THP performance improvement with MAP criterion over MIMO broadcast channels

The Tomlinson-Harashima Precoding (THP) is considered as one of the efficient techniques for achieving near Multiple Input Multiple Output (MIMO) channels capacity with reasonable complexity. However, at low Signal-to-Noise Ratios (SNR), the THP performance is dominated by modulo loss, which is proportional to the constellation points at the boundaries that may be mistaken for symbols at the opposite boundary of the constellation. This paper, addresses modulo loss and proposes using a Maximum A-Posteriori (MAP) detector rather than a modulo detector. A MAP detector recovers part of the modulo loss by optimizing the boundary regions that are considered to be fixed at the modulo receiver. The proposed method is based on a scalar operation rather than a vector operation that is used in typical MAP detectors, and is appropriate for multi-user applications. Moreover, simulation results show that the Bit Error Rate (BER) advantage of the MAP-THP receiver with less complexity compared with that of the modulo receiver.