Polarization Conversion -based Ambient Backscatter System

In Ambient Backscatter Communications (AmBC), a backscatter device communicates by modulating the ambient radio frequency (RF) signal impinging at its antenna. In many cases, the system setup is bi-static such that the receiver and the ambient signal source are separated in space. This configuration suffers from the direct path interference problem. The direct signal component can be several orders of magnitude stronger than the scattered one. This imposes a challenge for the receiver that needs to have high dynamic range in order not to lose the scattered signal component to the quantization noise. In this paper, we propose a novel AmBC system concept, in which a polarization conversion between the direct and scattered path is introduced at the backscatter device and exploited at the dual polarization based receiver antenna to mitigate the direct path interference. The proposed system is agnostic to the ambient signal source characteristics as long as it uses linearly polarized antennas. The backscatter device changes the polarization from linear to circular. The receiver antenna is a circularly polarized patch antenna with a 180-hybrid to obtain the difference between the left-and right-hand polarized fields. Ideally, this receiver antenna and 180-hybrid combination would completely remove linearly polarized direct path and reflected components. In this paper, we propose a robust design that can mitigate the direct path signal power more than 25 dB despite nonidealities in the antenna manufacturing.