Could we fit Small Cells underneath the streets?

I was intrigued by a plan to install Carrier Wi-Fi access points below the sidewalks. It's like underfloor heating for wireless coverage. Could the same approach be taken for small cells, avoiding the need to find deployment sites, bypassing planning permission and complex backhaul? We considered some of the issues and implications.

Carrier Wi-Fi below the street

Wi-Fi NOW reported a field trial where Carrier Wi-Fi is being installed below street level in the UK by Virgin Media, the UK’s largest cable network. They own a huge fibre infrastructure with many sub-pavement cabinets throughout towns and cities.

From the photos, it appears they’ve installed Wi-Fi access points directly below the manhole covers. It would surprise me if the metal cover doesn’t constrain throughput and capacity below the claimed 166Mbps rate quoted. Nonetheless, it’s a fairly quick and easy way to install access points in locations where fibre backhaul and power are already available.

Existing Virgin Media customers can use the service which is also initially provided free to others. Potentially this could evolve to a wholesale Wi-Fi service with interested mobile network operators.

You can view some pictures of the installation here

The article also notes that Ziggo, the Dutch cable operator, has some 200,000 street cabinets of which 2,200 are Wi-Fi equipped. These are above ground, sitting unobtrusively alongside other street furniture.

Key benefits

Virgin don’t have to negotiate for additional sites with a variety of landlords, they can install them pretty much anywhere they already have fibre.

Access is generally 24/7, no need to close or partition off the road.

Backhaul is inherent, using their existing fibre network.

Existing workforce could deploy them with relatively little additional training.

So why don’t we see more small cells embedded into street cabinets and under pavements already?

Restricted coverage footprint.

I’d guess the range would be very limited and coverage mostly projected upwards rather than sideways or into the adjacent buildings. While those close by in the street may be able to make good use of it, the volume of users per access point may be quite low. I’m not convinced the signal would travel far along the street, requiring many to be fitted at frequent intervals.

To be more commercially attractive, each small cell has to capture a substantial amount of data traffic – offloading the macrocells and incrementally increasing network capacity. The more limited footprint would restrict the number of users, meaning more units need to be installed and doing little for those inside.

This is why we see network planners generally wanting to position small cells above head height, giving range and in-building penetration.

Pinpoint  location

Positioning a small cell in exactly the right place makes the difference between being profitable or not. Even 50 meters away can be too far from where the highest capacity is required.

Although there are many miles of fibre under sidewalks, and therefore many thousands of potential locations, they may not precisely overlap with where the traffic needs to be served.

Undoubtedly the large number of possibilities would be a benefit to any small cell site database, but not exclusively. The option for small street side cabinets (especially if antennas can be a few metres away/higher) might be worthwhile.

Unpredictable/awkward coverage pattern

As with the restricted range, the actual coverage footprint itself may not map directly to that desired or planned. Carrier Wi-Fi access points do include sophisticated beam forming and MIMO technology which does allow some shaping and adaption of the RF power used in each direction. Whether that would be enough to deliver a truly seamless service remains to be seen. Adding additional access points may help in that regard.

I suspect that the metal manhole cover might have a dramatic effect on that. Perhaps a plastic or composite material may be used as a workaround.

Some of the latest urban small cells also have beam forming/shaping technology which tunes and adapts to match the coverage footprint required. This can be projected to stretch along a street canyon for example or inside specific buildings.

Conclusion

This is an interested new twist in public wireless deployment. We’ll be watching to see what the results of these trials are, and whether switching between so many Wi-Fi access points as you walk along the street results in a good or bad experience.

UPDATE: We reported on a separate trial by Swisscom, using a specially design antenna that replaces the manhole cover and a relative high power 5 Watt remote radio head. This was first announced and trial results presented at DAS Congress Europe in December 2015. Read our event report for details here.