Delay Spread and Doppler Spread

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.