Problems of missing neighbours

In WCDMA a missing neighbour leads to both downlink and uplink quality problems.

Downlink quality

In the downlink two effects can occur

1: Increased DL interference

2: Call drop because of excessive DL interference

Uplink quality

In the uplink the missing neighbour cell can experience an UL noise rise, hence cell shrinkage can occur leading to the possible deterioration of all calls on the cell.

Solution

The way to solve missing neighbour is straightforward. Any cells found to be missing neighbours need to be added in the neighbouring list of the source cell at the OMC.

The difficult part is to identify the missing neighbour. RF scanners are the best tool for this job and therefore it is recommended that they are actively used for WCDMA missing neighbour optimisation.

Missing Neighbour Detection with a scanner

When used with “Top N” functionality a scanner will report all decoded scrambling codes seen within the band.

With a post-processing tool we can list all required neighbour pairs fulfilling the condition:

A comparison can then be made between this list and the declared neighbour list and this will give the missing neighbour list for the corresponding drive.

Missing Neighbour Detection with a trace mobile

When only a trace mobile is available it is far more difficult to detect missing neighbours. This is why scanners are really required for this purpose. The method with a trace mobile is similar to that used to detect missing neighbours in GSM.

Basically the trace mobile will obviously not report the missing neighbour in its monitored set because this neighbour is not declared. The problem is identical in both connected and idle mode. But a few events may help identifying missing neighbours:

If the missing neighbour causes the call to drop, the UE will “lose the network” and revert to scanning mode in order to select a new suitable cell. In this case the UE will most probably select the missing neighbour cell. Correlating the last connected cell and the newly selected cell is likely to give a missing neighbour pair.

If cell_C is a missing neighbour to cell_A but declared as neighbour of cell_B, a handover from cell_A to cell_B will suddenly trigger the reporting of cell_C’s level. If when first reported, cell_C appears to be the best server then it was most probably a missing neighbour of cell_A.