# Wake-Up Receiver-Based Routing for Clustered Multihop Wireless Sensor Networks
## 1 Introduction  
The Internet of Things (IoT) is developing towards a significant and extensive technology. The vision of a global infrastructure of connected physical objects interacting anywhere and anytime is still one of the emerging concepts in the IT world. Due to their robust design and self-organising networking without the need for extensive infrastructure, Wireless Sensor Networks (WSNs) are the backbone of IoT. They are increasingly used in different applications, e.g., for monitoring and data transmission in areas such as structural and environmental monitoring [1, 2]. Further, healthcare, as well as smart home applications, such as real-time indoor air quality monitoring [3] and on-demand indoor localisation [4, 5], are realised through this technology in a cost-effective and low-maintenance way. WSN is also becoming increasingly important in indoor scenarios where accurate and reliable information about people's activities and behaviour is collected and disseminated to support assisted living in home environments [6, 7]. The provision of wireless sensors for use in the sense of IoT is accompanied by various technical challenges [8]. Energy consumption is the most important challenge since the lifetime of each sensor is tightly related to its energy-critical battery. Thus, new ways must be found to increase the energy-efficiency of sensors. One of the major drawbacks of recent wireless sensors is the power consumption of Radio Frequency (RF) transceivers with approximately 10-30 mA for routing processes or radio transmissions. In order to allow an energy autarkic operation, a duty-cycled approachwith fixed phases of transmitting, receiving, and sleeping are generally introduced [9; 10]. This leads to the increase of system latency. This latency is not acceptable, especially in time-restricted or quasi real-time applications with reaction times in the range of some milliseconds to some seconds [11]. Researchers, such as in [12; 13], are promoting power management to improve the battery life of individual nodes and extend the lifetime of the entire network. A very promising solution to face these problems and challenges is the use of a novel, energy-autonomous, on-demand communication hardware. In combination with the application-specific main sensors (temperature, fire, etc. ), the so-called Wake-up Receiver (WuRx) forms the enhanced sensor node [14]. In order to preserve energy, the sensor node can be set to the sleeping state until a signal is received by the WuRx to wake up the node. This strategy, when applied in the routing process, is energy efficient but has drawbacks when it is applied in time-critical applications. Effectively, such solutions require an additional time to wake up each intermediate node sequentially when it is needed in the routing process [15; 16]. To overcome these limits, this paper proposes an energy-efficient and delay minimising routing strategy using asymmetric data transmission with multicasting wake-ups in a heterogeneous and clustered WSN. Our proposed approach is made up of two layers:  
* The sensors layer: made up of sensors equipped with batteries, and in addition, each sensor node is equipped with a WuRx. * Fog computing layer: made up of Fog Nodes. These nodes are much more powerful than sensor nodes and are placed physically near the sensor nodes to assist them and help them in their complex tasks [17; 18]. Each Fog Node is responsible for a sensor cluster. It controls the data packets transmitted in the network between the different network structures (sink and cluster nodes). It maintains in its routing table all the source routes from each node in its cluster to it. Based on the frequent interactions, the sink and Fog Node are main powered. At the time that a packet is to be exchanged, the sink transmits a Request Packet (REQ) to the Fog Node, which sends a Wake-up Packet (WuPt) to start Data Packet (DataPt) transmission.