Choosing Free Space Optics or Radio Frequency Wireless
Speed of fiber — flexibility of wireless
Optical wireless, based on FSO-technology, is an outdoor wireless product category that provides the speed of fiber, with the flexibility of wireless. It enables optical transmission at speeds of up to 1.25 Gbps and, in the future, is capable of speeds of 10 Gbps using WDM. This is not possible with any fixed wireless or RF technology. Optical wireless also eliminates the need to buy expensive spectrum (it requires no FCC or municipal license approvals worldwide), which further distinguishes it from fixed wireless technologies. Moreover, FSO technology’s narrow beam transmission is typically two meters versus 20 meters and more for traditional, even newer radio-based technologies such as millimeter-wave radio. Optical wireless products' similarities with conventional wired optical solutions enable the seamless integration of access networks with optical core networks and helps to realize the vision of an all-optical network.
Challenges with first generation FSO bridges
While fiber-optic cable and FSO technology share many of the same attributes, they face different challenges due to the way they transmit information. While fiber is subject to outside disturbances from wayward construction backhoes, gnawing rodents, and even sharks when deployed under sea, FSO technology is subject to its own potential outside disturbances. Optical wireless networks based on FSO technology must be designed to combat changes in the atmosphere, which can affect FSO system performance capacity. And because FSO is a line-of-sight technology, the interconnecting points must be free from physical obstruction and able to "see" each other.
All potential disturbances can be addressed through thorough and appropriate network design and planning. Among the issues to be considered when deploying FSO-based optical wireless systems:
Heavy Fog: The primary challenge to FSO-based communications is dense fog in some locations of the world, although this problem has been addressed by LightPointe with its HyBridge Series with RF backup. Rain and snow have little effect on FSO technology, but fog is different. Fog is vapor composed of water droplets, which are only a few hundred microns in diameter but can modify light characteristics or completely hinder the passage of light through a combination of absorption, scattering, and reflection. The primary answer to counter fog when deploying FSO-based optical wireless products is through a network design that shortens FSO link distances and adds network redundancies, or through the adition of RF backup as in the LightPointe HyBridge Series. Having said that, FSO installations in extremely foggy cities such as San Francisco have successfully achieved carrier-class reliability.
Absorption: Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in the power density (attenuation) of the FSO beam and directly affects the availability of a system. Absorption occurs more readily at some wavelengths than others. However, the use of appropriate power, based on atmospheric conditions, and use of spatial diversity (multiple beams within an FSO-based unit) helps maintain the required level of network availability.
Scattering: Scattering is caused when the wavelength collides with the scatterer. The physical size of the scatterer determines the type of scattering. When the scatterer is smaller than the wavelength, this is known as Rayleigh scattering. When the scatterer is of comparable size to the wavelength, this is known as Mie scattering. When the scatterer is much larger than the wavelength, this is known as non-selective scattering. In scattering — unlike absorption — there is no loss of energy, only a directional redistribution of energy that may have significant reduction in beam intensity for longer distances.
Physical obstructions: Flying birds or construction cranes can temporarily block a single-beam FSO system, but this tends to cause only short interruptions, and transmissions are easily and automatically resumed. LightPointe's optical wireless products use multi-beam systems (spatial diversity) to address temporary obstructions, as well as other atmospheric conditions, to provide for greater availability.
Building sway/seismic activity: The movement of buildings can upset receiver and transmitter alignment. LightPointe's FSO-based optical wireless offerings use a divergent beam to maintain connectivity. When combined with tracking, multiple beam FSO-based systems provide even greater performance and enhanced installation simplicity.
Scintillation: Heated air rising from the earth or man-made devices such as heating ducts create temperature variations among different air pockets. This can cause fluctuations in signal amplitude which leads to "image dancing" at the FSO-based receiver end. LightPointe's unique multi-beam system is designed to address the effects of this scintillation. Called "Refractive turbulence," this causes two primary effects on optical beams.
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Beam Wander: Beam wander is caused by turbulent eddies that are larger than the beam.
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Beam Spreading: Beam spreading — long-term and short-term — is the spread of an optical beam as it propagates through the atmosphere.
Safety: To those unfamiliar with FSO technology, safety can be a concern because the technology uses lasers for transmission. The proper use and safety of lasers have been discussed since FSO devices first appeared in laboratories more than three decades ago. The two major concerns involve eye exposure to light beams and high voltages within the light systems and their power supplies. Strict international standards have been set for safety and performance, and LightPointe's optical wireless systems comply with these standards.
Free Space Technology in Communication Networks
Free-space optics technology (FSO) has several applications in communications networks, where a connectivity gap exists between two or more points. FSO technology delivers cost-effective optical wireless connectivity and a faster return on investment (ROI) for Enterprises and Mobile Carriers. With the ever-increasing demand for greater bandwidth by Enterprise and Mobile Carrier subscribers comes a critical need for FSO-based products for a balance of throughput, distance and availability. During the last few years, customer deployments of FSO-based products have grown. Here are some of the primary network uses:
Enterprise
Because of the scalability and flexibility of FSO technology, optical wireless products can be deployed in many enterprise applications including building-to-building connectivity, disaster recovery, network redundancy and temporary connectivity for applications such as data, voice and data, video services, medical imaging, CAD and engineering services, and fixed-line carrier bypass.
Mobile Carrier Backhaul
ing services, and fixed-line carrier bypass. Mobile Carrier Backhaul: FSO technology and optical wireless products can be deployed to provide up to 16xE1/T1 backhaul connectivity and Greenfield mobile networks.
Mobile Carrier Base Station “Hoteling”
FSO-based products can be used to expand Mobile Carrier Network footprints through base station “hoteling” in tandem with ADC’s Digivance™ solution.
