The prevalence of Internet-of-Things (IoT) applications leads to an increasing focus on the design and optimization of sensor nodes. Battery lifetime and associated costs of battery replacement often limits the long term operation and viability of sensor nodes. RF wireless energy harvesting on the other hand can be appealing since it enables continuous operation as long as an RF energy source is available. Meanwhile, tracking the spacial position of miniature objects is important and can be widely applied to people or asset tracking. For example, the foraging pattern of bumblebees is of great interest for studying in crop pollination and bee-colony population decline.
This research presents the design of a batteryless wirelessly powered ultra-wideband (UWB) system-on-a-chip (SoC) tag for area-and-volume-constrained localization applications such as insect tracking.
An antenna-rectifier co-design methodology is presented for sensitivity optimization under area constraints. The energy harvester demonstrates state-of-the-art -30.7 dBm sensitivity for 1-V output voltage with only 1.3 cm2 antenna area, representing a 2.3x improvement in sensitivity over previously published work, at 2.6x higher frequency with 9x smaller antenna area, translating into a 50% longer range at the same frequency. Tag measurements in typical office environments demonstrate 20-m-range RF-energy harvesting with 36-dBm effective-isotropic radiated power in the 2.4-GHz ISM band.
A second generation of energy harvester further addresses the challenge of continually increasing the sensitivity of wireless-powering approaches to achieve targeted output voltage for wirelessly-powered sensors with compact area. The design exploits a boot- strapping approach where the rectifier first stage and the antenna are reused to increase the rectifier output voltage following an initial charging phase. Area-constrained rectifier- antenna co-optimization results in a compact high-Q 2.4-GHz antenna with 1.2 cm2 area which is then used in two topologies (CC and CP) based on the proposed bootstrapping concept. Both topologies increase rectifier output voltage with no additional off-chip inductors or capacitors. The CC topology achieves input sensitivity of -34.5 dBm for 1.6-V output voltage with RLOAD of 1.8 MΩ, while the CP topology achieves -26.5 dBm input sensitivity with 2.5-V output voltage and RLOAD of 250 KΩ. These results confirm more than 2x longer distance for wirelessly powering sensors compared to state-of-the- art, with only 1.2 cm2 of rectifier-antenna area. An extended Wakeup Receiver (WuRX) based on this concept was proposed with the performance of -61.5 dBm sensitivity for 10e−3 bit-error-rate (BER) at 2.5 kbps at 2.4 GHz with 19.1 dB interferer-to-carrier ratio (ICR) for 3 MHz CW blockers.
Key challenges for wirelessly powered SoC for localization operation at 10-m range including the design of high-sensitivity rectifiers and low-voltage high-efficiency UWB transmitters (TX) are detailed. A 300-nA power management unit (PMU) and low- voltage (0.8-V) UWB TX increases tag operating range by ensuring good rectifier sensitivity under loaded conditions and reducing required rectifier output voltage. The rectifier, PMU, and UWB TX are integrated in 65-nm CMOS, and the 0.8-V UWB TX consumes 64 pJ/pulse at 28-MHz pulse repetition rate and achieves 2.4 GHz 10-dB bandwidth. Wireless measurements demonstrate sub-10-cm range resolution at ranges exceeding 10 m. In Vivo measurements successfully demonstrate the RF-In and RF-Out operation of wirelessly powered UWB SoC on a flying bee without auxiliary battery or pre-charging.