A Preliminary Study on the Solution of GSM Wireless System Call Drop
Abstract This paper discusses the causes of GSM wireless system call drop from the aspects of handover failure, interference, hardware and software failure, and puts forward a feasible solution to the GSM wireless system call drop based on actual maintenance experience.
1 Introduction
——Call drop in GSM wireless system is a problem that users often encounter when using mobile phones, and it is also a hot spot reported by users. In addition, the call drop rate of wireless system is an important indicator for evaluating the operation of the network, so how to reduce the call drop rate of wireless system , Improving the quality of network operations is a top priority. This article will analyze the causes of dropped calls in wireless systems and propose some solutions accordingly.
2 Causes of dropped calls
——Wireless system call dropout is divided into SDCCH call dropout and TCH call dropout. The main causes are summarized as follows:
(1) Call drop due to switching
——①When the base station performs handover to share the traffic, some handover requests will fail because the signal strength of the cut-in cell is too weak, and even if the handover is successful, the call will often be dropped because the signal strength is too weak. The reason is that in BSC, we set a minimum threshold for the received signal strength of mobile phone users (RX_LEV_ACC_MIN = -105dBm). When this threshold is lowered, the mobile phone cannot establish a call.
——②In some cells, the neighboring cells are very busy, so the target base station has no switching channel when busy or the switching conditions (including inter-BSC switching and cross-office switching) are missed in the topology relationship, so that mobile phone users cannot switch. Occupy the idle voice channel of the neighboring cell. At this time, the BSC will perform call reconstruction (Direct Retry). If the signal of the calling base station cannot meet the minimum working threshold or there is no idle voice channel at this time, the call reconstruction fails and the call is dropped. . When there is a leaky coverage or a blind zone between cells, it will also cause a handover failure and call drop.
——③ Island effect. If the field strength generated by the serving cell A due to the terrain covers the island C and the coverage of the cell B is around the island 1C, if cell B is not added to the topology table of A's neighboring cells, then the user As soon as the call is established in C, out of the island C, there will be dropped calls due to nowhere to switch.
(2) Call drop due to interference
——Interference mainly includes co-frequency, adjacent frequency and intermodulation interference. When the mobile phone receives a strong co-channel or adjacent-channel interference signal in the serving cell, it will cause the deterioration of the bit error rate, so that the mobile phone cannot accurately demodulate the BSIC code of the neighboring cell or receive the mobile station measurement report correctly. After the base station allocates the voice channel that should be used for the mobile phone through the SDCCH, it cannot determine which cell's voice channel to use because there is no BSIC code of the neighboring cell, thereby causing dropped calls. Intermodulation interference is mainly interference caused by the base station of the exponential mode co-site due to the influence of the simulated base station transmitter. The direct consequence of this interference is the waste of base station resources due to the inability to allocate time slots.
(3) Call drop due to antenna feeder
——①Call drop due to the different pitch angles of the two antennas—During the installation of the base station, each directional cell has two transceiver antennas as shown in Figure 1. When the parameter CCCH_CONF = 0 in the DATABASE of the cell, the cell The SDCCH and BCCH use NO-COMBINED MODE, so that the BCCH and SDCCH of the cell may be sent from two different antennas. When the pitch angles of the two antennas are different, the coverage of the two antennas will be different. That is, the situation shown in Figure 1 occurs. When the user is in Zone B, the BCCH signal can be received, but the Cannot occupy SDCCH and drop the call.
——②Call drop due to azimuth angle of antenna feeder——During the installation of base station, each directional cell has two antennas as shown in Figure 2. When the azimuths of the two antennas are different, it will be formed as shown in Figure 2. The cells shown in A and C in 2. The user in the cell A can receive the control signal SDCCH, but once the user is designated as the TCH transmitted by another antenna, it will cause a call drop. Users in cell C will not be able to receive the signal.
—— ③ dropped call due to the antenna feeder itself.
-Damage to the antenna feeder, water ingress, discounts and poor contact at the joints will all reduce the transmission power and the receiving sensitivity, resulting in serious dropped calls.
—— ④ dropped call due to the distance between the two antennas.
——A certain horizontal distance should be maintained between the two antennas to achieve diversity reception, otherwise it will reduce the receiving sensitivity and cause dropped calls. The horizontal distance (empirical value) between the two antennas should be one-tenth of the vertical distance and should be at least greater than 3m.
(4) Call drop caused by Abis interface failure
——Abis interface, including the BSC has not received the measurement report from the BTS, exceeded the TA limit, some signaling failure during the handover process and some internal reasons, in addition to the impact of the bit error rate of the Abis interface.
(5) Call drop caused by A interface failure
——A interface failure occurs less frequently, mainly due to the failure of handover (handover between BSC or MSC), because the data of the handover office is incomplete or the target base station does not have the entry conditions.
(6) Call drop caused by base station hardware and software failure
——The system's hardware failure or imperfect software, program or data errors and other reasons will cause dropped calls.
(7) Call drops due to the use of repeaters
——In order to reduce investment and expand coverage, small base stations in some county towns generally use repeaters to directly amplify their signals and use optical fiber or microwave transmission. Due to terrain and environmental impact, repeaters are often placed in remote towns with many surroundings. Because of the mountainous and hilly terrain, plus the quality of the project, the target requirements are not met, resulting in dropped calls.
(8) TA does not match the actual
——For some reason, when the time advance (TA) calculated by the BSC does not match the actual required TA, it will cause interference on the time slot, and will cause dropped calls when the interference is serious.
3 Failure analysis
——Now analyze the causes of the above failures and propose corresponding solutions.
(1) Analysis and resolution of handover
——The main reasons for switching are as follows:
â‘ Handover caused by the upstream and downstream receive level RX_LEVEL.
â‘¡ Handover caused by the downstream and downstream reception quality RX_QUAL.
â‘¢ Handover caused by uplink and downlink interference.
â‘£ Switching caused by power budget (PBGT).
⑤ Call reconstruction.
â‘¥ Switching caused by traffic reasons.
——If the high call drop rate involves handover problems, you can first use a test vehicle to conduct a larger range of tests, because the handover occurs between the cell and the base station. Switching between settings is unreasonable. For some cells that have a switching topology relationship with the cell and the congestion rate is high, it should be the focus of the test, and it is necessary to check whether there is a blind spot around the cell. There is coverage of the base station. For dropped calls caused by unreasonable switching settings, the switching parameters can be modified according to the actual measurement. For those traffic drops caused by the target base station without switching channels during busy hours due to the imbalanced traffic volume, the solution is to adjust the traffic volume.
(2) Analysis and resolution of interference
â‘ Uplink interference-Uplink interference mainly comes from co-channel interference, and may also be external interference. Co-channel interference is related to the traffic volume of the same-frequency cell. If the traffic volume is high, the interference is large. The external interference is mainly intermodulation interference. For upstream interference, the related frequency in the same frequency cell can be modified by analyzing the related report in DRIVE_TEST, and the spacing between the two same frequency cells can be increased (actual statistics indicate that the signal strength attenuates with a law of approximately 4th power exponent with distance) or use The spectrum analyzer locates the intermodulation interference, and can also reduce the interference through diversity reception and effective power control.
â‘¡ Downlink interference-Downlink interference is mainly caused by co-frequency interference and adjacent-frequency interference of some base stations due to improper frequency planning. The discovered method is to judge by obtaining the handover measurement report in OMC. Downlink interference will cause frequent handovers. If the same frequency and adjacent frequency interference are found through the measurement report and on-site measurement, the frequency planning of the cellular system needs to be optimized and adjusted again. For a cell without interference but with interference, use a spectrum analyzer to find the source of interference.
â‘¢Using discontinuous transmission (DTX) and frequency hopping technology-DTX is divided into uplink DTX and downlink DTX, which uses voice activation detection (VAD) technology to stop transmission when no voice signal is transmitted, limiting the useless information transmission, reducing The effective time of the transmission reduces the interference level of the system and can extend the battery life. Frequency hopping can effectively improve the transmission quality of wireless signals, especially the transmission quality of slow moving bodies. This is because frequency hopping makes the transmission carrier frequency hop based on the burst sequence, which can significantly reduce co-channel interference and Frequency selective fading effect.
(3) Analysis and solution of antenna feeder
——①For dropped call due to incorrect antenna azimuth or signal pitch angle, you should first go to the base station to make observations. If you can't find the problem, you can find the cause of the failure by calling the test (CQT) of the mobile phone that closes the faulty cell or by analyzing the relevant statistical parameters (RF_LOSS_RATE, SDCCH_CONGESTION_KEY, TCH_CONGESTION_KEY, etc.) obtained from the OMC, and adjust the antenna azimuth and elevation angle in time To reduce the call drop rate.
——②For dropped calls caused by damaged antenna feeder or poor contact of the connector, the transmission power and receiving sensitivity are reduced, the antenna feeder tester can be used to measure the antenna feeder to determine the cause and point of failure, and replace the faulty day in time Feeders and connectors.
(4) Analysis and solution of software and hardware failures
——For calls dropped due to hardware reasons, you can see related hardware alarms through OMC_R. If there is no hardware alarm information in OMC_R, it may be that a certain time slot of the channel disk or a certain channel in the compression encoder is damaged. This can be found by turning off other channel disks in the cell, dialing the suspected channel disk or turning off other XCDR boards in the compression encoder, and dialing the suspected XCDR board to find the point of failure. Once the faulty hardware is found, it should be replaced in time. If there is no spare part, the faulty board should be closed first to avoid the occurrence of dropped calls and affect the network operation quality. For dropped calls due to software reasons, it should be solved by patching or upgrading the software in time.
——In short, no matter what the cause of the dropped call, all kinds of test methods and analysis of various test reports obtained from OMC should be used to find the cause of the fault phenomenon, and it is recommended to do regular quantitative CQT and DRIVER_TEST tests , Especially for hot spots, so that problems can be discovered and resolved as soon as possible.
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