Fading
From Free net encyclopedia
- This is about the phenomenon of loss of signal in telecommunications. For the book, see Fading (Book)
Fading (or fading channels) are mathematical models for the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media. Short term fading is due to multipath propagation, and is also known as multipath induced fading. Fading results from the superposition of transmitted signals which have experienced differences in attenuation, delay and phase shift while travelling from the source to the receiver.
For example, consider the common experience of stopping at a traffic light and hearing a lot of static on your radio, which is immediately corrected if you move less than a meter. Cellular phones also exhibit similar momentary lapses. The reason for these losses of signal is the destructive interference that multiple reflected copies of the signal makes with itself. To understand how a signal can destructively interfere with itself, consider the sum of two sinusoidal waveforms (which are similar to carrier modulated signals) with different phases.
The best way to combat fading is to ensure that multiple versions of the same signal are transmitted, received, and coherently combined. This is usually termed diversity, and sometimes acquired through multiple antennas. Mathematically, the simplest model for the fading phenomenon is multiplication of the signal waveform with a time-dependent coefficient which is often modeled as a random variable, making the received signal to noise ratio a random quantity.
Fading channel models are often used to model electromagnetic transmission of information over wireless media such as cellular phones, and broadcast communications. However, even for underwater acoustic communications the notion of fading is useful in understanding the distortion caused by the medium.
Small-scale fading is usually divided into fading based on multipath time delay spread and based on Doppler spread.
There are two types of fading based on multipath time delay spread:
- Flat fading, where the bandwidth of the signal is less than the coherence bandwidth of the channel or the delay spread is less than the symbol period.
- Frequency selective fading, where the bandwidth of the signal is greater than the coherence bandwidth of the channel or the delay spread is greater than the symbol period.
There are two types of fading based on doppler spread:
- Fast fading, which has a high doppler spread, and the coherence time is less than the symbol period, and the channel variations are faster than baseband signal variations.
- Slow fading, which has a low doppler spread. The coherence time is greater than the symbol period and the channel variations are slower than the baseband signal variations.
In addition to the small scale fading that is described above, for which the change in the signal strength occurs on the order of a fraction of a meter, the signal can also undergo shadow fading, or shadowing. This is due to the presence of obstacles between the transmitter and the receiver, and the scale of distance required to experience shadowing is about an order of magnitude larger than that of multipath fading.
Examples of fading with reference to the distribution of the attennuation are:
Techniques used to overcome signal fading: