Spatial Modulation-Aided Covert Ambient Backscatter Communications: Modeling and Performance Analysis

The minimalist and ultra-low-power natures of ambient backscatter communications (AmBC) make it a compelling wireless paradigm for Internet of Things (IoT), which in turn puts AmBC at great risk of being monitored. Recently, covert backscatter systems have been explored to secure transmission without detection, but they mainly introduce artificial noise and beamforming, consuming additional energy. In this paper, we propose a spatial modulation (SM)-enabled multi-antenna covert AmBC system that guarantees covertness and raises the covert capacity without requiring additional energy consumption. Specifically, the multiple-antenna backscatter tag transmits covert information by adopting SM, while its remaining antennas superimpose overt messages to shelter the covert transmission. We derive the closed-form expression of the minimum detection error probability (DEP) to capture the covertness, that represents the worst-case for the tag. Furthermore, we formulate a covert capacity maximization problem by optimizing the power reflection coefficient under the constraints of the covertness requirement. By exploiting the objective function's monotonic properties and the constraints, we derive the optimal power reflection coefficient and the corresponding covert capacity. Due to the target expression is very complex and lacks a closed-form solution, we introduce a Taylor expansion to simplify it. Finally, numerical results demonstrate the performance and effectiveness of the proposed multi-antenna covert backscatter communication system. As a result, it shown that the covert capacity can be significantly improved compared with the benchmark scheme.