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Wireless channels are divided into different categories depending on its bandwidth, characteristics and mobility. The easiest is point to point fixed channel with line of sight (i.e. no object between the transmitter and receiver) and the worst case is a mobile user walking/driving with buildings obstructing the line of sight.
The parameters of the wireless channel are:
1) Attenuation: how much of the signal power is lost due to the distance. This is calculated via different equations such as Friss equation,2 rays model, Hata's equations ... etc. The most important parameters affecting the attenuation are the distance, frequency and antenna heights
2) Reflections: in a typical wireless link you receive a signal sent directly from the transmitter (if you have Line Of Sight LOS) along with copies of the same signal reflected from objects near the transmitter and/or the receiver. This is known as multipathing. These multipath copies travel longer distances than the LOS and hence may arrive in phase with the LOS signal (i.e. increasing the signal) or out of phase (hence cancelling the LOS signal). The results is a varying signal power at the receiver which is know as fading. If you have LOS, the fading is called Rician fading. If there is no LOS, then the fading is Rayleigh fading.
3) Bandwidth/data rate: If the bandwidth of the signal is large, the attenuation (fading) may not be equal for all frequencies. Such a channel is called frequency selective channel. Using in a frequency selective channel makes the receiver very complicated (has to use comething called equalizer or use complicated transmission techniques such as OFDM). Ideally you want the attenaution to be the same for the whole bandwidth. If the bandwidth of the signal is small such that all frequencies have approx. equal attenuation, the channel is known as flat fading channel. If the transmitter and/or the receiver are moving, the channel attenuation becomes variable with time. If the movement speed is high, the channel changes very quickly (in comparison to the data rate), the channel is said to experience fast fading. If the movement is slow (again compared to the data rate), the channel is expereincing slow fading.
There are a lot of other issues such as diffraction, absorption, Fresenl zones, Doppler shift, time and bandwidth coherence times and a lot of other parameters that affect the wireless link. You have to read a lot to fully understand it.
