NOMA/CA: NOMA-based Random Access with Pattern Detection and Collision Avoidance

Non-orthogonal multiple access (NOMA) is gaining popularity in 5G communication technologies, which meets the requirements of the massive connectivity and the high data rate. Specifically, code domain NOMA (CD-NOMA) allows multiple users to share the same time-frequency resources with different patterns or codebooks for transmission. However, the conventional CD-NOMA needs to allocate a specific code pattern for each user before it can access the channel. In this paper, we propose a novel random access protocol based on CD-NOMA in which no patterns will be pre-allocated and all the users make fair contention to all the sparse patterns, as does in CSMA/CA (Carrier Sensing Multiple Access with Collision Avoidance), a well-known random access scheme used in WLANs. In the so-called NOMA/CA protocol, multiple users access the channel non-orthogonally with different patterns by code domain multiplexing, and avoid further collisions based on proper pattern detection and backoff procedures. The simulation results show that the uplink saturated throughput can be greatly improved and more channel connections can be supported as in conventional NOMA.