I. Introduction
Internet of Things (IoT) aims to connect everything at any time and place, where the user terminals include smart home, wearable devices, unmanned aerial vehicles, high speed-trains, low-Earth-orbit satellites, and so on [1]–[4]. As a result, the wireless channels in future IoT may varied from static channel to high-mobility channel, or from time-invariant channel to fast time-variant channel, where reliable communication becomes challenging. Accordingly, designing efficient air-interface techniques plays a vital role for future IoT.