Frequency Division Multiplexing (FDM) System

Last Updated on April 18, 2016 by Sasmita

Frequency Division Multiplexing (FDM) System

The operation of frequency division multiplexing (FDM) is based on sharing the available bandwidth of a communication channel among the signals to be transmitted .

This means that many signals are transmitted simultaneously with each signal occupying a different frequency slot within a common bandwidth .

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

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

The spectrum of composite FDM signal has been shown in fig.1 .

frequency division multiplexing spectrum

Fig.1 : Spectrum of FDM Signal

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

FDM Transmitter

Fig.2 shows the block diagram of an FDM transmitter .

FDM transmitter

Fig. 2 : FDM Transmitter

The signals which are to be multiplexed will each modulate a separate carrier .The type of modulation can be AM, SSB, FM or PM .

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

Working Operation of the FDM Transmitter

Each signal modulates a separate carrier . The modulator outputs will contain the sidebands of the corresponding signals .

The modulator outputs are added together in a linear mixer or adder .

The linear mixer is different from the normal mixers. Here the sum and difference frequency components are not produced . But only the algebraic addition of the modulated outputs will take place .

Different signals are thus added together i the time domain but they have a separate identity in the frequency domain . This is as shown in fig.2 .

The composite signal at the output of mixer is transmitted over the single communication channel as shown in fig.2 . This signal can be used to modulate a radio transmitter if the FDM signal is to be transmitted through air .

FDM Receiver

The block diagram of an FDM receiver is  shown in fig.3 .

FDM receiver

Fig.3 : FDM Receiver

The composite signal is applied to a group of bandpass filters (BPF) .

Each BPF has a center frequency corresponding to one of the carriers. The BPFs have an adequate bandwidth to pass all the channel information without any distortion .

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

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