COMMUNICATION SYSTEM EEEB453 Chapter 7(Part I) MULTIPLEXING

1. COMMUNICATION SYSTEM EEEB453Chapter 7(Part I)MULTIPLEXING

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. 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. 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. Frequency Division Multiplexing (FDM) A number of signal, mi(t),i=1..,n are to be multiplexed onto the same Tx medium. Each signal mi(t),is modulate onto a carrier fi, refers as subcarrier. Modulated signals are then summed to produce a composite signal mb(t). Figure (b) shows the result – signal mi(t) is shifted to be centered of fi. fi 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 mb(t), which is then passed through n BPF.a

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

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

9. Analog Hierarchy

10. Voice fv+fc Balance Modulator fv fv-fc fv-fc 0- 4 kHz Ch 12 BPF 60 – 64kHz 56 – 64kHz Linear Mixer fc = 60kHz 0- 4 kHz Ch 11 BPF fc = 64kHz 104– 108kHz 100– 108kHz SSBSC 0- 4 kHz Ch 1 BPF DSBSC Selects USB fc = 104kHz 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.

11. 4kHz Channel No. 12 11 10 9 8 7 6 5 4 3 2 1 f(kHz) 60 64 68 72 76 80 84 88 92 96 100 104 108 FDM in Telephone System Carrier frequencies If more than 12 voice channels are needed, multiple basic groups are used. 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.

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?

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. • How many basic groups are required? • Draw the circuit diagram of your design • Draw the frequency spectrum of your multiplexed system Solution 13

14. 48 kHz 48 kHz f (kHz) 60 104 108 152 Given BWtotal = 96kHz;12 channels/basic group 1 channel = 4 kHz, then 12 channels = 12x4 = 48kHz/basic group Thus 96/48 = 2 basic group Circuit Diagram Ch 12 BPF Linear Mixer fc = 60kHz Voice Ch 1 BPF Linear Mixer fc = 104kHz Ch 24 BPF Linear Mixer fc = 108kHz Ch 13 BPF fc = 152kHz Frequency Spectrum

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