Analysis of Two-Step Random Access Procedure for Cellular Ultra-Reliable Low Latency Communications

Due to the emergence of Internet of Things (IoTs), it can be expected that the bandwidth provided by cellular systems might be consumed up soon. Some applications of them are delay-sensitive such that it would be critical to guarantee random access (RA) delay less than a threshold. Since the existing Long-Term Evolution-Advanced (LTE-A) RA procedure is a four-step signaling procedure, it may not be suitable for such delay-sensitive applications due to its time-consuming procedure. This work investigates a two-step RA procedure for 5G New Radio systems, where RA preamble and bandwidth request message are transmitted at the same time. First we show that the operating region of two-step RA procedure can be divided into three regions such as unsaturated stable, bistable, and saturated regions in terms of a packet generation probability, retransmission probability, the number of devices, and the number of RA preambles. To see whether RA delay requirement of delay-sensitive applications can be guaranteed, this work shows that the system should run under unsaturated region and derives RA delay distribution when IoT devices employ geometric probability backoff (GPB) or uniform window backoff (UWB) algorithm. We then examine the probability that the RA delay would be larger than some threshold depending on the operation regions.