Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: User-centric C-RAN Architecture for Ultra-dense 5G Networks: Challenges
  and Methodologies   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.

28
The categories of this paper are: alg-geom
23
The categories of this paper are: solv-int
1
The categories of this paper are: cs
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
4
The categories of this paper are: cond-mat
31
The categories of this paper are: ao-sci
12
The categories of this paper are: nucl-th
17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
26
The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph