05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 1 Modulation Conversion of digital information to analog signals –Example: Telephone lines are band-limited ( Hz) Modem = modulator/demodulator Characteristics of defining sine waves: –Amplitude –Frequency –Phase Change one or more of them to send information

05 - Winter 2005 ECE 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 rate Bit rate ≥ Baud rate –Carrier Signal Quadrature Amplitude Modulation (QAM)

05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 3 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 Amplitude Time baud 1 second

05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 4 Amplitude Shift Keying (ASK) Bandwidth requirement BW = (1+d) x N baud d ≥ 0  related to the condition of the line Minimum Bandwidth = N baud fcfc f c -(N baud /2) f c +(N baud /2) Frequency Amplitude

05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 5 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 Amplitude Time baud 1 second 1 baud

05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 6 Frequency Shift Keying (FSK) Bandwidth requirement How about duplex communication? N baud /2 f c0 Frequency Amplitude f c1 f c1- -f c0 N baud /2 BW=f c1- -f c0 +N baud

05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 7 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 Amplitude Time baud 1 second 1 baud Reference

05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 8 Phase Shift Keying (PSK) Differential PSK –0 = same phase as last signal element –1 = 180º shift from last signal element Constellation Diagram: Amplitude Time baud 1 second 1 baud 10

05 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 9 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) PSK (Q-PSK) PSK

05 - Winter 2005 ECE 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