Experimental Characterization of Narrowband Power Optimized Waveforms

Low power devices driven by energy harvesting are limited in working range by transmit power restrictions. Power optimized waveforms (POWs) have been proposed as a way to achieve a longer working range without increasing average transmit power. In the ultra high frequency (UHF) band, bandwidth occupancy needs to be small enough to satisfy relevant regulations: thus the wideband POWs explored in some prior work may be less practical than narrowband POWs. This paper experimentally evaluates the performance of narrowband multi-sine based POWs, varying number of sines from 2 to 20 and occupied bandwidth of 10 kHz, 100 kHz, and 1 MHz-all narrowband from a regulatory perspective. Our key result is that additional sine waves improve voltage sensitivity, but not Power Conversion Efficiency. In our experiments, we observed the voltage sensitivity benefits of the multi-sine POWs to be most pronounced at a power level of -13 dBm; at signal levels significantly below (-30 dBm) or above (0 dBm) this level, their benefits diminish. Finally, we also observe that in a real system (the WISP 5.1, which includes a rectifier and DC-DC converter), voltage sensitivity is a much more complex function of waveform and signal power than in simpler idealized rectifiers. The sensitivity is a non-monotonic and complex function of the number of sines, occupied bandwidth, and signal strength.