Hardware-Efficient All-Digital Architectures for OFDM Backscatter Modulators

Orthogonal frequency-division multiplexing (OFDM) backscatter communication promises to enable ultralow-power wireless devices that are robust to time-varying multipath channels. Traditional OFDM transmitters require the use of power-hungry digital-to-analog converters (DACs) and vector modulators to realize the OFDM signal, adding to the complexity and power consumption of wireless sensor nodes. In this work, we compare three all-digital architectures for OFDM backscatter communication that use RF switches and discrete loads to implement digitally controlled single-sideband OFDM backscatter modulators. We present design analysis, including simulations and measurements, for a selected implementation using five subcarriers having binary phase shift keying (BPSK) modulation at a symbol rate of 250 kSymbols/s and a throughput of 1.25 Mbit/s with a modulator energy consumption of 160 pJ/bit. We also present a five-subcarrier over-the-air validation with 195-kb/s throughput. We demonstrate how the number of RF switch states and the choice of impedances impact the metrics of subcarrier interference ratio and sideband suppression ratio, and we explore how reduced-numeric-precision inverse fast Fourier transform (IFFT) structures impact the theoretical bit-error rate. The all-digital architecture and analysis presented in this article enable new avenues of low-cost, digital OFDM, and multiple-access backscatter communication systems for use in challenging multipath environments.