Pilot domain NOMA for grant-free massive random access in massive MIMO marine communication system

We propose a pilot domain non-orthogonal multiple access (NOMA) for uplink massive devices grant-free random access scenarios in massive multiple-input multiple-output (MIMO) maritime communication systems. These scenarios are characterized by numerous devices with sporadic access behavior, and therefore only a subset of them are active. Due to massive potential devices in the network, it is infeasible to assign a unique orthogonal pilot to each device in advance. In such scenarios, pilot decontamination is a crucial problem. In this paper, the devices are randomly assigned non-orthogonal pilots which are constructed by a linear combination of some orthogonal pilots. We show that a bipartite graph can conveniently describe the interference cancellation (IC) processes of pilot decontamination. High spectrum efficiency (SE) and low outage probability can be obtained by selecting the numbers of orthogonal pilots according to the given probability distribution. Numerical evaluations show that the proposed pilot domain NOMA decreases the outage probability from 20% to 2e-12 at the SE of 4 bits/s/Hz for a single device, compared to the conventional method of slotted ALOHA with 1024 antennas at the BS, or increases the spectrum efficiency from 1.2 bits/s/Hz to 4 bit/s/Hz at the outage probability of 2e-12 in contrast with the Welch bound equality (WBE) non-orthogonal pilots.