What is Modulation?
Transmission of information by communication systems over large distances is quite a feat of human ingenuity. We can talk, video chat and text anyone on this planet! Communication system uses a very clever technique called Modulation to increase the reach of the signals. Two signals are involved in this process.

Message signals also known as baseband signals are the band of frequencies representing the original signal. This is the signal to be transmitted to the receiver. Frequency of such a signal is usually low. The other signal involved with this is a high frequency sinusoidal wave. This signal is called the carrier signal. The frequency of carrier signals is almost always higher than that of the baseband signal. The amplitude of the baseband signal is transferred to the high frequency carrier. Such a higher frequency carrier is able to travel much farther than the baseband signal.

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But that’s not the only advantage of modulation. What is the need for modulation?

1. Size of the Antennae
When the transmission occurs over free space, the antennae radiate the signal out and receiver receives it. In order to operate efficiently, antennae need to be in order of the magnitude of wavelength of the transmitted signal.
L=λ=uϑ=(3∗108)ϑHz
Speech frequencies range from 20 Hz to 20 kHz. Suppose this is a frequency of 20 kHz and it is radiated out to a receiver through a channel of free space.
Length of Antennae=3∗10820∗103=15000m=15km
It is impossible to build an antennae this big. Instead say we give modulation a chance and use a 1000 kHz carrier wave to carry the signal. Length of the antennae now would be;
Length of Antennae=3∗1081000∗103=300m
This is much more doable and this example clearly shows us how hugely the process of modulation is enabling communication systems.

2. Wireless Communication
By using modulation to transmit the signals through space to long distances, we have removed the need for wires in communication systems. The technique of modulation helped humans to become wireless. Telephones no longer had to be plugged to a wall. Using a mobile phone went from a dream to the next big thing.

3. Interference from other signals
This is a point from the practical side of things. Suppose you are transmitting the baseband signal to a receiver, say your friends phone. Just like you there will be thousands of people in the city using their mobile phones. There is no way to tell such signals apart and they will interfere with each other leading to a lot of noise in the system and a very bad output. By using a carrier wave of high frequencies and allotting a band of frequencies to each message, there is no mixing up of signals and the received signals are absolutely perfect.

There are three types of Modulation:
• Amplitude Modulation
• Frequency Modulation
• Phase Modulation

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Source：byjus