ModulationDemodulation Asst Prof Chaiporn Jaikaeo Ph D chaiporn

Modulation/Demodulation Asst. Prof. Chaiporn Jaikaeo, Ph. D. chaiporn. j@ku. ac. th http: //www. cpe. ku. ac. th/~cpj Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya, LKaya@ieee. org © 2009 Pearson Education Inc. , Upper Saddle River, NJ. All rights reserved.

Carrier Frequencies n In many long-distance communication systems, certain frequencies propagate better than others n n n 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 2

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

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

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

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

Phase Modulation (PM) n n 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 n Difference is clearer in digital modulation process 7

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

ASK and FSK Carrier Digital Input ASK FSK 9

Phase Shift Keying (PSK) 0 o phase shift 180 o phase shift 10

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

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

n-PSK n n 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? 13

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

QAM – Time Domain 15

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

Communication over Modems 17

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

Bandwidth of Dialup Modems 19