According to 3GPP 0505, the carrier-interference ratio should be more than 9dB in European GSM network. China GSM-R network refers to this European requirement. However, there are some reasons that GSM-R network system can't simply peculate this requirement. First of all, GSM and GSM-R are quite different in some aspects, such as the coverage, interference probability, quality of service, and call drop probability; secondly, there are many differences between China and Europe, the railway transport organization, flow characteristics, the train speed, and so on. Therefore, it is necessary to combine the specification with the practical application. A C/I threshold is needed to evaluate the co-channel interference of GSM-R in China.In this paper, the arithmetic to calculate the threshold for co-channel interference is brought forward under the background of Chinese railway. In this paper, a definition for the co-channel interference probability is given, a formula for co-channel interference probability is derived, and the relationship between bit error rate and C/I in the Rayleigh / Lognormal fading channel is figured out.The paper innovatively associated the co-channel interference probability and bit error rate, and designed an arithmetic which can successfully calculate the C/I threshold. The arithmetic is validate by GSM-R network of Daqin railway, on the basis of sampling, actual measurement, research, theoretical analysis and empirical method. At last, the reasonable C/I value is suggested. The arithmetic was proved to be simple, efficient and useale.
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