Frequency DivisionMultiplexing By : Ronel Culibar Figure 6.1 Dividing a link into channels 6.2 3 .Figure 6.2 Categories of multiplexing 6. 3 Frequency-division multiplexing (FDM) 6.4 .Figure 6. Note FDM is an analog multiplexing technique that combines analog signals.5 . 6. It uses the concept of modulation discussed in Ch 5. 4 FDM process 6.6 .Figure 6. 7 .FM 6. Figure 6.8 .5 FDM demultiplexing example 6. Then we combine them as shown in Figure 6. from 20 to 32 kHz.9 .1 Assume that a voice channel occupies a bandwidth of 4 kHz. 6. Show the configuration. Assume there are no guard bands. using the frequency domain. and the 28 to 32kHz bandwidth for the third one. We use the 20 to 24kHz bandwidth for the first channel.6. Solution We shift (modulate) each of the three voice channels to a different bandwidth. the 24 to 28kHz bandwidth for the second channel.6. as shown in Figure 6.Example 6. We need to combine three voice channels into a link with a bandwidth of 12 kHz. 1 6.6 Example 6.Figure 6.10 . we need at least four guard bands.7. each with a 100-kHz bandwidth.11 . 6.2 Five channels. This means that the required bandwidth is at least 5 × 100 + 4 × 10 = 540 kHz. are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference? Solution For five channels.Example 6. as shown in Figure 6. 7 Example 6.12 .Figure 6.2 6. 13 .Figure 6.9 Analog hierarchy 6. the band is divided into 832 channels. 42 channels are used for control. How many people can use their cellular phones simultaneously? Solution Each band is 25 MHz. we get 833.33. and 869 to 894 MHz is used for receiving. which means only 790 channels are available for cellular phone users. Each user has a bandwidth of 30 kHz in each direction. The first band of 824 to 849 MHz is used for sending. 6.4 The Advanced Mobile Phone System (AMPS) uses two bands.Example 6. If we divide 25 MHz by 30 kHz.14 . In reality. Of these. . . Formation of Groups (consist of 12 channels) To find the carrier frequency of a channel. use the formula Fc = 112-4n kHz Where: n=channel number Fc= carrier frequency . To find the output frequency band of a channel use the formula Fout = (Fc – 4kHz) where: Fc= carrier frequency=112-4n kHz Frequency band of a channel is from Fout to Fc ex: find the carrier frequency of channel 3 and the output frequency band Ans: 100kHz. 96kHz to 100kHz . 1 Group= 12 Voice Channels Total Bandwidth of 12 x 4kHz = 48Khz . Formation of Supergroup(consist of 5 groups) To find the carrier frequency of a group. use the formula Fc = 372 + 48n kHz Where: n=group number Fc= carrier frequency . To find the carrier and the frequency band of a group use the formula: Fout = (fc – 108kHz) to (fc-60kHz) where: fc= carrier frequency=372+48n kHz ex: find the carrier frequency of group 3 and the output frequency band Ans: 516kHz. 408kHz to 456kHz . 1 Supergroup= 5 Groups Total Bandwidth of 5 x 48kHz = 240kHz . Figure 6.9 Analog hierarchy 6.23 . References • Data Communications and Networking by Forouzan 4th edition • Advanced Electronics Communications By Wayne Tomasi .