# LTM-LoRaWAN: A Multi-Hop Communication System for LoRaWAN
## 2 Related Works  
LoRaWAN multi-hop communication approach has been investigated by several publications in the literature to find out its feasibility in increasing network coverage. It has been used in several LoRaWAN IoT applications, such as environmental monitoring [8], underground monitoring [9], oil pipeline detection [10], and smart cities [11]. This part of the paper covers current research related more to the relaying approaches rather than the routing. A comprehensive survey for multi-hop communication in the LoRaWAN field can be found in [3; 12; 13]. The work in [14] used a special node (Enhanced Node) to extend the range of the transmitted traffic. The Enhanced node requires a continuous power supply to stay ready for any incoming traffic. It operates at the link layer level. The Enhanced node duty is to replicate any received messages without enforcing any limitation if the incoming traffic does not require confirmation. However, the subsequent receive windows are used for confirmed traffic along with synchronizing the incoming and sending paths. In the case of downlink traffic, a gateway is required to transmit the downlink traffic after the first receive window of the e-node, whereas the e-node is required to send the downlink after the end of the second receive window of the initial uplink message. The feasibility of the e-node proposal was tested in 2800 m\({}^{2}\) industrial laboratory. The e-node was located about 100 m away from the gateway, and few nodes transmitting traffic were used. The work in [15] suggested the addition of a relay device after the deployment of the network. The relay devices are placed in locations to serve far away LoRaWAN end devices and work as intermediate entities, relaying the traffic toward the gateway. The proposed relay nodes are different from standard end devices, where their main task is to extend the coverage of the LoRaWAN network via receiving and forwarding operations. One of the proposal's main considerations was to select the most appropriate location for the relay device. The relay devices can switch between active and sleep mode based on the analysis of the uplink traffic pattern. In addition, they are proposed as transparent where the end devices and the gateways are not aware of the existence of such relay devices. The proposed approach assumed two phases for a relay device operation. The first is when the relay device stays in active mode to discover end nodes within its vicinity and observe uplink traffic. Once a relay device finishes its observation phase, it moves to the second phase, the data forwarding stage. In this stage, the relay device is in sleep mode and wakes up periodically to receive and forward uplink traffic. On-demand forwarding has been used in [16], where relay nodes forward the received packet if and only if the network server does not receive it. This was achieved using an on-demand feedback mechanism by the server to inform the relay nodes of the status of a particular packet. The proposal made use of priority-based forwarding as well. The priority is organized based on the RSSI, which is distributed by the network server in advance of transmitting the feedback message. The relaying node with the highest priority can forward a packet immediately when it receives a feedback message from the server. Other relaying nodes should stay in active mode all the time, listening to the transmission from the relaying node with higher priority. Once the relay with higher priority is finished, the other can start forwarding their messages. The proposal was evaluated in simulation and achieved a similar delivery ratio with a smaller number of retransmissions. The work in [11] proposed two architectures to serve multiple hops in LoRaWAN. The first architecture assumes that an end device can operate as a relay node to forward faraway node traffic toward a gateway. However, the paper lacks a description of how the forwarding mechanism works. It claimed that the relay node operates a decode and forward mechanism.