10 Feb 2011

Optical Wireless Communications: An Overview

Andrew Grieve, CEO fSONA describes how optical wire-less communications can be used to provide cost effective reliable communications

The global telecommunications network has seen massive expansion over the last few years, catalyzed by the telecommunications deregulation of 1996. First came the tremendous growth of the long-haul, wide-area network (WAN), followed by a more recent emphasis on metropolitan area networks (MANs).

Meanwhile, local area networks (LANs) and gigabit Ethernet ports are being deployed with a comparable growth rate. In order for this tremendous capacity to be exploited, and for the users to be able to utilize the broad array of new services becoming available, network designers must provide a flexible and cost-effective means for the users to access the telecommunications network.

Presently, however, most local loop connections are limited to 1.5 Mbps (a T1 line). As a consequence, there is a strong need for a high-bandwidth bridge (the "last mile" or "first mile") between the LANs and the MANs or WANs.

Optical wireless - a promising approach

Optical wireless systems represent one of the most promising approaches for addressing the emerging broadband access market and its "last mile" bottleneck. These robust systems, which establish communication links by transmitting laser beams directly through the atmosphere, have matured to the point that mass-produced models are now available.

Optical wireless systems offer many features, principal among them being low start-up and operational costs, rapid deployment, and high fibre-like bandwidths. Available systems offer capacities in the range of 100 Mbps to 2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps.

The ideal broadband access approach should include low installation cost as well as cost per bit/second associated with each subscriber and low first-in cost (i.e. the cost of launching access service for the first few subscribers). It should also offer rapid deployment, so carriers can begin generating revenue as quickly as possible. Another important attribute is the capability to provide a high capacity to each subscriber, thereby enabling multiple services to be utilized. Optical wireless complements both RF and wireline networks, providing fibre-like capacity at a low cost. Given that no spectrum license is required, the start-up costs are significantly lower than for RF wireless. Optical wireless systems can be rapidly deployed; once a suitable line of sight is identified, a point-to-point link can typically be installed and brought to operational status in approximately one hour or less. Well engineered optical wireless links, which properly account for the statistical occurrence of fog, can achieve an availability of 99.9 per cent, or even full carrier-class availability of 99.999 per cent if one installs a (lower capacity) RF link or DSL back-up. Finally, optical wireless systems are "network-friendly" in that they can be:

  • engineered to be protocol-independent

  • implemented in cellular or mesh architectures as well as point-to-point linkssent from roof-top to roof-top or through office windows

  • designed to be compatible with common monitoring protocols to ensure the highest level of successful implementation

  • redeployed to a different subscriber location if desired, for example, if an existing subscriber no longer requires an optical wireless connection due to receiving a direct fibre-optic connection

Optical wireless challenges

Optical wireless systems are not without challenges, however. First, we believe that such systems must be eye-safe, which means that they must pose no danger to people who might happen to encounter the communications beam. This requirement manifests itself in the form of legally mandated upper limits to the intensity of the transmitted laser beam. Second, it is well known from common experience that fog substantially attenuates visible radiation, and it has a similar effect on the near-infrared wavelengths that are employed in optical wireless systems. (Note that the effect of fog on optical wireless radiation is entirely analogous to the attenuation - and fades - suffered by RF wireless systems due to rainfall.)

These two challenges highlight one of the critical design trades that must be resolved to field a successful optical wireless system: the system must transmit sufficient power that the free-space communication link will be available a high percentage of the time, even with some degree of fog attenuation, but the power must not be so high as to exceed eye-safe limits.

The optical wireless hardware currently on the market can be classified into two broad categories - systems that operate near 800 nm wavelength and those that operate near 1550 nm. Laser beams at 800 nm wavelength are near-infrared and therefore invisible, yet like visible wavelengths the light passes through the cornea and lens and is focused onto a tiny spot on the retina. The collimated light beam entering the eye in this retinal-hazard wavelength region is concentrated by a factor of 100,000 times when it strikes the retina. Because the retina has no pain sensors, and the invisible light does not induce a blink reflex, at 800 nm the retina could be permanently damaged by some commercially available optical wireless products before the victim is aware that hazardous illumination has occurred. In contrast, laser beams at 1550 nm wavelength are absorbed by the cornea and lens, and do not focus onto the retina. It is possible to design eye-safe laser transmitters at both the 800 nm and 1550 nm wavelengths, but due to the aforementioned biophysics the allowable safe laser power is about fifty times higher at 1550 nm. This factor of fifty is important to the communication system designer, because the additional laser power allows the system to propagate over longer distances and/or through heavier attenuation, and to support higher data rates.

BER for optical communications

The ultimate measure of the value of any communications system of course is whether it can cost-effectively transmit broadband data across a link with an acceptable bit error rate (BER), typically taken as 10-9 or better. We have seen that the biophysics of the eye leads to an allowable power level at 1550 nm that is approximately 50 times greater than at 800 nm. This higher allowable power is a significant advantage of the 1550 nm wavelength, but a number of other performance-related factors should also be considered in this wavelength trade.

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About the author

Andrew Grieve is CEO of fSONA. He brings over 15 years management experience to fSONA gained in progressively senior roles in accounting and manufacturing management. As Production Manager for a high-tech manufacturing department he successfully developed the program from a start-up phase to full production and was subsequently promoted to Director of Operations where he had complete responsibility for both the Manufacturing and Customer Service departments. Andrew Grieve is a Certified Management Accountant and holds a Diploma in Operations Management from BCIT.

fSONA Networks is an innovative provider of next generation Free Space Optics (FSO) solutions that utilize a flexible point-to-point architecture and protocol transparent design. Founded in 1997 with a goal to develop premier, cost effective, eye-safe, optical transmission products for the broadband access market, fSONA has created the most robust and powerful free space optics systems ever brought to market - the SONAbeam family.

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