# <center><i class="fa fa-edit"></i> Introduction to Open RAN </center>
###### tags: `O-RAN`
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**References:**
- [BMW LAB Training Course](https://univindonesia-my.sharepoint.com/:p:/g/personal/jonathan61_office_ui_ac_id/EZgWNTXM3JlMuxi3TJmCwjQBQCLb4tz03FuJ_fntp-prOg?rtime=16cNgDXM2Eg)
- [O-RAN (parallelwireless.com)](https://www.parallelwireless.com/openran-resources/)
- [O-RAN overview by Nokia](https://www.nokia.com/about-us/newsroom/articles/open-ran-explained/)
- [O-RAN Architecture](https://docs.o-ran-sc.org/en/latest/architecture/architecture.html)
- [O-RAN 101](https://www.rcrwireless.com/20200708/open_ran/open-ran-101-ru-du-cu-reader-forum#prettyPhoto)
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## A. RAN
Before we talking about O-RAN, we need to understand the concept of RAN (Radio Access Network) and the history how O-RAN is shaping from RAN.

### I. About RAN
A Radio Access Network (RAN) is the part of a telecommunications system that connects individual devices to other parts of a network through radio connections. A RAN resides between user equipment, such as a mobile phone, a computer or any remotely controlled machine, and provides the connection with its core network. The RAN is a major component of wireless telecommunications and has evolved through the generations of mobile networking leading up to 5G.

A RAN provides access and coordinates the management of resources across the radio sites. A handset or other device is wirelessly connected to a backbone, or core network, and the RAN sends its signal to various wireless end points, so it can travel with other networks’ traffic. A single handset/phone could be connected at the same time to multiple RANs, which is sometimes called dual-mode handsets.

The term radio access network (RAN) has been in use since the beginning of cellular technology and has evolved through the generations of mobile communications (from 1G up to 5G today). Components of the RAN include a base station and antennas that cover a given region depending on their capacity, plus required core network items.

The RAN includes a base station and antennas that cover a specific region according to its capacity, design and propagation. Some types of radio access networks include:

- GRAN (GSM radio access network)
- GERAN (specifies the inclusion of EDGE packet radio services)
- UTRAN (universal mobile telephone system RAN)
- E-UTRAN (evolved universal terrestrial RAN)


**Component of RAN**

- Access Network : Evolved NodeB (eNodeB)
- Core Network : Evolved Packet Core (EPC)

**eNodeB consist of BBU and RRU**

- BBU : Baseband Unit, digital signal processing

A baseband unit (BBU) is a unit that processes baseband in telecomm systems. A typical wireless telecom station consists of the baseband processing unit and the RF processing unit (remote radio unit - RRU). The baseband unit is placed in the equipment room and connected with RRU via optical fiber. The BBU is responsible for communication through the physical interface. A BBU has the following characteristics: modular design, small size, low power consumption and can be easily deployed.

- RRU : Remote Radio Unit, RF signal processing

A Remote Radio Head (RRH) or Remote radio unit (RRU) is the RF circuitry of a base station enclosed in a small outdoor module. The RRH performs all RF functionality like transmit and receive functions, filtering, and amplification. It also contains analog-to-digital or digital-to-analog converters and up/down converters. An RRH can also provide advanced monitoring and control features that allow operators to optimize performance from a remote, centralized location.

The RRH is usually mounted near the antenna to reduce transmission line losses and is connected to the main, digital portion of the base station (BBU) with an optical fiber.

## II. RAN Evolution
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### Traditional mobile towers

Characteristic:
- Antenna connected by long RF cabling
- Separate cabinet containing hardware + software (BBU+RRU)


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### Contemporary RAN
Characteristic:
- BBU is placed in seperate cabinet (Hardware + Software)
- RRU is placed on the tower (Hardware)
- The design uses proprieary Hardware, Software and interfaces


C-RAN network

The advantage C-RAN give is:
- Reducing operationg expenses (OPEX) and capital expenditures (CAPEX)
- Enables Resource Sharing
- Services can be deployed on the network edge

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### Virtualized RAN (vRAN)
Characteristic:
- RRU still uses proprietary hardware
- BBU runs virtualized proprietary software that runs on COTS server
- The design uses proprietary interface and purpose built hardware


vRAN network

Advantage of vRAN:
- Improved Network Performance
- Reduced Cell Site Costs
- Increased Flexibility in Deployment Options

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### Open RAN
Characteristic:
- RRU uses GPP COTS based hardware that can work with any ODM/OEM/RAN Hardware vendor
- BBU runs virtualized proprietary software that runs on COTS server
- Interface between BBU and RRU is open software.


Open RAN network

(further explanation of ORAN will be explained in the next chapter)

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## B. Open RAN
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### Get to know about O-RAN
Open RAN is an open project to support multivendor RAN interoperability that focuses on openness, performance and fast deployments. It's a movement to define and build 2G, 3G, 4G and 5G RAN solutions based on a general-purpose, vendor-neutral hardware and software-defined technology.

The Open RAN movement applies to all generations of mobile technology, current and future (5G and beyond). The groups and organization focused on Open RAN area the Telecom Infra Project (TIP) formed by Facebook and O-RAN Alliance that consisted of group of Chinese Vendors (C-RAN alliance) and XRAN forum from other countries.

#### [O-RAN Alliance](https://www.o-ran.org/)
The O-RAN alliance have two high level goals which are:
- Openness to enable vendors and operators to customize the network to suit their needs
- Automating increasingly complex network by working on set of working groups

#### [OpenAirInterface (OAI) Software Alliance](https://openairinterface.org/)
The OSA is the community of OpenAir interface, that work together to build wireless cellular Radio Access Network and Core Network technology.

#### [O-RAN Software Community](https://www.o-ran.org/software)

The OSC is a collaboration between the O-RAN alliance and Linux Foundation with the goal to support the creation of O-RAN software, with a focus on the open interfaces.

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### O-RAN Architecture

O-RAN SC architecture follows O-RAN Alliance defined architecture.

In the O-RAN architecture, the radio side includes Near-RT RIC, O-CU-CP, O-CU-UP, O-DU, and O-RU. The management side includes Service Management and Orchestration Framework that contains a Non-RT-RIC function.

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### Components Definition

**near-RT RIC**: O-RAN near-real-time RAN Intelligent Controller: a logical function that enables near-real-time control and optimization of O-RAN elements and resources via fine-grained data collection and actions over E2 interface.

**non-RT RIC**: O-RAN non-real-time RAN Intelligent Controller: a logical function that enables non-real-time control and optimization of RAN elements and resources, AI/ML workflow including model training and updates, and policy-based guidance of applications/features in near-RT RIC.

**NMS**: A Network Management System

**O-CU**: O-RAN Central Unit: a logical node hosting RRC, SDAP and PDCP protocols

**O-CU-CP**: O-RAN Central Unit – Control Plane: a logical node hosting the RRC and the control plane part of the PDCP protocol

**O-CU-UP**: O-RAN Central Unit – User Plane: a logical node hosting the user plane part of the PDCP protocol and the SDAP protocol

**O-DU**: O-RAN Distributed Unit: a logical node hosting RLC/MAC/High-PHY layers based on a lower layer functional split.

**O-RU**: O-RAN Radio Unit: a logical node hosting Low-PHY layer and RF processing based on a lower layer functional split. This is similar to 3GPP’s “TRP” or “RRH” but more specific in including the Low-PHY layer (FFT/iFFT, PRACH extraction).

**O1**: Interface between management entities in Service Management and Orchestration Framework and O-RAN managed elements, for operation and management, by which FCAPS management, Software management, File management shall be achieved.

**O1***: Interface between Service Management and Orchestration Framework and Infrastructure Management Framework supporting O-RAN virtual network functions.

**xAPP**: Independent software plug-in to the Near-RT RIC platform to provide functional extensibility to the RAN by third parties.
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## Functional Split

The functional split concept was introduced for 5G, to split the Baseband Unit (BBU) into two functional units: DU and CU. In Open RAN, open interfaces enable RAN to connect with network functions. By distributing protocol stacks between different components, providers are able to get an optimal Fronthaul (FH) balance between the RU, CU and DU, the goal is optimizing spesific performance requirements at lower costs.

### [5G Functional Split](https://www.rcrwireless.com/20200708/open_ran/open-ran-101-ru-du-cu-reader-forum#prettyPhoto)


In a 5G RAN architecture, the BBU functionality is split into two functional units: a distributed unit (DU), responsible for real time L1 and L2 scheduling functions, and a centralized unit (CU) responsible for non-real time, higher L2 and L3. In a 5G cloud RAN, the DU’s server and relevant software could be hosted on a site itself or can be hosted in an edge cloud (datacenter or central office) depending on transport availability and fronthaul interface. The CU’s server and relevant software can be co-located with the DU or hosted in a regional cloud data center. The actual split between DU and RU may be different depending on the specific use-case and implementation.

### [RAN Functional Split](https://www.rcrwireless.com/20200708/open_ran/open-ran-101-ru-du-cu-reader-forum#prettyPhoto)
Mobile operators need the flexibility to pick and choose different splits based on the same COTS-based hardware and network components by using different software implementations. Different protocol layers will reside in different components based on fronthaul availability and deployment scenarios. This approach will reduce the cost of operations and TCO for mobile operators.

Higher functional splits are more desirable for capacity use cases in dense urban areas while lower functional splits will be the optimum solutions for coverage use cases. So, while lower functional splits utilize less than perfect fronthauls, there is a greater dependence on fronthaul performance for higher functional splits.
### [O-RAN Functional Split](https://www.rcrwireless.com/20200708/open_ran/open-ran-101-ru-du-cu-reader-forum#prettyPhoto)

- #### Traffic Split-7
Higher splits,as in 7.x, will be the best approach going forward for deploying future mobile networks in different deployment scenarios. It is ideal for 4G and 5G and can support traffic in dense urban areas.

- #### Traffic Split-8
With traffic split 8, all functions (from PHY to RRC layers) except for RF are handled by the DU, while the RF layer is located in the radio. This split is highly effective in 2G and 3G, where traffic rates are much lower, while allowing operators to use cost-optimized RUs with minimal logic and processing. The DU and RU should be interoperable with other third party DUs and RUs. This approach allows for centralized traffic aggregation from the RUs, which in turn enables a seamless migration path from the traditional LTE ecosystem to the NR ecosystem.