A UHF/UWB Hybrid RFID Tag With a 51-m Energy-Harvesting Sensitivity for Remote Vital-Sign Monitoring.

A novel ultra-high frequency (UHF)/ultra-wideband (UWB) hybrid RFID tag is reported for object-specific remote vital-sign monitoring application. The tag achieves a record energy-harvesting sensitivity at UHF band by codesigning a meander dipole antenna and a passive rectifier. The especially high quality-factor makes the frontend sensitive to near-field motions such as heartbeats and respiration in a wearable setting. The custom CMOS IC of approximately 1-μW power consumption builds around a low-power UWB transmitter and converts variations of the supply voltage to the impulse repetition rate. The tag consisting of the IC and UHF/UWB antennas requires no other discrete components and features a size of 4.2 cm × 2.9 cm and a weight of 0.93 g. A long-distance experiment verifies that the tag can be wirelessly powered up at 51 meters from a 4-W equivalent-isotropic-radiation-power (EIRP) UHF transmitter. Remote vital-sign monitoring is validated on a human subject, in which the UHF power source is placed 2 meters away from the subject with a power emission of less than 20 dBm. This work proposes a first-of-its-kind remote vital-sign monitoring scheme based on a noncontact wearable tag. The design of the far-field energy-harvesting frontend with a record sensitivity serves as a reference for future works on battery-free remote sensors.