EE 354
Modern Communication Systems
Digital Bandpass
ASK, FSK, PSK
Spring 2016
Instructor: C. R. Anderson
1
Recall: Modulation of a sinusoid
s  t   A cos  2 f ct   


Amplitude
Modulation
Frequency
Modulation
Message signal:
(Stochastic Process)
Data Bits:
Pulse Shape:

umessage  t  
Phase
Modulation

 b  n p  t  nT 
n 
b
b  0,1
p t 
2
1
Amplitude Shift Keying
u p  t   umessage  t  cos  2 f ct 
Unipolar
NRZ Data
0
0
1
1
0
0
1
1
0
0
1
Volts
1
time
OOK
Waveform
3
ASK Power Spectral Density
Recall: Frequency Shift Property of the Fourier Transform:
  x  t  cos  2 f c t   
1
1
X  f  fc   X  f  fc 
2
2
Which means that the PSD will be:
2
2
1
1
X  f  fc   X  f  fc 
2
2
1
1
SU  f   M U message  f  f c   M U message  f  f c 
4
4
SU  f  
M U message
is the baseband PSD of the
NRZ bitstream.
Square Pulse:
M U message  f   sinc2  f 
The Result:
For ASK: Translating the PSD of the
message signal up to the carrier.
4
2
Frequency Shift Keying
u p t  


 1  b  n p  t  nT  cos  2 f t    b  n p  t  nT  cos  2 f t 
b
n 
1
0
1
0
1
0
1
b
n 
1
0
0
1
1
1
0
0
1
Unipolar
NRZ Data
FSK
Waveform
5
FSK Power Spectral Density
Recall: Frequency Shift Property of the Fourier Transform:
  x  t  cos  2 f c t   
1
1
X  f  fc   X  f  fc 
2
2
Which means that the PSD will be:
2
2
1
1
X  f  fc   X  f  fc 
2
2
1
1
SU  f   M U message  f  f c   M U message  f  f c 
4
4
SU  f  
M U message
is the baseband PSD of the
NRZ bitstream.
Square Pulse:
M U message  f   sinc2  f 
Note: For FSK we have two different carrier frequencies f c0 & f c1 !
The Result:
For FSK: Effectively creating two ASK
signals at two different carrier
frequencies.
6
3
Phase Shift Keying
u p  t   cos  2 f ct  umessage  t   
u p  t   umessage  t  cos  2 f ct  ;
b  1, 1
BPSK Only!
Bipolar
NRZ Data
BPSK
Waveform
7
PSK Power Spectral Density
Recall: Frequency Shift Property of the Fourier Transform:
  x  t  cos  2 f c t   
1
1
X  f  fc   X  f  fc 
2
2
Which means that the PSD will be:
2
2
1
1
X  f  fc   X  f  fc 
2
2
1
1
SU  f   M U message  f  f c   M U message  f  f c 
4
4
SU  f  
M U message
is the baseband PSD of the
NRZ bitstream.
Square Pulse:
M U message  f   sinc2  f 
The Result:
For PSK: Translating the PSD of the
message signal up to the carrier.
8
4