free space optical communication - casestudy

7/31/2019 Free Space Optical Communication - Casestudy

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FREE SPACE OPTICAL COMMUNICATION

SIES College of Commerce and Economics | MSc-IT Part-1

2012

ABSTRACT

Free Space Optical Communication (FSO) is a line-of-sight technology thatenables optical transmission of data, voice, and video communications through the air. Free

space optical communication systems are different than Fiber optic system. Free space optics use

beams of light, such as laser beam, as optical communication signals, and therefore do not

require cables or fiber connected between transmitter and receivers.

Optical transmissions provide a wider bandwidth than other wireless communicationsmediums, such as RF frequency signals and optical signals can be more focused than RF signals,

and are thus more difficult to intercept and less likely to cause interference with othertransmissions.

Over the last two decades free-space optical communication (FSO) has become more and

more interesting as an adjunct or alternative to radio frequency communication.

I am going to explore this new technology regarding how does it actually work, its

applications , its utilization in the communication network , current drawbacks , compare it with

RF signals and predict whether it can be the answer for high speed communication for the future.


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Introduction

Communication, as it has always been relied and simply depended upon speed. The faster themeans ! the more popular, the more effective the communication is !

Presently in the twenty-first century wireless networking is gaining because of speed and ease of

deployment and relatively high network robustness. Modern era of optical communicationoriginated with the invention of LASER in 1958 and fabrication of optical fiber in 1970. When

we hear of optical communications we all think of optical fibers, what I have for u today is AN

OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS. Free space optics or FSOAlthough it only recently and rather suddenly sprang in to public awareness, free space optics is

not a new idea. It has roots that back over 30 years-to the era before fiber optic cable became thepreferred transport medium for high speed communication. FSO technology has been revived to

offer high band width last mile connectivity for todays converged network requirements.

Free space optical communication or FSO, free space photonics or optical wireless, refers to the

transmission of modulated visible or infrared beams through the atmosphere to obtain opticalcommunication. FSO systems can function over distances of several kilometers. FSO is a line-of-

sight technology, which enables optical transmission up to 2.5 Gbps of data, voice and video

communications, allowing optical connectivity without deploying fiber optic cable or securing

spectrum licenses. Free space optics require light, which can be focused by using either lightemitting diodes (LED) or LASERS(light amplification by stimulated emission of radiation). The

use of lasers is a simple concept similar to optical transmissions using fiber-optic cables, the only

difference being the medium..

As long as there is a clear line of sight between the source and the destination and enough

transmitter power, communication is possible virtually at the speed of light. Because light travels

through air faster than it does through glass, so it is fair to classify FSO as opticalcommunications at the speed of light. FSO works on the same basic principle as infrared

television remote controls, wireless keyboards or wireless palm devices.


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History

It is said that optical communication was first used in the 8th

century by the Greeks. They used fire as

the light source, the atmosphere as the transmission medium and human eye as receiver.

Later Alexander Graham Bell and Charles Sumner Tainter conducted experiments on optical wireless

communication in the late 19th centaury . Bells FSO experiment converted voice sounds to telephone

signals and transmitted them between receivers through free air space along a beam of light for a distance

of some 600 feet, - this was later called PHOTOPHONE. Although Bells photo phone never became a

commercial reality , it demonstrated the basic principle of optical communications. Essentially all of the

engineering of todays FSO or free space optical communication systems was done over the p ast 40 years

or so mostly for defense applications.

The Photophone used crystalline selenium cells at the focal point of its parabolic receiver.This

material's electrical resistance varies inversely with the illumination falling upon it, i.e., its resistance is

higher when it is in the dark, and lower when it is exposed to light. The idea of the Photophone was thusto modulate a light beam: the resulting varying illumination of the receiver would induce a corresponding

varying resistance in the selenium cells, which was then used to regenerate the sounds captured by the

telephone receiver.

The modulation of the transmitted light beam was done by a mirror made to vibrate by a person's voice:

the thin mirror would alternate between concave and convex forms, thus focusing or dispersing the light

from the light source.The Photophone functioned similarly to the telephone, except that the Photophone

used modulated light as a means of transmitting information, while the telephone relied on a modulated

electrical signal carried over a conductive wire circuit.


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How does FSO work ?

The concept behind FSO is simple. FSO uses a directed beam of light radiation between

two end points to transfer information (data, voice or even video). This is similar to OFC (optical

fiber cable) networks, except that light pulses are sent through free air instead of OFC cores. An

FSO unit consists of an optical transceiver with a laser transmitter and a receiver to provide fullduplex (bi-directional) capability. Each FSO unit uses a high power optical source ( laser ) plus a

lens that transmits light through the atmosphere to another lens receiving information. The

receiving lens connects to a high sensitivity receiver via optical fiber. Two FSO units can take

the optical connectivity to a maximum of 4kms.

The concept of transmitting information through the air by means of a modulated light signal is

quite old; and although significant advances have been made over the past 10 years, the concept

remains relatively simple: a narrow beam of light is launched at a transmission station,transmitted through the atmosphere, and subsequently received at the receive station. The

advances, which have led to what we now refer to as free-space optical communications, or FSO,

have come about in response to a need for greater bandwidth and improved communicationssystems. Inasmuch as FSO and fiber-optic transmission systems use similar infrared (IR)

wavelengths of light and have similar transmission bandwidth capabilities, FSO is often referred

to as fiberless optics or optical wireless transmission .

Furthermore, given the fact that the optical spectrum is unlicensed with frequencies of the orderof hundreds of terahertz, most FSO systems use simple ONOFF keying (OOK) as a modulation

format, the same standard modulation technique that is used in digital fiberoptics systems.


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Electromagnetic Spectrum :


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Modulation Scheme :

As we now know that the optical spectrum is unlicensed with frequencies of the order of

hundreds of terahertz, most FSO systems use simple ONOFF keying (OOK) as a modulationformat, the same standard modulation technique that is used in digital fiberoptics systems,

wherein data are typically transmitted in a digital format with light ONrepresenting a 1 and

light OFFrepresenting a 0.This simple modulation scheme allows FSO systems to be desiged as bandwidth and protocol

transparent physical layer connection.

For OOK, the exact wavelength of the carrier and its phase are irrelevant for the demodulation. The

receiver just directly detects the currently incoming power and compare it against a certain level. OOK is

sensitive to amplitude distortion (fading) and propagation through different routes, while the second one

is negligible for clear-sky conditions.

Atmospheric obscuration e.g. in clouds can lead to significant attenuation of the received signal but is less

important for FSO systems operating under clear-sky conditions.

There are three ways to realize the on-off keying modulation, including

(1)NRZ coding

(2)NRZI coding

(3) RZ coding


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TRANSMISSION

Generally, all of todays commercially available FSO systems operate in the near-IR

wavelength range between roughly 750 and 1600 nm, with one or two systems being developed

to operate at the IR wavelength of 10,000 nm. The physics and transmission properties of opticalenergy as it travels through the atmosphere are similar throughout the visible and the near-IRwavelength range.

The modulated light source, which is typically a laser or light-emitting diode (LED), provides the

transmitted opticalsignal and determines all the transmitter capabilities of the system. Only thedetector sensitivity plays an equally important role in total system performance. Fortelecommunication purposes, only lasers that are capable of being modulated at 20 Mbit/s to 2.5

Gbit/s can meet current marketplace demands. In addition, how the device is modulated and how

much modulated power is produced are both important to the selection of a device. Lasers in the780925-nm and 15251580-nm spectral bands meet frequency requirements and are available.

Although other operating wavelengths are used in commercial FSO systems,which lasers that

operate in the 850- and 1550-nm wavelength bands.

FSO systems should have the following characteristics:

Ability to operate at higher power levels (important for longer-distance FSO systems).

High-speed modulation (important for high-speed FSO systems). Low power consumption (important for overall system design and maintenance).

Ability to operate over a wide temperature range without major performance degradation

(important for outdoor systems).

Mean time between failure (MTBF) that exceeds 10 yr.


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APPLICATIONS OF FSO

Optical communication systems are becoming more and more popular as the interest and

requirement in high capacity and long distance space communications grow. FSO overcomes thelast mile access bottleneck by sending high bitrate signals through the air using laser

transmission. Applications of FSO system is many and varied but a few can be listed.

1.Metro Area Network ( MAN ):FSO network can close the gap between the last mile customers, there by providing access to

new customers to high speed MANs resulting to Metro Network extension.

2. Last Mile Access :End users can be connected to high speed links using FSO. It can also be used to bypass local

loop systems to provide business with high speed connections.

3. Enterprise connectivity :

As FSO links can be installed with ease, they provide a natural method of interconnecting LANsegments that are housed in buildings separated by public streets or other right-of-way property.

4. Fiber backup :FSO can also be deployed in redundant links to backup fiber in place of a second fiber link.

5.Backhaul :FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired

into the public switched telephone network.

6. Service acceleration :

instant services to the customers before fiber being layed.

Advantages :1. Straight forward deployment-as it requires no licenses.

2. Rapid time of deployment.

3. Low initial investment.4. Ease of installation even indoors in less than 30 minutes.

5. Security and freedom from irksome regulations like roof top rights and spectral licenses.

6. Re-deployability.

7. High Data rates upto 2.5 Gbps at present and 10 Gbps in the future.


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Free Space Optical Technology in Communication Network

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. With the ever-increasing demand for greater bandwidth

by Enterprise and Mobile Carrier subscribers comes a critical need for FSO-based products for abalance 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, and

temporary connectivity for applications such as data, voice and data, video services, medicalimaging.

Mobile Carrier Base Station HotellingFSO-based products can be used to expand Mobile Carrier Network footprints through base

station hoteling. By deploying the Base Station Hoteling architecture, carriers can centralize base -station equipment and rapidly deploy antennas where coverage and capacity issues exist. Digivance then

transports wideband RF via digital optical signals over fiber or free space optics to economically expand

coverage and capacity. This also solves the difficult challenge of covering entire buildings or campus

facilities. With the installation of this low-cost, high-quality solution, wireless carriers can quickly and

cost-effectively deliver coverage to corporate facilities and other buildings where mobile users demand

service.

This FSO technology approach has a number of advantages:

Requires no RF spectrum licensing. Is easily upgradeable, and its open interfaces support equipment from a variety of

vendors, which helps enterprises and service providers protect their investment in

embedded telecommunications infrastructures.

Requires no security software upgrades. Is immune to radio frequency interference or saturation. Can be deployed behind windows, eliminating the need for costly rooftop rights.


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CURRENT LIMITATIONS OF FSO

FOG AND FSO

Fog substantially attenuates visible radiation, and it has a similar affect on the near-infraredwavelengths that are employed in FSO systems. Rain and snow have little affect on FSO. Fog

being microns in diameter, it hinder the passage of light by absorption, scattering and reflection.

Dealing with fog which is known as Mie scattering, is largely a matter of boosting the

transmitted power. In areas of heavy fogs 1550nm lasers can be of more are. Fog can becountered by a network design with short FSO link distances. FSO installation in foggy cities

like san Francisco have successfully achieved carrier-class reliability.

PHYSICAL OBSTRUCTIONS

Flying birds can temporarily block a single beam, but this tends to cause only short interruptions

and transmissions are easily and automatically re-assumed. Multi-beam systems are used forbetter performance.


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SCINTILLATIONScintillation refers the variations in light intensity caused by atmospheric turbulence. Such

turbulence may be caused by wind and temperature gradients which results in air pockets of

varying diversity act as prisms or lenses with time varying properties. This scintillation affectson FSO can be tackled by multi beam approach exploiting multiple regions of space- this

approach is called spatial diversity.

SOLAR INTERFERENCE

This can be combated in two ways.

The first is a long pass optical filter window used to block all wavelengths below 850nmfrom entering the system.

The second is an optical narrow band filter proceeding the receive detector used to filterall but the wavelength actually used for intersystem communications.

SCATTERING

Scattering is caused when the wavelength collides with the scatterer. The physical size of the

scatterer determines the type ofscattering.

When the scatterer is smaller than the wavelength-Rayleigh scattering. When the scatterer is of comparable size to the wavelength Mie scattering. When the scatterer is much larger than the wavelength -Non-selective scattering

In scattering there is no loss of energy, only a directional redistribution of energy which maycause reduction in beam intensity for longer distance.

This is a free space optics laser linkThe receptor is the large disc in the middle and the transmitter the smaller ones


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Analysis With Radio Frequency :


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Current developments in FSO

Development and vendor selections are underway for an Optical Satellite System (OSS)

from Laser Light Communications (Laser Light) that is anticipated to be deployed in Q1 2017and operational by the middle of 2017. The free-space optical (FSO) communications network--

what is believed to be the world's first commercial satellite communications constellation basedentirely on optical wave technology--is to be comprised of 12 satellites (8 primary and 4 spares)

in medium Earth orbit (10,500 km) with an operating system capacity of 4.8 Tbps, includingsatellite-to-satellite optical crosslinks and satellite-to-ground optical up/down links of 200 Gbps,

without reliance on the radio frequency (RF) spectrum.

Conclusion

Presently we are faced with a burgeoning demand for high bandwidth and differentiated data

services. Network traffic doubles every 9-12 months forcing the bandwidth or data storing

capacity to grow and keep pare with this increase. The right solution for the pressing demand isthe untapped bandwidth potential of optical communications. Optical communications are in the

process of evolving Giga bits/sec to terabits/sec and eventually to pentabits/sec. The explosion of

internet and internet based applications has fuelled the bandwidth requirements.

Business applications have grown out of the physical boundaries of the enterprise and gone wide area

linking remote vendors, suppliers, and customers in a new web of business applications. Hence

companies are looking for high bandwidth last mile options. The high initial cost and vast time required

for installation in case of OFC speaks for a wireless technology for high bandwidth last mile connectivitythere FSO finds its place. Though not very popular in India at the moment, FSO has a tremendous scope

for deployment companies like CISCO, LIGHT POIN few other have made huge investment to promote

this technology in the market. It is only a matter of time before the customers realized, the benefits of

FSO and the technology deployed in large scale.

Free Space Optical Communication has some limitation to be covered before it could be utilized

in a full fledged basis. But with the growing need for speed and connectivity the futre stands with

FSO as the ultimate option available fulfilling these needs.


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Biblography :

Introduction to free space optical communication Hennes Henniger

www.fsona.com

www.freespaceoptics.com

http://www.lightpointe.com/
http://www.fsona.com/http://www.fsona.com/http://www.freespaceoptics.com/http://www.freespaceoptics.com/http://www.lightpointe.com/http://www.lightpointe.com/http://www.lightpointe.com/http://www.freespaceoptics.com/http://www.fsona.com/

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