  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 grantbased (GB)
(i.e., dynamic grant) random access, which can hardly support the URLLC
requirements. Grant-free (GF) (i.e., configured grant) access is proposed as a
feasible and promising technology to meet such requirements, especially for
uplink transmissions, which effectively saves the time of requesting/waiting
for a grant. While some basic GF access features have been proposed and
standardized in NR Release-15, there is still much space to improve. Being
proposed as 3GPP study items, three GF access schemes with Hybrid Automatic
Repeat reQuest (HARQ) retransmissions including Reactive, K-repetition, and
Proactive, are analyzed in this paper. Specifically, we present a
spatiotemporal analytical framework for the contention-based GF access
analysis. Based on this framework, we define the latent access failure
probability to characterize URLLC reliability and latency performances. We
propose a tractable approach to derive and analyze the latent access failure
probability of the typical UE under three GF HARQ schemes. Our results show
that under shorter latency constraints, the Proactive scheme provides the
lowest latent access failure probability, whereas, under longer latency
constraints, the K-repetition scheme achieves the lowest latent access failure
probability, which depends on K. If K is overestimated, the Proactive scheme
provides lower latent access failure probability than the K-repetition scheme.

Signal Processing;Networking and Internet Architecture