Describe fading in mobile environment in detail.Ripunjay Tiwari
Ans. Multipath waves in the radio channel combine at the receiver to give a resultant signal that can vary widely in amplitude and phase over a short period of time or over a small travel distance. This short- term rapid fluctuation of signal strength is superimposed on the local mean slow varying field due to log normal large scale power loss as shown in fig. 1.16.
Mobile radio systems cause time dispersion in the pulse due to their multipath nature. In addition, systems mobility modulates the carrier frequency due to random Doppler shift.
In such system, the signal received by the mobile at any point in space may consist of a large number of plane waves having randomly distributed amplitudes, phases and angles of arrival. These multipath fields combine vectorially to cause the signal strength to fluctuate with time due to the dynamic nature of the surroundings and mobile radios.
The mechanism of fading normally broken down into two different categories based on the position of the receiver relative to the transmitter. Fig. 1.17. Shown the differentiate between the small and large scale fading effects and the reasons for these effects.
(i) Large-scale Fading – It is due to the following reasons –
- Attenuation in Free Space – Power degrades with 1/d2.
- Shadows – Signals are blocked by obstructing structures.
The large-scale fading essentially represents the average attenuation of a wireless signal as it travel a long distance (several hundred wavelengths or more). Degradation due to the blockage by large objects is sometimes referred to as shadowing, because the fade area is very large and tends to blanked the area surrounding the antenna. An ideal signal travelling through free space would experience a path loss proportional to the distance squared. In the real world, a signal’s energy is absorbed and reflected by the atmosphere, the curvature of the earth and obstacles. The obstacle can be natural or human made.
(ii) Small-scale fading- it is due to the following reasons -
- Random frequency modulation due to varying Doppler shifts on different multipath signals.
- Time dispersion caused by multipath propagation delays due to nearby objects.
- Even when the mobile is stationary, the received signals may fade due to movement of surrounding object (see fig. 1.18.)