Contents I. Introduction
The Internet of Thing (IoT) plays a critical role in future 6G [1], [2]. In order to support IoT service, multi-tier computing is required [3], [4], [5], [6], [7]. Specifically, multi-tier computing integrates cloud, fog, and edge computing technologies, which can provide flexible computing capability for IoT [3], [4], [5]. Furthermore, one of the key features in 6G is the space-air-ground integrated network (SAGIN) [8], [9], [10]. The SAGIN is a multidimensional heterogeneous network consisting of three network segments, i.e., the space network (SN) composed of satellites, the aerial network composed of high altitude platforms (HAPs) and flying unmanned aerial vehicles (UAVs), and the terrestrial network composed of devices and ground stations [11], [12]. One of the important application scenarios of the SAGIN is to provide multi-tier computing services for IoT devices in remote areas where there is no cellular coverage [2], [13], [14], [15], [16]. Specifically, in the aerial network, the flying UAVs can serve as edge servers to provide edge computing capabilities for ground users [2], [13], while in the SN, satellites can provide access to cloud computing service through seamless network coverage [2], [13]. Hence, the IoT devices in remote areas can offload their missions with computation requirements, such as monitoring and video surveillance, to their nearby UAVs for edge computing or satellites for access to the cloud computing [2], [13], [14], [15].
