  Ultra-dense networks (UDN) constitute one of the most promising techniques of
supporting the 5G mobile system. By deploying more small cells in a fixed area,
the average distance between users and access points can be significantly
reduced, hence a dense spatial frequency reuse can be exploited. However,
severe interference is the major obstacle in UDN. Most of the contributions
deal with the interference by relying on cooperative game theory. This paper
advocates the application of dense user-centric C-RAN philosophy to UDN, thanks
to the recent development of cloud computing techniques. Under dense C-RAN,
centralized signal processing can be invoked for supporting CoMP transmission.
We summarize the main challenges in dense user-centric C-RANs. One of the most
challenging issues is the requirement of the global CSI for the sake of
cooperative transmission. We investigate this requirement by only relying on
partial CSI, namely, on inter-cluster large-scale CSI. Furthermore, the
estimation of the intra-cluster CSI is considered, including the pilot
allocation and robust transmission. Finally, we highlight several promising
research directions to make the dense user-centric C-RAN become a reality, with
special emphasis on the application of the `big data' techniques.
