Spatial modulation-based ambient backscatter: Bringing energy self-sustainability to massive internet of everything in 6G

Ambient backscatter communications (AmBC) is a new ultra-low-power communication paradigm, which holds great promise for enabling energy self-sustainability (ESS) to massive data-intensive Internet of Everything (IoE) devices in 6G. Recent advances improve throughput and reliability by adopting multiple-antenna techniques in conventional backscatter communications (CoBC), but they cannot be directly applied to AmBC devices for high spectral and energy efficiency due to the unknown RF source and minimalist design in backscatter tag. To fill this gap, we propose SM-backscatter, an AmBC-compatible system that greatly improves spectral efficiency while maintaining ultra-low-power consumption. Specifically, the SM-backscatter consists of two novel components: i) a multiple-antenna backscatter tag that adopts spatial modulation (SM), and ii) a joint detection algorithm that detects both backscatter and source signals. To this end, we theoretically obtain an optimal detector and propose two suboptimal detectors with low complexity. Subsequently, we derive the BERs of both the backscatter and source signals to analyze the communication performance by introducing a two-step algorithm. Our simulation results verify the correctness of the theoretical analysis and indicate that our system can significantly outperform existing solutions.