Modified Combinatorial Design for Sparse NOMA System

It is known that the Non-Orthogonal Multiple Access (NOMA) system can achieve higher spectral efficiency and massive connectivity. However, due to the linear combination of modulated symbols, the surjective-only mapping is raised with uncertainty and excessive the complexity in decoding process. In this paper, a construction method of a sparse structure for downlink NOMA system based on a modified combinatorial design with balanced incomplete block design (BIBD) is proposed, aiming at building systems with different diversity and more users. Our scheme can provide a one-to-one mapping pattern with a sparser multiple access structure. Then the complexity of the design and multiuser detection (MUD) can be leveraged owing to the distinctive discretizing distribution in reconstructed BIBD. This new scheme can easily transmit a large number of users within this system. Finally, numerical simulations show that this scheme can deploy more than 50 users with relatively low complexity.