# Open RAN--Radio Access Network Evolution, Benefits and Market Trends
## 3 Open RAN Landscape
### O-RAN Architecture  
The Open RAN vision of interface openness, disaggregation, and intelligent radio access networks was materialized by the O-RAN Alliance in its specifications of the O-RAN standard [40]. The O-RAN reference architecture is based on enhanced 3 GPP nodes, functions, layers, and interfaces [41]. In Figure 5, we present a general view of the O-RAN architecture that is composed, among others, of the following functional blocks and interfaces:  
1. Near-Real-Time and Non-Real-Time RAN Intelligent Controller (near-RT RIC/non-RT RIC) functions;
2. service management and orchestration (SMO) framework;
3. interfaces A1, E1, O1, O2;
4. 3 GPP extended nodes O-CU, O-DU, O-RU;
5. open FH interfaces together with Open FH Management plane (M-plane). Native 3 GPP interfaces (X2, Xn, XG) enabling connection towards other eNB/gNB elements are not shown in Figure 5. O-RAN specifications are published as outcomes of the official established Working Groups (WGs), each of which covers selected RAN aspect listed above. Apart from WGs, the O-RAN Alliance constitutes dedicated Focus Groups that cover topics related to whole organization aspects. Figure 4: Timeline of selected actions and events related to Open RAN development. Non-RT RIC is placed within SMO outside RAN. Such an approach enables to feed the RIC with external/contextual data and use it for RAN optimization purposes [43]. This component contains non-RT RIC Applications (rApps) and frameworks. By means of the A1 interface, non-RT RIC may send policy update information to steer RAN performance and receive feedback from near-RT RIC, e.g., traffic patterns or user mobility behaviour. Concurrently, via the O1 interface, it receives information concerning network status. The goal of rApps is to support the control and optimization of RAN elements and resources (via R1 interface) in use cases based on a control loop greater than 1 s. The decision process may be supported by machine learning models implemented in RIC that make use of collected data. In Figure 5, the control loops timing is presented to show the time range of decision process. Near-RT RIC is the logical function, perceived by the market as beneficial, enables Radio Resource Management (RRM) optimization based on fine-grained data collection and actions. Near-RT RIC hosts one or more xApps-dedicated applications (delivered also by the third parties) as microservices. xApps use cases encompass optimization of those areas which have been defined by 3 GPP as well as internal proprietary features related to RRM, quality of experience (QoE), mMIMO, mobility, energy efficiency, self-organized network (SON), interference mitigation, etc. [44]. The control loop for xApps is greater than 10 ms. Near-RT RIC is connected via the E2 interface with E2 nodes i.e., O-CU-CP, O-CU-UP, O-DU in NR and O-eNB in Evolved Universal Mobile Telecommunications System. The other main elements of the O-RAN architecture are as follows. 1. O-RAN Central Unit--Control Plane (O-CU-CP), which is an enhanced CU-CP defined by 3 GPP that hosts RRC (3 GPP TS 38.331 specification) and PDCP (control part, 3 GPP TS 38.323 specification) protocol;  
Figure 5: O-RAN reference architecture (adapted from [42]). O-RAN Central Unit--User Plane (O-CU-UP), which is an enhanced CU-UP defined by 3 GPP hosting SDAP (3 GPP TS 37.324 specification) and PDCP (user part) protocols;
* O-RAN Distributed Unit (O-DU), which is a logical node hosting RLC (3 GPP TS 38.322 specification), MAC (3 GPP TS 38.321 specification) protocols and High-PHY layer (3 GPP TS 38.201 specification);
* O-RAN Radio Unit (O-RU), which is a logical node hosting Low-PHY and RF processing;
* Open Fronthaul--O-DU and O-RU are connected via open FH interface implementing Option 7.2x split. As mentioned in Section 2, two version of the 7.2x split are defined in which a precoding function is placed either in O-DU (Category A) or O-RU (Category B).