Research On Onboard-Relay Based High Speed Mobile Broadband Wireless Access Technology

With the large-scale construction of railroads all over the world especially of China and continuous increase of train speed, the broadband wireless vehicle to land communications under fast moving environment attract the public attention. The broadband wireless vehicle to land communications faces a series of challenges caused by the high mobility of trains. One of them is severe Doppler Effect which not only leads to frequency shift that is hard to correct but also makes the channel fast fading to influence channel estimation. Others such as frequent handover and insufficient overlapping region also have bad effect on handover performance. Moreover, sealed carriages which result in penetration loss as well as the complicated operating environment of railway will influence the communications seriously. Therefore, it is important to choose proper broadband wireless communication technologies and adjust them to match the high-speed railway environment.After analyzing the existing broadband wireless communication systems, this thesis chooses to study the communication technology based on LTE (Long Term Evolution). Some helpful information which can improve the performance of broadband wireless vehicle to land communications, such as speed of trains, positioning information, moving direction, is emphasized after analyzing the specific scenarios of railway. It is discovered that the communications based on LTE faces five challenges as follows:handover, Doppler Effect, penetration loss, complicated environment and economic benefit. Most of them have been already well solved in some existing solutions. However, the fast fading channel caused by Doppler Effect and insufficient overlapping region in handover need to be studied further.To solve the problem of fast fading channel caused by Doppler Effect, a novel speed and positioning assisted channel estimation scheme is proposed. The accuracy of this scheme is guaranteed by verifying and revising the channel state information stored in base stations and onboard relay stations which is obtained by measuring repeatedly at certain location and speed. The proposed scheme is advantageous with low complexity, high efficiency and reliable accuracy.To solve the problem of insufficient overlapping region in handover, a handover scheme based on positioning and relay power control is proposed. Firstly, the traditional handover scheme of LTE networks is modeled and analyzed. Then the optimized strategies assisted by positioning and relay power control is discussed. In the proposal, diversity gain is obtained through relay stations to improve channel capacity and link reliability before the train arrives at handover location. When the train arrives at the handover location, the minimum communication requirement is satisfied by relay power control which also means that the overlapping region is widened and the handover is easy to trigger to improve success probability. The simulation results have shown that the handover success probability and channel capacity are increased effectively by the proposed scheme. The reliability and efficiency of LTE communication systems in high speed railway is improved significantly.