5G RAN Functional Split

A lot has changed from the previous generation cellular/mobile network. From 'voice-only' 2G network, to introduction of mobile internet on 3G, then the IP based 4G network. Our generation have seen many advancements and technological enhancements as far as cellular communication is concerned. We have come to an age of cloud based network that puts major emphasis on server virtualization, the use of containers, orchestration and network automation.

In this blog, we will go through the various 5G RAN Architecture options.

Traditional RAN

In traditional RAN deployments, the cell site generally consisted of a Base Band Unit (BBU) with optical fiber and power cable running over to the Remote Radio Head (RRH). The Radio Head used to be connected to Passive Antennas to propagate the RF Signal. In all these deployments, hardware, software and various interfaces remained either proprietary or 'locked' to the individual RAN vendor. This sometimes created vendor interoperability issues.

Few years later, virtualization of the RAN started with the C-RAN (cloud RAN or centralized RAN) methodology. With C-RAN, deployment was divided into two parts:

Base Band Unit sitting in a data center (Centralized) and
Radio or RRH (remote radio head unit) sitting miles away but connected to the Base Band Unit through a dedicated high bandwidth connection.

What's changing with 5G?

Short Answer - Lot of things are going to be changed that includes below major modifications / enhancements:

The overall architecture for both core and RAN.
5G Core Function virtualization using containers and Kubernetes (for orchestration).
Splitting of Base Band Unit (BBU) into two functions - Centralized Unit (CU) and Distributed Unit (DU).
Splitting of CPRI into two functions - eCPRI (CPRI over Ethernet) and PTP (for Timing)
And most importantly, extremely stringent timing requirements at the Mobile Fronthaul.

The following figure shows the various RAN Functional Split Options that an operator have for the deployment of their 5G RAN.

Fig 1. RAN Split Options

Among these splits, the most popular options are marked in blue below - Option 6 (nFAPI), Option 7-2x (O-RAN) and Option 8 (CPRI). However, we will go through each and every option mentioning their features.

Option 1 (eCPRI v1-A)

Low bandwidth requirements.
Separate User Plane and centralized Radio Resource Control/Radio Resource Management.
Suited for low latency and edge computing scenario.
Doesn't provide much scalability in terms of overall Radio function.

Option 2 (eCPRI v1-B)

Achieves PDCP-RLC split.
Option 2-2 allows a Separate User Plane and centralized Radio Resource Control/Radio Resource Management.
Since it achieves a high layer split between CU and DU, hence it is latency tolerant and helps enabling distances up to 40km.

Option 3

Very Low bandwidth requirements and low latency requirements.
More robust under non-ideal transport conditions.
Not a popular option and doesn't generate much traction.

Option 4 (eCPRI v1-C)

Low bandwidth requirements.
Bit rate scales with MIMO layers.
Same as option 3 in terms of popularity.

Option 5

Low bandwidth requirements.
Reduced latency requirements if HARQ processing and cell-specific MAC functionalities are performed in DU.
Efficient interference management across multiple cells and enhanced scheduling technologies such as CoMP,CA, etc.

Option 6 (nFAPI)

Ideal for Small Cells deployment.
Double Split RAN architecture (CU <--> DU <--> RU)
May require subframe-level timing interactions between MAC layer in the CU and Phy layers in the DU
Round trip fronthaul delay may affect HARQ timing and scheduling

Option 7-2x (O-RAN)

Simplified interfaces
Open interface protocol specifically designed to enable interoperability between DUs and RUs from different vendors.
Ideally suited for virtualized RAN and virtual DU running on general purpose processing platforms.
Cost effective Remote Radio Head
High bandwidth requirements and relatively high latency requirements

Option 8 (CPRI)

Small and cost-effective Radio Unit
Easy to centralize CU/DU enabling Coordinated MultiPoint (CoMP)

Based upon the various options, the following figure depicts the overall 5G RAN Architecture in terms of connectivity and distance between each entity.

Fig 2. 5G RAN Architecture

Please check my next blog on 5G RAN Timing and Synchronization - one of the most important aspects of the over all 5G network.

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