On the use of Multiple Grants for Contention-Based Uplink Transmissions in 5G NR

Employing Contention-Based (CB) Uplink (UL) transmissions is a potential way to reduce the initial access latency of mobile radio networks, as User Equipments (UEs) can transmit without the need of explicitly asking the network for a dedicated resource allocation. Many contributions have addressed this topic over the years with the main objective of limiting collisions. However, all these works neglect the implications of being compliant with the grant-based structure typical of mobile radio networks, like 5G NR, where UEs may perform padding to fully occupy the radio resources of the grant and this clearly impacts the performance of CB strategies. This paper then leverages ad-hoc Monte-Carlo simulations to assess (i) whether or not is convenient to divide the radio resources into multiple grants, (ii) how to do the split, and (iii) the rules with which UEs should exploit such grants. The performance analysis quantifies the average collision probability, average message success probability, and average waiting time as a function of the number of UEs, split techniques, number of retransmissions, and UL transmission rules. Numerical results show the beneficial effect of adopting multiple grants and reveal a tradeoff between collision probability and waiting time that affects the choice of the best-performing CB design for 5G NR.