  Enabling technologies for energy sustainable Internet of Things (IoT) are of
paramount importance since the proliferation of high data communication demands
of low power network devices. In this paper, we consider a Multiple Input
Single Output (MISO) multicasting IoT system comprising of a multiantenna
Transmitter (TX) simultaneously transferring information and power to low power
and data hungry IoT Receivers (RXs). Each IoT device is assumed to be equipped
with Power Splitting (PS) hardware that enables Energy Harvesting (EH) and
imposes an individual Quality of Service (QoS) constraint to the downlink
communication. We study the joint design of TX precoding and IoT PS ratios for
the considered MISO Simultaneous Wireless Information and Power Transfer
(SWIPT) multicasting IoT system with the objective of maximizing the minimum
harvested energy among IoT, while satisfying their individual QoS requirements.
In our novel EH fairness maximization formulation, we adopt a generic Radio
Frequency (RF) EH model capturing practical rectification operation, and
resulting in a nonconvex optimization problem. For this problem, we first
present an equivalent semi-definite relaxation formulation and then prove it
possesses unique global optimality. We also derive tight upper and lower bounds
on the globally optimal solution that are exploited in obtaining low complexity
algorithmic implementations for the targeted joint design. Analytical
expressions for the optimal TX beamforming directions, power allocation, and
IoT PS ratios are also presented. Our representative numerical results
including comparisons with benchmark designs corroborate the usefulness of
proposed framework and provide useful insights on the interplay of critical
system parameters.
