On the design and performance of scheduling policies exploiting spatial diversity for URLLC

In this paper, we study the performance of packet scheduling schemes for Ultra-Reliable Low-Latency Communications (URLLC) services. We exploit spatial diversity (i.e., redundant coverage of users) guaranteed in numerous 5G radio access network scenarios to examine the impact of multi-connectivity. Thus, we consider a set of URLLC users connected to two frequency layers or Radio Access Technologies (RATs) to ensure minimal queuing time. We review four packet scheduling and redundancy schemes, namely Join-the-Shortest-Queue (JSQ), the shortest expected delay (SED), systematic Redundancy (RED), and redundancy with Cancellation upon completion (CAN). We choose the outage probability as a metric, defined as the packet’s probability of arriving after some given target delay. We show that RED performs well at low load, whereas JSQ and SED are better when the load rises. Besides, CAN outperforms all other schemes. We then discuss the trade-off between performance and implementation complexity.