# Functional Split and Open Fronthaul Interface
[TOC]
## 5G Standards
3GPP defines new NR (New Radio) bands in FR1 and FR2 in 3GPP Rel. 15 NR
- FR1: 450 MHz – 7.125 GHz
- FR2: 24.25 GHz – 52.6 GHz
Ultra wide bandwidth
- Up to 100 Mhz in <= 7 GHz
- Up to 400 MHz in > 7 GHz

Impact of Frequency
- Higher Path Loss -> smaller coverage
- Higher Bandwidth -> higher data rate
- Shorter wavelength -> more antenna/unit area

5G New Radio (NR) can provide:
- Scalable numerologies for low latency and ultra reliability
- Beamforming
- New channel coding (LDPC for data channel and polar coding for control channel)
- Flexible and modular RAN architecture (split front haul, split CUPS)
- Native end to end support for network slicing

**5G System Architecture**
5G system architecture consists of 5G Core Network, 5G Access Network (AN), and UE. As shown below:

**5G Protocol Stack**

**5G Network Architecture**

Comparison Between 5G and 4G architecture are:

| 5G NR | LTE |
| -------- | -------- |
| AMF/SMF | MME |
| PCF | PCRF |
| AUSF + UDM| HSS |

## Fronthaul Split
- **Hardware and Software Disaggregation**
The purpose of O-RAN is to disaggregate the hardware and software in base station, so it has open interface that allow any vendor software to work on this hardware and software.

- **Control and User Plane Separation (CUPS) and CU/DU Split**
CUPS is a vertical split in 5G Core Technology. It is the first step to introduce SDN to the RAN. CUPS is used to separate CU into Control Plane (CP) and User Plane (UP) vertically. It has consistent CP in multi-vendor networks. Although it is more simple in higher layer, but more challenges for lower layer splits implementation. Meanwhile, CU/DU is a horizontal split in 5G Core Technology that used to centralization gain in terms of performance and scale. It try to shift functionalities to different locations in order to support different service (eMBB, mMTC, URLLC). CU/DU can reduce the cost of RAN by using commercial available hardware.

Functional Splits are divided into 2: Higher layer split and Lower layer split.
- **Higher Layer Split Options**
There several preferences of separating CU, DU, and RU for higher layer, such as:
- Option 1 (RRC/PDCP split): LTE R’12 DC Option 1A
- Option 2 (PDCP/RLC split): Option 3C for UP (F1)
- Option 3 (intra-RLC split):
- Most of the RLC located in the CU; all ARQ-related functionalities
- Real time functionalities (like aggregation) located in the DU)
- Option 5 (intra-MAC split):
- High-level scheduling decisions (e.g. ICIC, CoMP) are in the CU
- Time-critical MAC processing (e.g HARQ) is located in the DU
- Option 6 (MAC/PHY split):
- Complete MAC layer located in the CU
- PHY layer implemented in the DU
- Requires sub-frame-level timing interactions between CU and DUs. FH delays would affect HARG timing and scheduling.

- **Lower Layer Split Options**
- Option 7.1:
- IFFT and CP insertion/removal performed at DU
- Rest of PHY located at the CU
- I/Q samples in frequency domain are exchanged over the interface
- Compared to option 8 only the samples related to occupied sub-carriers need to be exchanged instead of time domain samples reflecting the whole system bandwidth.
- Option 7-2 and 7-2a:
- Pre-coding and digital beamforming or parts of it, are performed at the DU
- FH requirements scale with number of MIMO layers and not number of antenna ports, as it is the case in option 7.1
- Options 8:
- Legacy C-RAN

## O-RAN Fronthaul Interface
**Open Fronthaul** is a lower layer split defined by O-RAN Alliance. One of the main purpose of the open fronthaul is to reduce the bandwidth requirement when transporting between O-DU and O-RU.

**Fronthaul interface** is a protocol connecting the Baseband Unit (BBU) and Remote Radio Head (RRH). It carries signal along the base station and antenna using fiber ethernet. Fronthaul interface is used because the existing CPRI (Common Public Radio Interface) specifications implemented on C-RAN is vendor-specific, which is difficult for the interoperability of DU and CU in 5G. So, fronthaul interface defined by O-RAN aims to support the need for wider frequency bandwidths in FHI (Fronthaul Interface) and the main purpose is to achieve multi-vendor RAN in 5G era.

**The important function for Fronthaul:**
- Transporting data between DU and RU
- Splits functionality between O-RU and O-DU
- Enables multivendor RU and DU interoperability
- Increase transmission bandwitdth through Split Option 7-2x
- Downlink and uplink bits processing


From the image above, we can see that O-RAN fronthaul is using split option 7-2x because as we mention before, open fronthaul is lower layer split so it using split option 7-2x. Option 7.2 can implements functions up to Resouce Element (RE) mapping in O-DU and can support an O-RU that implements:
- Digital beamforming (easy to implement): Category A
- Digital beamforming + precoding: Category B

**Open Fronthaul Interface consists of three part:** CU-Plane, S-Plane, M-Plane


- CU-Plane: I/Q signal for control plane and user plane that transport using eCPRI
- S-Plane: synchronization protocol that use PTP (Precision Time Protocol) and SyncE
- M-Plane: configuration of network management. It will be related to O1 interface. M-Plane consist NETCONF protocol over SSH/TCP/IP.


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## Comment
5G has different interfaces and components to 4G because 5G enables multivendor interoperability.

## Personal Studying Report
- Create summary from yesterday and today topic.
- Learn more about Open Fronthaul Interface because it is my part for the final presentation.
- Make my PPT slide and share to other members about my sub-topic part.