1. COMMUNICATION SYSTEM EEEB453 Chapter 7(Part I) MULTIPLEXING

2. 2 MULTIPLEXING Multiplexing – methods of transmitting more than one signal along a single transmission path/stream i.e many to one. Demultiplexing – separate the stream back into its component transmission i.e one to many. Path– refers to the physical link. Channel – refers to a portion that carries a transmission between a given pair of devices. One path can have many channels.

3. 3 MULTIPLEXING Two common form of multiplexing are Frequency Division Multiplexing(FDM) and Time Division Multiplexing(TDM). Two variations of these basic methods are frequency division multiple access (FDMA) and time division multiple access (TDMA) Another form of multiple access is known as code-division multiple access (CDMA) Advantages:  Increase number of channels so that more info can be transmitted  Save cost by using one channel to send many info signals

4. 4 Frequency Division Multiplexing (FDM) FDM – multiple sources that originally occupied the same frequency spectrum are each converted to different frequency band and transmitted simultaneously. FDM is an analog technique – the information entering an FDM system must be analog. If the source is digital, it must be converted to analog before being frequency-division multiplexed. Split the total channel bandwidth into several smaller channels of different frequencies. Different signal travel over the medium concurrently. Guard bands keep the modulated signals from overlapping and interfering with one another. Modulation is used to lift the centre freq of the baseband signal up into a preassigned freq slot.

5. 5 Frequency Division Multiplexing (FDM) A number of signal, m i (t), i=1..,n are to be multiplexed onto the same Tx medium. Each signal m i (t),is modulate onto a carrier f i, refers as subcarrier. Modulated signals are then summed to produce a composite signal m b (t). Figure (b) shows the result – signal m i (t) is shifted to be centered of f i. f i must be chosen so that the BW of the various signals do not overlap i.e channel must be separated by unused BW (guard band). At the Rx end, the FDM signal is demodulated to retrieved m b (t), which is then passed through n BPF.a

6. Figure shows a simple FDM system where four 5kHz channels are frequency-division multiplexed into a single 20kHz combined channel. With FDM, each narrowband channels are stacked on top of one another in the frequency domain.

7. Figure (a) shows how a group is formed with A-type channel bank. Each voice band channel is bandlimited with an antialising filter prior to modulating the channel carrier. Figure (b) shows the output spectrum.

8. Analog Hierarchy ▀ Figure shows first stages of telephone mux ▀ Group multiplexer takes 12 voice ch and puts them on subcarriers at 64, 68,…108kHz using LSB. ▀ The resulting spectrum extends 48kHz starting at 60kHz. ▀ Five such carriers are combined by LSB on subcarriers at 420, 468,… 612kHz to produce a supergroup from 312 to 552kHz 8

9. Analog Hierarchy 9

10. FDM in Telephone System The original signal voice is in 300 to 3000Hz range. The voice is used to modulate subcarrier. Each subcarrier is on different frequency. These subcarriers are then added together to form a single channel. Balance Modulator Voice f c = 60kHz Ch 12 f c = 104kHz Ch 1 BPF f c = 64kHz Ch 11 BPF fvfv fv+fcfv+fc fv-fcfv-fc fv-fcfv-fc 56 – 64kHz 60 – 64kHz Selects USB 100– 108kHz DSBSC SSBSC 104– 108kHz 0 - 4 kHz Linear Mixer

11. Voice signal amplitude modulates 1 of 12 ch’s in the 60 to 108kHz range. The carrier freq begin at 60kHz with a spacing of 4kHz.(slightly higher than the highest typical freq of voice) Output of the balance modulator – DSBSC. The output of the filter is the SB containing the original voice signal. All 12 SSB signals are then summed in a linear mixer to produce a single frequency multiplexed signal – basic group. Basic group freq spectrum for FDM telephone mux system is shown below. 11 FDM in Telephone System 60727680 f (kHz) 646884889296100104108 123456789101112 Channel No. 4kHz Carrier frequencies If more than 12 voice channels are needed, multiple basic groups are used.

12. Example 1– A cable TV service uses a single coaxial cable with bandwidth of 860 MHz to transmit multiple TV signals to subscribers. Each TV signal is 6 MHz wide. How many channels can be carried? 12

13. Example 2– For a particular telephone company, the first sub carrier frequency is at 60 kHz and the total bandwidth is 96 kHz. Design a FDM system, given a general rule of 12 channels per basic group and 4 kHz per channel applies to the design. i. How many basic groups are required? ii. Draw the circuit diagram of your design iii. Draw the frequency spectrum of your multiplexed system Solution 13

14. 14 i.Given BW total = 96kHz;12 channels/basic group 1 channel = 4 kHz, then 12 channels = 12x4 = 48kHz/basic group Thus 96/48 = 2 basic group ii.Circuit Diagram Voice f c = 60kHz Ch 12 f c = 104kHz Ch 1 BPF Linear Mixer f c = 1 0 8kHz Ch 24 f c = 152kHz Ch 13 BPF Linear Mixer f (kHz) 60104108152 48 kHz iii.Frequency Spectrum

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