Study On ISI Suppression Methods For OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) is a high-speed multicarrier transmission techlonogy in wireless communications, and it has been regarded as the core technology of 4G mobile communication systems due to its high bandwidth efficiency and robustness combat multipath interference. When the carrier frequency of OFDM system is low and the speed of the terminal is slow, the parameters of the channel can be regarded as time-invariant or slow-varying. The intersymbol interference(ISI) and intercarrier interference(ICI) of OFDM can be eliminated by employing cyclic prefix long enough (greater than channel length), and therefore the distortion of received signals can compensated by just a single-tap equalizer in frequency domain.In OFDM system, when the length of the cyclic prefix(CP)is shorter than the length of channel impulse response, the orthogonality between OFDM sub-channels is lost because of the intersymbol interference(ISI) and interchannel interference(ICI). In this case, the one-tap frequency domain equalizer can not be used any more. In order to solve this problem, the ISI and ICI cancellation of the received signals must be considered in the receiver. Focus on the problem of OFDM ISI cancellation in frequeney selective fading channel, in this thesis when cyclic prefix is not enough, the mechanism of ISI and ICI generation is analyzed, a kind of frequency domain equalizer using the recursive least square (RLS) algorithm is researched in detail. Further more, theoretical analysis and simulation results show that this algorithm is effective and can greatly increase the frequency spectrum efficiency. In addition, this thesis also studies the parallel iterative equalization scheme and residual ISI cancellation algorithm, at the same time, an improved RISIC scheme for OFDM systems based on transform-domain processing channel estimation is researched, simulation results show that this scheme can lead to the reduction of bit error rate.