Automatic Neighbour Relation in LTE

Manually provisioning and managing neighbor cells in traditional mobile network is challenging task and it becomes more difficult as new mobile technologies are being rolled out while 2G/3G cells already exist. For LTE, task becomes challenging for operators, as in addition of defining intra LTE neighbour relations for eNBs operator has to provision neighboring 2G, 3G, CDMA2000 cells as well.

According to 3GPP specifications, the purpose of the Automatic Neighbour Relation (ANR) functionality is to relieve the operator from the burden of manually managing Neighbor Relations (NRs). This feature would operators effort to provision

Figure below shows ANR and its environment as per 3GPP. It shows interaction between eNB and O&M due to ANR.

The ANR function resides in the eNB and manages the conceptual Neighbour Relation Table (NRT). Located within ANR, the Neighbour Detection Function finds new neighbours and adds them to the NRT. ANR also contains the Neighbour Removal Function which removes outdated NRs. The Neighbour Detection Function and the Neighbour Removal Function are implementation specific.

An existing Neighbour cell Relation (NR) from a source cell to a target cell means that eNB controlling the source cell knows the ECGI/CGI and Physical Cell Identifier (PCI) of the target cell and has an entry in the NRT for the source cell identifying the target cell.

For each cell that the eNB has, the eNB keeps a NRT. For each NR, the NRT contains the Target Cell Identifier (TCI), which 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.

The ANR function relies on cells broadcasting their identity on global level, E-UTRAN Cell Global Identifier (ECGI) and allows O&M to manage the NRT. O&M can add and delete NRs. It can also change the attributes of the NRT. The O&M system is informed about changes in the NRT.

Intra-LTE/frequency ANR:

The eNB serving cell with ANR function, instructs each UE to perform measurements on neighbor cells, as a part of the normal call procedure. The eNB may use different policies for instructing the UE to do measurements, and when to report them to the eNB.

When UE discovers 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. The eNB adds this neighbour relation to NRT.

Inter-RAT/Inter-frequency ANR:

The eNB serving cell with ANR function can instruct a UE to perform measurements and detect cells on other RATs/frequencies .during connected mode. The eNB may use different policies for instructing the UE to do measurements, and when to report them to the eNB.

The UE reports the PCI of the detected cells in the target RATs/frequencies. When the eNB receives UE reports containing PCIs of cell(s), eNB may instruct the UE to read the CGI and the RAC of the detected neighbour cell in case of GERAN detected cells and CGI, LAC and, RAC in case of UTRAN detected cells. For the Interfrequency case, the eNB may instruct the UE to read the ECGI, TAC and all available PLMN ID(s) of the inter-frequency detected cell.

The eNB updates its inter-RAT/inter-frequency Neighbour Relation Table after receiving relevant info from UE.