The 5G Flexible RAN, also known as Cloud RAN (C-RAN) or Virtualized RAN (vRAN), is based on the disaggregation of the base station (gNB). This architecture splits the baseband processing functions into a Centralized Unit (CU) and a Distributed Unit (DU). The Radio Unit (RU) remains at the cell site to handle radio frequency (RF) transmission and reception. These three components—CU, DU, and RU—are the fundamental building blocks of the flexible RAN architecture as defined by standards bodies like 3GPP. An "Optical Unit" is not a standardized functional component within this architectural model, although optical fiber is the common transport medium for fronthaul and midhaul links connecting these components.

A. Radio Unit: The Radio Unit (RU) is a core component of the 5G RAN, responsible for handling the radio frequency (RF) functions at the antenna site.

B. Distributed Unit: The Distributed Unit (DU) is a key component in the disaggregated architecture, processing the real-time, lower-layer baseband functions.

C. Centralized Unit: The Centralized Unit (CU) is a key component that runs the non-real-time, higher-layer protocol stack and can be located centrally.

---

1. Nokia Official Documentation: Nokia. (2021). Cloud RAN: A Guide for Operators. White Paper. In Section "Cloud RAN architecture," the document details the functional split into Centralized Unit (CU), Distributed Unit (DU), and Radio Unit (RU), establishing them as the core components. It does not mention an "Optical Unit" as a functional block. (Specific document available via Nokia channels for partners and customers).

2. Standards Body Publication: 3GPP TS 38.401 V17.3.0 (2022-03). NG-RAN; Architecture description. Section 4.2, "NG-RAN Architecture," and Figure 4.2.2-1 explicitly define the gNB architecture and its potential split into a gNB-CU and one or more gNB-DUs. The RU is implicitly part of the physical layer handled by the DU.

3. Academic Publication: Polese, M., Jana, R., & Zorzi, M. (2017). A Survey on 5G Network Architecture and Its Technology Enablers. IEEE Communications Surveys & Tutorials, 19(4), 2541-2575. In Section IV-A, "Cloud-RAN," the paper describes the C-RAN architecture as comprising three main components: the BaseBand Unit (BBU) pool (which is further split into CU/DU in 5G), the Remote Radio Head (RRH) or RU, and the fronthaul transport network. It confirms RU, DU, and CU as the standard architectural elements. DOI: https://doi.org/10.1109/COMST.2017.2720758