Downlink Beamforming Design And Adaptive Pilot Allocation For High Speed Railway Wireless Communication Systems

With the development of high-speed railway communication technologies,railway wireless communication requirements have evolved from traditional voice services to various high data rate services,such as passenger-oriented mobile TV,Internet applications and other services.However,there are still many challenges in high-speed railway wireless communications,including complex electromagnetic propagation environment,frequent handovers,high Doppler frequency offset,penetration loss,and signaling storms caused by user concentration.In order to meet the demands of the new communication services,and ensure the safety and efficiency of train operation at the same time,it is necessary to conduct in-depth research on developing new low-latency and high-reliability high-speed railway communication technologies.Therefore,this thesis studies the downlink beamforming design and adaptive pilot frequency allocation of the high-speed railway communication systems.First of all,the problem of multi-cell cooperative beamforming in high-speed railway communication systems with imperfect channel state information(CSI)is studied.In order to maximize the data transmission rate of high-speed trains,the robust optimization problem of multi-cell cooperative beamforming for high-speed railway communication is formulated by considering CSI error,the inter-user interference,the minimum data transmission rate requirements of the low-speed users,and the maximum transmission power constraint.The semi-definite relaxation(SDR)effective approximation method is adopted to convert the nonconvex problem into a deterministic convex optimization problem,and iterates the optimal solution by the bisection method.The simulation results show that the proposed multi-cell cooperative beamforming algorithm can effectively improve the data transmission rate of the high-speed railway communication systems and reduce its outage probability while ensuring the Qo S of the low-speed users.Secondly,due to the lack of research on the applicability of 5G communication systems in ultra-high-speed scenarios(>600 km/h),this thesis first builds a 5G-R link simulation platform based on the 5G standard framework.Then,the applicability of the 5G system in ultra-high-speed scenarios is studied.The performance of bit error rate and the throughput under different channel delay spread,speeds,subcarrier spacing and modulation and coding scheme(MCS)index are analyzed.Moreover,based on the characteristics of location changes in high-speed railway scenarios,an adaptive DMRS pilot allocation scheme is proposed,that is,the mapping method of demodulation reference signal(DMRS)in the resource grid is dynamically adjusted according to the CSI feedback and the average block error rate(BLER)within the fixed frame length.The proposed scheme fully takes into account both the channel estimation accuracy and the DMRS overhead.Compared with the traditional fixed DMRS pilot pattern,the proposed scheme can well adapt to the rapid changes of the high-speed railway channel state,and can also effectively improve the 5G-R system throughput.