User pairing schemes for capacity maximization in non‐orthogonal multiple access systems

This paper addresses the issues related with conventional near–far user pairing in non‐orthogonal multiple access. Performance effects of near–far pairing on regions with negligible channel gain differences between users are investigated. These regions occur when pairing is performed between cell center and cell edge users, thus leaving the cell mid users to be either paired with each other or kept unpaired. Pairing these mid users with each other causes successive interference cancelation (SIC) performance degradation resulting in capacity reduction for these users. On the other hand, leaving these mid users unpaired perfectly avoids the SIC issue but makes these users unable to benefit from the capacity gains provided by non‐orthogonal multiple access. Therefore, two user pairing strategies have been proposed that can provide capacity gains to almost all the users by accommodating them in pairs, while avoiding or minimizing the mid users pairing problem. A generalized M‐users pairing scheme is also proposed. Simulations have been performed to investigate the performance of proposed schemes for both perfect and imperfect SIC receiver scenarios in comparison with conventional pairing where the mid users are kept paired with each other. Simulation results show that proposed schemes achieve high capacity gains, especially when imperfect SIC is considered. Copyright © 2016 John Wiley & Sons, Ltd. This paper addresses the mid users pairing problem caused by conventional near‐far user pairing in non‐orthogonal multiple access (NOMA). Two strategies for user pairing are proposed that can provide capacity gains to almost all the users by accommodating them efficiently in pairs such that the mid users pairing problem is also minimized/avoided. It is shown analytically and through simulations that the proposed schemes outperform conventional near‐far pairing, especially when imperfect successive interference cancellation (SIC) receivers are considered.