## 2 Passive Wake-Up Radio
### Passive Wake-Up Radio Hardware Architecture  
Although passive wake-up radios need power to operate, power is not required to be continuously supplied all the time. This can be achieved by harvesting energy from the incoming RF (wake-up) signal. Thus, more hardware needs to be attached at the wake-up radio to serve this function. This increases the size and complexity of the circuit.  
The energy harvesting process, however, causes a delay in the wake-up of the main radio due to the time needed to accumulate enough energy. This increases the latency and reduces throughput in the node, hence affecting the overall network performance. Passive wake-up radios also have shorter ranges than the active wake-up radios, typically by a few meters. The shorter ranges occur due to reduced sensitivity as compared to the active wake-up radio with active receiver components, thus lower the receiver sensitivity. Moreover, the energy harvesting process places extra work on the transceiver in the case where the wake-up signal is sent by the main radio. The transmitter needs to modulate the wake-up signal and transmit for long enough periods (normally for seconds) so that the signal can be detected at the receiving point. More power is thus consumed when the transmitter is active for a longer period.  
A generic sensor node with passive wake-up radio is shown in Figure 1. It consists of eight major units: antenna unit, energy harvesting wake-up radio, microcontroller unit, power management unit, memory unit, analog-to-digital converter unit, main RF transceiver unit, and the sensor node.  
The wake-up signal is first received by the antenna of the passive wake-up radio and then passed through the energy harvesting process. The harvested power is used to activate the microcontroller unit, which then sends an interrupt signal to wake-up the main radio to exchange data messages.  
A typical RF energy harvesting circuit block diagram is shown in Figure 2. It consists of four units: matching network, voltage multiplier, energy storage, and the antenna as the RF input. The functions and characteristics of each unit will be described in the next subsections.  
The harvested wake-up signal is of different polarization and frequency. The signal is passed through the matching circuit so that the input impedance obtained from the antenna is matched with the wake-up radio circuit load to allow the maximum power transfer to the capacitor. The RF energy is converted to DC voltage in the voltage multiplier and the output DC voltage is passed into a capacitor and stored there. Thus, the performance of the energy harvesting circuit is assessed based on its conversion rate and the available ambient RF energy [51]. However, the RF to DC conversion efficiency and the output DC voltage can be reduced by multiple dissipation and reflection in the terrain, which causes a decline of the available ambient RF energy. Recently, RF energy harvesting circuits are designed to work on multiple frequency bands as opposed to the previous models, which only operate on a single frequency band. These multiple frequency band circuits offer higher dc output voltage because they can harvest the energy from the various RF sources available.  
Figure 1: Sensor node with passive wake-up radio.  
Figure 2: Breakdown of the components for the energy harvesting circuit.  
Furthermore, a higher dc output voltage can be obtained by using wide-band harvesters, which can collect several RF signals at a time. However, on the wide frequency band range, it is difficult to maintain the high conversion efficiency and impedance matching. This is due to the mismatch of the diode input impedance and the received RF energy, resulting from nonlinearity of the devices [52; 53]. The basic components of the energy harvesting circuits are described below.