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BYTAMAL GHOSH ;Electronics & Communication ;AOT
IntroductionIntroduction
Why Free Space Optics?Why Free Space Optics?
How FSO works?How FSO works?
ChallengesChallenges
Transceiver DesignTransceiver Design
SafetySafety
Applications & Network IntegrationApplications & Network Integration
The Future of FSOThe Future of FSO
What is Free-Space Optics (FSO)?
• FSO is a wireless technology that transmits data via laser beams.
FSO uses light to transmit data between buildings that have clear a line of sight (LOS).
1. Originally developed by the
military and NASA.
2. The invention of lasers in the 1960s
revolutionized free space optics.
3. In 1880 Alexander Graham Bell and
his assistant Charles Sumner Tainter created the photophone .
FSO can send large amounts of data (around 2.5 Gbps of data).
No need to get a license, the spectrum used is an unlicensed worldwide.
Can transmit at distances around 4 km (almost 2 and one half miles).
The cost is often less than that of using fiber optic cables.
FSO systems can be installed quickly (in days).
because it uses light there is no RF interference.
Only about 5% of commercial buildings are lit with fiber
Wide Area Networks between major cities are extremely fast
• Fiber based• >2.5 Gbps
Local Area Networks in buildings are also fast
• >100Mbps
The connections in between are typically a lot slower
• 0.3-1.5 Mbps
Light Source
Glass Fiber Strands
Detector
NetworkDevice
• Pulses of light communicate the data
• “ON” = 1
• “OFF = 0
• Capable of more than 40 Gbps
• >7 CDs a second
Light Source
Detector
NetworkDevice
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 Reverse direction data transported the same way.
• Full duplex
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
Anything that can be done in fiber can be done with FSO
FSO systems use optical wireless link heads each having:a transceiver with a laser or LED transmittera lens or telescope (can have more that one)
shaping overcomes building movementa receiver usually a semiconductor May also employ servo motors, voice coils,
mirrors, CCD arrays, and even liquid crystals and micro-electromechanical systems (MEMS) for tracking and acquisition.
FSO operates in the infrared (IR) range around 850 and 1550 nm (frequencies around 200 THz).
FSO can use Power Over Ethernet (PoE).
Beams only a few meters in diameter at a kilometer
Allows VERY close spacing of links without interference
Highly secureEfficient use of energyRanges of 20m to more than 8km possible
Rapid installations without trenching and permitting
Direct connection to the end user
Bypasses the building ownerNo roof rightsNo riser rights
No interference
Unlicensed
Easy to install
Through the window (or from the rooftop)
No trenching, no permits
Fiber-like data rates
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)
1° ≈ 17 mrad → 1 mrad ≈ 0.0573°
A logarithmic ratio between
two values
In the optical world of Power in mW,
dB=10*Log(power2/power1)
Gain/Loss Multiplier
+30 db
+20 db
+10 db
0 db
-10 db
-20 db
-30 db
1000
100
10
1
.1
.01
.001
Sunlight
Building Motion
Alignment
WindowAttenuation
Fog
Each of these factors can “attenuate” (reduce) the signal. However, there are ways to mitigate each environmental factor.
Scintillation
RangeObstructions
Low Clouds
Absorption or scattering of optical signals due to airborne particles
FSO wavelengths and fog droplets are close to equal in size
Typical FSO systems work 2-3X further than the human eye can see
High availability deployments require short links that can operate in the fog.
Low Clouds Very similar to fog May accompany rain and snow
Rain Drop sizes larger than fog and
wavelength of light Extremely heavy rain (can’t see
through it) can take a link down
Heavy Snow May cause ice build-up on
windows Whiteout conditions
Sand Storms Likely only in desert areas; rare in
the urban core
• Beam spreading and wandering due to propagation through air pockets of varying temperature, density, and index of refraction.
• Almost mutually exclusive with fog attenuation.
• Results in increased error rate but not complete outage.
Challenges:
Scintillation >>Challenges:
Scintillation >>
• Uncoated glass attenuates 4% per surface due to reflection
• Tinted or insulated windows can have much greater attenuation
• Possible to trade high altitude rooftop weather losses vs. window attenuation
ChallengesWindow AttenuationChallengesWindow Attenuation
WAM
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
Results from Seattle Deployment:
• 15% of buildings move more than 4 mrad
• 5% of buildings move more than 6 mrad
• 1% of buildings move more than 10 mrad
License-free operation
High bit rates
Low bit error rates
Immunity to electromagnetic interference
Full duplex operation
Very secure due to the high directionality and narrowness of the beams
No Fresnel zone necessary
RONJA , a free implemantation of FSO utilizingHigh intensity LEDs
Beam dispersionAtmospheric absorptionRainFog (10..~100 dB/km attenuation)SnowScintillationShadowingPollution / smogIf the sun goes exactly behind the transmitter, it
can swamp the signal.
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.
Typically scenarios for use are:
• LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds.
• To cross a public road or other barriers which the sender and receiver do not own.
• Speedy service delivery of high-bandwidth access to optical fiber networks.
• Converged Voice-Data-Connection.
• Temporary network installation (for events or other purposes).
• Reestablish high-speed connection quickly (disaster recovery).
• For communications between spacecraft, including elements of a satellite constellation.
• For inter- and intra-chip communication.
Two solar-powered satellites communicating optically in space
via lasers.
LEDs and Fresnel type lenses help reduce power requirements
Wide-beam technology reduces effects of:Building movementscintillation, and shimmer
FSO and microwave hybrid systems to overcome distance, fog, and dust.
Parallel lasers help integrity and increase the amount of data that can be transmitted.
The FSO industry shows some strength, and the FSO market is growing, though with much less speed.
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.
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.
Free-Space Optics: Enabling Optical Connectivity in Today's NetworksBy Heinz Willebrand, Ph.D.,, Baksheesh S. Ghuman Sams Publishing 2001/12/21
“Free Space Optics (FSO), Optical Wireless, Infrared Fixed Wireless Access, Wireless Broadband, Laser”. Copyright 2000 CableFree Solutions Limited. Retrieved from http://www.cablefreesolutions.com/
Isaac I. Kim and Eric Korevaar, “Availability of Free Space Optics (FSO)and hybrid FSO/RF systems”
Rowe, Schuf. Computer Networking. (2005). Pearson Education, Inc.