A crosstalk detection scheme in GSM base stations

1 Introduction
Crosstalk is one of the more common fault phenomena in mobile communications. Since a third party's intelligible voice appears during the communication between the two parties, users are very disgusted with this phenomenon and the complaint rate to operators is very high.

2 GSM system structure
There are many reasons for the crosstalk phenomenon. To analyze the specific reasons for the crosstalk, we must first analyze the system structure. Figure 1 is a diagram of the GSM system structure.

In Figure 1, MS (Mobile Station) is a mobile user terminal; BTS (Base Station Transceivers) is a base transceiver station, which can be regarded as a wireless modem and is responsible for receiving and sending mobile signals; BSC (Base Station Controller) is a base station controller, which is the connection point between the base transceiver station and the mobile switching center, and also provides an interface for exchanging information between the base transceiver station and the operation and maintenance center. A base station controller usually controls several base transceivers, and its main functions are to manage wireless channels, implement the establishment and removal of calls and communication links, and control the handover of mobile stations within the control area; MSC (Mobile Switching Center) is a mobile switching center, which is the core of the network, and is a functional entity that controls mobile stations in the area it covers and completes voice channel switching. It is also the interface between the mobile communication system and other public communication networks; HLR (Home Location Register) is a home location register; VLR (Visiting Location Register) is a visiting location register; AUC (Authentication Center) is an authentication center; EIR (Equipment Identify Register) is an equipment identification register. BTS, BSC, MSC are the main parts of the system.

3 Crosstalk and its causes
Crosstalk can be divided into two types. One is double-talk, which means that during the normal communication between two parties, one party can hear the call from a third party. The other is single-talk, which means that one or both parties cannot talk normally and can only receive calls from a third party other than the user. There are many reasons for crosstalk:
(1) Crosstalk is related to the management and allocation of MSC and CIC (circuit identification code), the working status of the circuit board and the CIC connection between MSC and BSC. CIC is the circuit identification code, which refers to the voice circuit between MSC and BSC. The CIC value range is 0-4095. Each time slot in the circuit group corresponds to a CIC number.
(2) The 2Mbit/s link connection between MSC and BSC is incorrect, such as crossover between 2Mbit/s links or crossover between the transmission and reception of two 2Mbit/s links.
(3) When the MSC does not release the CIC after the call ends and then allocates it to other users, crosstalk may also occur.

4 Crosstalk Detection Solution
Every time a call is established, the TC will send a two-byte Call ID to the base station. The content of the ID will be different for each call. After receiving the Call ID, the base station will write the ID into the voice frame as an identifier for the call. When the voice data is transmitted from the base station to the TC (code converter) part of the BSC, the TC will take out the Call ID in the voice frame and compare it with the Call ID it has saved. If the ID numbers are consistent, the voice link is considered correct; if the ID numbers are inconsistent, it means that there is crosstalk between the two voice links, and the BSC will record the crosstalk log. When the voice frame is transmitted to the other base station side in the downlink, the DSP module in the base station will also check the Call ID in the voice frame and use the same method to determine whether crosstalk occurs in the downlink. The schematic diagram is shown in Figure 2:

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3.1 Generation of Call ID
In order to maximize the uniqueness of each Call ID, Call ID is generated based on the circuit identification code (CIC) assigned to each call. Because at the same time, the circuit identification codes assigned to different calls are different. In this way, it is guaranteed that each Call ID is different.
3.1 Injection of CaII ID
In order not to affect the call quality, Call ID will be written in an unused part of the voice frame. From the structural definition of the TRAU frame in the GSM protocol, it can be seen that the C18-c21 bits are idle. Therefore, the Call ID can be filled in here. As shown in Figure 3:

Since the size of a Call ID is 16 bits, and there are only 4 idle bits in a voice frame, if we divide the Call ID into 4 frames (4 bits per frame), we will not know which frame to start extracting the Call ID. Therefore, divide the Call ID into 6 parts, 3 bits each for the first 5 parts, and 1 bit for the last part. Set the C18 bit position of the voice frame with the first 3 bits to 1, and the C18 bit position of the remaining voice frames to 0. In this way, when DSP extracts the Call ID, it can obtain the first 3 bits of the Call ID by searching for the voice frame with C18 being 1, which is convenient for positioning. For example, the CallID of a call is 373, which is converted into binary and full of 16 bits: 0000 0001 01110101, then the distribution of this Call ID in the voice frame should be as shown in Figure 4:

5 Conclusion
By adding test bits to the voice frame to detect crosstalk, the service performance of the base station can be understood without affecting the service. The detection method is convenient and simple, saving time and money compared to the method of manually testing and dialing to locate crosstalk.