Rayleigh Fading Channel In Mobile Digital Communication System

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Rayleigh Fading Channel In Mobile Digital Communication System

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Rayleigh Fading Channel In Mobile Digital Communication System

  1. 1. Rayleigh Fading Channel in Mobile Digital Communication System Part I: Characterization Oum Saokosal Cambodian Graduate Student March 2009
  2. 2. Rayleigh Fading Channel <ul><li>We starts from additive white Gaussian noise (AWGN), with statistically independent Guassian noise sample corrupting data samples free of intersymbol interference (ISI). </li></ul><ul><li>Causes to degradation: </li></ul><ul><ul><li>Thermal noise at the receiver </li></ul></ul><ul><ul><li>External interference received by the antenna </li></ul></ul><ul><ul><li>Signal attenuation vs. distance </li></ul></ul>
  3. 3. Rayleigh Fading Channel <ul><li>Path loss or free space loss: </li></ul><ul><ul><li>L s (d) = ( 4  d ) 2 </li></ul></ul><ul><ul><li>where </li></ul></ul><ul><ul><li> L s (d): path loss </li></ul></ul><ul><ul><li> d: distance between transmitter and receiver </li></ul></ul><ul><ul><li>  : wavelength of the propagation signal </li></ul></ul><ul><li>For this case of idealized propagation, received signal power is very predictable. </li></ul>
  4. 4. Rayleigh Fading Channel <ul><li>In wireless sys, a signal can travels over multiple reflective paths, which is referred to as multipath propagation . It causes to multipath fading or scintillation . </li></ul><ul><ul><li>Scintillation described the multipath fading caused by physical changes in the propagation medium. </li></ul></ul>
  5. 5. Mobile Radio Propagation: Large-Scale and Small-Scale Fading <ul><li>Large Scale Fading at Fig. 1: </li></ul><ul><ul><li>Large scale fading due to motion over large areas </li></ul></ul><ul><ul><li>Mean signal attenuation vs. distance </li></ul></ul><ul><ul><li>Variation about the mean </li></ul></ul><ul><li>These above fadings are affected by prominent terrain contours. It is as being “ shadowing ”. </li></ul><ul><li>It described in terms of a mean-path loss ( n th-power law) & log-normally distribution variation . </li></ul>
  6. 6. Mobile Radio Propagation: Large-Scale and Small-Scale Fading <ul><li>Small Scale Fading at Fig. 1: </li></ul><ul><ul><li>Small-scale fading due to small changes in position </li></ul></ul><ul><ul><li>Time spreading of the signal </li></ul></ul><ul><ul><li>Time variance of the channel </li></ul></ul><ul><li>Small scale fading manifests itself in: </li></ul><ul><ul><li>Time-spreading of the signal (or signal dispersion) </li></ul></ul><ul><ul><li>Time-variant bahavior of the channel </li></ul></ul><ul><li>Small scale fading is also called Rayleigh fading </li></ul>
  7. 7. Mobile Radio Propagation: Large-Scale and Small-Scale Fading <ul><li>There are 3 basic mechanism impacting Signal Propagation in Mobile Com. Sys.: </li></ul><ul><ul><li>Reflection : a propagation electromagnetic wave impinges on a smooth face. </li></ul></ul><ul><ul><li>Diffraction : radio path is obstructed by a dense body. It is often termed “shadowing.” </li></ul></ul><ul><ul><li>Scattering : radio wave impinges on a large rough surface causing the reflected energy to scatter. </li></ul></ul>
  8. 8. Mobile Radio Propagation: Large-Scale and Small-Scale Fading <ul><li>(Fig.2)When estimating path loss for a link budget analysis in a cellular application: </li></ul><ul><ul><li>Mean path loss as a function of distance due to large-scale fading </li></ul></ul><ul><ul><li>Near-worse-case variation (6-10 dB) </li></ul></ul><ul><ul><li>Near-worse-case Rayleigh (20-30 dB) </li></ul></ul>
  9. 9. Mobile Radio Propagation: Large-Scale and Small-Scale Fading <ul><li>Generally: </li></ul><ul><ul><li>r(t) = s(t) * h c (t) , where: </li></ul></ul><ul><ul><li>* : convolution </li></ul></ul><ul><ul><li>r(t): receive signal </li></ul></ul><ul><ul><li>s(t): transmitted signal </li></ul></ul><ul><ul><li>h c (t): the impulse response of the channel </li></ul></ul>
  10. 10. Mobile Radio Propagation: Large-Scale and Small-Scale Fading <ul><li>In case of mobile radios: </li></ul><ul><ul><li>r(t) = m(t) x r 0 (t) , where: </li></ul></ul><ul><ul><li>m(t): large-scale-fading component or local mean or log-normal fading. </li></ul></ul><ul><ul><li>r 0 (t): Small-scale-fading component or multipath or Rayleigh fading. </li></ul></ul>

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