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

Marks: 10 M

Year: Dec 2014

1. Small scale fading or simply fading is used to describe the rapid fluctuations of the amplitudes, phases, or multi path delays of radio signal over a short period of time or travel distance, so that large scale path loss effects may be ignored.
2. Fading is caused by interference between two or more versions of the transmitted signal which arrive at receiver at slightly different times.

3. These waves are called as multipath waves combine at receiver antenna to give a resultant signal which can very widely in amplitude and phase, depending on distribution of the intensity and relative propagation time of waves and bandwidth of transmitted signal

   • Three most important small scale fading effects are: Rapid changes in signal strength over a small travel distance or time interval
   • Random frequency modulation due to varying Doppler’s shifts on multi path signals
   • Time dispersion (echoes) caused by multi path propagation delay.

4. For narrow band signal small scale Fading typically results in a Rayleigh fading distribution of signal strength over small distances. The signal fluctuates in a range of about 40dB (10 dB above and 40 dB below the average signal)

5. Microscopic diversity techniques can be used to prevent deep fades from occurring. For e.g.-If two antennas are separated by fraction of a meter, one may receive a null while the other receives a strong signal. By selecting the best signal at all times, a receiver can reduce small scale fading effects.

6. Factors influencing small scale fading :

   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.