Small scale fading is defined as rapid fluctuations of the amplitude, phase of a signal over short period f time interval. The factors influencing small scale fading are:
A. Multipath propagation:
- Reflecting objects and scattering in the channel creates a constantly changing environment that dissipates the signal energy in amplitude, phase and time. This results in multiple version of transmitted signal that arrive at the receiving antenna, displaced with respect to one another in time and space.
- The random phase and amplitude of the different multipath components cause fluctuations in signal strength causing small scale fading.
- Multipath propagation increase the time required for the baseband portion of signal to reach the receiver which cause signal smearing due to intersymbol interference.
B. Speed of mobile:
- The relative motion between the base station and the mobile results in random frequency modulation due to different Doppler shifts on each of the multipath components.
- Doppler shift is positive if mobile receiver is moving toward the base station and is negative if mobile receiver is moving away from the base station.
C. Surrounding of mobile:
- If objects in the radio channel are in motion, they induce varying Doppler shift on multipath components.
- If the surrounding objects move at a greater rate than the mobile, then this effect dominates the small-scale fading. Otherwise motion of surrounding objects can be ignored.
D. Transmission bandwidth of signal:
- If transmitted radio signal bandwidth is greater than the bandwidth of the multipath channel, the received signal will be distorted but the received signal strength will not fade much over a local area.
- If transmitted radio signal bandwidth is less than the bandwidth of the multipath channel, the received signal will not be distorted but the received signal strength will change rapidly.
- Bandwidth of the channel is quantified by the coherence bandwidth which is related to specific multipath structure of the channel. Coherence bandwidth is measure of maximum frequency difference for which signals are strongly correlated in amplitude.