# Leveraging Energy Harvesting and Wake-Up Receivers for Long-Term Wireless Sensor Networks
## 2 Related Work
### MAC Protocols Leveraging Wake-Up Receivers  
There has been a tremendous amount of research on the design and implementation of MAC protocols in WSNs [35]. WSN MACs can be classified into three paradigms: synchronous, pseudo-asynchronous and asynchronous. In the first approach, neighboring nodes are synchronized to wake up at the same time. However, in the context of EH-WSNs, environmental power sources provide energy that continuously varies over time and space, making synchronous approaches not optimal for such application scenarios [36]. Indeed, nodes powered by energy harvesting must be able to dynamically adapt their duty-cycle, and pseudo-asynchronous and asynchronous schemes allow each node to choose its active schedule independently of other nodes. Traditionally, pseudo-asynchronous schemes rely on duty-cycling [7], in which nodes are periodically powered on and off according to their own specific schedule while establishing on demand rendezvous using a beaconing approach. Pseudo-asynchronous schemes can be categorized as transmitter-initiated or receiver-initiated depending upon who initiates the rendezvous [37]. In the transmitter-initiated scheme, the receiving node periodically wakes up to monitor the channel and goes back to sleep after a short wake-up duration if the channel is found to be clear. When a node has a packet to send, it transmits request-to-send signals to the destination node, each followed by a listening period. The destination node, upon waking up according to its regular schedule acquires the transmit request and answers the transmitting node by a clear-to-send message. After this rendezvous process, the data packet is sent. In the receiver-initiated scheme, the receiving node periodically wakes up and send a clear-to-send beacon. It then monitors the channel for a short duration and goes back to sleep if no signal is detected. If a node needs to transmit data, it listens to the channel for the clear-to-send beacon from the receiver and, upon reception, starts sending its data packet. Many variations of these two approaches can be found in the literature [35], and because of the underlying wake-up scheme and rendezvous process, these approaches are referred to as pseudo-asynchronous. Fully-asynchronous communications can be achieved with the use of ULP WuRx. ULP WuRx enables the cancellation of the energy waste due to the rendezvous process and the periodic wake-ups [38] As ULP WuRx technology is still under development and relatively new, only a few research studies were conducted on designing communication protocols leveraging ULP WuRx. WUR-MAC [39] was the first MAC protocol that took advantage of ULP WuRx. The ULP WuRx and the main transceiver use separate channels, and a request-to-send/clear-to-send handshake with channel assignment is done using the ULP WuRx. By receiving each incoming request-to-send and clear-to-send frame, a node has therefore the information about which channel is used by its neighbors. When it wants to communicate, it randomly chooses a free channel and sends a request-to-send frame containing the chosen channel. As our approach does not require request-to-send/clear-to-send handshake or a similar mechanism, packet transmissions are energetically less expensive than with WUR-MAC. Zippy [18] is a flooding protocol that leverages WuRx. This protocol has been experimentally validated on real sensor nodes. However, in the context of data-gathering star networks, the flooding approach, in which each packet is sent to all neighboring nodes, is not well-suited as only the sink needs to receive the data sent by the nodes. In a previous work, we introduced OPWUM [40], an opportunistic forwarding MAC using timer-based contention for next hop relay selection and leveraging ULP WuRx. This MAC is nonetheless specifically designed for multi-hop networks, and is therefore not appropriated to star networks.