Multiplexing is the process of simultaneously transmitting two or more individual signals over a single communication channel. Due to multiplexing it is possible to increase the number of communication channels so that more information can be transmitted. The typical application of multiplexing are telemetry or telephone, satellite comm etc.

There are basic 3 types of multiplexing they are:

1] Frequency division multiplexing. (FDM)

2] Time division multiplexing. (TDM)

3] Wavelength division multiplexing (WDM)

In FDM many signal are transmitted simultaneously where each signal occupies a different frequency slot within a common BW.

Frequency Division multiplexing:

• The operation of FDM is based on sharing the available BW of a communication channel among the signals to be transmitted.

• Each signal to be transmitted modulates a different carrier, the modulation can be AM, SSB, FM or PM.

• The modulated signals are then added to form a composite signal which is transmitted over a single channel.

• The information of composite FDM signal is shown below:

• Generally FDM systems are used for multiplexing the analog signals.

• Below figure shows the block diagram of FDM transmitter, the signals which are to be multiplexed will each modulate a separate carrier.

• They are then added to form a composite signal which is transmitted over a simple channel.

• Each signal modulates a separate carrier which contains the sidebands of the signal. The modulator o/p are then added in linear mixer or adder.

• The composite signal at the o/p of the mixer is transmitted over the single communication channel.

• The block diagram of FDM receiver is shown below:

• The composite signal is applied to a group of BPF.

• Each BPF has a center frequency corresponding to one of the carriers.

• The BPF have an adequate BW to pass all the channel information without any distortion.

• Each filter will pass only its channel and reject all the other channels.

• The channel/demodulator then removes the carrier and recovers the original signal back.

Advantages of FDM:

• large number of signals can be transmitted.

• De modulation is easy.

• Due to slow narrow band fading only a single channel gets affected.

Disadvantages:

• The channel must have a very large BW.

• Inter modulation distortion takes place.

• FDM suffers from problem of cross talk.

Applications of FDM:

• Telephone system

• AM radio broad casting.

• TV broad casting etc.