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Covert communication promises a secure transmission mechanism for Backscatter Communication (BC) by hiding the transmission behavior of backscatter devices from potential eavesdroppers. This letter investigates the upper limit of the covert rate in a scenario where a multi-antenna eavesdropper is present in BC setup, considering the inherent structural reflection of a backscatter tag. Specifically, the transceiver can employ a Gaussian signal to illuminate the tag, with special considering two distinct signals: a modulated signal and an additional reflected signal from the tag’s antenna structure. This letter considers a powerful eavesdropper who is equipped with a massive antenna to enhance its ability to detect the tag’s transmission activities. The results reveal the presence of the square root law ( $\Theta (\sqrt {n})$ covert bits in n channel uses) across various scenarios. Theoretical analysis demonstrates the inhibitory effects of both the tag’s antenna structural reflection and the variance of the transceiver’s Gaussian signal on the eavesdropper’s detection capabilities. Furthermore, numerical results illustrate the impact of different parameters on the covertness of the system.

Ambient backscatter communication (AmBC) is becoming increasingly popular as a green Internet-of-things technology by enabling ultra-low-power data exchanges among simple tags. The bit-error-rate (BER) performance of the AmBC receivers is hampered by the low signal-to-interference-plus-noise ratio (SINR), which limits either the range or the data rate that can be supported by these systems. Among the alternatives, the solutions provided by a multiantenna receiver enable several practical methods to improve the SINR using array signal processing. In this paper, the optimum multiantenna AmBC receiver for any binary modulated tag signal is presented. Two receivers are derived from the maximum-a-posterior probability criterion for deterministic-unknown and for Gaussian distributed ambient signals. The closed-form cumulative distribution functions are derived for both of the receivers for performance evaluation purposes. It is discussed that these two receivers are equivalent under practical conditions, and the optimum receiver is a generalization of the multiantenna receivers available in the literature. Several implementation details are discussed, and numerical evaluation is provided to validate the development. Therefore, the presented receiver accommodates different tag modulations and achieves the best possible BER performance, which opens up new application possibilities by improving AmBC system flexibility.

The backscatter device (BD) in ambient backscatter communication (AmBC) systems is often illuminated by multiple ambient sources in reality. Although multiple ambient sources can benefit the AmBC systems, this scenario has been rarely studied. This letter gives an answer to the overlooked question of how to effectively obtain the gain from multiple ambient sources—using a proper signaling matrix on a multi-antenna BD. We optimize the BD signaling matrix based on the criterion of minimizing the AmBC system bit-error-rate (BER) performance. The derived signaling matrices combine the ambient signals and steer the backscatter signal toward the receiver to increase the received signal strength. The simulation results show that the multi-antenna BD effectively uses multiple ambient sources, and the derived BD signaling matrices robustly achieve a larger communication range or a higher datarate of AmBC systems.