Analyzing Uplink Outage Probability of Grant-Free Access and Retransmission for URLLC

Grant-free (GF) access is proposed as a feasible and promising technology to meet the stringent latency and reliability requirements. In this paper, we propose a system transmission scheme combining contention-based GF transmission and hybrid automatic retransmission request (HARQ) and analyze the system outage probability for the small packet transmission. Firstly, the system model is established and the transmission latency is analyzed. Based on the path loss model and the flat Rayleigh fading channel model, the signal-to-noise-plus-interference ratio (SINR) is derived for a given transmission power. Next, according to the SINR threshold, the closed expression of the outage probability is derived. Finally, simulation results demonstrate the performance of the derived uplink outage probability.