Channel Estimation for Non-Ideal OFDM Systems

Studies of channel estimation techniques for OFDM systems often assume idealized conditions such as a cyclic prefix (CP) long enough to absorb the entire channel delay spread, or perfect time and frequency synchronization at the receiver. This paper studies the impact of removing these assumptions. We characterize the mean square error (MSE) of various linear channel estimation methods, and we derive analytical expressions for the BER performance. The derivations are kept generic so that they apply to both time-division and frequency-division multiplexing of the pilot signal and the data subcarriers. The analysis is specialized to the BER performance characterization of OFDM-based UWB systems for different realistic channel estimation methods with residual time and frequency errors. The effect of inter-carrier interference (ICI) on channel estimation is shown to be small. However, inter-symbol interference (ISI) severely impacts the quality of the channel estimates and hence can yield a large (>2dB) data demodulation performance degradation.