Research on DFT-based Channel Estimation Algorithm for MIMO-OFDM System

Combining MIMO and OFDM system can not only enhance the capacity of wireless communication system and the transmission rate but also combat the interference. Accurate channel state information is required for MIMO equalization technique, the coherent demodulation, and space-time decoding in MIMO-OFDM system, so accurate channel estimation is the key to ensure the transmission quality in MIMO-OFDM system. This paper has investigated the channel estimation for MIMO-OFDM.Firstly, this paper analyzes the propagation characteristics of the wireless channel, and introduces the basic principles of OFDM and MIMO technologies. Then this thesis introduces the channel estimation methods in MIMO-OFDM system, and mainly studies pilot-assisted channel estimation methods including LS, LMMSE and DFT-based channel estimation algorithms. The analysis and comparion of these algorithms are made by computer simulation.This paper mainly studies DFT-based channel estimation method in MIMO-OFDM, and provides the following two improvements for the shortage of the original algorithm:(1) The conventional DFT-based channel estimation method will be affected by the energy leakage when the channel is non-sample-spaced multipath channel. It is analyzed in this paper. To solve estimation performance degradation caused by this problem, this thesis presents an improved algorithm. The improved method uses symmetric property and selects useful paths by calculating the changing rate of the leakage energy to improve the estimation performance. Through the simulation of the improved and conventional algorithms, it proves that the improved algorithm can effectively reduce the impact of energy leakage and improve the performance of channel estimation.(2) This thesis analyzes the energy leakage because of the existence of virtual subcarriers. This problem will cause the estimation performance degradation of the conventional DFT-based method. Aiming at this problem, this paper presentes an improved algorithm. The improved method used Wiener filter interpolation to estimate channel frequency response at virtual subcarriers. On this basis, improved method uses relationship factor to determine the useful subcarriers which are used to estimate channel frequency response at virtual subcarriers in order to simplify the complexity of the algorithm. Through the simulation of the improved algorithm, it proves that the improved method effectively reduces the impact of energy leakage by virtual subcarriers and improves the performance of channel estimation.