Bidirectional Soft-Decision Feedback Turbo Equalization for MIMO Systems

This paper proposes a bidirectional soft-decision feedback turbo equalizer for severe triply selective fading channels. The proposed bidirectional soft-decision feedback equalizer (Bi-SDFE) uses a time reversed SDFE in conjunction with a normal SDFE to harvest time-reverse diversity and mitigate error propagation. A simple and effective linear combining scheme is derived for combining the extrinsic information on the two SDFEs for multiple-input-multiple-output (MIMO) systems with multilevel modulation. The bit-error-rate (BER) simulation results show that the proposed Bi-SDFE exhibits a lower signal-to-noise-ratio (SNR) threshold than the original SDFE and outperforms the well-known exact turbo minimum mean square error (MMSE) linear equalizers (LEs) at medium-to-high SNRs. Moreover, both extrinsic information transfer (EXIT) chart analysis and BER simulation results show that the Bi-SDFE achieves better performance than the original SDFE, whereas the Bi-SDFE uses only half the number of iterations of the SDFE. Therefore, the total computational complexity of the Bi-SDFE is similar to that of the original SDFE, which is a linear function of the channel length, MIMO size, and modulation constellation size. The performance gain of the proposed Bi-SDFE is also verified through an underwater acoustic (UWA) communication experiment with tough channel conditions.