Reliability analysis of Uplink Grant-Free transmission over Shared Resources

OAPA Uplink grant-free schemes have the promise of reducing the latency of an User Equipment (UE)-initiated transmission by avoiding the hand-shaking procedure for acquiring a dedicated scheduling grant. However, the possibility of successfully delivering a payload within a latency constraint may be severely compromised in case of grant-free operations over shared radio resources. In this paper, we study the performance of two different uplink grant-free schemes over shared resources recently discussed within the 5th Generation (5G) new radio (NR) standardization, namely a solution based on a Stop-and- Wait (SAW) protocol and a blind retransmission approach. Performance is evaluated assuming Rayleigh fading channels with a Maximum Ratio Combining (MRC) multi-antenna receiver. Analytical results show the benefits of grant-free transmission with respect to the traditional grant-based approach for a tight latency constraint. A high order receive diversity is beneficial to leverage the MRC gain and enables the possibility of achieving the 10 & #x2013;5 outage probability target set for Ultra-Reliable Low- Latency Communication (URLLC) services. The blind retransmission approach is significantly penalized by identification and signaling errors, while a SAW solution with potentially scheduled retransmissions out of the shared bandwidth leads to the lowest outage probability, at least for frequent packet arrivals.