Grant-Free Access in URLLC with Combinatorial Codes and Interference Cancellation

When targeting ultra-reliability for randomly accessing uplink users, an irreducible error floor exists when employing random repetition coding in synchronised frames. Combinatorial Code Designs (CCDs) that produce deterministic access patterns have been shown to overcome this performance limit in MAC layer channel models, where the physical layer has been abstracted to a collision model. In this work, we demonstrate that CCDs remain a promising option for URLLC for a properly modelled physical layer with fading and collision recovery through diversity reception. We investigate uplink users arriving according to a Poisson process, attempting to directly transmit data packets in a grant-free manner. We measure the achievable rate using a system outage capacity formulation which takes into account the access intensity, the size of the data packets transmitted, and the packet error rate, with the packet loss rate constrained to be less than a given URLLC target. We compare random and deterministic access patterns by system simulation in a factory environment. The results show that considerable gains in system throughput can be achieved by using deterministic access patterns. Unlike random patterns, patterns based on CCDs have the potential to support considerable communication rates with very high success probability. With ideal channel estimation, CCDs enable an aggregate system spectral efficiency of 2 bps/Hz in a grant-free URLLC access scenario.