Some challenging areas in Free-Space Laser Communications

Dr. Arun K. Majumdara.majumdar@IEEE.org

Lecture Series,: 3

Brno University of Technology, Brno Czech RepublicDecember 1-6, 2009

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

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

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

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

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

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, Ө0 - Rytov Variance, σr2 - Greenwood Frequency, fG

Atmospheric Models

Hufnagel-Valley (HV) model

Modified Hufnagel-Valley (MHV) model:

•SLC-Day model:

CLEAR1 model:

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

Cn

2

M i s s i o n D a y / T i m e [ D a y s ]

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.61 10

18

1 1017

1 1016

1 1015

1 1014

MeasuredHufnagel-ValleyModified Hufnagel-ValleySLC-DayCLEAR1 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 2.61 10

18

1 1017

1 1016

1 1015

1 1014

MeasuredHufnagel-ValleyModified Hufnagel-ValleySLC-DayCLEAR1 Night

Cn2 Profile Comparison

Altitude (Km)

Cn2

(m

^-2/

3)

Comparison of ) Cn2 profile generated from tethered-blimp instrument measurement and various models.

Histogram of Cn2 : some typical examples

14.5 14 13.5 13 12.5 12 11.50

2

4

6

8

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

FR

EQ

UE

NC

Y (

%)

15.5 15 14.5 14 13.5 13 12.50

5

10

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

FR

EQ

UE

NC

Y (

%)

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

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 fit 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

Intensity

CDF

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

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

Review of Results and Conclusions

• Atmospheric Propagation Effects relevant to UV Communications

Monte Carlo Impulse Response Model

Atmospheric Propagation Effects relevant to UV Communications (contd..)

Parametric model (Gamma function) :

3-DB bandwidth:

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

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

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

Different communication scenarios

1. Line-of-sight communication link

2. A modulating retro reflector link

3. A reflective link

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:

Underwater optical wireless communication channel properties and link models (contd..)

3. Reflective communication link:

Approximate received power:

where

Bit Error Rate (BER):

Number of photons and BER as a function of transmitter receiver separation for clean ocean water with extinction

coefficient equal 0.15 m-1

Indoor Optical Communications

• Optical wireless communications as a complementary technology for short-range communications

Different Indoor link configurations

indoor

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

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

Source radiant intensity pattern:

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 is reflectiviytu of the Lambertiam source

Typical Impulse Responses for a Transmitter and Receiver separated by 0.8 m in a 4x4 m2

room

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

Recent developments and possibilities

– bandwidth >~90MHz within ‘typical’ room

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)

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)

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

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

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!

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..)• Aperture Averaging

Multiple-beam Free-Space Optical Communications

Scintillation Mitigation Techniques (contd..)

•Spatial Diversity

BER for space time block code for four optical transmitters

Scintillation Mitigation Techniques (contd..)

• Adaptive Optics

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 )

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