Impact of Decoding Order on User Pairing in NOMA-CoMP Joint Transmission

This correspondence investigates the optimal decoding order and its impact on user pairing in non-orthogonal multiple access (NOMA)-enabled coordinated multipoint (CoMP) joint transmission (JT) to maximize the sum rate of users. First, the optimal solution of power control for a user pair is obtained under different decoding orders, where closed-form expressions and a low-complexity algorithm are given by monotonic optimization. Then, a criterion for finding the optimal decoding order is derived for any given user pair in the considered scenario. Results show that the optimal decoding order is distinct for different user pairs, which can be achieved separately. This indicates that the decoding order is not a necessary prerequisite for user pairing in NOMA-enabled CoMP systems. Moreover, the optimal decoding order is shown to depend on the differentials of channel gains between the user served by JT and coordinated base stations.

I. Introduction

Non-orthogonal multiple access (NOMA) is recently integrated into coordinated multipoint (CoMP) systems to improve spectrum efficiency while alleviating severe interference in future ultra dense networks [1]. In a NOMA-enabled CoMP (NOMA-CoMP) system, the base station (BS) cluster performs joint transmission (JT) for one user meanwhile each coordinated BS in the cluster can use NOMA to associate with other users located in coverage on the same spectrum resources. It is shown in the literature that the decoding order of users has significant impacts on the transmission efficiency of NOMA-CoMP systems [2]. In general, the decoding order can be simplified to be determined by the channel gains between the BS and its associated users [3]. However, since JT expands the user association from single BS to multiple BSs, the traditional decoding order based on channel condition with single BS would be inappropriate for NOMA-CoMP systems. Thus, it is vital for NOMA-CoMP systems to explore suitable decoding order.