Joint User Pairing and Subchannel Allocation for Multisubchannel Multiuser Nonorthogonal Multiple Access Systems

In this paper, we investigate the issues of joint user pairing and subchannel allocation (JUP-SA) in a multisubchannel multiuser nonorthogonal multiple access (NOMA) system. We first investigate the uplink and propose a JUP-SA scheme so that the minimum achievable diversity order of users is maximized. By deriving the upper and lower bounds of the outage probability of the worst-performance user, the achievable diversity order of the scheme is obtained. Following this, the downlink of the system is considered and a JUP-SA scheme that maximizes the minimum diversity order is presented. The closed-form expression of the worst-case outage probability is then derived and validated by simulation results. Numerical results show that both schemes in uplink and downlink NOMA systems can achieve the same diversity order as that of exhaustive search.

I. Introduction

In future mobile systems, it is envisioned that traffic demand will be hundreds of folds of the current level, due to the emerging of new multimedia applications such as video streaming and online gaming [1], [2]. It is thus necessary to develop new multiple-access techniques to meet the demand of high spectral efficiency and massive connectivity in the fifth generation (5G) wireless communication systems [3]– [5]. As a promising candidate for 5G networks, non-orthogonal multiple access (NOMA) can further improve the spectral efficiency by accommodating more users in the same resource block as compared with the orthogonal multiple-access (OMA) system. Hence, NOMA has attracted intensive research interests recently [6]–[9]. Unlike conventional OMA schemes, users with different channel conditions can be transmitted at the same time, frequency or code domain, but with different power levels in a NOMA system [10], [11]. For the uplink, user signals with different channel conditions are transmitted in the same resource block, and the multiplexed signal is detected at the base station using the successive interference cancellation (SIC) scheme, where the high-power user signal is detected with a high priority [12], [13]. For the downlink, users are allocated with different power strengths in the same resource block according to certain power allocation algorithm, and the multiplexed signal is transmitted. At the receiver, the desired signal is obtained by SIC through the successive removal of the maximum-power interference signal[14], [15].