Analyzing Grant-Free Access for URLLC Service

5G New Radio (NR) is expected to support new ultra-reliable low-latency communication (URLLC) service targeting at supporting the small packets transmissions with very stringent latency and reliability requirements. Current Long Term Evolution (LTE) system has been designed based on grant-based (GB) (i.e., dynamic grant) random access, which can hardly URLLC Grant-free (GF) configured access proposed as a feasible promising technology meet such requirements, especially for uplink transmissions, effectively saves time of requesting/waiting grant. While some basic GF features have standardized in NR Release-15, there still much space improve. Being 3GPP study items, three schemes Hybrid Automatic Repeat reQuest (HARQ) retransmissions including Reactive, K-repetition, Proactive, are analyzed this article. Specifically, we present spatio-temporal analytical framework contention-based analysis. Based framework, define latent failure probability characterize performances. We propose tractable approach derive analyze typical UE under HARQ schemes. Our results show that shorter constraints, Proactive scheme provides lowest probability, whereas, longer K-repetition achieves depends K. If K overestimated, lower than scheme.