Research On ICI Self-cancellation Technology For FrFT-OFDM System

Orthogonal frequency division multiplexing(OFDM)system has the characteristics of strong anti-interference ability and high spectrum efficiency.It plays an important role in today’s wireless communication system.However,the transmission reliability of OFDM system depends on the strict orthogonality between its subcarriers.In the real transmission environment,factors such as carrier frequency offset and time-frequency selective fading will destroy the orthogonality relationship between subcarriers and cause inter-carrier interference(ICI)and reduce the communication performance of OFDM system.As an extension of OFDM system in fractional domain,fractional Fourier transform orthogonal frequency division multiplexing(Fr FT-OFDM)system uses a group of orthogonal Chirp signal basis to replace the exponential signal basis in OFDM system.Compared with exponential signal,Chirp signal has the structural advantage of variable chirp-rates.In different channel environments,it can adjust the chirprates to adapt to channel changes and select the optimal rotation angle to suppress ICI.Although Fr FT-OFDM system has stronger adaptability to complex channel environment,if the ICI is strong enough,the improvement of communication performance of the system is limited.Therefore,how to use the structural advantages of Fr FT-OFDM system to achieve better ICI suppression is a very important problem.This paper has carried out in-depth research on this problem.The research work of this paper is mainly divided into the following three parts:Firstly,in order to explore the ICI suppression method in Fr FT-OFDM system,this paper analyzed three typical ICI causes in wireless communication environment,deduced the physical causes of ICI in detail,and simulated and analyzed its impact on the system,which laid a theoretical foundation for the design of ICI elimination method in the following paper.Secondly,this paper analyzed the traditional ICI self-cancellation methods suitable for OFDM system,and compared the suppression effects of several methods with the flexibility of coding.Then,based on the traditional methods and the theoretical derivation of ICI in Fr FTOFDM system,this paper proposed a combined self-cancellation method based on ICI selfcancellation coding and frequency offset compensation.The method used the self-cancellation matrix with phase compensation to encode the signal.By using the structural advantage of adjustable subcarrier rotation angle of Fr FT-OFDM system,the fractional Fourier transform(Fr FT)was carried out at the appropriate rotation angle to compensate the frequency offset,so that this method achieved better ICI suppression effect than the traditional ICI self-cancellation methods under the same coding efficiency.Simulation results verified the performance of the scheme.Finally,in order to improve the practicability of the combined ICI self-cancellation method proposed in this paper,a new discrete algorithm was proposed.The algorithm used a new scale transformation dimensional normalization method to preprocess the signal,and its normalization factor changed adaptively with the transformation order.Then the signal was processed by DFr FT,and the search model of the new algorithm was established.New algorithm,Pei algorithm and Ozaktas algorithm were applied to the proposed self-cancellation method,discrete precisions and chirp-rates optimize-ranges were theoretically deduced respectively.Finally,the effectiveness of the algorithm was verified under different chirp-rates and signal-to-noise ratio(SNR)environments,and compared with the traditional algorithm.