5. Modulation

advertisement
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
Winter 2005
ECE
Change one or more of them
to send information
ECE 766
Computer Interfacing and Protocols
05 - 1
Modulation
• Modulation Techniques
– Amplitude Shift Keying (ASK)
– Frequency Shift Keying (FSK)
– Phase Shift Keying (PSK)
Quadrature
Amplitude
Modulation (QAM)
• Aspects of Modulation
– Bit rate vs. Baud rate
Bit rate ≥ Baud rate
– Carrier Signal
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
Amplitude
0
1
0
1
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 = Nbaud
Frequency
fc-(Nbaud/2)
fc
fc+(Nbaud/2)
• BW = (1+d) x Nbaud
• 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)
Amplitude
0
1
0
1
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
BW=fc1--fc0+Nbaud
fc1--fc0
Nbaud/2
Nbaud/2
Frequency
fc0
fc1
• 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
Amplitude
0
1
0
1
Time
1 baud
• Not practical
Winter 2005
ECE
1 baud
1 baud
1 baud
1 second
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
Amplitude
0
1
1
0
Time
1 baud
1 baud
1 baud
1 baud
1 second
• Constellation Diagram:
Winter 2005
ECE
1
ECE 766
Computer Interfacing and Protocols
0
05 - 8
Phase Shift Keying (PSK)
• Quadrature Phase Shift Keying (Q-PSK):
Do not only use 180º shift
010
01
011
10
001
00
100
11
000
111
101
110
4-PSK (Q-PSK)
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
Download
Related flashcards
Create Flashcards