Dr. Arun K. Majumdar

illnurturedtownvilleMobile - Wireless

Nov 21, 2013 (3 years and 11 months ago)

100 views

Copyright © 2009 Arun K. Majumdar

Some challenging areas in Free
-
Space Laser
Communications

Dr. Arun K. Majumdar

a.majumdar@IEEE.org



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, fog, rain, smoke, etc.) on optical and RF
communication links

Copyright © 2009 Arun K. Majumdar

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 space
-
worthy 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

H

Air
-
borne
Imaging
system

Aberrated
wavefront

Spherical wave from
point source

Turbulence

Point Source

Strength of Turbulence, Cn2 parameter



-

Coherence length,
r0




-

Isoplanatic Angle,

Ө
0



-

Rytov Variance,
σ
r2




-

Greenwood Frequency, fG


Atmospheric Models

Hufnagel
-
Valley (HV) model

Modified Hufnagel
-
Valley (MHV) model
:


SLC
-
Day model:

CLEAR1 model:


Copyright © 2009 Arun K. Majumdar

Temperature fluctuations and Cn2 from
scintillation measurements



1 6.6
1 6.8
1 7
1 7.2
1 7.4
1 7.6
1 0
- 1 5
1 0
- 1 4
1 0
- 1 3
1 0
- 1 2
Cn2
M i s s i o n D a y/T i m e [ D a y s ]
Copyright © 2009 Arun K. Majumdar


0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
1

10
18
1

10
17
1

10
16
1

10
15
1

10
14
Measured
Hufnagel-Valley
Modified Hufnagel-Valley
SLC-Day
CLEAR1 Night
Cn2 Profile Comparison
Altitude (Km)
Cn2 (m^-2/3)

0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
1

10
18
1

10
17
1

10
16
1

10
15
1

10
14
Measured
Hufnagel-Valley
Modified Hufnagel-Valley
SLC-Day
CLEAR1 Night
Cn2 Profile Comparison
Altitude (Km)
Cn2 (m^-2/3)
Comparison of ) Cn2 profile generated from tethered
-
blimp
instrument measurement and various models
.

Copyright © 2009 Arun K. Majumdar


Histogram of Cn2 : some typical examples



14.5
14
13.5
13
12.5
12
11.5
0
2
4
6
8
log10(Cn2 (m^-2/3))
FREQUENCY (%)

15.5
15
14.5
14
13.5
13
12.5
0
5
10
log10(Cn2 (m^-2/3))
FREQUENCY (%)
Copyright © 2009 Arun K. Majumdar

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


sought
-
for PDF is given by a gamma PDF modulated
by a series of generalized Laguerre polynomials:


0

2

4

6

8

10

12

-
0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

generalized
-
Laguerre f
it to log
-
Normal with 6 moments: 10000 data values

ideal PDF

PDF fit

PDF(x
)

Random Variable, x


0

2

4

6

8

10

12

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1



generalized
-
Laguerre fit to data LN5000 with 6 moments:
5000 data values

fit

nrm
histogram

I
nt
e
n
si
ty

C
D
F


0

2

4

6

8

10

12

-
0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

PDF

Intensity

generalized
-
Laguerre fit to data LN5000 with 6 moments:
5000 data values

fit

nrm
histogram

RESULTS : Simulation using 5000
data samples generated randomly
to follow a given distribution

Copyright © 2009 Arun K. Majumdar

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, Rice
-
Nakagami 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
Free
-
Space 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 S
j
, transmitting a signal X
j

using intensity
modulation, photodiode receiver responsivity r (direct detection), receiver R
i
,
and N
i
(t) is noise at the receiver, h
e
(t;S
j
,R
i
) is the impulse response of the

channel between source S
j

and receiver R
i
.

The signal received by receiver R
i

is

Source radiant intensity pattern:

Copyright © 2009 Arun K. Majumdar

Propagation Modeling (contd..)


Line of sight impulse response:


Where
is the distance between the source
and the receiver:, and A
ri

is the optical
collection area of the receiver.

Finally, for k bounces, the impulse response for each
source S
j

is


Where and represent element n acting as a


receiver and a source, and is reflectiviytu of the
Lambertiam source

Copyright © 2009 Arun K. Majumdar

Typical Impulse Responses for a Transmitter and
Receiver separated by 0.8 m in a 4x4 m
2

room

Copyright © 2009 Arun K. Majumdar

Visible light communications: Indoor links

Emission spectrum of white
-
light LED

Small
-
signal modulation bandwidth of LED

Transmitter: LED, lens and
driver; Channels: LOS and
diffuse paths; Receiver: Optics,
PD, and amplifiers

Copyright © 2009 Arun K. Majumdar

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
using time
-
delayed optical systems,” International Journal of Bifurcation and
Chaos< Vol.9, No.11,(1999)

Copyright © 2009 Arun K. Majumdar

Chaos
-
based optical communication at high bit rate

Reference
: Apostolos Argyris, et al, “Chaos
-
based communications at high
bit rates using commercial fibre
-
optic link,”Vol.438/17, Nature, November
2005.

Transmission rates in the Gigabit per second
range with bit
-
error rates below 10
-
7

achieved

Copyright © 2009 Arun K. Majumdar

Acousto
-
optic Chaos based secure Free
-
space
Optical Communication Links

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
Christopher C. Davis, 2009.

Acousto
-
optic system with electronic
feedback:

Shows bistable behavior and can generate
chaotic oscillations

Signal Modulation/Encryption with AO Chaos

Copyright © 2009 Arun K. Majumdar

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

-

Free
-
space optical communications possible!


Copyright © 2009 Arun K. Majumdar

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 free
-
space 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