High Mobility Channel Estimation With Interference Suppression

With the growing development of high speed train(HST)systems,highway vehicular communications systems,and low altitude flying object(LAFO)systems,wireless communications in high mobility environments have attracted considerable attentions.In recent years,HST has been increasingly developed in China and has made great impact on the national economic development.Meanwhile,there is a growing demand of offering passengers the data-rich wireless communications with high data rate and high reliability in HST systems.However,since the train travels at a high speed of 200–350km/h,the high mobility will incur many new challenges,e.g.,fast time-varying fading of channel,multiple interferences,high Doppler shift,fast and frequent handovers,and etc.These will seriously degrade the system performance and thus the current technologies cannot be directly applied to high mobility scenarios.Therefore,it is imperative to develop new theories and technologies to provide high quality broadband wireless communication for high mobility systems.Recently,orthogonal frequency-division multiplexing(OFDM)has been widely adopted for high mobility broadband wireless communications due to its high spectral efficiency.In this thesis,we focus on the channel estimation and interference suppression for high mobility OFDM systems.We deeply study the position information of the high mobility system,and try to improve the system performance by exploiting the position information.The main contributions of this thesis are summarized as follows:Firstly,for the high mobility OFDM system,we propose a new position-based high mobility channel model,in which the HST position information and Doppler shift are utilized to determine the positions of the dominant channel coefficients in delay-Doppler domain.Based on the proposed channel model,we propose a novel position-based compressed channel estimation method to utilize the sparsity of high mobility channels.This method utilizes the position information and the channel sparsity to reduce the complexity and pilot numbers,as well as to improve the spectrum efficiency.Then,we propose a joint pilot placement and pilot symbol design algorithm for compressed channel estimation.It aims to reduce the system average coherence and,hence,can improve channel estimation accuracy.In addition,we discuss the practical applicability and design a pilot codebook.Simulation results demonstrate the effectiveness of the proposed method.Secondly,for the high mobility single-input multi-output(SIMO)OFDM system,we exploit the position information of the high mobility channel modeled by basis expansion model(BEM),and study the relationship between the HST position and the dominant channel coefficients.Then,we show that the ICI caused by large Doppler shifts can be mitigated by exploiting the train position information.In specific,for the complex-exponential BEM(CE-BEM),we propose a novel position-based ICI suppression method by which the ICI can be completely eliminated to get the ICI-free pilots at each receive antenna with low complexity.In addition,this method does not need any guard pilot,which highly improves the spectrum efficiency as well.Then we propose a new pilot design to minimize the system average coherence,and the optimal pilot pattern is independent of the train speed,the train position,the Doppler shift,or the number of receive antennas.Simulation results prove the effectiveness of the proposed method in high mobility environments and also show that the proposed method is robust to high mobility.Thirdly,for the multi-cell high mobility OFDM system,we exploit the position information of the high mobility channel modeled by a general BEM,and propose a simplified position-based channel model.Then,a new position-based interference suppression method is proposed to eliminate both the multi-cell interference(MCI)and ICI for a general BEM.By utilizing the position information,we show that the MCI and ICI can be completely eliminated with the proposed method to get the ICIfree pilots at each receive antenna.In specific,this method does not need any guard pilots and can be applied to different BEMs.After that,for the considered multi-cell multi-antenna system,we propose a low-complexity compressed channel estimation method and consider the optimal pilot pattern design.Both the proposed interference suppression method and the optimal pilot pattern are independent of the train speed and position,as well as the multi-cell multi-antenna system.Simulation results demonstrate the benefits and robustness of the proposed method in the multi-cell HST system.In summary,this thesis investigates the channel estimation and interference suppression methods in high mobility OFDM systems,where the train position information is utilized to improve system performance and spectrum efficiency,as well as to reduce the complexity.Compared to the existing technologies,the proposed methods are more robust to high mobility.In addition,we also analyze the practical applicability of the proposed methods,which provides guidelines for the design of high mobility broadband communication systems.