A Two-Channel Frequency Selective 2.4 GHz Backscatter Modulator for Wi-Fi and Bluetooth Backscatter Using Bulk Acoustic Wave (BAW) Filters

We present a two-channel frequency selective 2.4 GHz backscatter modulator for Wi-Fi (IEEE 802.11) and Bluetooth backscatter communication. Prior backscatter modulators create unwanted spectral pollution by backscattering every signal incident over the entire antenna bandwidth. In contrast, the proposed frequency selective modulator employs narrowband bulk acoustic wave (BAW) filters having a 20 MHz bandwidth to enable backscatter modulation in either Wi-Fi channel 1 (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2401-2423 \text{MHz}$</tex>) or Wi-Fi channel 11 (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2451-2473 \text{MHz}$</tex>). The measured differential reflection coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\vert\Delta \Gamma\vert$</tex> exceeds 0.568 across Wi-Fi channel 1 and 0.601 across Wi-Fi channel 11. The opposite-channel rejection exceeds 47.02 dB from Wi-Fi channel 1 to Wi-Fi channel 11 and 48.99 dB from Wi-Fi channel 11 to Wi-Fi channel 1. Both simulated and measured performance are presented and compared. When generating BPSK modulation at 11 Mbps (as used by IEEE 802.11b), the modulator has an average power consumption of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$20.9 \mu ~\mathrm{W}$</tex> and a measured energy figure of merit as low as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.9 \text{pJ} / \text{bit}$</tex>; meanwhile, when generating FSK modulation at 1 Mbps (as used by Bluetooth Low Energy), the modulator has an average power consumption of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.9 \mu ~\mathrm{W}$</tex> and a measured energy figure of merit as low as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.9 \text{pJ} / \text{bit}$</tex>. By addressing the key unsolved problem of backscatter-induced spectral pollution, this work paves the way for backscatter devices to become “first-class citizens” of the wireless spectrum.