Signal Fading


  • Slow fading: In case shadow effect is caused by obstacles, and the receiving
    signal strength decreases but the field strength mid-value changes slowly with
    the change of the topography, the strength decrease is called “slow fading” or
    “shadow fading”. The field strength mid-value of slow fading takes on a
    logarithmic normal distribution, and is related to location/locale.  The fading speed is dependent on the speed of the mobile station.
  • Fast fading: In case the amplitude and phase of the combined wave change
    sharply with the motion of the mobile station, the change is called “fast
    fading”. The spatial distribution of deep fading points is similar to interval of
    half of wavelength. Since its field strength takes on Rayleigh distribution, the
    fading is also called Rayleigh fading. The amplitude, phase and angle of the
    fading are random.
  • Fast fading is subdivided into the following three categories:
    Time-selective fading: In case the user moves quickly and causes Doppler
    effect on the frequency domain, and thus results in frequency diffusion, timeselective
    fading will occur.
  • Space-selective fading: The fading features vary between different places and
    different transmission paths.
    Frequency-selective fading: The fading features vary between different
    frequencies, which results in delay diffusion and frequency-selective fading.
  • In order to mitigate the influence of fast fading on wireless communication,
    typical methods are: space diversity, frequency diversity, and time