Inter-Carrier Interference Cancellation For OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is the projected modulation of choice for fourth-generation broadband multimedia wireless systems. OFDM is robust against frequency selective fading due to the increase of the symbol duration. However, for mobile application channel time-variations in one OFDM symbol introduce intercarrier-interference (ICI) which degrades the performance. A frequency offset between the local oscillators at the transmitter and receiver causes a single frequency shift in the signal, while a time-varying channel can cause a spread of frequency shifts known as the Doppler spread ; Frequency shifts ruin the orthogonality of OFDM subcarriers and cause intercarrier-interference. Therefore, quickly diminishing the performance of the system.In this paper, I explain the generated reason and the affect of ICI. I investigate the effectiveness of the Extended Kalman Filter (EKF), Self-Cancellation (SC), and Maximum Likelihood Estimation (MLE) techniques for mitigation of ICI in OFDM systems. Matlab communication simulation will be carried out to the performance of the three techniques. I also analyze and compare the implement complexity of the above three schemes. Due to the BER curve I give some conclusion such as their disadvantages and advantages for the every scheme. Based on their theories, in this paper, I propose two improved schemes. The first scheme is the improved Maximum Likelihood Estimation. This scheme only uses one symbol to estimate the frequency shift and it can increase the bandwith efficiency but the BER performance of the OFDM system won't decrease. In the second scheme, I joint the Self-Cancellation scheme and Maximum Likelihood Estimation. It makes use of their advantages and overcome their disadvantage. Then the OFDM system won't be sensitive to frequency shift and modulation level. The performance will be improved greatly.