Research On OFDM With Index Modulation And Channel Estimation For High Speed Train Communication

Orthogonal frequency division multiplexing with index modulation(OFDM-IM)is a novel multisubcarrier transmission technology.It can achieve a tradeoff between frequency efficiency,and bit error performance by changing the number of active subcarriers.Furthermore,OFDM-IM has the potential of suppress intercarrier interference(ICI).OFDM-IM is introduced into high-speed train communication in this thesis.What's more,the improving scheme and channel estimation of OFDMIM is studied.The detailed work is as follows.First,the feasibility of applying OFDM-IM to the next generation railway communications for the high flexibility and reliability of data transmission is investigated in this thesis.Then the technical challenges for applying the OFDM-IM is further analysed,followed by providing the solutions to them and channel estimation methods.Next,the OFDM-IM-ID scheme featuring index information diversity transmission is proposed since the detection of active subcarrier index is susceptible to noisy in low signal to noise(SNR)region.Exactly to say,the subcarriers are divided into primary and auxiliary subcarriers according to first transmitted information.The remaining data information is transmitted by the primary subcarriers and the index information is transmitted by some auxiliary subcarriers respectively.In order to make a distinction between these two subcarriers at the receiver,this study designs the constellation mapping scheme of the auxiliary subcarriers in different situations.Besides,the detection algorithm based on log-likelihood ratio(LLR)is proposed to reduce the complexity.The OFDM-IM-ID is then integrated with interleaving subcarrier technique,which can further improve the bit error rate(BER)performance.Also,this research presents the hybrid-diversity OFDM-IM scheme containing the index information diversity in the low SNR region and data information diversity for the high SNR region.The simulation results indicate that the proposed OFDM-IM-ID scheme can suppress ICI and improve the BER performance.Finally,considering the uncertain active subcarrier indices of OFDM-IM and high-speed train runs along the fixed track,a channel estimation scheme is proposed which adopts historical channel state information,Basis Expansion Model(BEM)and expectation maximization algorithm to estimate the time varying channel.Specifically,the frequency channel response can be expressed as the multiplication of basis matrix and basis coefficient by employing BEM.As the basis matrix changes slowly while the basis coefficient changes relatively quickly,the former can be derived by the collected data in the base station whereas the latter can be sought through the expectation maximization algorithm.In doing so,the OFDM-IM channel estimation method can be applied to the high-speed train commucation.The simulation results show that such method has a higher precision than traditional data-aided ones such as least square and linear minimum mean square error.