5G Fronthaul Architecture | Time Sensitive Networking
5G fronthaul architecture
Now one of the popular interfaces in 5G RAN is between RU and DU in 5G fronthaul architecture. RU and DU functions are described as below.
RU functions:
- Analog to digital conversion,
- Filtering,
- Power amplification,
- TX/RX; using massive multiple input/multiple output (MIMO),
- The RU is integrated with the antenna.
DU Functions:
- Digital processing,
- Signal modulation,
- Encoding,
- Scheduling etc.
RU and DU must be integrated very closely to meet expected performance.
In 4G network CPRI interface is used between RU and DU (BBU. BBU and RU are place at cell site and CPRI interface which is run over fiber from cell site cabinet to RU. This is very low distance, probably 10 to 20 meters distance. While we are talking about C-RAN architecture then it will be approximately 10-15 km.
Limitation of CPRI interfaces:
- It is very much rely on transport and demands wavelengths or dark fiber pairs;
- It is, in efficient and vendor proprietary;
- It is not scalable with massive MIMO and some other new RAN technologies.
Below table provide the information about various industry and working group working on fronthaul, midhaul interface and fronthaul transport architecture.
| Interface | Description about interface | Comments |
| CPRI | Radio frequency digitally over optical interface between radio & baseband. It is basically designed for cell site deployment; In case of C-RAN it is not that much of efficient. | Vendor proprietary, greater transport requirement. Not flexible. |
| eCPRI | Advance verson of CPRI that may run on Ethernet switch. Efficiency is reduced. Management plan issue is not resolved. | Not standardized properly. Many options are available. |
| xRAN Fronthaul Interface | It uses eCPRI style frame carried over IP. Addressed user and management plan interoperability. It shall be used by ORAN group. | Standardized, Popular for lower layer split. |
| Time Sensitive Networking | Mapping interface to TSN is IEEE 1914.3 TSN for fronthaul profile is IEEE 802.1CM It supports multiple higher- and lower-layer splits Supports critical time synchronization | TSN is gaining popularity as Ethernet transport for 5G RAN split. |
| TIP Open Fronthaul | Lower-layer split for vRAN fronthaul project It will work over non-ideal fronthaul. | Great contribution. May be it will become xRAN/ORAN |
| 3GPP Higher- Layer Split | Higher layer split, F1 interface between DU-CU and the interface used between CU-UP and CU-CP is E1. Specified in TS 38.47x | 3GPP open interface and standardizes. Very popular for midhaul. |
| TTA | The lower layer split between gNB-CU and gNB-DU Is Fx. This is recommended by SK Telecom. | Support issue compared to other lower layer split. |
There are two most popular architecture are mentioned below.
In case of lower-layer split (fronthaul) – xRAN interface over TSN-based Ethernet transport. This is one of the most popular architecture. It will take some time for maturity.
In case of higher-layer split (midhaul) – the 3GPP functional split 7x (corresponds to the F1 interface) over IP/Ethernet transport, this approach is easier to deploy and will be popular for most early split-RAN 5G architectures.
Time Sensitive Networking
Time sensitive networking is belongs to 802.1Q standard. It is developed to give specific messaging service over standard Ethernet network. The time sensitive networking plays an important role in case of aerospace, automotive, transportation and utilities and manufacturing industry where real time and deterministic communications are required.
There is some great functionality that time sensitive networking is providing for 5G RAN transport.
- There is no loss in congestion,
- Meet the criteria of ultra-low latency,
- It provides hitless 1+1 redundancy and
- It is able to carry sync almost <1 micro second.
- It has an ability to prioritize different type of traffic.
- Ethernet is widely accepted technology.
Therefore time sensitive networking shall serve the X-haul transport network requirement for 5G RAN.