# WG4 overview
###### tags: `WG4`
The Open Fronthaul Interfaces Workgroup

## Fronthaul specification overall
There are some specifications in the WG4. And it can be classified into tow basic categories at the following, respectively test and plane.

### test
1. Open Fronthaul Conformance Test Specification Version 1.0
2. Fronthaul Interoperability Test Specification (IOT) v02.00
### plane
1. Control, User and Synchronization Plane Specification v03.00
4. Management Plane Specification v03.00
5. Cooperative Transport Interface Transport Control Plane Specification v01.00
## What is RAN ?
Before we introduce the fronthoul, we have something need to know.

### RAN (Radio Access Network)
RAN (Radio Access Network) is an interface that user's cell phone can via it transmit some messages to the core network.

**Developing:** D-RAN -> C-RAN -> O-RAN


### D-RAN (Distribute RAN)
It is compose of RRH and BBU ,which RRH usually is receiver or antenna and BBU is used to baseband processing. When the baseband processing is completed, the signal will be sned to the core network by backhaul.

D-RAN architecture
- RRH : antenna or transciever
- BBU : baseband process
- CPRI: connect between RRH and BBU

**CPRI:** Internal interface specification for radio base stations



### C-RAN(Centralized RAN)
---
The C-RAN architecture is combines each BBU from base station and centralized together into the house which named BBU hotel. It has some advantages in the below.
- reduce the cost
- space
- heating
- cooling
- power
- test access
- efficiently manage
- loading balance
- SDN
- VNF


### O-RAN(open RAN)
---
The O-RAN architecture is in the below and the RRU and the BBU will be opened by three components, respectively RU, DU and CU.

- RU (Radio Unit)
- include Low-PHY and RF signal process
- DU (Distributed Unit)
- **baseband process :** include RLC、MAC and High-PHY
- CU (Central Unit)
- manage one or more DU and connect to core network

## What is the Fronthaul
The fronthaul is an interface that connected between RU and DU .



### Split Option 7-2x
In order to resolve the bandwidth problem, O-RAN fronthaul specification include a new provision functional splitting called Split Option 7-2x. It split between O-DU and O-RU. An overview of the split option 7-2x in shown in the below.

#### Downlink
- O-DU
1. Encoding
1. Scrambing
1. Modilation
1. Layer mapping
1. precoding (type A)
1. Resource element mapping
- O-RU
1. Precoding (supported by type B)
2. Digital Beamforming(option)
3. IFFT
4. Analog conversion
5. Analog Beamforming (option)


### eNB/gNB architecture with O-DU and O-RU
---

**LLS(Lower Layer Split):** logical interface between O-DU and O-RU when using a lower layer (`intra-PHY based`) functional split

**LLS-U(Lower Layer Split User-plane):** logical interface between O-DU and O-RU when using a lower layer functional split.
**LLS-C( Lower Layer Split Control-plane):** logical interface between O-DU and O-RU when using a lower layer functional split

#### category A
precoding not support in O-RU


#### category B
precoding support in O-RU



### C/U/S/M plane(between O-DU and O-RU)
---
**C-Plane(Control Plane):**
refers specifically to **real-time** control between O-DU and O-RU, and should not be confused with the UE’s control plane.
- Scheduling and beam-forming commands
- DL precoding configuration
- Mixed numerology and PRACH handling
> transferring control data
> need to setup M-plane and S-plane

**U-Plane(User Plane):**
User plane messages for efficient data transfer within the strict time limits of 5G numerologies
- Support Data Compression
- I/Q data transfer
- DL data precoding

**S-Plane(Synchronization Plane):**
Synchronization plane is responsible for the **timing** and **sync** aspects between the O-DU and O-RU.
In Cloud RAN deployments, a high accurate synchronization is required between O-DU and O-RUs to achieve controlled linking for
- inter-O-RU sync operation for TDD,
- Carrier Aggregation using multiple O-RUs,
- MIMO

O-DU has a high-performance clock, which synchronizes the other inter-O-RUs and support some protocol to achieve high-accuracy synchronization.
- PTP
- SyncE

> A protocol for achieving synchronization between multiple unit of equipment

- Synchronization Typologies
- PTP and SyncE profiles for Synchronization
- Time and Frequency Sync guidelines

**[M-Plane(Management Plane)](https://hackmd.io/q1_WHoSUTG6yLt9NinlcuA):**

refers to **non-real-time** management operations between the O-DU and the O-RU

> start up,(SW, configuration, performance ,Fault , File ) management

#### Model
- Hierarchical
- reduce NMS processing load
- In the event that existing NMS does not yet support NETCONF, this module has the advantage of enabling network construction without affecting the existing system since O-DU support NETCONF in this M-plane
- Hybrid model
- NMSs can monitor/control other network devices in addition to O-RUs enabking yniform maintenance, monitoring, and control of all equioment.

### fronthaul network topology
---
Two type network topology
1. point to point
2. L2 switch


This topology can be used to increase the fronthaul transmission capacity without having to increase the capacity per port of the O-DU and O-RU by simply increasing the number of ports each having the standard amount of capacity.

1. case (1): increasing transmission capacity
2. case (2): redundancy. If transmission has path falls, the L2 switch changes to another path.
3. case (3): O-DU use one port connect to one or more O-RU by L2 switch