Efficient User Grouping for Grant-Free URLLC

Ultra-Reliable Low-Latency Communications (URLLC) is a new type of service supported in 5G systems, which imposes very strict requirements on packet delivery time and reliability. Many URLLC applications generate sporadic traffic in the uplink. To satisfy strict URLLC requirements for such a type of traffic, a 5G base station can use the Grant-Free channel access method. According to this method, the base station configures transmission parameters and allocates channel resources for users in advance. Most existing papers analyzing Grant-Free channel access for sporadic traffic consider that all users share common frequency resources. This paper studies whether it is possible to increase the network capacity by dividing users into several groups and allocating separate resources to each group. The open questions are: (i) which criteria should be used to divide users into groups, and (ii) how to allocate resources to different groups. The paper proposes and analyzes two grouping approaches based on path losses between users and the base station. The numerical results obtained with NS-3 show that the proper user grouping can significantly increase the network capacity for URLLC traffic compared with sharing common resources.