# Wake-Up Receiver-Based Routing for Clustered Multihop Wireless Sensor Networks
## 3 Related Work  
The authors in [23] proposed two flooding protocols FLOODWUP and GREEN-WUP. FLOOD-WUP uses different broadcast addresses for opportunistic routing to forward messages to receivers that are not in range of the source node. Avoiding retransmission, each node changes its wake-up address upon reception of a data packet. To implement a  
Figure 1: A wireless sensor node equipped with a WuRx, according to [19]. further reliability mechanism, a unique sequence number is assigned to each broadcast packet. However, it may happen that a node loses the correct sequence, and additional control packages are required to reorganise the communication procedure. In GREEN-WUP, the researchers have added a parameter to the address of the sensor nodes in order to determine which sensor nodes should be used as relays. The address contains information about the level of harvested energy, based on which the sensor node with the highest level is selected. Unlike FLOOD-WUP, the sink goes into sleep mode after sending the WuPt. Therefore, a possible relay must first wake up the sink again by direct addressing it to indicate that it is acting as a relay. Due to this, this strategy requires additional WuPts and acknowledgements that are time and energy-consuming. The approach we have adopted, on the other hand, ensures that delay minimisation takes place by synchronously waking up the sensor nodes and the energy-efficient communication is achieved by means of no additional coordination packets. Researchers extended in [24] the Collection Tree Protocol (CTP) [25], the de facto standard for data collection in WSN to work with nodes equipped with WuRx. CTP-WUR utilises WuRx to relay wake-up requests and reduce end-to-end data latency. By reducing the number of hops needed to relay data packets, the proposed approach improves the performance of using short-range WuRx scenarios. Each node in the network is assigned a unique WuRx address, a unique WuRx relay address, and a broadcast WUR address shared by all nodes in the network. In broadcast mode, each node in the sender's neighbourhood will wake up and wait for receiving a data packet. On the contrary, in the unicast mode, the WuPts will be relayed without the need for waking up their main radio. Relay of WuPts is requested by means of a WuRx address containing the node's unique address and an additional flag indicating that the WuPt needs to be forwarded to the receiver's own parent. Compared to our work, the authors of this paper use the WuRx as a fully functional radio. They are able to receive and send WuPts with the WuRx, leaving the main radio in sleep mode. Even if the relay of the WuPt takes place without activating the main radio, energy is consumed for the forwarding of the WuPts. In WHARP [26], authors propose a data forwarding strategy for green wireless networks that exploits the self-powered Wake-up Radio capabilities of the network nodes. The described strategy offers the possibility to send data to the destination by making decentralised and proactive decisions. The decision is made based on forecasting energy and expected traffic. To start communication, a WuPt is broad-casted and all recipients then decide whether they can become active and act as a relay. Sending a large number of coordination packets results in a lot of data traffic and an increased time of the sensor nodes in the active state. Since sending and receiving in wireless sensor networks is very energy consuming, this is to be avoided. As our approach determines which nodes are to be woken up or used as relays before communication begins, there is no unnecessary energy consumption due to unnecessary waking ups. In addition, our strategy does not generate unnecessary communication traffic and thus favours a time-efficient network. A cross layer routing protocol called T-ROME was proposed in [27]. Authors introduce a set of parameters to optimise the relaying of data by dynamically choosing the most appropriate stopover nodes in case the sink is not reachable within one hop.