Kronecker-Based Non-Orthogonal Sequences of Small Phases for Uplink Grant-Free Access Using the Genetic Algorithm

In massive machine-type communications (mMTC), grant-free access is a key enabler for a massive number of users to be connected to a base station with low signaling overhead and low latency. In this letter, we present a set of Kronecker-based non-orthogonal sequences and study the performance of uplink grant-free access using the sequences. The user-specific, non-orthogonal, unimodular sequences of small phases are designed by subsampling the Kronecker product of Hadamard and Fourier matrices, where we use the genetic algorithm (GA) to find the subsampling index set. Simulation results demonstrate that the Kronecker-based non-orthogonal sequences exhibit excellent performance for compressed sensing based joint activity detection and channel estimation in uplink grant-free access. Consequently, the Kronecker-based sequences can be a good option for mMTC, thanks to the superior performance and the practical benefits of arbitrary sequence length and small phases.