CodeScatter: Distortion-Aware Collision Resolution for Concurrent Multiple Access in Backscatter Networks

Concurrent transmission with collision resolution has been considered as the multiple access solution in backscatter networks. However, designing practical collision resolution mechanisms in backscatter networks is challenging for the following reasons. First, interference from multiple tags accumulates, resulting in strong and complicated inter-tag interference. Second, transmitted signals are distorted by various factors (e.g., imperfect digital filter), which have not been considered in previous collision resolution schemes. Due to these distortions, tag-side mechanism like orthogonal codes, which assume undistorted conditions, are not effective for resolving collision at the receiver. Third, variation in received signal levels across tags often degrades performance in concurrent transmissions (i.e., near-far problem). Although transmit power control was proposed as a solution, it increases tag complexity and energy consumption. To address the challenges, we propose CodeScatter, a distortion-aware design for collision resolution that is robust to near-far problem in backscatter networks. The innovative features of CodeScatter are 1) distortion-aware design and 2) robustness to near-far problem without transmit power control. CodeScatter achieves these features by employing two mechanisms: 1) Distortion-aware Successive Decoding (DSD), which iteratively isolates individual tag signals while accounting for distortions and 2) Distortion-aware Orthogonal Codes (DOC), which ensures orthogonality even in the presence of distortions. To our knowledge, CodeScatter is the first distortion-aware design for collision resolution in backscatter networks. We implement a prototype using COTs RF switch and USRPN210. Experimental results demonstrate that CodeScatter outperforms previous proposals in terms of BER and throughput.