pilot pollution analysis algorithm 2 CNA pilot pollution analysis algorithm 2 is used to check the test data of the UE. Pilot pollution refers to the condition that the pilots that exceed the pilot pollution threshold are not included in the active set after certain lag time. When the UE is used in the test, the condition that the missing neighboring cells that cannot join the active set in a short period of time deteriorates the Ec/Io of the serving cell and generates extra interference. The pilot pollution analysis algorithm 2 can speedily locate the potential area with missing neighboring cells. Then, the pilot set of the UE can be used to check whether the problem exists. As shown in the figure below, you can locate the pilot pollution area by pilot pollution analysis algorithm 2, and then use the UE to judge whether the pilot pollution is caused by missing neighboring cells. From the pilot set of the UE, we can judge that Cell194 is the missing neighboring cell.
Figure Error! No text of specified style in document.-1 Using pilot pollution analysis algorithm 2 to check for missing neighboring cells
Solution for missing neighboring cells Once the missing neighboring cells are detected, you should add the cells to the neighboring cell list in the OMC-R. Note that more configured neighboring cells does not necessarily represent the network performance is better. It is the quality of the neighboring cells that impacts the network performance. If too many neighboring cells are configured, the neighboring cell searching period will be
prolonged, and then the equipment performance will be impacted; on the contrary, if some neighboring cells are missing, unnecessary interference will exist, and call drop may occur. At the initial stage of network construction, the network engineers set the neighboring cell relation based on field inspection and distribution of the base stations. After the network is put into commercial operation, the network load is added with the increase of subscribers. In this condition, the engineer can optimize the neighboring cell configuration by tracing the detected set and MR.
1.1.1.1
Pilot Pollution Judgment of pilot pollution Pilot pollution is the most common problem in UMTS. In plain terms, it refers to the condition that the pilot signals received from different cells are similar (either strong or weak) at one testing point, and there is no primary pilot signal. At present, most UMTS terminals support a maximum of three active sets. In other words, if more than three cells have similar Ec/Ios, the three RLs in the active set will be interfered. Pilot pollution is generally caused by improper design of the network coverage. The following causes may lead to poor coverage: overshooting of high sites, ringshaped NodeB distribution, wave-guide effect, and large reflector. All these factors lead to the deformation of signals. Areas with severe pilot pollution will have low call setup success rate, low call setup success rate for high-speed data services, high handover failure rate, and compromised capacity. CNA has defined three algorithms to calculate the pilot pollution. Click Tools > Analysis Parameter Setup to set the parameters related to the algorithms. These algorithms can speedily and accurately analyze the pilot pollution problem, and timely locate the pilot pollution area.
Figure Error! No text of specified style in document.-2 Parameter setting for pilot pollution analysis algorithms
The above figure shows the setting of parameters related to pilot pollution and their default value. Algorithm one considers the number of pilots in the active set, and is applicable to the test data of the Scanner and the UE. Algorithm one is defined as the number of pilots that has exceeded the pilot pollution threshold and the active set threshold. In algorithm one, pilot pollution is defined as follows: RSCP > -95 dBm (RSCP Threshold), and Ec/Io > 13 dB (Ec/Io Threshold). We can get the number of pilot pollutions by subtracting 3 (active set) from the total number of pilots that have exceeded the pilot pollution threshold.
As shown in the figure below, when the parameters related to pilot pollution is set as shown in the figure above, three pilots have exceeded the pilot pollution threshold, and the number of the active set is set to 3, so the number of pilot pollution is 0. If the RSCP Threshold is set to be larger than -105 dBm, and the Ec/Io Threshold is set to be larger than -18 dB, then four pilots have exceeded the pilot pollution threshold. Because the active set is set to 3, the number of pilot pollution is 1.
Figure Error! No text of specified style in document.-3 Example of pilot pollution analysis algorithm one
Algorithm two does not consider the number of pilots in the active set, and it is applicable to the test data of the UE. Pilot pollution refers to the condition that the pilots that exceed the pilot pollution threshold are not included in the active set after certain lag time. In algorithm two, pilot pollution is defined as follows: RSCP > -95 dBm (RSCP Threshold), and Ec/Io > 13 dB (Ec/Io Threshold). Number of the pilot pollutions refers to the pilots that have exceeded the pilot pollution threshold, and are not included in the active set after certain lag time.
When the parameters related to pilot pollution is set as shown in the figure below, three pilots have exceeded the pilot pollution threshold. After 1 s, pilot 222 and pilot 123 still are not included in the active set, so the number of pilot pollution is 2.
Figure Error! No text of specified style in document.-4 Example of pilot pollution analysis algorithm two
Algorithm three, defined by China Unicom, is applicable to the test data of the Scanner and the UE. Here, pilot pollution refers to the number of pilots that has exceeded the pilot pollution threshold and the active set threshold. In algorithm three, pilot pollution is defined as follows: RSCP > -100 dBm (RSCP Threshold), and Ec/Io > 1BestServingCell - 5dB (Ec/Io Threshold). We can get the number of pilot pollutions by subtracting 3 (active set) from the total number of pilots that has exceeded the pilot pollution threshold. As shown in the figure below, when the parameters related to pilot pollution is set as shown in Figure Error! No text of specified style in document.-2, six pilots have exceeded the pilot pollution threshold. Because the active set is set to 3, the number of pilot pollution is 3.
Figure Error! No text of specified style in document.-5 Example of pilot pollution analysis algorithm three
Note: The parameter settings of the three algorithms are different, so their conclusions also may differ. Pilot pollution analysis can help to locate the area with possible pilot pollution problems. Therefore these three algorithms with different parameter settings can be used to analyze the condition of the network from different aspects. Then, we can put forward a more customized network optimization solution. For China Unicom projects, it is recommended to use algorithm three to evaluate the pilot pollution level.
The following figure shows the analysis chart of pilot pollution algorithm three. 0, 1, 2, and 3 represent the number of pilot pollution.
Figure Error! No text of specified style in document.-6 Analysis chart of pilot pollution algorithm three
Solution for pilot pollution To solve the pilot pollution problem, we have to find one main coverage pilot within the pilot pollution area, which can be performed by the following steps: 1.
Add base stations. If pilot pollution happens in areas with strong coverage signals, reconstruct the ambient base stations to omni cells or 2-sector base stations to reduce the number of pilots; if pilot pollution happens in areas with poor coverage signals, add a new base station in the polluted area. It is common to add base stations in the pilot polluted areas. The working principle is as follows: the path loss of a newly constructed site will be far less than that of the cells in the pilot polluted areas owning to the distance causes. So the pilot power of this site will obviously larger than that of the other sites, and the new site will work as the main serving cell of the UE. For the sectors of the original sites, the introduction of the new site helps to increase the Io value of this area, so the Ec/Io reduces, and the pilot pollution problem is solved. This method has its own shortcomings, for example, it may waste some resources, PSC resources, or even capacity. What’s more, it makes the PSC planning become more complicated, and increase the investment.
2.
Adjust TX power of the cell. Increase or decrease the TX power of one or multiple cells within the pilot polluted area, then a main pilot will appear. If the TX power of the cell is reduced, the Io value within the area also will reduce. Then, the EC/Io will be increased under the
precondition that the power of the rest pilots remains unchanged. In this way, you can enlarge the difference between the main pilot and the rest PSCs in Ec/Io, thereby removing the pilot pollution. ZTE has proved that there will not be significant changes in the cell capability when the TX power of the pilot is reduced. Similarly, increase of the TX power of one or two sectors can help to increase the Ec/Io of the sector(s), and then Ec/Io of the other sectors is reduced for the Io value is increased. Thereby, the pilot pollution problem is solved. Different from reducing the TX power, when you increase the TX power of the sectors, you must ensure that this operation will not generate extra interference to other cells or even new pilot pollutions in other areas. All these problems may appear. Surely, this adjustment method also has its own shortcomings: If the power of the pilot increases, the sync channel power and paging channel power also increase, and then the traffic channel power will reduce. If the pilot power is reduced, the penetration of the signals will be greatly reduced, and then the communication quality will be affected. Therefore increasing rather than decreasing of the power is recommended. Because the TX power of the sector is adjusted, the coverage of the cell and ambient cells may be affected. When optimizing the pilot pollution issue, you must fully consider the impact of this operation on system coverage. 3.
Adjust the antenna parameters. This mainly refers to the azimuth and downtilt. The solution is to generate a main pilot signal strong enough for the pilot pollution area, and to reduce the pilot quality of the other signals. The adjustable range of the antenna downtilt is small. Therefore you can adjust the downtilts of multiple cells to expand the adjustment range. This may also influence the coverage of the adjusted cell and its ambient cells. Compared with adjusting the TX power of the sectors, adjusting the antenna downtilt will have limited influences on coverage. Only the coverage of the ambient cells will be slightly impacted for the cell breathing effect. Similarly, if you adjust the azimuth adequately, the signal power of this sector in the polluted area will decrease or increase, then the difference among the signal power of different sectors will be enlarged, and the pilot pollution problem can be solved. However, it is not easy to control the TX power of the sector within the range by adjusting the sector azimuth. What’s more, the coverage of this sector may be affected. Actually, azimuth is mainly used to adjust the coverage. From the reasons above, we can see that adjusting antenna azimuth and downtilt are good optimization methods to solve the pilot pollution problem. It requires few project loads, and has little impact on system coverage. It is recommended to use the planning software with advanced algorithms and high accuracy to do the simulation. During the DT, several engineers can work together to modify the parameters.
4.
Replace the original antenna with Remote Electrical Tilt (RET). When the downtilt of mechanical antennas is increased, the antenna beam will be flattened, and more signals cover the side lobe. Then coverage of the side lobe to other cells will increase, and the new pilot pollution may occur. For the RET, this kind of problem never appears. Therefore, you can replace the mechanical antenna with RET when the other methods do not work.
5.
Adjust handover parameters. From the handover aspect, you can increase the handover threshold properly so signals with poor Ec/Io cannot enter the active set. However, this may cause the problem that the handover cannot be completed timely, and the handover failure rate may arise.
