Mitigation of intercarrier interference in OFDM systems over slow fading channels based on the EM algorithm

Orthogonal Frequency Division Multiplexing (OFDM) is a transmission technique that allows simple multipath channel equalization schemes. On the other side it is sensitive to frequency offset at the local oscillator, phase noise and Doppler shift which may destroy the orthogonality among subcarriers causing Inter Carrier Interference (ICI). It is possible to represent this interference in the frequency domain by means of an ICI matrix. Since these types of errors greatly affect the performance of an OFDM system, estimation of this matrix is crucial. In this work we propose an iterative method to mitigate the interference among subcarriers exploiting the presence of pilot tones in the frequency domain. It can be explained referring to the Expectation-Maximization (EM) paradigm. The proposed technique is very effective in multipath fading channels and a simplified version is also introduced in case of AWGN channel with frequency offset at the local oscillator. Simulation results show that the presented algorithm converges very quickly and looks promising to be used in actual implementations.