## 5 What Is C-RAN?
### Working Principle of C-RAN  
Centralization is a concept that has been adopted in many sectors to achieve higher utilization of limited resources, healthcare, for example. But centralization always comes with a price; the transport. In the case of mobile networks, it is data that needs to be transported from the RU on the cell site to the DU in a centralized data center. The current distributed RAN installations use the Common Public Radio Interface (CPRI) protocol for transport between the RU and BBU to encapsulate the very raw signal sent from the RU. The signal transmitted in the CPRI protocol has only been through a few operations inside the RU, namely frequency down conversion, sampling, and analog-to-digital conversion [111]. Hence, due to the very raw nature of the signal transmitted between the RU and the DU, the load is very high and continuously occupies the transport medium, while the latency requirements are strict [111]. Thus, the distance between RU and DU is bound to the latency requirements as well as the availability of high-capacity infrastructure. Several solutions to overcome the huge transport capacity problem of C-RAN exist; for example, compression [112], high capacity infrastructure like fiber or mmWaves [113], or to add more functions to the RU [111]. The latter solution is referred to as functional splits, and multiple approaches exist. The low layer split, separating the RU and DU functions, has not yet been determined, and multiple approaches exist. On the other hand, the high layer functional split separating the DU and CU has already been settled, to include the Packet Data Convergence Protocol (PDCP) and Radio Resource Control (RRC/IP) in the CU [110].  
When the data finally reach the data center, they will meet a virtualized format of the baseband processing. Baseband processing functions can be virtualized either as virtual machines or containers, both enabling multiple virtual software instances to share the same hardware. The hardware used can be Commercial-Off-The-Shelf (COTS) servers, either in a private enterprise data center or in a public cloud where capacity is rented on demand. In  
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Category** & **Centralized RAN** & **Virtualized RAN** & **Cloud RAN** \\ \hline
**Concept** & Baseband processing is moved away from the cell sites to a central baseband pool. & Baseband processing is virtualized, running as software instances independently from the underlying hardware. & Virtualized baseband processing is centralized in a datacenter. Deployment options are agile. \\ \hline
**Benefits** & Reduced site footprint, improved cell cooperation, and shared cooling mechanisms. & Load balancing, agile service deployment, faster updates. & In addition to the benefits derived from centralized and virtualized RAN, Cloud-RAN also benefits from dynamic capacity assignment, improved scalability, and increased resource utilization. \\ \hline
**Drawbacks** & Large capacity and latency requirements for the transport network connecting radio functions to centralized baseband processing (fromthaul network). & Complexity of virtualized functions and challenges in running time-critical RAN functions on COTS hardware. & Cloud-RAN faces the same drawbacks as virtualized RAN, but its agile deployment options can reduce from virtualized RAN. \\ \hline \hline \end{tabular}
\end{table}
Table 4: Difference between Centralized, Virtualized, and Cloud-RAN.  
Figure 14: The future mobile network architecture where all radio processing is divided into three units: Radio Unit (RU), Distributed Unit (DU), and Centralized Unit (CU), as specified by 3 GPP [110].  
both cases, using COTS hardware for baseband processing will also make it possible for other applications to use the same hardware when the load in the mobile network is low.