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EE 354Modern Communication Systems
Digital BandpassASK, FSK, PSK
Spring 2016
Instructor: C. R. Anderson
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Recall: Modulation of a sinusoid
cos 2 cs t A f t
Amplitude
ModulationFrequency Modulation
Phase Modulation
Message signal:
(Stochastic Process)
message bn
u t b n p t nT
0,1bData Bits:
Pulse Shape: p t
2
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Amplitude Shift Keying
1 0 0 1 1 0 0 1 1 0 0 1
Vo
lts
time
Unipolar
NRZ Data
OOK
Waveform
cos 2p message cu t u t f t
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ASK Power Spectral Density
1 1cos 2
2 2c c cx t f t X f f X f f
The Result:
For ASK: Translating the PSD of the message signal up to the carrier.
Recall: Frequency Shift Property of the Fourier Transform:
Which means that the PSD will be:
2 21 1
2 2
1 1
4 4message message
U c c
U U c U c
S f X f f X f f
S f M f f M f f
messageUM
2sincmessageUM f f
is the baseband PSD of the NRZ bitstream.
Square Pulse:
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Frequency Shift Keying
1 0 1 1 0 1 1 0 0 1 1 0 0 1
Unipolar
NRZ Data
FSK
Waveform
0 11 cos 2 cos 2p b bn n
u t b n p t nT f t b n p t nT f t
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FSK Power Spectral Density
1 1cos 2
2 2c c cx t f t X f f X f f
The Result:
For FSK: Effectively creating two ASKsignals at two different carrier frequencies.
Recall: Frequency Shift Property of the Fourier Transform:
Which means that the PSD will be:
Note: For FSK we have two different carrier frequencies !0 1
&c cf f
2 21 1
2 2
1 1
4 4message message
U c c
U U c U c
S f X f f X f f
S f M f f M f f
messageUM
2sincmessageUM f f
is the baseband PSD of the NRZ bitstream.
Square Pulse:
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Phase Shift Keying
cos 2p c messageu t f t u t
cos 2 ; 1, 1p message cu t u t f t b
Bipolar
NRZ Data
BPSK
Waveform
BPSK Only!
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PSK Power Spectral Density
1 1cos 2
2 2c c cx t f t X f f X f f
The Result:
For PSK: Translating the PSD of the message signal up to the carrier.
Recall: Frequency Shift Property of the Fourier Transform:
Which means that the PSD will be:
2 21 1
2 2
1 1
4 4message message
U c c
U U c U c
S f X f f X f f
S f M f f M f f
messageUM
2sincmessageUM f f
is the baseband PSD of the NRZ bitstream.
Square Pulse:
