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.

翻译：长距离（LoRa）技术最广泛用于在非授权频段上实现低功耗广域网。尽管其数据速率不高，但能为低功耗设备提供广泛覆盖，使其成为众多物联网应用的理想通信系统。通常，LoRa被视为物理层，而LoRaWAN是LoRa协议栈中介质访问控制层，采用星型拓扑实现多个终端设备与网络网关之间的通信。啁啾扩频调制技术应对LoRa信号干扰并确保远距离通信，同时，自适应数据速率机制允许终端设备动态调整LoRa的部分特征参数，如扩频因子、编码率和载波频率，以应对密集网络中通信条件的时变特性。尽管LoRa连接需求旺盛，但LoRa信号干扰和并发传输碰撞是其主要限制。因此，为提升LoRaWAN容量，LoRa联盟发布了多个LoRaWAN版本，学术界也提供了大量解决方案来发展可扩展的LoRaWAN技术。为此，我们深入研究了LoRaWAN的可扩展性挑战及物理层与MAC层的最先进解决方案。这些方案主要基于扩频因子、逻辑信道与频域信道分配，另有一些方案提出新型网络拓扑或采用信号处理机制来消除干扰，使LoRaWAN能高效连接更多终端设备。本文末尾总结了现有文献中的解决方案，描述了各方案的优缺点，并提出了可能改进的未来研究方向。