# LTM-LoRaWAN: A Multi-Hop Communication System for LoRaWAN
## 2 Related Works
Moreover, the work evaluated the proposal using real experiments with a few devices that were placed only a few hundred meters from each other. The second architecture divides the network topology into several clusters. Each cluster has its gateway, which forwards the traffic towards a main gateway. The architecture was evaluated in a 290 m \(\times\) 210 m topology covering two clusters. An opportunistic protocol for LoRa, named LoRaOpp, was proposed in [17]. It uses some opportunistic mechanisms, such as source routing and spray and focus, which have been studied to handle disruption and opportunistic networking in the literature. These mechanisms were used to limit the number of forwarded packets in the network. The proposal assumes three different LoRa devices: end node, peer node, and gateways. It assumed that LoRa devices are capable of storing data in case the next node in the path is not reachable until the following contact opportunity. LoRaOpp used broadcasting to allow gateways and end devices to announce their existence via broadcasting dedicated packets frequently. The protocol uses a collision detection mechanism and retransmission technique to avoid data loss. It was evaluated experimentally in an outdoor environment with seven end devices, including the gateway. An emulation was also used to evaluate the performance of the proposal in larger networks that contain up to 100 LoRa devices. Based on the LoRa physical layer, the work in [18] proposed a mesh LoRaWAN named LoRaMesh aiming to extend the coverage of LoRaWAN networks. To implement multi-hop functionality, the proposed system modified several components of LoRaWAN, including the implementation of the network server, reception windows, and packet headers to facilitate retransmission. Bacon flooding is initiated by Gateways and is used to keep time synchronization among devices. The synchronization bacon is then retransmitted by one hop device to other LoRaWAN devices. A rule-based mechanism is used to set a fixed transmission schedule for every single device at the beginning of the network, where each child device is assigned a slot time for transmission. The proposal assumes a routing procedure with a global knowledge of locations and path loss between devices and the gateway to calculate routing paths and build tree structure routing for the whole network. The network server determines parent devices as well as children's devices. The proposal was implemented in NS-3, evaluated, and compared to single-hop LoRaWAN. A genetic algorithm was used in [19] to optimize the performance of the LoRaWAN network in terms of latency and coverage probability. The study investigated the performance of dual-hop LoRaWAN following the cooperative communication strategy where devices cooperate to carry out data transmission. The study then utilized a genetic algorithm to obtain the LoRa device's optimal transmission configuration in terms of received power, where it is at its maximum value. The achieved values are then utilized to enhance the performance regarding coverage and latency. The proposal was simulated in MATLAB, and it is reported that it improved the coverage of the network by about 20%. A two-hop real-time LoRa was proposed in [20] and extended in [21]. The proposed protocol contains two main phases: tree topology construction and slot scheduling. In the first phase, the protocol constructs a tree topology of the network based on the quality of the links between end devices and the gateway. The tree topology is then used by the network server to perform slot scheduling between one-hop and two-hop neighbors of the gateways. The one-hop end devices can transmit their uplink traffic directly, and the two-hop devices use one of the one-hop devices as a relaying node to transmit their traffic on the scheduled slots. The proposal optimizes broadcasting to build the tree topology where a gateway is required to broadcast a Tree Construction Request that will be received by all possible one-hop devices.