A transmitted signal changes as it travels to the receiver. The effect of these changes is commonly referred to as fading. The fading results from the superimposition of transmitted signals, in which differences in attenuation, delay and phase shift have occurred.
- 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 diversity.
If destructive interference occurs, the signal power can be significantly reduced. This phenomenon is called fading. Large-area fading represents the average attenuation of the signal power or the path loss due to movement over large areas.