## 5 Evolution of Opportunistic Routing in Asynchronous WSN
### Evolution of Opportunistic Routing Protocols
#### 5.2.6 Lora  
Recently, in 2019, [PERSON] et al. proposed Load-Balanced Opportunistic Routing for Asynchronous duty-cycled WSN (LORA) [16], an opportunistic routing protocol for balancing load issues in asynchronous duty-cycled WSNs. In this protocol, the authors applied significant changes and introduced new measurements for opportunistic routing. This protocol combines two main parts. First, each node defines a candidate zone (CZ) [16] using a regular geometric shape with four corners, as shown in Figure 17. Second, the candidates within the CZ are prioritized based on the OR metric. The OR metric is defined as the multiplication of four distributions: direction distribution, transmission distance distribution, perpendicular-distance distribution and residual energy distribution [16]. The authors focused on two major issues: the duplicate packet problem (on account of multiple forwarders) and sender waiting time (increased due to ACK collisions). The main objective of this protocol is to control the number of candidates in the network layer, and design an OR protocol that can counterbalance the sender waiting time with the multiple forwarder node problem. It mainly works with a duty cycle and a forwarder set. The size of the forwarder set is controlled by the network density.  
In LORA, the authors identified that the energy required for routing a packet depends on three major components: (i) the transmission distance of each hop in the path; (ii) the length of the perpendicular distance from every selected forwarder node to the virtual line linking the source node and sink node; and (iii) the width of the direction angle between the sender node and the forwarder node, with respect to the location of the sink node. The nodes with smaller perpendicular distances are preferably selected as forwarders, because these nodes construct a straighte  
Figure 17: Packet transmission in LORA [16].  
direction angle between the sender and forwarder means that the forwarder is closer to the sink, and potentially provides routing progress at a lower cost. A forwarder with a smaller transmission distance from the sender is preferable, as the energy consumption is related to transmission distance. Hence, the authors concluded that an energy-efficient path between the source and the sink nodes should select forwarders with smaller perpendicular distances, transmission distances and direction angles. They added the energy-balancing factor to avoid the energy depletion of some nodes earlier than others. Therefore, their routing metric (or link quality estimator) is defined as a multiplication of these four factors (four distributions).  
However, in order to obtain this metric, the calculation cost is too high for every forwarder node because with an increase in network density, the number of multiple forwarders also increases. Moreover, they use a longer duty cycle in order to reduce the sender waiting time, which however increases the number of forwarders. For this reason, they focused on reducing the size of the forwarder set. On the other hand, the overhead of the calculations is higher and more time-consuming. In addition, this protocol did not consider the E2E latency. Compared with the previous OR protocols for asynchronous duty-cycled WSNs, LORA did not show any improvement in reducing sender waiting times.