The transmission of information through short distances does not require complicated techniques. The energy of the information signal is sufficient enough to be sent directly. However if the information, for example, audio frequency (20 to 20,000 Hz) needs to be transmitted to long distances across the world, certain techniques are required to transmit the information without any loss.
For long distance transmission, the low frequency baseband signal (input signal) is superimposed onto a high frequency radio signal by a process called modulation. In the modulation process, a very high frequency signal called carrier signal (radio signal) is used to carry the baseband signal.
As the frequency of the carrier signal is very high, it can be transmitted to long distances with less attenuation. The carrier signal is usually a sine wave signal. Also, the carrier signal will be more compatible with the communication medium like free space and can propagate with greater efficiency.
Carrier signal does not have information.
A sinusoidal carrier wave can be represented as ec = Ec sin (2πνct + ϕ), where Ec is the amplitude, νc is the frequency and ϕ is the initial phase of the carrier wave at any instant of time t.
Three characteristics in the carrier signal can be modified by the baseband signal during the process of modulation: amplitude, frequency and phase of the carrier signal.
There are 3 types of modulation based on which parameter is modified.
(i) amplitude modulation,
(ii) frequency modulation and
(iii) phase modulation.