The Baseband Unit (BBU) functionality in 4G/LTE is split in 5G into a Centralised Unit (CU) and Distributed Unit (DU). The DU is generally located close to a RU (in 5G a RRU/RRH is referred to simply as RU), either on site or in an Edge Cloud/data centre. The CU however can be either located in the Regional Cloud/data centre (e.g. C-RAN), or with the DU (e.g. D-RAN).
The most appropriate location of the CU and DU is determined by many factors including, but not limited to: the expected latency requirements of applications/services, site lease costs, availability of backup battery/generator capacity, high capacity fronthaul availability, the location of data centres and bandwidth costs.
For 5G, each CU can connect to one or more DUs, each of which in turn can connect to one or more RUs. In Carrier Aggregation (CA) scenarios, a single DU connects to all the RUs of the carriers being aggregated. In Dual Connectivity (DC) scenarios, two DUs are present, and can be connected to either the same or different CUs.
For distributed units and centralised units therefore, the key issues for Open RAN are:
- Comparable performance to incumbent systems
- Cost compared to incumbent systems
- True openness and interoperability with a range of hardware platforms
- True openness and interoperability with a range of radio units, centralised units and distributed units
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 distributed units and centralised units:
FRANC academic partners in this area include:
- University of Bristol: their 5G Test Network has been designed to provide a multi-tenancy cloud network infrastructure with its resources including a mix of proprietary and open-source network functions, and interfaces using multi-vendor end-to-end network solutions. They are part of both the O-RANOS and Proteus projects.
- University of Edinburgh: through their work in the Towards AI Powered and Secure Carrier-Grade Open RAN Platform project, they aim to realise a carrier-grade cloud solution, enabling operators to deploy Open RAN network functions easily and securely, with a focus on lowering the barrier to entry for new entrants.
- University of York: through their work in the DU-Volution project, they will evolve distributed unit devices to meet industry requirements including reduced power, smaller form factors, improved spectrum efficiency and reduced latency.
The following commercial organisations are actively undertaking R&D into Open RAN in the distributed units and centralised units space, through the FRANC projects.
