The Research Of Frequency Domain Equalization Technique In SC-FDE System

How to resist the inter-symbol interference caused by the frequency selective fading effectively while achieving high speed and high-capacity communication transmission is a key research direction in wireless communications. The traditional single-carrier time-domain equalizer mode is too complex, and the orthogonal frequency division multiplexing (OFDM) technology based on FFT/IFFT can resist frequency selective fading effectively but with the disadvantage of excessive Peak-to-Average ratio.Single carrier frequency domain equalization (SC-FDE) technology adopts the similar signal processing method to OFDM, by moving the IFFT module of OFDM transmitter to the receiving end. The equalization in frequency domain, which not only overcome the disadvantage of excessive peak average power rate (PAPR) , but also achieve the similar performance to OFDM at the same time keep the complexity unchanged, becomes one of important technologies of physical layer of broadband wireless access system. SC-FDE is getting more and more attention, and already has been brought into the IEEE 802.16 standards.Firstly, the characteristics of the wireless communication channel are analyzed and SUI channel is modeled and simulated in this paper. Then, the basic principle of SC-FDE system is described and two algorithms of frequency-domain equalization MMSE-based frequency-domain decision feedback equalization (MMSE-DFE) and LMS adaptive frequency-domain equalization algorithm are researched in detail. Based on this, two new frequency-domain equalization algorithms are proposed which are frequency domain LMS adaptive decision feedback equalizer (LMS-DFE) algorithm and variable step size LMS frequency domain equalizer (VLMS-DFE) algorithm. LMS-DFE algorithm uses LMS algorithm to update the equalizer filter coefficients in the frequency domain and make them adaptive to the channel transmission characteristics. Traditional of the horizontal filter is adopted to void the inherent delays in FFT block signal processing in time-domain feedback part. By reducing the error diffusion, a lower error rate is obtained ultimately. But the LMS-DFE algorithm exist contradiction between convergence speed and steady-state error in step length requirements. According to this problem, the variable step-size ideology is introduced into VLMS-DFE algorithm, using improved MSE-based sigmoid function to replace the traditional fixed-step to achieve faster convergence rate and smaller steady state error. Finally, MATLAB simulations show that, VLMS-DFE algorithm have the optimal equalization performance and lowest bit error rate, followed by the LMS-DFE algorithm.