Research On Reliable Transmission Link Of Lte System In High Speed Railway

The existing railway communication system, GSM-R (GSM for Railway), is facing with lots of challenges and requirements from the high-density of high speed trains, such as high-definition video surveillancing trains’ safety, high redundant communication system with error correction, always ensuring communication reliability. LTE (Long Term Evolution) system, with its remarkable high data rate and low delay, has been adopted as the next generation of railway communication system by International Union of railway. In a period of transition, LTE can be used as the redundant hot backup, when GSM-R is abnormal, the transmission of voice and part of data service can be replaced through LTE network. Thus the railway communication can be operated correctly.First of all, this thesis introduces the high-speed railway communication system based on LTE, describes its network architecture, and analyzes the key technologies. It also discusses feasibility of the GSM-R system’s evolution to LTE-R and proposes the possible evolution model.In order to overcome the effect of time-varying wireless channel attenuation to the signal, LTE adopts HARQ (Hybrid Automatic Retransmission reQuest) to improve the system throughput and reliability. However, in high-speed railway environment, the HARQ performance of LTE wireless communication system is deteriorated due to the high speed and the complicated railway environment. In this thesis, the HARQ performance in high-speed railway environment is analyzed. It shows that the system with HARQ technology has lower block error rate and higher throughput, compared to the case without it. However, the HARQ performance in high-speed environment is not as good as that in the scenario of public network, where most of the users are with low-speed. And then, a novel HARQ scheme is proposed, which could enhance the HARQ performance in the overlapping region. In this scheme, the number of maximum retransmission is reduced when the train arrives at a predefined location, and the packets, which suffer transmission error or can not be acknowledged before handover triggering, will be saved by the system, and retransmitted by the target eNodeB (after the train accesses it), and then combined and decoded by the onboard UE (User Equipment). Performance analysis shows that the error rate decreases by55%when these packets are retransmitted in the target cell. Compared to the situation with fixed number of maximum retransmission, the proposed scheme provides more opportunities for new packets to be transmitted before handover, thus the throughput increases by about 12%when target eNodeB retransmits those forward packets.In the meantime, the QoS guarantee for LTE Voice faces severe challenge in the time-varying channel of high-speed railway. Some possible solutions of reducing delay and improving reliability are proposed and analysed. This thesis fucoses on the scheduling, discusses the impact of modulation and coding scheme on the reliability of VoIP in AWGN first, and then extends to high-speed scenario. The VoIP transmission combined with HARQ is modeled. The performance results show that a low-level modulation and coding scheme can reduce the outage probability and decrease the number of transmissions, thereby the delay is decreased. Finally, the thesis presents a novel MCS selection criterion for LTE VoIP, which can improve reliability without consuming more resource.