Signature-Based Nonorthogonal Massive Multiple Access for Future Wireless Networks: Uplink Massive Connectivity for Machine-Type Communications

The problem of providing massive connectivity in the Internet of Things (IoT) with a limited number of available resources motivates nonorthogonal 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 nonorthogonal 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 nonorthogonally superimposed signals at the receiver. The bit error rate (BER) of different S-NOMA schemes is simulated in impulsive noise environments, which can be important in machine-type communications (MTCs). 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.