8 Optimization issues

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 thatReporting Range 1a: threshold for addition window/2 _

difference of the pilot power settings of two neighboring cells _

Reporting Range 1a: threshold for addition window/2It 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 thatReporting Range 1a: threshold for addition window/2 _

difference of the pilot power settings of two neighboring cells _

Reporting Range 1a: threshold for addition window/2It 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 pilotpower 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 PPilot/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.