  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.
