While comprehensive Cloud-RAN deployments like the one envisaged by China Mobile are on a very distant horizon for most operators, there are increasing moves to use virtualization in at least some areas of the RAN, in order to increase efficiency and enable truly heterogeneous networks (HetNets).
This is especially seen in small cell projects. Recent Rethink operator surveys indicated that, for two-thirds of MNOs planning virtualized RAN implementations in the next five years, the work would start with small cells.
This is for several reasons. One, as operators embark on densification programs, the small cell layer can be deployed from scratch and so a new architecture is possible without having to risk disruption to the established LTE macro network – which will need to perform several more years of useful service before it would be justifiable to replace it again. Two, virtualization may improve the economics of a new small cell roll-out far more obviously than those of the macro RAN, because very low cost cell site equipment can be combined with virtualized gateways. And three, the main justification for large-scale small cell projects is to enable the HetNet, and a virtualized approach, combined with self-optimizing network (SON) technology, can make it far more viable to integrate different cellular and WiFi platforms into a seamless pool of capacity.
Not all the work in this area is coming from the big movers like China Mobile, Telefonica, Intel and the major OEMs. Several start-ups are active too, and are often more nimble in demonstrating what virtualization can achieve in small cell networks today, as well as for the future vision. Quortus is one example, with its virtualized mobile core, which can be implemented on a small cell to create a remote or locally controlled RAN.
Another is Parallel Wireless, whose ‘C-RAN’ architecture is powering rural roll-outs by EE in the UK. In recent weeks, it has added significantly to its platform, announcing an NFV-compliant HetNet Gateway (HNG) and a content delivery alliance with Saguna.
The HNG claims to make it easier to deploy multiple technologies from different vendors within a unified network. The firm says the platform combines the benefits of gateway VNFs (virtual network functions) with real time SON and centralized scheduling. The HNG supports many virtualized gateway network functions based on the ETSI Open VNF specifications, interworking with one another across 3G, 4G, and WiFi.
This fits into a rising interest in combining virtualization and SON to make C-RAN and small cell HetNet- both of which are targeting huge numbers of cell sites and users, and so require considerable levels of automation – easier to deploy and manage. By virtualizing both C-RAN and SON on the same platform, Parallel says that HNG provides centralized and elastic scheduling over standard X2 interfaces.
Reverb is another pioneer in combining C-RAN and SON, and announced Predictive SON last year, as a stepping stone to full Cloud SON. CEO Zoran Kehler said then: “The Cloud SON proposal isn’t a trivial detail, like hosting your SON software on a cloud hosting service. It’s a fundamentally new network architecture for self-optimizing networks that is in tune with the trend towards cloud-RAN and NFV.”
The aim is to align the SON domain with the move towards centralizing and virtualizing network intelligence, turning it into a full network operating system which can predict cause and effect, and coordinate responses, dynamically. “Many of today’s operational systems are analogous to the muscles and bones of the body, the basic electromechanical fibers. SON is the brain,” boasted Kehler.
Companies like Reverb and Parallel aim to go beyond current centralized SON offerings and create a fully unified set of intelligent capabilities – a sort of ‘SON of SONs’ for a complex multilayer network of the future. For an operator with a mixture of legacy macrocells, small cell and WiFi zones, and cloud-RAN islands, there will be a significant challenge to make all these elements interwork and provide a common pool of resource. The solution may be to run distributed SON on standalone base stations and virtual network elements, to manage local issues like neighbor relations; hybrid or centralized SON at the network level; and Cloud SON on each C-RAN island to pull everything together.
That balance, between centralized and local network functions, will be the most important architectural decision for many operators as they move towards densification, virtualization and eventually 5G. While C-RAN, in its classic form, is inherently centralized – with baseband functions running on central servers and supporting hundreds of simplified cell site systems – an important shift is towards Mobile Edge Computing (MEC), which pushes a great deal of network intelligence out to small cells, and as close to the user as possible.
Parallel’s HNG highlights the need to decide which functions should be centralized and which run on the edge-based cells themselves. The HNG “takes advantage of Moore’s Law for baseband chips and allows operators to keep BBU functionality on site instead of centrally pooled” the company says. This enables operators to reap the benefits of virtualization without the hefty expense of ultra-low latency fronthaul, as required by classic C-RAN.
Also focused on the MEC is Parallel’s alliance with PeerApp and Saguna. The PeerApp/Saguna solution focuses on content delivery in the MEC environment, and it will now be able to run with Parallel’s HNG and its software-defined Converged Wireless System (CWS) base station. This will offer an affordable small cell platform for the content system, with flexible backhaul options, including resilient wireless mesh when multiple CWS base stations are interlinked in a multipoint-to-multipoint formation (the system EE is using to blanket villages with wireless connectivity).
The CWS uses two backhaul modes which draw on the same principles as LTE-LAA (Licensed Assisted Access) in the RAN. These are License Assisted Backhaul (LAB) and Unlicensed Assist (ULA). The former performs similarly to LAA by aggregating LTE and unlicensed spectrum for backhaul and making decisions as to which carrier to use according to the required QoS level (for voice, video and so on). Those decisions are handled by the HNG.
A CWS with Unlicensed Assist backhaul works on licence-exempt spectrum only and complements existing wired or microwave backhaul links with backhaul capacity, while enabling resilience in a case of fixed backhaul failure.
Lior Fite, CEO of Saguna Networks, said in a statement: “The innovative RAN architecture created by combining Mobile Edge Computing and License Assisted Backhaul offers highly efficient content delivery and scalability to mobile operators. This open, standard-based solution can be easily expanded to deliver new MEC services and applications while supporting the diverse requirements posed by HetNet environments.”