Brno University Lecture3

8/12/2019 Brno University Lecture3

1/44

Copyright 2009 Arun K. Majumdar

Some challenging areas in Free-Space Laser

Communications

Dr. Arun K. [email protected]

Lecture Series,: 3

Brno University of Technology, BrnoCzech RepublicDecember 1-6, 2009
mailto:[email protected]:[email protected]


2/44


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


3/44


Background, need and recent R&D directionsNeeds 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 shapingtechnologies

laser propagation and tracking in the atmosphere

atmospheric effects on high-data-rate free-space optical datalinks (including pulse broadening)

long wavelength free-space laser communications

adaptive optics and other mitigation techniques for free-spacelaser communications systems

techniques to mitigate fading and beam breakup due toatmospheric turbulence/scintillation: spatial, temporal,polarization, and coding diversity strategies, and adaptiveapproaches

error correction coding techniques for the atmospheric channel

characterization and modeling of atmospheric effects

(aerosols, turbulence, fog, rain, smoke, etc.) on optical and RFcommunication links


4/44


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 toFSO

free space optical communications in indoorenvironments

underwater and UV communications: applications andconcepts of FSO in sensor networks for monitoringclimate change in the air and under water


5/44


Basic Free-Space Optics (FSO)

communication system and parameters

A typical free-space laser communicationssystem

Communications Parameters

- Modulation Techniques for FSO communications

- Received signal-to-noise ratio (SNR)

- Bit-Error-Rate


6/44


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 UVCommunications


7/44


Review of Results and Conclusions

Atmospheric Turbulence Measurements over Desert

site relevant to optical communications systems

H

Air-borne

Imaging

system

Aberratedwavefront

Spherical wave from

point source

Turbulence

Point Source

Strength of Turbulence, Cn2 parameter

- Coherence length, r0

- Isoplanatic Angle,0

- Rytov Variance, r2

- Greenwood Frequency, fG

Atmospheric Models

Hufnagel-Valley (HV) model

Modified Hufnagel-Valley (MHV) model:

SLC-Day model:

CLEAR1 model:


8/44


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

C

n

2

M is s io n D a y / T im e [ D a y s ]


9/44


0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.61 10

18

1 10 17

1 10 16

1 10 15

1 10 14

MeasuredHufnagel-Valley

Modified Hufnagel-Valley

SLC-DayCLEAR1 Night

Cn2 Profile Comparison

Altitude (Km)

Cn2(m^-2/3

)

Comparison of ) Cn2 profile generated from tethered-blimp

instrument measurement and various models.


10/44


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 12.50

5

10

log10(Cn2 (m^-2/3))

FREQU

ENCY(%)


11/44


SUMMARY AND CONCLUSIONS

New results of atmospheric turbulence measurements

over desert site using ground-based instruments andtethered-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 lasercommunications and imaging systems

Because of the complexity and variability of the nature of

atmospheric turbulence, accurate measurements of

turbulence strength parameters are essential to designthe system for operating over a wide range


12/44



Reconstruction of Unknown Probability Density

Function (PDF) of random Intensity Fluctuationsfrom 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 12-0.0500.05

0.10.150.20.250.30.350.40.45

generalized-Laguerre fit to log-Normal with 6 moments: 10000 data valuesideal PDFPDF fit

x

Random Variable, x

0 2 4 6 8 10 1200.10.20.30.40.50.60.70.80.9

1 generalized-Laguerre fit to data LN5000 with 6 moments:fitnrm

C

D

F

0 2 4 6 8 10 12-

00.050.1

0.150.2

0.250.3

0.35

PDF

Intensity

generalized-Laguerre fit to data LN5000 with 6 moments:

fitnrm

RESULTS : Simulation using 5000

data samples generated randomly

to follow a given distribution


13/44


CONCLUSIONS AND SUMMARY

A new method of reconstructing and predicting anunknown probability density function (PDF) is presented

The method is based on a series expansion ofgeneralized Laguerre polynomials and generates thePDF from the data moments without any prior knowledge

of specific statistics, and converges smoothly

We have applied this method to both the analyticalPDFs and simulated data, which follow some knownnon-Gaussian test PDFs such as Log-Normal, Rice-Nakagami and Gamma-Gamma distributions

Results show excellent agreement of the PDF fit wasobtained by the method developed

The utility of reconstructed PDF relevant to free-space

laser communication is pointed out


14/44



Atmospheric Propagation Effects relevant to UV

CommunicationsMonte Carlo Impulse Response Model


15/44


Atmospheric Propagation Effects relevant to UV

Communications (contd..)

Parametric model (Gammafunction) :

3-DB bandwidth:


16/44


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


17/44


18/44


Underwater optical wireless communications

The present technology of underwater acoustic

communication cannot provide high data ratetransmission

Optical wireless communication has been

proposed as the best alternative to meet thischallenge

Using the scattered light it is possible to mitigate

the communication performance decrease due

to absorption only; thus a high data rateunderwater optical wireless is a feasible solution


19/44


Different communication scenarios

1. Line-of-sight communication link

2. A modulating retro reflector link

3. A reflective link


20/44


Underwater optical wireless communication channel

properties and link models

Reference: Shlomi Arnon, an underwater optical wireless communication Network, in Free-

Space Laser Communications IXedited 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:


21/44


Underwater optical wireless communication channel

properties and link models (contd..)

3. Reflective communication link:

Approximate received power:

where

Bit Error Rate (BER):

N b f h t d BER f ti f t itt i


22/44


Number of photons and BER as a function of transmitter receiver

separation for clean ocean water with extinction coefficient equal

0.15 m-1


23/44


Indoor Optical Communications

Optical wireless communications as a

complementary technology for short-range

communications


24/44


Different Indoor link configurations


25/44


indoor


26/44


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
http://iss.bu.edu/jbc/Publications/jbc-j7.pdfhttp://202.194.20.8/proc/PIMRC2008/content/papers/1569135393.pdfhttp://202.194.20.8/proc/PIMRC2008/content/papers/1569135393.pdfhttp://iss.bu.edu/jbc/Publications/jbc-j7.pdfhttp://iss.bu.edu/jbc/Publications/jbc-j7.pdfhttp://iss.bu.edu/jbc/Publications/jbc-j7.pdf


27/44


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 Xjusing 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 Sjand receiver Ri.

The signal received by receiver Riis

Source radiant intensity pattern:


28/44


Propagation Modeling (contd..)

Line of sight impulse response:

Where

is the distance between the source

and the receiver:, and Ariis the optical

collection area of the receiver.

Finally, for k bounces, the impulse response for each

source Sjis

Where and represent element n acting as a

receiver and a source, and is reflectiviytu of the

Lambertiam source


29/44


Typical Impulse Responses for a Transmitter and

Receiver separated by 0.8 m in a 4x4 m2room


30/44


Visible light communications: Indoor linksEmission 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


31/44


Recent developments and possibilities

bandwidth >~90MHz within typical room


32/44


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)

Fib i b d h i i h


33/44


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 andChaos< Vol.9, No.11,(1999)


34/44


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 10-7achieved


35/44


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 electronicfeedback:

Shows bistable behavior and can generate

chaotic oscillations

Signal Modulation/Encryption with AO Chaos

Basic schemes for optical comm nications ith


36/44


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!


37/44


Scintillation Mitigation Techniques for Free-Space

Optical Communications

Aperture Averaging

Spatial Diversity

Adaptive Optics Partially Coherent beams

Long Wavelength

Wavelength diversity Modulation Schemes

Scintillation Mitigation Techniques (contd )


38/44


Scintillation Mitigation Techniques (contd..)

Aperture Averaging


39/44


40/44



Spatial Diversity


41/44


BER for space time block code for four optical

transmitters


42/44



Adaptive Optics


43/44



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 )


44/44


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

Date post:	03-Jun-2018
Category:	Documents
Upload:	garry-mehrok
View:	227 times
Download:	0 times

Brno University Lecture3

Documents