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The strongest link?

Optical antennas could provide huge bandwidth improvements. By Eric Russell.

A communications system is being developed that could displace Bluetooth. The concept uses omnidirectional infrared (IR) instead of rf for short range communications across an office or other localised site. This requires an optical antenna with all round coverage for both transmitter and receiver.
Designing a very wide angle lens that is efficient through 360o at IR frequencies is apparently not as simple as designing its photographic counterpart, the fish eye lens. But the prize is huge bandwidth. One source says that, whilst radio transmission is limited to less than 250Mbit/s, optical links can offer 10Gbits/s.
Three independent camps are known to be developing systems but technical details, unsurprisingly, are sketchy. What has emerged is that a Fresnel lens with its series of concentric stepped rings is the most likely design. But there is very little modelling software and experience to help create the optimum design.
Optical links are more secure than rf as it is not possible to eavesdrop on the link; there are no health risks; operating licences are not needed; links are not susceptible to interference; and IR light can be polarised to improve channel separation. But the link requires clear line of sight to function, so it would not activate a device in a coat pocket, for example, nor pass through office walls and screens.

Researchers
One researcher is Professor Roger Green of Warwick University. He has developed an optical antenna with a 120o capture angle. The antenna includes filters to eliminate background interference from ambient light such as the sun and room lighting. This improves sensitivity and range of the antenna.
Prof Green has also pioneered a bandwidth enhancement technique. The techniques employed in his optical antenna can be extended to ultrasonic, microwave and terahertz level systems.
He says IR could become an option on mobile ’phones that will choose automatically between RF and IR to link into the strongest signal, especially where radio reception is poor. IR is at a stage to complement Bluetooth and rival it in some circumstances.
Green quotes applications for blanket IR coverage such as hospitals. IR links between patients’ equipment and a central monitoring point would not cause interference with other hospital systems. IR ’phones could also be used on aircraft without jeopardising navigation and control systems.
Professor Alwyn Seeds at University College London is also working on optical antennae while Infranet Technologies is assessing the viability of IR links. Derek Wilson, Infranet’s managing director, says that, for really wide scale adoption, local wireless links need a range of 100m and bandwidth of 1Gbit/s to realistically handle the traffic that will comprise future communications. He finds that Bluetooth does not come near this figure but he can guarantee that Infranet’s products will.
The company designs and manufactures its own point to point communications products and also resells and acts as consultant. Its point to point free space communications equipment uses diffused beam diodes, as used in television remote controllers.
Wilson says light from diffused beam diodes offers higher reliability of links during bad weather. The hard light from lasers is too easily disturbed by atmospheric conditions, it bounces and is refracted.
Infranet is currently working with local authorities to interlink sites with broadband access using rf, microwave and optical links as appropriate. Its latest contract covers 600 sites.
The work on omnidirectional IR communication systems is known to the Infrared Data Association, IrDA, which has been discussing protocols and compatibility with Bluetooth. IrDA is very active in promoting IR and has convened a number of special interest groups to deliver low cost, high speed wireless connectivity for a large number of applications.
Whilst the case for IR is strong, all parties agree it will not replace RF systems completely. It will provide choice in certain situations and the only answer in others but, overall, represents a powerful new contender in the short range communications sector.

Author
Vanessa Knivett

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