Wireless power transfer with transmit diversity

Background: Wireless power transfer is important for energizing and recharging the Internet-of-Things (IoT) cordlessly. Harnessing energy effectively from radio waves has become a crucial task. It is known that diversities at the transmitting antenna and waves (i.e. simultaneous continuous waves with center frequencies separated apart) can enhance the radio frequency (RF) to direct current (DC) energy conversion. What remains unknown is the extent of which the wave diversity enhances the conversion gain. This study attempts to examine the RF-to-DC conversion gain of applying wave diversity. This paper investigates the effects of wave diversity on the energy conversion efficiency, and contributes the analytical expression that relate the conversion efficiency to the diversity count, i.e. the number of simultaneously transmitted sinewaves. Methods: We adopted a theoretical approach to the problem. First, we derived and presented a theoretical model that incorporated different forms of transmit diversity, i.e. antenna and wave diversities. This model then connected a RF-to-DC energy conversion model resulting from polynomial fitting on circuit simulation results. With the availability of these two models, we determined the theoretical energy conversion gain of simultaneously transmitting multiple sinewaves. Results: The results showed that transmitting multiple sinewaves simultaneously yields diversity gain and higher energy conversion efficiency. Most importantly, the gain and conversion efficiency can now be theoretically quantified. For example, at certain RF power measured at the receiver circuit, the diversity gain of transmitting four sinewaves is 2.6 (as compared to transmitting single sinewave). In fact, both the diversity gain and conversion efficiency increased with the number of simultaneously transmitted sinewaves. In another example, the conversion efficiency of transmitting four sinewaves is 0.1 as compared to 0.075 of two sinewaves. Conclusions: In summary, this paper presents a novel analytical expression for wave diversity in the context of wireless power transfer.