Optimization Issues Ericsson UMTS

8 Optimization issues 8.1 Pilot channel failure - high downlink interference If inter-RAT mobility function is activated, i.e. FDD GSM handover support = 1, and works properly, this problem should not happen.

8.1.1 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. Please read appendix C.

8.1.2 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, i.e. high Fcch value. Please read equation 10 for Fcch equation.

Figure 15. Example of many cells overlapping, the observation is from the pilot scanning. 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.

Figure 16. Result of increasing the pilot power of the desired cell e.g. pilot 3. 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 It should be noted that the unit for the Reporting Range 1a: threshold for addition window is 0.5dB. For example, if the Reporting Range 1a: threshold for addition window is 6, i.e. 3dB, the margin for pilot power modification is equal to “-3dB _ difference of the pilot power settings of two neighboring cells _ 3dB. Please read appendix B for the restrictions to modify the pilot channel power. - In case the pilot power of a cell is increased, the power of common channels in that cell will simultaneously increase because their parameter settings are relative to the pilot power value. At the same time, the required power for the downlink DPCHs in that cell also increase, please see equation 69. 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, please read chapter 8.5 for details.

8.1.3 Reason 2 – dominant interferer An undesired cell with very high signal strength is found in the problem area.

Figure 17. Example of the overshooting problem from the undesired cell, the observation is from the pilot scanning. 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.

Figure 18. Result of adding the overshooting cell into the neighboring cell list. 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. - Handover event detection in the UE becomes very slow if the list of the neighboring cells is very long. - Additional radio link will cause out of hardware resources in both interferer and interfered cell. - Additional radio link will cause cell blocking in both interferer and interfered cell. 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, see Figure 19.

Figure 19. Downlink interference is reduced by tilting down the antenna at the interfering site. The drawbacks of the solution 2 are: - Time and cost consumption. - The desired coverage of the interferer is modified. Coverage hole might occur. Verification of the coverage should be done again. - 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. After decreasing the pilot power, the total downlink power for the common channels of the interferer decreases. When the pilot power is reduced, the power of all other common channel decreases simultaneously because their parameter settings are relative to the pilot power value. Moreover, the total DL DPCH Tx power of the interferer will probably decrease because the interferer will covers fewer UEs.

Figure 20. Downlink interference is reduced by reducing the pilot power The drawbacks of this solution are: - It is not a solution for long term. The pilot power reduction does not affect the physical pathloss. Therefore, when the load of the interfering cell gradually increases, the interference issues will occur again at the same area as previously. - This solution is not suitable for a capacity limited interferer. The total transmission power of the downlink control channels that is saved by reducing the pilot power, is used up by new DPCHs. - Like the problem in increasing pilot power, the uplink is not optimized due to uneven pilot power setting. 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 It should be noted that the unit for the Reporting Range 1a: threshold for addition window is 0.5dB. For example, if the Reporting Range 1a: threshold for addition window is 6, i.e. 3dB, the margin for pilot power modification is equal to “-3dB _ difference of the pilot power settings of two neighboring cells _ 3dB. Please read appendix B for the restrictions to modify the pilot channel power. - 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 simultaneously because this parameter setting is relative to the pilot power value. Then, outage of the downlink DPCH will be higher if the pilot power is reduced too much. Please read appendix B for the restrictions to modify the pilot channel power. - The desired coverage of the interferer is modified. Coverage hole might occur. Verification of the coverage should be done again. - 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 coverage are not well confined.

8.1.4 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.

Figure 21. Example of too low best serving PPilot/PTot, the observation is from the pilot scanning. Solution 1 The best solution is to add a new site with “good coverage control” at the problematic area. It is a cost- and time-consuming solution because installation work is needed. After having a new site, the optimization engineers have to verify the coverage once again to ensure that the coverage of the new site is well under control. “Good coverage control” is extremely important to WCDMA system. The probability of interference problems can be minimized with a suitable degree of overlapping. Note: In TDMA or FDMA systems, the problems due to poor coverage control (excessive overlap) can be hidden by frequency planning. Solution 2 Another possible solution is to shift the downlink load of the problematic cell to its adjacent cells. Solution 3 It is an issue due to high total downlink Tx power. Solution 4 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. Therefore, the total transmission downlink power of the cell will consequently increase. At the end, the ratio of the P Pilot/PTot does not increase much. Please read chapter 8.1.2 for the drawbacks after the pilot power is increased.

8.2 Pilot channel failure - out of pilot coverage If inter-RAT mobility function is activated, i.e. FDD GSM handover support = 1, and works properly, this problem should not happen.

8.2.1 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 very low, e.g.