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.