Joining the dots — the backhaul challenge behind the HetNet

The business case for backhauling small cells is much more complicated than for macrocells, which usually rely on fibre. But this will not be feasible for the thousands of small cells needed to meet future capacity demands. Steve Greaves, CEO of CCS looks at the challenges associated with wireless backhaul

Joining the dots — the backhaul challenge behind the HetNet

With analysts predicting urban small cell deployments will ramp up in the next 12 months, mobile operators realise that city networks need to evolve and change in order to deliver the capacity and throughput that subscribers are demanding. At the same time, equipment vendors have moved to appreciate the ‘toolkit’ approach that operators will need to take, incorporating different technologies each suited for different locations and tasks.

In the face of unprecedented levels of network congestion in the coming years, mobile operators are looking at ways to improve spectral efficiency through deploying multi-layered radio access networks.

These ‘new-look’ networks comprise macrocells to deliver wide-area coverage, and small cells and Wi-Fi to cover very specific areas and hotspots. Combining these components into a single heterogeneous network — or HetNet — is regarded as the best way to deliver seamless high-capacity mobile broadband coverage to subscribers regardless of location (indoors, outdoors or even underground) but brings with it several new areas of complication.

For an industry that has traditionally operated on a single vendor model, managing the integration of multiple technologies and radios is a significant challenge. A key part of this integration is in connecting these nodes back into the core network, getting data to and from the busiest areas of cities quickly and efficiently to ensure a high quality of service for subscribers.

Moving away from fibre
Macrocells have typically been linked back to the core network using fibre wherever possible. However, with potentially hundreds or even thousands of small cells and Wi-Fi access points to connect to lampposts or sides of buildings across a busy city, vendors can’t practically or affordably take the same approach.

So far we’re seeing operators try a variety of different backhaul solutions, working out what offers the best fit for each location. If fibre is not a viable option then operators must find a suitable wireless alternative — one that needs to be a carrier-class solution that delivers high capacity and low latency cost-efficiently.

Wireless alternatives also need to cope with new challenges with synchronisation and timing and have a discreet enough form factor that does not obstruct a potentially complex planning permission process.

The business case for macrocell backhaul was much more straightforward compared to the small cell equivalent, where low-cost deployment is crucial with thousands of nodes potentially being deployed.

Operators are also facing similar challenges in planning permission that they were facing over 20 years ago for macros. These issues arise when dealing with local authorities and councils that are likely to have limited knowledge of telecoms and instead have a clear focus on protecting the look and feel of their town and city centres.

These concerns should not be underestimated, whether that’s in helping maintain the aesthetics of an area, or take into account that certain lampposts will be earmarked for Christmas decorations once a year!

It’s also important to minimise disruption during installation of small cells, keeping both local residents and businesses onside throughout the implementation process. These considerations can pose a problem for conventional point-to-point wireless backhaul systems, which typically require two or even three nodes per site to deliver the connectivity.

It’s essential therefore that both the small cells and backhaul system are quickly and easily installable — ideally without requiring specialist engineers. Subsequent additions to a hetnet should also be as easy as possible to minimise disruption, which is a significant challenge because small cell deployments are likely to be gradual, and having engineers re-aligning multiple nodes each time is simply not cost-effective or practical.

Timing and synchronisation has gone from being relatively simple in the macro-based network, to significantly challenging for small cell and hetnet deployments. With new MIMO standards on the way and operators looking to carrier aggregation, achieving tight synchronisation is critical for managing interference and handing off between macro cells and small cells or Wi-Fi.

Another big trend is virtualisation and it’s something that the Small Cell Forum is paying close attention to. Virtualisation will ultimately require a split in the hetnet architecture between centralised and distributed elements. The connection between these elements is often referred to as ‘fronthaul’ and traditional implementations require links with very high capacity and low latency.

However, with a more pragmatic functionality split between distributed and centralised elements, it becomes feasible and much more cost-effective to use microwave-based links with limited impact on network performance.

Spectrum is finite
While sub-6GHz non-line-of-sight (NLOS) backhaul systems can typically meet many operator requirements, spectrum is in short supply and won’t deliver the necessary capacity on a longer-term basis. Managing interference is also set to become more of an issue as small cell networks expand — a significant problem in a multi-band, carrier-aggregated hetnet scenario.

By contrast, V/E-band systems offer the requisite capacity and low latency, but deployment is slow and costly with precise alignment required for each link and multiple nodes may be required at each site. The ideal backhaul system should be a single unit that supports multiple links, with the ability to self-organise, making it easy for local, less-skilled contractors to install and enable the network to automatically adapt when they add new nodes.

Hetnets are coming and, despite the challenges, they are the best way for operators to deliver reliable, high-speed data services to subscribers in busy locations. Deployments will be more successful, however, if operators and vendors select the right backhaul solution that is both cost effective and easy to deploy.

The optimal solution needs to do more than just connect multiple small cells back into the core network. It also needs to help smooth planning approvals, offer highly accurate timing and synchronisation and make it easy to expand the network as needed. It’s no easy task, but it’s one that at CCS we think we’re on the right path to solving.

About the author: Steve Greaves is co-founder and CEO of CCS (Cambridge Communication Systems).

CCS was founded in 2010 with the specific purpose of creating a backhaul system for small cells. Its Metnet solution uses a highly resilient multipoint-to-multipoint architecture, which can be deployed in a hybrid mesh, Point-To-Multi-Point and Point-To-Point topologies. The CCS Metnet system uses LOS, area-based microwave spectrum. It operates in a single frequency channel – at 26 and 28 GHz currently.

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