Swapped Feeders Analysis in New England

T- Mobile Market

George Karim

Introduction• One of the most common faults with a new site build is

swapped feeders. This occurs when the feeder(s) for one sector is connected to a different sector.

• The majority of sites will have two feeders per sector. The first feeder is for Tx/RxA, the second feeder for RxB (receive diversity). Either of these feeders could be swapped between sectors. It is relatively easy to identify a swapped Tx/RxA feeder with a drive test. However, it is more difficult to identify a swapped RxB feeder.

• Figure 1 shows the correct feeder connections on top of the RBS cabinet. The connections on the right (J1, J2, J3) are for the Tx/RxA feeders of each sector. The connections on the left (H1, H2, H3) are for the RxB diversity feeders of each sector. Problems arise when the sector numbering or diversity paths get mixed up. This is usually caused by incorrect labeling on the feeders.

Sectors connections inside Node-B Cabinet

Methods of identifying Swapped Feeders

Method 1. identify swapped feeders by display Best server Ec / Io for scrambling codes (by signal strength) along the drive route.

Method 2. identify swapped feeders by displaying Best server RSCP for each sector along the drive route.

Figure below shows the CPICH RSCP for scrambling code 440 along the drive route. The signal is a lot weaker than predicted.From the CPICH RSCP plot below, It is clear that SC456 is swapped with SC440. the Tx/RxA feeder for Sector 3 (SC456) has been mistakenly connected to the sector 1 antennas

SC440: Weaker than predicted CPICH RSCP.

Conclusions

• Unloaded cluster drives will be used to troubleshoot sites.

• The most common problems expected are swapped feeders, wrong tilts or wrong azimuths. Other problems could be damaged hardware on the sites or other nodes, parameter errors, sites going down during the drive, or wrong antenna types installed.

• It is important to handle the drive test equipment with care, and to perform inspections before each drive. Loose connections or damaged equipment will give a false indication of network problems when in fact the fault is with the drive test equipment.

• The UE could also be faulty.