Research On Diversity Reception And Doppler Frequency Estimation Technology For OFDM Communications

Orthogonal Frequency Division Multiplexing （OFDM） technology is considered tobe the mainstream technology for next-generation communications system, which hashigher spectrum utilization and strong anti-multipath fading ability. However, In themobile communication environment, the Doppler effect will destroy the orthogonalitybetween the various sub-carriers in the OFDM system. It can generate the interferencebetween the various inter-subcarriers, and greatly reduce the performance of system.Therefore, in order to resist the harmful influence of the Doppler shift of the OFDMsystem effectively, the paper has studied the OFDM Doppler diversity receptiontechnology and the Doppler shift estimation techniques. The main contents include:The paper presents an improved OFDM Doppler diversity reception technology.First, the classical OFDM Doppler diversity reception technology selects frequencyoffset value （f_x=0.75） in the condition of the interference between two branchesuncorrelated, and selected the weighted value, which making the interference power ofthe branches equaled, so that the interference can be suppressed maximally, and thesignals are combined through the maximum ratio; Secondly, the paper presents animproved Doppler diversity technology, which selected frequency offset value （f_x=0.8）as each interference of two of the branches uncorrelated, and selected the weightedvalue, which making the interference power and the noise power of the branchesequaled, so that the interference and noise can be suppressed maximally, and the signalsare combined through the maximum ratio. Finally, the method gains the coefficient ofthe maximum ratio combining, η_q (i, f_xq)=H^*(i, f_xq), and verified it can obtain theMaximum Signal Interference noise radio, by theoretical analysis. The situation resultsshows that: the improved Doppler diversity receiver gain better bit error rateperformance than the classic one.the Doppler shift estimation techniques has been researched, and after studying acyclic prefix-based Doppler shift estimation algorithm, the paper presents an improvedjoint synchronization-Doppler estimation techniques, which obtains the synchronization based on conjugate symmetry training symbol, and then estimate theDoppler shift value. The situation results show that: assumed symbol synchronization,the maximum Doppler shift estimation algorithm based on a cyclic prefix will has highestimation accuracy. However, the algorithm is difficult to accurately estimate thefrequency shift if the synchronization information unknown. In this situation, theimproved Synchronization-Doppler estimation algorithm can be able to estimate theDoppler shift value accurately. Moreover, the greater the signal to noise ratio, the betterthe estimation value of the improved method.