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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 generalized-Laguerre fit to data LN5000 with 6 moments: 1 5000 data values fit nrm 0.9 histogram 0.8 0 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