Transmit Power-Efficient Beamforming Design for Integrated Sensing and Backscatter Communication

Ambient Internet of Things networks use low-cost, low-power backscatter tags in various applications, and sensing is necessary to introduce perceptive intelligence to the network. In this work, an integrated sensing and backscatter communication (ISABC) system is thus introduced, comprising multiple backscatter tags, a user (reader), and a full-duplex base station (BS) with integrated sensing and communication (ISAC). The BS simultaneously detects backscatter tags and communicates with the user using the same time and frequency resources. The tag-reflected signals provide data to the user and allow the BS to sense the tags' state information. The communication rates for both the user and tags and the BS sensing rate are derived. To minimize the total BS transmit power, the BS communication beamformer, sensing signal, tags' reflection coefficients, and sensing combiners are jointly optimized using the alternating optimization technique. Closed-form solutions are provided for the sensing combiners, while semi-definite relaxation is applied to the BS communication beamformer and sensing signal, and slack optimization is used for the tags' reflection coefficients. For instance, with ten BS antennas, ISABC achieves a 75% gain in combined communication and sensing rates compared to traditional backscatter, with only a 3.4% increase in transmit power. Additionally, ISABC with active tags requires just a 0.24% increase in power compared to conventional ISAC systems.