## 2 Passive Wake-Up Radio
### Review of the Existing Passive Wake-Up Radio Hardware Design
#### 2.3.1 ID-Based Wake-Up Radio Design
This design can reach a wake-up range of up to 37 ft (11.3 m). It removes the extra hardware necessary for ID-based communication in the WISP mote, making it suitable for broadcast communication only. The dual stage energy harvesting circuit in [81] is composed of a seven-stage and ten-stage design for a reception in low and high input power, respectively. New strategies for ultralong wake-up radio were implemented in [82] with a new RF energy harvesting circuit to achieve longer wake-up range. Enhance RFID devices were used to realize this using ultralow-power components and a new generation I\({}^{2}\)C RFID chip. The authors were able to come up with three different strategies. First is the WWU (Write Wake-up) strategy obtained from the WWU mode functionality of the Monza X-2K IC. The WWU is fully passive and ID-based. The sleeping Iris mote detects a transition on the serial clock (SCL) line from high to low, which is used as the wake-up signal targeted to the particular Monza X-2K (ID), which is write-accessed by the reader. The second strategy is E-WWU (Enhanced WWU), which is also ID-based, but works on battery. E-WWU exploits the boosted sensitivity of the battery-assisted Monza X-2K and the ultralow power consumption of the TI FRAM MCU to prolong the wake-up range significantly. The third strategy is MWU (Multicast Wake-up), which are fully passive but broadcast-based. MWU uses the RF energy harvesting circuit to produce a wake-up interrupt for the sleeping Iris mote. The mechanism is considered a broadcast-based because any Iris mote equipped with a harvester and within the coverage of the reader is awakened. The longest wake-up range achieved was 22 m with a delay of 198 ms under 30 dBm transmission power. In [83], the authors described a simple oscillator free wake-up radio design. The proposed system uses the envelope detector and baseband amplifier with a passive RF voltage transformer preceding the receiver. This offers a robust design, which is easy to implement on an ordinary CMOS without the need for external added components. A data rate of 200 kbps and \(-\)47 dBm sensitivity was measured with a 2.3 \(\upmu\)A current consumption from a 1 V supply. An energy harvesting power management unit (EH-PMU) was recently presented in [84] to solve the problem of custom wake-up signal needed by wake-up radios, which cannot be produced by a standard-compliant radio [85]. The proposed system initiated the implementation of the backchannel communication from Bluetooth low energy (BLE) [86] compliant to an ultralow power wake-up-radio. The chip fabrication was implemented in a 65 nm CMOS process consisting of a broadband, noncoherent RF front-end [87] and a 32 Hz crystal oscillator. The design harvests energy up to 30 mV and enabled fully passive operation. The systems support RF energy harvesting mode, cold-start mode [88] during start-up and normal boost operation. In [89], ultralow power wake-up receiver (WuRx) based on direct active-RF detection architecture. The receiver is designed through a single-ended topology that reduces the number of circuit components to minimize the system complexity. The 2.4 GHz RF COK passive wake-up receiver in this design is implemented 65 nm CMOS process. The receiver achieves a total power consumption of 12 \(\upmu\)W at \(-\)50 dBm sensitivity and data rate of 100 kbps.