2.1 Multipath propagation
2.2 Characterization of Fading Channels
2.2.1 Time variance and Doppler spread.
2.2.2 Frequency selectivity and delay spread
2.2.3 Time- and frequency-variant channels.
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2.1 Multipath propagation
One of the main characteristics of mobile radio is multipath propagation. The electromagnetic wave is scattered, diffracted, and reflected, and reaches the antenna via various ways as an incoherent superposition of many signals. This phenomenon causes so-called the fading effect, which is the fluctuation of the received signals. This fading effect can be categorizes into two as time-variance fading and time-spreading fading based on the cause of those effects. The change of displace of transmitter or receiver can make time-variance fading. Usually, how fast, or velocity, is mainly related to the time-variance fading. Hence, the time-spreading fading is deeply related to the Doppler frequency shift. This time-variance fading is again categorized into fast-fading and slow-fading based on the degree of fluctuation. In the case of the time-spreading fading, the different path length of multipath signals and subsequent delay time are the reason of this fading. Again, this time-spreading fading can be categorized into flat fading and frequency-selective fading based on whether the fading effect influences through whole signal or part of it.
We can explain the time-variance fading with the parameters. As explained above, the time-variance fading affected by the change of displacement of transmitter or receiver and deeply related to the Doppler frequency shift. Doppler frequency shift is the change of the frequency by the change of the velocity of transmitter or receiver. As a signal’s direction of the velocity is toward to the receiver, the receiver gets the signal earlier than when both the transmitter and receiver do not move. We can consider the increase of speed or decrease of speed by the calculation of two directional vectors of transmitter and receiver. In other words, the speed we consider is relative speed. The maximum Doppler frequency shift is given by
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