Mobile Communication - Wireless Channel Fading
This post is a summary and analysis of various wireless channel fading.
Radio wave propagation effect
The wireless communication channel is a time-varying channel. When the radio signal passes through the channel, it will suffer from fading from different channels. The total power of the received signal is a combination of path loss, shadow fading and multipath effect. (multipath propagation: when radio waves encounter various obstacles, reflection, diffraction and scattering will occur, which will interfere with direct waves, that is, there are multiple paths between transceivers.)
- Path loss: the signal strength changes with distance (hundreds or thousands of wavelengths) in a large range, which should be proportional to the square of the distance, and in essence, it is a wave energy diffusion phenomenon.
- Shadowing: the median value of the signal level in the medium range changes slowly (hundreds of wavelengths). Due to the slow fading caused by the topographic relief in the propagation environment and the shielding of buildings and other obstacles, the median value of the signal changes slowly. The fading depth is related to the frequency and obstacles.
- Multipath effect (fading): the instantaneous value of the signal in a small range changes rapidly (tens of wavelengths). Due to the fast fading caused by multipath propagation, the instantaneous value of the field strength of the received signal changes rapidly.
Wireless channel fading
According to the propagation effect of radio wave, the fading of radio channel is generally divided into two categories: large-scale fading and small-scale fading (the small scale is generally the same order of magnitude as the signal wavelength). The scale refers to the size of time or distance.
Large-scale fading
(including transmission loss and shadow fading; large-scale fading is slow fading, but slow fading is not necessarily large-scale fading.)
Transmission loss (path loss): when a radio signal is transmitted through a large-scale distance channel, with the increase of the transmission path, the radio wave energy diffuses, resulting in the average power attenuation of the received signal. The attenuation is related to the transmission distance. The greater the distance, the more the attenuation.
Shadow fading: when the radio signal is transmitted in the medium-scale distance channel, the shadow area is formed behind the obstacles due to the undulation of the terrain or the blocking of tall buildings, resulting in the random change of the average power of the received signal. Its fading characteristics obey lognormal distribution.
Small-scale fading
(caused by multipath effect or Doppler effect. When the transmission channel changes in small scale (distance or time), the radio signal is reflected, diffracted and scattered by surrounding obstacles during transmission, and its amplitude or phase changes rapidly.)
According to the delay spread caused by multipath effect, small-scale fading is divided into frequency selective fading (the channel has constant gain and the bandwidth range of linear phase is smaller than the bandwidth of the transmitted signal) and frequency non selective / flat fading (the wireless channel bandwidth is larger than the bandwidth of the transmitted signal and has constant gain and linear phase within the bandwidth range);
According to the Doppler (frequency domain) spread generated by Doppler effect, small-scale fading is divided into fast fading (the coherence time of the channel is shorter than the period of the transmitted signal, and the bandwidth of the baseband signal is smaller than the Doppler spread) and slow fading (the coherence time of the channel is much longer than the period of the transmitted signal, and the bandwidth of the baseband signal is much larger than the Doppler spread).
