Introduction
During the lone range communication of critical radio links, the fading of radio waves will affect the communication distance. In the article, we will introduce it in details from its characteristics and classification.
The Fading Characteristics of Radio Waves
The characteristics of direct radiation, refraction, reflection, scattering, diffraction and absorption of radio waves make radio waves attenuate gradually with the increase of propagation distance.
(1) If radio waves propagate to larger and larger distances and spatial regions in free space, the energy of the radio waves will become more and more dispersed, causing diffusion attenuation (ie, path loss). It is defined as the ratio of the power density at a certain propagation distance from the radiation source to the power density at a unit distance, and its value is inversely proportional to the square of the propagation distance.
(2) While propagating in the medium, in addition to the diffusion attenuation, the radio wave energy will also be consumed by the medium, resulting in absorption attenuation and refraction attenuation. The refractive index n and absorption attenuation constant ɑ of electromagnetic waves propagating in different media are different.
The propagation path loss of the radio signal sent by the base station is greatly affected by the terrain and objects on the ground. The higher the base station is, the farther the signal will travel.
Radio wave propagation is also related to frequency. The higher the frequency, the greater the path loss, the weaker the diffraction ability, and the closer the propagation distance. Conversely for low frequencies.
Classification of radio wave fading
(1) According to the influence of fading on wireless signals, the fading of radio waves during transmission in wireless space is divided into 3 categories:
●Average Path Loss—The average received signal strength varies inversely with some power of increasing distance length.
●Shadow fading - when the radio wave encounters the electromagnetic field shadow caused by different obstacles on the propagation path, its local median level changes gently with location, time and moving speed, which is called shadow fading (due to the change slow, so it is also called slow fading).
●Multipath Fading - Fading due to multipath propagation. The amplitude and phase of the synthesized wave at the receiving station fluctuate rapidly with the movement of the mobile station. This phenomenon is usually called multipath fading (it is also called fast fading because the received signal strength changes rapidly).
(2) According to the magnitude of the field strength change of the received signal, the fading of radio waves is divided into 3 types:
●Large-scale fading - describes the attenuation of the signal caused by distance, and the characteristics of the received signal strength in a large-scale interval (hundreds or kilometers) that vary with the transmitting and receiving distance.
●Mesoscale Fading - The slow-moving characteristic of the median received signal strength over a medium-scale interval (hundreds of wavelengths).
●Small-scale fading—the rapid change characteristic of the instantaneous value of the field strength of the received signal in a small-scale interval (tens of wavelengths).
In a cellular environment there are two effects:
1. Multipath, short-term fast fading caused by reflection and scattering from building surfaces or other objects.
2. The long-term slow change in the strength of the dominant signal produced by the directly visible path. The channel works in the fast fading that obeys the Rayleigh distribution and is superimposed with the slow fading that the signal amplitude conforms to the logarithmic normal distribution.
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Post time: Aug-15-2023
