Research On Channel Estimation Techniques In OFDM Systems In Timing Varying Channel

Orthogonal frequency multiplexing technique (OFDM) has been widely used in modern wireless communication area due to the high spectrum efficiency, simplicity to implementation, etc. Many wireless communication standards including digital audio and video broadcasting (DAB, DVB), wireless LAN standard IEEE802.11a and 802.16, have adopted OFDM as one of the main techniques.Channel estimation plays a significant role in OFDM systems. Via channel estimation, we can carry out coherent detection which can help to maintain low BER performance even in low SNR environments. This is vital in actual OFDM systems.This thesis investigates a variety of issues of channel estimation in time-varying channel, especially in fast time-varying channel.First, we discuss the frame model of OFDM systems and introduce the characteristics of wireless channel. Especially, we expound the multipath fading and Doppler effect in detail. Then we study the various existing channel estimation methods. For the method using pilot, we have discussed the pilot patterns and interpolation algorithms in channel reconstruction process and compared these algorithms through simulation.Second, this thesis investigates the mechanism of the formation of inter-carrier-interference; especially the ICI induced by fast varying characteristic of wireless channel and explains ICI through mathematical derivation. Then we study the channel estimation methods in fast varying wireless channel and various ICI suppression measures.Finally, this thesis proposes a joint channel estimation method in time and frequency domain. In time domain we adopt an estimation method based on adaptive filtering principle and discuss the corresponding time-domain equalization methods. We mainly focus on the application of Kalman filter in channel estimation in time varying environments. Based on MMSE principle, Kalman filter can make optimal estimation as to the channel information and adjust its own parameters dynamically, making it able to track the change of the channel. In frequency domain, we adopted relatively simply Zero-Forcing algorithm to estimate the response value in pilot positions and linear interpolation to reconstruct the channel information. As the simulation result shows, this method can enhance the system performance significantly. When the Doppler frequency is 80Hz, 120Hz and SNR equals to 20dB, the BER decreases more than an order of magnitude after applying this method.