Study On Channel Estimation And OFDM Signals Detection Under High Speed Mobile Environment

Demand for high spectral efficiency has led to Orthogonal Frequency Division Mul-tiplexing (OFDM) as a key technology in the 4th Generation communication system. As the development of demand on the communication, OFDM systems are required to provide Quality of Service guarantee, not only in quasi-static environment, but also in high speed mobile environment. According to the problems that the demodulation under high speed mobile environment has poor performance and could not meet the requirement of practical application in OFDM systems. This dissertation is mainly concerned with the key technologies of demodulation in high speed mobile environment, which including the maximum Doppler frequency shift estimation, channel estimation and OFDM signals detection. The author’s major contributions are outlined as follows:1、According to the problems that the traditional maximum Doppler frequency shift estimation methods have low estimation accuracy, a novel maximum Doppler frequency shift estimation method based on level crossing rate is proposed. Introducing iterative design to make the bandwidth of the receiving filter achieve optimal, the proposed method can estimate the maximum Doppler frequency shift more accurate than the tradition methods in Rice channels. Simulation results also show that the proposed method can estimate the maximum Doppler frequency shift accurately and effectively.2、According to the problems that the traditional channel estimation schemes have poor performance in the time-variant channels, two channel estimation algorithms are proposed. The first algorithm is proposed under the assumption that the time-variant channels could be approximated by the polynomial models. By using a repeated-pattern preamble and pilots, two second-order polynomial (SOP) models are constructed. As the channel information can be obtained from the second SOP model which has shorter estimation period than the general SOP model, the proposed scheme is superior to the existing methods. Simulation results are presented to illustrate the superiority of the proposed approach. In addition, a joint channel estimation and OFDM signals detection algorithm using total least square is proposed under the assumption that the time-variant channels could be approximated by the basis expansion models. Firstly, the initial channel information is obtained by performing the pilots. Then the total least square is employed in the OFDM signals detection and channel estimation, and as such the effect of iterative model error can be effectively alleviated. The proposed algorithm is able to accelerate the rate of convergence, improve the accuracy of the channel estimation, and sequentially reduce the bit error rate of the OFDM systems. Both the derived Cramer Bound of the channel estimation and simulation results show that the proposed algorithm is better than the existing joint channel estimation and OFDM signals detection methods as well.3、It is well known that iterative channel estimation and OFDM signals detection can significantly improve the performance of communication system. However, its per-formance is poor due to the modelling error of basis expansion model being large enough and can not being ignored in rapidly time-variant channels. In order to solve this prob-lem, a novel joint channel estimation and OFDM signals detection algorithm is proposed. In this algorithm, channel estimation and OFDM signals detection are integrated into a real non-linear least squares (NLS) problem. Then the modified Broyden-Fletcher-Goldfarb-Shanno algorithm is adopted to search the optimal solution. Simulation results are presented to illustrate the superiority of the proposed approach.4、After a dissection of the relationship between inter-carrier interference (ICI) coef-ficients and normalised Doppler frequency shift, the relationship between the bandwidth of the nearly banded channel matrix and normalized Doppler frequency shift is deduced under certain power of negligible ICI coefficients. Then, according to the problems that the existing equalisers has poor detection performance in the traditional nearly banded channel matrix, a new structure of nearly banded channel matrix is proposed. The struc-ture takes account of all the ICI coefficients when the nearly banded channel matrix is divided into blocks. Lastly, the detection methods proposed by Panayirci are applied to detect the OFDM signals in each block. Simulation results show that although a little more computational complexity is cost, the detection methods using the proposed struc-ture can achieve better performance than the detection methods using the traditional structure.5、In the light of the fact that the performance of the existing signals detection meth-ods for BPSK-OFDM systems are poor, a novel signals detection algorithm, i.e., modified minimum mean square error (M-MMSE), based on the MMSE criterion is therefore pro-posed. To find the proper equalizer matrix, the redundant imaginary component is not considered in the cost function. Consequently, its interference on the signal detection could be eliminated. In order to further improve the detection performance, the succes-sive detection technique is used in the M-MMSE algorithm. The reason for its success is that, it not only minimize both the residual interference and the noise enhancement, but also make good use of the time diversity in time-variant channels. Both the computation-al complexity investigation and simulation results illustrate that the proposed algorithms have complexity advantages and significant performance over the existing signals detection methods.