Spatial User Clustering and Power Control for Downlink MIMO-NOMA Systems

The proliferation of Internet of Things (IoT) devices and the attractive underlying market has motivated the integration of IoT into the fifth generation (5G) of wireless communication. As the number of connected devices continues to increase, wireless communication networks are facing challenges in terms of throughput and massive connectivity. To tackle these challenges, this paper investigates the utilization of Massive Multi-Input Multi-Output (mMIMO) over millimeter-wave (mmWave) frequency bands as well as Non-Orthogonal Multiple Access (NOMA), which is considered a key technology enabling multiple users to share the same frequency/time/code resource. Specifically, it is performed to separate users served through the same beam in IoT scenarios where the number of active users exceeds the number of Radio Frequency (RF) chains at the Base Station (BS). By incorporating NOMA on top of MIMO, the system's spectral efficiency can be increased due to the introduction of additional degrees of freedom. We propose two joint user clustering and power control algorithms to address the aforementioned challenges. The proposed algorithms take into account the spatial channel correlation between users to maximize the number of allocated users while considering their Quality of Service (QoS) requirements. Extensive simulations illustrate that the proposed techniques improve the efficiency of wireless resource utilization, especially when there is a large set of active users and a limited number of RF chains.