A BCJR-based turbo equalizer for multicarrier time-varying fading channels

Abstract

A cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) system that transmits information over a time-varying fading channel is considered. In channels with large Doppler spreads, the frequency orthogonality assumption of OFDM is not valid. Hence, inter-carrier interference (ICI) is introduced, which leads to higher bit error rates (BERs). We propose a channel equalizer based on the BCJR algorithm (BCJRE) for mitigating ICI. Its performance is contrasted against a previously proposed approximate iterative maximum likelihood equalizer (IMLE) under a range of channel and system parameters. Our simulation results show that the BCJRE consistently outperforms the IMLE. For slow-varying channels with perfect channel state information at the receiver (CSIR), the gain is about 0.25 dB, while for fast-varying channels, it is 6 dB, revealing that the BCJRE is much more robust in high-ICI scenarios due to Doppler diversity. By using turbo iterations, equalizer performance can be further improved between 0.5 and 2 dB. We also evaluate the performance of the BCJR and the IMLE with noisy channel estimates at the receiver, and show that the BCJRE is more robust to channel estimation noise.