Sparse Multiuser Signatures for Massive Grant-Free Transmissions

In this paper, we propose a design of sparse multiuser signatures to increase the capacity of grant-free high-overloading transmissions for massive machine-type-communication (mMTC). The proposed sparse signature is composed of a short spreading sequence and a sparse pattern. The design not only lowers the correlation between the signatures but also brings in additional randomness and sparsity to the sequences. Therefore, it benefits from less-interfered symbol streams to present geometric characteristics of the modulation constellation at the receiving end, which enables blind multiuser detection even if the number of active users is large. The design method can also be applied to highly-correlated spreading sequences, and the correlation is lowered and more users can be supported. Numerical results are provided in terms of block error rate, which demonstrates that the proposed sparse signature has a similar error performance to the optimized schemes while the number of active users is increased sianificantly.