Equalizer based on the BCJR algorithm for underwater acoustic channels in OFDM systems

In an acoustic communication system that employs orthogonal frequency division multiplexing (OFDM), the presence of considerable Doppler spread destroys the frequency orthogonality of OFDM, and thus inter-carrier interference (ICI) is introduced, which increases the bit error rate (BER). This work proposes a channel equalizer based on the BCJR algorithm (BCJRE) for mitigating ICI. Its performance is compared against a maximum likelihood equalizer (IMLE) previously proposed on the bibliography under identical channel and system parameters. The simulations show that the BCJRE consistently outperforms the IMLE. For channels with little Doppler spread, the gain is about 0.25 dB, and this gain increased to 6 dB when there is severe Doppler spread. Using turbo iterations, the performance of the equalizer can be improved between 0.5 and 2 dB. In addition, the behavior of the equalizers with imperfect channel information is explored, and it is shown that the BCJRE is more robust against channel estimation noise.