An adaptive two-dimensional non-orthogonal multiple access technique using multi-level modulation and interleaving

Non-orthogonal multiple access (NOMA) is a promising technique that can dramatically increase radio resource utilization efficiency and network capacity of 5G networks. However, due to the non-orthogonal resource sharing among concurrent users, deployment of NOMA relies on the effective cancellation of inter-user interference and recovery of superimposed signals from multiple users. To mitigate the interference and increase the Euclidean distance of superimposed signals, a two-dimensional NOMA (2D-NOMA) scheme is proposed in this paper by collaborative design of power-domain and code-domain NOMA. In the power domain, different power ratios are allocated to multiple users according to their channel qualities to realize multi-level modulation. In the code domain, sparse spreading codes superimpose only a subset of users on an identical resource block. Since the channel quality of each NOMA user could change during the power allocation, the quadrature components of multiple signals are further interleaved to overcome the deep fading, in which the interleaving depth is adapted based on the channel fading characteristic. In addition, a two-dimensional non-orthogonal code sequence that can maximize the network throughput and energy efficiency under throughput requirements and power constraints is also designed for the proposed 2D-NOMA system. Simulation results confirm that the proposed 2D-NOMA can improve the throughput rate by using the designed code sequence with appropriate power allocation.