Compare impulse response of channel with frequency selective fading

Hello Researchers, In this section we will discuss about “Compare impulse response of channel with frequency selective fading“. In Mobile Radio Propagation specially in Wireless communication it is very much important topic for Researchers.  


The channel creates frequency selective fading on the received signal when the channel possesses a constant gain and linear phase response over a bandwidth, which is smaller than the bandwidth of the transmitted signal.

⇒ Under these conditions the channel impulse response has a multipath delay spread which is greater than the reciprocal bandwidth of the transmitted message waveform

⇒ So the received signal includes multiple versions of the transmitted waveform, which are attenuated and delayed in time, and hence the received signal is distorted.

Figure: Frequency Selective Fading


  • For frequency selective fading, the spectrum S(f) of the transmitted signal has a bandwidth which is greater than the coherence bandwidth Bcof the channel
  • Frequency selective fading is caused by multipath delays which approach or exceed the symbol period of the transmitted symbol. Thus the channel induces intersymbol interference (ISI).
  • These channels are also known as wideband channels since the bandwidth of the signal s(t) is wider than the bandwidth of the channel impulse response
  • As time varies, the channel varies in gain and phase across the spectrum of s(t), resulting in time varying distortion in the received signal r(t)


Signal undergoes Frequency Selective Fading if:

  • Bs>Bc


Bs is bandwidth and

Bc is the coherence bandwidth of the channel

  •  Ts<st


Ts is the reciprocal bandwidth and

st rms delay spread.


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