Baseband and Passband Communication Systems:

Communication system can be classified into 2 groups depending on the range of frequencies they use to transmit information.

1)Baseband Communication

2)Passband Communication

Baseband transmission sends the information as it is without modulation (without shifting or increasing the frequency) while Passband transmission shifts the signal to be transmitted to a higher frequency and then transmits it, where at the receiver the signal is shifted back to its original frequency (Demodulation).

Example:

The Telephone system used for homes and offices may transmit the baseband signal as it is when the call is local. When the telephone call is a long-distance call that is transmitted via satellite links, the baseband signal (audio) becomes unsuitable for transmission and the communication system becomes a passband system.

Baseband system which is easier than Passband transmission is usually used when communicating over wires while over the air transmission requires passband transmission.

What is the need for Modulation?

If directly Baseband signals (low frequency) are transmitted there are several factors due to which effective transmission fails.

1)Size of an Antenna:

Analysis of Antenna shows that for optimum transmission of signal the size of the Antenna must be at least Lambda/4, where Lambda is the wavelength.

We know that Lambda=Velocity of light/frequency of the signal to be transmitted. 

For example, if we consider low frequency signal say 3000Hz and substitute in the above formula then we will get the result as height of antenna should be 25km which is practically impossible. If we substitute the high frequency signal say 1000 MHz then we will get the antenna height as 7.5 cm which can be easily installed practically

2)Mixing of Signals:

If the baseband audio signals are transmitted without using modulation by using more than one transmitter, then all the signals will be in the same frequency range (20Hz to 20 KHz). Therefore, all the signals get mixed together and a receiver cannot separate them from each other.

If each Baseband signal is used to modulate different carrier signal (high frequency) then they will occupy different slots in the frequency domain. Thus, modulation avoids mixing of signals.

3)Range of Communication:

The frequency of baseband signal is low and the low frequency signals cannot travel long distance because they get heavily attenuated. The attenuation reduces with increase in frequency of the transmitted signal as they travel longer distance.

The modulation process increases the frequency of the signal to be transmitted. Therefore, it increases the range of communication.