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BY TAMAL GHOSH ;Electronics & Communication ;AOT

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<ul><li>Introduction </li></ul><ul><li>Why Free Space Optics? </li></ul><ul><li>How FSO works? </li></ul><ul><li>Challenges </li></ul><ul><li>Transceiver Design </li></ul><ul><li>Safety </li></ul><ul><li>Applications & Network Integration </li></ul><ul><li>The Future of FSO </li></ul>

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<ul><li>What is Free-Space Optics (FSO)? </li></ul><ul><li>FSO is a wireless technology that transmits data via laser beams. </li></ul><ul><li>FSO uses light to transmit data between buildings that have clear a line of sight (LOS). </li></ul>

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<ul><li> 1. Originally developed by the </li></ul><ul><li>military and NASA. </li></ul><ul><li>2. The invention of lasers in the 1960s </li></ul><ul><li>revolutionized free space optics. </li></ul><ul><li>3. In 1880 Alexander Graham Bell and </li></ul><ul><li>his assistant Charles Sumner Tainter </li></ul><ul><li>created the photophone . </li></ul>

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<ul><li>FSO can send large amounts of data (around 2.5 Gbps of data). </li></ul><ul><li>No need to get a license, the spectrum used is an unlicensed worldwide. </li></ul><ul><li>Can transmit at distances around 4 km (almost 2 and one half miles). </li></ul><ul><li>The cost is often less than that of using fiber optic cables. </li></ul><ul><li>FSO systems can be installed quickly (in days). </li></ul><ul><li>because it uses light there is no RF interference. </li></ul>

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Only about 5% of commercial buildings are lit with fiber <ul><li>Wide Area Networks between major cities are extremely fast </li></ul><ul><ul><li>Fiber based </li></ul></ul><ul><ul><li>>2.5 Gbps </li></ul></ul><ul><li>Local Area Networks in buildings are also fast </li></ul><ul><ul><li>>100Mbps </li></ul></ul><ul><li>The connections in between are typically a lot slower </li></ul><ul><ul><li>0.3-1.5 Mbps </li></ul></ul>

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Light Source Glass Fiber Strands Detector Network Device <ul><li>Pulses of light communicate the data </li></ul><ul><ul><li>“ ON” = 1 </li></ul></ul><ul><ul><li>“ OFF = 0 </li></ul></ul><ul><li>Capable of more than 40 Gbps </li></ul><ul><ul><li>>7 CDs a second </li></ul></ul>Light Source Detector Network Device

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Anything that can be done in fiber can be done with FSO 1 Network traffic converted into pulses of invisible light representing 1’s and 0’s 2 Transmitter projects the carefully aimed light pulses into the air 5 <ul><li>Reverse direction data transported the same way. </li></ul><ul><ul><li>Full duplex </li></ul></ul>3 A receiver at the other end of the link collects the light using lenses and/or mirrors 4 Received signal converted back into fiber or copper and connected to the network

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<ul><li>FSO systems use optical wireless link heads each having: </li></ul><ul><ul><li>a transceiver with a laser or LED transmitter </li></ul></ul><ul><ul><li>a lens or telescope (can have more that one) </li></ul></ul><ul><ul><ul><li>shaping overcomes building movement </li></ul></ul></ul><ul><ul><li>a receiver usually a semiconductor </li></ul></ul><ul><ul><li>May also employ servo motors, voice coils, mirrors, CCD arrays, and even liquid crystals and micro-electromechanical systems (MEMS) for tracking and acquisition. </li></ul></ul><ul><li>FSO operates in the infrared (IR) range around 850 and 1550 nm (frequencies around 200 THz). </li></ul><ul><li>FSO can use Power Over Ethernet (PoE). </li></ul>

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<ul><li>Beams only a few meters in diameter at a kilometer </li></ul><ul><li>Allows VERY close spacing of links without interference </li></ul><ul><li>Highly secure </li></ul><ul><li>Efficient use of energy </li></ul><ul><li>Ranges of 20m to more than 8km possible </li></ul>

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<ul><li>Rapid installations without trenching and permitting </li></ul><ul><li>Direct connection to the end user </li></ul><ul><li>Bypasses the building owner </li></ul><ul><ul><li>No roof rights </li></ul></ul><ul><ul><li>No riser rights </li></ul></ul>

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<ul><li>No interference </li></ul><ul><li>Unlicensed </li></ul><ul><li>Easy to install </li></ul><ul><li>Through the window (or from the rooftop) </li></ul><ul><li>No trenching, no permits </li></ul><ul><li>Fiber-like data rates </li></ul>

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1 ° ≈ 17 mrad -> 1 mrad ≈ 0.0573° 1 mrad 1 km 1 m Small angle approximation: Angle (in milliradians) * Range (km)= Spot Size (m) Divergence Range Spot Diameter 0.5 mrad 1.0 km ~0.5 m (~20 in) 2.0 mrad 1.0 km ~2.0 m (~6.5 ft) 4.0 mrad (~ ¼ deg) 1.0 km ~4.0 m (~13.0 ft)

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<ul><li>A logarithmic ratio between two values </li></ul><ul><li>In the optical world of Power in mW, </li></ul><ul><li>dB=10*Log(power2/power1) </li></ul>Gain/Loss Multiplier +30 db +20 db +10 db 0 db -10 db -20 db -30 db 1000 100 10 1 .1 .01 .001

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Sunlight Building Motion Alignment Window Attenuation Fog Each of these factors can “attenuate” (reduce) the signal. However, there are ways to mitigate each environmental factor. Scintillation Range Obstructions Low Clouds

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<ul><li>Absorption or scattering of optical signals due to airborne particles </li></ul><ul><li>FSO wavelengths and fog droplets are close to equal in size </li></ul><ul><li>Typical FSO systems work 2-3X further than the human eye can see </li></ul><ul><li>High availability deployments require short links that can operate in the fog. </li></ul>

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<ul><li>Low Clouds </li></ul><ul><ul><li>Very similar to fog </li></ul></ul><ul><ul><li>May accompany rain and snow </li></ul></ul><ul><li>Rain </li></ul><ul><ul><li>Drop sizes larger than fog and wavelength of light </li></ul></ul><ul><ul><li>Extremely heavy rain (can’t see through it) can take a link down </li></ul></ul><ul><li>Heavy Snow </li></ul><ul><ul><li>May cause ice build-up on windows </li></ul></ul><ul><ul><li>Whiteout conditions </li></ul></ul><ul><li>Sand Storms </li></ul><ul><ul><li>Likely only in desert areas; rare in the urban core </li></ul></ul>

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<ul><li>Beam spreading and wandering due to propagation through air pockets of varying temperature, density, and index of refraction. </li></ul><ul><li>Almost mutually exclusive with fog attenuation. </li></ul><ul><li>Results in increased error rate but not complete outage. </li></ul>Challenges: Scintillation >>

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<ul><li>Uncoated glass attenuates 4% per surface due to reflection </li></ul><ul><li>Tinted or insulated windows can have much greater attenuation </li></ul><ul><li>Possible to trade high altitude rooftop weather losses vs. window attenuation </li></ul>Challenges Window Attenuation WAM

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Type Cause(s) Magnitude Frequency Tip/tilt Thermal expansion High Once per day Sway Wind Medium Once every several seconds Vibration Equipment (e.g., HVAC), door slamming, etc. Low Many times per second

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<ul><li>Results from Seattle Deployment: </li></ul><ul><ul><li>15% of buildings move more than 4 mrad </li></ul></ul><ul><ul><li>5% of buildings move more than 6 mrad </li></ul></ul><ul><ul><li>1% of buildings move more than 10 mrad </li></ul></ul>

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<ul><li>License-free operation </li></ul><ul><li>High bit rates </li></ul><ul><li>Low bit error rates </li></ul><ul><li>Immunity to electromagnetic interference </li></ul><ul><li>Full duplex operation </li></ul><ul><li>Very secure due to the high directionality and narrowness of the beams </li></ul><ul><li>No Fresnel zone necessary </li></ul>RONJA , a free implemantation of FSO utilizing High intensity LEDs

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<ul><li>Beam dispersion </li></ul><ul><li>Atmospheric absorption </li></ul><ul><li>Rain </li></ul><ul><li>Fog (10..~100 dB/km attenuation) </li></ul><ul><li>Snow </li></ul><ul><li>Scintillation </li></ul><ul><li>Shadowing </li></ul><ul><li>Pollution / smog </li></ul><ul><li>If the sun goes exactly behind the transmitter, it can swamp the signal. </li></ul>

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<ul><li>To those unfamiliar with FSO technology, safety can be a concern because the technology uses lasers for transmission. 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. </li></ul>

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<ul><li>Typically scenarios for use are: </li></ul><ul><li>LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds. </li></ul><ul><li>To cross a public road or other barriers which the sender and </li></ul><ul><li>receiver do not own. </li></ul><ul><li>Speedy service delivery of high-bandwidth access to </li></ul><ul><li>optical fiber networks. </li></ul><ul><li>Converged Voice-Data-Connection. </li></ul><ul><li>Temporary network installation (for events or other purposes). </li></ul><ul><li>Reestablish high-speed connection quickly (disaster recovery). </li></ul><ul><li>For communications between spacecraft, including elements of a satellite constellation. </li></ul><ul><li>For inter- and intra-chip communication. </li></ul>Two solar-powered satellites communicating optically in space via lasers.

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<ul><li>LEDs and Fresnel type lenses help reduce power requirements </li></ul><ul><li>Wide-beam technology reduces effects of: </li></ul><ul><ul><li>Building movement </li></ul></ul><ul><ul><li>scintillation, and shimmer </li></ul></ul><ul><li>FSO and microwave hybrid systems to overcome distance, fog, and dust. </li></ul><ul><li>Parallel lasers help integrity and increase the amount of data that can be transmitted. </li></ul>

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<ul><li>The FSO industry shows some strength, and the FSO market is growing, though with much less speed. </li></ul><ul><li>In spite of this, the commercial future of free-space optical communications remains uncertain.Perhaps the best overall prospects are in space, where progress is being made in improving acquisition and tracking. Once these are perfected, the bandwidth advantages of optical free-space communications should open up a substantial market. </li></ul><ul><li> The FSO industry consists of mostly established vendors that manufacture equipment for various distances and speeds of transmission. The highest speed of 2.5 Gb/s promises to be increased to 10 Gb/s in future. </li></ul>

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<ul><li>Free-Space Optics: Enabling Optical Connectivity in Today's Networks By Heinz Willebrand, Ph.D.,, Baksheesh S. Ghuman Sams Publishing 2001/12/21 </li></ul><ul><li>“ Free Space Optics (FSO), Optical Wireless, Infrared Fixed Wireless Access, Wireless Broadband, Laser”. Copyright 2000 CableFree Solutions Limited. Retrieved from http://www.cablefreesolutions.com/ </li></ul><ul><li>Isaac I. Kim and Eric Korevaar, “Availability of Free Space Optics (FSO)and hybrid FSO/RF systems” </li></ul><ul><li>Rowe, Schuf. Computer Networking. (2005). Pearson Education, Inc. </li></ul>