High-speed Wireless Ofdm Mobile Communication Systems, Interference Cancellation Technology

With the rising requirement of the mobile multimedia and wireless Internet accessing in the B3G communications system, the wireless transmitting symbol rate becomes much higher. However, the wireless spectrum is unique and cannot be regenerated in the nature. At the same time, the ISI becomes more and more serious especially after higher speed symbol passed through the multi-path wireless channel. Wireless OFDM system is widely studied because of its higher spectrum efficiency, multi-path overcoming and easy digital realization. However, long guard intervals in OFDM system decrease the spectrum usage. So it is necessary to design an efficient interference canceller (IC) to overcome the interference introduced by non-enough GI in high speed wireless OFDM system.Firstly, the IC design in time domain is presented. By analyzing the various ISI and ICI introduced by non-enough GI in high speed wireless OFDM system, the channel shortening IC method and the maximum capacity method by using MMSE criterion are given. Then, a new modified MMSE-TEQ IC algorithm and a fast algorithm based on the FFT structure, which is benefited from the advantages of the special transmit channel matrix.Secondly, the IC processing method in frequency domain is given. By analyzing the frequency equalization proceeding, when the GI is removed thoroughly in high rate wireless OFDM system, the block equalizer IC method in frequency domain in wireless OFDM system is given. Also, a fast algorithm for frequency IC method and its iterative realization based on Turbo equalization are given.Thirdly, time domain and frequency domain method are combined, two new IC methods are proposed. These are a new doubly adaptive loading with channel shorten IC method and a new frequency constraint channel shorten IC method in high speed wireless OFDM system. These two proposed methods may not only improve the BER performance but also enhance the OFDM system capacity.Then, a multistage IC (MSIC) iterative method is included into high rate wireless OFDM system, which refers to the iterative algorithm in multi-user detection. The new MSIC method, including an error-diffusion-combated MSIC method and a adaptive filter MSIC method, can be easily realized in real OFDM communications system.The simulations and comparison of the proposed algorithms are also given.