Which type of communication is employed in mobile phones?

The space wave propagation is employed in mobile phones, then; Space wave propagation: This transmission is limited to the line of sight. Waves generally travel in a straight line from the transmitter to the receiver. It is used for propagation of waves of frequency greater than 40 MHz . The ground wave propagation is not used for low range, it is used in landlines. And in the case of sky wave propagation the frequency is more than instead of reflection, refraction takes place and the waves are not received by the receiver.

Mention the basic methods of modulation.

The basic methods of modulation in which a signal can be modulated are: Amplitude modulation (AM): This form of modulation involves modulating the amplitude or intensity of the signal. Frequency modulation (FM): This form of modulation varies the frequency in line with the modulating signal. Phase modulation (PM): Phase modulation varies the phase of the carrier in line with the modulating signal.

Mention the frequency range of speech signals.

Speech signal are those which emerges from a speaker's mouth, nose and cheeks, is a one- dimensional function (air pressure) of time. The speech signal don't have much information to carry. So, their frequency range is low, 300 Hz t o 3100 Hz .