Fading

When fading ( fading ) is called by interference, shadowing, multipath and Doppler effect caused by fluctuations in the received signal strength at radio transmissions.

Causes

The interference is caused by that the radio signal phase achieved in several ways due to refraction or reflection at the ionosphere or terrestrial objects ( buildings, hills, etc.), the receiving antenna. Here, Selective carrier loss occur, the highly distorted modulation.

Fading, which occurs due to the superposition of different propagation phenomena of the terrestrial objects will be referred to as a slow, long-term or lognormal fading. That which occurs due to the frequency used, referred to as fast, short-term Rayleigh fading or by virtue of the multipath reception and the Doppler effect. Reception drops in fast fading occur periodically at intervals of about λ / 2. Thus the signal fades fast fading always depend on the frequency used.

Compensation

So-called fading Mitigation Techniques (FMT ) to compensate the fading so as to maintain the communications link in case of malfunctions.

To compensate for the fading at the reception of amplitude modulated radio transmitter ( long, medium and short wave ) the receivers are equipped with a so-called gain control. In practice, an average signal power is generated at the demodulator and applied as a control variable. With increasing signal voltage, the so-called control voltage increases by the IF branch is throttled in its amplification. A decreasing signal quality makes itself felt in increased noise, while the volume is adjusted.

To ensure data transmission of a radio link, it is not enough to increase the sensitivity of the receiver. Instead, be used:

• Adjusting the transmit power
• Adaptation of coding and modulation, see Adaptive Coding and Modulation
• Changing to a different frequency, see also Frequency hopping.

With the frequency of the disturbances in the atmosphere are increasing. While satellites in L-, S- and C-band send almost unhindered, the transmission characteristics of the K - bands strongly depend on the weather. By appropriately FMT frequencies can also be used above 20 GHz for data communication with satellites, without experiencing excessive restrictions in the rain.