View Full Version : GSM Problems10

01-30-2014, 05:01 PM
[Hardware problem positioning and elimination]
When the interference is doubted in a cell, you should first check if the BTS where the cell locates works normally. In the remote end, you should check if there is antenna alarm, TRX alarm, or BTS clock alarm generated. In the near end, you should check if there is antenna problem, water penetration, feeder (jumper) damage, CPU problem, TRX problem, wrong jumper connection or clock problem occurred.
- Antenna performance decline
Antenna a passive component and its damage probability is small. However, if the antenna is damaged or its performance declines, the voice quality will become poor.
- Antenna connector problem
GSM RF signals are micro wave signals. If the connections between TRX, CDU, feeder, and antenna have any problem, both the standing wave ratio and the intermodulation will increase. In this case, the interference will be resulted.
- Inverse antenna connection.
The inverse antenna connection is a commonly seen problem. If the antenna is inversely connected, the channel numbers used by the cell and the planned channel numbers are completely inconsistent. In this case, intra-frequency interference, inter-frequency interference, and handover difficulty will be resulted. Especially for the networks that have inadequate frequency resource, the inverse antenna connection has great effect against network quality.
- Jumper problem.
Many jumpers locate between antennas, so they are often wrongly connected. In this case, high call drop rate will be resulted.
- TRX problem.
If TRX problems occur, the interference will increase, the coverage distance area will decrease, and the access is difficult.
- Clock failure.
If the clock deviation is too great, it is hard for the MS to lock the frequencies of the BTS, so the handover failure always occurs, or the MS cannot camp on any cell of the BTS. In addition, if the clock deviation is too great, the BTS cannot understand the signals of the BTS, which will result in bit errors. However, the clock failure will not really introduce interference, but it is the transmission errors that make the voice quality decrease.
- Conclusion
Any problem concerning the TRX, CDU, feeder, antenna, jumper, and connector may cause interference or call drop. Therefore, if interference appears, you should check the hardware of the BTS. In addition, BTS clock failure will also cause interference and call drop.
It is easy to solve the hardware problems through changing the boards or adjusting traffic data. If there is spectrum analyzer available, you can position the problem more efficiently. Especially when the interference appears without any modification of network data, you should focus on checking the hardware.
[Intra-Network Interference]
The intra-network interference is mainly from intra-frequency interference and neighbor cell interference. When C/I is smaller than 12dB or the C/A is smaller than -6dB, the interference is unavoidable. However, the aggressive frequency reuse technology will increase of the occurrence probability of interference.
- Same-frequency and neighbor frequency interference
In GSM system, the frequency reuse is unavoidable. When the frequency reuse distance of two cells using the same frequency is smaller than cell radius, same-frequency interference will be easily caused. Past experiences show that the frequency reuse must be avoided in many cases.
The interference against the uplink channel numbers can be judged by the interference band data in the traffic statistics.
For the interference against the downlink channel numbers, the existing drive test equipments can be indirectly used to measure if the same-frequency interference is present. First you should lock the test MS in the service cell and enable make the MS work in conversation mode during drive test. If you find that the Rxlev in an area is high but the Rxqual is low, it is likely that the same-frequency is present in this area.
- Interference caused by cross coverage.
In a properly designed network, each cell covers the areas around the BTS only and the MS camps on or holds conversation in the nearest cell. Cross coverage means that the coverage of a cell is too large and the cell can cover the areas under the control of other BTSs. If cross coverage occurs, irrational traffic absorption, interference, call drop, congestion, and handover failure may arise.
- Interference caused by aggressive frequency reuse
Capacity and quality always contradicts to each other. In urban areas, the aggressive frequency reuse technology must be used for the number of subscribers in urban areas are great. In this case, the network quality will surely decrease. In the areas where BTSs are irrationally distributed, the aggressive frequency reuse technology may cause the collision of same frequency and neighbor frequencies.
- Interference caused by repeater
It is convenient to use repeater for special coverage. However, if a repeater is not qualified or it is not properly installed, it will cause interference.
- Interference caused by outside environment.
Outside environment, such as TV station, big-power radio station, micro wave, radar, high voltage wire, analog BTS, and so on, will cause interference.