Cloud RAN and eCPRI fronthaul in 5G networks

EventHelix
Feb 5, 2018 · 5 min read

5G networks will move towards centralizing the radio function using an eCPRI based fronthaul. The main objective here is to limit the electronics needed at the antenna site. This lowers cost as air conditioning is not needed at the antenna site.

Cloud-RAN and CPRI

4G LTE networks have started moving towards a distributed Cloud RAN based architecture. The C-RAN approach is summarized in CPRI overview in IEEE Communications (Antonio de la Oliva, et al):

The C-RAN approach advocates for the separation of the radio elements of the base station (called remote radio heads, RRHs) from the elements processing the baseband signal (called baseband units, BBUs), which are centralized in a single location or even virtualized into the cloud. This approach benefits from simpler radio equipment at the network edge, easier operation, and cheaper maintenance, while the main RAN intelligence (BBUs) is centralized in the operator-controlled premises. The challenge of C-RAN deployments is that such a functional split requires these two elements to be connected through a high-speed, low-latency, and accurately synchronized network, the so-called fronthaul.

The network is split into three parts:

  • C-RAN: Cloud-RAN that houses the Core Network, RRC, PDCP, and RLC layers. Standard cloud computing platforms may be used to house the C-RAN.
CPRI in the transmit chain (Credit: CPRI overview, IEEE Communications)

The link between the cloud-hosted C-RAN and REC is via a traditional IP based backhaul. The link between REC and RE is served using CPRI (Common Public Radio Interface). The CPRI link requires:

  • High bandwidth as IQ samples are being transferred between the REC and RE.

Splitting the 5G RAN

5G networks will bring new functional splits between the baseband and radio. Some of the split options under discussion at the 3GPP are shown below.

5G network split options under discussion (Credit: eCPRI overview)

The industry seems to be converging on a split 2+split 7 NG-RAN architecture:

  • Control Unit (CU) hosts RRC and PDCP layers in a telco cloud.
5G Core, CU, DU and RU split (credit: Xilinx)

The above figure shows the 5G Core, CU, DU, and RU splits. The links connecting these units are referred to:

Backhaul

  • Connects the 4G/5G core to the CU.

Midhaul

  • Connects the CU with the DU.

Fronthaul

  • Connects the DU with the RU.
5G Backhaul, Midhaul, and Fronthaul based network architecture (credit: Xilinx)

Fronthaul Bandwidth

The fronthaul bandwidth depends on the exact split point between the DU and the RU. The following figure compares the fronthaul bandwidth needs for a 64 Transmit-64 Receive Massive MIMO installation with 100 MHz system bandwidth.

  • Split 6: PHY is completely implemented in the RU. This option requires a 3Gbps link.
Splitting the 5G PHY layer between the Data Unit and the Radio Unit (credit: Xilinx)

eCPRI: Fronthaul for 5G

eCPRI is designed to handle such diverse fronthaul types. eCPRI supports service points for:

  • User plane traffic
eCPRI architecture (Credit: CPRI)

These service points are handled by the eCPRI protocol stack over IP/Ethernet.

eCPRI protocol stack (Credit: eCPRI)

Advantages of eCPRI

  • Ten-fold reduction of required bandwidth

eCPRI presentation

Fronthaul design

The following video from Xilinx provides a good coverage of the challenges involved in fronthaul design.

5G NR

5G New Radio