The Study Of Channel Estimation Methods In OFDM Wireless Access Systems

Orthogonal Frequency Division Multiplexing (OFDM) transmission over mobile communications channels can alleviate the problem of multipath propagation, and has high bandwidth efficiency. OFDM has been widely used in Wireless Local Area Network (WLAN) and is proposed to be the next generation mobile communication system. However, either the coherent demodulation of OFDM or the combiner at the receiver needs information of the channel. So it's very important to study the channel estimation methods. The channel estimation methods in OFDM WLAN systems are studied in this thesis.Under analyzing several characteristics of frequency-selective fast fading channels, such as large Doppler spread and multi-path interference, we study the basic principles of WLAN and OFDM matrix representation and the necessary elements for constructing an OFDM system and thus analyze some non-ideal effects on the performance of OFDM digital communications, in order to choose optimal parameter of OFDM systems in pratical environment.The implementation for carrier frequency estimation and synchronization and channel estimation of OFDM WLAN systems are discussed. Then a channel estimation method based on vector KALMAN filter and pilot tracking for OFDM WLAN systems with mobile terminals is proposed. The result of training sequence estimation is used as the initialization and observationof KALMAN filter. Simulation results show that this combined estimation method makes use of the characteristics of IEEE 802.11a frame format and has good Symbol Error Rate (SER) performance. Based on the research and simulation of OFDM system and the existing channel estimation methods, a low-dimensional Kalman filter method based on pilot signals is presented for the channel estimation of OFDM systems with large number of carriers in Beyond 3 Generation (B3G) outdoor environment. For simplicity, a one-order AR process is used to model the time-varying channel and the LS algorithm based on pilot signals is adopted to track the time-varying channel fading factor a. In order to utilize the relationship of fading channel in frequency domain, a Minimum Mean Square Error (MMSE) combiner is used to refine the estimation results. The simulation results show that the proposed method achieves a good SER performance close to the theoretic bound of ideal channel estimation.