Signature-based Non-orthogonal Multiple Access (S-NOMA) for Massive Machine-Type Communications in 5G

The problem of providing massive connectivity in Internet-of-Things (IoT) with a limited number of available resources motivates the non-orthogonal multiple access (NOMA) solutions. In this article, we provide a comprehensive review of the signature-based NOMA (S-NOMA) schemes as potential candidates for IoT. The signature in S-NOMA represents the way the data stream of an active device is spread over available resources in a non-orthogonal manner. It can be designed based on device-specific codebook structures, delay patterns, spreading sequences, interleaving patterns, and scrambling sequences. Additionally, we present the detection algorithms employed to decode each device's data from non-orthogonally superimposed signals at the receiver. The bit error rate of different S-NOMA schemes is simulated in impulsive noise environments, which can be important in machine-type communications. Simulation results show that the performance of the S-NOMA schemes degrades under such conditions. Finally, research challenges in S-NOMA oriented IoT are presented.