Communications between the Remote Radio Unit (RRU) and the Baseband Unit (BBU) or Distributed Unit (DU) in a network occur via a fibre optic cable; this is referred to as fronthaul.
The most commonly used protocol for fronthaul is the Common Public Radio Interface (CPRI). CPRI was designed for the RRU and BBU to be co-located, meaning there is a limit on the physical distance that can exist between them. With centralised and cloud RAN (both abbreviated as C-RAN) becoming ever more popular, this physical distance limitation can become challenging for locations outside of built-up areas.
As CPRI is specified for point-to-point topology and is antenna dependent, it has ended up with many different proprietary ‘flavours’, creating a barrier to multi-vendor deployments. CPRI can also only support limited fronthaul data rates, rendering it unsuitable in high data rate scenarios for 5G. Enhanced CPRI (eCPRI) was developed to overcome these and other limitations.
eCPRI replaces the synchronous data transfer in CPRI with an ethernet/IP based protocol, removing the vendor-specific dependency. It supports point-to-multipoint and multipoint-to-multipoint communication, in addition to providing a point-to-point topology, meaning operators can mix and match vendors.
eCPRI identifies three planes as a part of the protocol: user (U), synchronization (S), and control and management (C&M). U-plane specific transport definitions in eCPRI standardise the data frame, packet, and header formats; the synchronization and control and management planes are not explicitly limited by the protocol.
The O-RAN Alliance ‘Open Fronthaul Interfaces’ working group (WG4) aims to deliver truly open fronthaul interfaces, where multi-vendor DU-RRU interoperability can be realised. It includes the Control User Synchronization (CUS) plane and the Management (M) plane over Low Layer Split interfaces.
For fronthaul therefore, the key issues for Open RAN are:
- Supporting multiple vendors on shared physical infrastructure
- Supporting a range of Open RAN splits cost effectively including most challenging high bandwidth and low latency of CPRI
FRANC (Future RAN Competition) run by DSIT, has allocated up to £30 million of R&D funding to projects that support the goals of the government's 5G Supply Chain Diversification Strategy. The competition aims to help to incentivise industry to create new products and services to unlock the full potential of Open RAN. Several of these projects are exploring fronthaul:
FRANC academic partners in this area include:
University of Leeds - working in the ECORAN project on a use case where processing capabilities are steered in an energy efficient manner to where they are needed in space and time
Aston University - working in the Scalable Optical Fronthaul for 5G OpenRAN project to develop ground-breaking, UK-made, scalable, cost-effective optical interface technology to enable dense roll out of optical fibre 5G radio access networks (RANs) with open digital interfaces for interoperability and low latency.
The following commercial organisations are actively undertaking R&D into Open RAN in the fronthaul space, through the FRANC projects.
