Research On Capacity Optimization And Handover Algorithms In High-speed Railway Wireless Communication Systems

In the field of high-speed train wireless communications,it is an important to provide smooth,stable and reliable communication services for users.However,the high speed and complex wireless environment bring challenges to the existing high speed train wireless communication system.Therefore,it is very important to design good capacity optimization and handover algorithms.Based on the Distributed Antenna System(DAS),the capacity optimization and handover problems are studied in this thesis.The main contributions are summarized as follows:1)In the high-speed mobile wireless communication environment,in order to solve the problems of poor quality of users’ communication and the bottleneck of Base Station(BS)edge capacity,a Remote Antenna Unit(RAU)power allocation method is proposed based on Channel State Information(CSI)between RAUs and Train Mobile Relay Station(TMRS)in the Cooperate Distributed Antenna System(CDAS),namely the CDAS-CSI power allocation algorithm.Simulation results show that compared with the existing Equal Power Allocation(CDAS-EPA)algorithm,the CDAS-CSI algorithm can significantly enhance the system channel capacity.In addition,in order to obtain the antennas’ combination method suitable for cooperate transmission of high-speed train scenarios quickly,an antenna selection algorithm is proposed.Simulation results show that the channel capacity will become suboptimal when the number of RAUs is 2 or 3 in CDAS,and it does not always increase with the increase of number of RAUs.2)In the high speed train handover scenario,due to the single handover mechanism and untimely handover,the success rate of handover trigger is low and results in the outage of communication link.Based on the joint handover decision,an improved handover algorithm is proposed by applying the power adjustment scheme.In this algorithm,the Received Signal Strength(RSS)of the train can be improved in the adjacent BS overlapping area through the power adjustment scheme on the premise of no additional energy consumption.So,the "certainty" of handover decision is increased and the phenomenon of ping-pong handover is avoided.At the same time,according to the position and speed of the train,trigger threshold was adaptively adjusted,to ensure sufficient handover success probability.Simulation results show that the improved handover scheme based on power adjustment is better than traditional handover schemes in terms of triggering handover probability,communication outage probability and handover success probability.