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Carrier-grade optical wireless networks: an overview

By Sceptre Communications

In an ideal world, every home and business would be connected to a high capacity telecomms infrastructure using optical fibre. In practise, deploying fibre in the ‘last mile’ is extraordinarily expensive, and extremely time consuming. Even where fibre is installed, the cost of running a second fibre via a diverse route to provide an alternative connection on failure is a cost that no one is willing to bear.

As a result, telecomms service providers large and small have sought alternative, cheaper and more flexible ways to provide broadband services over that last mile. In the consumer & small business arena ADSL and related technologies offer a great deal of hope. However in the medium and large business sectors, where bandwidth demand is orders of magnitude larger than the typical household, this is not a viable solution.

The tried and tested wireless solution in the last mile is line-of-site radio at microwave frequencies. This also has drawbacks: Microwave solutions are still relatively expensive to purchase and running costs can be high. They also require a licence to operate, which complicates fast deployment. Finally, the public perception is that radio antennae on the sides of buildings are a health risk, which further complicates the issue of planning consent and consequently rollout.

The Optical Wireless Networking market (or sometimes Free Space Optics market) grew in response to these problems. The concept is that rather than sending light through an optical fibre, that signal can be sent a short distance directly through the atmosphere. In this way, so-called ‘last mile’ links at typical speeds of 2 to 10meg & up to 155 Mbit/s can be provisioned at a tiny percentage of the cost of an optical fibre link, and in a fraction of the time.

However the technology, which to date has used lasers in the transmitting unit, has historically had one significant drawback: It is unreliable in adverse weather conditions, especially fog. Service providers to large business have contracts based on service level agreements. Part of the SLA will define the worst-case long-term bit error rate on the link, as well as its availability or ‘up-time’. Normally the bit error rate is defined as one data bit lost in every 1 million transmitted (BER = 1 x 10-6), and the link availability as 99.99% (or better!). For much of its history, laser-based optical wireless technology has been able to offer no such guarantees, and hence has been written off by the majority of telecomms service providers.

Carrier-Grade Optical Wireless Networks
Through unique and patented technology, Sceptre Communications is able to solve this problem, delivering 99.99% reliability, and 1 10-9 BER, at bandwidths of up to 155Mbit/s, whatever the weather. As importantly, Sceptre technology does this at an exceptional price point.

Independent market analysts Gartner Group estimate that by 2005 the total optical wireless marketplace will be worth $3.8 billion & & Pioneer Consulting estimating that by 2005 the middle mile wireless backhaul will be worth $3.9 billion, driven in large part by the availability of reliable technologies, mobile communications operators, alternative fixed-line service providers and traditional telephone companies will all find applications for the technology.

It also means a potentially more competitive marketplace. It is mainly the incumbent telcos that have the fibre infrastructure in the last mile. A new, reliable and affordable technology allows competitors to bypass the incumbent’s local infrastructure. This should mean more, better and cheaper services for business

Science and Technology
Existing laser-based optical wireless systems operate on a single wavelength; typically 850 Nm, 950 Nm or 1,550 Nm. Laser-light between 600-1600 Nm suffers severe attenuation in fog.

Sceptre has developed an alternative optical wireless technology, based on light-emitting diodes operating in the infrared spectrum. A combination of some original materials science, unique technology and excellent systems design has allowed Sceptre to build optical wireless systems that can truly deliver carried-grade availability.

Unique LED Technology
Using Sceptre’s technology, a single LED simultaneously transmits across a broad wavelength of 820-890 Nm. This is the first unique proposition. The second is that the transmitted signal is continuous across the entire waveband (rather than using several fixed, discreet and narrower bands). There are transmission windows in even the most adverse atmospheric conditions, including fog. However the wavelengths at which the windows occur vary greatly depending on density, droplet size and so on. The Sceptre approach is simply much more likely to find these windows.

Attenuation (dB/km)
  Rain Snow Heavy Fog
Sceptre LED 15 15 20
Competitive Laser Product 20-50 50-150 30-300

Table 1: Typical Attenuation Compared with laser company

A third unique is derived from an LED design that allows the combination of high frequency with (comparatively) high power output. The highest frequency generated determines the eventual data rate supported, whereas higher power translates as greater availability over any given distance.

In summary, by using a broad wavelength and higher transmitted energy, Sceptre systems maximise the reliability of the link by exploiting the existence of transmission windows that exist in even the most extreme atmospheric conditions.

Sceptre LED Data rates Max Distance Availability Bit Error Rates
Generation 1 2 - 34 Mbit/s 1200 - 1600m 99.99% 1 x 10-9
Generation 2¹ 2 - 155 Mbit/s 2000 - 2500m 99.99% 1 x 10-9

Table 2: LED Performance¹

This is not the end of the technology story, since the LED is just one component of a system. The system includes highly sensitive receivers and amplifiers with very low signal-to-noise ratios, and some very sophisticated signal processing technology. This enables Sceptre receivers to extract the transmitted data in a highly reliable fashion from very low signal levels, which obviously enhances the performance of the system during adverse weather.

Building Sway
There are other interesting advantages derived from the LED approach. A laser source is coherent, spreading very little before hitting its target. This helps to maximise signal strength at the receiver, so long as it is not scattered first. However the LED beam is conical, much more like a torch beam, spreading about 1 meter every 100 meters travelled. When combined with highly sensitive receivers and sophisticated signal processing, this characteristic means that alignment is much less critical (2-3 minutes of arc), and unlike lasers the systems are therefore tolerant of wind, mechanical vibration and ‘building sway’.

Sceptres LEDs are defined as Class 1 eye safe, which is the safest rating that any optical device can be given. In fact the energies used are very similar to those use in infrared TV remote controls in millions of homes worldwide. There is therefore no actual or perceived health risk from the technology.

As a consequence, Sceptre delivers a safe and highly affordable last mile broadband solution that delivers ‘telco grade’ availability whatever the weather.

Sceptre had patents covering:

  • The use of broad wavelength (820 – 890nm), high-power (100 – 350mW) LEDs for full duplex optical wireless applications.
  • The design for broad wavelength (720-900nm) LEDs
  • High Power filters

With the ability to guarantee availability and performance over set distances, carrier-grade optical wireless technology is broadly applicable within telco, new entrant, mobile and enterprise-type networks. The table below outlines some of the application areas where the technology has, or shortly will prove itself.

  Telco New Entrant Mobile Operator Local Authority Large Enterprise
First/Last Mile Broadband Voice and Data Yes Yes   Yes  
Back-up Connections Yes Yes   Yes  
Middle Mile DSL Backhaul Yes Yes      
Other Data Backhaul Yes Yes Yes Yes  
Base Station Aggregation     Yes    
Core Backbone Connections (Metro)   Yes   Yes Yes
Alternative Routes   Yes   Yes Yes

Table 3: Sceptre OWN Applications

OKB MEI, the Communications R&D arm of the Russian Space Agency, originally developed the predecessor of the technology in the 1980’s. The original use was to allow on-board computers in spacecraft to communicate with each other for the purposes of docking etc. In space, the technology regularly works over a distance of 10,000km.

Sceptre and OKB MEI have been working together since 1998 to develop the technology for commercial applications. To that end Sceptre has created a joint venture R&D company, OKB MEI Telekom.

For more information, please contact:
Romane Smith: rsmith@sceptre-communications
Steve Jones: sjones@sceptre-communications.com

¹ Available end 2003

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