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

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 1

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)

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 2

Amplitude Shift Keying (ASK)

• Basic technique (like AM)  change the amplitude of the signal to transfer logical values

1 1 0 0

Amplitude Time 1 baud 1 baud 1 baud 1 baud 1 second • On-Off Keying (OOK) • Highly susceptible to noise • Inefficient

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 3

Amplitude Shift Keying (ASK)

• Bandwidth requirement Amplitude

Minimum Bandwidth = N

baud

Frequency

f c

-

(N baud /2)

f c f c

+

(N baud /2)

• • BW = (1+

d

) x

N baud d

≥ 0  related to the condition of the line

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 4

Frequency Shift Keying (FSK)

• Represent each logical value with another frequency (like FM)

1 0 0 1

Amplitude Time 1 baud 1 baud 1 baud 1 baud 1 second • Not susceptible to noise as much as ASK • Limiting factor: Physical capabilities of the carrier

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 5

Frequency Shift Keying (FSK)

• Bandwidth requirement Amplitude

N baud /2

BW=

f c1-

-

f c0

+

N baud

f c1-

-

f c0

N baud /2

Frequency

f c0 f c1

• How about duplex communication?

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 6

Phase Shift Keying (PSK)

• Use different phased signals to represent binary values Reference – 0 = in phase with reference – 1 = out of phase with reference

1 1 0 0

Amplitude Time 1 baud 1 baud 1 baud 1 baud 1 second • Not practical

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 7

Phase Shift Keying (PSK)

• Differential PSK – 0 = same phase as last signal element – 1 = 180º shift from last signal element

1 1 0 0

Amplitude 1 baud 1 baud 1 baud 1 second • Constellation Diagram:

1

1 baud

0

Time

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 8

Phase Shift Keying (PSK)

• Quadrature Phase Shift Keying (Q-PSK): Do not only use 180 º shift

010 01 011 10 00 001 000 100 111 11

4-PSK (Q-PSK)

101 110

8-PSK • Min. BW requirement: same as ASK!

• Self clocking (most cases)

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 9

Quadrature Amplitude Modulation (QAM)

• Combine ASK and PSK such that each signal corresponds to multiple bits 3 amplitudes 12 phases • More phases than amplitudes • Minimum bandwidth requirement same as ASK or PSK

Winter 2005

ECE

ECE 766 Computer Interfacing and Protocols 05 10