The utilization of unlicensed spectrum presents a promising solution to the issue of spectrum scarcity in densely populated areas, while also offering a cost-effective means to connect underserved regions. In response to this potential, both academia and industry are actively exploring innovative applications of unlicensed spectrum. This work offers a thorough overview of unlicensed spectrum bands below 8 GHz, including TV White Spaces, Civil Broadband Radio Services, Industrial Scientific Medical bands, and the Unlicensed National Information Infrastructure. The paper focuses on three key aspects: regulations, existing technologies, and applications. It is essential to recognize that "unlicensed" does not equate to "unregulated"; therefore, a clear understanding of permissible and prohibited activities is crucial. From a technological perspective, we examine the current technologies, their capabilities, and relevant applications. Additionally, the shared nature of this spectrum introduces challenges related to interference among users. These collisions can be managed through two primary strategies, that we described: a database-driven approach and coexistence mechanisms at the MAC and PHY layers. This work may serve as a starting point for those who are interested in the unlicensed spectrum, both in academia and industry.

翻译：非授权频谱的利用为解决人口密集区域的频谱稀缺问题提供了一种有前景的方案，同时也为连接未覆盖区域提供了一种经济有效的手段。针对这一潜力，学术界和工业界正积极探索非授权频谱的创新应用。本文全面概述了8 GHz以下的非授权频段，包括电视空白频段、民用宽带无线电业务、工业科学医疗频段以及非授权国家信息基础设施。本文聚焦于三个关键方面：法规、现有技术及应用。需明确认识到，“非授权”并不等同于“无监管”，因此清晰理解允许与禁止的行为至关重要。从技术角度出发，我们审视了当前技术、其能力及相关应用。此外，该频谱的共享特性引入了用户间干扰的挑战。这些冲突可通过两种主要策略加以管理，本文对此进行了描述：基于数据库的方法以及MAC和PHY层的共存机制。本文可作为学术界和工业界对非授权频谱感兴趣的从业者的入门参考。