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Star-QAM Signaling Constellations for Spatial Modulation

The performance of spatial modulation (SM)-assisted multiple-input-multiple-output (MIMO) communication systems is highly dependent on the specific amplitude/phase modulation (APM) signal constellation adopted. In this paper, we conceive new star-quadrature amplitude modulation (star-QAM)-aided SM schemes. Our goal is to minimize the system's average bit error probability (ABEP). More specifically, a new class of star-QAM constellations is introduced for SM, which is capable of flexibly adapting ring ratios of the amplitude levels. Then, under a specific MIMO configuration and a predetermined transmission rate, a simple and efficient ring-ratio optimization algorithm is proposed to minimize the ABEP. Moreover, to improve further the performance of our star-QAM-aided SM scheme, a diagonal precoding technique is proposed, and a low-complexity minimum-distance-based approach is conceived for extracting the precoding parameters. Our numerical results show that the proposed star-QAM-aided SM arrangement provides beneficial system performance improvements compared with the identical-throughput maximum-minimum distance (MMD) QAM and phase-shift keying (PSK) benchmarkers. Moreover, our precoding scheme is capable of further improving the attainable system performance at a modest feedback requirement.