Before understanding the architecture of ORAN, First we need to understand the LTE architecture which consist of EPC,eNB and the RF Antenna Unit.
The functions in LTE RAN are split so that the Baseband Unit (BBU) and the Remote Radio Head (RRH) can be separated physically. This allows the RRH to be located closer to the antenna, while the BBU can be located in center shown in fig.1a
Now Coming to 5G RAN Architecture as show in fig 1b
5G Architecture consist of :
a) 5G CORE
b) 5G RAN divided in to two parts – BBU and RU.
c) RF antenna Unit
BBU is split into CU and DU. Now the question comes What is CU and DU?
CU: CU is the centralized unit that runs the RRC,SDAP and PDCP layers.CU further divided into CU CP & CU UP
CU-CP: Central Unit – Control Plane: a logical node hosting the RRC and the control plane part of the PDCP protocol
CU-UP: Central Unit – User Plane: a logical node hosting the user plane part of the PDCP protocol and the SDAP protocol
DU: Distributed Unit: a logical node hosting RLC/MAC/High-PHY layers based on a lower layer functional split.
RU: Radio Unit: a logical node hosting Low-PHY layer and RF processing based on a lower layer functional split. More specific in including the Low-PHY layer (FFT/iFFT, PRACH extraction)
Now Question comes in mind what is ORAN? What is the need of ORAN?
The objective of the O-RAN Alliance is to clearly define requirements for an open, virtualized and interoperable RAN. It has defined two core principles to guide this mission, namely openness and intelligence.
In the case of intelligence, the O-RAN architecture has introduced a new type of Software Defined Network (SDN) controller called the RAN Intelligent Controller (RIC), which is responsible for automating the deployment of RAN functions in response to service needs. This includes a near real-time (near-RT) RIC and a non-near-real-time (non-RT) RIC. Both RICs make decisions based on analysis of data collected in the network using deep learning and artificial intelligence.
In the case of openness, the O-RAN architecture is based on well defined, open interfaces to enable interoperability between implementations from multiple vendors. This includes the fronthaul interface between the O-DU and O-RU based on split option 7.2x.
Near-RT RIC: O-RAN near-real-time RAN Intelligent Controller: a logical function that enables near-real-time control and optimization of RAN elements and resources via fine-grained data collection and actions over E2 interface. It may include AI/ML workflow including model training, inference and updates.
Non-RT RIC: O-RAN non-real-time RAN Intelligent Controller: a logical function within SMO that enables non-real time control and optimization of RAN elements and resources, AI/ML workflow including model training, inference and updates, and policy-based guidance of applications/features in Near-RT RIC.
Overall Architecture of O-RAN
Figure below provides a high-level view of the O-RAN architecture. It shows that the four key interfaces –namely, A1, O1, Open Fronthaul M-plane and O2 – connect SMO (Service Management and Orchestration) framework to O RAN network functions and O-Cloud. Figure below also illustrates that the O-RAN network functions can be VNFs (Virtualized Network Function), i.e., VMs or Containers, sitting above the O-Cloud and/or PNFs (Physical Network Function) utilizing customized hardware. All O-RAN network functions are expected to support the O1 interface when interfacing the SMO framework.
The Open Fronthaul M-plane interface, between SMO and O-RU, is to support the O-RU management in hybrid model.It is an optional interface to the SMO that is included for backward compatibility purposes. It is intended
for management of the O-RU in hybrid mode only.
O1: Interface between management entity and O-RAN managed elements.
SMO a Service Management and Orchestration system.
A1-Interface between Non RT RIC & Near RT RIC.
O2- Interface between SMO & O cloud
Now question here comes in mind Why, What , When ORAN.
Most of the CAPEX required to build a wireless network is related to the RAN segment, reaching as high as 80% of the total network cost. Any reduction in the RAN equipment cost will significantly help the bottom line of wireless operators as they struggle to cope with the challenges of ever increasing mobile traffic and flat revenues. So the deployment cost could drop if if an open architecture is used. Operators are also realizing that opening up the RAN for only 5G will not reduce the overall network cost. The operators believe that modernizing their legacy networks, in addition to deploying 5G, will reduce their overall network OPEX as they
will have one unified network to run and will be able to make time and cost-saving remote upgrades to the overall site.
What is different types of RAN ?
C RAN- It is a deployment model where a BBU that was doing digital processing could be located in a data centre and not on the site itself – under the radio or RRH (remote radio head unit) where the radio processing was happening – and was instead connected to the baseband unit via a dedicated high-bandwidth connection.
The C-RAN required a new fronthaul interface, and various industry standards such as the Common Public Radio Interface (CPRI) and the Next Generation Fronthaul Interface (NGFI) evolved to enable these new interfaces between the radios and baseband.
V RAN- With vRAN, the proprietary hardware remains as it is, but the BBU gets replaced by a COTS server rather than proprietary hardware. The software that runs on the BBU is virtualized to run on any COTS server.
The proprietary interfaces remain as they are.
So its very important to understand that VRAN is not necessarily ORAN. VRAN still contains proprietary interfaces and purpose-built hardware.
Whereas Open RAN is a movement to define and build 2G, 3G, 4G and 5G RAN solutions based on a generalpurpose, vendor-neutral hardware and software-defined technology. Open RAN Is the disaggregation of hardware and software: the RRU / RRH hardware becomes a COTS hardware that can be purchased from any OEM or RAN hardware vendor. The BBU is the same as in the case of vRAN: COTS server + vendor’s proprietary software with virtualized functions.
The Open RAN vision is that the RAN is open within all aspects, with the interfaces and operating software separating the RAN control plane from the user plane, building a modular base station software stack that operates on commercial-off-the-shelf (COTS) hardware The software enabled Open RAN network architecture enables a “white box” RAN hardware – meaning that baseband units, radio units and remote radio heads can be assembled from any vendor and managed by Open RAN software to form a truly interoperable and open
The Open RAN TIP group in 2016 has brought together operators, traditional equipment vendors and startups that are using open source technologies and open approaches to get the solution for high costs in the telecom equipment required during deployment.
As part of the Telecom Infra Project (TIP), Facebook is working with telecom service providers and operators to accelerate innovation, new technology, to help the industry build the networks of the future. Through this program, Facebook is working with operators in areas that have not been covered with any kind of communication services in various geographies throughout the globe. The cost attractions of Open RAN enabled by interoperability could prove important in such diversified markets from high-income to low-income markets.
The O-RAN Alliance was formed after the merger of the C-RAN alliance and XRAN. The O-RAN Alliance publishes new RAN specifications, releases open software for the RAN, and supports its members in integration and testing of their implementations.