Fading is most important thing when you design any RF Design lets understand what is fading and how its effect.

  • The communication between the base station and mobile station in mobile systems is mostly non-LOS.
  • The LOS path between the transmitter and the receiver is affected by terrain and obstructed by buildings and other objects.
  • The mobile station is also moving in different directions at different speeds.
  • The RF signal from the transmitter is scattered by reflection and diffraction and reaches the receiver through many non-LOS paths.
  • This non-LOS path causes long-term and short term fluctuations in the form of log-normal fading and rayleigh and rician fading, which degrades the performance of the RF channel.

fading effectLONG TERM FADING

  • Terrain configuration & man made environment causes long-term fading.
  • Due to various shadowing and terrain effects the signal level measured on a circle around base station shows some random fluctuations around the mean value of received signal strength.
  • The long-term fades in signal strength, r, caused by the terrain configuration and man made environments form a log-normal distribution, i.e the mean received signal strength, r, varies log-normally in dB if the signal strength is measured over a distance of at least 40l.
  • Experimentally it has been determined that the standard deviation, s, of the mean received signal strength, r, lies between 8 to 12 dB  with the higher s generally found in large urban areas.

RAYLEIGH FADING

  • This phenomenon is due to multipath propagation of the signal.
  • The Rayleigh fading is applicable to obstructed propagation paths.
  • All the signals are NLOS signals and there is no dominant direct path.
  • Signals from all paths have comparable signal strengths.
  • The instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna.

RAYLEIGH FADING

RICEAN FADING

  • This phenomenon is due to multipath propagation of the signal.
  • In this case there is a partially scattered field.
  • One dominant signal.
  • Others are weaker.

 RICEAN FADING