Energy Efficiency Maximization in Multi-User Miso Mixed RF/VLC Heterogeneous Cellular Networks

In this work, we consider coordinated beamforming design in a multi-user multiple-input single output (MISO) downlink heterogeneous cellular network (HCN) that combines radio frequency (RF) and visible light communication (VLC). VLC can exploit the illumination efficiently but cannot transmit signal in the absence of a line-of-sight (LOS) since the performance will be degraded significantly. Our goal in this paper is to maximize energy efficiency (EE) of the entire communication system with quality-of-service (QoS) constraints of all users in the femtocell. We formulate the problem as a fractional programming problem, which is difficult to solve due to the non-convex objective function and constraints. To tackle this problem, we introduce slack variables and use several approximation techniques to reformulate the original problem into an approximated convex problem. To further improve EE of the system, we apply such convex approximation successively to devise an successive convex approximation (SCA) algorithm. Based on extensive numerical experiments, we observe that the EE performance of the proposed mixed RF/VLC network architecture is significantly better than that without VLC by as much as 67.6% and 43.5% when the RF and VLC circuit power are low (1 W for both cases). In summary, we demonstrate that the RF/VLC network architecture is much more energy efficient than those without VLC.