## 2 O-RAN Architecture and Development
### O-RAN Architecture  
The O-RAN architecture is a set of open interfaces and protocols designed to enable multi-vendor interoperability and support a wide range of use cases and services in RAN. The architecture defines a modular and disaggregated approach to building RAN systems, where different functional components can be developed and deployed independently by a variety of vendors. Such an approach intends to increase innovation, reduce costs, and enable faster deployment of new services and features. The O-RAN architecture specifies main components and interfaces connecting the components. The interfaces allow the Service Management and Orchestration (SMO) framework to connect with O-RAN network functions and O-Cloud. Figure 2 illustrates the high level O-RAN architecture, which can be viewed as Virtualized Network Functions (VNFs) placed above the O-Cloud and/or Physical Network Functions (PNFs). The A1 Interface between the Non-RT RIC in the SMO and the Near-RT RIC used for RAN Optimization, O1 Interface between the SMO and the O-RAN Network Functions used for Fault, Configuration, Accounting, Performance, Security (FCAPS) support and in the hybrid model. The Open Fronthaul M-plane interface between SMO and O-RU supports FCAPS too. While the O2 Interface between the SMO and the O-Cloud provides platform resources and workload management, the O-Cloud Notification interface allows event consumers such as an O-CU implemented on O-Cloud to subscribe to events or status. Moreover, the Y1 interface permits the Y1 consumers to subscribe or request the RAN analytics information delivered by Near-RT RIC. Where the Y1 consumer stands for an entity or more, within or outside of the public land mobile network (PLMN) trust domain that ingests analytics information services after mutual authentication and authorization by subscribing to or requesting the RAN analytics information via the Y1 service interface. There are three main control loops that run simultaneously in O-RAN, depending on the use cases, which are real-time (RT), which is limited to a maximum of 10 ms execution time \(t_{\textit{exe}}\), Near-RT, with \(10\leq t_{\textit{exe}}\leq 1000\) ms; and None-RT, which can take \(1000\leq t_{\textit{exe}}\) ms. Multi-vendor Slices use case targets enabling functions that belong to different vendors; there are many possible configurations to deploy the Multi-vendor slicing, all of which share that one O-RU is connected to one or more O-DUs. The advantages of such a use-case include a higher flexibility and rapid deployment of services to market by network operators, sharing RAN equipment among operators, optimizing CAPEX and OPEX among their existing assets, and future investments. In addition, reducing the supply chain risk; for example, if an existing vendor supplies a certain pair of vO-DU and vO-CU functions and if, for business reasons or even political situations, it has to withdraw from a certain market, then the operator can outsource and deploy alternative vO-DU and vO-CU that support multi-vendor slicing functions. The O-RAN specification work has been covered by eleven technical Work Groups (WG) to covers all the O-RAN Architecture parts, each WG has been supervised by the O-RAN alliance technical steering committee. Below is a brief overview of each WG:  
* WG1: Use Cases and Overall Architecture. This WG is responsible for defining the overall architecture of Open RAN and the use cases that it will support;
* WG2: Non-Real-Time RAN Intelligent Controller and A1 Interface. This WG is responsible for defining the specifications for the Non-Real-Time RIC (Non-RT-RIC) and the A1 interface. The Non-RT-RIC is a centralized controller that manages the non-real-time aspects of the RAN. The A1 interface is the interface between the Non-RT-RIC and the radio units;
* WG3: Near-Real-Time RIC and E2 Interface. This WG is responsible for defining the specifications for the Near-Real-Time RIC (nRT-RIC) and the E2 interface. The nRT RIC is a centralized controller that manages the near-real-time aspects of the RAN.