Minimum mean square error turbo equalization of doubly selective channels using the BEM

In this paper, linear minimum mean square error (LMMSE) turbo equalization for transmission over doubly selective channels is proposed for single carrier (SC) as well as for multi-carrier (MC) systems. The complex exponential basis expansion model (CE-BEM) is used to approximate the time-varying multi-path fading channel. In this BEM, the time-varying channel is viewed as a bank of time-invariant FIR filters and the time-variation is captured by means of time-varying complex exponential basis functions. Based on the Gaussian approximation, exact expressions for the a-priori, a-posteriori and extrinsic information are derived for QPSK modulation. The MC system is implemented using orthogonal frequency-division multiplexing (OFDM). The performance of the proposed turbo equalization schemes are evaluated using numerical simulations.