Recent Advances in Cloud Radio Access Networks: System Architectures, Key Techniques, and Open Issues

Abstract

Abstract-As a promising paradigm to reduce both capital and operating expenditures, the cloud radio access network (C-RAN) has been shown to provide high spectral efficiency and energy efficiency. Motivated by its significant theoretical performance gains and potential advantages, C-RANs have been advocated by both the industry and research community. This paper comprehensively surveys the recent advances of C-RANs, including system architectures, key techniques, and open issues. The system architectures with different functional splits and the corresponding characteristics are comprehensively summarized and discussed. The state-of-the-art key techniques in C-RANs are classified as: the fronthaul compression, large-scale collaborative processing, and channel estimation in the physical layer; and the radio resource allocation and optimization in the upper layer. Additionally, given the extensiveness of the research area, open issues and challenges are presented to spur future investigations, in which the involvement of edge cache, big data mining, socialaware device-to-device, cognitive radio, software defined network, and physical layer security for C-RANs are discussed, and the progress of testbed development and trial test are introduced as well.

Dynamic Resource Allocation for Virtualized Wireless Networks in Massive-MIMO-Aided and Fronthaul-Limited C-RAN

“…Virtualized wireless networks (VWN), cloud radio access network (C-RAN) and massive MIMO are the three key technologies envisioned for the fifth generation of wireless networks (5G) [1], [2], [3]. While they can considerably improve the resource utilization as well as increase the spectrum and energy efficiency of 5G, each of them suffers from their own implementation issues.…”

“…Due to the importance of C-RAN in the architecture of 5G networks, there is a surge of research to study how to utilize the C-RAN resources in the optimal manner [3]. C-RAN facilitates the centralized resource allocation, however, due to the interference in the wireless networks, the resource allocation problems are more complex and non-convex, especially due to the new set of cloud-structure variables, e.g., front-haul assignment parameters.…”

Multi-domain SDN controller federation in hybrid FiWi-MANET networks

“…For example, assuming that both devices apply a common protocol such as Ethernet as the common interface protocol, no special requirements are needed and no additional complex hardware is necessary. Moreover, the recent proliferation of the C-RAN concept [50,51], which was first introduced by IBM [52] and is based on the concept of distributed wireless communication systems, established the separation of the BS in a radio unit, called RRH, and a signal processing unit, called baseband unit (BBU). While RRH offers the interface to the fiber and performs digital processing, e.g., digital to analog conversion and vice versa, the BBU equipment can be placed in a more flexible way, meaning that it is not required to be co-located with the BS.…”

Fronthaul-Limited Uplink OFDMA in Ultra-Dense CRAN With Hybrid Decoding

“…with the objective function given by (15). Problem (17) is nonconvex due to the integer constraints (17c), (17e) and (17f) and also due to the coupled variables in the objective and the constraint (17b).…”

“…On each SC, the RRHs can choose either to: decode the assigned user's message locally and then forward it to the CP (namely, decode-and-forward (DaF)), or simply quantize the signal and forward it to the CP for joint decoding (termed forward-and-decode (FaD)), or not process the received signal at all (to save fronthaul capacity). Most of the existing work on CRAN considers single-channel systems [3]- [15], where all the users transmit in the same bandwidth. Previously, a hybrid compression and message-sharing strategy was considered in [21] for the downlink transmission in a single-channel CRAN, where the CP shares the uncompressed messages of some users to a subset of RRHs, in addition to transmitting compressed messages of all users to each RRH.…”