© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. Modulation/Demodulation Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya,

2 Carrier Frequencies In many long-distance communication systems, certain frequencies propagate better than others These frequencies are used as carriers The system makes small changes to the carrier that represent information being sent This process is known as Modulation

3 Modulation/Demodulation  Communication Channel  modulated signal input signal carrier wave modulator demodulator Modulator and demodulator are usually integrated into a single device, called a modem

4 Modulating Analog Signals Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

5 Amplitude Modulation (AM) AM varies the amplitude of a carrier in proportion to the information being sent

6 Frequency Modulation (FM) FM varies the frequency of a carrier in proportion to the information being sent

7 Phase Modulation (PM) PM varies the phase shift of a carrier in proportion to the information being sent For analog signals, PM is considered a special form of FM Difference is clearer in digital modulation process

8 Modulating Digital Signals Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Combinations of the above

9 ASK and FSK Carrier Digital Input ASK FSK

10 Phase Shift Keying (PSK) 0 o phase shift180 o phase shift

11 PSK Constellation Diagrams Constellation diagram is used to express the exact assignment of data bits to specific phase changes

12 4-PSK Uses 4 different phase shifts Each phase shift can represent 2 data bits

13 n-PSK In theory, it is possible to increase the data rate by increasing the range of phase shifts Thus, a 16-PSK mechanism can send twice as many bits per second as a 4-PSK mechanism In practice, noise and distortion limit the ability of hardware to distinguish among minor differences in phase shifts How can data rate increase furthur?

14 Quadrature Amplitude Modulation Known as QAM or or Quadrature Amplitude Shift Keying (QASK) Combining ASK and PSK Uses both change in phase and in amplitude

15 QAM – Time Domain

16 Modem Hardware Modem  Modulator/Demodulator Modems are designed to provide communication over long distances Such as telephone system Acoustically coupled modem PCI modem ADSL modem Wireless/3G modem Pictures from Wikipedia

17 Communication over Modems

18 Dial-up Modems A dialup modem uses data to modulate an audible carrier which is transmitted to the phone system

19 Bandwidth of Dialup Modems