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Backscattering ambient signals: Assisting radios and tracking planes

Vardakis Iosif

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Summary

This work’s main objective was to exploit ambient signal backscattering to aid radio communication, through a smart reflective surface or detect airplane reflections using a passive radar system. First, ultra-low cost, commodity, radio frequency identification (RFID) tags are utilized as elements of a reconfigurable intelligent surface (RIS). Such batteryless tags are powered and controlled by a software-defined radio (SDR) reader, with properly modified software, so that a source-destination link is assisted, operating at a different band. Signal model includes small-scale and large-scale fading, direct link, as well as specific parameters relevant to reflection (i.e., backscatter) radio, such as antenna structural mode and reflection efficiency, typically overlooked in the literature. An algorithm is offered that computes the optimal RIS configuration with complexity of O(M log M) in number of elements M, instead of intractable exponential complexity of exhaustive search, while accommodating any number K ≥ 2 of loads. With the proposed algorithm, it is shown that performance gains reach a plateau for constant element spacing and increasing number of elements, suggesting that the weak, passive nature of backscattered links limits the performance gains, even with perfect channel estimation. A concrete way is offered to design and prototype a wireless, batteryless, RF-powered, reconfigurable surface and a proof-of-concept is experimentally demonstrated. Then, an FM passive radar station capable of tracking airplanes in Cartesian 3D coordinates was developed. The station was constructed using a low-cost SDR, based on the LimeSDR USB platform; the latter is a relatively low-cost SDR, in the order of 300$. The station is complemented with a 5-element Yagi antenna for observation and another 3-element Yagi antenna, for observing the FM radio broadcasting stations. Signals from different FM broadcasting stations were collected and amplitude-range-Doppler(ARD) plots were created. Then, a bistatic tracker was designed for each broadcasting station, estimating the bistatic range and Doppler shift from each ARD plot. Finally, the estimated range Doppler frequency shift pairs from each station were fed as input to the Cartesian tracker, which was designed to offer 3D tracking. Simulation results showed an error below 100m in each direction. The system exploits space diversity, due to the multiple spatially separated broadcasting stations required as input, and frequency diversity, due to processing of multiple frequency bands of interest. The system is scalable, since increasing the number of broadcasting stations utilized requires no change in the processing chain. Experimental results with the low-cost LimeSDR platform corroborate theoretical findings and demonstrate the utility of this work, as a testbed for further research and development.

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