In the CDMA 1X system, the pilot intensity (Ec/Io) is an important parameter indicating network forward coverage and same-frequency interference. Due to the interference-restricted characteristic of the CDMA system, how to control the pilot intensity (Ec/Io) is especially important. We take a site engineering case as the example to analyze and describe how to control pilot intensity (Ec/Io), as shown in Figure 1.
| 皋兰路1 | Gaolan Rd. |
| 天吴2 | Tianwu2 |
| 天吴3 | Tianwu3 |
| 百汇百货 | Baihui Department Store |
| 昆仑宾馆 | Kunlun Hotel |
Figure 1
As shown in Figure 1, this test point has five intense pilot branches, while a mobile phone has only three RAKE receivers to receive signals. The signals of the forth and fifth branches will not enter the RAKE receiver, i.e. will not enter the activation centralization, thus becoming interference and resulting in pilot pollution.
The same frequency band is used between various base stations of the CDMA system. Therefore, for inter-cell signals, it is a key factor in CDMA network construction to avoid interference to one another. Suppose one sector is selected, with its Ec given. The increase of signals in any cell in the network means the increase of Io, which will result in the decrease of Ec/Io of the cell. In the soft handover area, the effective soft handover branch can obtain soft handover gain, while other branches will produce interference as they fail to enter the soft handover area. When the intensity of these branches is large enough, the interference on signal becomes an extremely important factor. Therefore, the effective control on the signal of every sector becomes an important means of interference control in CDMA. In CDMA, on one hand, due to the introduction of soft handover, soft handover can be applied effectively in the handover area to obtain the gain of soft handover, so as to improve the network performance; on the other hand, since the number of Rake receivers of a mobile phone is limited (Currently, a mobile phone can effectively receive three soft handover braches), when the number of received signal branches exceeds the number of Rake receivers, the mobile phone will not be able to use these signals effectively. These signals result in interference on the valid signals. If the number of these signals exceeds the given threshold, they will result in serious interference on the valid signals. This is a kind of pilot pollution, i.e. there are excessive intense pilots in the receiving location.
Therefore, as pilot pollution is concerned, one case is: the number of pilots exceeding the given threshold > the number of Rake receivers. Generally, the value of this given threshold is the set value of T-Add. Currently, since a mobile phone has three valid branches, if there are more than 4 intense branches exceeding T-Add, it is regarded that pilot pollution exists.
Seen from the above analysis, pilot pollution is mainly the result of mutual interference of signals between multiple sectors. In ideal condition, the signals of various sectors should be strictly controlled in the designed range. However, due to the complexity of radio environment, including landform and appearance, building distribution, street distribution and waters etc., it is very difficult to control signals, and reach the ideal condition.
Since pilot pollution is mainly the result actions of multiple base stations, pilot pollution mainly occurs in the urban environment with dense base stations. In normal condition, several types of typical areas in which pilot pollution is like to occur are: high buildings, wide streets, viaducts, crossroads and the areas around waters.
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