25-05-2012, 05:13 PM
Small Scale Fading
Small-Scale Fading.pptx (Size: 1.1 MB / Downloads: 33)
Rapid Changes in Signal Strength, Random frequency modulation, time dispersion (Echoes) that is :
Rapid fluctuations of radio signal amplitude, phase, or delays
Occurs on short time period or short travel distance
Large-scale path loss effects can be ignored
Caused by arrival of two or more waves from the source combining at the receiver
Resultant detected signal varies widely in amplitudes and phase
Bandwidth of transmitted signal is important factor
Small-scale Multipath Propagation
Fading: The rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance.
Fading is caused by interference between two or more versions of the transmitted signal, which arrive at slightly different times.
Multipath in the radio channel creates small-scale fading effects.
Phenomenon :
1. Rapid changes in signal strength over a small travel distance or time interval.
2. Random frequency modulation due to varying Doppler shifts on different multipath signals.
3. Time dispersion caused by multipath propagation delays.
If objects in the radio channel are static, and motion is considered to be only due to that of the mobile, then fading is purely a spatial phenomenon.
Antenna space diversity can prevent deep fading nulls.
Factors influencing Small-scale fading
Multipath propagation: multipath propagation often lengthens the time required for the baseband portion of the signal to reach the receiver which can cause signal smearing due to inter-symbol interference.
Draw a figure to explain ISI
Speed of the mobile: generate random Doppler shifts.
Train passing
Speed of surrounding objects: if the surrounding objects move at a greater rate than the mobile, then this effect dominates the small-scale fading.
The transmission bandwidth of the signal: if signal’s bandwidth bandwidth of the multipath channel received signal will be distorted.
The coherent bandwidth is a measure of the maximum frequency difference for which signals are still strongly correlated in amplitude.
Doppler Shift
Distance difference
Phase difference
Doppler frequency shift
Frequency shift is positive when mobile moves toward source
In a multipath environment, frequency shift for each ray may be different, leading to a spread of received frequencies.
For example, for pure sinusoid, the signal blurred in frequency.