Energy-Efficient Uplink Power Control in NOMA Enhanced Cell-Free Massive MIMO Networks

The ever-increasing communication requirement of the next generation network imposes great challenges in the existing cellular architecture and requires energy-efficient resource allocation. In this paper, we consider a cell-free (CF) massive multiple-input multiple-output (MIMO) network, where a large number of distributed access points with multiple antennas simultaneously serve a smaller number of users in a geographic coverage without cell boundary. To alleviate the limitation of orthogonal resource and further enhance network capacity, non-orthogonal multiple access is applied to the CF massive MIMO network in both channel training and uplink transmission phases. First, we derive the closed-form expression of uplink transmission rate by taking into account the imperfect channel state information (CSI) caused by pilot contamination and channel statistics. Second, based on the obtained uplink rate expression, an energy efficiency (EE) maximization problem is formulated by optimizing power control scheme, and we propose a low-complexity algorithm to address the non-convex optimization. Finally, simulation results validate that the designed algorithm can improve EE significantly compared to benchmarks in different CSI cases and network setups.