Optimized Handover Schemes For The High Speed Train Wireless Communication System

In recent years, with the large-scale construction in high speed railway around the world, especially in our country, the high speed train-ground wireless communication gains much attention. The high mobility of the train results in a series of challenges to the high speed train-ground wireless communication. The handover problem can influence the handover performance seriously. The train’s high speed causes Doppler Effect and fast fading, which makes channel estimation more difficult. Additionally, body loss caused by closed compartment and the operating environment of high speed railway also affect the high speed train-ground wireless communication. Therefore, how to solve all these problems is of great importance.Several wireless communication access schemes for high-speed railway are introduced and analyzed. After that, some problems the high speed train-ground wireless communication faced are discussed in detail, such as Doppler Effect, frequent handover, body loss, the operating environment of high speed railway, co-existents of different systems and etc. In addition, some schemes and their advantages and disadvantages are analyzed. In all of these schemes, how to implement fast, accurate and reliable handover schemes need to be further studied.In order to improve the reliability and delay performance of handover schemes for high speed railway communications, an optimized handover scheme based on selective diversity reception technology is proposed. Firstly, traditional hard handover schemes are introduced. These schemes would lead long handover delay and outage, which are the main reasons of handover failure. Then, an optimized scheme is proposed. In the optimized scheme, the train keeps connection to the source base station and the target base station when the train is driving through the overlapping area. The target base station synchronizes with the train and caches the data before the handover is triggered. Once the handover is triggered, the serving base station only needs to update the train’s information. Then, the whole handover procedure is completed. This scheme has many advantages. On one hand, handover delay and outage time can be lowered efficiently, so that, fast handover can be implemented. Furthermore, the outage probability on mobile station side and network side can be reduced greatly and the reliability of handover can be improved. The simulation results show that the proposed scheme can improve handover success probability and the reliability of high speed railway communications.Another problem existing in present handover schemes designed for high speed railway communications is that handover triggering time is lagged and inexactly. In traditional cellular networks, hysteresis margin is used to avoid Ping-Pong effect. However, if the hysteresis margin is too high, handover triggering would be lagged and handover process can’t be completed in the overlapped region. If the hysteresis margin is too low, Ping-Pong effect would happen. In order to solve this problem, a handover scheme suitable for GSM-R system is proposed. Handover decision is assisted by velocity and position of the train. So that handover failure problem can be improved and Ping-Pong effect can be avoided significantly. Simulation result shows that the proposed scheme increases handover triggering probability in overlapped region and the handover success probability.