The Effect of User Pairing Strategies on the Downlink Throughput of NOMA

The ability of serving many users with high- speed transmission has made Non-Orthogonal Multiple Access (NOMA) as one of the promising candidates of future generation of a wireless communication system. In order to take the best out of NOMA, two users with more distinctive channel conditions are paired together where user with strong (near/cell centred) channel condition is paired with the weak (far/cell edge) one. However, pairing the near-far users will leave the middle users to be paired among them leading to the possibilities of near-zero channel condition. Thus, this paper has proposed two user pairing strategies and analysed the performance to determine which strategy could overcome the issue. The first strategy is to pair the weakest user with the strongest one while, the second strategy is to pair the weakest user with the next strongest. Several path loss (PL) values and subcarriers allocation has been configured during the simulation set-up. To assure the Quality of Service (QoS), the metric of throughput is evaluated. The performance of the user with strong channel condition outperformed the users with weak channel condition by 82%. The lower the PL values, the higher the throughput will be and vice versa. Meanwhile, all users show similar throughput values even both strategies have been implemented. In addition, when it comes to unequal subcarriers allocation, higher throughput value is observed when the user is allocated with more subcarriers at high PL value. Thus it can also be concluded that the performance of throughput is affected by the PL values and the allocation of the subcarriers. This shows the importance of an efficient subcarriers allocation in order to improve the cell edge user performance. For future work, it is recommended to propose a new pairing strategy that can balance the performance between the cell edge and cell centred users.