Small Cell Backhaul

Revisiting Free Space Optics: A low power, low cost approach for small cell backhaul

Free Space Optical Small Cell BackhaulThere are many technologies contending for use in outdoor Small Cell Backhaul today, and one thing is certain – none of them is suitable for all scenarios and the industry consensus is that a mix of different technologies will be needed. We've seen free space optics (using infra-red beams) being proposed before, but it's often dismissed out of hand. A Norwegian company is setting out to change that perception, with a novel approach to reduce cost and improve effectiveness.

The traditional approach to Free Space Optical

When I've looked at this technology in the past, the usual approach is to take an industrial laser costing up to $1000 and project an infra-red light beam with a footprint of perhaps 4-6 feet at the far end. Companies such as SkyFiber have been demonstrating shoe-box size equipment at Small Cell conferences using two parallel beams delivering up to 1Gbps data rates over short distances. Lightpointe are one of the world's most successful FSO vendors, but their equipment typically costs several thousand per link and wouldn't be as suitable as 60GHz millimetre products.

Despite large investments in FSO in the past, it is still perceived as an "exotic" technology, perhaps because vendors have tried to compete/match the range and performance of microwave. While several kilometres range is possible in sunshine, fog quickly causes outages.

The attractive features (on paper at least) for optical links include:

  • No spectrum licence required. It will be hard for authorities to tax the use of light.
  • High data security: Difficult or impossible to intercept/eavesdrop
  • No interference issues with competitors
  • Low latency and jitter
  • Up to 1Gbps rates or higher

Commonly perceived downsides

  • Links are affected by fog and sleet
  • Potential for outages during sunrise/set if aligned in path of sun
  • Alignment, especially during installation
  • Cost: Not significantly cheaper than microwave technologies
  • High power lasers may not be eye-safe, e.g. for Window cleaners who might inadvertently stare directly into the beam
  • Must have completely clear line of sight with no foliage or other obstructions.

A novel approach, focussing on low cost and low power

Polewall, a Norwegian company, has taken an unusual approach to the topic, focussing on short range, low cost and simple installation. They have developed a product using existing, known and mass market components:

  • The laser from a standard DVD player pickup,
  • The housing and stepper motors from city centre surveillance (PTZ) cameras
  • Wi-Fi/Smartphone app for alignment during commissioning

The beam width is quite narrow, only 10cm at the far end, and has to be kept aligned. Large adjustments, including during installation, are made using stepper motors that normally rotate the surveillance camera. Minor adjustments, such as pole sway compensation, are made by electromagnetically adjusting the lens.

The use of such a low power laser also has benefits in total power consumption, with each transceiver box consuming less than 4 Watts.

The design isn't intended or suitable for long ranges of 1km or more, aiming at a maximum of 250 meters or less in street canyons. The current product delivers 100Mbps symmetrically and consistently.

I'm told the price per link today is $1500 in low volume, with a view to getting that down below $1000 in the medium term. Multiple links may be terminated in a central hub in a future design.

Simplifying installation

Apolewall-installations we've heard from many backhaul and metrocell vendors, it's not just the cost of the equipment itself that is important – installation costs must also be kept low to make a good business case. This means not needing specialist engineering expertise to install or calibrate the links. NSN offer a Bluetooth link to communicate with their small cells from the street below. Polewall offer a smartphone App or PC application for this purpose.

They've chosen Wi-Fi rather than Bluetooth because the longer range allows one engineer to control both ends of a link from the same point. A Wi-Fi access point is temporarily connected to each end of the link during this "No Touch" commissioning process.

Experts in Grand Theft AutoTM and similar video games rather than Ph.D's would be highly qualified to align the links, which during installation project a small red dot on the target. The app is used to move this around until it lights up the far end link, after which the system auto-aligns. There are two lasers, visible red and infrared, with the former being switched on only during commissioning.

Not just in the lab

While this sounds an interesting academic concept, the proof is whether it works in the real world. With more than 50 links actively in use by mostly small and regional ISPs across the Nordic region, the kit is proving itself. While it always takes a lot of trialling and verification before operators will accept new technology into their networks, the results so far are promising.

Remote management uses SNMP, a common protocol that is easily integrated with operator's Network Management Systems.

Polewall have been awarded an EU grant to further develop and market this equipment, so we should expect to hear more about it. It's already being trialled by Tier 1 US operators. Their experience in the harsh snowy environmental conditions of Norway provide a sound basis for perfecting the design.

Fits into the urban cityscape

las-vegas-lightpoleThe spread of surveillance cameras throughout our city centres means that this unit fits unobtrusively into the street environment. Few question the safety of these installations and would think twice about damaging them because they think they would be recorded on video. Perhaps co-locating these links with existing surveillance camera installations makes sense – there may already be fibre backhaul and power to that point.

Summary

I think it will take a lot to persuade a sceptical industry to adopt Free Space Optical instead of more familiar microwave and millimetre wave alternatives. Concerns about fog and link availability will need to be overcome.

However, the relatively low cost and high performance of this approach makes it worth considering. Particularly where operators take a view that they could accept less than five nines (99.999%) availability for individual small cells because they could install more of them. The NGMN have indicated that three nines (99.9%) might be adequate for a meshed environment, and virtually everywhere on the planet could achieve that using this technology.

It also sets a target price point of $1000 per link for competing technologies, such as the 60GHz and microwave vendors.

Our thanks to Carl-Fredrik Lehland of Polewall for sharing his insights into this technology.

Polewall are sponsors of ThinkSmallCell

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Comments   

#1 C.Joy Hepzibah said: 
In fso ,how can i transmit high data rate with low power?
+1 Quote 2014-02-25 05:39
 
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