Semi-Grant-Free Uplink NOMA with Contention Control: A Stochastic Geometry Model

Grant-free (GF) transmission holds promise in terms of low latency communication by directly transmitting messages without waiting for any permissions. However, collision situations may frequently happen when limited spectrum is occupied by numerous GF users. The non-orthogonal multiple access (NOMA) technique can be a promising solution to achieve massive connectivity and fewer collisions by multiplexing users in power domain. We utilize a semi-grant-free (semi-GF) NOMA scheme for enhancing network performance by enabling grant-based (GB) and GF users to share the same spectrum resources. With the aid of semi-GF protocols, uplink NOMA networks are investigated by invoking stochastic geometry techniques. We utilize open-loop protocol to interpret which part of the GF users are paired in NOMA transmissions. As a further advance, we propose a novel dynamic protocol, which provides more accurate access thresholds than open-loop protocol, thereby the interference from the GF users is reduced to a large extent. We analyze the outage performance and diversity gains. Numerical results demonstrate that dynamic protocol is capable of enhancing the outage performance comparing to open-loop protocol.