Impact of User Pairing on 5G Non-Orthogonal Multiple Access

Non-orthogonal multiple access (NOMA) represents a paradigm shift from conventional orthogonal multiple access (MA) concepts, and has been recognized as one of the key enabling technologies for 5G systems. In this paper, the imp act of user pairing on the performance of two NOMA systems, NOMA with fixed power allocation (F-NOMA) and cognitive radio inspired NOMA (CR-NOMA), is characterized. For FNOMA, both analytical and numerical results are provided to demonstrate that F-NOMA can offer a larger sum rate than orthogonal MA, and the performance gain of F-NOMA over conventional MA can be further enlarged by selecting users whose channel conditions are more distinctive. For CR-NOMA, the quality of service (QoS) for users with the poorer channe l condition can be guaranteed since the transmit power alloca ted to other users is constrained following the concept of cogniti ve radio networks. Because of this constraint, CR-NOMA has different behavior compared to F-NOMA. For example, for the user with the best channel condition, CR-NOMA prefers to pair it with the user with the second best channel condition, whereas the use r with the worst channel condition is preferred by F-NOMA. I. I NTRODUCTION Multiple access in 5G mobile networks is an emerging research topic, since it is key for the next generation netwo rk to keep pace with the exponential growth of mobile data and multimedia traffic [1] and [2]. Non-orthogonal multiple acc ess (NOMA) has recently received considerable attention as a promising candidate for 5G multiple access [3]–[6]. Partic ularly, NOMA uses the power domain for multiple access, where different users are served at different power levels. The users with better channel conditions employ successive int erference cancellation (SIC) to remove the messages intended for other users before decoding their own [7]. The benefit of usin g NOMA can be illustrated by the following example. Consider that there is a user close to the edge of its cell, denoted by A, whose channel condition is very poor. For conventional MA, an orthogonal bandwidth channel, e.g., a time slot, will be allocated to this user, and the other users cannot use this time slot. The key idea of NOMA is to squeeze another user with better channel condition, denoted by B, into this time slot. SinceA’s channel condition is very poor, the interference fromB will not cause much performance degradation to A, but the overall system throughput can be significantly improved since additional information can be delivered between the base station (BS) andB. The design of NOMA for uplink transmissions has been proposed in [4], and the performance of NOMA with randomly deployed mobile stations has been characterized in [5]. The combination of cooperative diver sity with NOMA has been considered in [8]. Z. Ding and H. V. Poor are with the Department of Electrical En gineering, Princeton University, Princeton, NJ 08544, USA. Z. Ding is a l o with the School of Computing and Communications, Lancaster Univers ity, LA1 4WA, UK. Pingzhi Fan is with the Institute of Mobile Communicatio ns, Southwest Jiaotong University, Chengdu, China. Since multiple users are admitted at the same time, frequency and spreading code, co-channel interference will be strong in NOMA systems, i.e., a NOMA system is interference limited. As a result, it may not be realistic to ask all the users in the system to perform NOMA jointly. A promising alternative is to build a hybrid MA system, in which NOMA is combined with conventional MA. In particular, the users i n the system can be divided into multiple groups, where NOMA is implemented within each group and different groups are allocated with orthogonal bandwidth resources. Obviously the performance of this hybrid MA scheme is very dependent on which users are grouped together, and the aim of this paper is to investigate the effect of this grouping. Particularly , tn this paper, we focus on a downlink communication scenario with one BS and multiple users, where the users are ordered according to their connections to the BS, i.e., the m-th user has them-th worst connection to the BS. Consider that two users, the m-th user and then-th user, are selected for performing NOMA jointly, wherem < n. The impact of user pairing on the performance of NOMA will be characterized in this paper, where two types of NOMA will be considered. One is based on fixed power allocation, termed F-NOMA, and the other is cognitive radio inspired NOMA, termed CR-NOMA. For the F-NOMA scheme, the probability that F-NOMA can achieve a larger sum rate than conventional MA is first studie d, where an exact expression for this probability as well as its high signal-to-noise ratio (SNR) approximation are obtain ed. These developed analytical results demonstrate that it is a lmost certain for F-NOMA to outperform conventional MA, and the channel quality of then-th user is critical to this probability. In addition, the gap between the sum rates achieved by FNOMA and conventional MA is also studied, and it is shown that this gap is determined by how different the two users’ channel conditions are, as initially reported in [8]. For ex ample, if n = M , it is preferable to choose m = 1, i.e., pairing the user with the best channel condition with the user with the worst channel condition. The reason for this phenomenon can be explained as follows. When m is small, them-th user’s channel condition is poor, and the data rate supported by thi s user’s channel is also small. Therefore the spectral efficie ncy of conventional MA is low, since the bandwidth allocated to thi s user cannot be accessed by other users. The use of F-NOMA ensures that then-th user will have access to the resource allocated to them-th user. If(n−m) is small, then-th user’s channel quality is similar to the m-th user’s, and the benefit to use NOMA is limited. But ifn >> m, then-th user can use the bandwidth resource much more efficiently than the m-th user, i.e., a larger (n−m) will result in a larger performance gap between F-NOMA and conventional MA. The key idea of CR-NOMA is to opportunistically serve the n-th user on the condition that the m-th user’s quality of service (QoS) is guaranteed. Particularly the transmit pow er