Low-Complexity Turbo Block Decision Feedback Equalization for MIMO Systems

This paper proposes a low-complexity turbo block decision feedback equalizer (BDFE) with reliability-based successive soft interference cancellation (SSIC) for multiple-input multiple-output (MIMO) systems. Turbo equalization in MIMO systems needs to combat both intersymbol interference (ISI) due to frequency-selective fading and multiplexing interference (MI) among data streams transmitted by different antennas. The proposed MIMO BDFE performs SSIC on both ISI in the time domain and MI in the space domain. For an SSIC receiver, the order in which the symbols are detected and canceled is critical to the system performance, yet it is often overlooked in the literature. We propose a new reliability-based detection ordering scheme, where symbols with higher a priori reliability are detected before those with lower a priori reliability. The reliability information can be obtained from the a priori probability at the input of the soft-decision equalizer. The reliability-based ordering scheme is enabled by the turbo equalization structure, and it yields an extra equalization gain that is unavailable in a conventional equalization system. Furthermore, the adoption of non-linear BDFE in MIMO systems enables a low-complexity sequence-based log-likelihood ratio (LLR) evaluation, which is superior to the symbol-based LLR evaluation method employed by most other low-complexity turbo equalizers.