What is fading? Explain small scale fading and multipath fading.Ripunjay Tiwari
Ans. Since the antenna height of the mobile unit is lower than its typical surrounding, and the carrier frequency wavelength is much less than the sizes of the surrounding structure multipath waves are generated. At the mobile unit, the sun of the multipath waves causes a signal-fading.
Multipath fading – multipath fading also called rayleigh fading in the mobile radio environment. When these multipath waves bounce back and fourth due to the buildings and houses, they from many standing-wave pairs In space. Those standing-wave pairs are summed together and become an irregular wave-fading structure. When a mobile unit is standing still, its receiver only receives a signal strength at that spot, so a constant signal is observed. When the mobile unit is moving the fading structure of the wave in the space is received. It is multipath fading. The recorded fading becomes fast as the vehicle moves faster (see fig. 1.15).
Small-scale fading- it is also known as fast fading. If the channel changes during the smallest time interval of interest for an applied signal the channel is said to be fast fading. Fast fading has movement over small distances of the order of λ/2 about 15 cm at 1 GHz.
Small scale fading is caused by changes in phase. It can result in significant phase changes in the scattered components within a ray and cause components. In fast fading the received signal amplitude/power fluctuated rapidly.