Multiplexing is a technique in which several message signals are combined into a composite signal for transmission over a common channel.
These signals to be transmitted over the common channel must be kept apart so that they do not interfere with each other, and hence they can be separated easily at the receiver end.
Basically, multiplexing is of two types such as :
- Frequency division multiplexing (FDM)
- Time division multiplexing (TDM)
Now we will discuss each type of multiplexing in detail.
1. Frequency division multiplexing (FDM)
The FDM scheme is shown in figure below with the simultaneous transmission of three message or baseband signals.
The spectra of the message signals and the sum of the modulated carries are indicated in the figure.
Double side band (DSB) modulation is used in illustrating the spectra in the above fig.Any type of modulation can be used in FDM as long as the carrier spacing is sufficient to avoid overlapping.However, the most widely used method of modulation is single side band (SSB) modulation.
At the receiving end of the channel the three modulated signals are separated by band pass filters (BPFs) and then demodulated.
FDM is used in telephone system, telemetry, commercial broadcast, television, and communication networks.
2. Time Division Multiplexing (TDM)
In case of time division multiplexing (TDM), the complete channel bandwidth is allotted to one user for a fixed time slot.
For example, if there are ten users,then every user can be given the time slot of one second. Thus, complete channel can be used by each user for one second time i every ten seconds.
This technique is suitable for digital signals. Because digital signals are transmitted intermittently and the time spacing between two successive digital codewords can be utilized by other signals.
There is possibility of crosstalk in FDM whereas intersymbol interference is possible in TDM. These problems can be taken care of by some special arrangements.