Orthcatter: High-throughput In-band OFDM Backscatter with Over-the-Air Code Division

The existing ambient backscatter systems suffer from either more spectrum utilization or low throughput. we propose Orthcatter, the first in-band OFDM backscatter system that provides a higher throughput while consuming fewer spectrum resources. Our key innovation is the designed over-the-air code division technique that enables the cancellation of the co-channel interferences, solving the core challenge of the in-band backscatter communication. Unlike the common code-division systems that generate orthogonal codewords locally, we construct the quasi-orthogonal backscatter codewords by swapping the subcarriers of each excitation OFDM symbol and concrete this design passively with a double side-band symbol construction method. Armed with these quasi-orthogonal codewords, we design a two-step interference cancellation scheme, significantly improving reliability. We prototype and test Orthcatter. The results show that Orthcatter can achieve throughput of 248kbps and a BER of 10^-4 under OFDM WiFi exciter, improving by over 4.6× and 300× compared with the state-of-the-art in-band backscatter system. Our throughput and BER can even be 11kbps higher and 59× better than the prior side-band backscatter systems, and the exciter-to-tag communication range is 3× of prior OFDM backscatter systems.