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FRONTHAUL

Optical signals are used for communications between the Remote Radio Unit (RRU) and the Baseband Unit (BBU) or Distributed Unit (DU) 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) but this does have some limitations and challenges. 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’, which provide 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. TEnhanced CPRI (eCPRI) project 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, enabling operators to mix and match vendors.

eCPRI identifies three planes as a part of the protocol: user (U), synchronization (S), and control and management (C&M) planes. 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’ workgroup (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

Active Projects

FRANC (Future RAN Competition) run by DCMS, 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:

Academic Institutes

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.