Expanded Soft Demapper for LDPC Coded GMD-THP MIMO System

Tomlinson-Harashima precoding (THP) can be combined with geometric mean decomposition (GMD) to decouple a multiple input multiple output (MIMO) channel into multiple subchannels with identical signal-to-noise ratio (SNR). The combined system is called GMD-THP MIMO system. As the subchannels have identical SNR, it is more convenient for GMD-THP MIMO systems to design modulation/demodulation and coding/decoding schemes than singular value decomposition (SVD) MIMO systems. In this paper, we consider low-density parity-check (LDPC) coded GMD-THP MIMO system. Since the modulo operation at the receiver may generate modulo errors, the log likelihood ratio (LLR) values provided by conventional soft demapper can be very inaccurate. To overcome this problem, we propose an expanded soft demapper scheme to obtain better LLR values, by which significant performance improvement can be achieved for LDPC decoding as compared to conventional soft demapper. Computer simulations are presented to show that the expanded soft demapper indeed improves the error rate performance of the LDPC coded GMD-THP systems and reduces the noise floor caused by the modulo errors