Multipath and Fading
Application: Typical terrestrial cellular and PCS Propagation Paths fade, that is, at times the signal power is much lower than it is at other times separated by just tens of milliseconds. However, reliable communications performance must be maintained even during the fades when the signal power is weak. Interleaving and deinterleaving, FEC, and wideband Pseudo-random Noise (PN) Spread Spectrum Modulation are all used in part to achieve reliable communications in spite of fading.
Multipath and Fading
Example: Assume a signal arrives at the receiver over paths of lengths Dm equal to 2000, 2900, and 3600 feet with m equal to 1, 2 and 3 respectively. The three phase values in radians resulting from these delays equal 2pfcDm/c, where c is the speed of light. But since the wavelength l at the carrier center frequency equals c/fc, the three phase values equal 2pDm/l. Only the fractional portion of phase modulo 2p matters in determining the vector sum. At a carrier center frequency fc of 881.52 MHz, the wavelength equals about 1.1163 feet. In this case, the three multipath components add together with angles of 40.53, 55.90, and 4.19 degrees. The magnitude of the vector sum depends on the magnitudes of the individual signal components. Now suppose the paths lengths change by just one foot to 1999, 2901, and 3601 feet. The phases of the three signal components in degrees now equal 46.49, 49.93, and 55.52 respectively. The vector sum may change significantly with changes in the path distances of only 1 foot at frequencies used by terrestrial cellular systems.
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