# Rayleigh channel, Les Channel and Gaussian channel model

Source: Internet
Author: User

1 definition and classification of mobile wireless channels
After all kinds of signals are sent out from the transmitter, all the paths experienced before reaching the receiving end are collectively referred to as channels. If a radio signal is transmitted, the path through which the electromagnetic wave travels is called a wireless channel. Compared with other communication channels, the wireless channel is the most complex one. The wireless communication environment is the basic factor affecting the wireless communications system. The wireless transmission path between the transmitter and receiver is very complicated by the reflection, diffraction and scattering propagation, which is caused by the spread of simple visual range and the encounter of complex objects such as buildings, mountains and forests. In addition, the movement of the mobile station in relation to the launch pad direction and speed, and even the two sides of the mobile object near the reception of the signal has a great impact. Therefore, this makes the wireless channel extremely random.
The decline of the mobile communication signal in the space propagation can be divided into 2 categories, that is, large scale fading and small scale fading, 1 shows. Large-scale fading is due to the attenuation of signal strength caused by the distance between the transmitter and the receiver and the shielding effect of the obstacles (such as hills, forests, buildings, etc.), which reflects the decrease in the average energy of the moving signal in the larger area or the path loss. The small-scale fading refers to the rapid change of the amplitude, phase and arrival angle of the receiving signal when the mobile station moves in a smaller range.

Figure 1 Classification of mobile wireless channels
The signal in the process of transmission, affected by various environments will produce reflection, diffraction and scattering, so that the arrival of the receiver signal is the superposition of many path signals, so that the superposition of these multipath signals in the absence of sight propagation in the case of the envelope obeys Rayleigh distribution. When the multipath signal contains a line of sight propagation path, the multipath signal obeys the LES distribution.

In simple terms:

`1. No direct path signal arrives at the receiving end, is the Rayleigh channel; mainly used to describe multipath channels and Doppler frequency shift phenomena`
`2. The channel is when the mobile station and the base station there is a direct wave signal, there is a main path, through the main path transmitted by the received signal is a stable amplitude ak and phase φk, the remaining multipath transmitted over the signal is still like "Rayleigh fading probability model" described.`
`3. Gaussian channel (AWGN) is mainly additive Gaussian white noise, used to describe the constant parameter channel, such as satellite communication, fiber channel, coaxial cable, etc.`
`Baidu Encyclopedia Information:`
`In the wireless communication channel environment, the intensity of the total signal is subject to the Rayleigh distribution after the propagation of the electromagnetic wave through the reflection refraction scattering and other paths reaches the receiver. At the same time, due to the movement of the receiver and other reasons, the signal strength and phase characteristics are also fluctuating, so called Rayleigh fading.`
`If the received signal in addition to the reflected refraction scattering and other signals, but also from the transmitter directly to the receiver (such as from the satellite directly to the terrestrial receiver) signal, then the total signal strength to obey the sub-Bryce, it is called the decline of the Lai.`
`In general, when a multi-channel signal arrives at the receiver, there is a relative time (inter) delay (late). If these relative delays are much less than the time of a symbol, then the multipath signal can be considered to reach the receiver almost simultaneously. Multipath does not cause interference between symbols in this case. This decline is called flat fading, because the frequency response of this channel is flat within the band used.`
`Conversely, if the relative delay of the multipath signal is not negligible compared with the time of a symbol, then when the multi-channel signal is iterated, the symbols of different times overlap, resulting in the interference between the symbols. This decline is called frequency-selective fading because the frequency response of this channel is uneven within the band used.`
`As for fast fading and slow fading, it usually refers to how quickly the signal changes relative to a symbolic time. Roughly speaking, if there is little change in the time of a symbol, it is considered to be slow fading. Conversely, if there is a significant change in the time of a symbol, it is considered to be fast fading. Theoretically, there is a strict mathematical definition of what is fast and what is slow.`

Rayleigh channel, Les Channel and Gaussian channel model

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