Research On Inter Carrier Interference Suppression In OFDM System

Orthogonal frequency division multiplexing(OFDM)has the outstanding anti-jamming ability and high spectrum utilization ratio, so it becomes a key technology in the Long Term Evolution(LTE)system. However, the OFDM system requires accurate frequency synchronization to maintain the orthogonality between subcarriers, while the time-varying channel and the Doppler frequency offset can cause the frequency offset between the transmitted symbols, which leads to the Inter Carrier Interference(ICI). And under the condition of high transmission rate, the effect of ICI on the system will be greater. Therefore, it is necessary to study the method of eliminating ICI in OFDM system.Firstly, the mechanism of ICI in OFDM system is analyzed. Then the ICI caused by the carrier frequency offset and multipath time-varying channel is studied comprehensively, and the effect of ICI on the OFDM system is illustrated by simulations.Secondly, the current algorithms of ICI cancellation are studied. The adjacent symbol self cancellation algorithm reduces the interference coefficient between the subcarriers by modulating the opposite symbols onto the adjacent inter carriers. The time domain windowing algorithm can effectively reduce the sidelobe of the signal power spectrum. The Zero Forcing(ZF)equalization and Minimum Mean Square Error(MMSE)equalization methods can be used to eliminate the ICI in the time varying channel.Finally, for solving the ICI caused by frequency deviation, the paper proposes an improved ICI cancellation algorithm which is based on symmetric inter carrier. The proposed algorithm modulates the opposite symbols onto the symmetric inter carriers, at the receiver, the demodulated symbols are combined linearly. For reducing the ICI further, the algorithm is combined with windowing in time domain. It is that the symbols are multiplied by raised cosine function before they are transmitted. The simulation experiments show that the symmetric algorithm can reduce the interference coefficient between the subcarriers and improve Carrier to Interference power Ratio(CIR)performance of the system. And the side lobe of the OFDM symbol spectrum attenuates fast after windowing, which reduces the sensitivity to the frequency offset. Finally, for solving the ICI caused by time varying channel, the algorithm is combined with frequency domain equalization. It is, at the transmitter side, the symbols are symmetrically modulated. At the receiver side, the ZF equalizer is added to recover the required transmitted symbols. The simulation results show that, the ICI caused by time varying channel can be effectively suppressed, and the anti-jamming performance of the system is also improved after combining with the frequency domain equalization.