The attached narrated power point presentation attempts to explain the block diagram, working principle, different architectures, advantages, disadvantages and applications of free space optical communications apart from the comparison of free space optics with fiber optics and other counterparts such as RF and metallic cables. The material will be extremely useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Free Space Optical
Communications
MEC

2. 2
Contents
• Introduction.
• Block Diagram.
• Benefits.
• Challenges.
• Comparison.
• Applications.

3. 3
Optical Wireless Technology

4. 4
Terms and Definitions
• Optical wireless communication (OWC) -
transmission in unguided propagation
media through the use of optical carriers,
i.e., visible, infrared (IR), and ultraviolet
(UV) bands.
• Free-Space Optics (FSO) also known as
fiber-free or fiberless optics.
• Transports data via laser technology.

5. 5
Free Space Optics
• Line-of-sight technology - enables optical
transmission up to 2.5 Gbps of data, voice
and video through air to long distances (4
km).
• No requirement of optical fiber cables.
• Optical connectivity without deploying
fiber-optic cable or securing spectrum
licenses.
• Successful in LAN/campus connectivity
market.

6. 6
Free Space Optics
• Information sent through free space instead
of OFC cores (optical fibers).
• Realistic alternative to laying fiber in access
networks.
• Laser beams used, non-lasing sources like
light-emitting diodes (LEDs) or IR light
emitting diodes (IR-LEDs) can also serve
the purpose.
• Line-of-sight to a target within a couple of
miles.

7. 7
Free Space Optics
• Prefer unobstructed path.
• If no direct line-of-sight, strategically
positioned mirrors used to reflect energy.
• Beams can pass through glass windows
with little or no attenuation.
• Can function over several hundred meters
upto 4 km, depending on system
characteristics & environmental conditions.

8. 8
Free Space Optics
• Optical transmission at speeds of upto 2.5
Gbps and upto 10 Gbps using WDM.
• Optical communications at the speed of
light.
• No digging, no delays and associated
costs to lay optical fibers.
• Address the "last mile" bottleneck,
connects to fiber backbone infrastructure
directly to customer premises.

9. 9
Free Space Optics
• FSO network designed for short optical
links (range from 200 to 2000 m).
• Levels of bandwidth comparable to fiber
optic cable.
• High connectivity and dispersion-free
dynamic optical paths.
• Helpful when physical connection between
transmitter and receiver locations is difficult.
• Full Duplex Communication possible.

10. 10
Typical FSO System

11. 11
Typical FSO System
• High-power optical source - Laser or LED.
• Visible/IR energy modulated with data to
be transmitted.
• A telescope that transmits light through the
atmosphere to another telescope that
receives the information.
• Receiving telescope connects to a high-
sensitivity receiver through an optical fiber.

12. 12
Typical FSO System
• Beam intercepted by a photodetector.
• Data extracted from visible or IR beam
(demodulation).
• Resulting signal amplified, sent to
hardware.
• Lens collimation if energy source does not
produce sufficiently parallel beam to travel
required distance.

13. 13
FSO Block Diagram

14. 14
FSO Coverage Area
Θ – Beam
divergence angle, r-
incident light radius,
d- distance.

15. 15
FSO Requirements
• Ability to operate at higher power levels for
longer distances.
• High speed modulation.
• Small footprint, low power consumption.
• Ability to operate over wide temperature
range.
• Low performance degradation for outdoor
systems.
• Mean time between failures (MTBF) shall
be high (say > 10 years).

16. 16
FSO Architectures
• Point to point architecture - higher
bandwidth, less scalable.
• Mesh architecture – redundancy, high
reliability, easy node addition, restricted
distances.
• Point to multipoint architecture - cheaper
connections, facilitates node addition,
lower bandwidth than point to point.
• Ring topology.

17. 17
FSO Architectures

18. 18
FSO Architectures
• Point-to-point arrangement:
- can support speeds between 155 Mbps
to 10 Gbps at a distance of 2 to 4 km.
- dedicated connection with higher
bandwidth.
- does not scale cost-effectively.

19. 19
FSO Architectures
• Point-to-multipoint/star configuration:
- can support same speeds at a distance
of 1 to 2 km.
- cheaper.
- less bandwidth.
- single point failure.
- costs related to hub development.
- lower reliability because of longer
distances.
- issues with hub location.

20. 20
FSO Architectures
• Mesh topology:
- can support 622 Mbps at a distance of
200 m to 450 m.
- can transmit data to a node from
several directions.
- can avoid an obstructed path.
- service restoration (redundancy) via
multiple network nodes.
- scalability : add/remove nodes easily.
- alternate routing to overcome single point
failure.

21. 21
FSO Architectures
• Ring Topology :
- used by metropolitan service providers.
- backbone is represented by high-speed
rings.
• Possible to combine architectures.

22. 22
FSO Merits
• Better speed than broadband.
• Easy installation, less time consuming.
• Low initial investment.
• No need for spectrum license or frequency
coordination between users.
• Secure due to line of sight operation.
• No security system upgradation needed.

23. 23
FSO Merits
• High data rate, comparable to the optical
fiber cables.
• Very low error rate.
• Extremely narrow laser beam permits
unlimited number of FSO links which can
be installed in a specific area.
• Immunity to electromagnetic and RF
interference.

24. 24
FSO Merits
• Offers dense spatial reuse.
• Low power usage per transmitted bit.
• Relatively high bandwidth.
• Flexible rollouts.
• Communication at the speed of light.

25. 25
FSO Limitations
• Environmental challenges unavoidable.
• Line of sight limitations.
• Physical obstructions - flying birds, trees,
and tall buildings temporarily block a single
beam.
• Scintillation - temperature variations among
different air packets due to heat rising from
the earth and the man-made drives like
heating ducts, temperature variations can
cause fluctuations in signal amplitude.

26. 26
FSO Limitations
• Image dancing at FSO receiving end due
to temperature fluctuations.
• Geometric losses : optical beam
attenuation due to beam spreading,
reduced signal power level as it travels
from transmitter to receiver.
• Absorption caused by suspended water
molecules, CO2 in the terrestrial
atmosphere.

27. 27
FSO Limitations
• Atmospheric turbulence : atmospheric
disturbance due to weather & environment
structure caused by wind and convection,
mixes air parcels at different temperatures.
• Fluctuations in the density of air leads to
change in air refractive index, refraction of
beam at different angle and spreading.
• Beam wander : displacement of optical
beam spot rapidly, when size of turbulence
cell is of larger diameter than optical beam.

28. 28
FSO Limitations
• Intensity fluctuation/scintillation of optical
beam if size of turbulence cell is of smaller
diameter than optical beam.
• Atmospheric attenuation due to fog, haze,
dust and rain, can be wavelength
dependent .
• Attenuation in fog weather condition is
wavelength independent.

29. 29
FSO Limitations
• Scattering when optical beam & scatterer
collide, wavelength dependent, energy of
optical beam not changed.
• Scattering Types:
- Rayleigh scattering or molecule
scattering.
- Mie scattering or aerosol scattering.
- Nonselective scattering or geometric
scattering.

30. 30
FSO Limitations
Type of scattering depends on physical
size of the scatterer.
• Rayleigh scattering – scatterer size
smaller than the size of wavelength.
• Mie scattering - Scatterer size comparable
to wavelength.
• Nonselective scattering – scatterer size
larger than the size of wavelength.

31. 31
FSO Limitations
• Atmospheric weather conditions cause
attenuation.
• Optical beam of light absorbed, scattered
and reflected by hindrance caused by fog,
scattering caused by fog, also known as
Mie scattering.
• Rain attenuation due to rain fall -
nonselective scattering, wavelength
independent.

32. 32
FSO Limitations
• Haze particles can stay longer in the air,
lead to atmospheric attenuation.
• Smoke affects visibility of transmission
medium.
• Sandstorms disrupt outdoor links.
• Clouds can completely block fractions of
optical beam transmitted from earth to the
space.
• Snow has larger particles, cause geometric
scattering.

33. 33
Atmospheric Effects on FSO

34. 34
FSO Applications
• Outdoor wireless access.
• Storage Area Network - to provide access to
consolidated, block level data storage.
• Last-mile access - bypass local-loop
systems.
• Enterprise connectivity – interconnecting LAN
segments.
• Fiber backup link during transmission failure.
• Metro-network extensions – extending the
fiber rings of an existing metropolitan area.

35. 35
FSO Applications
• Service acceleration - instant service to
customers.
• Backhaul – carry the traffic of cellular
telephone from antenna towers back to
PSTN.
• Bridging WAN Access – as backbone for high
speed trunking network.
• Point-to-point/multipoint communication links.
• Military access - secure and undetectable
systems

36. 36
FSO vs Fiber Optics

37. 37
FSO vs Counterparts

38. 38
Thank You