Prepared By:Naveen Kumar

OUTLINE What is Free Space Optics (FSO) ?

History of Free Space Optics

Architecture.

Basic link designs.

Advantages and disadvantages of links

Advanced approaches towards link performance improvement

Techniques based on diversity

Adaptive signal processing in a nondirected los link

Performance Limited Effects.

FSO advantages and Disadvantages.

Security Issues.

Conclusion

Free Space Optics communications, also called Free Space Photonics (FSP) or

Optical Wireless, refers to the transmission of visible and infrared (IR) beams

through the atmosphere to obtain optical communications.

Like fiber, Free Space Optics uses lasers and LED to transmit data, but instead

of enclosing the data stream in a glass fiber, it is transmitted through the air

Free space optical communication is an effective means of communication

at high bit rates over short distances.

Wireless optical holds the promise of delivering data rates that can meet the

broadband requirements.

• In the late nineteenth century, Alexander Graham Bell’s "photo-phone”.

• Late 1950‘s~early 1960's, several scientists theorized and developed

laser.

• In the mid-1960's NASA initiated experiments to utilize the laser as a

means of communication between the Goddard Space Flight Center and the

Gemini-7 orbiting space capsule.

• In the early 1980's United States and its military research.

• Germany, France and Japan made significant advancements in free space

optics for satellite communications.

The safety standard recommends that these systems should be located

where the beam cannot be interrupted or viewed inadvertently by a

person or receiver.

ELEMENTS OF THE OPTICAL WIRELESS LINK.

EXPLANATION

The electrical signal is converted to optical power and

transmitted through the air.

After undergoing the influences of the time-dispersive channel

and ambient light, the optical signal is directly translated into a

photocurrent at the detector.

The electrical SNR in optical links depends on the square of

the optical power, which has a deep impact on both design and

performance of OW systems.

KEY COMPONENTS

AT TRANSMITTER:

One or more laser diodes (LD) or light emitting diodes (LED) are used. The

choice between LED and LD is determined by standard factors that influence price

and performance as known from traditional optical communications

AT RECIEVER:

An optical concentrator ( collect and concentrate incoming radiation) and an optical

filter (to reject ambient light), a photo detector (PD, to convert radiation/ optical

power into a photocurrent), and an electrical front-end (performing amplification,

filtering etc.)

PROPERTIES OF DETECTORSLarge area, low-capacitance Si Photo Diodes(PD) with peak

responsivity in the near IR window are available at low-cost.

PIN Photo Diodes are cheaper but limit the link range due to

their poor sensitivity.

APDs have a better sensitivity than PINs and offer link

performance when the ambient light is weak.

BASIC LINK DESIGNS

There are two basic link designs:

Directed line-of-sight (LOS) :

The beam of a directed link travels from a narrow-beam Tx to a

narrow field-of-view (FOV) receiver(Rx) via the LOS.

Diffuse link:

Communication between a wide-beam Tx and a wide FOV Rx relies

on numerous signal reflections off the surfaces in the room, instead

of a LOS.

SCHEME OF LOS AND DIFFUSED LINK

ADVANTAGES AND DISADVANTAGES OF TWO LINKS

Directed LOS experiences minimal effects of multipath

dispersion, noise and path loss but its disadvantage is

shadowing.

Diffused link is simpler, more robust to shadowing and offers

better mobility than the directed one (no tracking and pointing

is needed) but its speed is less due to multiple reflections.

ADVANCED APPROACHES TOWARDS LINK PERFORMANCE IMPROVEMENT

It is often desired to combine the mobility of diffuse and the

high speed capability of LOS systems. For this techniques are:

Techniques based on diversity.

Adaptive signal processing in a non-directed LOS link.

TECHNIQUES BASED ON DIVERSITY Techniques based on diversity is called multi-spot

diffusing approach. Tx Sends multiple moderate-width or collimated beams to

ceiling, where they are diffusely reflected to an angle diversity receiver.

A way to secure the directed link against connection failures.

Here multiple LOS occur between the diffusing spots and the receiver.

SIGNAL PROCESSING IN A NON DIRECTED LOS LINK

• Here LOS and diffuse signal components are simultaneously

present at the receiver.

• These systems exploit the LOS for transmission speed and

reflections for good coverage.

• Here it is difficult to sufficiently maintain the SNR for high bit

rates. So it makes sense to have rate-adaptive links.

• Free space loss due to unguided media

• Clear air absorption

• Scattering

• Inter-Symbol-Interference (ISI)

FREE SPACE LOSSFree space loss defines the proportion of optical power

arriving at the receiver that is usually captured within the

receiver’s aperture. A typical value for a point-to-point system

that operates with a slight divergent beam would b20 dB.

• It is equivalent of absorption loss in optical fibers.

• It is a wavelength sensitive process that gives rise to low-

loss transmission windows centered on 850nm, 1310nm, and

550nm, essentially the same as that in optical fiber.

SCATTERING

This is due to rain, fog, mist, and snow. This attenuation is

continually in the state of flux.

Atmospheric scattering can be Rayleigh due to molecular sized

particles. Mie scattering is due to larger sized particles such as

smoke and fog.

Rayleigh scattering is much less than Mie scattering.

ADVANTAGES1. Unregulated Spectrum Leads to virtually unlimited use of

spectrum by individual networks.

2. Huge Bandwidth Great Support for high-speed applications.

3. No Strict Laws License-free operation.

4. Optoelectronic Leads to manufacturing

Technology inexpensive components and little power consumption

5. Less Interference Facilitates system design and results in a significant cost savings.

6. Fading Immunity Results in less power loss to attenuation.

7. Reusability Enables use of same communication equipments and

wavelengths by nearby systems.

8. Low Power Consumption Leads to less energy requirements

DISADVANTAGE High Launch Power represents eye hazard.

Light Interference negatively affects system performance.

Blockage Leads to design challenges.

Low Power Source requires high sensitive receivers.

Signal Scattering results in multipath impairment.

Alignment Leads to more operation constraints.

FSO laser beams cannot be detected with spectrum analyzers

or RF meters.

FSO laser transmissions are optical and travel along a line of

sight path that cannot be intercepted easily.

The laser beams generated by FSO systems are narrow and

invisible, making them harder to find and even harder to intercept

and crack.

Data can be transmitted over an encrypted connection adding

to the degree of security available in FSO network transmissions.

CONCLUSIONSFor future short-range applications, optical wireless

communications present a viable and promising supplemental

technology to radio wireless systems.

Recent advances in the performance of optical links (in terms

of both transmission speed and spatial coverage) are based

above all on efficient digital signal processing techniques.

REFERENCES[1] Transparent optical networks, 2007. Icton '07, 9th international conference

[2] Infrared Data Association: http://www.irda.org.

[3] O. Bouchet et al.: Free-space optics: Propagation and communication, London: ISTE

Ltd., 2006.

[4] S.T. Jivkova, M. Kavehrad: Multispot diffusing configuration for wireless infrared

access, IEEE Trans. On Communications.

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