LAUSR

This paper investigates an uplink grant-free non-orthogonal multiple access (NOMA) system with a multiple-antenna base station (BS), in which each user autonomously transmits its own data by multiplying its unique but non-orthogonal spreading sequence, namely code-domain NOMA, and proposes two receivers efficiently performing active user detection (AUD), channel estimation (CE), and data detection, simultaneously. We first propose a receiver based on multiple measurement vector approximate message passing (MMV-AMP), and the threshold for AUD is theoretically analyzed. Numerical results show that the symbol error rate (SER) of our receiver with the designed threshold is comparable to that of grant NOMA in which active users are ideally known. To reduce the complexity, we further propose a receiver based on Boosted AMP, which converts the original multiple-vector reconstruction problem into a smaller-sized vector reconstruction problem. Upon AUD via Boosted AMP, this receiver performs both CE and data detection via well-known linear minimum mean squared error (MMSE). Computer simulations reveal that our low-complexity receiver exhibits SER performance comparable to that of a conventional scheme based on block sparse adaptive subspace pursuit (BSASP), while the complexity is significantly reduced.