Deterministic Patterns for Multiple Access in Latency-Constrained Ultra-Reliable Communications

The grant-free access is envisioned as one of the enablers of the ultra-reliable low-latency communications. Yet, when there are many devices that tend to be active only intermittently, the fully orthogonal resource allocation is largely inefficient. The solution is to employ a common, shared pool of resources and account for the fact that some collisions and interference will inevitably occur. In this contribution we study the reliability aspects of such multi-user uplink communication scenario over a shared pool of channel resources, where intermittently active devices utilize multiple transmissions (K-repetition coding) to achieve diversity. We focus on two access methods -- one where devices choose the K slots at random and one where the access patterns are deterministic and follow a specific code design, namely the Steiner System. We analyze the problem under two signal models that involve different complexity for the receiver. Firstly, a model which treats collisions as destructive, i.e. only those K' among K transmissions that do not contain interference can be used and combined. Second, where receiver is capable of utilizing all K replicas and applies maximum ratio combining (MRC) treating interference as noise. Furthermore, in both cases we investigate the receiver with and without successive interference cancellation (SIC) capabilities. As one of the main contributions of this work, we develop useful approximations and bounds for the outage probabilities in the aforementioned scenarios that match very closely the simulation results. We also show that deterministic patterns have the potential to significantly outperform fully random selection, both in terms of raw performance and by simplifying the system design.