Low Complexity Maximum-Likelihood Based QRD-M for MIMO Systems with S-QAM or APSK

In this work, we propose a rotation-based transformation method and apply it to the QRD-M detector for MIMO systems with the commonly used square quadrature amplitude modulation (S-QAM) or amplitude and phase-shift keying (APSK). The QRD-M detector based on proposed rotation transformation principle method achieves 50% computation reduction for MIMO systems with 4-QAM compared with the original QRDM algorithm (25% for those with 16-QAM) without sacrificing any performance. The proposed rotated QRD-M method achieves performance close to the maximum-likelihood (ML) detector with computational complexity lower than the linear detector for MIMO systems with 4-QAM. The underlying principle of the proposed rotation-based transformation method can be extended to S-QAM (or APSK) modulated MIMO systems with employing sphere decoding (SD), list sphere decoding (LSD), list sequential sphere decoder (LISS) (or other quasi-ML methods), and the full ML based methods in a straightforward manner.