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Modulation

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Modulation

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  1. Modulation • Conversion of digital information to analog signals • Example: Telephone lines are band-limited (300-3300Hz) • Modem = modulator/demodulator • Characteristics of defining sine waves: • Amplitude • Frequency • Phase Change one or more of them to send information ECE 766 Computer Interfacing and Protocols

  2. Modulation • Modulation Techniques • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK) • Aspects of Modulation • Bit rate vs. Baud rateBit rate ≥ Baud rate • Carrier Signal Quadrature AmplitudeModulation (QAM) ECE 766 Computer Interfacing and Protocols

  3. 1 1 0 0 Amplitude Time 1 baud 1 baud 1 baud 1 baud 1 second Amplitude Shift Keying (ASK) • Basic technique (like AM)  change the amplitude of the signal to transfer logical values • On-Off Keying (OOK) • Highly susceptible to noise • Inefficient ECE 766 Computer Interfacing and Protocols

  4. Amplitude Minimum Bandwidth = Nbaud Frequency fc fc-(Nbaud/2) fc+(Nbaud/2) Amplitude Shift Keying (ASK) • Bandwidth requirement • BW = (1+d) x Nbaud • d ≥ 0  related to the condition of the line ECE 766 Computer Interfacing and Protocols

  5. 1 1 0 0 Amplitude Time 1 baud 1 baud 1 baud 1 baud 1 second Frequency Shift Keying (FSK) • Represent each logical value with another frequency (like FM) • Not susceptible to noise as much as ASK • Limiting factor: Physical capabilities of the carrier ECE 766 Computer Interfacing and Protocols

  6. Amplitude BW=fc1--fc0+Nbaud fc1--fc0 Nbaud/2 Nbaud/2 Frequency fc0 fc1 Frequency Shift Keying (FSK) • Bandwidth requirement • How about duplex communication? ECE 766 Computer Interfacing and Protocols

  7. Reference 1 1 0 0 Amplitude Time 1 baud 1 baud 1 baud 1 baud 1 second Phase Shift Keying (PSK) • Use different phased signals to represent binary values • 0 = in phase with reference • 1 = out of phase with reference • Not practical ECE 766 Computer Interfacing and Protocols

  8. 1 1 0 0 Amplitude Time 1 baud 1 baud 1 baud 1 baud 1 second 1 0 Phase Shift Keying (PSK) • Differential PSK • 0 = same phase as last signal element • 1 = 180º shift from last signal element • Constellation Diagram: ECE 766 Computer Interfacing and Protocols

  9. 010 01 011 001 10 00 100 000 111 101 11 110 4-PSK (Q-PSK) 8-PSK Phase Shift Keying (PSK) • Quadrature Phase Shift Keying (Q-PSK):Do not only use 180º shift • Min. BW requirement: same as ASK! • Self clocking (most cases) ECE 766 Computer Interfacing and Protocols

  10. Quadrature Amplitude Modulation (QAM) • Combine ASK and PSK such that each signal corresponds to multiple bits • More phases than amplitudes • Minimum bandwidth requirement same as ASK or PSK 3 amplitudes 12 phases ECE 766 Computer Interfacing and Protocols