Performance Analysis of Repetition-based Grant-free Access for URLLC

In the context of ultra-reliable low latency communication (URLLC) becoming the dominant demand of wireless communication, the method of enhancing the reliability of Grant-free (GF) transmission technology through hybrid automatic retransmission request (HARQ) needs to be further improved. In this paper, a GF-HARQ transmission scheme combined with repetitions is proposed and the outage probability of the system under sporadic small packets arrive is studied. Firstly, we establish the system model and analyze the transmission delay of a HARQ round trip according to the transmission principle. Then, combined with the delay constraint of QoS delivery under URLLC short packet transmission, the maximum transmission number of times M is obtained. Next, the system outage probability is deduced based on random geometry and probability theory. Finally, we verify the accuracy of the outage probability by simulation and compare the performance with the GF-HARQ scheme. Our results show that the proposed scheme provides higher reliability.