I. Introduction
With the rapid growth of data traffic, the limited resources of spectrum and energy have brought great challenges. Especially, with the development of internet-of-things (IOT), the 5th generation mobile communication (5G) is aiming to serve the massive terminals to share the same resources (e.g. spectrum, power, space) at the same time. To achieve higher spectral efficiency and higher capacity for 5G cellular networks, non-orthogonal multiple access (NOMA) has been recognized as one of the key radio access techniques [1].