Abstract
Energy efficiency (EE) and spectral efficiency (SE) have been identified as key performance indicators in the design of future cellular networks. However, the available radio frequency (RF) spectrum is becoming highly saturated, thus making it difficult for network operators to achieve significant throughput and SE enhancement without increasing their power consumption. To that end, exploiting the abundant unlicensed spectrum in the visible light band to complement RF communication has become an important research direction in the design of wireless systems. Visible light communication (VLC) combines illumination and communication while significantly reducing the power consumption and related carbon footprint of wireless systems. This paper investigates the introduction of a VLC system in a two-tier RF heterogeneous network (HetNet). The EE and SE performance of the resulting three-tier HetNet is investigated, and a novel energy efficient resource allocation scheme is proposed. More specifically, the joint problem of user association and power control to maximize the EE is formulated as a fractional programming problem under the transmit power and quality-of-service requirements constraints. To tackle the nonconvexity of the problem, the original EE problem is first transformed into a parametric subtractive form. Then, the joint problem is separated into a user association and power control sub-problems. An efficient iterative algorithm is proposed to solve these two sub-problems, alternately. The performance of the proposed algorithm in terms of total network throughput, EE, and SE for different user densities is verified using simulation results.