Understanding Modulation & Demodulation in communication system (Why?)

Communication is the process of transmitting information.

Baseband communication: Communication that directly transmit the baseband signal without any modulation is known as baseband communication.

The term baseband is used to designate the band of frequencies of the signal delivered by the source or the input transducer.( The transducer is a general term used for representing a device which can convert physical signal into electrical signal or electrical signal into physical signal). In baseband communication, baseband signals are transmitted without modulation because they have sizeable power at low frequencies. They can’t be transmitted over a radio link but are suitable for transmission over a pair of wires, coaxial cables or optical fibers. 

Carrier communication: Communication that uses modulation to shift the frequency spectrum of a signal is known as carrier communication.  

Modulation: The process of changing some characteristics (amplitude, frequency or phase) of a carrier wave in accordance with the intensity of the signal is known as modulation.

 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 wavelength. For low frequency the wavelength ( ƛ = 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 signals bandwidth is low.

In nature the generated signals (like sound range 20 Hz - 20 kHz) have closely similar frequency bands. So the efficiency of the signal can also decrease by their interference.

Therefore 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

Demodulation:- The process of recovering the message signal from the modulated signal is known as demodulation or detection.

Why Demodulation?: 

The modulated wave has large frequency and consists of carrier and sideband frequencies. If the modulated wave is directly fed to the operating device, it wouldn’t at all able to respond to such a high frequencies. So we must separate the higher carrier frequency. In the channel, it is also possible to add some noise to the modulated wave which should be rejected. That is why demodulation is needed.