What Is a Shading Coefficient?

In mobile communications, the shadow effect caused by obstructions decreases the strength of the received signal, but the median field strength changes slowly with geographic changes, also known as slow fading.

In mobile communications, the shadow effect caused by obstructions decreases the strength of the received signal, but the median field strength changes slowly with geographic changes, also known as slow fading.

Chinese name

Shadow fading

Foreign name

Shadow fading

Synonyms

Slow fading

Part of speech

noun

Shadow fading overview

The atmospheric refraction changes with time due to climatic reasons, resulting in a change in the path difference d of the reflected wave. As can be seen from Section 21.1.1, the phase difference also changes with time. At this time, even if the receiver is stationary, this change will cause the field strength at the receiving antenna to change. But this change is relatively slow, so it is called slow multipath fading. In mobile communications, the influence of slow multipath fading on the quality of received signals is generally not considered.

Because the movement of the mobile station will significantly change its surrounding environment (such as terrain and features), the degree of obstruction of obstacles such as buildings and woods on the radio wave propagation path will also vary. When the mobile station is in the shadow area, the signal field strength is weak; when the mobile station passes through the shadow area, the signal field strength is strong. This results in a relatively slow change in the median field strength of the received signal. This is shadow fading.

Log fading obeys the log-normal distribution.

Causes of shadow fading

(1) Path loss, which is the main cause of slow fading.

(2) Obstacles block the shadow area generated by electromagnetic waves, so slow fading is also called shadow fading.

(3) Weather changes, the relative speed of obstacles and mobile stations, and the operating frequency of electromagnetic waves.

In the environment of mobile communication transmission, radio waves encounter obstacles such as undulating hills, buildings, and woods on the propagation path, forming a shadow zone of the radio waves, which will cause a slow change in the median field strength and cause fading. This phenomenon is often referred to as the shadow effect, and the resulting fading is also referred to as slow shadow fading. In addition, due to changes in meteorological conditions, the radio wave refraction coefficient changes slowly with time, so that the median field strength of the signal received at the same location also changes slowly with time. However, because the slow change with time in land mobile communications is much smaller than the slow change with terrain, the slow change with time is often ignored in engineering design, and only the slow change with terrain is considered.

It is a loss due to a shadow effect caused by being blocked by a building or a hill on the radio wave transmission path. It reflects the loss caused by the average change of the reception level of hundreds of wavelengths in the medium range, and generally follows a log-normal distribution.

Shadow fading calculation

Shadow fading is a relatively slow macro change in nature compared to fast fading. Its fading rate has nothing to do with the operating frequency, but only depends on the moving speed of the mobile station. But the fading depth depends on the state of the obstacle and the frequency of signal operation.

Because the roles of the various objects on the propagation path are independent of each other, if there are N objects, the decibel values of the attenuation caused by each object are L1, L2, ..., LN, and the entire attenuation value can be expressed in decibels as

In the formula, Li is a random variable. According to the central limit theorem, when N is large, Li is a Gaussian random variable. Therefore, when the attenuation value caused by each object is represented by a true number, it follows a logarithmic normal distribution; if the attenuation value caused by each object is represented by a logarithm, it follows a normal distribution. That is, the slow fading in which the median field strength changes with the position of the mobile station follows a normal distribution (the signal is expressed in decibels)

In the formula: the signal U (t) is the decibel value of the median signal; m is the decibel value of the mean value of the signal U (t); and the decibel value of the standard deviation of the signal median U (t).

From the above conclusions, it can be seen that the problem of radio wave propagation in mobile communication systems is more complicated, so its propagation characteristics cannot be simply applied to the radio wave propagation mode of fixed-point wireless communications, but must be based on the characteristics of mobile communications and different propagation environments and terrain characteristics. Using statistical analysis combined with actual measurement methods, find the method of radio wave propagation under moving conditions to obtain a more accurate method for predicting signal field strength. Similarly, there are special requirements for the information transmission technology used in mobile communications.