I. Introduction
Future 6th Generation (6G) mobile communication systems are expected to possess a sensing capability to enable various connected service applications [2], such as unmanned aerial vehicles (UAVs) and intelligent automobiles [3]. Such applications require larger amounts of spectrum, which makes it unaffordable to assign independent bands to the radio-frequency (RF) systems. Therefore, integrated sensing and communications (ISAC), as a technology with improved spectrum efficiency, lower power consumption and reduced cost, will play a crucial role in 6G and beyond [4].