Single Sideband Noise Shaping for All-Digital Delta-Sigma OFDM Backscatter Modulators

Delta-sigma modulation (DSM) enables the use of all-digital switched impedance modulators to generate arbitrary backscatter signals. For example, a DSM-based backscatter modulator having only two or four impedance states can generate quadrature amplitude modulation (QAM) with e.g., 64 states, or multicarrier orthogonal frequency division multiplexed (OFDM) signals having many subcarriers. This paper describes potential improvements to in-channel spectral characteristics by adding single sideband (SSB) and double sideband (DSB) noise shaping to the DSM signal path. Using numerical simulation and hardware validation, we demonstrate that noise-shaped DSM can improve the spurious-free dynamic range (SFDR) of OFDM subcarriers generated by a low-resolution two-state or four-state impedance digital-to-analog converter. The noise shaping approaches are validated using a prototype OFDM backscatter uplink based on an FPGA driving a single-pole-four-throw (SP4T) CMOS RF switch that serves as the backscatter modulator. The SSB and DSB noise shaping techniques are compared by over-the-air transmission of five-subcarrier OFDM backscatter symbols with a four-times oversampling DSM at up to 1.25 Mbps. With this approach, we find that DSB noise shaping yielded a 6.2 dB improvement in SFDR relative to SSB noise shaping, at the cost of 9.8 dB higher peak out-of-band quantization noise. These results confirm that an all-digital modulation approach with noise-shaped DSM can be used to balance in-band vs. out-of-band quantization noise and thus optimize the spectral characteristics of hardware-efficient, all-digital backscatter modulators for low-power wireless communication.