A LoRa-Based Mesh Network for Peer-to-Peer Long-Range Communication

LoRa is a long-range and low-power radio technology largely employed in Internet of Things (IoT) scenarios. It defines the lower physical layer while other protocols, such as LoRaWAN, define the upper layers of the network. A LoRaWAN network assumes a star topology where each of the nodes communicates with multiple gateways which, in turn, forward the collected data to a network server. The main LoRaWAN characteristic is the central role of the gateways; however, in some application scenarios, a much lighter protocol stack, relying only on node capabilities and without the presence of gateways, can be more suitable. In this paper, we present a preliminary study for realizing a LoRa-based mesh network, not relying on LoRaWAN, that implements a peer-to-peer communication between nodes, without the use of gateways, and extends node reachability through multi-hop communication. To validate our investigations, we present a hardware/software prototype based on low-power-consumption devices, and we preliminarily assess the proposed solution.

1. Introduction and Background

The Internet of Things (IoT) has become an essential and pervasive means in our society. Nowadays, more and more industrial, commercial and customer applications rely on data collected by a multitude of heterogeneous devices located at the extreme borders of the network. Given their high density, their limited battery availability and their inaccessibility in most of the cases, the research community has struggled to design lightweight solutions to ensure long-range, low-power and low-bitrate wireless transmissions to/from those devices, opening the door to the deployment of the so-called LPWANs (low-power wide-area networks) [1].

A well-known wireless technology for LPWANs is LoRa (long range) [2]. LoRa implements a physical layer that combines the chirp spread spectrum (CSS) radio modulation with integrated forward error correction (FEC) for enabling robust long-range communications on unlicensed industrial, scientific and medical (ISM) frequency bands. Given its robustness and versatility, LoRa has quickly become the most widely adopted physical layer for LPWANs. Concerning the upper layers, the LoRa Alliance has then proposed LoRaWAN [3], an open media access control (MAC) and network protocol that allows LoRa-based devices to communicate and that inherently adopts a well-defined network architecture [4].

A LoRaWAN network includes three architectural components: the end devices, the gateways and a remote network server. These components are inter-connected in a “star-of-stars” topology, where end devices communicate with one or more gateways (using LoRa as the physical layer) and where each gateway dispatches LoRaWAN frames to the network server using a higher-throughput backhaul interface (e.g., WiFi or 5G). Then, applications interfacing with the server can make the best use of the collected data. LoRaWAN is the most widely adopted L2/L3 protocol for LPWANs, although some limitations have been identified [5,6].

One of the strongest limitations of LoRaWAN is the adopted topology, where only direct single-hop communication is allowed between end devices and gateways. Even though this configuration is suitable for many applications, in some cases (e.g., when data must be gathered/exchanged from/in difficult-to-access areas) it is far from being the optimal solution. Many works in the literature have then dealt with enabling mesh networking and multi-hop communication in LoRaWAN or on similar alternative LoRa-based architectures, where end devices can act as relay nodes, to extend network coverage and improve energy consumption [7].

Even though a significant step forward has been made by these works, we believe that it is just an intermediate step. In fact, all of them still partially embrace a LoraWAN or LoRaWAN-like network architecture, and extend it towards supporting a “star-of-meshes” topology. This means that gateways still play a central role as concentrators, and that data need to be finally conveyed through Internet/broadband access, to a remote location before being made accessible to applications. This is clearly not ideal in application scenarios where it would be better to keep data local for privacy or performance reasons (e.g., in the case of emergency applications for disaster recovery or first responders support). Additionally, also in the case of a privacy-preserving and high-speed private network infrastructure where data collected by the gateway are not conveyed through the public Internet but through dedicated backhaul links, the gateway is a single point of failure whose malfunctioning would compromise the operation of the whole LPWAN network.

In this paper we pave the way towards filling this gap. We propose and preliminarily evaluate a hardware/software LoRa-based solution that, being completely gateway-free, enables peer-to-peer communication among LoRa end devices, while also preserving multi-hop and mesh networking functionalities as proposed in previous works. The solution, developed on top of the LoRa physical layer, is meant to provide a lighter network stack than LoRaWAN, so that low-cost, flexible and easy-to-configure “out-of-Internet” communication can be ensured wherever and whenever needed.

Our proposal is extremely cheap (no LoRaWAN gateway needs to be bought and configured) and effective: each of the end devices acts as a simplified gateway, which can be accessed through its USB serial port by a more powerful device (e.g., a laptop). In this case, the limited computational capacity of the single node can be increased in order to embrace more computational demanding application scenarios. Our preliminary results have shown how, by exploiting different LoRa transmission setups (i.e., modulations), it is possible to strike the most desirable balance between network coverage and end-to-end transmission delay.

The remainder of this paper is structured as follows. Section 2 recalls the related work. Section 3 describes the proposed architecture, while Section 4 summarizes our preliminary results. Finally, Section 5 concludes the paper and reports on the planned future work.