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.
- REC: Radio Equipment Controller that handles the MAC and most of the PHY layer. The REC transmit-chain handles channel coding, interleaving, modulation, MIMO, and transmit power control are handled. The individual channels are then multiplexed using an Inverse FFT operation. Finally, the In-phase (I) and Quadrature (Q) samples are sent over to CPRI link to the Radio Equipment (RE).
- RE: The Radio Equipment handles the analog processing on the channel. The RE transmit chain receives the I and Q, performs the digital to analog conversion and transmits the resultant channel to the antenna. The RE is hosted at the antenna site.
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.
- Low latency as any delay on this list adds to the overall link delay. The low latency requirements also place a limit on the distance between REC and RE.
- Low jitter as variation in IQ sample delivery may result in burst decode failure at the UE or the eNodeB.
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.
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.
- Data Unit (DU) serves the RLC, MAC, and parts of the PHY layer.
- Radio Unit (RU) handles the digital front end (DFE) and the parts of the PHY layer.
The above figure shows the 5G Core, CU, DU, and RU splits. The links connecting these units are referred to:
- Connects the 4G/5G core to the CU.
- A latency of ~40ms may be tolerable on this link.
- The 5G core may be up to 200 km away from the CU.
- Connects the CU with the DU.
- The latency on the link should be around 1ms.
- A centralized CU may be controlling DUs in an 80 km radius.
- Connects the DU with the RU.
- Fronthaul latency is constrained to 100 microseconds.
- A DU may be serving RUs up to 10 km away.
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.
- Split 7: PHY is split between the DU and the RU. The bandwidth need varies between ~10 Gbps and 140 Gbps. The 7.2 and 7.3 splits seem more realistic as they keep the Massive MIMO beamforming at the Radio Unit.
- Split 8: PHY is moved completely to the DU. This split seems impractical as it requires an eye-popping 236 Gbps link.
eCPRI: Fronthaul for 5G
eCPRI is designed to handle such diverse fronthaul types. eCPRI supports service points for:
- User plane traffic
- Control and Management
These service points are handled by the eCPRI protocol stack over IP/Ethernet.
Advantages of eCPRI
- Ten-fold reduction of required bandwidth
- Required bandwidth can scale according to the to the user plane traffic
- Ethernet can carry eCPRI traffic and other traffic simultaneously, in the same switched network
- A single Ethernet network can simultaneously carry eCPRI traffic from several system vendors.
- Ethernet-OAM may be used for operation, administration, maintenance, provisioning, and troubleshooting of the network
- The new interface is a real-time traffic interface enabling use of sophisticated coordination algorithms guaranteeing best possible radio performance
- The interface is future proof allowing new feature introductions by SW updates in the radio network
- Jitter and latency will be reduced for high priority traffic using Time Sensitive Networking standard IEEE 802.1CM. The 802.1CM supports preemption of a low priority packet to schedule a high priority delay and jitter sensitive transmission.
The following video from Xilinx provides a good coverage of the challenges involved in fronthaul design.