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Prof. Brandt-Pearce Lecture 2 Channel Modeling PowerPoint PPT Presentation


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Optical Wireless Communications. Prof. Brandt-Pearce Lecture 2 Channel Modeling. Channel Effects. Attenuation (Loss) Absorption Scattering Rayleigh scattering (atmospheric gases molecules) Mie scattering (aerosol particles ) Beam divergence Pointing Loss

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Prof. Brandt-Pearce Lecture 2 Channel Modeling

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Prof brandt pearce lecture 2 channel modeling

Optical Wireless Communications

Prof. Brandt-Pearce

Lecture 2

Channel Modeling


Prof brandt pearce lecture 2 channel modeling

Channel Effects

  • Attenuation (Loss)

    • Absorption

    • Scattering

      • Rayleigh scattering (atmospheric gases molecules)

      • Mie scattering (aerosol particles)

    • Beam divergence

    • Pointing Loss

  • Atmospheric (refractive) turbulence

    • Scintillation

    • Beam wander

  • Background light (Sun)


Prof brandt pearce lecture 2 channel modeling

Attenuation

  • Atmospheric attenuation: loss of part of optical energy when traversing atmosphere

    • : Transmitted Power

    • : Received Power

    • : Path Length

  • Attenuation is due to absorption and/or scattering

    • : molecular absorption coefficient

    • : Aerosol absorption coefficient

    • : molecular scattering coefficient

    • : Aerosol scattering coefficient

  • An aerosol is a suspension of solid or liquid particles in a gaseous medium, with sizelarger than a molecule.


Prof brandt pearce lecture 2 channel modeling

Attenuation

Weather conditions and their visibility range values 1

1Free-space optics by Willebrand and Ghuman, 2002


Prof brandt pearce lecture 2 channel modeling

Attenuation

Signal Attenuation coefficient at λ = 850 nm.

Thick fog

Clear air


Prof brandt pearce lecture 2 channel modeling

Attenuation

  • Low Clouds

    • Very similar to fog

    • May accompany rain and snow

  • Rain

    • Drop sizes larger than fog and wavelength of light

    • Extremely heavy rain (can’t see through it) can take a link down

    • Water sheeting on windows

  • Heavy Snow

    • May cause ice build-up on windows

    • Whiteout conditions

  • Sand Storms

    • Likely only in desert areas; rare in the urban core


Prof brandt pearce lecture 2 channel modeling

Attenuation due to Absorption

  • Absorption: the energy of a photon is taken by gas molecules or particles and is converted to other forms of energies

  • This takes place when there is an interaction between the propagating photons and molecules (present in the atmosphere) along its path

  • Primarily due to water vapor and carbon dioxide

  • Wavelength dependent

  • This leads to the atmosphere having transparent zones (range of wavelengths with minimal absorptions) referred to as the transmission windows

  • It is not possible to change the physics of the atmosphere, therefore, wavelengths adopted in FSO systems are basically chosen to coincide with the atmospheric transmission windows


Prof brandt pearce lecture 2 channel modeling

Attenuation due to Absorption

Atmospheric absorption transmittance at sea level over 1820 m horizontal path1

1Free-space optics by Willebrand and Ghuman, 2002


Prof brandt pearce lecture 2 channel modeling

Attenuation due to Scattering

  • Scattering: dispersion of a beam into other direction due to particles in air

  • This results in angular redistribution of the optical field with and without wavelength dependence

  • Depends on the radius of the particles

  • Two type of scattering:

    • Rayleigh scattering (Molecule): elastic scattering of light by molecules and particulate matter much smaller than the wavelength of the incident light.

    • Mie Scattering (Aerosol): broad class of scattering of light by spherical particles of any diameter.

  • Scattering phase function at angle θis (μ=cos θ)1

  • 1Zachor, A. S., “Aureole radiance field about a source in a scattering-absorbing medium,” Applied Optics, (1978).


    Prof brandt pearce lecture 2 channel modeling

    Rayleigh Scattering (Molecular)

    • Elastic scattering of light by molecules and particulate matter much smaller than the wavelength of the incident light.

    • Rayleigh scattering intensity has a very strong dependence on the size of the particles (it is proportional the sixth power of their diameter).

    • It is inversely proportional to the fourth power of the wavelength of light: the shorter wavelength in visible white light (violet and blue) are scattered stronger than the longer wavelengths toward the red end of the visible spectrum.

    • The scattering intensity is generally not strongly dependent on the wavelength, but is sensitive to the particle size.

    • Responsible for the blue color of the sky during the day


    Prof brandt pearce lecture 2 channel modeling

    Rayleigh Scattering

    • For a single molecule, the scattering phase function at angle θ is 1

    • where

    • ρ is the depolarization parameter

    • A simplified expression describing the Rayleigh scattering 1

    • : number of particles per unit volume

    • : the cross-sectional area of scattering

    1Bucholtzr, A., “Rayleigh-scattering calculations for the terrestrial atmosphere,” Applied Optics 34 (1995).


    Prof brandt pearce lecture 2 channel modeling

    Mie Scattering (Fog. Haze, Rain)

    • Broad class of scattering of light by spherical particles of any diameter.

    • The scattering intensity is generally not strongly dependent on the wavelength, but is sensitive to the particle size.

    • Mie scattering intensity for large particles is proportional to the square of the particle diameter.

    • Coincides with Rayleigh scattering in the special case where the diameter of the particles is much smaller than the wavelength of the light; in this limit, however, the shape of the particles no longer matters.

    • The scattering phase function at angle θ is 1

    • g: aerosol asymmetry parameter given by the mean cosine of the scattering angle

    • f: aerosol hemispheric backscatter fraction

    1Zachor, A. S., “Aureole radiance field about a source in a scattering-absorbing medium,” Applied Optics, (1978).


    Prof brandt pearce lecture 2 channel modeling

    Attenuation due to Beam Divergence

    • One of the main advantages of FSO systems is the ability to transmit a very narrow optical beam, thus offering enhanced security But due to diffraction, the beam spreads out This results in a situation in which the receiver aperture is only able to collect a fraction of the beam.

    • The remaining uncollected beam then results in beam divergence loss


    Prof brandt pearce lecture 2 channel modeling

    Attenuation due to Beam Divergence

    • Transmitter effective antenna gain:

    • : Diffraction limited beam divergence angle in radians

    • : Aperture diameter

    • In diffuse channels and FSO networks, is non-diffraction limited and determined by transmitter optics

    • : Radiation solid angle

    • Receiver effective antenna gain:

    • : Receiver effective aperture areas

    • Free-space path loss:

    • : Path length

    • For transmitted power , received power, , is (Friistransmission equation)


    Prof brandt pearce lecture 2 channel modeling

    Attenuation due to the Pointing Loss

    • When the received signal is not centered on the detector, a part of received signal may fall outside the detector area

    • Additional power penalty is usually incurred due to lack of perfect alignment of the transmitter and receiver

    • For short FSO links (<1 km), this might not be an issue

    • For longer link ranges, this can certainly not be neglected

    • Misalignments could result from building sway or strong wind effect on the FOS link head stand

    • The ratio of the received beam spot size and detector area becomes important

    • Lenses and their focal length play an important role in determining the spot size

    • Small spot size requires low receiver field of view (FoV)


    Prof brandt pearce lecture 2 channel modeling

    Total Link Loss

    • Atmospheric link with receive spot larger than the receive aperture:

    • ηt: transmit optics efficiency

    • ηA: transmit aperture illumination efficiency

    • At: effective area of transmit optics

    • Ar: effective area of receive optics

    • ηr: receive optics efficiency

    • Ltp; transmit pointing loss

    • Lrp: receive pointing loss

    • Latm: atmospheric loss

    • Lpol: polarization mismatch

    • L: link length

    16


    Prof brandt pearce lecture 2 channel modeling

    Attenuation: Link Budget Example

    Example

    Typical link budget for 2.5 Gbps, 2 km link, and 1550 nm wavelength

    “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.


    Prof brandt pearce lecture 2 channel modeling

    Turbulence

    • Beam spreading and wandering due to propagation through air pockets of varying temperature, density, and index of refraction.

    • Almost mutually exclusive with fog attenuation.

    • The interaction between the laser beam and the turbulent medium results in random phase and amplitude variations of the information-bearing optical beam which ultimately results in fading of the received optical power

    • Results in increased bit-error-rate (BER) but not complete outage.


    Prof brandt pearce lecture 2 channel modeling

    Turbulence

    • Atmospheric turbulence results in random fluctuation of the atmospheric refractive index

    • Lens-like eddies result in a randomized interference effect between different regions of the propagating beam causing the wavefront to be distorted in the process


    Prof brandt pearce lecture 2 channel modeling

    Turbulence

    • Atmospheric turbulence effects include

      • Beam wander: caused by a large-scale turbulence

      • Beam scintillation

      • In imaging detector they causes speckle pattern


    Prof brandt pearce lecture 2 channel modeling

    Turbulence – Experimental Results

    21


    Prof brandt pearce lecture 2 channel modeling

    Turbulence

    • Due to the turbulence a fluctuation is introduced on the received irradiance

    • A measure of irradiance fluctuations can be given by the scintillation index:

    • For weak fluctuations, it is proportional, and for strong fluctuations, it is inversely proportional to the Rytov variance:

      • is the refractive-index structure parameter

    Y. Tian, S.G. Narasimhan, A. J. Vannevel ,Proc. of Computer Vision and Pattern Recognition (CVPR), Jun, 2012.


    Prof brandt pearce lecture 2 channel modeling

    Turbulence

    • Three most reported models for irradiance fluctuation in turbulent channels:

      • Log-normal (weak regimes)

      • Gamma–gamma (weak-to-strong regimes)

      • K-distribution (very strong regimes)

      • Negative exponential (saturated regimes)


    Prof brandt pearce lecture 2 channel modeling

    Turbulence

    Values of α and β under different turbulence regimes: weak, moderate to strong and saturation

    Log-normal

    Negative exponential

    Gamma–gamma


    Prof brandt pearce lecture 2 channel modeling

    Mitigating Turbulence Effects

    • Multiple Transmitters Approach

      (Courtesy Jaime Anguita: Ref. Jai Anguita, Mark A. Neifeld and Bane Vasic, “Multi-Beam Space-Time Coded Communication Systems for Optical Atmospheric Channels,” Proc. SPIE, Free-Space Laser Communications VI, Vol. 6304, Paper # 50, 2006)

    • Aperture averaging and multiple beams is effective in reducing scintillation, improving performance

    • Adaptive Optics approach can be incorporated to mitigate turbulence effects for achieving free space laser communications


    Prof brandt pearce lecture 2 channel modeling

    Background Light

    • In FSO systems is divided into two types

      • Localized point sources, such as the Sun

        • Irradiance (power per unit area):

        • W(λ): the spectral radiant emittanceof the sun

      • Extended sources, such as sky or lighting in urban areas

        • Irradiance:

        • N(λ): spectral radiance of the sky

        • Ω: photodetector’sfield of view angle in radians

    • Celestial bodies such as stars affect deep space FSO systems


    Prof brandt pearce lecture 2 channel modeling

    Other Effects

    • There can be other effects

      • Dispersion: wavelength dependence of refraction index can cause optical signals with different wavelengths travel with different speed.

      • Multipath: reflections can occur for low altitude beams, especially from sea surface for shipboard applications and for underwater FSO links

      • Nonlinearity: strong transmitted powers can cause nonlinear effects in the channel


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