Pilot Sequences Aided Spatial Modulation for Grant-Free Multiple Access Systems

Currently, grant-free multiple-access (GFMA) has been paid much attention, especially for a random access fading channel. This letter presents pilot sequences aided spatial modulation (SM) for GFMA systems, which can take advantages of spatial information and achieve well-behaved bit error rate (BER) performance with low detection complexity. We propose a sphere decoding (SD) based active user detection (AUD) algorithm, and then an interference cancellation with equalization (IC-EQ) multiuser detection (MUD) algorithm. Besides, we deduce the optimal threshold of the AUD algorithm, and investigate the complexity of the proposed algorithms. Simulation results show that our proposed system can simultaneously support 5 users, and the average BER is about 10−4, when E_{b}/N_{0} = 20 dB. At this moment, it has only 0.23 dB gap away from the optimal maximum likelihood detector (MLD), and the complexity reduces up to 71%, revealing the validity of our proposed schemes.