Pairing Distance Resolution and Power Control for Massive Connectivity Improvement in NOMA Systems

One of the main objectives of 5G is to ensure very high spectral efficiency. Non-Orthogonal Multiple Access (NOMA) is one of the promising techniques to address this issue. Generally, NOMA uses criteria based on distance difference between users or power allocation to pair the users. However, when the distance difference becomes small, the interference caused by users proximity can seriously affect the system performance. Many users would thus apply the former Orthogonal Multiple Access (OMA) rather than NOMA. Therefore, it is thus crucial to adopt a good pairing policy in NOMA systems. In this paper, we investigate the minimum pairing distance which separates the far and near users in order to promote massive connectivity. We provide an analytical expression of the pairing distance threshold assuming a fixed power allocation. Then, we propose a new user pairing scheme called Minimum Distance-NOMA (MD-NOMA) which is based on the resolved pairing distance threshold. In the second phase, we extend our study by a joint power control scheme which considers the channel gain. As Compared to the performances of other algorithms Conventional-NOMA, Uniform Channel Gain Difference-NOMA and OMA, simulation results showed that MD-NOMA achieves a significant gain in terms of number of paired users and total achieved throughput.