Latticing the interference: Non-linear pre-coding for non-orthogonal multiple access in downlink multi-user MIMO system

Spectrally efficient multiple access technologies, such as Multi-User Multiple-Input-Multiple-Output (MU-MIMO) system and Non-Orthogonal Multiple Access (NOMA), are the key enablers for the next generation communication systems to accommodate tremendous devices. Combining NOMA in MU-MIMO systems (NOMA-MIMO) is expected to boost the spectral efficiency even further. However, in order to solve the Inter-Antenna-Interference (IAI) and the Inter-Cluster-Interference (ICI) resulted from multiple-antenna and non-orthogonal transmissions, computation-intensive interference cancellation schemes are usually required in the receivers, which prohibit the employment of NOMA-MIMO system in systems with simple receivers, such as sensor networks. In this work, we propose a non-linear pre-coding method for two-cluster NOMA-MIMO systems that allows simple receiver implementation with only one antenna. Our main contribution is twofold: first, we adopt IAI-oriented pre-coding to spare the receiver from IAI cancellation; second, we propose to regulate the conventionally unsolvable IAI-corrupted ICI onto lattice points, so that the ICI can be removed by simple modulo operations instead of computational-intensive Successive Interference Cancellation (SIC) schemes. The ergodic sum capacity and whole system bit-error-rate (BER) performance are provided to show the validity of the proposed approach.