Transcript Uv communications Test slides

Some challenging areas in Free-Space Laser
Communications
Dr. Arun K. Majumdar
[email protected]
Lecture Series,: 3
Brno University of Technology, Brno
Czech Republic
December 1-6, 2009
Copyright © 2009 Arun K. Majumdar
Review of last lecture: :
• Background, need and recent R&D directions
• Basic Free-Space Optics (FSO) communication
system and parameters
• Some areas of current interest
• My own recent research and results
• Conclusions and recommendations for solving
complex problems
Copyright © 2009 Arun K. Majumdar
Background, need and recent R&D directions
•
•
•
•
•
•
•
•
•
Needs for improvements and advanced technologies
laser and hybrid (combination of laser and RF)
communications: advanced techniques and issues
advances in laser beam steering, scanning, and shaping
technologies
laser propagation and tracking in the atmosphere
atmospheric effects on high-data-rate free-space optical data
links (including pulse broadening)
long wavelength free-space laser communications
adaptive optics and other mitigation techniques for free-space
laser communications systems
techniques to mitigate fading and beam breakup due to
atmospheric turbulence/scintillation: spatial, temporal,
polarization, and coding diversity strategies, and adaptive
approaches
error correction coding techniques for the atmospheric channel
characterization and modeling of atmospheric effects
(aerosols, turbulence,Copyright
fog, rain,
smoke, etc.) on optical and RF
© 2009 Arun K. Majumdar
communication links
Background, need and recent R&D directions
(Continued…)
• communication using modulated retro-reflection
• terminal design aspects for free-space optical link (for
satellite- or land-mobile-terminals)
• integration of optical links in networking concepts (e.g.
inter-aircraft MANET)
• design and development of flight-worthy and spaceworthy optical communication links
• deep-space/ inter-satellite optical communications
• multi-input multi-output (MIMO) techniques applied to
FSO
• free space optical communications in indoor
environments
• underwater and UV communications: applications and
concepts of FSO in sensor networks for monitoring
climate change in the
air and under water
Copyright © 2009 Arun K. Majumdar
Basic Free-Space Optics (FSO)
communication system and parameters
• A typical free-space laser communications
system
Communications Parameters
- Modulation Techniques for FSO communications
- Received signal-to-noise ratio (SNR)
- Bit-Error-Rate
Copyright © 2009 Arun K. Majumdar
Some areas of current interest
• Atmospheric Turbulence Measurements over Desert site
relevant to optical communications systems
• Reconstruction of Unknown Probability Density Function
(PDF) of random Intensity Fluctuations from Higher-order
Moments
• Atmospheric Propagation Effects relevant to UV
Communications
Copyright © 2009 Arun K. Majumdar
Review of Results and Conclusions
• Atmospheric Turbulence Measurements over Desert
site relevant to optical communications systems
Strength of Turbulence, Cn2 parameter
- Coherence length, r0
- Isoplanatic Angle, Ө0
- Rytov Variance, σr2
CLEAR1 model:
- Greenwood Frequency, fG
Air-borne
Imaging
system
Aberrated
wavefront
H
Turbulence
Atmospheric Models
Spherical wave from
point source
Hufnagel-Valley (HV) model
Point Source
Modified Hufnagel-Valley (MHV) model:
•SLC-Day model:
Copyright © 2009 Arun K. Majumdar
Temperature fluctuations and Cn2 from
scintillation measurements
1 0
C n 2
1 0
1 0
1 0
-1 2
-1 3
-1 4
-1 5
Copyright © 2009 Arun K. Majumdar
1 6 .6
1 6 .8
1 7
M is s io n
1 7 .2
1 7 .4
D a y / T im e [ D a y s ]
1 7 .6
Valley
Hufnagel-Valley
Comparison of ) Cn2 profile generated from tethered-blimp
instrument measurement and various models.
Cn2 (m^-2/3)
Night
1 10
14
1 10
15
1 10
16
1 10
17
1 10
18
Cn2 Profile Comparison
0.8
1
1.2
Measured
Hufnagel-Valley
Modified Hufnagel-Valley
SLC-Day
CLEAR1 Night
1.4
1.6
1.8
Altitude (Km)
2
2.2
Copyright © 2009 Arun K. Majumdar
2.4
2.6
Histogram of Cn2 : some typical examples
FREQUENCY (%)
8
6
4
2
0
14.5
14
13.5
13
12.5
log10(Cn2 (m^-2/3))
14.5
14
13.5
log10(Cn2 (m^-2/3))
12
11.5
FREQUENCY (%)
10
5
0
15.5
15
13
Copyright © 2009 Arun K. Majumdar
12.5
SUMMARY AND CONCLUSIONS
• New results of atmospheric turbulence measurements
over desert site using ground-based instruments and
tethered-blimp platform are presented
• An accurate model of the complex optical turbulence
model for profile is absolutely necessary to analyze and
predict the system performance of free-space laser
communications and imaging systems
• Because of the complexity and variability of the nature of
atmospheric turbulence, accurate measurements of
turbulence strength parameters are essential to design
the system for operating over a wide range
Copyright © 2009 Arun K. Majumdar
Review of Results and Conclusions
• Reconstruction of Unknown Probability Density
Function (PDF) of random Intensity Fluctuations
from Higher-order Moments
PROPOSED METHOD BASED ON HIGHER-ORDER MOMENTS
(x
•sought-for PDF is given by a gamma PDF modulated
by a series of generalized Laguerre polynomials:
RESULTS : Simulation using 5000
data samples generated randomly
to follow a given distribution
generalized-Laguerre fit to log-Normal with 6 moments: 10000 data values
0.45
ideal PDF
PDF fit
0.4
generalized-Laguerre fit to data LN5000 with 6 moments:
0.35 5000 data values
fit
nrm
0.3
histogram
0.35
0.3
0.25
0.25
0.2
PDF
0.15
0.2
0.15
0.1
0.05
0.1
0
generalized-Laguerre fit to data LN5000 with 6 moments:
1 5000 data values
fit
nrm
0.9
histogram
0.8
0.05
-0.05
0
2
4
6
8
Random Variable, x
10
12
0
0.050
2
4
6
8
Intensity
10
12
0.7
0.6
C 0.5
0.4
D 0.3
F 0.2
Copyright © 2009 Arun K. Majumdar
0.1
0
0
2
4
6
8
10
12
CONCLUSIONS AND SUMMARY
• A new method of reconstructing and predicting an
unknown probability density function (PDF) is presented
• The method is based on a series expansion of
generalized Laguerre polynomials and generates the
PDF from the data moments without any prior knowledge
of specific statistics, and converges smoothly
• We have applied this method to both the analytical
PDF’s and simulated data, which follow some known
non-Gaussian test PDFs such as Log-Normal, RiceNakagami and Gamma-Gamma distributions
• Results show excellent agreement of the PDF fit was
obtained by the method developed
• The utility of reconstructed PDF relevant to free-space
laser communication is pointed out
Copyright © 2009 Arun K. Majumdar
Review of Results and Conclusions
• Atmospheric Propagation Effects relevant to UV
Communications
Monte Carlo Impulse Response Model
Copyright © 2009 Arun K. Majumdar
Atmospheric Propagation Effects relevant to UV
Communications (contd..)
Parametric model (Gamma
function) :
3-DB bandwidth:
Copyright © 2009 Arun K. Majumdar
Related other challenging areas of research and recent
developments
•
•
•
•
•
Optical RF Free-Space communications
Underwater optical wireless communications
Indoor optical wireless communications
Chaos-based secure communications
Mitigation of atmospheric turbulence for
communications
Copyright © 2009 Arun K. Majumdar
Optical RF Free-Space communications
• There is a need for high-capacity communication
networks for many applications where it is possible to
integrate RF and free space optical hybrid
communications
• A robust network
• The network is expected to operate under a variety of
weather conditions and through atmospheric distortions
Copyright © 2009 Arun K. Majumdar
Underwater optical wireless communications
• The present technology of underwater acoustic
communication cannot provide high data rate
transmission
• Optical wireless communication has been
proposed as the best alternative to meet this
challenge
• Using the scattered light it is possible to mitigate
the communication performance decrease due
to absorption only; thus a high data rate
underwater optical wireless is a feasible solution
Copyright © 2009 Arun K. Majumdar
Different communication scenarios
1. Line-of-sight communication link
2. A modulating retro reflector link
3. A reflective link
Copyright © 2009 Arun K. Majumdar
Underwater optical wireless communication channel
properties and link models
•
Reference: Shlomi Arnon, “an underwater optical wireless communication Network,” in FreeSpace Laser Communications IX edited by Arun K. Majumdar, Christopher Davis, Proc. SPIE Vol.
7464 (2009).
• Extinction coefficient:
Propagation Loss:
Optical signal at the receiver:
1. LOS communication link:
2. Modulating retro-reflector
communication link:
Copyright © 2009 Arun K. Majumdar
Underwater optical wireless communication channel
properties and link models (contd..)
3. Reflective communication link:
Approximate received power:
where
Bit Error Rate (BER):
Copyright © 2009 Arun K. Majumdar
Number of photons and BER as a function of transmitter receiver
separation for clean ocean water with extinction coefficient equal
0.15 m-1
Copyright © 2009 Arun K. Majumdar
Indoor Optical Communications
• Optical wireless communications as a
complementary technology for short-range
communications
Copyright © 2009 Arun K. Majumdar
Different Indoor link configurations
Copyright © 2009 Arun K. Majumdar
indoor
Copyright © 2009 Arun K. Majumdar
Website References for Indoor Optical Communications
• Website for “Propagation modeling… Jefffrey Carruthaers ,..):
• http://iss.bu.edu/jbc/Publications/jbc-j7.pdf
• Website for Dominic Obrien “visible light communications:
challenges and possibilities”
http://202.194.20.8/proc/PIMRC2008/content/papers/1569135393.pdf
Copyright © 2009 Arun K. Majumdar
Propagation Modeling for Indoor Optical Wireless
Communications
• Impulse response of optical wireless channels
• Many receiver or transmitter locations
The transmitter or source Sj, transmitting a signal Xj using intensity
modulation, photodiode receiver responsivity r (direct detection), receiver Ri,
and Ni(t) is noise at the receiver, he(t;Sj,Ri) is the impulse response of the
channel between source Sj and receiver Ri.
The signal received by receiver Ri is
Copyright ©
Source radiant intensity pattern:
2009 Arun K. Majumdar
Propagation Modeling (contd..)
• Line of sight impulse response:
Where
is the distance between the source
and the receiver:, and Ari is the optical
collection area of the receiver.
Finally, for k bounces, the impulse response for each
source Sj is
Where
and
represent element n acting as a
receiver and a source, and
Lambertiam source
is reflectiviytu of the
Copyright © 2009 Arun K. Majumdar
Typical Impulse Responses for a Transmitter and
Receiver separated by 0.8 m in a 4x4 m2 room
Copyright © 2009 Arun K. Majumdar
Visible light communications: Indoor links
Emission spectrum of white-light LED
Small-signal modulation bandwidth of LED
Copyright ©
Transmitter: LED, lens and
driver; Channels: LOS and
2009 Arun K.diffuse
Majumdar
paths; Receiver: Optics,
PD, and amplifiers
Recent developments and possibilities
– bandwidth >~90MHz within ‘typical’ room
Copyright © 2009 Arun K. Majumdar
Chaos-based Free-space Optical Communications
• Chaotic communication using time-delayed
optical systems with EDFRL (erbium-doped fiber
ring laser) producing chaotic fluctuations
• Laser with external feedback
chaotic optical
signal : Optical to opto-electronic feedback
• Mostly fiber optic. Free-space optical
communication also (2002 and then 2008)
Copyright © 2009 Arun K. Majumdar
Fiber-optics based chaos-communications research
Experimental setup for chaotic communication
Transmitted and received signals
35 km of single-mode fiber at up to 250 Mbit/s data rate
Reference: Gregory D. Vanwiggeren abd Rajashri Roy, “Chatic communication
Copyright International
© 2009 Arun K. Majumdar
using time-delayed optical systems,”
Journal of Bifurcation and
Chaos< Vol.9, No.11,(1999)
Chaos-based optical communication at high bit rate
Transmission rates in the Gigabit per second
range with bit-error rates below 10-7 achieved
Reference: Apostolos Argyris, et al, “Chaos-based communications at high
bit rates using commercial fibre-optic link,”Vol.438/17, Nature, November
2005.
Copyright © 2009 Arun K. Majumdar
Acousto-optic Chaos based secure Free-space
Optical Communication Links
Acousto-optic system with electronic
feedback:
Shows bistable behavior and can generate
chaotic oscillations
Signal Modulation/Encryption with AO Chaos
Reference: A.K. Ghosh et al, “Design of Acousto-optic Chaos based secure Free-space
optical communication links, ”Proc.
SPIE Vol.7464,
edited
by Arun K. Majumdar and
Copyright
© 2009 Arun
K. Majumdar
Christopher C. Davis, 2009.
Basic schemes for optical communications with AO
Chaos
-Simpler than laser based chaos encryption systems (external modulator
type approach)
- Numerically shown that decryption of the encoded data is possible by
using an identical acousto-optic system in the receiver
Copyright © 2009 Arun K. Majumdar
- Free-space optical communications possible!
Scintillation Mitigation Techniques for Free-Space
Optical Communications
•
•
•
•
•
•
•
Aperture Averaging
Spatial Diversity
Adaptive Optics
Partially Coherent beams
Long Wavelength
Wavelength diversity
Modulation Schemes
Copyright © 2009 Arun K. Majumdar
Scintillation Mitigation Techniques (contd..)
• Aperture Averaging
Copyright © 2009 Arun K. Majumdar
Multiple-beam Free-Space Optical
Communications
Copyright © 2009 Arun K. Majumdar
Scintillation Mitigation Techniques (contd..)
•Spatial Diversity
Copyright © 2009 Arun K. Majumdar
BER for space time block code for four optical
transmitters
Copyright © 2009 Arun K. Majumdar
Scintillation Mitigation Techniques (contd..)
• Adaptive Optics
Copyright © 2009 Arun K. Majumdar
Scintillation Mitigation Techniques (contd..)
• Other Mitigation Techniques
– Various Modulation schemes (one example: Polarization Shift
Keying Modulation (POLSK) versus OOK modulation for freespace optical communication) and Forward Error Correction
(FEC), Various Coding Schemes
– Partially coherent and Partially polarized beam : for
communication
– Long wavelength laser communications (for example: 3.5 μ )
Copyright © 2009 Arun K. Majumdar
Conclusions
• Challenges exist for Free-Space Optical
communications both from theoretical and
experimental point of view
• Accurate atmospheric modeling, efficient
techniques to mitigate atmospheric effects
will lead to improved system design and
performance
Copyright © 2009 Arun K. Majumdar