I. Introduction
Ultra-reliable low-latency communication (URLLC) is one of the three main service categories that have been defined in 5G, with the other two being enhanced mobile broadband and massive machine-type communication [1]. URLLC provides communication support with stringent constraints on reliability and end-to-end latency and has attracted extensive attention and significant research interest, since information transmission with low-latency and high reliability is crucial for enabling various mission-critical services, such as machine-to-machine communication, remote surgery, augmented reality, vehicle automation, industrial robotics, factory automation, and smart-grid [2].