## 4 Key Application Areas for Passive Wake-Up Radio
### Remote Environmental Monitoring  
Environmental monitoring performed by wireless sensor networks (WSN) is suitable to showcase the capability of a passive WUR technique [5]. It is a combination of a large number of sensor nodes deployed all over a city or rural location to assist humans in easy management and administration of such an environment [115]. However, these sensor nodes need to have a stand-alone energy supply to offer continuous transmissions and ideally without the necessity for a schedule battery change. An example of remote environmental monitoring is shown in Figure 9a.  
Figure 9: (**a**) Description of passive wake-up radio for remote environmental monitoring. (**b**) Description of passive wake-up radio for medical applications.  
For instance, the work in [104] proposed three polling service-based MAC scheduling schemes, known as PSMACs, which have radio triggered hardware attached to the node. The scheme supports diverse QoS as well as mobility, which shows that the protocol can be implemented for environmental monitoring. The protocol features demonstrate the feasibility of passive wake-up radio in environmental monitoring, whereby the mobile node can move around the environment to collect data from the sensor nodes. The sensor nodes are equipped with the passive wake-up radios so that they remain in a deep sleep and only wakes up to send data after receiving a wake-up call from the mobile sink. Relative high data rates and communication range as well as robustness and energy efficiency are other features of the passive wake-up radios, which are required in the implementation of the environmental monitoring.  
Wildlife monitoring is another potential environmental application of the passive wake-up radio. Animals are equipped with sensor nodes such as using a collar sensor. Data collectors (sink nodes) are placed at strategic locations where the animals are expected to gather or move around. The radio on the animals is awakened the moment they get within the data collector range, and the radio starts transmitting the sensed data. In [77], the authors simulated a similar scenario in a 1000 m \(\times\) 1000 m area with a data collector at the center. The data collector is set to continuously send out the broadcast-based wake-up call. The monitoring system using this passive wake-up process resulted in a significant reduction in the energy wastage in the sensor nodes, and hence the overall energy efficiency is increased.  
The protocol in [112] is designed primarily for wireless aerial-ground sensing network for remote environmental monitoring applications in large-scale geospatial or challenging spaces. Likewise, the ID-based enhanced WISP mote (EH-WISP) mote design in [80] is a suitable candidate for periodic (for event-triggered and nonevent-triggered) environmental monitoring applications. With the enhanced wake-up range of up to 5 m, the MH-REACH can be used for indoor living monitoring and greenhouse monitoring. This protocol utilizes its ID capability to allow for a single sink node to wake-up and aggregate the data of multiple sensor nodes individually, without having to wake-up all sensor nodes within its range.