# Leveraging Energy Harvesting and Wake-Up Receivers for Long-Term Wireless Sensor Networks
## 1 Introduction  
Wireless Sensors Networks (WSNs) are today a mature technology enabling a large variety of cyber-physical system applications in environmental monitoring, healthcare, security and industrial domains. They are composed of multiple wireless sensor nodes that monitor an environment and wirelessly send information data to one or more remote hosts called sinks. A wireless sensor node is made of several components: a processing unit, memory, sensors, a transceiver and an energy source [1]. Usually, these devices are battery-powered and therefore have a limited lifetime, making energy one of the most precious resources, especially in scenarios where the network is expected to work for several months or even years. To tackle this problem, a successful approach is Energy Harvesting (EH), which allows the nodes to be powered by environmental energy sources such as sunlight, wind, vibration, water flow, etc. Using EH, it is possible to increase the WSN lifetime by an order of magnitude with respect to traditional battery-powered approaches and to achieve the Energy Neutral Operation (ENO) state, i.e., the amount of harvested energy is greater than or equal to the amount of consumed energy over longperiods of time [2; 3; 4; 5]. Obviously, adequate energy management techniques remain a key feature for enabling EH-WSNs. Harvested energy is by nature unpredictable and intermittent [5], making energy management strategies crucial in EH-WSNs. Efficient energy management requires both careful hardware design (by selecting optimal circuits) and energy-aware algorithms (e.g., network protocols). As wireless communication is usually the primary consumer of energy in WSNs [6], many energy management techniques focus on improving the efficiency of radio communication. One of the most widespread techniques is duty-cycling, in which the transceiver is periodically turned on and off according to a schedule [7], and the proportion of time during which the node is active is termed the duty-cycle. If duty-cycling considerably increases the lifetime of WSNs, it does not cancel the two primary sources for unnecessary energy consumption in WSNs: overhearing, i.e., reception of frames intended for other nodes, and idle listening, i.e., time during which the transceiver is active, but not receiving nor transmitting. A widespread solution to reduce these energy wastes is the design of adaptive communication schemes that modify their duty-cycling according to the available energy [8; 9; 10]. In this work, the mainstream contribution is to annul these wastes by associating EH-WSN and the Ultra Low Power (ULP) Wake-up Receiver (WuRx). Recent circuit-level advances have made possible the realization of ULP WuRx circuits that can efficiently \"wake up\" a node when a specific signal, called the Wake-up Beacon (WuB), is sent by a neighboring node [11; 12; 13; 14; 15; 16]. Moreover, many ULP WuRx embed address matching features, which allow nodes to wake up only a specific node and not all their neighbors [17; 18]. The main feature of ULP WuRx is the continuous listening of the wireless medium while keeping the main transceiver in the sleep state. When the node is in sleep state, the ULP WuRx is the only active component listening for WuBs and consuming power in the micro- or nano-Watt range, which is negligible when compared with the main radio that consumes milliwatts when active [11; 12; 13; 17; 18]. ULP WuRx were already successfully used in the context of wireless body area networks [19; 20] in which the limitations in terms of power budget are stringent [10]. Combining ULP WuRx with recent improvements in EH allows a significant increase of network lifetime [21]. However, in order to efficiently take advantage of this combination, this paper proposes to jointly design dedicated energy management strategies and ad hoc communication protocols.