At this stage, repeaters have become an effective means of addressing specific area coverage in CDMA networks. However, after the introduction of the repeater, if it is not handled properly, it will have a certain impact on the network. How to make the repeater fully function without adversely affecting the whole network is a problem that all repeater manufacturers must face.
In the construction of CDMA repeater, several main types of repeaters used are: wireless co-frequency repeaters, fiber optic repeaters and frequency shift repeaters. Here, combined with the engineering practice, briefly introduces several key issues that need to be considered in the engineering design of CDMA repeater at this stage.
Issues considered in the design of repeaterIn the engineering design of CDMA repeaters, any type of repeater needs to consider the following issues:
Uplink noise
Any repeater accessing the base station will cause interference to the original CDMA network, that is, introducing certain noise to the donor base station, causing the sensitivity of the base station to decrease.
It is well known that under the CDMA system, we want the noise power to be connected to the base station receiver inlet to be less than -113 dBm. This requires reducing the impact on the base station by adjusting the uplink gain of the repeater during link budget and repeater commissioning. During the commissioning, the uplink gain should be adjusted and the noise power of the noise arriving at the base station receiver via the effective path loss should be controlled within -113 dBm. As long as the uplink noise is controlled, the repeater will not form interference to the base station.
2. Adjustment of search window parameters
After the repeater station is added, the coverage area of ​​the donor base station is changed, and the search window range originally set by the base station may be too small. In order to make the system work normally, the corresponding parameters need to be modified. The main parameters of the general adjustment are: Search A, N, R and the search window of the reverse access channel.
The use of a repeater in a CDMA system produces timing delay and signal delay spread. If the delay is large, the phase of the pilot code of the CDMA system will change, causing problems such as dropped calls. Adjust the access channel search window width of the donor base station according to the local situation, the reverse traffic channel search window width, and activate the pilot channel and the search window width. At the same time, the donor cell should be added to the neighbor cell list of the neighboring cell, and the adjacent pilot channel and the search window width should be appropriately adjusted.
3. Output power
The purpose of installing the repeater is to solve the known coverage area, and the repeater only needs to solve the coverage of the range. When the repeater is commissioning, the output power setting should not be too large, otherwise it will bring new pilot pollution and other problems. Therefore, the transmission power of the antenna should be reduced as much as possible in the case where the coverage effect can be achieved. If the repeater is used in an indoor coverage system, the indoor signal is required to be controlled as much as possible to avoid signal leakage affecting the outdoor signal. Below, we will separately Repeater three primary engineering design to be applied to analyze
Wireless co-frequency repeaterThe wireless co-frequency repeater is favored by operators because of its easy construction, quick opening and low cost. It is widely used in outdoor coverage and indoor distribution systems, and it is widely used in the entire repeater application. proportion.
When designing the wireless co-frequency repeater, you need to fully consider the following points:
1. Signal source selection
In order to ensure good call quality, the reasonable selection of the donor base station is very important. When the repeater picks up the donor signal, the donor signal is required to be single and stable (the donor signal strength is generally required to be greater than the minimum inlet level of the repeater by about 5 dB). This is a prerequisite for determining whether a wireless co-frequency repeater can achieve a good turn-on effect.
When selecting the donor signal, it should be avoided that there are multiple pilot signals with equivalent signal strength in the direction of the donor antenna to avoid the ping-pong effect of the coverage area. If there are multiple base station signals that can be used as donor signals, it is necessary to consider the traffic volume, the number of CEs on the base station, and the like.
In selecting the donor signal, the method of testing the signal strength together with the test cell phone and the spectrum analyzer should be used. At present, most manufacturers use only test mobile phones for testing during electrical testing. We believe that mobile phones vary from model to model, and the displayed level is not as accurate as the spectrum analyzer, especially when it is weak. It is better to use the mobile phone to determine the PN of the donor signal and then use the spectrum analyzer to determine the strength of the donor signal. Practice has proved that the only way to accurately measure the strength of the donor signal to be selected is to make the source a good source.
2. The isolation of the transceiver antenna
Transceiver antenna isolation refers to the attenuation of the output port signal of the repeater to the output port signal, which is the key to determine whether the wireless co-frequency repeater can work stably. If the isolation of the donor antenna and the retransmission antenna is insufficient, it will cause the self-excitation of the repeater, causing the repeater to be unable to work normally. If the design is not well considered, any remedy is very limited.
The isolation of the repeater is related to the gain of the donor antenna and the retransmission antenna, the front-to-back ratio, the sidelobe suppression ratio, the installation and the surrounding environment. In design, ensure that the isolation of the transceiver antenna is about 15dB greater than the actual gain after the system is turned on. In actual engineering, it is necessary to use the source and receiver for simulation test according to the actual situation on site.
Fiber optic repeaterDue to the use of optical fiber to realize signal transmission and transmission and transmission, the optical fiber repeater overcomes many factors such as terrain influence and transmission instability, especially to solve the problem of signal source clutter, and can easily realize large-scale coverage of base station signal hopping. When designing, the following three points should be considered:
1. How to introduce the source
When the fiber optic repeater picks up signals from the base station, it should avoid using the same PN value in the overlap zone and the neighboring cell of the donor base station. If the repeater picks up the same PN value as the adjacent sector, then the dropped call rate will be high in the overlapping area with the base station coverage.
2. Base station search window adjustment
Due to the particularity of fiber construction, the actual fiber length of the base station to the repeater is generally greater than the linear distance. Even if the length of the fiber obtained from the operator is due to the quality of the fusion of the fiber, the true delay and the theoretical value are quite different. The most ideal method is to use the drive test equipment to accurately measure the signal delay, and then calculate the correction value of each window to adjust the search window of the original base station. In engineering, the theoretical calculation is generally adopted, plus a certain margin to determine the signal delay.
In the engineering commissioning, the adjustment of the forward search window is generally noticed. However, the phenomenon that the mobile phone often has too high transmission power and dropped calls is usually caused by the small setting of the reverse search window, because the transmission power of the mobile phone reaches the base station due to timing in the demodulation process. Some of them are not fully demodulated, and the base station mistakenly believes that the mobile phone's transmit power is insufficient, while other powers interfere with the base station as noise. Therefore, the adjustment of the search window is subject to both front and back adjustments.
3. Avoid the occurrence of island stations
A station with excessive time errors or delays is called an island station. In the process of handover, the mobile phone uses a time reference with a large time error, searches for a neighboring cell in a predetermined time window, and finds only unrelated noise, so that the call is dropped. This phenomenon is usually caused by a long transmission distance of the optical fiber or an increase in transmission delay due to other reasons. In this case, the useful signal of the neighboring cell may be mistaken for the noise signal in the detection of the mobile phone. At this time, when the call is made on the boundary of the cell, whether it is cut or cut, there is a hidden danger of dropped calls. Therefore, when using a fiber optic repeater, the occurrence of an island station should be avoided, and the fiber distance should be reasonably considered. It is recommended not to exceed 20Km.
Frequency shift repeaterWireless co-frequency repeaters are limited by source selection and isolation. As the density of base stations increases, the range of applications in urban areas becomes smaller and smaller; and optical fibers cannot reach in many places. In this case, frequency shifting is adopted. The repeater can solve the above problems very well. The frequency shift repeater can carry out the long-distance extension coverage of the donor base station signal, and solve the coverage of the area where the optical fiber and the pilot signal are not polluted.
The engineering design of the frequency shift repeater mainly considers the following two points:
1. Transmission distance should not be too large
Due to the influence of terrain, terrain and transmission path attenuation on the transmission path, the line of sight of the wireless transmission link should be smooth, and the transmission distance should not be too large (to ensure that the RF input level is greater than -60dBm, the general station distance should be about 15km). ).
2. How the source is coupled
As a frequency shift repeater, the coupling mode can be directly coupled to the base station RF port, or can be coupled by means of airborne wireless relay. In order to avoid introducing too many signals and causing new pilot pollution, we recommend direct coupling. This will ensure the uniqueness of the source and make full use of the original organic room and supporting facilities.
Typical case analysisAs shown in the figure, the repeater B is a fiber optic repeater, the length of the fiber from the base station A is 6.2 Km, and the donor signal is taken from the base station A: PN12. Starting from base station A, driving 3.8Km, entering the coverage area of ​​the repeater, there is a strong repeater signal (using the instrument test, PN is 13), Ec/Io is -3.4dB, and the signal used by the mobile phone is still at this time. For the base station signal, it cannot be "switched" to the signal of the repeater (strictly speaking, the repeater signal should be understood as the multipath component of the base station signal with large delay and high strength, and should not be regarded as switching. For convenience of description, here is referred to as switching), the Ec/Io is -23.62 dB, the FER is increased to 80.53%; when the FER is raised to 100%, a dropped call occurs. After the phone is re-dialed, it can use the repeater signal to talk.
Because the fiber optic repeater brings a certain delay, the instrument test found at the repeater station, the signal delay is 55 chips, and the signal of the repeater is PN13, that is, the base station signal PN12 is delayed by 55 chips and becomes PN13. This way, at 3.8Km:
The delay of the base station signal is: 3.8 & TImes; 4.1 = 16 chips
The delay after the signal passes through the repeater is: 55+(6.2-3.8)&TImes;4.1=65chips
Therefore, SRCHWIN A should be: (65-16+1) & TImes; 2=100 chips
Therefore, it is judged that the original base station search window parameter needs to be adjusted. After the original CHCHWINA is changed from 10 to 10, the signal of the base station and the repeater station is normally "switched" at the original call drop, and no call drop occurs, and the problem is solved.
Issues to considerAt this stage of the increasingly perfect CDMA network, when a repeater needs to be introduced in the network, the preliminary survey work must be done first, and the actual situation on the site should be fully considered and weighed in the engineering design to avoid design problems.
After the equipment is opened, it is also necessary to pay attention to network optimization to avoid the bad coverage area of ​​the repeater or affect the base station. Only the repeater manufacturers will take the technology in the application of the CDMA repeater, and at the same time get the cooperation between the operators and the base station manufacturers, do the optimization work of the whole network, improve the operation quality of the network, and then fully play the direct stand. Utility in a phase CDMA network.
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