I. Introduction
Emerging technologies are being developed to improve the wireless communication performances in the fifth-generation (5G) mobile network and beyond [7], [8], [9], [10], [11], [12], [13], [14]. There are two main technical directions for the 5G and beyond. One realistic direction is continually pursuing extremely high data rate network access with super high-frequency bands, such as -band and -band [15], [16], [17]. Another promising direction is non-terrestrial networks with extreme coverage extension, and low energy and cost [10], [18], [19], [20], [21], [22]. This article mainly focuses on the direction of wide coverage and low-energy non-terrestrial networks for the small satellite communication system.