无线通信中，信号因多路径传播经历反射、折射和散射，导致接收端接收到多个延迟不同、幅度不一的信号副本，这种现象称为延迟传播。延迟传播影响了无线系统的相干带宽和相干时间。另一方面，信号在传播过程中由于路径差异产生多普勒频移，导致接收信号的频谱展宽，这一现象称为多普勒频移，也称为衰落率。这两个因素对于评估无线系统的性能至关重要。
Delay Spread:
As we know that radio frequency signal takes different path to reach the destination due to multiple paths(unbounded medium). This multiple paths cause reflection, refraction and scattering of radio signal. Hence when the signal is transmitted from one place to the other, multiple copies of the signal is received with different amplitudes and different delays (leads to different time of arrival) at the receiver.
For example, if an impulse is transmitted then it will be no longer a impulse when it is received at the other end, but it will become a pulse with spreading effect. The effect which makes this spreading of signal is known as Delay spread.
To measure performance of a wireless system different scenarios from low to medium to high delay spreads are considered for test purpose. Delay spread helps determine coherence bandwidth and coherence time of a wireless system.
Doppler Spread:
Doppler frequency shift is usually different from path to path when signal arrives at the wireless receiver. Hence transmitted signal frequency will experience Doppler spreading and is seen as spectral widening or broadening in received signal power spectrum. This width of the spectrum is known as Doppler Spread or fading bandwidth.
Doppler Spread is also known as fading rate.
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