Research On Wireless Access Technology In High-speed Railway Environment

As one of the typical implementation scenarios for high-speed mobile environment,High-speed railway communication system needs to be more stable and reliable than that in the relatively low mobility cases.High mobility aggravates the time-varying of wireless channels.And in such a hostile environment,providing users with reliable and stable high data transmission experience has become a topic for in-depth study of the subject.Around the high-speed mobile environment and its implementation scenarios,this paper studies an adaptive adjusting reporting period algorithm for Channel Quality Index(CQI)in Adaptive Modulation and Coding(AMC)system and the frequency synchronization algorithm.The details are presented as follows.Firstly,this paper not only introduces the development of mobile communication system and the evolution of high-speed railway communication system,but also analyzes the existing problems for communication system within high-speed mobile environment and the research hotspot on high mobile environment.Secondly,it introduces the propagation characteristics of wireless channel and the fading channel in high-speed environment.Meanwhile,the channel characteristics and the existing channel models for high-speed railway environment are presented.What's more,we introduce several high-speed wireless access network coverage schemes,such as the coverage of single Base Band Unite(BBU)+ multiple Remote Radio Unite(RRU).Thirdly,According to the 15 kinds of modulation and coding scheme in the specification,it simulates the relation of Signal to Noise Ratio(SNR)with target Block Error Ratio(BLER)in Additive White Gaussian Noise(AWGN)channel and establishes the switching threshold table of CQI.Meanwhile,we also introduce the effective SNR mapping algorithm,which maps the multistate channels to a single SNR value to reduce the system complexity.For the problem of constant CQI reporting period in AMC,an adaptive adjustment method for CQI reporting period in AMC system within high speed mobile environment is proposed.It evaluates the channel variation by calculating the differences of signal-to-noise ratio(SNR)between adjacent symbols in total at the same frame and uses the evaluation result to predict future SNR variation.Then the CQI feedback period adjustment value is calculated based on the quantization granularity of CQI,which achieves the performance of CQI feedback load and throughput better.Finally,the feasibility and validity of the proposed scheme are verified by computer simulation.Finally,this paper analyzes the performance of frequency synchronization algorithm in high-speed mobile environment.In the case of the Doppler shift and frequency offset aroused by crystal oscillator existing simultaneously,an orthogonal angle domain subspace projection algorithm is adopted in high speed mobility environment.First,the algorithm is utilized to divide the Doppler spread aroused by high speed mobility into equivalent Doppler shift on each orthogonal subspace.Then,the equivalent Doppler shift is estimated to pre-shift the received signal on each orthogonal subspace for frequency synchronization.Simulation results show that the adopted algorithm without AR filter acquires great performance because the Doppler shift is rapid time-varying,which means the AR filter will loss tracking of the real Doppler shift value and even enhance the estimation noise.In addition,the algorithm can effectively reduce the number of bit error at most sampling positions,except in some special regions with rapid changing angle of arrival(AOA)such as AA region and handover region.