Mobile small cells create new network challenges

CCS CEO Steve Greaves discusses the backhaul challenge behind next-generation mobile networks in Electronics Weekly.

Steve-Greaves

Faced with unprecedented levels of network congestion in the not-too-distant future, mobile operators will be looking at ways of improving spectral efficiency through the deployment of multi-layered radio access networks.

These new-look networks are comprised of macrocells to deliver wide-area coverage and small cells and WiFi to cover very specific areas and hotspots. Combining all these 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 it’s one that brings with it several new areas of complication.

Managing the integration of multiple technologies and radios is a significant challenge, particularly when subscribers shouldn’t need to know or care about which particular radio technology is providing their service. A significant challenge is how to connect all of these new cells back into the network, and doing so in a way that delivers the best possible performance for subscribers and network operators alike.

Connecting the dots

Macrocells have typically been linked back to the core network using fibre wherever possible, but with potentially hundreds or even thousands of small cells and WiFi access points to connect on lampposts or sides of buildings across a busy city, it is unlikely to be practical or affordable to take the same approach.

If fibre is not a viable option for every new cell on the network, then a suitable wireless alternative needs to be found. However, it needs to be a carrier-class solution that delivers high capacity and low latency in a cost-efficient manner. It also needs to cope with new challenges in synchronisation and timing and not obstruct a potentially complex planning permission process.

Operators will ultimately need to roll out thousands of small cells, so rapid, low-cost deployment is crucial. They are also facing similar challenges in planning permissions that were faced over 20 years ago for macros, dealing with local authorities and councils that may have limited knowledge of telecoms and a clear focus on protecting the look and feel of their town and city centres.

These concerns should not be underestimated and those involved in town planning often won’t like the idea of new boxes being attached to lampposts, or the disruption that comes with installing hundreds of units across a busy city. This 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 needed.

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

While sub-6GHz non-line-of-sight backhaul systems can typically fulfill the brief, spectrum is in short supply and won’t deliver the capacity ultimately required 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 and be able to self-organise, making it easy for local, less-skilled contractors to install and enable the network to automatically adapt when new nodes are added.

Timing and synchronisation is another key challenge and one that has gone from being relatively simple in the macro-based network, to a significant one for small cell and HetNet deployments. With new MIMO standards on the way and more operators looking to carrier aggregation, achieving tight synchronisation is critical in both managing interference and the hand-off between macro cells and small cells or WiFi.

This will ultimately require a split in the HetNet architecture between centralised and distributed elements. The connection between these 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.

Despite the challenges, HetNets are coming and it’s the only real way for operators to deliver reliable, high-speed data services to subscribers in busy locations. The key to a successful HetNet rollout will be in selecting a carrier-grade backhaul solution that’s cost-effective and easy to deploy. However, this 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.

This article first appeared in Electronics Weekly.

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