## 2 Passive Wake-Up Radio
### Review of the Existing Passive Wake-Up Radio Hardware Design
#### 2.3.1 ID-Based Wake-Up Radio Design  
The first ever passive wake-up receiver concept [72; 73] is presented in 2004 by [PERSON] and [PERSON], which is facilitated by a radio triggered circuit. The circuit collects energy from electromagnetic waves in the incoming radio signal. The receiver then identifies the wake-up signal and distinguishes them from other signals. The design operates at 433 MHz frequency and has the addressing capability. The ID system is designed by applying a multiple-frequency technique called radio triggered ID (RTID), determined by the combination of several frequencies. These combinations are assigned to nodes in the network to serve as group IDs or locally unique IDs of network nodes. SPICE circuit simulation is used to evaluate the technique using experimental data. The design achieved a maximum wake-up range of 3 m and the idle mode power consumption of 145 \(\mu\)W. An optimized, discrete-based wake-up radio for the Internet of Things networks application is designed in [74] with embedded addressing capability. The wake-up circuit is composed of an antenna, an impedance matching network, and a rectifier with a comparator to convert the RF power to a dc voltage. The solution offers a receiver sensitivity of \(-\)28 dBm with a power consumption of 39 \(\mu\)W and a data rate of 1 kbps. WISP [75] is the first microcontroller to be incorporated as part of a passive UHF RFID tag. It is a battery-free system that serves as an enhanced RFID tag. The connected MSP430 programmable microcontroller performs the task of measuring the attached sensors and also the implementation of the EPC Class 1 Generation 1 communication between the WISP and an RFID reader. The model is developed for research purpose by Intel. WISP can achieve an operating distance of 4.5 m.  
Another passive system solution is the RFID impulse [76]. RFID tag and reader are attached on the motes. The reader transmits a wake-up signal and the tag collects the RF signal and uses it to transmit an interrupt to the main radio to wake it up. The results show that RFID Impulse outperforms the previous method [75] in terms of energy efficiency and transmission rate. However, their energy  
Figure 3: Classification of the previous works on passive wake-up radio hardware design. consumption analysis does not include the energy consumed by the nodes to wake-up. In addition, the implementation described in the RFID Impulse uses a coil instead of an RFID tag for proof of concept. In [21; 77], the authors integrated the passive tag WISP with the Tmote sky mote to develop a passive wake-up mote, called WISP-Mote. The design used an off the shelf UHF Gen2 Speedway RFID-reader by Impini) [78], which produced the interrupt signal to the Tmote-sky mote. This assembly results in an extended wake-up range of up to 5 m, which is consistent with range on the Tmote sky datasheet. The design supports both broadcast and ID-based wake-ups. The WISP's microcontroller unit has an output signal of 1.8 V, which is more than the 0.92 V required for an external interrupt to be triggered. After the node is active, the sensor data is transmitted by a 2.4 GHz CC2420 main radio. In 2015, the same authors also proposed a new passive wake-up system called MH-REACH-Mote (Multi-hop Range EnhAnCing energy Harvester-Mote) [79]. It uses RFID with energy harvesting as well as multi-hop capabilities. To achieve multi-hop, the system has the UHF RFID serving as the WuTx and also a WISP-Mote used as the WuRx which supports ID based wake-up. The system test results shown that for a wake-up signal transmission of 10 s, it consumes the energy of 23.52 J with 9.4 m wake-up range. In [80], the WISP-mote is integrated with an energy harvesting circuit [81] to further improve the wake-up range. The RF energy harvesting circuit, known as enhanced WISP (EH-WISP) mote, enhanced the wake-up range of the WISP mote. Furthermore, the design of a new wake-up radio node known as REACHMote (Range EnhAnCing Energy Harvester-Mote) is described in [80].