Communications Over Unlicensed Sub-8-GHz Spectrum: Opportunities and Challenges

The use of unlicensed spectrum offers a promising solution to spectrum scarcity in densely populated areas and a cost-effective way to connect underserved regions. Recognizing this potential, both academia and industry are actively exploring innovative uses of unlicensed spectrum. This work presents a broad overview of unlicensed spectrum bands below 8 GHz, including TV White Spaces, Civil Broadband Radio Services, Industrial, Scientific, and Medical bands, and the Unlicensed National Information Infrastructure. The paper examines three main aspects: regulations, existing technologies, and applications. Regulations play a central role, as understanding these rules is vital since “unlicensed” does not mean “unregulated”. From a technological perspective, we review current technologies, standards, and products, their features, and related applications. Furthermore, the shared nature of these spectrum bands introduces challenges related to user interference. The communication collisions can be managed through two primary strategies 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 interested in the unlicensed spectrum, both in academia and industry.