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）技术是在非授权频段实现低功耗广域网（WAN）最广泛采用的技术。尽管其数据速率有限，但能为低功耗设备提供广域覆盖，使其成为众多物联网（IoT）应用的理想通信系统。通常，LoRa被视为物理层，而LoRaWAN是LoRa协议栈中的介质访问控制（MAC）层，采用星型拓扑结构实现多个终端设备（ED）与网络网关之间的通信。啁啾扩频调制技术处理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扩频因子（SF）、编码速率和载波频率等LoRa参数，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，但信号干扰和并发传输冲突仍是主要限制因素。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可扩展LoRaWAN技术的解决方案。本文系统研究了LoRaWAN在物理层和MAC层的可扩展性挑战及前沿解决方案。这些方案主要基于扩频因子、逻辑信道与频率信道分配，另一些则提出新型网络拓扑或实现信号处理方案来消除干扰，使LoRaWAN能够更高效地连接更多终端设备。本文末尾对现有文献中的解决方案进行了总结，阐述了各方案的优缺点，并提出了未来研究方向中可能的改进措施。