The growth of the Internet of Things (IoT) led to the deployment of many applications that use wireless networks, like smart cities and smart agriculture. Low Power Wide Area Networks (LPWANs) meet many requirements of IoT, such as energy efficiency, low cost, large coverage area, and large-scale deployment. Long Range Wide Area Network (LoRaWAN) networks are one of the most studied and implemented LPWAN technologies, due to the facility to build private networks with an open standard. Typical LoRaWAN networks are single-hop in a star topology, composed of end-devices that transmit data directly to gateways. Recently, several studies proposed multihop LoRaWAN networks, thus forming wireless mesh networks. This article provides a review of the state-of-the-art multihop proposals for LoRaWAN. In addition, we carried out a comparative analysis and classification, considering technical characteristics, intermediate devices function, and network topologies. This paper also discusses open issues and future directions to realize the full potential of multihop networking. We hope to encourage other researchers to work on improving the performance of LoRaWAN mesh networks, with more theoretical and simulation analysis, as well as practical deployments.

LoRaWAN is a widely adopted protocol for Internet of Things applications with energy constraints and large coverage area demands. The protocol standardization defines a star-of-stars topology, where end-devices send their packets directly to a gateway that forwards the messages to the network server, where they are handled depending on the application. LoRaWAN is at the top of the Long Range (LoRa) physical layer based on the Chirp Spread Spectrum (CSS) modulation, where signals are spread along the frequency by the Spreading Factor (SF). The modulation technique gives the LoRaWAN long range coverage characteristics, that could reach 45 kilometers in theory. Furthermore, the protocol adopts the Industrial, Medical, and Scientific (ISM) frequency band plans, which impose spectrum occupation limitations, and the constraints are defined by a device Duty Cycle (DC). The DC imposes a restriction of 0.1% up to 10% in transmission time, limiting the total amount of messages an end-deceive could send. In order to increase even more the coverage area, many works proposed the adoption of multihop techniques in LoRaWAN networks. Proposed solutions are based on routing and relay approaches, both with benefits and limitations. However, the literature does not explore the DC constraints on the multihop LoRaWAN solutions since packets might be enqueued. The packet delay increases due to the time spent in the buffer, and the delivery ratio decreases because packets are dropped when the buffer is full. To understand such impact, we provide a model to generalize the DC behavior on multihop LoRaWAN networks, focused on the packet delivery ratio and delay metrics. We evaluate our model analytically and experimentally. First, we conducted a numerical comparison between the standard network with the multihop Long Range Wide Area Network (LoRaWAN) over duty cycle constraints. The comparison considers a rural area and the coverage area limited by the 802.11 ah propagation model. The results elucidated the tradeoff between packet size, number of packets, and delay. The two-hop solution could decrease the delay compared with direct transmission using high spreading factors. However, in dense networks, the intermediate device buffer increases the packet delivery delay. It could reduce the packet delivery ratio because the device must respect the duty cycle to forward packets. On the other hand, higher spreading factors decrease the payload size and need more packets to transmit the same message. Then, we compared analytical and experimental results in four relayed scenarios with different numbers of devices and different intermediate device buffer sizes. The results show the DC significantly increases the network delay and packet losses. The intermediate device also causes packet prioritization, making the network unfair. The multihop strategies are relevant for many applications, including coverage area extension, but the solutions must consider the DC constraints as a limitation to their adoption.

LoRaWAN is one of the protocols responsible for connecting devices to the IoT ecosystem. The protocol provides connectivity with low energy consumption and a large coverage area. However, many scenarios impose difficulties on the transmission, and sometimes it is necessary to increase the coverage using a multihop approach. The objective of this work is to introduce a gateway-based relay device for a multihop LoRaWAN network. The called relay-gateway is an intermediate device capable of receiving data from many sources due to its hardware capabilities. It is reliable in terms of energy consumption because it uses a fixed energy source. The experimental evaluation conducted at the university campus shows that the relay solution is suitable for extending the network and providing connectivity to remote areas. The results show a 50% increase of packet delivery for some places using the relay solution compared with direct transmission using SF7.

IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) is the only standardized routing protocol for Low Power and Lossy Networks. Therefore, RPL is the routing protocol chosen for most Internet of Things (IoT) applications. However, the RPL protocol has some open issues when the scenario includes mobile nodes. Since many IoT applications include mobility, this is an important problem to address. Node mobility does not permit the network to stabilize and because of this keep exchanging control messages. Mobility increases the packet loss because mobile nodes may leave the coverage area before transmitting the data. This problems also increase the power consumption. Another important factor affects the RPL performance. To save nodes energy, the data link layer implements a radio duty cycle. These duty cycles turn the radio off when it is not used. One common and used radio duty cycle protocol is the ContikiMAC. Changes in the frequency of the duty cycle modifies the RPL performance. This work shows how the change of radio duty cycle of ContikiMAC on a hybrid scenario, composed by fixed and mobile nodes, interferes on RPL performance. Results show that the nodes mobility degrade the metrics tested. The choice of radio duty cycle is very important to the network performance. The results show that the RPL performance is strictly dependent of the duty cycle.