Interference Elimination Techniques For OFDM System In Multinath Fading Channel

Orthogonal frequency division multiplexing eliminates Inter-symbol interference and inter-carrier interference by adding cyclic prefix. When channel delay is long, there should be long cyclic prefix. But it will reduce the data transmission rate. Therefor combating interference under the condition of short cyclic prefix is important technology of OFDM systems.The main contents of this work is to combat interference in the condition of short cyclic prefix and multipath fading channel with large delay spread, the details are listed as follows:The system model and algorithm of channel shortening equalization based on MMSE criteria are investigated. The MMSE channel shortening equalization algorithm is studied for OFDM systems with unequal subcarrier powers. The equalizer coefficients and the expressions of signal correlation matrix are derived. The relationship between the performance and equalizer parameters is analyzed through simulations.In order to increase OFDM system bitrates, the system model and algorithms of maximum bitrates equalization is investigated. A low-complexity variable-length per-tone equalization algorithm is proposed, the method to calculate equalizer length and coefficients of each sub-carrier is given. A variable length per-group equalization algorithm is proposed for the systems with limited memory. With different CP lengths, the relationship between the performance and parameters of equalizer under different multipath fading channels is analyzed through simulations.In order to combat interference under the condition of short cyclic prefix with low complexity, iteration time reversal algorithm is discussed, and the method of cutoff length selection is proposed. The applicability of iteration time reversal algorithm in OFDM systems is proved theoretically, and an OFDM system model adopting iteration time reversal is presented. The relationship between the iterations and effective channel signal-to-interference-ratio is given through simulations, and the effect of iterations on system bit error rate is also presented.