Phase Cancellation in Backscatter-Based Tag-to-Tag Communication Systems

In this paper, we investigate a unique phase cancellation problem that occurs in backscatter-based tag-to-tag (BBTT) communication systems. These are systems wherein two or more radio-less devices (tags) communicate with each other purely by reflecting (backscattering) an external signal (whether ambient or intentionally generated). A transmitting tag modulates baseband information onto the reflected signal using backscatter modulation. At the receiving tag, the backscattered signal is superimposed to the external excitation and the resulting signal is demodulated using envelope detection techniques. The relative phase difference between the backscatter signal and the external excitation signal at the receiving tag has a large impact on the envelope of the resulting signal. This often causes a complete cancellation of the baseband information contained in the envelope, and it results in a loss of communication between the two tags. This problem is ubiquitous in all BBTT systems and greatly impacts the reliability, robustness, and communication range of such systems. We theoretically analyze and experimentally demonstrate this problem for devices that use both ASK and PSK backscattering. We then present a solution to the problem based on the design of a new backscatter modulator for tags that enables multiphase backscattering. We also propose a new combination method that can further enhance the detection performance of BBTT systems. We examine the performance of the proposed techniques through theoretical analysis, computer simulations, and laboratory experiments with a prototype tag that we have developed.