1. 디지털통신
Bandpass Modulation 1
임 민 중
동국대학교 정보통신공학과

2. Digital Bandpass Modulation
Baseband Modulation
Bandpass Modulation
Signal
Mapping
TX Filter
RX Filter
Decision
TX Data
RX Data
binary
discrete
pulse
pulse
discrete
binary
Noise
Analog
version
> 0 → 1
< 0 → 0
1 → 1
0 → -1
Signal
Mapping
TX Filter RF
Modulation RF
Demod
ulation
RX Filter
Decision
TX Data
RX Data
binary
discrete
pulse
wave
wave
pulse
discrete
binary
Noise
Analog
version
> 0 → 1
< 0 → 0
1 → 1
0 → -1

3. Digital Bandpass Modulation

4. Bandpass Modulation - 1 Digital modulation Baseband modulation
The process by which digital symbols are transformed into waveforms that are compatible with the characteristics of the channel
Baseband modulation
Waveforms are pulses
Bandpass modulation
Information signal modulates a sinusoid called a carrier
For radio transmission the carrier is converted to an electromagnetic field for propagation to the desired destination
Digital symbol: a sinusoid of duration T
Why is it necessary to use a carrier?
antenna size, frequency division
Carrier wave
Amplitude, Frequency, Phase

5. Amplitude Shift Keying Frequency Shift Keying
Bandpass Modulation - 2
Bandpass modulation
Amplitude Shift Keying
(ASK) t T T T
ASK is similar to DSB-SC
FSK is similar to FM
PSK is similar to PM
Frequency Shift Keying
(FSK) t T T T
Phase Shift Keying
(PSK) t T T T

6. Bandpass Modulation - 3 ASK, PSK, FSK Signals 2ASK (= 2PSK) 4ASK t t

7. Bandpass Modulation - 4 Bandpass demodulation
the reverse of the modulation process
Coherent demodulation
The receiver exploits knowledge of the carrier’s phase to detect the signals
Optimal method
Noncoherent demodulation
The receiver does not utilize phase reference information
Phase estimation is not required
Suboptimal method RF
Modulation RF
Demodulation
cos(2fct)
cos(2fct) RF
Modulation RF
Demodulation
cos(2fct)
cos(2fct+)
Coherent demodulation is more difficult but produces better results
Modulation for
Coherent Receiver
Coherent
Demodulation
Some transmission methods require coherent demodulation and will be called coherent method
Other transmission methods allow both coherent and noncoherent demodulation and will be called noncoherent method
Modulation for
Noncoherent Receiver
Noncoherent
Demodulation

8. Quadrature Amplitude Modulation
Bandpass Modulation - 5
Linear Modulation
ASK, PSK, QAM
Baseband signal (M-ary PAM signal)  Carrier
Bandwidth efficient
high data rate transmission
High cost
Large peak-to-average power ratio
Constant Envelope Modulation
FSK
Power efficient
Low cost
Low peak-to-average power ratio
Quadrature Amplitude Modulation

9. Modulation/Demodulation Procedures
1 → 1
0 → -1
Signal
Mapping
Pulse
Shaping
Filter RF
Modulation
Power Amp
binary data
discrete symbol
baseband pulse
sinusoidal wave
Decision
Matched
Filter RF
Demodulation
Low Noise
Amp
> 0 → 1
< 0 → 0

10. Signal Space Diagram - 1 Signal Space Diagram (Constellation)
for ASK, PSK, QAM (not for FSK)
sine and cosine waves are orthogonal
Assuming that carrier frequency is not changing
and we only consider amplitude and phase
sin(2fct)
Quadrature
(Amplitude, Phase)
Constellation
sin
cos(2fct)
-cos
cos
2cos
In-phase
(I,Q)
For example,
-2  -2 cos(2fct)
j  cos(2fct - /2) = sin(2fct)
1 + j  sqrt(2) cos(2fct - /4)

11. Signal Space Diagram - 2
Example

12. Amplitude Shift Keying
ASK, ASK-SSB, OOK
ASK-SSB 
M-ASK = M-PAM x Carrier
OOK 
ASK 
unipolar 
QAM 
bandwidth efficient 

13. Amplitude Shift Keying - 1
Transmitted signal waveforms
sm(t) = Amcos2fct, m = 1,2,…,M
Am = amplitude signal =-M, -M+2, …-1, 1, …, M-2, M
similar to DSB-SC
Baseband signal Am
Bandpass signal
Am cos2fct
2ASK
4ASK
8ASK
2PAM
4PAM
8PAM
2ASK = 2PAM x Cosine Carrier
4ASK = 4PAM x Cosine Carrier
8ASK = 8PAM x Cosine Carrier
Carrier
cos2fct 1 1
2ASK t t 11 01 10 00
4ASK t t

14. Amplitude Shift Keying - 2
2ASK signal
4ASK signal

15. Amplitude Shift Keying - 3
Received signal
r(t) = Amcos2fct + n(t), 0  t  T
n(t) = bandpass noise process
ASK Receiver
coherent receiver is required, in other words, knowledge of carrier phase is required at ASK receiver
cos2fct
Since the average of cosine or sine is zero, the integration of high-frequency cosine or sine is zero

16. Amplitude Shift Keying - 4
ASK modulation and demodulation
Message signal
x(t)
Carrier at transmitter
2cos2fct
After RF modulation (ASK signal)
x(t) 2cos2fct
Carrier at receiver
2cos2fct
After RF demodulation
low pass filter can extract the baseband signal
x(t) 2cos2fct 2cos2fct

17. Amplitude Shift Keying - 5
Symbol Decision Boundaries
2-ASK
4-ASK
8-ASK

18. Amplitude Shift Keying - 6
Gray Coding and Bit Decision Boundaries
adjacent binary values differ in only one bit
2-ASK 1
4-ASK 11 10 00 01
Left bit
Right bit
8-ASK
110
111
101
100
000
001
011
010
Left bit
Right bit
Middle bit
Due to Gray coding, one symbol error implies one bit error out of log2M bits in M-ary modulation
Hence, M-ASK BER = M-ASK SER / log2M
2ASK SER = 1 x 2ASK BER
4ASK SER = 2 x 4ASK BER
8ASK SER = 3 x 8ASK BER

19. Amplitude Shift Keying - 7
With fixed average symbol energy
distance between two adjacent points
= immunity to noise
When symbol energy = 1
2-ASK 1
most robust
(-A A)
symbol energy = A2 = 1
 A = 1 -A A
4-ASK 11 10 00 01
-3B -B B 3B
(-3B -B B 3B)
symbol energy = (B2 + (3B)2)/2 = 5B2= 1
 B = 1/sqrt(5)
8-ASK
110
111
101
100
000
001
011
010
high data rate
vulnerable to noise
high SNR is required
-7C
-5C
-3C -C C 3C 5C 7C
(-7C -5C -3C -C C 3C 5C 7C)
symbol energy = (C2 + (3C)2 + (5C)2 + (7C)2)/4 = 21C2 = 1
 C = 1/sqrt(21)

20. (rotation version of TX signal + noise)
On-Off Keying - 1
On-Off Keying (OOK)
Unipolar
Noncoherent method
Envelope detection or energy detection can be used
2ASK ~ bipolar
OOK ~ unipolar 1 t T T T
2ASK requires coherent receivers
OOK allows coherent or noncoherent receivers (coherent demodulation produces better performance)
TX signal
(no transmission)
RX signal
(noise)
Advantage:
- Simple receiver
- If 0 occurs with high probability, this method can save energy
Disadvantage:
- When 0 and 1 occur with equal probability, this method wastes power
RX signal
(rotation version of TX signal + noise)
TX signal

21. (rotated TX signal + noise)
On-Off Keying - 2
Coherent receiver
Noncoherent receiver
TX signal
1  1
0  0
> T/4  1
< T/4  0
cos2fct
RX signal
(TX signal + noise)
cos2fct
The performance is affected by real noise
TX signal
1  1
0  0
| |2
> T/4  1
< T/4  0
cos2fct
cos(2fct+)
RX signal
(rotated TX signal + noise)
The carrier phase is not known
The performance is affected by complex noise
sin(2fct+)

22. On-Off Keying - 3 Performance
2ASK > OOK with coherent receiver > OOK with noncoherent receiver
Bipolar vs. Unipolar
Coherent vs. Noncoherent
Bipolar -1 1
Unipolar

23. Carrier at transmitter
ASK-SSB - 1
SSB Transmission
Message signal
Carrier at transmitter
ASK wastes bandwidth
Double-sideband
ideal bandpass filter
SSB filter
Transmitted signal
single sideband

24. ASK-SSB - 2
SSB Reception
Received signal
Carrier at receiver

25. Bandwidth and Symbol Rate
Baseband
Passband
SSB
Example
8PAM, 10 MHz, r = 0
data rate = 3 bit/symbol  2 symbol/Hz  10 MHz = 60 Mbit/sec
8ASK, 10 MHz, r = 0
data rate = 3 bit/symbol  1 symbol/Hz  10 MHz = 30 Mbit/sec
8ASK-SSB, 10 MHz, r = 0
PAM
B: bandwidth (Hz)
Rs: symbol rate (symbol/sec)
r: roll-off factor
Rb: data rate (bit/sec)
= Rs log2M
for M-ary modulation
bandwidth
ASK
bandwidth
ASK-SSB
bandwidth