Analysis Of ICI And ICI Cancellation Method In OFDM Systems

Orthogonal Frequency Division Multiplexing(OFDM) is one of the multi-carriermodulation technology. It breaks high-speed data streams down into many sub-low-rate datastreams, and transmits information at the same time using a number of orthogonal andoverlapped subcarriers. Therefore, OFDM system, which has been widely used in the mobilewireless communication systems, has advantages of high spectrum efficiency, anti-multipathcapability and high data rate transmission.For the OFDM mobile communication system with the multi-carriers paralleltransmission, the subchannel spectrum of the output signal are overlapped each other, whichneed to have a strict orthogonality. However, because of the existence of time-varyingwireless channel, the frequency of the signal will shift during the transmission. For example,Doppler frequency expansion and transmitter and receiver oscillator frequency offset, whichwill make the orthogonality between subcarriers destroy in OFDM system, results ininterference between subcarriers. The interference makes performance of the system declineseriously. Thus, ICI(Inter-Carrier Interference)must be suppressed to improve theperformance of the system.First, the characteristics of wireless channel are introduced. Delay spread, Dopplerexpansion and the OFDM basic principles are summarized particularly. Second, for theoscillator frequency inconsistent of transceiver and receiver and time-varying channelcausing of the ICI. Further study is discussed, and mathematical explanation and analysis aregiven. ICl will not only result in loss of signal to noise ratio and bit error rate performancedegradation for OFDM systems, it will bring serious floor effect. Finally, the paper presentsthe principle of ML, EKF and the traditional self-cancellation ICI algorithm in detail, and animproved algorithm is given on this basis.The advantage of these methods is proved throughtheoretic analysis and computer simulation using Matlab．We simulated OFDM system withAWGN channel as well as Rayleigh channel to test these methods. Then, self-cancellation ICIalgorithm relevant to channel estimation is studied and simulated. The simulation resultsshow that carrier-to-interference ratio and error bit ratio of the improved algorithms areimproved compared with that of the traditional algorithm.The self-cancellation ICI algorithm is suitable for small frequency offset, EKF is suitable for large frequency offset.The methodof self-cancellation ICI algorithm relevant to channel estimation also improves BERobviously.