Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

7.2 FREQUENCY DIVISION MULTIPLEXING

In frequency division multiplexing (FDM), the signals are translated into different frequency bands and sent over the medium. The communication channel is divided into different frequency bands, and each band carries the signal corresponding to one source.

Consider three data sources that produce three signals as shown in Figure 7.2. Signal #1 is translated to frequency band #1, signal #2 is translated into frequency band #2, and so on. At the receiving end, the signals can be demultiplexed using filters. Signal #1 can be obtained by passing the multiplexed signal through a filter that passes only frequency band #1.

Figure 7.2: Frequency division multiplexing.

FDM is used in cable TV transmission, where signals corresponding to different TV channels are multiplexed and sent through the cable. At the TV receiver, by applying the filter, a particular channel's signal can be viewed. Radio and TV transmission are also done using FDM, where each broadcasting station is given a small band in the frequency spectrum. The center frequency of this band is known as the carrier frequency.

Figure 7.3 shows how multiple voice channels can be combined using FDM.

Figure 7.3: FDM of voice channels.

Each voice channel occupies a bandwidth of 3.4kHz. However, each channel is assigned a bandwidth of 4kHz. The second voice channel is frequency translated to the band 4–8kHz. Similarly, the third voice channel is translated to 8–12 kHz and so on. Slightly higher bandwidth is assigned (4kHz instead of 3.4kHz) mainly because it is very difficult to design filters of high accuracy. Hence an additional bandwidth, known as guard band, separates two successive channels.

In FDM, the signals from different sources are translated into different frequency bands at the transmitting side and sent over the transmission medium. In cable TV, FDM is used to distribute programs of different channels on different frequency bands. FDM is also used in audio/video broadcasting.

FDM systems are used extensively in analog communication systems. The telecommunication systems used in telephone networks, broadcasting systems, etc. are based on FDM.