I. Introduction
To improve the spectral efficiency (SE) for the 5th generation (5G) mobile communication systems, non-orthogonal multiple access (NOMA) has been proposed [1], in which the receiver separates the super-imposed signals via successive interference cancellation (SIC) technique. When it is used for the downlink, a base station (BS) constructs the super-imposed signal for a group of users in the same radio resource and allocates different transmission powers to each user equipment (UE). The multiplexed signal experiences the same (small-scale) fading and path-loss collectively over the downlink, and then it can be successfully separated by each UE with SIC technique if the BS properly allocates different levels of powers to the UEs. For the uplink, in contrast, the BS receives the super-imposed signals from different UEs, each of which may experience independent fading and path-loss due to their different locations.