Energy Efficiency Optimization of Heterogeneous Networks with SWIPT: A Cell-Load Perspective

In a heterogeneous cellular network (HetNet) consisting of multiple different types (tiers) of base stations (BSs), the void cell event in which a BS does not have any users has been shown to exist due to user-centric BS association and its probability is dominated by the cell load of each tier. Such a void cell phenomenon has not been well characterized in the modeling and analytical framework of simultaneous wireless information and power transmission (SWIPT) in a HetNet. This paper aims to accurately analyze the energy efficiency of the HetNet with SWIPT by modeling the cell-load impact on the downlink and uplink transmissions of each BS. We first characterize the power-splitting receiver architecture of SWIPT at a user and then specify the model of energy harvesting. First, the rates of users in the downlink and uplink are accurately found for the generalized user association scheme and then they are used to define the energy efficiency of users in the downlink and uplink. We formulate an optimization problem to maximize the energy efficiency under the constraints on the outage probability of energy harvesting and the defined self-power sustainability. Finally, we show that the feasible solutions to the optimization problem indeed exist if certain conditions are satisfied.