On the Banded Approximation of the Channel Matrix for Mobile OFDM Systems

In mobile orthogonal frequency-division multiplexing (OFDM) systems, a frequency-domain channel matrix represents the same-carrier channel frequency response in the diagonal and intercarrier interference (ICI) between the subcarriers in the off-diagonals, respectively. A variety of the banded equalizers manipulated the banded approximation of the channel matrix to be exploited by the low-complexity equalizations. In this paper, we derive a simple and tight lower bound on the variance of the individual coefficients in the channel matrix for insights into the banded approximations. We obtain the errors introduced with the banded approximation and the ICI-mitigation gains of the banded equalizers in simple closed forms. The derivations of the banded approximation errors (BAEs) are beneficially applicable to the equalizers that perform the minimum mean square error (MMSE) estimation with the banded channel matrix. Simulations show that both the block MMSE banded equalizers and the block turbo MMSE banded equalizers significantly reduce the error floors by considering the BAEs.