In this second of two articles, Andy Sutton, Principal Network Architect at EE, walked around central London with me. He pointed out the practicalities of Small Cell backhaul for our urban streets. As the network densifies, more street-level small cell sites will be needed. Andy explains how these will be connected back into the network, and clarifies the technical requirements for this rollout.

Staged evolution towards LTE Urban Small Cells

In our first article, we reported how Andy envisages a three stage densification of the network – first expanding existing macrocell sites, then microcells and thereafter rolling out urban small cells. These would mostly be LTE only, but capable of matching the features of the macro/micro network.

High data rates using Carrier Aggregation (EE have spectrum in both 1800MHz and 2600MHz bands) and high quality voice using VoLTE should be easy to enable, but both will require adequate backhaul capacity and design.

I took this to mean that backhaul links will be dimensioned towards peak throughput speeds rather than just the bare minimum.

Dark fibre hubs feasible for many locations

EE have recently announced a deal with CityFibre, who will provide dark fibre to many existing cellsites in urban areas. They are also in discussion with other suppliers.

Even where dark fibre is used, Andy expressed a preference to use Small Cells rather than Cloud RAN with remote radio heads in the initial phase of deployment.

A combination of Gigabit Ethernet leased lines, supplied by BT and Virgin, along with self-deployed point to point microwave radio links connect most urban macrocells and small cells today. Looking up the data for one macrocell rooftop site on their database, backhaul capacity is scheduled to be upgraded from around 400Mbps to over 700Mbps to keep pace with demand. There's no point in installing a lot of expensive radio equipment if the backhaul isn't dimensioned to make best use of it.

When I asked about the high cost of laying new fibre to a site, this is offset by much lower OPEX costs of dark fibre when compared with third party Ethernet leased lines. There is also the option of upgrading capacity in the future by simply upgrading the equipment connected at each end of the link.

Rooftop to street level

Urban small cells would be connected either to a nearby macrocell rooftop site or to a fibre hub in the street. It's unlikely that all small cells would have their own dark fibre connection, with wireless links used to connect between them.

Where rooftop sites are used as the hub, wireless line-of-sight links are preferred, and there may be issues with tree foliage and temporary obstacles (buses, signage, festive decorations etc.). A more practical issue is that links have vertical constraints as depicted below. Even if mounted on the side of the building, they may not reach the same side of the street.

It's only by physically walking down the street and making a site survey that these aspects are fully appreciated.

Hub and Spoke from the rooftop

A popular arrangement is hub-and-spoke where each small cell has a direct connection to the nearby rooftop macrocell. The diagrams below illustrate the same street layout with and without buildings blocking the LoS path – the central rooftop scenario is shown on the right of the chart.

Walking around the streets of London, it becomes clear that such direct line of sight from the rooftop isn't always feasible. This might not be the case in other metropolitan layouts with wider streets, lower buildings and/or a more standard grid format.

It's also clear that the challenges of traversing at street level are somewhat different to those of connecting to a rooftop, which may lead to different technologies being more appropriate.

Daisy-chaining at street level

The left side of the chart above shows small cells positioned in exactly the same places but connected at street level. Daisy chaining means that the capacity of the links closest to the hub must handle the total traffic from four or five cells, where the links on the right only had to cater for one.

Point-to-point, point-to-multipoint and multi-point to multi-point wireless backhaul will be used. The number of backhaul devices varies considerably between these as shown below:

Red connections on the multi-point to multi-point diagram indicate automatic back-up paths enabled through the use of this technology. EE are actively testing a variety of wireless backhaul technologies from multiple vendors including 60GHz point-to-point, microwave (multi)point-to-multipoint and sub 6GHz non-line-of-sight. While Andy wouldn't be drawn on which particular vendor equipment he favours most, it seems thatnone will dominate exclusively.

LTE-Advanced Urban Small Cells will need Phase Sync

LTE-Advanced features such as eMBMS (LTE Broadcast) and eICIC will need phase synchronisation, which ensures time alignment between cells of all sizes. Andy's view is that the groundwork should be laid down in advance so that any of these features can be used as required, rather than needing a retrofit.

When I asked which method might be used for this, it seems that a combination of GNSS (including GPS and in the future; Galileo) and IEEE1588v2 PTP will be used, underpinned by Synchronous Ethernet that provides a stable frequency clock. Inherently, all backhaul transmission, whether wired or wireless, will need to cater for that.

Summary

Highlights from this short walk around London's city centre streets:

• There are already significant numbers of cellsites in our urban areas. These will be expanded before the next wave of urban small cells are deployed.
• Capacity and performance from small cells must match that of the macrocell network. This means avoiding backhaul bottlenecks and using LTE Carrier Aggregation, especially where daisy-chaining is used.
• Phase sync will be implemented throughout the network, fully in place prior to large scale LTE small cell rollout
• Fibre will become more prevalent, but even dark fibre does not predicate a switch to Cloud RAN architecture
• Rooftop to street level wireless backhaul presents a different challenge to that of traversing along and around street canyons

My thanks to Prof. Andy Sutton, Principal Architect at EE, for taking the time to educate me on the streets of London and providing the diagrams used above.

in case you missed it, read Andy's previous article from last month where he explained EE's three stage evolution towards Urban Small Cells.