High-Efficient Receiving Techniques In OFDM Systems

This dissertation is devoted to the research on the OFDM communication techniques in the wireless mobile communication environments, including some high-efficient transmission techniques.This dissertation begins with the basic introduction of OFDM techniques and a summary of single carrier technique and several existing multicarrier techniques in order to clarify the difference between the OFDM and the other multicarrier techniques. The research on the OFDM system is of great practical values since many of the existing multicarrier techniques can be fulfilled through OFDM module and many useful results in OFDM system are also applicable to other multicarrier systems. This dissertation mainly focuses on some high-efficient transmission techniques, including the high-efficient time-frequency synchronization algorithm, the high-efficient channel estimation algorithm based on pilot arrangement and the high-efficient adaptive channel estimation algorithm, in OFDM systems.In order to illustrate the necessary of OFDM high-efficient communication techniques in the wireless mobile communication environments, this dissertation analyses the performance influence of OFDM systems due to the time and frequency synchronization errors and detailedly introdues the existing time and frequency synchronization algorithms and their corresponding merits and demerits. This dissertation also clarifies the characteristics of wireless mobile communication environments in OFDM systems, summarizes the existing channel estimation algorithms and detailedly describes the channel estimation algorithms based on pilot arrangement and their corresponding merits and demerits.In the research of high-efficient time-frequency synchronization algorithms of OFDM systems, this dissertation presents a new time and frequency synchronization method used in wireless mobile communication environments. This method is based on the frequency domain correlation and the maximum likelihood theories and can solve the problems in the existing time and frequency synchronization algorithms. This method uses one training sequence of just one symbol and the design of the symbol is presented. The time synchronization and carrier frequency offset estimation can be achieved simultaneously and the frequency synchronization algorithm can extend the frequency acquisition range to nearly the whole useful OFDM signal bandwidth. The frequency synchronization algorithm can also overcome the ambiguity problem existing in carrier frequency offset estimation. A fast realizing algorithm is presented in the dissertation to reduce the calculating complexity in order to make the method easier to be implemented. The simulation results demonstrate the efficacy of the method.In the research of the high-efficient channel estimation algorithm based on pilot arrangement, this dissertation proposes a robust pilot-assisted channel estimation method for OFDM signals in Rayleigh fading channel. The method is based on the theory that the calculating complexity can be greatly reduced when the filter for channel estimation is performed in Dopler spreading domain of wireless mobile communication environments in OFDM systems. Using the way of Most Significant Tap (MST) selection and polynomial filtering, the method conducts the time-frequency filtering for the Least Square (LS) estimation results of pilot symbols. The method is simple and need not know the statistic characteristic of the practical channel, such as the channel correlation matrix and signal-to-noise rate(SNR). The method can be used in rectangular pilot-assisted geometry and can be easily realized in practical OFDM systems. The simulation results show that the mean square error(MSE) performance of the method is greatly improved when the method is compared with LS and MST estimation method in comb pilot-assisted geometry and the bit error rate(BER) performance of the method approaches that of the ideal channel estimation in rectangular pilot-assisted geometry.