Space-Time Coded Generalized Spatial Modulation for Sparse Code Division Multiple Access

Space-time coded generalized spatial modulation-aided sparse code division multiple access (STC/GSM-SCDMA) is proposed, which exploits the two-dimensional transmit diversity potential of both the spatial and of the frequency domain. Hence, it constitutes a promising solution for the pervasive connectivity of devices in next-generation nonorthogonal multiple access (NOMA) systems. More explicitly, our STC/GSM scheme achieves diversity in the spatial-domain, while the sparse signal-spreading action of SCDMA results in frequency-domain (FD) diversity. A single-user bit error rate (BER) bound is derived as the benchmark of the BER performance of our STC/GSM-SCDMA system. Furthermore, a pair of novel detectors, namely a bespoke message passing aided (MPA) detector and a tailor-made approximate message passing (AMP) detector are conceived by designing a new factor graphs for our proposed STC/GSM-SCDMA system. The performance of these detectors is characterized in terms of their BER vs. complexity. Our simulation results show that the proposed AMP detector is capable of operating within 2 dB of the MPA detector's signal-to-noise ratio (SNR) requirement, while supporting a normalized user load of 150%, despite its appealing low complexity, which is about 1000 times lower than the MPA detector.