  Long-range (LoRa) technology is most widely used for enabling low-power wide
area networks (WANs) on unlicensed frequency bands. Despite its modest data
rates, it provides extensive coverage for low-power devices, making it an ideal
communication system for many internet of things (IoT) applications. In
general, LoRa is considered as the physical layer, whereas LoRaWAN is the
medium access control (MAC) layer of the LoRa stack that adopts a star topology
to enable communication between multiple end devices (EDs) and the network
gateway. The chirp spread spectrum modulation deals with LoRa signal
interference and ensures long-range communication. At the same time, the
adaptive data rate mechanism allows EDs to dynamically alter some LoRa
features, such as the spreading factor (SF), code rate, and carrier frequency
to address the time variance of communication conditions in dense networks.
Despite the high LoRa connectivity demand, LoRa signals interference and
concurrent transmission collisions are major limitations. Therefore, to enhance
LoRaWAN capacity, the LoRa Alliance released many LoRaWAN versions, and the
research community has provided numerous solutions to develop scalable LoRaWAN
technology. Hence, we thoroughly examine LoRaWAN scalability challenges and
state-of-the-art solutions in both the physical and MAC layers. These solutions
primarily rely on SF, logical, and frequency channel assignment, whereas others
propose new network topologies or implement signal processing schemes to cancel
the interference and allow LoRaWAN to connect more EDs efficiently. A summary
of the existing solutions in the literature is provided at the end of the
paper, describing the advantages and disadvantages of each solution and
suggesting possible enhancements as future research directions.
