Small Cell Forum takes a stand on virtualization June 10 2015


The Small Cell Forum has unveiled the latest instalment of its Release Program, this time focused on virtualization, a trend which will have a deep impact on small cell deployment and may, in time, spur adoption of new HetNet architectures.

Release 5.1, like its predecessors, consists of case studies, technical primers, best practice guides, business cases and other resources, designed to set out technical and commercial options and to lower the barriers to deployment for would-be small cell adopters. It is a point update to Release 5, and adds 12 documents, five of them focused on virtualization (other topics include LTE-LAA and 5G).

The SCF put virtualization on its agenda a year ago, and now it has laid out its roadmap. At the heart of this is nFAPI (Network Functional Application Platform Interface), which underpins the Forum's preferred architecture split for a virtualized small cell RAN. It provides the interface bwteen the radio head, where most of the latency-specific RF activity remains, and the centralized media access controller, in which most of the intelligence is concentrated, and which may be in the cloud or more localized.

One of the big debates around vRAN, of course, is where the architectural split should occur, and how many functions should be centralized or distributed. Real world choices will depend on the individual operator's objectives and resources (for instance, access to dark fiber), but the Forum believes a common framework approach will help some carriers accelerate and simplify their projects. But it acknowledges that one approach will not suit all, and also plans to explore a second approach, centered on PDCP (Packet Data Convergence Protocol - the layer of the LTE stack which performs IP header compression and decompression).

As David Chambers explains in his ThinkSmallCell blog this week (, the main advantage of the PDCP split would be to avoid increase in backhaul overheads. Backhaul and fronthaul capacity and latency requirements are among the barriers to vRAN deployments, but the Forum says its nFAPI approach limits the impact on this, as backhaul can still be carried over IP, though with lower latency tolerance. The body said the approach was "well aligned with the packet switched backhaul service conventionally used to support small cell deployment".

Forum chairman Alan Law said: "We've found clear benefits and drivers towards centralisation and virtualization of the small cell network. These facilitate the scalability of small cells and enable functions to be moved around depending on loading conditions or availability of compute and transport resources."

A Rethink survey of mobile operators in 2014 found a clear overlap between small cell densification and vRAN in carriers' plans for next generation network architectures. With about 50% of MNOs planning some elements of vRAN or Cloud-RAN in their five-year plans for LTE, 90% of those also had urban small cells on their roadmap. And over one-third of those respondents said vRAN activities would be specifically motivated by the need for densification. Reasons to kick off vRAN projects in the small cell layer include the belief that virtualization will enable resources to be assigned more flexibly and cost-efficiently; that large numbers of cells will require centralized and automated resource allocation to avoid over-capacity and network chaos; and that there will be less risk, and more immediate benefits, from applying virtualization to a new and somewhat discrete small cell layer, rather than the well-established macro network.

Sue Monahan, CEO of the SCF, said: "Some of our most influential work to date has been around integrating small cells with not just the macro network, but with service integration, WiFi technologies and enterprise networks. Our new roadmap will help operators accelerate the delivery of integrated HetNet deployments, while also leveraging new trends from virtualization through to 5G, M2M and LAA." [Licensed Assisted Access - LTE in unlicensed spectrum.]

The Forum says 11m small cells have been deployed as of March 31, though the deployment of public access networks remains in its infancy - 95% of the base is residential, while there are 450,000 enterprise small cells installed, 16,500 in urban environments, and 20,000 for rural and remote applications.

Some players, such as ip.access's CTO Nick Johnson, believe that deployment of public access small cells will only be jumpstarted when there are better platforms to support multi-operator services. The ability for multiple providers to offer services on a common small cell network requires better management and quality control than the standard, MOCN, enables, since it takes a 'free-for-all' approach. Frameworks should allow a primary operator to build out a network (possibly working with an integrator, 'as-a-service provider, or vertical market partner) and then open it up to other MNOs and MVNOs.

The Forum's next major Release will appear in about a year's time. Release 6 will focus on the broad issues surrounding the evolution of the HetNet. This may look at some of the emerging architectures which their developers hope will be part of '5G'. For instance, NTT Docomo's Phantom Cell concept integrates small cells and virtualization, in a currently proprietary way which could feed into standards.The Forum believes one of its contributions will be to collate and distil the requirements of a large base of operators, and then feed the common themes into the vendor ecosystem and into standards bodies and specifications.