Can Spatial Modulation Outperform Spatial Multiplexing in Practical Band-Limited Coded Systems?

Spatial modulation (SM) is one of the transmission schemes in multiple-input multiple-output (MIMO) systems, which utilizes the antenna indices to carry additional information bits. The most attractive aspect of SM is its higher minimum Euclidean distance property compared to the conventional spatial multiplexing (SMX) under the same transmission rate. It thus leads to better uncoded bit error rate (BER) performance even with lower computational complexity for signal detection. In practice, the use of channel coding is mandatory for reliable communications, but the unique utilization of antenna selection significantly complicates its design. Therefore, the coded error rate performance of SM systems needs to be investigated. Inter-symbol interference (ISI) and out-of-band (OOB) radiation caused by antenna switching is another crucial issue in band-limited wireless communications. In this article, we first review the limitations of SM as practical band-limited coded MIMO systems. We then introduce a new coded modulation by observing SM as SMX with non-binary constellation containing the null signal point (i.e., origin), which we refer to as null-point assisted spatial multiplexing (NSMX). It is demonstrated that NSMX outperforms SM in terms of both coded BER performance and computational complexity for signal detection, even though the resulting transmitted signal constellations of these schemes are identical.