Mumbai University > Electronics and Telecommunication > Sem 7 > Mobile Communication

Marks: 10 M

Year: May 2013, May 2015

A. Fast fading:

• A channel is classified as fast fading or slow fading depending upon how the transmitted signal changes as compared to the rate of change of the channel. In multipath component fast fading occurs due to speed of mobile terminal and bandwidth of the signal.
• In fast fading the channel impulse response changes rapidly within the symbol duration i.e. the coherence time of the channel is smaller than the symbol period of the transmitted signal.
• This causes frequency dispersion due to Doppler spreading which leads to signal distortion.

• In the frequency domain, signal distortion due to fading increases with increasing Doppler spread relative to the bandwidth of the transmitted signal. Therefore a signal undergoes fast fading if

  Ts> Tc and Bs <bd <="" p="">

  Ts= symbol period of transmitted signal,

  Tc= coherence time of channel,

  Bs=signal bandwidth,

  BD= Doppler spread

  Thus due to Doppler spreading the frequency dispersion or time selective fading occurs.

• Fast fading only deals with the rate of change of channel due to motion. Fast fading channel is the channel in which the amplitude of the delta function varies faster than the rate of change of the transmitted baseband signal.

• In case of frequency selective fast fading channel the amplitudes, phase and time delays of any of one of the multipath components vary faster than the rate of change of the transmitted signal.
• Fast fading occurs for very low data rates.

B. Slow fading:

• In slow fading the channel impulse response changes at a rate much slower than the transmitted baseband signal. In this channel is assumed to be static over one or several bandwidth interval.

• In frequency domain in slow fading Doppler spread is much less than the bandwidth of the baseband signal. Thus the signal undergoes slow fading if

  Ts<< Tc and Bs >>BD

  Ts= symbol period of transmitted signal,

  Tc= coherence time of channel,

  Bs=baseband signal bandwidth,

  BD= Doppler spread.

• It occurs when the channel variations are slower than base station signal variation. If channel variations are less Doppler spread is less and thus slow fading takes place.