Content for TS 38.300 Word version: 18.4.0

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15.5 Self-optimisation 15.5.1 Support for Mobility Load Balancing 15.5.1.1 General 15.5.1.2 Load reporting for intra-RAT and intra-system inter-RAT load balancing 15.5.1.3 Load balancing action based on handovers 15.5.1.4 Adapting handover and/or reselection configuration 15.5.1.5 Load reporting for inter-system load balancing 15.5.2 Support for Mobility Robustness Optimization 15.5.2.1 General 15.5.2.2 Connection failure 15.5.2.2.1 General 15.5.2.2.2 Connection failure due to intra-system mobility 15.5.2.2.3 Connection failure due to inter-system mobility 15.5.2.3 Inter-system Unnecessary HO 15.5.2.4 Inter-system Ping-pong 15.5.2.5 O&M Requirements 15.5.2.6 PSCell addition/change failure 15.5.2.7 Successful HO 15.5.2.8 Successful PSCell Addition/Change Report 15.5.3 Support for RACH Optimization 15.5.4 UE History Information from the UE 15.5.5 Support for Coverage and Capacity Optimisation 15.5.5.1 General 15.5.5.2 OAM requirements 15.5.5.3 Dynamic coverage configuration changes 15.5.6 Support for PCI Optimisation 15.5.6.1 Centralized PCI Assignment 15.5.6.2 Distributed PCI Assignment
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15.5 Self-optimisation |R16| p. 140

15.5.1 Support for Mobility Load Balancing p. 140

15.5.1.1 General p. 140

The objective of mobility load balancing is to distribute load evenly among cells and among areas of cells, or to transfer part of the traffic from congested cell or from congested areas of cells, or to offload users from one cell, cell area, carrier or RAT to achieve network energy saving. This can be done by means of optimization of cell reselection/handover parameters and handover actions. The automation of such optimisation can provide high quality user experience, while simultaneously improving the system capacity and also to minimize human intervention in the network management and optimization tasks.

Intra-RAT, intra-system inter-RAT and inter-system load balancing scenarios are supported.

In general, support for mobility load balancing consists of one or more of following functions:

Load reporting for intra-RAT and intra-system inter-RAT load balancing;
Load balancing action based on handovers;
Adapting handover and/or reselection configuration;
Load reporting for inter-system load balancing.

15.5.1.2 Load reporting for intra-RAT and intra-system inter-RAT load balancing p. 140

The load reporting function is executed by exchanging load information over the Xn/X2/F1/E1 interfaces.

The following load related information should be supported which consists of:

Radio resource usage (per-cell and per SSB area PRB usage: DL/UL GBR PRB usage, DL/UL non-GBR PRB usage, DL/UL total PRB usage, and DL/UL scheduling PDCCH CCE usage; PRB usage for slice(s): DL/UL GBR PRB usage, DL/UL non-GBR PRB usage, and DL/UL Total PRB allocation);
TNL capacity indicator (UL/DL TNL offered capacity and available capacity);
Cell Capacity Class value (UL/DL relative capacity indicator);
Capacity value (per cell, per SSB area and per slice: UL/DL available capacity);
HW capacity indicator (offered throughput and available throughput over E1, percentage utilisation over F1);
RRC connections (number of RRC connections, and available RRC Connection Capacity);
Number of active UEs;
NR-U channel load (DL/UL channel occupancy time percentage, DL/UL energy detection threshold, radio resource usage).

To achieve load reporting function, Resource Status Reporting Initiation & Resource Status Reporting procedures are used.

15.5.1.3 Load balancing action based on handovers p. 140

The source cell may initiate handover due to load. The target cell performs admission control for the load balancing handovers. A handover preparation related to a mobility load balancing action is distinguishable from other handovers, so that the target cell is able to apply appropriate admission control.

15.5.1.4 Adapting handover and/or reselection configuration p. 141

This function enables requesting of a change of handover and/or reselection parameters at target cell. The source cell that initialized the load balancing estimates if it is needed to change mobility configuration in the source and/or target cell. If the amendment is needed, the source cell initializes mobility negotiation procedure toward the target cell.

The source cell informs the target cell about the new mobility setting and provides cause for the change (e.g. load balancing related request). The proposed change is expressed by the means of the difference (delta) between the current and the new values of the handover trigger. The handover trigger is the cell specific offset that corresponds to the threshold at which a cell initialises the handover preparation procedure. Cell reselection configuration may be amended to reflect changes in the HO setting. The target cell responds to the information from the source cell. The allowed delta range for HO trigger parameter may be carried in the failure response message. The source cell should consider the responses before executing the planned change of its mobility setting.

All automatic changes on the HO and/or reselection parameters must be within the range allowed by OAM.

15.5.1.5 Load reporting for inter-system load balancing |R17| p. 141

The load reporting function for inter-system load balancing is executed by exchanging load information between NG-RAN and E-UTRAN. Both event-triggered and periodic inter-system load reporting are supported. Event-triggered inter-system load reports are sent when the reporting node detects crossing of cell load thresholds.

The following load related information should be supported:

Cell Capacity Class value (UL/DL relative capacity indicator);
Capacity value (per cell: UL/DL available capacity);
RRC connections (number of RRC connections, and available RRC Connection Capacity);
Number of active UEs;
Radio Resource Status (per cell PRB usage: UL/DL GBR PRB usage for MIMO, DL/UL non-GBR PRB usage for MIMO, DL/UL total PRB usage for MIMO).

NGAP procedures used for inter-system load balancing are Uplink RAN Configuration Transfer and Downlink RAN Configuration Transfer.

S1AP procedures used for inter-system load balancing are eNB Configuration Transfer and MME Configuration Transfer.

15.5.2 Support for Mobility Robustness Optimization p. 142

15.5.2.1 General p. 142

Mobility Robustness Optimisation aims at detecting and enabling correction of following problems:

Connection failure due to intra-system or inter-system mobility;
Inter-system Unnecessary HO (too early inter-system HO from NR to E-UTRAN with no radio link failure);
Inter-system HO ping-pong;
PSCell change failure;
Inter-system voice fallback failure;
Fast MCG recovery failure or a sub-optimal Fast MCG recovery.

MRO provides means to distinguish the above problems from NR coverage related problems and other problems, not related to mobility.

For detection of a sub-optimal successful handovers, MRO additionally enables observability of:

Successful HO due to intra-NR mobility;
Successful HO due to inter-RAT mobility.

For detection of a sub-optimal successful PSCell addition/change, MRO additionally enables observability of:

Successful PSCell addition/change.

15.5.2.2 Connection failure p. 142

15.5.2.2.1 General p. 142

For analysis of connection failures, the UE makes the RLF Report available to the network.

The UE stores the latest RLF Report, including both LTE and NR RLF report until the RLF report is fetched by the network or for 48 hours after the connection failure is detected.

The UE only indicates RLF report availability and only provides the RLF report to the network if the current RPLMN is a PLMN that was present in the UE's EPLMN List or was the RPLMN at the time the connection failure was detected. In case RLF happens in an E-UTRA cell, the UE makes the LTE RLF Report available to NG-RAN nodes and eNB(s), and in case RLF happens in an NR cell the UE makes the NR RLF Report available to gNB(s).

If the LTE RLF Report is reported to a NG-RAN node, and the last serving node is an E-UTRAN node, the NG-RAN node may transfer it to the E-UTRAN node by triggering the Uplink RAN configuration transfer procedure over NG and the E-UTRAN node can take this into account as defined in TS 36.300.

15.5.2.2.2 Connection failure due to intra-system mobility p. 142

One of the functions of Mobility Robustness Optimization is to detect connection failures that occur due to Too Early or Too Late Handovers, or Handover to Wrong Cell. These problems are defined as follows:

Intra-system Too Late Handover: an RLF occurs after the UE has stayed for a long period of time in the cell; the UE attempts to re-establish the radio link connection in a different cell.
Intra-system Too Early Handover: an RLF occurs shortly after a successful handover from a source cell to a target cell or a handover failure occurs during the handover procedure; the UE attempts to re-establish the radio link connection in the source cell.
Intra-system Handover to Wrong Cell: an RLF occurs shortly after a successful handover from a source cell to a target cell or a handover failure occurs during the handover procedure; the UE attempts to re-establish the radio link connection in a cell other than the source cell and the target cell.

In the definition above, the "successful handover" refers to the UE state, namely the successful completion of the RA procedure.

In case of CHO, the Too Late Handover, Too Early Handover and Handover to Wrong Cell in the definition above means Too Late CHO Execution, Too Early CHO Execution and CHO Execution to Wrong Cell.

Detection mechanism

A failure indication may be initiated after a UE attempts to re-establish the radio link connection at NG-RAN node B after a failure at NG-RAN node A. NG-RAN node B may initiate the Failure Indication procedure towards multiple NG-RAN nodes if they control cells which use the PCI signalled by the UE during the re-establishment procedure. The NG-RAN node receiving this selects the UE context that matches the received Failure Cell ID and C-RNTI, and, if available, uses the shortMAC-I to confirm this identification, by calculating the shortMAC-I and comparing it to the received IE.

A failure indication may also be sent to the node last serving the UE when the NG-RAN node fetches the RLF REPORT from UE by triggering:

The Failure Indication procedure over Xn;
The Uplink RAN configuration transfer procedure and Downlink RAN configuration transfer procedure over NG.

The detailed detection mechanisms for too late handover, too early handover and handover to wrong cell are carried out through the following in the NG-RAN node that served the UE before the reported connection failure:

Intra-system Too Late Handover: there is no recent handover for the UE prior to the connection failure e.g. the UE reported timer is absent or larger than the configured threshold (e.g. Tstore_UE_cntxt), or if CHO is configured but the CHO execution is not initiated for the UE prior to the connection failure, e.g. the UE reported timer is absent or larger than the configured threshold (e.g. Tstore_UE_cntxt).
Intra-system Too Early Handover: there is a recent handover for the UE prior to the connection failure e.g. the UE reported timer is smaller than the configured threshold (e.g. Tstore_UE_cntxt), and the first re-establishment attempt cell/the successful re-connect cell is the cell that served the UE at the last handover initialisation or fall back to the source cell configuration in case of DAPS HO.
Intra-system Handover to Wrong Cell: there is a recent handover for the UE prior to the connection failure e.g. the UE reported timer is smaller than the configured threshold (e.g. Tstore_UE_cntxt), and the first re-establishment attempt cell/ the cell UE attempts to re-connect/the cell UE attempts CHO recovery is neither the cell that served the UE at the last handover initialisation nor the cell that served the UE where the RLF happened or the cell that the handover was initialized toward.

The "UE reported timer" above indicates the time elapsed since the last handover initialisation until connection failure or the time elapsed since the CHO execution until connection failure.

In case of Too Early Handover or Handover to Wrong Cell, the NG-RAN node receiving the failure indication may inform the NG-RAN node controlling the cell where the mobility configuration caused the failure by means of the Handover Report procedure over Xn or the Uplink RAN Configuration Transfer procedure over NG. This may include the RLF report.

For MRO analysis, in case of RLFs or handover failures, the source gNB may take into account the information regarding the LBT failures occurred in UL and DL for a specific UE. For UL, the information may be collected and provided by the UE to the network in the RLF report. For DL, the information may be collected at the source and target gNBs. The target gNB may provide the information to the source gNB via the Access and Mobility Indication procedure over Xn, if requested by the source gNB during the handover preparation.

Retrieval of information needed for problem analysis

In order to retrieve relevant information collected at the network side as part of the UE context, the UE provides C-RNTI used in the last serving cell. If the cause for the failure is identified as a "Too Early HO" or a "HO to Wrong Cell", the NG-RAN node controlling the last serving cell shall, include in the HANDOVER REPORT message the C-RNTI used in the source cell of the last completed handover before the failure. If the NG RAN node controlling that source cell provided the Mobility Information, it is also included in the HANDOVER REPORT message. If used, the Mobility Information is prepared at the source NG RAN node of a handover and may refer to or identify any handover-related data at this NG RAN node. In the Handover Preparation procedure, the source gNB can request the target gNB to provide information on DL LBT failures at the target gNB during handover execution.

Handling multiple reports from a single failure event

In case the RRC re-establishment fails and the RRC connection setup succeeds, MRO evaluation of intra-RAT mobility connection failures may be triggered twice for the same failure event. In this case, only one failure event should be counted.

15.5.2.2.3 Connection failure due to inter-system mobility p. 144

One of the functions of Mobility Robustness Optimization is to detect connection failures that occurred due to Too Early or Too Late inter-system handovers or inter-system Mobility Failure for Voice Fallback. These problems are defined as follows:

Inter-system/ Too Late Handover: an RLF occurs after the UE has stayed in a cell belonging to an NG-RAN node for a long period of time; the UE attempts to re-connect to a cell belonging to an E-UTRAN node.
Inter-system/ Too Early Handover: an RLF occurs shortly after a successful handover from a cell belonging to an E-UTRAN node to a target cell belonging to an NG-RAN node or a handover failure occurs during the handover procedure; the UE attempts to re-connect to the source cell or to another cell belonging to an E-UTRAN node.
Inter-system Mobility Failure for Voice Fallback: an RLF occurs shortly after a successful handover triggered due to Voice Fallback, or a failure occurs during an handover triggered due to Voice Fallback, from a cell belonging to an NG-RAN node to a cell belonging to an E-UTRAN node; the UE attempts to re-connect to a cell belonging to an E-UTRAN node, or an NG-RAN node.

Detection mechanism

A failure indication may be sent to the node last serving the UE when the NG-RAN node fetches the RLF REPORT from UE by triggering:

The Failure Indication procedure over Xn;
The Uplink RAN configuration transfer procedure and Downlink RAN configuration transfer procedure over NG.

In case the last serving node is an E-UTRAN node, the detection mechanism proceeds as defined in TS 36.300.

In case the last serving node is an NG-RAN node, the detection mechanisms for Too Late Inter-system Handover, Too Early Inter-system Handover, and Inter-system Mobility Failure for Voice Fallback are carried out through the following:

Too Late Inter-system Handover: the connection failure occurs while being connected to a NG-RAN node, and there is no recent handover for the UE prior to the connection failure i.e., the UE reported timer is absent or larger than the configured threshold, e.g., Tstore_UE_cntxt, and the first node where the UE attempts to re-connect is a E-UTRAN node.
Too Early Inter-system Handover: the connection failure occurs while being connected to a NG-RAN node, and there is a recent inter-system handover for the UE prior to the connection failure i.e., the UE reported timer is smaller than the configured threshold, e.g., Tstore_UE_cntxt, and the first cell where the UE attempts to re-connect and the node that served the UE at the last handover initialisation are both E-UTRAN node.
Inter-system Mobility Failure for Voice Fallback: in case the connection failure occurs during an inter-system handover for voice fall back from NR, the RLF Report from the UE includes a voice fallback indication, as defined in TS 38.331.

The "UE reported timer" above indicates the time elapsed since the last handover initialisation until connection failure. The UE may make the RLF Report available to an NG-RAN node. The NG-RAN node may forward the information using the FAILURE INDICATION message over Xn or by means of the Uplink RAN configuration transfer procedure and Downlink RAN configuration transfer over NG to the node that served the UE before the reported connection failure.

In case the failure is a Too Early Inter-system Handover, the NG-RAN node receiving the failure indication may inform the E-UTRAN node by means of the Uplink RAN Configuration Transfer procedure over NG. This may include the RLF report.

15.5.2.3 Inter-system Unnecessary HO p. 145

One of the purposes of inter-system Mobility Robustness Optimisation is the detection of a non-optimal use of network resources. In particular, in case of inter-system operations and when NR is considered, the case known as Unnecessary HO to another system is identified. The problem is defined as follows:

UE is handed over from NR to E-UTRAN even though quality of the NR coverage was sufficient for the service used by the UE. The handover may therefore be considered as unnecessary HO to another system (i.e. EPS) (too early inter-system HO without connection failure).

In inter-system HO, if the serving cell threshold (NR cell) is set too high, and cell in another system (i.e. EPS) with good signal strength is available, a handover to another system may be triggered unnecessarily, resulting in an inefficient use of the networks. With a lower threshold the UE could have continued in the source system (5GS).

To be able to detect the Unnecessary HO to another system, a gNB node may choose to put additional coverage and quality condition information into the HANDOVER REQUIRED message in the Handover Preparation procedure when an inter-system HO from gNB to another system occurs. The RAN node in the other system, upon receiving this additional coverage and quality information, may instruct the UE to continue measuring the cell(s) in source system during a period of time, while being connected to another system, and send periodic or single measurement reports to the node in other system. When the period of time indicated by the node in source system expires, the RAN node in the other system, may evaluate the received measurement reports with the coverage/quality condition received during the inter-system HO procedure and decide if an inter-system unnecessary HO report should be sent to the gNB in the source system.

The inter-system unnecessary HO report shall only be sent in cases where, in all UE measurement reports collected during the measurement period, any cells in the source system exceed the radio coverage and/or quality threshold (the radio threshold RSRP, RSRQ or/and SINR and the measurement period are indicated in the additional coverage and quality information in the Handover Preparation procedure). If an inter-system handover towards 5GS is executed from EPS within the indicated measurement period, the measurement period expires. In this case, the eNB in EPS may also send the HO Report. No HO Report shall be sent in case no NR cell could be included, or if the indicated period of time is interrupted by an inter-system handover to a system different than 5GS.

The RAN node in the source system (5GS) upon receiving of the report, can decide if/how its parameters (e.g., threshold to trigger Inter-system HO) should be adjusted.

15.5.2.4 Inter-system Ping-pong p. 145

One of the functions of Mobility Robustness Optimization is to detect ping-pongs that occur in inter-system environment. The problem is defined as follows:

A UE is handed over from a cell in a source system (e.g. 5GS) to a cell in a target system different from the source system (e.g. EPS), then within a predefined limited time the UE is handed over back to a cell in the source system, while the coverage of the source system was sufficient for the service used by the UE. The event may occur more than once.

The solution for the problem may consist of the following steps:

Statistics regarding inter-system ping-pong occurrences are collected by the responsible node;
Coverage verification is performed to check if the mobility to other system was inevitable.

The statistics regarding ping-pong occurrence may be based on evaluation of the UE History Information IE in the HANDOVER REQUIRED message. If the evaluation indicates a potential ping-pong case and the source NG_RAN node of the 1st inter-system handover is different than the target NG-RAN node of the 2nd inter-system handover, the target NG-RAN node may use the HANDOVER REPORT message or the UPLINK RAN CONFIGURATION TRANSFER message to indicate the occurrence of potential ping-pong cases to the source NG-RAN node.

If NG-RAN coverage during the potential ping-pong event needs to be verified for the purpose of determining corrective measures, the Unnecessary HO to another system procedure may be used.

15.5.2.5 O&M Requirements p. 146

All automatic changes of the HO and/or reselection parameters for mobility robustness optimisation shall be within the ranges allowed by OAM and specified below.

The following control parameters shall be provided by OAM to control MRO behaviour:

Maximum deviation of Handover Trigger: this parameter defines the maximum allowed absolute deviation of the Handover Trigger, from the default point of operation defined by the parameter values assigned by OAM.
Minimum time between Handover Trigger changes: this parameter defines the minimum allowed time interval between two Handover Trigger change performed by MRO. This is used to control the stability and convergence of the algorithm.

Furthermore, in order to support the solutions for detection of mobility optimisation, the parameter Tstore_UE_cntxt shall be configurable by the OAM system.

15.5.2.6 PSCell addition/change failure |R17| p. 146

For analysis of PSCell addition/change failures, the UE makes the SCG Failure Information available to the MN. If the MN can perform an initial analysis, it transfers the SCG Failure Information together with the analysis results to the relevant SN as defined in TS 37.340.

15.5.2.7 Successful HO |R17| p. 146

One of the functions of Mobility Robustness Optimization is to detect a sub-optimal successful handover event. The aim is to identify underlying conditions during successful ordinary handovers, successful DAPS handovers, or successful Conditional handovers.

For analysis of successful handover, the UE may collect Successful Handover Report (SHR) based on configuration by network, if stored, and makes the SHR available to the network as specified in TS 38.331. The UE stores the SHR until it is fetched by the network or for 48 hours after the SHR is recorded.

For SHR collected during intra-NR handover, if the target NR node fetches the SHR from the UE and the trigger of SHR is T310/T312, it may forward the information to the source NR node, i.e. the node handling the cell reported as source cell in this SHR, by using the ACCESS AND MOBILITY INDICATION message over Xn or by means of the Uplink RAN configuration transfer procedure and Downlink RAN configuration transfer procedure over NG.

If the NG-RAN node that fetches the SHR from the UE is neither the source node nor the target node of the handover, it forwards the information to the node(s) which configured the SHR trigger causing the SHR to be generated, by using the ACCESS AND MOBILITY INDICATION message over Xn or by means of the Uplink RAN configuration transfer procedure and Downlink RAN configuration transfer procedure over NG.

In case of failure shortly after successful Handover, the same mobility event may generate both a SHR and a RLF report. In this case, the node(s), which configured the SHR trigger causing the SHR, may take the duplication into account e.g. ignore the SHR.

Upon retrieval of an SHR, the receiving node may analyse whether its mobility configuration needs adjustment.

The SHR report can be used to detect one case of Intra-system Too Late Handover, namely when DAPS HO is configured but an RLF is detected in the source cell during a successful DAPS HO.

The SHR report can be collected for intra-NR handover and for handover from NR to E-UTRA.

15.5.2.8 Successful PSCell Addition/Change Report |R18| p. 147

For the analysis of successful PSCell addition/change, the UE may collect Successful PSCell Addition/Change Report (SPR) based on the configuration by network, if received, and makes the SPR available to the network as specified in TS 38.331.

The UE stores the SPR until the SPR is fetched by the network or for 48 hours after the SPR is recorded.

The UE makes the SPR available to gNB(s).

When a gNB fetches the SPR from UE, it forwards the SPR to the MN that was serving the UE at the time the SPR was generated by using the ACCESS AND MOBILITY INDICATION message over Xn or by means of the Uplink RAN configuration transfer procedure and Downlink RAN configuration transfer over NG.

15.5.3 Support for RACH Optimization p. 147

RACH optimization is supported by UE reported information made available at the NG RAN node as specified in TS 38.331 and by PRACH parameters exchange between NG RAN nodes.

The contents of the RA Report comprise of the following:

Contention detection indication per RACH attempt;
Indexes of the SSBs and number of RACH preambles sent on each tried SSB listed in chronological order of attempts;
Indication whether the selected SSB is above or below the configured RSRP threshold per RACH attempt;
2-step RACH information as specified in clause 5.7.10.4 of TS 38.331;
Indication of LBT failures detected during the random access.

SN RA Reports

The UE may also support collection of SN RA Reports.

The SN RA report retrieval and forwarding is specified in TS 37.340.

15.5.4 UE History Information from the UE p. 147

The source NG-RAN node collects and stores the UE History Information for as long as the UE stays in one of its cells.

The UE may report the UE history information when connecting to a cell of the NG-RAN node, consisting of PCell and PSCell mobility history information, as specified in TS 38.331.

When information needs to be discarded because the list is full, such information will be discarded in order of its position in the list, starting with the oldest cell record. If the list is full, and the UE history information from the UE is available, the UE history information from the UE should also be discarded.

The resulting information is then used in subsequent handover preparations by means of the Handover Preparation procedures over the NG and XN interfaces, which provide the target NG-RAN node with a list of previously visited cells and associated (per-cell) information elements. The Handover Preparation procedures also trigger the target NG-RAN node to start collection and storage of UE history Information and thus to propagate the collected information.

In case of CHO, the target NG-RAN node updates the time UE stayed in cell of the latest PCell entry (i.e. the source cell) when the UE successfully accesses a candidate cell of the target NG-RAN node. The updated value of the time UE stayed in the source cell is equal to the value received from the source NG-RAN node during the Handover Preparation plus the time from receiving the Handover Request message from the source NG-RAN node to receiving the RRC Reconfiguration Complete message from the UE. When the target NG-RAN node receives the SCG UHI from the source NG-RAN node via the Handover Request message, the target NG-RAN node also updates the time UE stayed in cell of the latest PSCell entry (i.e. the source PSCell) as specified in TS 37.340.

15.5.5 Support for Coverage and Capacity Optimisation |R17| p. 148

15.5.5.1 General p. 148

The objective of NR Coverage and Capacity Optimization (CCO) function is to detect and resolve or mitigate CCO issues, e.g. coverage and cell edge interference issues.

15.5.5.2 OAM requirements p. 148

Each NG-RAN node may be configured with alternative coverage configurations by OAM. The alternative coverage configurations contain relevant radio parameters and may also include a range for how each parameter is allowed to be adjusted.

15.5.5.3 Dynamic coverage configuration changes p. 148

An NG-RAN node may autonomously adjust within and switch between coverage configurations. When a change is executed, a NG-RAN node may notify its neighbour NG-RAN nodes using the NG-RAN NODE CONFIGURATION UPDATE message with the list of cells and SSBs with modified coverage included. The list contains the CGI of each modified cell with its coverage state indicator and optionally the SSB index of each modified SSB with its coverage state indicator.

The coverage state indicator may be used at the receiving NG-RAN node to adjust the functions of the Mobility Robustness Optimisation, e.g. by using the coverage state indicator to retrieve a previously stored Mobility Robustness Optimisation state. The coverage state indicator may also be used at the receiving NG-RAN node to adopt coverage configurations matching with neighbouring cells coverage configurations.

If the list includes indication about planned reconfiguration and possibly a list of replacing cells, the receiving NG-RAN node may use this to avoid connection or re-establishment failures during the reconfiguration. Also, if the sending NG-RAN node adds cells in inactive state, the receiving NG-RAN node may use this information to avoid connection or re-establishment failures. The receiving NG-RAN node may also use the notification to reduce the impact on mobility. The receiving NG-RAN node should avoid triggering handovers towards cell(s) that are indicated to be inactive.

15.5.6 Support for PCI Optimisation |R17| p. 148

The PCI Optimization Function in split gNB case is specified in TS 38.401.

15.5.6.1 Centralized PCI Assignment p. 148

For centralized PCI assignment in gNB, the OAM assigns a single PCI for each NR cell in the gNB, and the gNB selects this value as the PCI of the NR cell.

15.5.6.2 Distributed PCI Assignment p. 148

For distributed PCI assignment in gNB, the OAM assigns a list of PCIs for each NR cell in the gNB, and the gNB selects a PCI value from the list of PCIs. The gNB may restrict this list by removing some PCIs that are reported by UEs, reported over the Xn interface by neighbouring gNBs, and/or acquired through other methods, e.g. detected over the air using a downlink receiver.