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1. Communication Theory
Single-sideband Suppressed carrier
(SSB-SC) Modulation

2. f-B -B
M( f )
2
SSB-SC: Principle (1)
 Either USB or LSB is transmitted
 Required BW: B Hz
 Spectral efficiency (SE) is improved by 100%
Transmitter Side:
(Frequency Domain)
ffc- fc
- fc
f- fc
fc
ffc- fc

3. 3
SSB-SC: Principle (2)
 Can be detected coherently
Receiver Side (Frequency Domain):
After the multiplication by the carrier
f
f2fc-2fc
fc-fc
f

4. 4
SSB: Time Domain Representation (1)
Hilbert Transform:
        





t
txd
t
x
txHtxh





1
*
1
         fXfHffjXfXh  sgn
   








0,.1
0,.1
sgn
2
2
fej
fej
fjfH
j
j


 Thus, a Hilbert transformer is an ideal phase shifter that shifts the phase of every spectral
component by -π/2
 Difficult to achieve such sharp change in phase response
 fj
t
sgn1 

Thus,
f
θh (f)
f
|H (f)|

5. 5
SSB: Time Domain Representation (2)
          
    fjMfM
ffMfufMfM
h

2
1
sgn1
2
1
          
    fjMfM
ffMfufMfM
h

2
1
sgn1
2
1
SSB:
M (f)
M+ (f)
M_ (f)
f
f
f
B
-B
-B
B
     ffjMfMNote h sgn: 

6. 6
SSB: Time Domain Representation (3)
     
         chchcc
ccUSB
ffMffM
j
ffMffM
ffMffMf

 
2
1
2
1

Hence,
Similarly,
M+ (f-fc)
M+ (f+fc) M- (f-fc)
M- (f+fc)
fc f
fc f-fc
-fc
USB
LSB

7. 7
Generation of SSB-SC
Generation of SSB-SC:
1. Phase-shift method: requires ideal Hilbert phase shifter
2. Selective filtering method (most commonly used): First, DSB-SC signal is
generated and then passed through a band pass filter for selecting the desired
band. Requires ideal filter or null around DC.
3. Weaver method: Uses two stages of modulation (SSB using selective filtering)
– first using a smaller carrier frequency fc1 and then again using a higher carrier
frequency fc. Thus achieves a gap of 2fc1 for second stage selective filtering.

8. 8
SSB-SC: Detection
      
        


c
ttmttmtm
tttmttmtt
chc
cchccSSB



2carrierwithsignalSCSSB
2sin2cos
cos2sincoscos2



Demodulation using a coherent detector:
 An LPF will suppress unwanted SSB terms and produce m(t)
 Any of the synchronous DSB-SC demodulators can be used for demodulating
SSB-SC signal

9. 9
Comparison between AM, DSB-SC and SSB-SC
AM (DSB-WC) DSB-SC SSB-SC
Modulation Simple Simple Costly and Complex
(difficult to generate)
Demodulation Both envelope
detection and
coherent
detection
Coherent detection
=> Costly and
complex
Coherent detection
=> Costly and complex
Power Efficiency Max 33 %
=> Inefficient
Better
=> Efficient
Better
=> Efficient
Bandwidth (BW)
Requirement
Twice the signal
BW
=> Inefficient
Twice the signal
BW
=> Inefficient
Equal to the signal BW
=> Efficient

10. 10
Single-Sideband With Carrier
(SSB+C) Modulation

11. 11
SSB+C
Envelope detection:
SSB+C Signal:
From Taylor series expansion,
Envelope:
 Not power efficient as significantly higher amplitude is required for carrier

12. 12
Quadrature Amplitude
Modulation (QAM)

13. 13
QAM: Principle
 SSB signals are difficult to generate
 QAM is an attractive alternative to SSB
 Two base band signals, each of bandwidth B Hz, are sent over the same band of
bandwidth 2B Hz (Modulation: DSB –SC)
 The two carriers are of the same frequency with a phase difference of π/2
 QAM is also known as quadrature multiplexing (QM)
Synchronous
detector

14. 14
QAM: Detection (1)
 In-phase (I) Channel
 Quadrature (Q) Channel
        
      ttmttmtm
tttmttmtttx
cc
ccccQAM


2sin2cos
sinsincos2sin2
122
212


        
      ttmttmtm
tttmttmtttx
cc
ccccQAM


2sin2cos
cossincos2cos2
211
211



15. 15
QAM: Detection (2)
 Impact of loss of synchronization
 Loss of power
 interference
Output of the I-channel
Q. Derive the output of the quadrature (Q) channel.

16. 16
Vestigial Sideband (VSB)
Modulation

17. 17
VSB (1): Principle
 SSB signals are difficult to generate and DSB requires twice the signal bandwidth
 VSB is a compromise between DSB and SSB
 VSB inherits the advantages of DSB and SSB, but avoids their disadvantages at a small cost
 Bandwidth of VSB is little (typically 25%) greater than SSB
 VSB is also known as asymmetric sideband system

18. 18
VSB(2): Generation and Detection
        fHffMffMf iccVSB 
        cVSBcVSBcVSB ffffttte   cos2
        fHfffffM ocVSBcVSB  
BPF
Hi(f)
LPF
Ho(f)
Coherent detection:
VSB signal generation:
 
   
Bf
ffHffH
fH
cici
o 

 ||,
1
Hi(f) = Vestigial
shaping filter Synchronous
detection

19.  fHi
   cici ffHffH 
19
VSB (3): Example
The carrier frequency is 20 kHz. Baseband signal bandwidth is 6 kHz. Hi(f) is
shown if fig (a). Determine H0(f).
Solution:
 fHo

20. 20
VSB Application: Broadcast Television
 Video signal:
 large bandwidth (4.5 MHz) – DSB requires 9 MHz
 contains significant low-frequency component – SSB is not feasible
 The demodulation of the TV signal must be simple and cost effective – envelope detector is
preferred
 So, VSB modulation with the carrier is chosen for TV broadcast
DSB Spectrum
Transmitted
Spectrum

21. 21
End of AM