Low-complexity time-domain channel estimators for mobile wireless OFDM systems

This paper proposes low-complexity time-domain channel estimators for OFDM systems. Time-domain channel estimation combined with frequency domain equalization is an attractive approach, since it can provide lower complexity and better estimator performance than pure frequency domain processing. In the particular case of constant modulus (M-PSK) modulation, we show that the transmission matrix is unitary. Therefore, the simple correlator channel estimator turns out to be optimal. For the case of non-constant envelope the transmission matrix is not unitary any more, but its corresponding correlation matrix is strongly diagonal. Therefore, a low-complexity approximate LS solution is proposed which avoids matrix inversion. A study of decision-feedback errors is performed, which suggest that iterative re-decoding of the data further improves the channel estimates. Simulations carried out for a wide range of mobile speeds demonstrate that the proposed estimators, using only 2% of pilot data, outperform recently proposed [T. Roman et al, 2003] time-and frequency-domain approaches.