Inter-Carrier Interference Cancellation For High-Mobility OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is a kind of multi-carrier modulation technique. It distributes high-speed data stream into many low-rate sub-data streams, makes use of overlapping multiple orthogonal sub-carriers to transmit information. OFDM has been widely used in wireless communication system, with high spectrum efficiency, resistance to multi-path, and high transmission rate.One of OFDM system's main disadvantages is sensitive to frequency offset. It is built on strict orthogonality between sub-carriers. Carrier frequency offset will destroy this orthogonality. Any carrier signal waveform distortion in transmission process will affect the orthogonality between sub-carriers, cause inter-carrier interference (ICI), make system performance fall rapidly. In high-speed mobile environment, large Doppler frequency offset will cause more severe ICI. So high-rate OFDM system requires high-performance, good universality ICI cancellation method.This paper introduces OFDM basic principle and system's FFT realization, analyzes the advantages and disadvantages of OFDM system, studies carrier frequency offset reasons, and analyzes ICI caused by carrier frequency offset through concrete mathematical derivation. Based on WiMAX, OFDM system's simulation environment is built on Matlab platform. This paper classifies existing ICI cancellation methods as three classes—first, according to characteristic of code shape; second, receiver frequency offset compensation; third, Doppler diversity techniques. Respectively, take self-cancellation method (SC), maximum likelihood estimation method (MLE), joint multipath-Doppler diversity method (JMPD) for each class method to perform algorithm research and simulation. Then, research Kalman filtering method and its recent progress. Study Extended Kalman Filtering (EKF) in ICI cancellation field. As to SC be adapted to second-order modulation, MLE be suitable for large frequency offset, EKF needing high SNR, this paper develops Kalman filtering method in this field of application.This paper firstly put forward introducing Unscented Kalman filter (UKF) to cancel ICI in high-mobility OFDM system. Simulation results prove that UKF can get accurate estimated value, stable performance and good universality. In the end, this paper compares SC, MLE, EKF, UKF methods from modulation order; frequency offset size, received SNR, transmission efficiency, algorithm complexity and universality, and compares EKF and UKF from algorithm complexity, accuracy and convergence rate. It concludes that using UKF method can effectively cancel ICI, greatly improve system performance in high-mobility and high-rate OFDM systems.