What is C-RAN?

Cloud/Centralized Radio Access Network (C-RAN) is a next-generation RAN architecture in which the Baseband Units (BBUs) are moved from cell sites to a centralized location. This centralized location is commonly known as BBU hotel. The centralization of BBUs also enables network virtualization, in which the BBU hotel can be replaced by a server.

A Distributed RAN cell site consists of a Remote Radio Head (RRH) and Baseband Unit (BBU). The RRH connects mobile devices to the network i.e it performs all RF functionality like transmit and receive functions, filtering, and amplification. The BBU processes calls and forwards traffic to the Mobile Switching Centre (MSC).

C-RAN was introduced to tackle the rapid increase in mobile data traffic. The rise in mobile users over the years has led to an increase in radio access networks being deployed by network operators. In C-RAN architecture, the BBU is shifted from cell site to a centralized location, often called a BBU hotel. This has several benefits for network operators, such as reduced space requirement at cell sites, reduced expenditure for cooling solutions at cell sites, improved overall network performance, easier test access along with faster deployments. Other advantages of C-RAN include network virtualization, easier to implement advanced BBU technologies and dynamic resource allocation.

BBU Hotel

A BBU hotel houses the baseband units of a distributed Radio Access Network. It can be located far away from the RRH (usually up to 20 Km) and is connected by fibers running CPRI (Common Public Radio Interface) protocols between the two nodes. Once the BBUs are centralized, they can be replaced by a server thus enabling SDN (Software-Defined Networking) and NFV (Network Function Virtualization).

Benefits of network virtualization include:

• Reduced Expenditure
• Easier maintenance and troubleshooting 
• Dynamic resource allocation depending on the requirement of RRUs
• Faster network deployment

BBU virtualization in C-RAN architecture (Image Credit: GeeksforGeeks)

C-RAN System Structure: The C-RAN System Structure can be divided into 3 categories:

Fully Centralized: In a fully centralized C-RAN structure, all physical, MAC and network layers are moved into Baseband Unit. The BBU handles all the functions of managing and processing resources.

Partially Centralized: In a partially Centralized structure, the physical layer functioning is accomplished at BBU while the MAC layer is performed at RRU. This reduces the overhead of RRUs-BBUs communication as the physical layer takes the major computation burden of C-RANs.

Hybrid Centralized: In this centralized system, physical layer functioning is done at RRU while others are done at BBU layer. The Remote Radio Unit (RRU) takes on most of the user responsibility which is mainly concerned with signal processing.