The document discusses free space optics (FSO) communication systems. It covers topics such as the basic concepts of FSO systems, propagation through the atmosphere, configurations of FSO links, considerations for data security and safety, and challenges related to signal propagation through various atmospheric conditions. The key advantages of FSO systems over other communication technologies are also summarized.

1. OPTICAL WIRELESS
COMMUNICATION :
FREE SPACE OPTICS
P R E P A R E D B Y : G A D I . N A G A S A I
M A N I K A N T A
Guided by: Dr. G. Santra

2. Optical Wireless Communication : Types
Introduction to the concepts of Free Space Optics (FSO).
Propagation concepts, Link Budget calculations.
FSO: Last Mile Bottleneck Solution.
Configurations of FSO systems.
Chaining in FSO Systems
DATA security/ Safety considerations for FSO systems.
Signal Propagation impediments.
Advantages of FSO as regards to other widely used
systems.
Physical Applications of FSO systems
Manufacturers/Players in field of FSO.
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3. 3
OPTICAL
COMMINICATIONS
WIRED WIRELESS
Optical Fibre
Communications
Photonic
switching
Indoor Free Space
Optics (FSO)
 Chromatic dispersion
 Compensation using
optical signal
processing
 Pulse Modulations
 Optical buffers
 Optical CDMA
• Fast switches
• All optical
routers
• Pulse Modulations
• Equalisation
• Error control coding
• Artificial neural network
&
Wavelet based receivers
Subcarrier
modulation
 Spatial diversity
 Artificial neural
network/Wavelet
based receivers

4. Abundance of unregulated
bandwidth – 200 THz in the 1500-
700 nm range.
No multipath fading – Intensity
Modulation and Direct Detection.
High data rate – in particular line
of sight(in and out doors).
Improved wavelength
reuse capability.
Flexibility in installation -
Deployment in a wide variety of network
architecture and installation on roof to roof,
window to window, roof to window, etc.
Secure transmission.
4
4

5. Multipath induced dispersion (non-
line of sight, indoor) - Limiting data
SNR can vary significantly with the
distance and the ambient noise
Limited transmitted power - Eye safety
(indoor)
Receiver sensitivity
Large area photo-detectors - Limits the
bandwidth
May be high cost - Compared with RF
Limited range: Indoor: ambient noise is the
dominant (20-30 dB larger than the signal level .
Outdoor: Fog and other factors
High transmitted power - Outdoor
5
5

6. • High Bandwidth
• Low BER
• High SNR
• Power efficient
• Provide Data Security.
• Low cost
• Easy to install and maintain.
6

7. • FSO is a line-of-sight technology which uses
LASERS and Photo detectors to provide optical
connections between two points—without the
fiber.
• FSO can transmit data, voice or video at speeds
capable of reaching 2.5 Gbps. Products capable
of speeds upto 10 Gbps are expected to hit the
markets within one year.
• FSO units consist of an optical transceiver with a
laser (transmitter) and a Photo detector
(receiver) to provide full duplex (bi-directional)
capability.
• FSO systems use invisible infrared laser light
wavelengths in the 750nm to 1550nm range. 7

8.  Narrow low power transmit beam- inherent security
 Narrow field-of-view receiver
 Similar bandwidth/data rate as optical fibre
 No multi-path induced distortion in LOS
 Efficient optical noise rejection and a high optical signal
gain
 Suitable to point-to-point communications only (out-door
and in-door)
 Can support mobile users using steering and tracking
capabilities
 Used in the following protocols:
- Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI, ATM
- Optical Carriers (OC)-3, 12, 24, and 48.
 Cheap (cost about $4/Mbps/Month according to fSONA)
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9. 9
9
In addition to bringing huge bandwidth to businesses /homes FSO also finds
applications in :
Multi-campus university
Hospital
Others:
 Inter-satellite
communication
 Disaster recovery
 Fibre communication
back-up
 Video conferencing
 Links in difficult terrains
 Temporary links
e.g. conferences
Cellular communication back-haul FSO challenges…

10. 10
 Disaster management as was exhibited
during the Sept 11 attacks.
 Merill Lynch & Co. has set up FSO system
from its Vesey Street office towers across
the Hudson River to an alternate site in
New Jersey.
 TeraBeam, a major producer of FSO
equipment, successfully deployed FSO at
the Sydney Summer Olympic Games.
 A network of FSO devices is fast coming up
in Seattle which is touted as the Capital of
Fog. Manufacturers believe that if an FSO
system can successfully work in Seattle
then it can do so in any part of the world.
 Affordably extend existing fiber network.
 Disaster recovery and temporary
applications

11. • No licensing required.
• Installation cost is very low as compared to
laying Fiber.
• No sunk costs.
• No capital overhangs.
• Highly secure transmission possible.
• High data rates, upto 2.5 Gbps at present and 10
Gbps in the near future.
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12. 12

13. 13

14.  A source producing data input is to be transmitted to a remote
destination. This source has its output modulated onto an optical
carrier; laser or LED, which is then transmitted as an optic al field
through the atmospheric channel.
 The important aspects of the optical transmitter system are size,
power, and beam quality, which determine laser intensity and
minimum divergence obtainable from the system.
 At the receiver, the field is optically collected and detected, generally
in the presence of noise interference, signal distortion, and
background radiation. On the receiver side, important features are
the aperture size and the f/-number, which determine the amount of
the collected light and the detector field-of-view (FOV).
 The transmit optics consists of lens assembly ( Plano convex lenses
) and receiver Optics consist of telescope units to receive the
incident light.
14

15. The choice of LED vs. Laser Diode as a light source in a
wireless optical transmission product depends on the
target application, and the related performance, cost and
reliability requirements of the overall solution being
designed.
Long range, very high speed (gigabit or more) point-to-
point FSO systems require laser diodes. Such products
compete with high-speed RF point-to-point solutions
often based on milimeter wave transmission in the 60,
70, 80 and 90 GHz bands.
 However, shorter range LED based systems can
achieve high-speed optical system performance, while
dramatically reducing the overall system size and cost. 15

16. Compared with transmitters, receiver choices are much
more limited.
The two most common detector material systems used
in the near-IR spectral range are based on Si or indium
gallium arsenide (InGaAs) technology.
 Germanium is another material system that covers the
operating wavelength range of commercially available
FSO systems.
 However, germanium technology is not used very often
because of the high dark current values of this material.
All these materials have a rather broad spectral
response in wavelength, and, unlike lasers, they are not
tuned toward a specific wavelength.
16

17.  Usually a trans-impedance amplifier is used after the detector because in
most cases they provide the highest gain at the fastest speed.
 If CCD, CMOS, or quad cell detectors are used as tracking detectors, these
relatively large area devices are easy to align to the tracking optics.
However, care must be taken in manufacture to co-align these optics with
the transmit and receive optical axes.
 For building-mounted free-space optical systems, the tracking bandwidth
can be very low—sub-hertz—because the bulk of building motion is due to
the building’s uneven thermal loading and these effects occur in a time
scale of hours.
 For systems that are to be mounted on towers or tall poles, the tracking
bandwidth should be higher—most likely on the order of several hertz at
least—to remove wind-induced vibrations.
 Acquisition systems can be as crude as aligning a gunsight to very
sophisticated GPS based, high accuracy, fully automated systems. The
choice of this subsystem really depends on the application and number of
devices to be put into a network. 17

18. On and Off Keying (OOK) Modulation : On-off keying (OOK)
the simplest form of modulation that represents digital data as the
presence or absence of a carrier wave. In its simplest form, the
presence of a carrier for a specific duration represents a binary one,
while its absence for the same duration represents a binary zero.
Pulse Position Modulation : Pulse-position modulation (PPM)
is a form of signal modulation in which M message bits are encoded
by transmitting a single pulse in one of possible time-shifts. This is
repeated every T seconds, such that the transmitted bit rate is M/T
bits per second. It is primarily useful for optical
communications systems, where there tends to be little or
no multipath interference.
18

19. 19

20. 20

21. 21

22.  Firstly, the incoming data stream is serial to parallel converted into
"n" independent streams. These streams are encoded in parallel by
an encoder.
 In the parallel encoder, a data block is composed by taking one bit
out of each data sequence, each time the data blocks are encoded.
 The parity check bits are added and transmitted on "k" exclusive
channels, which have same rate as the data sequence and are also
generated by the encoder.
 Hence, this parallel encoder makes an (11 + k, n) code, where n + k
is the codeword length. Secondly, these n + k codeword sequences
are modulated into 00K or PPM codes on each channel.
 At the optical modulator, these code sequences modulate each
diode with a different wavelength and are multiplexed. In the
multiplexer, each optical signal from channels is focused on an
optical fiber.
 The optical pulses from the fiber are spread on the optical channel
22

23. At the receiver, the transmitted pulses are received
together with the ambient light noise. These multiplexed
signals are separated in accordance with their carrier
wavelength.
 The optical filter is used as the de-multiplexer. These
optical band-pass filters are usually constructed of
multiple thin dielectric layers, and can achieve narrow
bandwidths.
These separated signals passed to the photo diode array,
demodulated by pulse demodulator, and then decoded in
parallel by the parallel decoder. Finally, these parallel
data blocks are parallel to serial converted to retrieve the
original data. 23

24. DRIVE
R
CIRCU
IT
SIGNAL
PROCESSI
NG
PHOTO
DETECTOR
Link Range L
FSO - Basics
 Cloud
 Rain
 Smoke
 Gases
 Temperature variations
 Fog and aerosol
Transmission of optical radiation through the atmosphere obeys the Beer-
Lamberts’s law:
α : Attenuation coefficient dB/km – Not controllable and is roughly independent of
wavelength in heavy attenuation conditions.
d1 and d2: Transmit and receive aperture diameters (m)
D: Beam divergence (mrad)(1/e for Gaussian beams; FWHA for flat top beams),
This equation fundamentally ties FSO to the atmospheric weather conditions
Dominant term at
99.9% availability
24

25. 25

26. Less then 5% of all buildings in the US have a direct
connection to the very high speed (2.5-10 Gbps) fiber
optic backbone, yet more than 75% of businesses are
within 1 mile of the fiber backbone.
Most of these businesses are running some high speed
data network within their building, such as fast Ethernet
(100 Mbps), or Gigabit Ethernet (1.0 Gbps).
Yet, their Internet access is only provided by much lower
bandwidth technologies available though the existing
copper wire infrastructure (T-1 (1.5 Mbps), cable modem
(5 Mbps shared) DSL (6 Mbps one way) ), etc.
The last mile problem is to connect the high bandwidth
from the fiber optic backbone to all of the businesses with
high bandwidth networks.
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27. 27

28. 28
 DSL and cable modems cannot provide
true broadband services. Cable modems
enjoy higher capacity, yet the channel is
shared and the amount of bandwidth at any
given time is not guaranteed.
 Copper lines provide data rates to a
fraction of 1 Mbps.
 T1 lines can reach upto a few Mbps but
are still far away from the Gbps speed which
the fiber backbone can support.
 The chart below shows how these
technologies address different market
segments based on technology, technical
capabilities (reach, bandwidth), and
economic realities.

29. 29

30. Point to Multipoint Topology
Point to Point Topology
Ring with Spurs Topology
Mesh Topology
Metro Network
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31. 31

32. 32

33. 33

34. 34

35. 35

36. 36

37. To overcome the security in a
network two conditions are
necessary:
(1) Intercept enough of the signal to
reconstruct data packets and
(2) Be able to decode that
information.
37

38. 38
Preventing Interception of the Signal

39. 39

40. 40

41.  Physical Obstruction
 Atmospheric Losses
 Free space loss
 Clear air absorption
 Weather conditions (Fog, rain, snow, etc.)
 Scattering
 Scintillation
 Building Sway and Seismic activity
41

42.  Construction crane or flying bird comes in path
of light beam temporarily
Solution:
Receiver can recognize temporary loss of
connection
In packet-switched networks such short-duration
interruptions can be handled by higher layers
using packet retransmission
42

43. 43
 Proportion of transmitted
power arriving at the receiver
 Occurs due to slightly
diverging beam
Solution:
 High receiver gain and large receiver aperture
 Accurate pointing

44.  Equivalent to absorption loss in optical fibers
 Wavelength dependent
 Low-loss at wavelengths ~850nm, ~1300nm
and ~1550nm
 Hence these wavelengths are used for
transmission
44

45.  Adverse atmospheric conditions increase Bit Error Rate
(BER) of
an FSO system
 Fog causes maximum attenuation
 Water droplets in fog modify light characteristics or
completely
hinder the passage of light
 Attenuation due to fog is known as Mie scattering
 Solution:
 Increasing transmitter power to maximum allowable
 Shorten link length to be between 200-500m
45

46.  Caused by collision of wavelength with
particles in atmosphere
 Causes deviation of light beam
 Less power at receiver
 Significant for long range communication
46

47. Heated air rising from the earth or man-made devices
such as heating ducts creates temperature variations
among different air pockets. This can cause fluctuations
in signal amplitude which leads to image fluctuations at
the FSO receiver end.
Caused due to different refractive indices of small air
pockets at different temperatures along beam path
Air pockets act as prisms and lenses causing refraction
of beam
Optical signal scatters preferentially by small angles in
the direction of propagation
Distorts the wavefront of received optical signal causing
‘image dancing’
Best observed by the simmering of horizon on a hot day 47

48. Solution:
 Large receiver diameter to cope with image
dancing
 Spatial diversity: Sending same information
from several laser transmitters mounted in same
housing
 Not significant for links < 200m apart, so
shorten link length
48

49.  Movements of buildings upsets transmitter-receiver
alignment
Solution:
 Use slightly divergent beam
 Divergence of 3-6 milliradians will have diameter
of 3-6 m after traveling 1km
 Low cost
 Active tracking
 Feedback mechanism to continuously align
transmitter-
receiver lenses
 Facilitates accelerated installation, but expensive49

50.  Use lasers ~850 nm for short distances and
~1550 nm for long distance communication with
maximum allowable power
 Slightly divergent beam
 Large receiver aperture
 Link length between 200-1000m in case of
adverse weather conditions
 Use multi-beam system
50

51.  Clear, still air -1 dB/km -5 dB/km
 Scintillation 0 to -3 dB/km 0
 Birds or foliage Impenetrable 0 to -20 dB
 Window (double-glazed) -3 dB -1 dB
 Light mist (visibility 400m) -25 dB/km -1 dB/km
 Medium fog (visibility 100m) -120 dB/km -1 dB/km
 Thick fog (visibility 40m) -300 dB/km -1 dB/km
 Light rain (25mm/hour) -10 dB/km -10 dB/km
 Heavy rain (150mm/hour) -25 dB/km -40 dB/km
51

52.  Requires line-of-sight
 Limited range (max ~8km)
 Unreliable bandwidth availability
BER depends on weather conditions
 Accurate alignment of transmitter- receiver necessary
52

53. Manufacturers/ Players in the Field of
FSO:
LightPointe: A San Diego based company which
received contributions from Cisco Systems and
Corning to the tune of $33 million. It has raised a
total of $51.5 million.
AirFiber: Another San Diego based company which
has received contributions from Nortel Networks to
the tune of $50 million. It has raised a total of $92.5
million.
Terabeam: A Kirkland, WA based company has
received funding from Luscent technologies to the
tune of $450 million and has raised $585 million to
date. 53

54. • Lighpointe’s “The phyiscs of free space optics”
white paper
• http://en.wikipedia.org/
• Lightpointe Communications Corp., "Free Space
Optics: A Viable Last-Mile Alternative,"white
paper.
• "Optical Wireless: Low-Cost, Broadband, Optical
Access," white paper, fSONA Communications
Corp.
54

55. • http://www.lightpointe.com/
• http://www.freespaceoptic.com/
• http://www.fsonews.com/
• http://www.cablefreesolutions.com/
• http://www.thefoa.org/
• http://www.free-space-optics.org/
• http://www.freespaceoptics.com/
• http://www.opticsreport.com/
55

56. I thank our Seminar Guide Dr G. Santra for her valuable
guidance and directions in making the seminar
resourceful.
56