triggered mobility (LTM).

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure.

It is an objective of the present disclosure to provide a technical solution to reduce unnecessary RS (e.g., CSI-RS) transmissions and configurations associated with each prepared target or candidate cell, resulting in a reduction in unnecessary RS (e.g., CSI-RS) measurements performed by UEs.

The objective above is achieved by the features of the independent claims in the appended claims. Further embodiments and examples are apparent from the dependent claims, the detailed description, and the accompanying drawings.

According to a first aspect, a UE in a Radio Access Network (RAN) is provided. The UE comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the UE to operate at least as follows. At first, the UE receives control information from a network node of the RAN. The control information comprises a first indication for the UE to collect RS measurements to be performed by the UE during at least one handover event within a predefined time period. The RS measurements relate to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell. The control information further comprises a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period. Then, the UE creates the mapping based on the control information and the RS measurements. Next, the UE determines, among the first set of RSs, at least one strongest RS in terms of a signal strength. Further, the UE uses the at least one strongest RS and the created mapping to determine, among the second set of RSs, at least one target RS to be measured. After that, the UE performs next measurements on the at least one target RS. By using this mapping, it is possible to provide a relationship between the RSs of the serving cell and the RSs of the non-serving cell (which may be a potent target cell for a next handover event). The knowledge of this relationship may allow the UE to measure only the most relevant RS(s) of the non-serving cell, thereby reducing the number of unnecessary RS measurements which the UE would otherwise perform. In one example embodiment of the first aspect, the mapping between the first set of RSs and the second set of RSs comprises at least one of: a one-to-one mapping between one RS from the first set of RSs and one RS from the second set of RSs; a one-to-many mapping between one RS from the first set of RSs and a subset of RSs from the second set of RSs; a many-to-one mapping between a subset of RSs from the first set of RSs and one RS from the second set of RSs; and a many-to-many mapping between a subset of RSs from the first set of RSs and a subset of RSs from the second set of RSs. Such different types of the mapping may provide flexibility in determining which RS(s) of the non-serving is(are) to be measured, which may be beneficial in some use scenarios. For example, in scenarios where cell handover occurs in the area of the serving cell where at least two beams are received with similar power, for a large majority of observations, then the mapping between the RSs of the serving cell and the RSs of the non-serving cell should involve all such multiple RSs from the serving cells, since the mapping becomes then more accurate.

In one example embodiment of the first aspect, the UE is caused to determine, among the first set of RSs, at least two strongest RSs in terms of the signal strength. In this embodiment, the UE is caused to determine the at least one target RS based on which of the at least two strongest RSs is stronger. In other words, this embodiment involves ordering the at least two strongest RSs according to their signal strength, and the ordered list of the at least two strongest RSs is then used by the UE together with the received mapping to determine one or more target RSs among the second set of RSs. This embodiment may be beneficial when it is required to provide the mapping between multiple RSs of the serving cell and multiple RSs of the non-serving cell.

In an alternative example embodiment of the first aspect, the UE is caused to determine, among the first set of RSs, at least two strongest RSs in terms of the signal strength. In this embodiment, the UE is caused to determine the at least one target RS based on a relative value of the signal strength among the at least two strongest RSs. This embodiment may also be beneficial when it is required to provide the mapping between multiple RSs of the serving cell and multiple RSs of the non-serving cell.

In one example embodiment of the first aspect, the serving cell and the non-serving cell are both controlled by the network node. In an alternative example embodiment of the first aspect, the serving cell is controlled by the network node, and the non-serving cell is controlled by a different network node. Thus, the mapping may be applied with respect to the RSs of the cells served by the same network node or by different network nodes, which may be beneficial in some use scenarios.

In one example embodiment of the first aspect, the UE is caused to receive the control information via an RRC message. By using the existing RRC signaling, the UE may be properly informed of the mapping to be applied for RS measurements. The use of the existing RRC signaling may also avoid unnecessary increase of signaling overhead in the network.

In one example embodiment of the first aspect, the UE is further caused to transmit a measurement report comprising the measurements on the determined at least one target RS to the network node and, in response, receive a switching instruction from the network node. The switching instruction may cause the UE to switch to a target beam of the multiple beams of the non-serving cell. The target beam is selected by the network node based on the measurements on the at least one target RS. Alternatively, the switching instruction may cause the UE to determine, among the multiple beams of the non-serving cell, a beam that is the best in terms of a beam quality, and switch to the beam. Thus, the measurements performed by the UE on one or more target RSs found by using the mapping may contribute to the network node making a correct handover decision.

According to a second aspect, a network node in a RAN is provided. The network node comprises at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the network node to operate at least as follows. At first, the network node generates control information. The control information comprises a first indication for a UE to collect RS measurements to be performed by the UE during at least one handover event within a predefined time period. The RS measurements relate to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell. The control information further comprises a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period. Then, the network node transmits the control information to the UE. By using this mapping, it is possible to provide a relationship between the RSs of the serving cell and the RSs of the non-serving cell (which may be a potential target cell for a next handover event). The knowledge of this relationship may allow the UE to measure only the most relevant RSs of the non-serving cell, thereby reducing the number of unnecessary RS measurements which the UE would otherwise perform.

In one example embodiment of the second aspect, the mapping between the first set of RSs and the second set of RSs comprises at least one of: a one-to-one mapping between one RS from the first set of RSs and one RS from the second set of RSs; a one-to-many mapping between one RS from the first set of RSs and a subset of RSs from the second set of RSs; a many-to-one mapping between a subset of RSs from the first set of RSs and one RS from the second set of RSs; and a many-to-many mapping between a subset of RSs from the first set of RSs and a subset of RSs from the second set of RSs. Such different types of the mapping may provide flexibility in determining which RS(s) of the non-serving is(are) to be measured, which may be beneficial in some use scenarios. For example, in scenarios where cell handover occurs in the area of the serving cell where at least two beams are received with similar power, for a large majority of observations, then the mapping between the RSs of the serving cell and the RSs of the non-serving cell should involve all such multiple RSs from the serving cells, since the mapping becomes then more accurate.

In one example embodiment of the second aspect, the serving cell and the non-serving cell are both controlled by the network node. In an alternative example embodiment of the first aspect, the serving cell is controlled by the network node, and the non-serving cell is controlled by a different network node. Thus, the mapping may be applied with respect to the RSs of the cells served by the same network node or by different network nodes, which may be beneficial in some use scenarios.

In one example embodiment of the second aspect, the network node is caused to transmit the control information via an RRC message. By using the existing RRC signaling, the network node may properly inform the UE of the mapping. The use of the existing RRC signaling may also avoid unnecessary increase of signaling overhead in the network.

In one example embodiment of the second aspect, the network node is further caused to receive a measurement report from the UE. The measurement report comprises measurements on at least one target RS of the second set of reference signals. In response to this report, the network node determines whether the UE needs to switch to one of the multiple beams of the non-serving cell. If it is determined that the UE needs to switch one of the multiple beams of the non-serving cell, the network node transmits a switching instruction to the UE. The switching instruction may cause the UE to switch to a target beam of the multiple beams of the non-serving cell. The target beam is selected by the network node based on the measurements on the at least one target RS. Alternatively, the switching instruction may cause the UE to determine, among the multiple beams of the non-serving cell, a beam that is the best in terms of a beam quality, and switch to the beam. Thus, the measurements performed by the UE on one or more target RSs found by using the mapping may contribute to the network node making a correct handover decision.

According to a third aspect, a method for operating a UE in a RAN is provided. The method starts with the step of receiving control information from a network node of the RAN. The control information comprises a first indication for the UE to collect RS measurements to be performed by the UE during at least one handover event within a predefined time period. The RS measurements relate to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell. The control information further comprises a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period. The method then proceeds to the step of creating the mapping based on the control information and the RS measurements. Next, the method goes on to the step of determining, among the first set of RSs, at least one strongest RS in terms of a signal strength. Further, the method proceeds to the step of using the determined at least one strongest RS and the created mapping to determine, among the second set of RSs, at least one target RS to be measured. After that, the method goes on to the step of performing next measurements on the at least one target RS. By using this mapping, it is possible to provide a relationship between the RSs of the serving cell and the RSs of the nonserving cell (which may be a potential target cell for a next handover event). The knowledge of this relationship may allow the UE to measure only the most relevant RSs of the nonserving cell, thereby reducing the number of unnecessary RS measurements which the UE would otherwise perform.

According to a fourth aspect, a method for operating a network node in a wireless communication network is provided. The method starts with the step of generating control information. The control information comprises a first indication for a UE to collect RS measurements to be performed by the UE during at least one handover event within a predefined time period. The RS measurements relate to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell. The control information further comprises a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period. Then, the method proceeds to the step of transmitting the control information to the UE. By using this mapping, it is possible to provide a relationship between the RSs of the serving cell and the RSs of the nonserving cell. The knowledge of this relationship may allow the UE to measure only the most relevant RSs of the non-serving cell, thereby reducing the number of unnecessary RS measurements which the UE would otherwise perform.

According to a fifth aspect, a computer program product is provided. The computer program product comprises a computer-readable storage medium that stores a computer code. Being executed by at least one processor, the computer code causes the at least one processor to perform the method according to the third aspect. By using such a computer program product, it is possible to simplify the implementation of the method according to the third aspect in any UE, like the UE according to the first aspect.

According to a sixth aspect, a computer program product is provided. The computer program product comprises a computer-readable storage medium that stores a computer code. Being executed by at least one processor, the computer code causes the at least one processor to perform the method according to the fourth aspect. By using such a computer program product, it is possible to simplify the implementation of the method according to the fourth aspect in any network node, like the network node according to the second aspect.

According to a seventh aspect, a UE in a wireless communication network is provided. The UE comprises a means for receiving control information from a network node of the RAN. The control information comprises a first indication for the UE to collect RS measurements to be performed by the UE during at least one handover event within a predefined time period. The RS measurements relate to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell. The control information further comprises a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period. The UE further comprises a means for creating the mapping based on the control information and the RS measurements and a means for determining, among the first set of RSs, at least one strongest RS in terms of a signal strength. The UE further comprises a means for using the at least one strongest RS and the created mapping to determine, among the second set of RSs, at least one target RS to be measured. The UE further comprises a means for performing next measurements on the at least one target RS. By using this mapping, it is possible to provide a relationship between the RSs of the serving cell and the RSs of the non-serving cell (which may be a potential target cell for a next handover event). The knowledge of this relationship may allow the UE to measure only the most relevant RSs of the non-serving cell, thereby reducing the number of unnecessary RS measurements which the UE would otherwise perform.

According to an eighth aspect, a network node in a wireless communication network is provided. The network node comprises a means for generating control information. The control information comprises a first indication for a UE to collect RS measurements to be performed by the UE during at least one handover event within a predefined time period. The RS measurements relate to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell. The control information further comprises a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period. The network node further comprises a means for transmitting the control information to the UE. By using this mapping, it is possible to provide a relationship between the RSs of the serving cell and the RSs of the non-serving cell (which may be a potential target cell for a next handover event). The knowledge of this relationship may allow the UE to measure only the most relevant RSs of the non-serving cell, thereby reducing the number of unnecessary RS measurements which the UE would otherwise perform.

Other features and advantages of the present disclosure will be apparent upon reading the following detailed description and reviewing the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is explained below with reference to the accompanying drawings in which:

FIG. 1 shows a signaling diagram for L1/L2 inter-cell mobility in accordance with the prior art;

FIG. 2 shows a block diagram of a UE in a wireless communication network in accordance with one example embodiment;

FIG. 3 shows a flowchart of a method for operating the UE of FIG. 2 in accordance with a first example embodiment;

FIG. 4 shows a block diagram of a serving network node in a wireless communication network in accordance with one example embodiment;

FIG. 5 shows a flowchart of a method for operating the serving network node of FIG. 4 in accordance with a first example embodiment;

FIG. 6 shows a signaling diagram for L1/L2 inter-cell mobility, in which the UE of FIG. 2 and the serving network node of FIG. 4 interact with each other in accordance with the methods of FIGs. 3 and 5, respectively;

FIG. 7 schematically shows CSI-RS beams across two neighboring cells, with the UE of FIG. 2 being handed over from one cell to another;

FIG. 8 shows a Probability Mass Function (PMF) of target cell CSI-RS indices for a given serving cell CSI-RS index;

FIG. 9 shows a flowchart of a method for operating the UE of FIG. 2 in accordance with a second example embodiment;

FIG. 10 shows a flowchart of a method for operating the serving network node of FIG. 4 in accordance with a second example embodiment;

FIG. 11 shows a signaling diagram for L1/L2 inter-cell mobility, in which the UE of FIG. 2 and the serving network node of FIG. 4 interact with each other in accordance with the methods FIGs. 9 and 10, respectively; FIG. 12 shows a flowchart of a method for operating the serving network node of FIG. 4 in accordance with a third example embodiment;

FIG. 13 shows a block diagram of a neighboring network node in a wireless communication network in accordance with one example embodiment;

FIG. 14 shows a flowchart of a method for operating the neighboring network node of FIG. 12 in accordance with one example embodiment; and

FIG. 15 shows a signaling diagram for L1/L2 inter-cell mobility, in which the UE of FIG. 2 and the serving network node of FIG. 4 interact with each other in accordance with the methods FIGs. 12 and 14, respectively.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are further described in more detail with reference to the accompanying drawings. However, the present disclosure can be embodied in many other forms and should not be construed as limited to any certain structure or function discussed in the following description. In contrast, these embodiments are provided to make the description of the present disclosure detailed and complete.

According to the detailed description, it will be apparent to the ones skilled in the art that the scope of the present disclosure encompasses any embodiment thereof, which is disclosed herein, irrespective of whether this embodiment is implemented independently or in concert with any other embodiment of the present disclosure. For example, the apparatus and method disclosed herein can be implemented in practice by using any numbers of the embodiments provided herein. Furthermore, it should be understood that any embodiment of the present disclosure can be implemented using one or more of the elements presented in the appended claims.

Unless otherwise stated, any embodiment recited herein as "example embodiment" should not be construed as preferable or having an advantage over other embodiments.

According to the example embodiments disclosed herein, a User Equipment (UE) may refer to an electronic computing device that is configured to perform wireless communications. The UE may be implemented as a mobile station, a mobile terminal, a mobile subscriber unit, a mobile phone, a cellular phone, a smart phone, a cordless phone, a personal digital assistant (PDA), a wireless communication device, a desktop computer, a laptop computer, a tablet computer, a gaming device, a netbook, a smartbook, an ultrabook, a medical mobile device or equipment, a biometric sensor, a wearable device (e.g., a smart watch, smart glasses, a smart wrist band, etc.), an entertainment device (e.g., an audio player, a video player, etc.), a vehicular component or sensor (e.g., a driver-assistance system), a smart meter/sensor, an unmanned vehicle (e.g., an industrial robot, a quadcopter, etc.) and its component (e.g., a self-driving car computer), industrial manufacturing equipment, a global positioning system (GPS) device, an Internet-of-Things (loT) device, an Industrial loT (I loT) device, a machine-type communication (MTC) device, a group of Massive loT (MIoT) or Massive MTC (mMTC) devices/sensors, or any other suitable mobile device configured to support wireless communications. In some embodiments, the UE may refer to at least two collocated and inter-connected UEs thus defined.

As used in the example embodiments disclosed herein, a network node may refer to a node in any of a Radio Access Network (RAN) and a Core Network (CN). It should be noted that the CN may refer to a network intended for connecting different RAN nodes by providing proper interfaces therebetween. The CN may also provide a gateway to other networks, for example, a Data Network (DN).

Being part of the RAN, the network node may be implemented as a fixed point of communication/communication node for a UE in a particular wireless communication network. More specifically, the RAN node may be used to connect the UE to the DN through the CN and may be referred to as a base transceiver station (BTS) in terms of the 2G communication technology, a NodeB in terms of the 3G communication technology, an evolved NodeB (eNodeB) in terms of the 4G communication technology, and a gNB in terms of the 5G New Radio (NR) communication technology. The RAN node may serve different cells, such as a macrocell, a microcell, a picocell, a femtocell, and/or other types of cells. The macrocell may cover a relatively large geographic area (for example, at least several kilometers in radius). The microcell may cover a geographic area less than two kilometers in radius, for example. The picocell may cover a relatively small geographic area, such, for example, as offices, shopping malls, train stations, stock exchanges, etc. The femtocell may cover an even smaller geographic area (for example, a home).

Being part of the CN, the network node may also refer to any of CN network functions, such as an Access and Mobility Management Function (AMF), a Session Management Function (SMF), Unified Data Management (UDM), User Plane Function (UPF), Policy Control Function (PCF), etc. The AMF supports termination of Non-Access Stratum (NAS) signalling, NAS ciphering and integrity protection, registration management, connection management, mobility management, access authentication and authorization, security context management. The SMF supports session management (session establishment, modification, release), UE IP address allocation and management, Dynamic Host Configuration Protocol (DHCP) functions, termination of NAS signalling related to the session management, downlink (DL) data notification, traffic steering configuration for the UPF for proper traffic routing. UDM supports Authentication and Key Agreement (AKA) credentials generation, user identification handling, access authorization, subscription management. The UPF supports packet routing and forwarding, packet inspection, Quality of Service (QoS) handling, acts as an external Protocol Data Unit (PDU) session point of interconnect to the DN, and is an anchor point for intra- and inter- Radio Access Technology (RAT) mobility. The PCF supports a unified policy framework, providing policy rules to Control Plane (CP) functions, access subscription information for policy decisions in a Unified Data Repository (UDR).

According to the example embodiments disclosed herein, a wireless communication network, in which a UE and a network node communicate with each other, may refer to a cellular or mobile network, a Wireless Local Area Network (WLAN), a Wireless Personal Area Networks (WPAN), a Wireless Wide Area Network (WWAN), a satellite communication (SATCOM) system, or any other type of wireless communication networks. Each of these types of wireless communication networks supports wireless communications according to one or more communication protocol standards. For example, the cellular network may operate according to the Global System for Mobile Communications (GSM) standard, the Code-Division Multiple Access (CDMA) standard, the Wide-Band Code-Division Multiple Access (WCDM) standard, the Time-Division Multiple Access (TDMA) standard, or any other communication protocol standard, the WLAN may operate according to one or more versions of the IEEE 802.11 standards, the WPAN may operate according to the Infrared Data Association (IrDA), Wireless USB, Bluetooth, or ZigBee standard, and the WWAN may operate according to the Worldwide Interoperability for Microwave Access (WiMAX) standard.

FIG. 1 shows a signaling diagram 100 for L1/L2 inter-cell mobility in accordance with the prior art. More specifically, the signaling diagram 100 relates to one exemplary implementation of Ll/2 inter-cell mobility from a serving cell in Distributed Unit 1 (DU1) to a target cell in DU2 (i.e., an inter-DU intra-Control Unit (CU) scenario). The same diagram would apply as well in case of intra-DU intra-CU cell change where DU1 would be the same as DU2.

The signaling diagram 100 starts with a step S102, in which a UE sends a measurement report to DU1. The measurement report contains the cell quality measurements of serving and neighboring cells. The UE can be configured by DU1 (its serving network node in this case) to send the measurement report early when it still has a good connection to DU1.

Then, the signaling diagram 100 proceeds to a step S104, in which DU1 forwards the measurement report to the CU. The CU uses the reported cell quality measurements to identify a potential set of candidate target cells to which the UE can be handed over. In this example, the CU identifies candidate target cells that are served by DU1 and another DU2 controlled by the same CU.

Next, the signaling diagram 100 goes on to a step S106, in which the CU requests the preparation of a candidate target cell controlled by DU1 by sending a UE Context Modification Request message.

In response, in a step S108, DU1 provides the configuration of the UE in a UE Context Modification Response message containing a container from DU1 to the CU.

Next steps S110 and S112 of the signaling diagram 100 are similar to the steps S106 and S108, respectively, except that they are performed using DU2 to prepare the target cell(s) that are controlled by DU2.

Having received the UE configurations for the candidate target cell(s) controlled by DU1 and DU2, the CU generates an RRC Reconfiguration message in a step S114. The RRC Reconfiguration message is further sent to the UE in a step S116. Among other information, the RRC Reconfiguration message contains: (1) a measurement reporting configuration for Ll/2 handover, i.e., configuration on how to report the LI beam measurements of serving and target cells (in a step S120), and (2) the configuration of the prepared candidate cell(s) which the UE needs to execute when it receives a MAC Control Element (CE) command to change the serving cell (i.e., perform handover) (in a step S122).

After confirming the RRC Reconfiguration to the CU in a step S118, the UE starts to report periodically the LI beam measurement of the serving and candidate target cells in the step S120.

Subsequently, the signaling diagram 100 goes on to a step S122, in which upon determining that there is a target candidate cell having a better radio link/beam measurement than the serving cell, e.g., [Ll-RSRP of target beam measurement] > [Ll-RSRP of serving beam measurement] + [Offset for an amount of time e.g., Time-to-Trigger (TTT)], DU1 sends the MAC CE command or a LI message to the UE to trigger the cell change to the target candidate cell. The handover from the serving cell to the target cell is executed by the UE in a last step S124.

In order to trigger the cell change in the step S122, the serving DU (i.e., DU1) needs to indicate a Transmission Configuration Index (TCI) State which includes QCL information for receiving on a Physical Downlink Control Channel (PDCCH)/Physical Downlink Shared Channel (PDSCH) from the target cell (DL reception) or/and for transmitting on a Physical Uplink Control Channel (PUCCH)/Physical Uplink Shared Channel (PUSCH) (UL transmission). The QCL information contains: 1) a Reference Signal (RS), 2) a QCL type (which may be any of typeA, typeB, typeC and typeD), and 3) the bandwidth part (bwp) in which the RS is located. The definitions of the QCL types are provided in TS 38.214 as follows:

- 'typeA': {Doppler shift, Doppler spread, average delay, delay spread},

- 'typeB': {Doppler shift, Doppler spread},

- 'typeC: {Doppler shift, average delay},

- 'typeD': {Spatial Rx parameter}.

For switching a serving beam, one of the following options are allowed for the QCL information: (1) qcl-Typel: referencesignal = Tracking RS (TRS) index where TRS is a special configuration of a CSI-RS index (see TS 38.331) and qcl-Type = A, qcl-Type2 (applicable only in Frequency Range 2 (FR2)): referencesignal = same TRS index and qcl-Type = D.

(2) qcl-Typel: referencesignal = Tracking RS (TRS) index and qcl-Type = A, qcl-Type2 (applicable only in the FR2): referencesignal = CSI-RS index with repetition and qcl-Type = D.

For example, in option (1) and for the FR1, DU1 points to the TRS index as the QCL source which shall be used by the UE to estimate the channel properties (i.e., the doppler shift, doppler spread, average delay, delay spread) for receiving the PDCCH/PDSCH or/and transmitting the PUCCH/PUSCH.

However, in order for the serving DU (i.e., DU1) to indicate the TCI state including the QCL- information with the CSI-RS index as the source RS, the target DU (i.e., DU2) controlling the prepared target cells can provide the list of CSI-RSs that shall be measured by the UE. Herein, DU2 shall be also aware of the configuration to select an appropriate TCI state of the target cell for the UE to be used upon cell switch.

To account for all possible target CSI-RSs measured by the UE, DU2 needs to configure them all for the UE. In other words, DU2 has to transmit all CSI-RS indices even if some of them are not needed (e.g., if their parent SSB-index is not detectable by the UE). All of this will lead to unnecessary RS measurements performed by the UE.

The example embodiments disclosed herein provide a technical solution to reduce unnecessary RS (e.g., CSI-RS) transmissions and configurations associated with each prepared target (candidate or non-serving) cell, resulting in a reduction in unnecessary RS (e.g., CSI-RS) measurements performed by UEs. For this purpose, a mapping (e.g., in the form of a mapping table) is used, which associates each RS of a serving cell with one or more RSs of each non-serving cell in a wireless communication network (e.g., RAN). The mapping may be created by a network node of the RAN based on past measurements relating to previous handover events, or a UE may be configured by the network node to do so. By using the mapping, the UE determines a subset of RSs of each non-serving cell which is associated with n strongest (or best in terms of a signal strength, and/or arranged in a specified order according the signal strength) RSs of the serving cell, whereupon the UE performs measurements on the determined subset of RSs. In one other alternative embodiment, the mapping created by the network node is provided to a target network node rather than the UE, and the target network node configures only those RSs which are associated with the n strongest RSs of the serving cell.

FIG. 2 shows a block diagram of a UE 200 in a RAN in accordance with one example embodiment. As shown in FIG. 2, the UE 200 comprises a processor 202 and a memory 204. The memory 204 stores processor-executable instructions 206 which, when executed by the processor 202, cause the processor 202 to perform the aspects of the present disclosure, as will be described below in more detail. It should be noted that the number, arrangement, and interconnection of the constructive elements constituting the UE 200, which are shown in FIG. 2, are not intended to be any limitation of the present disclosure, but merely used to provide a general idea of how the constructive elements may be implemented within the UE 200. For example, the processor 202 may be replaced with several processors, as well as the memory 204 may be replaced with several removable and/or fixed storage devices, depending on particular applications. Furthermore, in some embodiments, the processor 202 may perform different operations required to perform data reception and transmission, such, for example, as signal modulation/demodulation, encoding/decoding, etc. Alternatively, the UE 200 may further comprise an individual transceiver which can be configured to perform the required operations for data reception and transmission based on commands from the processor 202.

The processor 202 may be implemented as a CPU, general-purpose processor, singlepurpose processor, microcontroller, microprocessor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), digital signal processor (DSP), complex programmable logic device, etc. It should be also noted that the processor 202 may be implemented as any combination of one or more of the aforesaid. As an example, the processor 202 may be a combination of two or more microprocessors.

The memory 204 may be implemented as a classical nonvolatile or volatile memory used in the modern electronic computing machines. As an example, the nonvolatile memory may include Read-Only Memory (ROM), ferroelectric Random-Access Memory (RAM), Programmable ROM (PROM), Electrically Erasable PROM (EEPROM), solid state drive (SSD), flash memory, magnetic disk storage (such as hard drives and magnetic tapes), optical disc storage (such as CD, DVD and Blu-ray discs), etc. As for the volatile memory, examples thereof include Dynamic RAM, Synchronous DRAM (SDRAM), Double Data Rate SDRAM (DDR SDRAM), Static RAM, etc.

The processor-executable instructions 206 stored in the memory 204 may be configured as a computer-executable program code which causes the processor 202 to perform the aspects of the present disclosure. The computer-executable program code for carrying out operations or steps for the aspects of the present disclosure may be written in any combination of one or more programming languages, such as Java, C++, Python, or the like. In some examples, the computer-executable program code may be in the form of a high- level language or in a pre-compiled form and be generated by an interpreter (also pre-stored in the memory 204) on the fly.

FIG. 3 shows a flowchart of a method 300 for operating the UE 200 in accordance with a first example embodiment. The method 300 starts with a step S302, in which the processor 202 receives control information from a network node (e.g., gNB) of the RAN. The step S302 may be performed by using RRC signaling, for example. Furthermore, the step S302 may be performed after the UE 200 establishes a connection to the network node. Alternatively, the network node may broadcast the control information, e.g., as part of a System Information Block (SIB) message. The control information comprises a mapping between a first set of RSs, i.e., {RSi, ..., RSM}, associated with multiple beams of a serving cell and a second set of RSs, i.e., {RS'i, ..., RS'N}, associated with multiple beams of a non-serving cell, where M and N are natural numbers. For example, the mapping may associate each RS of {RSi, ..., RSM} with one or more RSs of {RS'i, ..., RS'N}, or the mapping may associate a (properly ordered according to the signal strength) subset of RSs of {RSi, ..., RSM} with a subset of RSs of {RS'i, ..., RS'N}. The serving cell and the non-serving cell may be both controlled by the network node with the UE 200 has established the connection. Alternatively, the serving cell may be controlled by the network node, while the non-serving cell may be controlled by a different network node of the RAN. Next, the method 300 proceeds to a step S306, in which the processor 202 determines, among {RSi, ..., RSM}, one or more strongest RSs in terms of a signal strength. The signal strength may refer to a Signal to Interference plus Noise Ratio (SINR), Reference Signal Received Quality (RSRQ) and/or Reference Signal Received Power (RSRP). Further, the method 300 goes on to a step S308, in which the processor 202 uses said one or more strongest RSs and the received mapping to determine, among {RS'i, ..., RS'N}, one or more target RSs to be measured. After that, the method 300 proceeds to a step S310, in which the processor 202 starts performing measurements on the target RS(s). The measurements may involve SINR, RSRQ, and/or RSRP measurements on the subset of RSs.

The mapping may imply a one-to-one mapping between one RS from the first set of RSs and one RS from the second set of RSs (e.g., RSi -> RS' 2, which means that RSi is mapped to RS'2). The mapping may additionally or alternatively imply a one-to-many mapping between one RS from the first set of RSs and a subset of RSs from the second set of RSs (e.g., (e.g., RSi -> {RS'2, RS'3}). The mapping may additionally or alternatively imply a many-to-one mapping between a subset of RSs from the first set of RSs and one RS from the second set of RSs (e.g., {RS'i, RS'2} -> RS'2). The mapping may additionally or alternatively imply a many-to-many mapping between a subset of RSs from the first set of RSs and a subset of RSs from the second set of RSs (e.g., {RSi, RS3} -> {RS'2, RS'3}).

In one example embodiment, the processor 202 may determine, in the step S306, at least two strongest RSs among {RSi, ..., RSM} and determine, in the step S308, the target RS(s) based on which of the at least two strongest RSs is stronger. That is, the processor 202 may order the at least two strongest RSs first according to their signal strength, whereupon it may use the ordered list or subset of the at least two strongest RSs in the step S308. Alternatively, the processor 202 may determine, in the step S306, at least two strongest RSs among {RSi, ..., RSM}, but now determine, in the step S308, the target RS(s) based on the relative value of the signal strength among the at least two strongest RSs.

In one example embodiment, the method 300 may comprise additional steps, in which the processor 202 transmits a measurement report comprising the measurements on the determined subset of RSs to the network node and, in response, receive a switching instruction from the network node. The switching instruction may cause the processor 202 of the UE 200 to switch to a target beam of the multiple beams of the non-serving cell. The target beam is selected by the network node based on the measurements on the target RS(s). Alternatively, the switching instruction may cause the processor 202 of the UE 200 to determine, among the multiple beams of the non-serving cell, a beam that is the best in terms of a beam quality, and switch to the beam. The measurement report may be transmitted to the network node, for example, by using LI signaling. FIG. 4 shows a block diagram of a serving network node 400 in the wireless communication network in accordance with one example embodiment. The serving network node 400 is intended to communicate with the UE 200 in the RAN. As shown in FIG. 4, the network node 400 comprises a processor 402 and a memory 404 coupled to the processor 402. The memory 404 stores processor-executable instructions 406 which, when executed by the processor 402, cause the processor 402 to implement the aspects of the present disclosure, as will be described below in more detail. It should be again noted that the number, arrangement, and interconnection of the constructive elements constituting the serving network node 400, which are shown in FIG. 4, are not intended to be any limitation of the present disclosure, but merely used to provide a general idea of how the constructive elements may be implemented within the serving network node 400. In general, the processor 402, the memory 404, and the processor-executable instructions 406 may be implemented in the same or similar manner as the processor 202, the memory 204, and the processor-executable instructions 206, respectively.

FIG. 5 shows a flowchart of a method 500 for operating the serving network node 400 in accordance with a first example embodiment. The method 500 starts with a step S502, in which the processor 402 collects past RS measurements performed by the UE 200 (i.e., the processor 202) during one or more past handover events in the RAN. The past RS measurements relate to the set of RSs, i.e., {RSi, ..., RSM}, and the second set of RSs, i.e., {RS'i, ..., RS'N}. The past RS measurements may relate to one or more RSs among {RSi, ..., RSM} and one or more RSs among {RS'i, ..., RS'N}. Additionally or alternatively, the past measurements may relate to one or more RSs of one or more cells similar to the serving cell and/or non-serving cell. Furthermore, the past measurements may be performed by the UE 200 and/or any other UEs in the serving and non-serving cells. In a next step S504, the processor 402 uses the past RS measurements to generate the control information for the UE 200. The mapping contained in the control information may be obtained by applying a conditional probability mass function or a joint probability mass function to the past RS measurements. As one non-limiting example, the mapping may be in the form of a mapping table. Next, the method 500 goes on to a step S504, in which the processor 402 transmits the control information to the UE 200 (i.e., the processor 202). As noted earlier, the step S504 may be performed by using RRC signaling. In one example embodiment, the method 500 may comprise additional steps, in which the processor 402 receives the measurement report from the UE 200, determines whether the UE 200 needs to switch to one of the multiple beams of the non-serving cell, and if so, transmits the switching instruction to the UE 200.

FIG. 6 shows a signaling diagram 600 for L1/L2 inter-cell mobility, in which the UE 200 and the serving network node 400 interact with each other in accordance with the methods 300 and 500, respectively. In the signaling diagram 600, the network node 400 is assumed to have a split or disaggregated architecture, in which DU1 serves a serving cell in which the UE 200 is currently present, DU2 serves at least one non-serving cell, and a CU controls each of the DU1 and DU2. It should be noted that the present disclosure is not limited to the split or disaggregated architecture shown in FIG. 6 - in other embodiments, there may be no CU, and DU1 may be replaced by the network node 400 and DU2 may be replaced by another (e.g., neighboring) network node, without departing from the teachings of the present disclosure.

The signaling diagram 600 starts with a step S602, in which the CU generates the control information for the UE 200 in accordance with the step S502 of the method 500. In this example, the mapping is assumed to in the form of a mapping table that associates each CSI- RS of the serving cell with one or more CSI-RSs of the non-serving cell, and the mapping table itself is obtained based on past handover-related measurements pertaining to the CSI-RS(s) of the serving cell at the point the handover is triggered (equivalently, at the point the MAC CE command is sent in a step S626 of the signaling diagram 600). Next steps S604-S614 of the signaling diagram 600 are the same as the step S102-S112 of the signaling diagram 100, respectively. In a next step S616, the CU generates an RRC reconfiguration message and indicates the control information with the mapping table in the RRC reconfiguration message. The CU then sends the RRC reconfiguration message to the UE 200 in a step S618. In general, the step S618 of the signaling diagram 600 is similar to the step S116 of the signaling diagram 100, except that the RRC Reconfiguration message further contains the mapping table. In a next step S620 (similar to the step S118 of the signaling diagram 100), the UE 200 confirms the RRC Reconfiguration to the CU, whereupon the UE 200 applies the mapping table (or, in other words, mapping rules indicated in the mapping table) to associate each strongest CSI-RS of the serving cell with one or more CSI-RSs of the non-serving cell (also hereinafter referred to as the target CSI-RS(s) of the non-serving cell) in a step S622. After the step S622, the signaling diagram 600 goes on to a step S624, in which the UE 200 starts to report periodically the measurements of the target RS(s) of the non-serving cell to DU1. In a next step S626 (which is similar to the step S122 of the signaling diagram 100), the UE 200 receives the MAC CE command from DU1. The signaling diagram 600 ends up with a step S628, in which the handover from the serving cell to the selected (by DU1) non-serving cell controlled by DU2 is executed by the UE 200.

FIG. 7 schematically shows CSI-RS beams across two neighboring cells, with the UE 200 being handed over from one cell to another. In FIG. 7, the serving cell is assumed to be served by the network node 400. What is shown in FIG. 7 is a typical inter-cell mobility scenario, in which the UE 200 is handed over from the serving cell to the neighboring (or non-serving) cell once a mobility event is triggered (e.g., when the MAC CE command is received by the UE 200 - see the step S122 in FIG. 1 and the step S626 in FIG. 6). As a rule, this occurs once the UE 200 enters the coverage area of the neighboring cell "well enough", such that the received signal power from the neighboring cell is sufficiently larger than that of the serving cell.

According to FIG. 7, the transition from the serving cell to the neighboring cell takes place at the cell edge and is typically associated with given combinations of serving CSI-RSs and neighboring CSI-RSs. This implies that the unnecessary CSI-RS measurements of the (prepared for handover) neighboring cell may be reduced by properly associating the neighboring CSI-RS(s) with the serving CSI-RS, i.e., the serving CSI-RS that the UE 200 measured as the best one and reported it to the network node 400 before exiting the serving cell.

More specifically, the serving network node 400 may, in accordance with the method 500, use the past measurements from the UE 200 (or other UEs) to configure only a subset of the neighboring CSI-RSs, which is mapped to the last reported CSI-RS of the serving cell before the MAC CE command triggering the cell change. By doing so, the network node 400 creates the mapping table, by which it may configure the UE 200 to perform measurements only on certain CSI-RS(s) of the neighboring cell. In this case, the mapping table may associate a subset of serving CSI-RSs from the serving cell (i.e., those strongest CSI-RSs which are measured above a threshold - see FIG. 8 and its description below) with a subset of neighboring CSI-RSs from the neighboring cell (i.e., those CSI-RSs which are associated with the strongest CSI-RSs of the serving cell according to the mapping table). This may help to filter and select only those neighboring CSI-RSs which are to be measured.

As follows from FIG. 7, CSI-RSs 3, 4 and 5 of the neighboring cell are of particular interest, since the UE 200 is located along their propagation paths and the handover is likely be decided based on the measurements of these CSI-RSs. This, in turn, means that the other CSI-RSs (especially, CSI-RSs 10-12) can be excluded from consideration (i.e., there is no need for the UE 200 to perform the measurements on these CSI-RSs).

With reference to FIG. 7, the mapping table may look as follows (in this example embodiment, the mapping table associates one serving CSI-RS of the serving cell with a set of neighboring CSI-RSs of (at least one) neighboring cell):

In one other example embodiment, the mapping table may associate a set of serving CSI-RSs of the serving cell (e.g., an ordered list of serving CSI-RSs, which are ordered according to their qualities, such as CSI-RSRPs or CSI-RSRQs) with a set of neighboring CSI-RS(s) of at least one neighboring cell. With reference to FIG. 7, such a mapping table may look as follows:

FIG. 8 shows a Probability Mass Function (PMF) of target (or non-serving) cell CSI-RS indices for a given serving cell CSI-RS index. In FIG. 8, each target cell CSI-RS is denoted as "entry CSI- RS", while the serving cell CSI-RS is denoted as "exit CSI-RS". As noted above, the PMF may be used to create the mapping table. The PMF may be in the form of a conditional PMF PRs₂\Rs₁ .^rs2 \^rsi) providing the probability of an entry CSI-RS (beam) rs₂ given a strongest exit CSI-RS (beam) rs_x. Alternatively, it can be in the form of a joint PMF P_RS1,RS₂ (,^rsi> ^rsi) providing the joint probability of an exit CSI-RS (beam) rs_x and an entry CSI-RS (beam) rs₂.

With reference to FIG. 7, the strongest exit CSI-RS is CSI-RS 4, and the entry CSI-RSs that are configured for the UE 200 are those entry CSI-RSs whose PMF lies above a pre-defined threshold (see FIG. 8). As follows from FIG. 8, the configured entry CSI-RS indices from the candidate cell are those with indices #3, #4, and #5 (i.e., CSI-RSs 3, 4, and 5).

Thus, the association between the exit (serving cell) CSI-RS beams and entry (candidate cell) CSI-RS beams is realized by means of defining, based on the past measurements, the entry CSI-RS beams with the PMF above a given threshold, and configuring those entry CSI-RS beams depending on the reported exit CSI-RS.

FIG. 9 shows a flowchart of a method 900 for operating the UE 200 in accordance with a second example embodiment. In the second example embodiment, it is implied that the mapping (e.g., in the form of the mapping table discussed above) may be created by the UE 200 itself if the UE 200 is allowed to do so by the network node 400. In this case, the UE 200 is configured with the legacy CSI-RS measurement procedure for candidate cells, yet it applies a filtering mechanism to select only the subset of candidate CSI-RSs for measurements based on the pre-created mapping table. This alternative embodiment particularly applies to scenarios where the UE 200 learns its mobility patterns and the associated transition of CSI-RS measurements from the serving cell to the target (nonserving) cell. For example, the UE 200 may be configured with a large set of CSI-RSs of one or more candidate cells for measurements. Using the proposed filtering mechanism based on the mapping table, the UE 200 may attempt to perform RRM measurements only on a subset of CSI-RSs. The measurement results may be then reported to the network node 400, if required and if the UE 200 is configured to do so.

Referring to FIG. 9, the method 900 starts with a step S902, in which the processor 202 receives control information from the processor 402 of the network node 400. The control information comprises a first indication for the UE 200 to collect RS measurements to be performed by the processor 202 during one or more handover events within a pre-defined time period. The collected RS measurements relate to the first set of RSs, i.e., {RSi, ..., RSM}, associated with the multiple beams of the serving cell and the second set of RSs, i.e., {RS'i, ..., RS'N}, associated with the multiple beams of the non-serving cell. Again, the serving and non-serving cells may be controlled by the network node 400, or the serving cell may be controlled by the network node 400, while the non-serving cell may be controlled by a different (e.g., neighboring) network node. The control information further comprises a second indication for the processor 202 to create the mapping, e.g., in the form of a mapping table (like the ones discussed above) based on the collected RS measurements after and/or during the pre-defined time period. Next, the method 900 proceeds to a step S904, in which the processor 202 creates the mapping table based on the control information and the collected RS measurements. For example, the step S904 may be performed using the PMF, as discussed above, and the control information may further instruct the UE 200 to use a certain PMF type. After that, the method 900 goes on to a step S906, in which the processor 202 determines, among {RSi, ..., RSM}, one or more strongest RSs in terms of their signal strength. Further, the method 900 proceeds to a step S908, in which the processor 202 uses the determined strongest RS(s) and the created mapping to determine, among {RS'i, ..., RS'N}, one or more target RSs to be measured. In a next step S910, the processor 202 performs next measurements on the target RS(s).

Thus, if the UE 200 is allowed and configured to store the past RS measurements across multiple (i.e., serving and non-serving) cells, the mapping of a set of exit (serving cell) CSI- RSs to a set of entry (target/candidate cell) CSI-RSs may be done at the UE 200 itself. In such case, the UE 200 may deploy any training process (depending on UE implementation) to determine the PMF of the entry CSI-RS beams for each exit CSI-RS beam.

In one example embodiment, the method 900 may comprise additional steps, in which the processor 202 transmits, to the network node 400, a measurement report like the one discussed above with reference to the method 300 and, in response, receives a switching instruction like the one discussed above.

FIG. 10 shows a flowchart of a method 1000 for operating the serving network node 400 in accordance with a second example embodiment. In this embodiment, the serving network node 400 is assumed to interact with the UE 200 operating in accordance with the method 900. The method 1000 starts with a step S1002, in which the processor 402 generates the control information which comprises the above-described first and second indications for the processor 202. Then, the method 1000 goes on to a step S1004, in which the processor 402 transmits the control information to the processor 202 of the UE 200.

In one example embodiment, the method 1000 may comprise additional steps, in which the processor 402 receives the measurement report from the processor 202 of the UE 200, determines whether the UE 200 needs to switch to one of the multiple beams of the nonserving cell, and if so, transmits the switching instruction to the UE 200.

FIG. 11 shows a signaling diagram 1100 for L1/L2 inter-cell mobility, in which the UE 200 and the serving network node 400 interact with each other in accordance with the methods 900 and 1000, respectively. Like in the signaling diagram 600, in the signaling diagram 1100 the network node 400 is assumed to have a split or disaggregated architecture, in which DU1 serves a serving cell in which the UE 200 is currently present, DU2 serves at least one nonserving cell, and a CU controls each of the DU1 and DU2. It should be again noted that the present disclosure is not limited to the split or disaggregated architecture shown in FIG. 11 - in other embodiments, there may be no CU, and DU1 may be replaced by the network node 400 and DU2 may be replaced by another (e.g., neighboring) network node, without departing from the teachings of the present disclosure.

The signaling diagram 1100 starts with a step S1102, in which the UE 200 is assumed to have received the control information from the CU and creates the mapping in the form of a mapping table (like the ones discussed above). That is, the table mapping associates each CSI-RS of the serving cell with one or more CSI-RSs of the non-serving cell, and the mapping table itself is obtained based on past handover-related measurements pertaining to the CSI- RS(s) of the serving cell at the point the handover is triggered (equivalently, at the point the MAC CE command is sent in a step S1126 of the signaling diagram 1100). Next steps S1104- S1120 of the signaling diagram 1100 are the same as the step S102-S118 of the signaling diagram 100, respectively. In a next step S1122, the UE 200 applies the mapping table (or, in other words, mapping rules indicated in the mapping table) to associate each strongest CSI- RS of the serving cell with one or more CSI-RSs of the non-serving cell (also hereinafter referred to as the target CSI-RS(s) of the non-serving cell). After the step S1122, the signaling diagram 1100 goes on to a step S1124, in which the UE 200 starts to report periodically the measurements of the target RS(s) of the non-serving cell to DU1. In the next step S1126 (which is similar to the step S122 of the signaling diagram 100), the UE 200 receives the MAC CE command from DU1. The signaling diagram 1100 ends up with a step S1128, in which the handover from the serving cell to the selected (by DU1) non-serving cell controlled by DU2 is executed by the UE 200.

FIG. 12 shows a flowchart of a method 1200 for operating the (serving) network node 400 in accordance with a third example embodiment. In this embodiment, the network node 400 is again responsible for creating the mapping table (like the ones discussed above) but transmits it to a neighboring (non-serving) network node rather than the UE 200. By using the mapping table, the non-serving network node may configure and broadcast only those RSs which are associated with a certain strongest RS of the serving cell in accordance with the mapping table.

More specifically, the method 1200 starts with a step S1202, in which the processor 402 collects past RS measurements performed by the UE 200 during one or more previous handover events in the RAN. The past RS measurements relate to the set of RSs, i.e., {RSi, ..., RSM}, and the second set of RSs, i.e., {RS'i, ..., RS'N}. The processor 402 uses the past RS measurements to create the mapping table in a next step S1204. The mapping table maps the first set of RSs, i.e., {RSi, ..., RSM}, associated with the multiple beams of the serving cell to the second set of RSs, i.e., {RS'i, ..., RS'N}, associated with the multiple beams of the nonserving cell. For example, the step S1204 may be performed by using the PMF, as discussed above. After that, the method 1200 goes on to a step S1206, in which the processor 402 1 transmits the mapping table and the past RS measurements to the non-serving network node.

FIG. 13 shows a block diagram of a non-serving network node 1300 in the RAN in accordance with one example embodiment. The non-serving network node 1300 is intended to communicate with the UE 200 and the serving network node 400 in the RAN. As shown in FIG. 13, the non-serving network node 1300 comprises a processor 1302 and a memory 1304 coupled to the processor 1302. The memory 1304 stores processor-executable instructions 1306 which, when executed by the processor 1302, cause the processor 1302 to implement the aspects of the present disclosure, as will be described below in more detail. It should be again noted that the number, arrangement, and interconnection of the constructive elements constituting the non-serving network node 1300, which are shown in FIG. 13, are not intended to be any limitation of the present disclosure, but merely used to provide a general idea of how the constructive elements may be implemented within the non-serving network node 1300. In general, the processor 1302, the memory 1304, and the processorexecutable instructions 1306 may be implemented in the same or similar manner as the processor 202, the memory 204, and the processor-executable instructions 206, respectively.

FIG. 14 shows a flowchart of a method 1400 for operating the non-serving network node 1300 in accordance with one example embodiment. The method 1400 starts with a step S1402, in which the processor 1302 receives, from the processor 402 of the serving network node 400, the control information with the mapping table, as well as the past RS measurements performed by the UE 200 during the previous handover event(s) in the RAN. The method 1400 further proceeds to a step S1404, in which the processor 1302 uses the past RS measurements to determine, among {RSi, ..., RSM}, one or more strongest RSs in terms of their signal strength. After that, the method 1400 goes on to a step S1406, in which the processor 1302 uses the determined strongest RS(s) and the mapping table to determine, among {RS'i, ..., RS'N}, one or more target RSs to be measured by the UE 200. In other words, the target RS(s) is(are) associated with the strongest RS(s) via the mapping table. Next, the method 1400 proceeds to a step S1408, in which the processor 1302 configures and transmits each target RS during a next handover event for the UE 200. In one example embodiment, the method 1400 may comprise an additional step, in which the processor 1302 of the non-serving network node 1300 informs the processor 402 of the network node 400 when it begins and ends said transmission of the target RS(s) towards the UE 200.

FIG. 15 shows a signaling diagram 1500 for L1/L2 inter-cell mobility, in which the serving network node 400 and the non-serving network node 1300 interact with each other in accordance with the methods 1200 and 1400, respectively. Like in the signaling diagrams 600 and 1100, in the signaling diagram 1500 the network node 400 is assumed to have a split or disaggregated architecture, in which DU1 serves a serving cell in which the UE 200 is currently present, DU2 serves at least one non-serving cell, and a CU controls each of the DU1 and DU2. It should be again noted that the present disclosure is not limited to the split or disaggregated architecture shown in FIG. 15 - in other embodiments, there may be no CU, and DU1 may be replaced by the network node 400 and DU2 may be replaced by another (e.g., neighboring) network node, without departing from the teachings of the present disclosure.

The signaling diagram 1500 starts with a S1502, in which the CU generates the control information for the UE 200 and DU2 in accordance with the step S1202 of the method 1200. In this example, the mapping is assumed to in the form of a mapping table that associates each CSI-RS of the serving cell with one or more CSI-RSs of the non-serving cell, and the mapping table itself is obtained based on past handover-related measurements pertaining to the CSI-RS(s) of the serving cell at the point the handover is triggered (equivalently, at the point the MAC CE command is sent in a step S1530 of the signaling diagram 1500). Next steps S1504-S1514 of the signaling diagram 1500 are the same as the step S102-S112 of the signaling diagram 100, respectively. In a next step S1516, the CU generates an RRC reconfiguration message and indicates the control information with the mapping table in the RRC reconfiguration message. The CU then sends the RRC reconfiguration message to the UE 200 in a step S1518. In next steps S1520 and S1522, the CU sends the same mapping table to DU2, thereby triggering DU2 to transmit target one or more CSI-RSs of the non-serving cell (in accordance with the method 1400), and, in response, DU2 sends a CSI-RS transmission ACK/NACK and (if ACK) starts broadcasting target CSI-RS(s). It should be noted that the steps S1518 and S1520 may be performed by the CU in parallel, if required. Further, the signaling diagram 1500 goes on to a step S1524 (similar to the step S118 of the signaling diagram 100), in which the UE 200 confirms the RRC Reconfiguration to the CU, whereupon the UE 200 applies the mapping table (or, in other words, mapping rules indicated in the mapping table) to associate each strongest CSI-RS of the serving cell with one or more CSI-RSs of the nonserving cell (also hereinafter referred to as the target CSI-RS(s) of the non-serving cell) in a step S1526. After the step S1526, the signaling diagram 1500 goes on to a step S1528, in which the UE 200 starts to report periodically the measurements of the target RS(s) of the non-serving cell to DU1. In the next step S1530 (which is similar to the step S122 of the signaling diagram 100), the UE 200 receives the MAC CE command from DU1. The signaling diagram 1500 ends up with a step S1532, in which the handover from the serving cell to the selected (by DU1) non-serving cell controlled by DU2 is executed by the UE 200.

It should be noted that each step or operation of the methods 300, 500, 900, 1000, 1200, and 1400 and the signaling diagrams 600, 1100, and 1500, or any combinations of the steps or operations, can be implemented by various means, such as hardware, firmware, and/or software. As an example, one or more of the steps or operations described above can be embodied by processor executable instructions, data structures, program modules, and other suitable data representations. Furthermore, the processor-executable instructions which embody the steps or operations described above can be stored on a corresponding data carrier and executed by the corresponding processor 202, 402, or 1202. This data carrier can be implemented as any computer-readable storage medium configured to be readable by said at least one processor to execute the processor executable instructions. Such computer-readable storage media can include both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, the computer- readable media comprise media implemented in any method or technology suitable for storing information. In more detail, the practical examples of the computer-readable media include, but are not limited to information-delivery media, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile discs (DVD), holographic media or other optical disc storage, magnetic tape, magnetic cassettes, magnetic disk storage, and other magnetic storage devices.

Although the example embodiments of the present disclosure are described herein, it should be noted that any various changes and modifications could be made in the embodiments of the present disclosure, without departing from the scope of legal protection which is defined by the appended claims. In the appended claims, the word "comprising" does not exclude other elements or operations, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A User Equipment (UE) in a Radio Access Network (RAN), comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the UE at least to: receive, from a network node of the RAN, control information comprising: a first indication for the UE to collect Reference Signal (RS) measurements to be performed by the UE during at least one handover event within a predefined time period, the RS measurements relating to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell; and a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period; create the mapping based on the control information and the RS measurements; determine, among the first set of RSs, at least one strongest RS in terms of a signal strength; based on the at least one strongest RS and the created mapping, determine, among the second set of RSs, at least one target RS to be measured; and perform next measurements on the at least one target RS.

2. The UE of claim 1, wherein the mapping between the first set of RSs and the second set of RSs comprises at least one of: a one-to-one mapping between one RS from the first set of RSs and one RS from the second set of RSs; a one-to-many mapping between one RS from the first set of RSs and a subset of RSs from the second set of RSs; a many-to-one mapping between a subset of RSs from the first set of RSs and one RS from the second set of RSs; and a many-to-many mapping between a subset of RSs from the first set of RSs and a subset of RSs from the second set of RSs.

3. The UE of claim 1 or 2, wherein the UE is caused to: determine, among the first set of RSs, at least two strongest RSs in terms of the signal strength; and determine the at least one target RS based on which of the at least two strongest RSs is stronger.

4. The UE of claim 1 or 2, wherein the UE is caused to: determine, among the first set of RSs, at least two strongest RSs in terms of the signal strength; and determine the at least one target RS based on a relative value of the signal strength among the at least two strongest RSs.

5. The UE of any one of claims 1 to 4, wherein the serving cell and the non-serving cell are controlled by the network node.

6. The UE of any one of claims 1 to 4, wherein the serving cell is controlled by the network node, and the non-serving cell is controlled by a different network node.

7. The UE of any one of claims 1 to 6, wherein the UE is caused to receive the control information via a Radio Resource Control (RRC) message.

8. The UE of any one of claim 1 to 7, wherein the UE is further caused to: transmit a measurement report comprising the measurements on the at least one target RS to the network node; and receive, from the network node, a switching instruction that causes the UE to: switch to a target beam of the multiple beams of the non-serving cell, the target beam being selected by the network node based on the measurements on the at least one target RS; or determine, among the multiple beams of the non-serving cell, a beam that is the best in terms of a beam quality, and switch to the beam.

9. A network node in a Radio Access Network (RAN), comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the network node at least to: generate control information comprising: a first indication for a User Equipment (UE) to collect Reference Signal (RS) measurements to be performed by the UE during at least one handover event within a predefined time period, the RS measurements relating to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell; and a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period; and transmit the control information to the UE.

10. The network node of claim 9, wherein the mapping between the first set of RSs and the second set of RSs comprises at least one of: a one-to-one mapping between one RS from the first set of RSs and one RS from the second set of RSs; a one-to-many mapping between one RS from the first set of RSs and a subset of RSs from the second set of RSs; a many-to-one mapping between a subset of RSs from the first set of RSs and one RS from the second set of RSs; and a many-to-many mapping between a subset of RSs from the first set of RSs and a subset of RSs from the second set of RSs.

11. The network node of claim 9 or 10, wherein the serving cell and the non-serving cell are controlled by the network node.

12. The network node of claim 9 or 10, wherein the serving cell is controlled by the network node, and the non-serving cell is controlled by a different network node.

13. The network node of any one of claims 9 to 12, wherein the network node is caused to transmit the control information via a Radio Resource Control (RRC) message.

14. The network node of any one of claims 9 to 13, wherein the network node is further caused to: receive, from the UE, a measurement report comprising measurements on at least one target RS of the second set of RSs; based on the measurements on the at least one target RS, determine whether the UE needs to switch to one of the multiple beams of the non-serving cell; and if it is determined that the UE needs to switch to one of the multiple beams of the non-serving cell, transmit a switching instruction to the UE; wherein the switching instruction causes the UE to: switch to a target beam of the multiple beams of the non-serving cell, the target beam being selected by the network node based on the measurements on the at least target RS; or determine, among the multiple beams of the non-serving cell, a beam that is the best in terms of a beam quality, and switch to the beam.

15. A method for operating a User Equipment (UE) in a Radio Access Network (RAN), comprising: receiving, from the network node, control information comprising: a first indication for the UE to collect Reference Signal (RS) measurements to be performed by the UE during at least one handover event within a predefined time period, the RS measurements relating to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell; and a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period; creating the mapping based on the control information and the RS measurements; determining, among the first set of RSs, at least one strongest RS in terms of a signal strength; based on the determined at least one strongest RS and the created mapping, determining, among the second set of RSs, at least one target RS to be measured; and performing next measurements on the at least one target RS.

16. A method for operating a network node in a Radio Access Network (RAN), comprising: generating control information comprising: a first indication for a User Equipment (UE) to collect Reference Signal (RS) measurements to be performed by the UE during at least one handover event within a predefined time period, the RS measurements relating to a first set of RSs associated with multiple beams of a serving cell and a second set of RSs associated with multiple beams of a non-serving cell; and a second indication for the UE to create a mapping between the first set of RSs and the second set of RSs based on the RS measurements during and/or after the predefined time period; and transmitting the control information to the UE.

17. A computer program product comprising a computer-readable storage medium, wherein the computer-readable storage medium stores a computer code which, when executed by at least one processor, causes the at least one processor to perform the method according to claim 15.

18. A computer program product comprising a computer-readable storage medium, wherein the computer-readable storage medium stores a computer code which, when executed by at least one processor, causes the at least one processor to perform the method according to claim 16.