Mobile communications have deeply changed people¡¯s life, but the demand for higher performance of mobile communications has never stopped. To satisfy people¡¯s desire for explosive growth of data traffic and continuous emergence of new services, the fifth-generation (5G) mobile communications system emerges at the right moment. This vision implies an average area capacity of 25 Gb/s/km2, which is 100 times higher than current fourth generation (4G) systems. Meanwhile, to minimize power consumption, a 1000¡Áimprovement in energy efficiency (EE) is anticipated by 2020. To meet such challenging goals, revolutionary approaches involving new wireless network architectures as well as advanced signal processing and networking technologies are anticipated.
Heterogeneous networks (HetNets) have attracted intense interest from both academia and industry . The low-power node (LPN) (e.g., pico base station, femto base station, small cell base station) is identified as one of the key components to increase capacity of cellular networks in dense areas with high traffic demands, as well as to enhance the coverage of cellular networks in remote areas. In actuality, these performance gains are achieved at the cost of consuming too much energy. Meanwhile, too dense LPNs incur severe interference, which restricts performance gains and commercial development of HetNets.
On the other hand, Cloud radio access networks (C-RANs) are proposed by CMCC which consists of cloud-computing-based centralized baseband unit (BBU) pool and Remote radio Heads (RRHs). In C-RANs, RRHs are mainly deployed to provide high capacity in special zones and operate as soft relay by compressing and forwarding the received signals from UEs to the centralized BBU pool through the high capacity optical links. Only the front radio frequency (RF) and simple symbol processing functionalities are implemented in RRHs, while the other important baseband physical processing and procedures of the upper layers are executed jointly in the BBU pool. . To distinguish the advantages of C-RANs, the joint decompression and decoding schemes are executed in the BBU pool through cloud-computing-based cooperative signal processing.
To guarantee backward compatibility with the existing 4G cellular system, macro base stations (MBSs) are introduced in C-RAN to establish the heterogeneous cloud radio access networks (H-CRANs). With the help of macro base stations, the multiple heterogeneous radio networks can be converged, and all system control signaling are delivered wherein. Consequently, H-CRANs are proposed to take full advantage of both HetNets and C-RANs, in which cloud computing capabilities are exploited to solve the aforementioned challenges in HetNets. Different from C-RANs, the BBU pool in H-CRANs is interfaced with macro base stations to mitigate the cross-tier interference between RRHs and macro base stations through centralized cloud-computing-based cooperative processing techniques. Furthermore,