Before answering, let me give you two rough and practical instances below are they:

First instance: Imagine that  you’re on a bank of one side of a river and I’ll give you a paper and tell you to throw it and send it to the other side of the bank. so my question is following  Is it possible to throw the paper which doesn’t have suitable mass to go to the other side of the river?

The answer is No. So in order to make the paper reach the other side of the river I’ll use a stone with the paper and throw it, now it’ll reach the other side.

In communication the message signal will be of low frequency which is like paper (light weight) in order to make it heavy, a carrier signal(stone) is used to send data from transmitting station to the receiving station.

Second instance: Consider you have food and you have to take it to school for lunch, will you carry the food with your hand? No right? You’ll use a carrier box and you’ll eat the food not the carrier box.

In communication food(message signal) cannot be transmitted as it is, it needs a carrier box(carrier signal)to which is used to enclose the message signal from the transmitter end and carry the modulated signal through the medium and at the receiver the carrier box(carrier signal) is removed by demodulation it removes the envelope and obtains the food(message signal).

This is why modulation and demodulation is required in communication systems.

Now let me explain the technical reason why modulation is needed and what is the advantage ?

Firstly, Transmission of error-free digital data.

The need for modulating baseband signal arises because of limitations in the wired communication which is governed by the Shannon-Hartley law :

Where R is rate of data transmission in bits/sec, B is the signal bandwidth and S/N is the signal to noise ratio (in terms of power). This Shannon-Hartley law tells the maximum amount of error-free digital data that can be transmitted over a wired communications channel (e.g., a copper wire or an optical fiber) with a specified bandwidth in the presence of noise.

So, We can see from the equation that the more the bandwidth the more the rate of data transmission.

Secondly, modulation reduces the height of the antenna .

For the transmission of radio signals, the antenna height must be multiple of λ/4 ,where λ is the wavelength .

λ = c /f        where c : is the velocity of light   3×10^8

f: is the frequency of the signal to be transmitted

The minimum antenna height required to transmit a baseband signal of f = 10 kHz is calculated as follows : λ = 3×10^8/10=30km, Antenna hight =λ/4=30000/4=7.5km .

So 7.5 km of long antenna is impossible to install.

Now, let us consider a modulated signal at high frequency f = 1 MHz .

The antenna hight is calculated in such way:

λ = c /f=3×10^8/1000000=300m

antenna hight : 300/4=75m

This antenna can be easily installed practically . Thus, modulation reduces the height of the antenna .

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