# Opportunistic and On-Demand Network Coding-Based Solutions for LPWAN Forwarding
## 2 Related Works  
### Forwarding Architecture  
There are three kinds of multi-hop approaches for LPWAN forwarding: fixed path, flooding, and opportunistic, as shown in Figure 2. Fixed-path forwarding is a topology based routing, according to which each node exchanges information to maintain a routing table. We group the routing discovery into proactive-based (Destination-Sequenced Distance-Vector (DSDV) and Optimized Link State Routing (OLSR)), reactive-based (Dynamic Source Routing (DSR) and Ad-hoc On-demand Distance Vector (AODV)), and Hybrid-based (Hybrid Wireless Mesh Protocol (HWMP)) [16; 17; 18]. Fixed-path forwarding determines the best path using routing metrics, such as Expected Transmission Count (ETX) and Expected Transmission Time (ETT) [19; 20; 21; 22]. The LPWAN multi-hop solutions in [9; 10; 11; 13] are based on routing protocol for low-power and lossy networks (RPL) routing. The authors in [12] adopt HWMP hybrid forwarding to reduce high control overhead in the proactive-based and high latency in the reactive-based routing discovery mechanism. However, this fixed-path based approach shows poor performance in an unstable wireless environment, especially when the next destination cannot receive the transmitted packet [8].  
Figure 2: LoRa forwarder anatomy.  
The combination of flooding architecture and concurrent transmission (CT) is presented in [23]. The flooding originates from a transmission broadcast by an initiator; subsequently, any receiver will rebroadcast the packet immediately as the next wave, which leads to a broadcast storm problem [24]. In LPWAN multi-hop, this approach is adopted by LoRaBlink [14] and CT flooding [15]. Unlike LBT (Listen Before Talk) or CSMA/CA (Carrier-sense multiple access with collision avoidance) [25] that avoids collisions, CT synchronizes the duplicate packet to recover the packet. Although CT exhibits excellent performance in energy consumption, reliability, and latency, flooding transmission reduces network capacity.  
ExOR [8] is the first conception of opportunistic routing that exploits multiple-receiving relays to forward an overheard packet based on a strict timing schedule to prevent collisions. This approach is proved to increase network reliability and throughput more efficiently. A Simple Opportunistic Adaptive Routing (SOAR) [26] improves Exor by reducing the diverging path and removing strict scheduling. In SOAR, each packet brings a list of relay candidates calculated from the ETX metric. The relay candidates coordinate and exchange transmission information state to prevent duplication. A Cooperative Power and Energy-efficient routing (COPE) [27] introduces network coding to prevents strict regulation. Each relay encodes multiple packets together, and then intelligently transmit the coded packets and reuse the duplicate packets for facilitating packet recovery. The authors in [7] propose a cooperative relaying for the LoRa network by exploiting the benefit of packet redundancy. Instead of performing a routing mechanism, the relay combines the overheard packet and then forwards it to the gateway following the duty cycle limitation. Although it achieves high-reliability performance, this approach is only good to combine a small packet of sensor data in a short distance. As it uses the maximum size of LoRa payload for packet relaying, these works consume a high amount of time resources in a higher SF.