Frequency Diversity Technology Based On OFDM

Orthogonal Frequency Division Multiplexing(OFDM) is regarded as a reliable wireless communication scheme in multi-path channel environment, which converts a frequency selective channel into several orthogonal flat fading sub-channels, thus obtaining high spectrum efficiency and good anti-fading performance. So far, it has been the most widely applied multi-carrier modulation scheme. Besides, it is easy to combine OFDM with Space Time Coding(STC), diversity, interference restraint and smart antenna techniques, which, to large extent, improving information transmission reliability in physical layer.This thesis proposes a modified OFDM channel model of which the correlation varys placidly in frequency domain. Based on different correlation hypothesis, a simulation and analysis is done about comparison among Maximal Ration Combination(MRC) OFDM, Direct Sequence Spread Spectrum(DS) OFDM, Space Frequency Block Code(SFBC) and Code Joint(CJ) SFBC. On this basis, a novel OFDM diversity scheme is proposed which can obtain frequency diversity by maximizing the Hamming distance(MHD) of signal vector, and then simulation is made in different correlation hypothesis.The thesis introduces the fundamental theory of OFDM and diversity in the first place. Then, a placid OFDM channel model is proposed in frequency domain. With this correlation hypothesis, the simulation and analysis is done about comparison among MRC-OFDM, DS-OFDM, SFBC and CJ-SFBC.Thirdly, a novel OFDM frequency diversity scheme is proposed based on MHD. This scheme maximizes the sending vectors’ Hamming distance through multiple symbols jointed design, thus frequency diversity is obtained via the relative independence of each sub-channel fading, so the anti-fading performance of the OFDM system is increased. In the thesis, three specific design examples are given, which can provide 2-order frequency diversity, 3-order frequency diversity and 4-order frequency diversity, respectively. And the high-order diversity scheme implementation is introduced. The simulation result shows that the proposed frequency diversity scheme is better than the other diversity schemes such as MRC-OFDM, DS-OFDM, SFBC and CJ-SFBC. In addition, the thesis compares and analyses the DS-OFDM, MRC-OFDM, SFBC, CJ-SFBC and MHD-OFDM from the diversity order, information transmission rate and the decoding complexity. The result indicates that the comprehensive performance of the proposed frequency diversity scheme is better than the rest scheme mentioned in the paper. The proposed scheme has good theoretical and practical application value.