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(1) The GPS signal receives interference from outside The GPS operating frequency band is 1575MHz. Since the GPS signal is very weak after it is launched from the satellite to the ground, it is easily affected by external interference. Many factors will cause interference to the GPS signal, such as Interference from solar flares in space, interference from the ionosphere and the atmosphere, and the effects of abnormal weather such as lightning. In the presence of interference, the signal quality of the satellites received by the receiver will be degraded, the signal to noise ratio will be reduced, and the bit error rate will increase. In some cases, satellite signals will not be received.
(2) Causes of Project Construction When real-time large-scale establishment of the station, if there are obstacles near the installation location of the GPS antenna or the construction process issues cause the feeder impedance is too large, the feed line head process problems, the feed line water and other factors, making the GPS received by the base station side The signal is weak.
Long-term synchronization failure will result in timing deviation between base stations. Excessive timing offset will affect cell search in neighboring cell, cell handover, downlink steering time slot (DwPTS) interference to UpPTS and service time slot crossover. These will further affect the quality of the network, causing handover failures, dropped calls, and decreased call-through rates, which will seriously affect users' perceptions in the network. To this end, we test the influence of the timing offset caused by GPS out of step on the performance and quality of the network by testing and determining the timing offset of GPS out-of-step that can be tolerated by the TD-SCDMA system, and further provide a solution for selecting an alternative GPS synchronization system. in accordance with.
1. Theoretical analysis of GPS out-of-step causes the GPS timing offset between base stations to be too large. From the analysis of the TD-SCDMA frame structure, terminals, and system implementation methods, GPS desynchronization mainly affects the system in three aspects.
(1) Handover and cell reselection related neighbor measurement (or neighbor search)
In the normal state, the user terminal (UE) needs to use the current cell DwPTS timing as a reference to perform DwPTS search of the neighboring cell. If the timing offset of the neighboring cell is too large, the UE cannot search for the DwPTS of the cell in the DwPTS search window. Or even if a neighboring cell can be found, the PCCPCH signal of the neighboring cell is found to be poor, and the signal to interference ratio (SIR) is low, which seriously affects the performance of the network's key parameter indicator (KPI), resulting in reselection and handover of the terminal. The problem is shown in Figure 1.
In addition, relay switching in TD-SCDMA reduces the uplink uplink UpPTS access process, speeds up the handover process, and increases the success rate of handover. However, the synchronization requirements between base stations are relatively high. Therefore, once the GPS between base stations has lost synchronization, the terminal The dedicated channel is not synchronized (the terminal sends special burst data on the dedicated channel, the base station receives the confirmation, indicating that the uplink synchronization is successful, and then the base station sends special burst data, and the terminal receives that the downlink synchronization is successful). Easy to switch failed.
(2) DwPTS interference to UpPTS time slots TD-SCDMA In order to avoid downlink DwPTS interference to UpPTS between cells, a 96-chip protection (GP) time slot is reserved between two time slots. In the case of GPS out-of-synchronization, the effective protection time between the DwPTS and UpPTS time slots is reduced. as shown in picture 2.
Uplift of the UpPTS timeslot interference will cause the uplink UpPTS channel to shrink. (In the TD-SCDMA system, the circuit switched domain 64k videophone service (CS64k) service has the least coverage, so the UpPTS service channel coverage must be at least the same as the CS64k. Coverage) affects the uplink access of users at the edge of a single cell. However, in a practical network, since the proportion of areas where the PCCPCH received signal code power (RSCP) is less than -95 dBm is very small, the impact on the call-through rate is relatively small.
(3) Cross-Interference of Service Timeslots There is a 16chip at the end of each time slot of a TD-CDMA system.
In TD-SCDMA, each service slot has a length of 864 chips. Therefore, cross-time slots caused by GPS out-of-step will only interfere with part of the chip time slots in the service slot. Only when the GPS out-of-step is large can cause significant interference. .
2. Test Design In order to quantitatively analyze the impact of GPS out-of-step on network performance and quality, we conducted test verification in a real network environment.
(1) Selection of test environment Select a high site in the real network, load the base station software that can perform GPS out-of-step set-up, cause human GPS out-of-step, GPS out-of-step timing deviation can be controlled and modifiable, there is 1 to Two rounds of base station GPS synchronization is normal, 30 to 40 contiguous cell coverage.
(2) The test terminal selection software uses Dingli road test software. The road test terminal uses ZTE U85 and Datang 8120 to support videophone.
(3) Simulated loading of the test area in the test The simulation load specification: 75% simulation load, that is, 75% code channel in a single time slot, and the power is 27dBm.
(4) Design of test cases Based on the above theoretical analysis, a total of eight test cases were designed.
(a) Base station GPS timing forward deviation
Analysis of GPS Synchronization Failure Factors in TD-SCDMA Network
In the three major standards of 3G, the base station synchronization system and the TD-SCDMA system are the entire network synchronization systems. It is self-evident that the importance of maintaining the time synchronization between all the base stations strictly for the TD-SCDMA communication system is self-evident. Because of the lack of advanced network synchronization technology, TD-SCDMA base stations generally adopt Global Positioning System (GPS) synchronization [At present, in TD-SCDMA networks, due to some reasons, the base station cannot receive GPS satellite signals, and the phenomenon of synchronous failure occurs.