# Opportunistic and On-Demand Network Coding-Based Solutions for LPWAN Forwarding
## 1 Introduction  
Low-power and wide-area networks (LPWAN) technology has gained significant attention from the standpoint of IoT services as it facilitates long-range communication with low-power consumption. This technology works on the sub-GHz band, allowing wider coverage up to 20 km in the line-of-sight (LoS). Typically, the data rate is limited to 0.3 kbit/s to 50 kbit/s [1]. However, it is sufficient for internet-of-things (IoT) applications, such as smart city or telemetry services that send small packets over wide areas. Several LPWAN technologies are available in the market, based on licensed bands (LTE-M and NB-IoT ) and unlicensed bands (Sigfox and LoRa). Among them, LoRa [2] or long-range radio, using a CSS (Chirp Spread Spectrum) modulation technique, is a popular LPWAN radio technology that has been adopted worldwide. LoRaWAN [3] is a Medium Access Control (MAC) layer specification used to build LPWAN networks based on LoRa radio. LoRaWAN is gaining popularity because it is an open standard that allows researchers and developers to improve and modify the specifications according to their needs. LoRaWAN adopts single-hop star-of-stars network topology where the end-devices transmit its packet directly to one or many gateways via LoRa modulation. The gateway acts as a bridge forwardthe received packet to the network server using a standard IP connection. Although simple and robust, it trade-off the data transmission rate to achieve longer coverage. There are 6 available spreading factor (\(SF\)) (7-12) and 3 Bandwidth (\(BW\)) selection (125 kHz, 250 kHz, and 500 kHz) [3]. As the SF is increased and the BW is decreased, greater coverage can be achieved. However, the use of a slower data transmission rate in LoRaWAN with an Aloha based protocol increases the collision probability. Field-testing conducted by [PERSON] et al. [4] shows that SF 12 is required to achieve a 90% packet delivery ratio (PDR) for coverage below 3 km. However, this low data rate settings can only support 120 end-devices [5]. Stationing more gateways near the end-device may solve reliability and scalability issues in LoRaWAN. However, a gateway requires internet access, which increases cost and not suitable for a location where internet infrastructure is extremely limited (e.g., basements). An alternative solution is to employ a forwarding mechanism by placing a relay between the end-device and the gateway. In this study, we investigated the benefit of stationing a relay between the end-devices and a gateway. The use of relay extends the coverage and, in addition, increases the scalability via utilization of a faster data transmission rate. However, random placement of the end-devices in a real-world case makes it hard to put the relay at the right location. Each end-device facing different interference, and some of them could transmit packet directly to the gateway using the best data rate. Although a packet duplication caused by unnecessary forwarding can increase reliability [6; 7], it also reduces the availability of network resources that prevent transmission from the end-devices. For a massive IoT, this unnecessary forwarding can be a severe problem that decreases the overall reliability. Motivated by the side effect of unnecessary forwarding, we design a relay forwarding mechanism to forward a packet from the end-devices that cannot communicate directly to the gateway, as shown in Figure 1. We also consider that wireless communication is unpredictable and showing poor dynamic characteristics owing to dynamic propagation [8]. Sometimes, the end-device can transmit directly to the gateway. However, when a forwarder is needed, the selected relay can fail to receive the packet. When several relays are present, it is beneficial to utilize multiple-receiving of one or more relays that overheard the packet. Instead of asking retransmission from the end-devices that are unnecessary, any relay that receives the packet can forward the packet.