Small Scale Fading - Wireless and Mobile Communication

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Small Scale Fading  - Wireless and Mobile Communication Powered By Docstoc
					            Small-Scale Fading
Also known by other names such as
    ‘Fading’; multipath and Rayleigh fading
Rayleigh fading is a result of constructive and destructive interference
  between several versions of the same signal at the receiver, leading
  to attenuation of signal power or amplitude
    – Usually over a fraction of the signal wavelength
    – Attenuation between 20 to 30 dB
    – multipath fading manifests as time spreading or time variation of the
      signal (due to motion, foliage, reflections and scattering)
         Rayleigh Distribution
If the impulse response h( , t) of the mobile
  radio station is time invariant and has zero
  mean, then the envelope of the impulse
  response has a Rayleigh distribution given as:
                          r       r2 
                   pr   2 exp  
                                  2 2 
                                        
                                      
where      is the total power in the multipath
  signal
                     Rice Fading
If however the impulse response has a non zero mean then there is a
   significant component of the direct path (line of sight, specular
   component) signal and the magnitude of the impulse response has a
   Ricean distribution
Ricean distribution is the combination of Rayleigh signal with the direct
   line of sight signal. The distribution is:
                         r       r 2  s 2   rs 
                 pr   2 exp  
                                           I
                                        2  0      
                                   2    2 
s2 is the power of the line of sight signal and I0 is a Bessel function of the
    first kind
      Characteristics of Small-Scale
                              Fading
Small-scale fading occurs as either of 4 types:
      • frequency selective fading in which the bandwidth of the signal
        is greater than the coherence bandwidth and the delay spread is
        greater than the symbol rate; Signals at some frequency
        components experience more fading than others - (caused by
        multipath delay spread)
      • flat fading when the bandwidth of the signal is less than the
        coherence bandwidth and the delay spread is less than the
        symbol rate - (caused by multipath delay spread)
      • fast fading when the Doppler spread is high and the coherence
        time is less than the symbol period and
      • slow fading with a low Doppler spread and coherence time is
        greater than the symbol period - (caused as well by Doppler
        spread)
              Summary of Small-scale fading
     Correct for small-scale fading with
     • adaptive equalizers
     • modulation techniques such as spread
       spectrum

Type of fading               Frequency Effects               Time Effects
                             Effects of Multipath Delay Spread
Frequency Selective fading   BW of signal > coherence BW     Delay spread > symbol period
Flat fading                  BW of signal < coherence BW     Delay spread < symbol period
                                 Effects of Doppler Spread
Slow fading                  Low Doppler spread              Coherence time > symbol period
Fast fading                  High Doppler spread             Coherence time < symbol period
        Propagation of Cellular
        Communication Signals
• Cellular communication is mostly land
  based.
     • There are a few applications on ships and airlines
       using networks in a box and satellites

• Propagation considerations are therefore
  based on
  – urban, rural, suburban and
  – in a few cases water and desert terrain
  – sky or space propagation (satellites)
   Free Space Propagation Model
• Free space propagation Model is used to
  predict received signal stength when the
  transmitter and receiver have clear,
  unobstructed line-of-sight path.
• The free space model predicts that received
  power decays as function of TR separation
  distance raised to some power.
                            Pr
             Pt

                   Lp



              Gt            Gr


                        d



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