I. Introduction
With a growing demand for long service lifetime of devices in IoT [1], [2], supplying sufficient power continuously to the IoT devices is a crucial problem in practice. The lifetime of IoT devices can be prolonged by replacing or recharging batteries but the cost may be high or it is sometimes impossible under certain circumstances such as unreachable devices after installation. One of the practical approaches for prolonging the lifetime of IoT devices is to harvest energy at a device from external sources, such as solar, wind, geothermal, but which are usually neither controllable nor reliable [3]. As another approach, in ambient backscatter communication, the IoT device can transmit information without an internal power source by modulating ambient RF signals [4]–[6]. RF signals, which are man generated and thus more manageable, can be used for energy harvesting in a more active way by WPT technologies, i.e., directly transferring the energy to IoT devices via RF signals [7]–[9]. In the SWIPT system, a transmitter transmits RF signals for transferring information and power, and the receivers then perform information decoding and energy harvesting [10]–[18]. There are three different SWIPT techniques: 1) power splitting [10]–[14]; 2) time switching [10], [13], [15]; and 3) antenna switching [16], [17]. In the power-splitting approach, a receiver divides its received signal into two parts for energy harvesting and information decoding. In the time switching approach, each time slot is used for either energy harvesting or information decoding alternatively. In the antenna switching approach, a set of antennas are used for energy harvesting while the others are used for information decoding.