This book constitutes the refereed post-conference proceedings of the 12th International Conference on Broadband Communications, Networks, and Systems, Broadnets 2021, which took place in October 2021. Due to COVID-19 pandemic the conference was held virtually. The 24 full papers presented were carefully reviewed and selected from 49 submissions. The papers are thematically grouped as a session on broadband communications, networks, and systems; 5G-enabled smart building: technology and challenge; and 5G: The advances in industry.