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Sunday, February 26, 2017

Pilot Pollution Problem Analysis

Pilot Pollution Definition and Judgment Standards

Definition

The pilot pollution is that excessive strong pilots exist in a point but no primary pilot is strong enough.

Judgment Standards

Pilot pollution exists if all the following conditions are met:
l   The number of pilots that meet the following condition is more than ThN
CPICH_RSCP > ThRSCP_Absolute
l   (CPICH_RSCP1st - CPICH_RSCP(ThN +1)th)< ThRSCP_Relative
Assume that ThRSCP_Absolute = –100 dBm, ThN = 3, and ThRSCP_Relative = 5 dB, and then pilot pollution exists if all the following conditions are met:
l   More than three pilots meet the following condition
CPICH_RSCP > –100 dBm.
l   (CPICH_RSCP1st - CPICH_RSCP4th) < 5 dB

Causes and Influence Analysis

Causes Analysis

Ideally the signals in a cell is restricted within its planned range. However the signals cannot reach the ideal state due to the following factors of radio environment:
l   Landform
l   Building distribution
l   Street distribution
l   Waters
Pilot pollution is the result of interaction among multiple NodeBs, so it occurs in urban areas where NodeBs are densely constructed. Normally typical areas where pilot pollution occurs easily include:
l   High buildings
l   Wide streets
l   Overhead structure
l   Crossroad
l   Areas round waters

I. Improper Cell Distribution

Due to restriction to site location and complex geographic environment, cell distribution might be improper. Improper cell distribution causes weak coverage of some areas and coverage by multiple strong pilots in same areas.

II. Over High NodeB or Highly-mounted Antenna

If a NodeB is constructed in a position higher than around buildings, most areas will be with in the line-of sight range. Therefore signals are widely transmitted. Over high site cause difficult control of cross-cell coverage, which causes pilot pollution.

III. Improper Antenna Azimuth

In a network with multiple NodeBs, the antenna azimuth must be adjusted according to the following factors:
l   NodeB distribution of the entire network
l   Coverage requirements
l   Traffic volume distribution
The sector azimuth of each antenna is set to cooperate with each other. If the azimuth is improperly set:
l   Some factors might cover the same area. This causes excessive pilot pollution.
l   Weak coverage exist in some areas without primary pilot.
The previous two situations might lead to pilot pollution. Therefore you must adjust the antenna according to actual propagation.

IV. Improper Antenna Down Tilt

Setting antenna down tilt depends on the following factors:
l   Relative height to around environment
l   Coverage range requirements
l   Antenna types
If the antenna down tilt is improper, signals are received in the areas which are covered by this site. Therefore interferences to other areas causes pilot pollution. Even worse, interferences might cause call drop.

V. Improper PICH Power

When the NodeBs are densely distributed with a small planned coverage rang and the PICH power is over high, the pilot covers an area larger than the planned area. This causes pilot pollution.

VI. Ambient Factors

The signals cannot reach the planned state due to the following factors of radio environment:
l   Landform
l   Building distribution
l   Street distribution
l   Waters
The ambient factors include:
l   High buildings or mountains block signals from spreading
The signals of a NodeB to cover a target area are blocked by high buildings or mountains, so the target area will have no primary pilot. This causes pilot pollution.
l   Streets or waters influences signals
When the antenna direction is pointing a street, the coverage range is expanded by the street. When the coverage range of a NodeB overlaps with the coverage range of other NodeBs, pilot pollution occurs.
l   High buildings reflect signals
When high glassed buildings stand near a NodeB, they will reflect signals to the coverage range of other NodeBs. This causes pilot pollution.

Influence Analysis

Pilot pollution causes the following network problems.

I. Ec/Io Deterioration

Multiple strong pilots interferes useful functional signals, so Io increases, Ec/Io decreases, BLER increases, and network quality declines.

II. Call Drop Due to Handover

More than three strong pilots or no primary pilot exists in multiple pilots, frequent handover occurs among these pilots. This might cause call drop.

III. Capacity Decline

The interference of the areas with pilot pollution increases, the system capacity declines.

Solutions to Pilot Pollution

Antenna Adjustment

According to the test, change pilot signal strength of an area with pilot pollution by adjusting antenna down tilt and azimuth. This changes the distribution of pilot signals in the area. The principle for adjustment is enhancing primary pilot and weakening other pilots.
To enhance pilot coverage of an area, adjust the antenna azimuth pointing the area. To weakening pilot coverage of an area, adjust the antenna azimuth pointing the opposite direction of the area. Adjusting down tilt is similar. You can increase the cell coverage range by reducing antenna down tilt and reduce cell coverage range by increasing antenna down tilt.
Adjusting antennas is restricted to a range. If the down tilt is over small, you might enhance cell coverage but causes cross-cell coverage. If the down tilt is over large, you might weaken cell coverage but you might change the antenna pattern.

Figure shows the pilot pollution due to improper antenna azimuth.


In Figure, the area marked in black encounters pilot pollution due to improper azimuth of the antenna of SC100 sector (scramble No. is 100). The SC100 sector covers the area with an antenna azimuth of 90°, so the coverage is poor with weak signals and no primary pilot, which cause pilot pollution.
After adjustment of the antenna azimuth from 90° to 170°, the primary pilot signals become stronger and pilot pollution is eliminated.
Figure 6-2 shows the pilot pollution due to improper antenna down tilt.



In Figure the area marked in blacked encounters pilot pollution due to improper antenna down tilt. The down tilt of SC360 cell is 2°, so the coverage area is large, cross-cell coverage is difficult to control, and interferences to other areas form.
After adjustment of antenna down tilt of SC360 cell from 2° to 7°, the cross-cell coverage by SC360 cell is eliminated and pilot pollution is eliminated.
Some areas with pilot pollution is inapplicable to the previous adjustment. You can use the following methods based on actual situation:
l   Change the antenna to a different type
l   Add reflection device or isolation device
l   Adjust installation position of antenna
l   Adjust NodeB location

PICH Power Adjustment

Pilot pollution is caused by the coverage by multiple pilots. A direct method to solve the problem is to form a primary pilot by increasing the power of a cell and decreasing the power of other cells.
An over large down tilt causes aberration of antenna pattern. To reduce coverage range by pilot, you can decrease PICH power. Over small down tilt causes cross-cell coverage. To increase coverage range by pilot, you can increase PICH power. Adjusting power and adjusting antenna must cooperate.
Figure, shows the pilot pollution due to improper distribution of cells.



In Figure,
l   The distance between NodeB A and NodeB B is 1260 meters.
l   The distance between NodeB A and NodeB C is 2820 meters.
l   The distance between NodeB B and NodeB C is 2360 meters.
The distances is unbalanced, so the pilot pollution is difficult to eliminate.
The optimization is to reduce weak pilot strength and eliminate pilot pollution, detailed as below:
l   Ensure seamless coverage between cells by not adjusting transmit power of SC20 and SC30 cells.
l   Decrease the PICH power of SC10, SC40, and SC50 cells by 3 dB. These cells have little impact on seamless coverage.



Using RRU or Micro Cells

If adjusting power and antenna is not effective to solving pilot pollution, use RRU or micro cells.
Using RRU or micro cells aims to bring a strong-signal coverage in the area with pilot pollution, so the relative strength of other signals decreases.
When a network expansion is necessary or more requirements is on network quality, using RRU or micro cells is recommended. Micro cells are used in traffic hot spot areas, they support multiple carriers. Micro cells are used if large capacity is needed. Compared with using RRU, using micro cells is more expansive.
Figure,shows pilot pollution due to ambient factors.







The area marked in black encounters pilot pollution due to ambient factors. The area is covered by SC60 cell of NodeB A, SC110 cell or NodeB B, and SC130 cell of NodeB C. However, shown in Figure, hills prevent NodeB A from transmitting signals, high buildings prevent NodeB B and NodeB C from transmitting signals, so the signals from NodeB A, NodeB B, and NodeB C are weak. On the contrary, SC240 and SC250 cells of NodeB D have good propagation conditions in this direction. Therefore the cross-cell coverage is serious and pilot pollution occurs.



High buildings or hills blocks the area, so no strong pilot is present in the area. For this problem, adjusting antenna down tilt has little effect on eliminating pilot pollution. Instead adding RRU helps solve the problem.

Process for Analyzing Pilot Pollution Problem

The process for analyzing pilot pollution problem proceeds as below:
1)      Start Assistant. Analyze scanner-based RSCP for 1st Best ServiceCell and EcIo for 1st Best ServiceCell. Select the areas with high RSCP and poor EcIo as candidate areas with pilot pollution.
2)      Analyze scanner-based Whole PP. Select the areas corresponding to candidate areas as the key areas with pilot pollution.
3)      Locate the cells that cause pilot pollution of the key areas.
4)      Based on RSCP for 1st Best ServiceCell, judge whether the pilot pollution is caused by existence of multiple strong pilots or lack of a strong pilot. For the former cause, you can solve the problem by weakening other strong pilots. For the latter cause, you can solve the problem by strengthening some strong pilot.
5)      Analyze the RSCP and Ec/Io distribution of areas related to pilot pollution and confirm the cells that need eliminating the coverage of an area and that need enhancing the coverage of an area. Based on the actual environment, analyze the specific causes to pilot pollution (for analyzing causes, see 6.2.1  ). For specific causes, provide solutions to pilot pollution (for solution, see 6.3  ). While eliminating pilot pollution in an area, consider the influence to other areas and avoid causing pilot pollution or coverage voids to other areas.
6)      Retest after adjustment. Analyze RSCP, Ec/Io and Whole PP. If they cannot meet KPI requirements, re-optimize the network by selecting new key areas until KPI requirements are met.






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