I. Introduction
In order to satisfy the exponential growth of wireless traffic in the next decade, the transmission data rate of the fifth generation (5G) mobile communication system is required to increase by 1,000 times [1]. Increasing the density of base stations in the system can support higher capacity and more connections [2], and hence, ultra-dense network (UDN) has been evolved as one of the key technologies of 5G [3]. UDN reduces the distance between users and base stations and hence the link quality can be improved. However, in addition to amplifying the interference problem, such dense deployment can also raise huge computational complexity and require huge signaling overhead [4], and according to [5], the density of small base stations in UDN may exceed 1000 cells/km2. Therefore, most of the existing methods (e.g., resource allocation and interference mitigation) will not be suitable for UDN systems due to the large-scale of the network and the real-time requirement.