Traditional thinking is that each cell has its own physical base station, and that adding capacity in a cellular network involves creating more cells by installing more base stations. CommScope (who acquired Airvana in October 2015) have developed a technique called Cell Virtualisation, by which they can grow capacity by reusing the same frequencies within a single cell. We spoke with Vedat Eyuboglu, Small Cells CTO, to understand out how it works.
What is Cell Virtualisation?
Spectrum is a precious asset. We designed our OneCell solution to ensure it could utilise the available spectrum in the most efficient way, while delivering a consistently good experience for all users and maximum possible capacity.
There is a lot of emphasis across the industry today around Network Function Virtualisation – running telecom applications on Intel-architecture hardware. This brings flexibility, scalability and cost savings. We saw that similar benefits can be accrued by virtualising the cells, which traditionally have been implemented as static physical cells tied to individual base stations. Cell Virtualisation brings consistent user experience, higher capacity and improved economics.
OneCell centralises most of the baseband functions of a base station in a controller node, which serves many radio nodes distributed across a building. These Radio Points form a single large cell radiating the same cell identifiers in a synchronised manner. The controller can tightly coordinate the radio transmissions and receptions of every radio node. The controller also oversees the detailed interactions with the smartphones that are in the coverage area and tracks their RF signatures as seen by the radio nodes. This allows the controller to serve different users on the same frequency at the same time even though these users belong to the same physical cell.
Using this method, we are able to serve users in the most efficient way, dynamically adjusting the serving radio nodes based on users’ positions and load conditions across the network. Effectively, we create additional virtualised cells and grow the capacity while delivering the best possible user experience, all within a single physical cell ID (PCI). In OneCell we call this feature Smart Reuse.
What kind of performance improvements can you deliver?
This is what we are most excited about. In common deployment scenarios throughput seen across all users also increases by up to 50%, and OneCell can deliver 100-300% improvements for 10-percentile users in poor RF conditions. This is especially critical when delay-sensitive traffic such as VoLTE is involved.
This sounds a lot like multi-user MIMO
In multi-user MIMO (MU-MIMO), two or more users are served from the same set of co-located antennas and transmissions to different users are separated through carefully crafted spatial filters (precoders) in the base station. This is a special case of the cell virtualisation in OneCell, where transmitting antennas can also be physically distributed.
An important aspect of Cell Virtualisation in OneCell is that it works for all LTE devices in use today. True MU-MIMO with spatial multiplexing was introduced into the LTE standard later and is not supported by most smartphones today. As MU-MIMO support becomes more widely available in devices, it will become part of OneCell’s cell virtualisation capability.
Could this work with LTE-Advanced features such as Carrier Aggregation?
Yes, of course.
Perhaps more significant however, is that cell virtualisation reduces the need for CA. CA by itself does not address the underlying issues of interference between densely-deployed small cells – rather it compensates for them by making two RF bands available to users. By contrast, cell virtualisation coordinates spectrum among users to reduce interference.
In a system with both CA and Smart Reuse, both carriers will have the advantage of cell virtualisation.
Are there any other benefits?
There are two I’d particularly like to emphasise. One is the quality of service at cell boundaries, where standalone/independent small cells may not achieve such seamless performance. The co-ordination allows OneCell to provide consistently high quality and throughput much more uniformly in the areas between radio nodes. These benefits apply equally to downlink and uplink transmissions. The latter is especially important in high-density venues where photo/video uploads can dominate traffic and bring a traditional system to its knees.
The distributed C-RAN architecture has a number of other benefits. On the downlink we can serve users from multiple radio nodes at the same time. Likewise, on uplink we can algorithmically combine the RF signals received from multiple radio nodes and in the process we not only improve the received signal strength but also suppress interference from other users nearby.
Operating as a single physical cell also simplifies the network planning and deployment. It streamlines any interactions with operator macro cell networks and enables reliable heterogeneous network deployments where macro cells and small cells share the same RF channel.
Is this purely for venues with extremely demanding requirements?
We’ve found that even in less extreme scenarios such as enterprise office buildings, there are still issues at the cell borders that cause interference and delayed handovers. A coordinated approach will deliver a better overall user experience even when the system is not capacity challenged.
Just as important for enterprise is the ease of deployment. Instead of dozens of PCIs and complex RF planning, as well as arduous optimization tasks and tweaking of handover parameters in different parts of the building, OneCell eliminates all of that extra deployment cost by using a single PCI and no handovers.
Does this avoid the need for D-SON (Distributed Self Organising Network)?
One can think of the OneCell architecture as delivering the ultimate in SON because it coordinates transmissions and receptions from/to our radio nodes in a fully dynamic manner. OneCell also adapts its operation to external cells (macro or small). This is done through a combination of network listen, user listen capabilities, as well as the use of the standardised X2 interface. These SON processes are greatly streamlined because an entire OneCell installation consisting of perhaps 30 Radio Points will appear as a single cell to the rest of the network.
What’s next for CommScope and OneCell?
C-RAN has proven to be a powerful architecture for enterprise small cells – Cell Virtualisation is the latest example of this. As we look at other important areas for the industry such as efficient use of license-exempt spectrum and improved coordination of HetNets, but stay tuned - more on those advantages later.
For more information about CommScope’s OneCell solutions, visit their website