Research On The Clock Frequency Offset In Railway Wireless Access Network And Its Impact On QoS Of Train Control Service

With the development of the mobile communication system, the digital signal is switched and transmitted at a higher rate. The effect of the frequency offset of the clock signal is more and more obvious. The mobile communication network, especially the radio access network, has a higher requirement for the clock synchronization.In China, GSM-R is mainly used in railway mobile communication system to carry all kinds of services related to train operation. As the scale of GSM-R network is getting larger, due to the fault and network performance degradation caused by clock synchronization problem, some occasions like call dropout, transmission error and switch failure often happen in the actual operation. However, in a long period of time, the importance of clock synchronization has not been fully recognized and the research on clock synchronization technology in the railway is not enough and thorough either.At present, the analysis of frequency offset in wireless access network for railway mobile communication system are mostly concentrated in the research of Doppler frequency shift, which rarely involves the uplink and downlink carrier, the Doppler frequency shift relative offset between MS and base station caused by the relative frequency offset of base station reference clock. The impact of clock frequency offset on the performance of the network has not been fully verified, and it also lack of effective testing methods. In view of the above situation, the main contributions of this paper are as follows:(1) Based on the realization of clock synchronization for radio access network, the factors of frequency deviation of clock signal which is transmitted from the core network to the radio access network are analyzed, and the effect of wireless access network performance and QoS parameters because of frequency deviation are researched.(2) When the reference clock frequency offset exists, the frequency and the relative frequency of base station clock, uplink and downlink carrier, and Doppler frequency shift are researched during frequency offset transmitted in the wireless access network. Meanwhile we also establish the functional relationship of relative frequency offset between reference clock and each node.(3) Using the function above, we quantitatively analyze the frequency variation due to the frequency offset of each node in the GSM-R 900MHz system.(4) The test environment is set up in the laboratory, and the influence of frequency offset for base station downlink carrier frequency, the QoS of CSD service are tested by adding frequency offset to the reference clock frequency.