Doppler Diversity Techniques Over Fast Varying Channel In The OFDM System

Due to the high spectrum efficiency and resistance to the frequency selectivefading and so on, orthogonal frequency division multiplexing (OFDM) has become animportant technology in the field of wireless communication in recent years. Butmobility support is increasingly becoming an important characteristic widely consideredby the next generation wireless communication systems, especially the rapidpopularization of high-speed railway. Fast varying channel destroys the orthogonalitybetween OFDM sub-carriers when communication terminal moves fast, which results inICI that affects system performance seriously. However, fast varying channel bringsutilizable diversity gain as well. Therefore the study of Doppler diversity techniquesover fast varying channel in the OFDM system is of great significance.This thesis makes a study of the existing Doppler diversity techniques in theOFDM system. It is found out that this kind of diversity techniques can only achieve apart of the Diversity gain through analysis, although they can improve the systemperformance over fast varying channel. Therefore this thesis proposes a newtransmission scheme in the OFDM system, which can achieve the full diversity gain bymeans of proposed precoding in the transmitter and equalization in the receiver.Simulation results show that the proposed algorithm can achieve the maximum diversitygain and outperforms the existing Doppler diversity techniques evidently. In addition,larger diversity order can get more diversity gain.Multipath diversity and Doppler diversity in OFDM system are usually achievedwith the help of channel coding and precoding. Although it is possible to get excellentperformance, codec in the transmitter and receiver is overly complex. Thereforeoversampling technique is introduced to OFDM system in this thesis. Taking the highcomplexity of optimal receiver-maximum likelihood receiver and the error propagationand residual interference of the suboptimal receiver-Doppler domain block decisionfeedback equalization into account, this thesis proposes to adopt iterative parallelinterference cancellation in the receiver, which achieves both multipath diversity gainand Doppler diversity gain in doubly selective fading channels with low complexity.And simulation results show that the proposed receiver is able to achieve a goodcompromise between performance and complexity.