What Is Multiplexing
Multiplexing is a technique in which several message signals are combined into a composite signal for transmission over a common channel.
In order to transmit a number of these signals over the same channel, the signals 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 as under:
- Frequency Division Multiplexing (FDM)
- Time Division Multiplexing (FDM)
Frequency Division Multiplexing (FDM)
The FDM scheme is illustrated in figure 1 with the simultaneous transmission of three messages or base band signals.
The spectra of the message signals and the sum of the modulated carriers are indicated in the figure.
Any type of modulation can be used in FDM as long as the carrier spacing is sufficient to avoid spectral overlapping. However, the most widely used method of modulation is SSB modulation.
FDM is used in telephone system, telemetry, commercial broadcast, television, and communication networks.
Commercial AM (Amplitude Modulation) broadcast stations use carrier frequency spaced 10kHz apart in the frequency range from 540 to 640 kHz. This separation is not sufficient to avoid spectral overlap for AM with a reasonably high fidelity (50 Hz to 15kH) audio signal. Therefore, AM stations on adjacent carrier frequencies are placed geographically far apart to minimize interference.
Commercial FM (Frequency-Modulation) broadcast uses carrier frequencies spaced 200kHz apart. In a long distance telephone system, upto 600 or more voice signals (200 Hz to 3.2 kHz) are transmitted over a coaxial cable or microwave links by using SSB modulation with carrier frequencies spaced 4 kHz apart.
In practice, the composite signal formed by spacing several signals in frequency may, in turn , be modulated by using another carrier frequency. In this case, the first carrier frequencies are often called sub-carriers.
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.
As an 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 in 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 y other signals.
There is possibility of crosstalk in FDM whereas Intersymbol Interference is possible in TDM. These problems can be overcome by some special cares.