User-Centric Clustering for Designing Ultradense Networks: Architecture, Objective Functions, and Design Guidelines

User-centric (UC) clustering, an innovative design principle for ultradense networks (UDNs), supports dynamically fluctuating adaptive network topologies. In this article, we introduce the UC-UDN architecture and provide a tutorial on UC clustering design by generalizing the problem under practical constraints. In the context of UC clustering, we briefly present various promising methods that represent constraint options and also provide a pair of case studies focused on design tradeoffs. Finally, the salient future directions of UCUDNs are identified.