Communication is the process of transmitting information. Why modulation? :-
Antenna height and radiation property is related to transmitted signal frequency. Theory shows that in order to transmit a wave effectively or efficiently, the length of the transmitting antenna should be at least approximately equal to or greater than the (1/10)* wavelength of the wave. Actually different antennas height are related differently with wavelenfth. For low frequency the wavelength ( ф = v/f) becomes very high. So physical length of antenna becomes impractically large. At low frequency radiation is poor and signal gets highly attenuated. Also the energy of a wave depends upon its frequency.
So the greater the frequency, the greater the distance of transmission. Also at low frequency sinals bandwidth is low. In nature the generated signals (like sound range 20Hz – 20 kHz) have closely similar frequency bands. So the efficiency of the signal can also decrease by their interference. for low frequency baseband signals, it is hard to transmit at large distance. All these problem can be overcome by modulating low frequency signal into a much higher frequency signal in term of carrier wave with additional facilities : 1. Allows multiplexing 2. Allows adjustments in bandwidth 3.
Avoids mixing of signals 4. Improve quality of reception and SNR 5. Reduce height of the antenna 6. Increase the range of communication The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a filter limits the frequency range to between 300 and 3400 Hz) or a limited radio frequency band. The aim of analog modulation is to transfer an analog lowpass signal, for example an audio signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or a cable TV network channel. Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate bandpass channels.
The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a lowpass channel, typically a non-filtered copper wire such as a serial bus or a wired local area network. The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband lowpass channel or, in some of the schemes, as a bit stream over another digital transmission system.