What you need to know about Intra-RAT ANR in 5G

Before dive into the topic, there are 2 main terms you need to know about Intra-RAT ANR in 5G which also applies for Inter-RAT ANR.

NCL (Neighboring Cell list)

This list contains information about external cells. An external cell is the one that belongs to a different gNodeB. You may have several gNodeB in the same base station (very uncommon, but still possible). Basically if the cells exist in different gNodeB (even if they are in the same physical Rack), they are still external cells.

The NCl contains information about the external cell, in this case for Intra-RAT ANR in 5G the usual information in the NCL are:

Mcc: mobile country code (MCC)
Mnc: mobile network code (MNC)
gNBId: gNodeB ID
CellId: cell ID
Tac: tracking area code (TAC)
PhysicalCellId: physical cell identifier (PCI)
SsbFreqPos: frequency-domain position of the synchronization signal and PBCH block (SSB)
SsbDescMethod: description method of the SSB’s frequency-domain position
NrNetworkingOption: NR networking option
FrequencyBand: frequency band

As you may guess, for cells is the same gNodeB, there is no need to add them in the NCL because the cell information is available for all cells in the gNodeB.

NRT (Neighboring Relation Table)

The neighboring relation table (NRT) of a cell contains information about all neighboring cells of the cell. In contrast with NCL, NRT is a table and not a list. This is because the NRT contains the neighbor relationships whereas NCL is only a list of each external cells.

The NRT for Intra-RAT in 5G contains only the neighboring relationship with the following information:

NrCellID: NG-RAN cell ID
Mcc: MCC
Mnc: MNC
gNBId: gNodeB ID
CellId: cell ID

As you can see, to create a relationship between cells from different gNodeB, the external cell must exist in the NCL in the gNodeB which you are trying to create the NRT relation.

Now that we know these 2 basic terms for Intra-RAT in 5G, let’s deep dive into the working principle of this feature.

Intra-RAT ANR in 5G enables automatic management of intra-NR NCLs and intra-NR NRTs. In Huawei equipment, there are 3 basic working principles to understand when setting up the Intra-RAT ANR in 5G.

Optimization Mode

This is mode works as either controlled or uncontrolled. The main difference is how the neighbor cell is add or remove. In controlled escenarios, before adding/removing automatically the neighbor, the RF engineer must confirm the process in the OSS. In uncontrolled scenarios, the neighbor is add/remove immediately without user intervention.

Control Mode

This control mode can be Manual or Automatic. This depends on whether the gNodeB can automatically modify or remove neighbor relationships and external cells with ANR.

Auto Mode is the common setting to allow the algorithm to work automatically. Manual control can be set in those cases where you would like to keep the control over modifying or removing some specific neighbor relations or external cells.

Penalty on reportCGI Measurements

During the period of reportCGI measurements, the gNodeB cannot process the measurement reports sent by the UE. To prevent continuous reportCGI measurements from affecting UE mobility, the gNodeB automatically imposes penalty on reportCGI measurements.

Now, let’s review how the Intra-RAT ANR in 5G add, remove and updates NCL and NRTs.

Intra-RAT ANR in 5G: Automatic detection and addition of missing neighboring cells

Event-triggered measurements or periodic measurements can trigger this process. Basically there are 3 possible functions:

Event-Triggered ANR: gNodeB detects and add missing neighbors based on the information in intra and inter frequency measurement reports. For Intra-RAT measurements, this includes mobility measurements and carrier aggregation measurements. This type of ANR is triggered during: Coverage-based intra/inter frequency handovers, frequency priority based inter-frequency handovers, service-based inter-frequency handovers, during SCell addition of unknown neighboring cells.
Fast ANR: This is a not an even-based ANR. In this case the gNodeB randomly selects some UEs in RRC_Connected mode in a serving cell and sends measurement configurations, including intra-RAT frequencies to the UEs. The gNodeB detects missing neighboring cells based on this periodic measurement reports sent from those selected UEs. This options is usually active at the initial deployment of the network. This is because it helps to build the neighbor cell tables. Once the network is stable, this functionality is disable to avoid signalling overload in the RF interface. In those escenarios event-triggered ANR should remain active.
PCI-specific ANR: In this scenario, measurements for specific PCIs are triggered. Once this function is active, the gNodeB selects some UEs in RRC_Connected mode in a serving cell and sends measurement configurations including intra-RAT frequencies associated with the specific PCIs to the UEs. This algorithm works similar as Fast ANR, so the recommendation is to not keep this function active to avoid signalling overhead.

For Fast ANR and PCI-specific ANR, the UEs selected should meet certain criteria such as: To be in RRC_Connected mode with a PDU session, UE should support NR neighboring cell measurements with the report type reportCGI.

In NSA scenarios, the eNodeB works as the master eNodeB connected to the EPC. The gNodeB works as a secondary gNodeB connected to the eNodeB through the X2 interface. The PSCell will change when a UE moves between NG-RAN cells, requiring existing neighbor relationships. If no neighbor relationship exists, neighboring cells will be automatically configured. There is no direct signaling bearer between the gNodeB and the UE. Therefore, the eNodeB is responsible for forwarding the RRC messages between the gNodeB and the UE.

As you can see, the eNodeB works as an intermediary between the UE and the gNodeB during the Intra-RAT ANR in 5G.

Intra-RAT ANR in 5G: Automatic removal of neighboring cells

This feature considers the automatic removal of neighbor relations and external cells (NRT and NCL). In Huawei, to configure this functionality there are cell- and frequency-level switches.

1. Automatic removal of neighbor relations

During Intra-RAT ANR in 5G, the automatic removal of the neighbor relations depends on:

– If the maximum number of neighboring cells of a cell is reached, the gNodeB automatically removes the neighbor relation that are least frequently used from the NRT.

– If the maximum number of neighboring cells of the gNodeB is reached but not for the cell trying to add the neighbor relation, the gNodeB automatically removes the neighbor relationship that are least frequently used from the NRTs of all cells.

– Removal of redundant neighbor relations. A neighbor relationship is redundant if it meets the following criteria: Total number of handover executions from the local cell to all intra-rat neighboring cells is greater than or equal to 50, the number of intra-rat neighboring cells of the local cell is greater than or equal to 32 and the number of times the neighboring relationship is used is 0.

When a neighbor relationship is automatically delete by ANR, the gNodeB imposes a penalty on the neighbor relationship. This is to prevent adding back the neighbor to the NRT within a short period.

2. Automatic removal of external cells

If the number of external cells in the NCL reaches its maximum value, the gNodeB automatically removes the external cells that are least frequently used from the NCL so that new external cells can be added.

The gNodeB can also automatically removes redundant external cells from the NCL in following scenarios:

– There is no neighbor relation in the NRT for that specific external cell.

– The external cell has not been used for a period of 7 days and none of the cells in the gNodeB have neighbor relations pointing to that external cell.

Intra-RAT ANR in 5G: Automatic neighbor cell update

NCL updates procedures occur by measurement reports from UEs, messages transmitted over the Xn interface between gNodeBs, or queries from the MAE-Access.

– Updates to NCL by measurements reports from UEs is triggered after receiving an NGCI, TAC, frequency band list etc. The CGI-Info IE of the measurement report from a UE carries this information. The gNodeB searches its NCL for external cell corresponding to the NGCI and performs the following operations

Check if the PCI and SSB frequency of the external cell in the NCL are the same as those carried in the CGI-Info IE. If it is the same, there is no further operations, otherwise the gNodeB updates the NCL using the SSB frequency or PCI carried by the CGI-Info IE.
The gNodeB checks whether the TAC, frequency band, additional frequency banc and PLMN Reserve flag of the external cell in the NCL are the same as those in the CGI-Info IE. If it is the same, there is no further operation, otherwise gNodeB updates the NCL using the information from the CGI-Info IE.

– Updates to NCL by messages transmitted in the Xn interface is only applicable in SA escenarios (there is no Xn interface in NSA). This kind of NCL updates happen during the Xn setup between gNodeBs or during gNodeB configuration change. When the Xn interface is being setup, both gNodeB exchanges information.

After receiving an NCGI, SSB frequency, PCI, TAC, and frequencyBandList carried in the Served Cell Information NR IE in an XN SETUP REQUEST or XN SETUP RESPONSE message, the gNodeB updates the NCL in the same way as it does in NCL update triggered by measurement reports from UEs. During gNodeB configuration change process, both gNodeB exchange information through the NG-RAN node configuration update procedure.

After receiving an NCGI, SSB frequency, PCI, TAC, and frequencyBandList carried in the Served Cell Information NR IE in an NG-RAN NODE CONFIGURATION UPDATE or NG-RAN NODE CONFIGURATION UPDATE ACKNOWLEDGE message, the gNodeB updates the NCL in the same way as it does in NCL update triggered by measurement reports from UEs.

– Updates to NCL by queries from MAE-Access is triggered in some specific cases where gNodeB queries MAE-Access during UE handover. It updates the the networking configuration option of the NCL entry based in the response from the query. If the MAE returns information, the gNodeB updates the networking option of the external NG cell to SA, NSA or SA_NSA depending on the query result. If the MAE does not return any information, the update cannot be performed.

And that’s all the basics of Intra-RAT ANR in 5G. There are still some advanced Huawei features related to Intra-RAT ANR in 5G that we will be addressing in future posts such as:

– Optimization of Neighbor Relationship Attributes Based on Zero Handover Success Rate

– Optimization of Neighbor Relationship Attributes Based on Identification of Over-Distant Neighboring Cells

– External-Cell PLMN List Configuration

– Neighboring Frequency Configuration

– Management of NRT/NCL blacklist