How can we solve the channel pollution problem in the CDMA wireless network system?

So far, wireless technology has not only made technological advances, but also has been widely used. However, there are also some problems. Everyone should have heard about the word "pilot pollution. Then, we will analyze the pilot pollution problem of the CDMA wireless network system in detail below.

With the rapid development of CDMA wireless network technology, more and more users are using wireless network services, and the coverage of network services is becoming wider and deeper. The number of base stations in CDMA wireless network systems is also increasing. Because CDMA is a self-interference system, with the increase in the number of base stations, the base station spacing is getting smaller and smaller, and it is easy to cause weak coverage, no dominant frequency, and other phenomena to form a pilot contaminated area, this affects KPI indicators such as system capacity, user call quality, connection rate, and disconnection rate. Therefore, when planning and optimizing CDMA wireless networks, pilot pollution becomes a problem that needs special attention and needs to be solved.

1. Definition of pilot pollution

In CDMA wireless networks, pilot pollution is divided into pilot phase pollution and pilot intensity pollution. The pilot pollution we usually mention is usually guided by the Frequency Intensity pollution.

1.1 Pilot phase pollution

In the forward channel of the CDMA wireless network system, different PN offsets are used for each sector to distinguish different PN short codes. The pseudo-random sequence generated by the 15th-order shift register contains a total of 215 chips. The CDMA wireless network system requires 64 chips as a group, so a total of 215 chips 64 = 512 groups, that is to say, there are 512 PN Codes. If two sectors with the same PN cover a region together, then the mobile phone in this region will receive signals from two sectors with the same PN at the same time, the mobile phone will not be able to distinguish which base station is communicating, this phenomenon is caused by the pilot phase pollution. If the signal of a certain sector passes through the Path Transmission and the delay is greater than 64 chip, the mobile phone will consider it as another PN, it may cause the same phase when the slices of different phases are transmitted through the path to the mobile phone, which leads to the pilot phase pollution. Due to the MM mobility management when the pilot phase pollution is generated) the system cannot correctly identify the sector in which the pilot communicates with the mobile phone. The interference is very large, communication between the mobile phone and the base station cannot be normal, and channel allocation must fail or be dropped. Currently, the Pilot Inc parameter can be set in the network to set the multiplexing distance between PN. Generally, two identical PN nodes will not overwrite the same region or cannot recognize the slice, the probability of phase contamination of the pilot in the current network is small.

1.2 boot intensity pollution

For a CDMA wireless network system, in simple terms, the pilot intensity pollution means that when the mobile station is located in a region with multiple pilot signals, and these pilot signals are both very strong or weak, this may cause poor EcIo or frequent switching. There are two situations: one is that when we receive more than three pilot signals, because the Rake receiver of CDMA mobile phones can only demodulated signals from three pilot frequencies at the same time, the existence of more than three strong pilot signals will interfere with the system and cause EcIo to decrease, it is considered that there is a pollution of the pilot intensity. It is considered that the mobile station is located in the region where multiple pilot signals exist, but the signal strength of each pilot is relatively weak and EcIo is similar, that is to say, there is not a channel with sufficient signal to take the dominant position. The rapid decline of the signal will lead to frequent switching between business channels in different sectors during the call of the mobile station, which can easily lead to disconnection.

From the perspective of Dingli road testing software, the performance is usually that the mobile phone receives a good enough or similar pilot power, but the Ec/Io in each sector is weak, from the real-time display of the activation set, candidate set, and adjacent set information of the software, if the intensity is good or there are more than three derivative frequencies with similar intensity, the EcIo may decrease or frequent soft switching may occur. In this case, we can see that more than three pilot signals interfere with the three Service pilot signals in the activation set.

From the comprehensive background analysis system of Dingli, the analysis of the pollution caused by the pilot frequency provides several custom pilot frequency pollution definition conditions, through the combination of different parameter conditions and decision threshold conditions, we can quickly locate the region where the pilot pollution problem exists for different pilot pollution phenomena for targeted analysis and optimization, provides suggestions for network planning and optimization.

The following describes the causes, impacts, and solutions of the pilot intensity pollution. The pilot intensity pollution mentioned in this article is referred to as pilot pollution.

2. Causes of pilot pollution

Pilot contamination is mainly caused by signal interference between multiple sectors. Due to the complexity of the wireless environment, the signal is very difficult to control due to the effects of topography, building distribution, street distribution, and water area. The pilot pollution mainly occurs in the urban environment with dense base stations. Because the base stations in cities are densely distributed, it is easy to generate pilot signals from several sectors to cover the same region, at the same time, because indoor signals will leak to the outdoors and the signals will be reflected and reflected by the high-rise buildings in the city, it will also easily lead to frequency pollution. In addition, when the theoretically strong guide signal is blocked, it may lead to weak guide for the main user in the area covered by the Guide signal, resulting in receiving a few weak guide signals at the same time, this also causes the pilot frequency pollution.

Therefore, the pilot frequency planning and fan area coverage are very important in the city. As long as the pilot frequency planning and sector coverage are well controlled in engineering construction and network optimization, the pilot frequency pollution can be avoided as much as possible.

3. Influence of pilot pollution on the System

When the pilot signal is contaminated, the background noise may be increased due to interference caused by other channel signals that cannot be demodulated to the signals that can be demodulated, and the Ec/Io value may be reduced, with the increase in the value of FER, the mobile phone will increase the transmit power when demodulation useful signals. As the transmit power of the mobile phone increases, the mobile phone user will cause greater interference to other users, for the base station, the sensitivity of the base station is reduced due to the higher transmission power of the mobile phone. When a mobile phone user's FER reaches a very large value, because the frame rate is too high, the mobile phone can no longer call out useful signals, it will produce a drop.

If the call is performed in the pilot contaminated area, the pilot frequency will be frequently switched during the call process due to the pilot pollution. However, there are few access switching functions in the current system, this will make the phone call start more frequently than the usual Preamble probe, so that the connection time is longer, the transmission power is too large during access, the non-connection rate increases, but also affects the call access of High-Speed businesses.

Mobile phone users can perform soft or more soft switching between these channels due to the pilot pollution, although the soft switching technology is conducive to reverse diversity reception in the CDMA wireless network system, it will occupy the system resources CE, the power of the white hat code and the base station), reducing the system capacity.

When the mobile station moves in this region, due to the large number of strong-guide signals and fast mutual changes, it is bound to lead to frequent switching of the Mobile Station. When the mobile station is in the soft switching status, it needs to communicate with several base stations at the same time. Although the diversity gain can improve the call quality of the Mobile Station, the switching gain is negative at the moment of switching, that is, not only does not have the gain, but it increases the possibility of switching failure.

4. Solution to pilot pollution

To solve the pilot pollution, it is mainly to highlight the strong pilot signal, reduce the pilot frequency entering the activation set, and weaken some of the pilot signals, which can reduce the interference caused by these pilot signals, solve the Problem of pilot pollution.

To solve the pilot pollution problem, we must find the source of the pilot pollution. The following is a summary of the main solutions:

1) Reduce the coverage overlap between base stations to avoid overlap between the coverage boundaries of each base station.

2) Avoid the case where the base station overpasses the area.

3) by adjusting the antenna azimuth, pitch angle, and base station transmit power, the signal strength of the main service area is enhanced in the area with pilot pollution to reduce the signal strength of other residential areas.

4) The signal strength of the main service area in the pilot contaminated area is enhanced through RRU and fiber optic direct station.