Small-Scale Fading and its effects
Small-scale fading, or simply fading, is used to describe the rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance, so that large-scale path loss effects may be may beignored.
Cause for Small Scale fading:
Fading is caused by interference between two or more versions of the transmitted signal which arrive at the receiver at slightly different times.
These waves, called multipath waves, combine at the receiver antenna to give a resultant signal which can vary widely in amplitude and phase, depending on the distribution of the intensity and relative propagation time of the waves and the bandwidth of the transmitted signal.
Small-scale fading effects:
- The three most important effects of small-scale fading are:
- Rapid changes in signal strength over a small travel distance or time interval
- Random frequency modulation due to varying Doppler shifts on different multipath signals
- Time dispersion (echoes) caused by multipath propagation delays
Figure 2.1Small-scale and large-scale fading
- Figure 2.1 shows typical rapid variations' in the received signal level due to small-scale fading as a receiver is moved over a distance of a few meters.
- Maintaining good communications can then become very difficult, although passing vehicles or people walking in the vicinity of the mobile can often disturb the field pattern, thereby diminishing the likelihood of the received signal remaining in a deep null for a long period of time.
- Antenna space diversity can prevent deep fading nulls
- Doppler shift: Due to the relative motion between the mobile and the base station, each multipath wave experiences an apparent shift in frequency.
The shift in received signal frequency due to motion is called the Doppler shift, and is directly proportional to the velocity and direction of motion of the mobile with respect to the direction