Toward Ubiquitous Connectivity via LoRaWAN: An Overview of Signal Collision Resolving Solutions

The ever increasing demand for energy-efficient, long-range, and license-free Internet of Things (IoT) communication networks has spurred great interest in unlicensed low-power wide area networks (LPWANs). Such a revolutionary technology represents a paradigm shift in the development of wireless networks, as it can enable low-power (milliwatts) IoT devices to transmit at low data rates (kilobits per second) over long distances (several kilometers), which is clearly different from conventional IoT networking technologies (e.g., Wi-Fi and ZigBee) that only support short-range communications. Specifically, long-range WAN (LoRaWAN) is a leading and representative LPWAN technology that has been gathering remarkable attention from both academia and industry because of its open source software and installation ease. Despite this interest, however, current LoRaWAN is not yet a mature communication platform to provide ubiquitous connectivity for a wide variety of IoT applications. This is mainly because LoRaWAN typically serves a large volume of coexisting end devices among which signal collisions frequently occur at the physical layer (LoRa) of LoRaWAN, thereby leading to severe degradation of overall network capacity. In this context, the aim of this article is to provide a comprehensive overview of up-to-date physical-layer solutions for LoRa collision resolution. We particularly investigate different types of LoRa collisions resulting from the spreading factor network parameter. Moreover, the potential visions of physical-layer design to resolve LoRa collisions that might be considered for future research are also discussed.