All About Automatic Neighbor Relation (ANR)

Automatic Neighbor Relation (ANR):
- Automatic Neighbor Relation (ANR) function is to relieve the operator from the burden of
manually managing Neighbor Relations (NRs).
- The ANR function resides in the eNB and manages the conceptual Neighbor Relation Table (NRT).
- The Neighbor Detection Function finds new neighbors and adds them to the NRT.
- ANR also contains the Neighbor Removal Function which removes outdated NRs.
- An existing Neighbour Relation from a source cell to a target cell means that eNB controlling the source cell:

• Knows the ECGI/CGI and PCI of the target cell.
• Has an entry in the Neighbour Relation Table for the source cell identifying the target cell.
• Has the attributes in this Neighbour Relation Table entry defined, either by O&M or set to default values.

- NRT contains : 

• Target Cell Identifier (TCI)
• Identifies the target cell. 
• For E-UTRAN, the TCI corresponds to the E-UTAN Cell Global Identifier (ECGI) and Physical Cell Identifier (PCI) of the target cell. 

Each NR has three attributes: 

• NoRemove : eNB shall not remove the Neighbor cell Relation from the NRT.
• NoHO : Neighbor cell Relation shall not be used by the eNB for handover reasons.
• NoX2 attribute : Neighbor Relation shall not use an X2 interface in order to initiate procedures towards the eNB parenting the target cell.

NR Table

- Part of Self-Configuring Networks in SON.
- Manual Addition of cells in the neighbor list is a tough task. It is becomes very difficult when there are exiting networks like 2G,3G.
- For LTE there can be Intra LTE, Inter LTE and Inter RAT neighbors.
- There can be:

• Intra-Freq Automatic Neighbor Relations. 
• Inter-Freq/Inter System Automatic Neighbor Relations. 

Intra-Freq Automatic Neighbor Relations:

- Intra-Frequency neighbors can be added automatically as part of Intra-Freq handover procedure.

-  As a process for Intra Frequency HO procedure the serving eNodeB instructs each UE  to perform Intra-Frequency Measurement on neighboring cell by sending RRC Connection Reconfiguration message with measurement control information.

Automatic Neighbour Relation Function

- Consider as in the above picture, Serving Cell is CellA(UE is in RRC Connected State) and Cell B is the Target Cell: 
1.

• The UE sends a measurement report regarding cell B. 
• This report contains Cell B’s PCI, but not its ECGI. 
• eNodeB checks if the reported PCI is already included in the Neighbor Database, then the HO proceeds in the normal way.
• If reported PCI is not included in the Neighbor Database then eNB proceeds to add the PCI to its NRT.

2.

• Once eNB receives a UE measurement report containing the PCI, the eNB instructs the UE with another RRC Connection Reconfiguration Message, using the newly discovered PCI as parameter.
• Instruct UE to read the ECGI, the TAC and all available PLMN ID(s) of the related neighbor cell. 
• To do so, the eNB may need to schedule appropriate idle periods to allow the UE to read the ECGI from the SIB1 of the detected neighbor cell. 
• The UE reads the requested information from SIB1 on PDSCH. UE needs to read MIB on PBCH, then DCI with SI-RNTI on PDCCH to read SIB1 on PDSCH.

3. 

• When the UE has found out the new cell’s ECGI, the UE reports the detected ECGI to the serving cell eNB. 
• In addition the UE reports the tracking area code and all PLMN IDs that have been detected. 
• If the detected cell is a CSG or hybrid cell, the UE also reports the CSG ID to the serving cell eNB.

4. 
The eNB decides to add this neighbor relation, and can use PCI and ECGI to:

• Look up a transport layer address to the new eNB.
• Update the Neighbor Relation List.
• If needed, setup a new X2 interface towards this eNB. 

Inter-Freq/System Automatic Neighbor Relations:
- Can be done as a part of the normal inter-frequency and inter-system handover procedure.
- Inter-frequency and Inter-RAT measurement requires compressed mode to be configured.
- The eNodeB instructs UE to start inter-frequency and inter-RAT measurement using RRC Connection Reconfiguration message.
- The UE searches for the neighbor cells, identifies and reports them to eNodeB.
- The format of PCI depends upon the RAT of the Cell being measured.

Measurement Info Reported By UE

- Neighbor cell addition procedure:

Automatic Neighbour Relation Function in case of Inter Frequency/System Neighbor

- Once UE receives the Measurement Report for the Inter Frequency/RAT cell, 

• eNodeB checks if the reported PCI is already included in the Neighbor Database, then the HO proceeds in the normal way.
• If reported PCI is not included in the Neighbor Database then eNB proceeds to add the PCI to its NRT

- eNodeB instruct UE using another RRC Connection Reconfiguration message to decode the Global Cell Identity (CGI) from the system information.
- The UE may be required to report additional information depending on the system being measured.
- The eNB updates its inter-RAT/inter-frequency Neighbour Relation Table.
- In the inter-frequency case and if needed, the eNB can use the PCI and ECGI for a new X2 interface setup towards the new eNB.