Compand Diversity Technolegies For High Speed Train-Ground Wireless Communications

With the expansion of the high-speed railway industry and the improvement of railway speed worldwide, the reliability and efficiency of the high speed train to ground wireless communication, which is dedicated to railway transportation, attract the public attention. Based on the features of the high speed train to ground wireless communications, this thesis point out that the high speed train to ground wireless communications face several problems: train penatration loss, Doppler effect and frequent handoff, etc. All these problems may lead to the train to ground wireless communications drop, which seriously affects the user’s communications experience. Therefore the handoff and Doppler effect require special investigation.Current LTE (Long Term Evolution) technology only supports hard handoff, which have long time-delay. Due to the shadow fading, multipath and Doppler effect, the handoff success rate of the system would be seriously affected with the quality of the received signal reduced. In that handoff mode, more frequent handover of high speed train to ground communication brings about more frequent data drop which seriously affects the communication experience of travelers. Therefore, we must optimize the existing LTE hard handoff scheme to improve the success rate of the handoff. In this thesis, a high speed railway communication scheme based on the CoMP (Coordinated Multi-Point transmission/reception) which is one of the important technologies of LTE-A (Long Term Evolution-Advanced) is proposed to optimize the system handover performance. And then the handover performance is analyzed and the comparison with the traditional handoff scheme is made. The results show that the proposed scheme could increase the handover success probability effectively with high mobility. Therefore it obviously improves the reliability and efficiency of train to ground communications.On the other hand, another problem in this thesis studied is that whether the diversity technique can obtain corresponding gain like CoMP technology in the high speed time-varying channel environment. Due to the high speed mobility, Doppler effect influences to the signal design, signal joint and signal detection of today widely used diversity technology, certainly will further influence the implementation of the diversity techniques. This thesis discusses the suitable condition of the time diversity, space transmit diversity and space receive diversity in high speed train to ground communication based on the Orthogonal Frequency Division Multiplexing (OFDM) system, and then presents the corresponding performance evaluation.