Abstract

The increasingly ubiquitous applications of Ultra-Reliable Low-Latency Communications (URLLC) require innovative solutions that can only be achieved through a flexible communication system such as the The Fifth Generation (5G) New Radio (NR). Recent studies on the resource allocation for URLLC have proposed the Grant-Free (GF) scheduling instead of the traditional high latency Grant-Based (GB) scheduling, adopted in 4G Long Term Evolution (LTE). Although the GF scheduling over shared resources offers reduced latency, the possibility of achieving the reliability requirement of URLLC may be compromised due to the increased likelihood of collisions. Therefore, we propose a solution for the uplink transmissions that is capable of realizing the reliability requirement in compliance with URLLC’s stringent latency budget.
The main strategy of the proposed solution is to transmit multiple uplink copies of the same packet, utilizing both dedicated and shared resources. In order to avoid additional delays, retransmissions are carried out independent of the conventional feedback from the Base Station (BS). Therefore, each packet is transmitted a pre-determined number of times, resulting in a fixed latency value for packets in the network. The network considered in this study consists of users with both periodic and sporadic traffic. Users in the network are grouped into classes according to their packet generation probabilities. Classes with high packet generation rates are characterized as periodic-traffic classes, while sporadic-traffic classes have low generation rates.
Users gain access to the available resources in the network via three different scheduling schemes. While all users access shared resources through GF scheduling, access to dedicated resources is done in two different ways, namely, Periodic Scheduling (PS) and GB scheduling. To avoid under-utilization of resources, the PS scheme is only assigned for users with high packet generation rates, while sporadic-type users access dedicated resources through the GB scheme. Although recent studies were disinclined towards the GB scheme due to its high latency, we show that the exploitation of 5G NR’s new scalable numerology results in significant reductions to GB’s latency, making it suitable for the URLLC use case. Following this latency examination, we present probabilistic expressions representing the reliability of our proposed solution.
The main contribution of this thesis to the available literature of URLLC is the presented system optimization. We optimize the system’s performance in terms of minimizing the required bandwidth or maximizing the supported traffic capacity, while satisfying the reliability requirements. Optimal performance of the system is achieved through determining the optimal allocation of resources between the considered scheduling schemes, as well as the optimal classification of user classes in the network as periodic-type or sporadic-type classes. In addition, we find the optimal packet length (for a fixed number of information bits) that results in the minimum amount of bandwidth required.