UMTS Pilot channel failure ,high downlink interference

Symptoms
From the drive test, following symptoms will be observed by using TEMS:
• Received Ec/No of the pilot channel is less than –16dB and
• Received RSCP of the pilot channel is high enough to maintain the connection, e.g. > -100dBm and
• DL RSSI is very high and
• The connection finally drops.

Reason 1 – no dominant cell
There are many overlapping cells at the problem area. The received signal strengths of these pilots are almost the same.



Solution 
The most direct and effective way to solve this problem is to increase the pilot channel power Primary CPICH power of the desired cell.



The drawbacks of this solution are:
• Due to uneven pilot power setting, some UEs might no longer be connected to the “closest” cell with respect to the pathloss. Then they transmit with high UE powers. As a result, the uplink interference level of the carrier is consequently increased. It means uplink is not optimized and this phenomenon is called as uplink near-far problem. It is recommended that

–Reporting Range 1a: threshold for addition window/2 ≤ difference of the pilot power settings of two neighboring cells ≤ Reporting Range 1a: threshold for addition window/2

• In case the pilot power of a cell is increased, the required power for the downlink DPCHs in that cell also increase. Finally, the load of the cell becomes high and then cell blocking may happen.
• The downlink interference level of the carrier will be higher.
• The cell with higher pilot power will absorb more UEs from its adjacent cells. Then the load of the cell will be higher.
• Pilot power changes may lead to uplink coverage and pilot coverage imbalance problems.

Reason 2 – dominant interferer

An undesired cell with very high signal strength is found in the problem area.



Solution 1
The simplest solution to overcome this problem is to include the overshooting cell into the neighboring cell list. This means the interferer now becomes a useful radio link.
The drawbacks of the solution 1 are:
• It creates more unnecessary handovers and excessive numbers of UEs are in soft handover.
• If the overshooting cell is physically far way to the problem area, the handover sequence might be messed up after including it into the neighboring cell list.

Solution 2
An alternative solution is to change the antenna configuration of the overshooting cell, e.g. tilting down the antenna, re-directing the antenna orientation, reducing the antenna height.
With this solution, UL/DL coverage imbalance problem will not occur in the interferer because both UL/DL pathloss is modified simultaneously. Moreover, the interferer probably will cover fewer UEs, and transmit a lower total downlink power. This means that its downlink interference contribution might be further decreased.

The main drawback of the solution 2 is:
• Neighboring cells of the interferer will cover a larger area and will thus absorb additional UEs. The risk of high blocking rate therefore increases in these cells. Moreover, due to transmit high Tx power, they might become interferers if their coverages are not well confined.

Solution 3
The third possible solution is to decrease the pilot power Primary CPICH power of the overshooting cell.
The drawbacks of the solution 3 are:
• Reducing the pilot power, the downlink channel estimation in the UE is affected. This influences the downlink quality. In the end, the UE might request more power from base stations.
• When the pilot power is reduced, the maximum allowed DL DCH power decreases. Then, outage of the downlink DPCH will be higher if the pilot power is reduced too much.

Reason 3 – low best serving PPilot/PTot
The received Ec/No of the best serving pilot channel is very low (near or less than –16dB) even though there is no other cell. It means the pilot power setting is not large enough to fulfill existing downlink load.

Solution 1
The best solution is to add a new site with “good coverage control” at the problematic area.

Solution 2 
The direct but ineffective solution is to increase the pilot channel power Primary CPICH power of the problematic cell. With high pilot power, the common channel powers and the required power for the downlink DPCHs will be increased. At the end, the ratio of the PPilot/PTot does not increase much.