A battery-less, multi-node wireless body area network (WBAN) system-on-a-chip (SoC) is demonstrated. An efficiency tracking loop is proposed that adjusts the rectifier's threshold voltage to maximize the wireless harvesting operation, resulting in a minimum RF sensitivity better than -20 dBm at 904.5 MHz. Each SoC node is injection-locked and time-synchronized with the broadcasted RF basestation power (up to a sensitivity of -33 dBm) using an injection-locked frequency divider (ILFD). Hence, every sensor node is phase-locked with the basestation and all nodes can wirelessly transmit TDMA sensor data concurrently. Designed in a 65 nm-CMOS process, the fabricated sensor SoC contains the energy harvesting rectifier and bandgap, duty-cycled ADC, digital logic, as well as the multi-node wireless clock synchronization and MICS-band transmitter. For a broadcasted basestation power of 20 dBm (30 dBm), experimental measurements verify correct powering, sensor reading, and wireless data transfer for a distance of 3 m (9 m). The entire biomedical system application is verified by reception of room and abdominal temperature monitoring.
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Xia et al. (2014) studied this question.