Single Sideband Suppressed Carrier (SSB-SC)

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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) f f c - f c - f c f - f c f c f f c - f c

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

4. 4 SSB: Time Domain Representation (1) Hilbert Transform: 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 Thus, f θ h ( f ) f |H ( f )|

5. 5 SSB: Time Domain Representation (2) SSB: M ( f ) M + ( f ) M _ ( f ) f f f B -B -B B

6. 6 SSB: Time Domain Representation (3) Hence, Similarly, M + ( f- f c ) M + ( f+f c ) M - ( f- f c ) M - ( f+f c ) f c f f c f - f c - f c 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 f c1 and then again using a higher carrier frequency f c . Thus achieves a gap of 2 f c1 for second stage selective filtering.

8. 8 SSB-SC: Detection 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 2 B 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

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 BPF H i ( f ) LPF H o ( f ) Coherent detection: VSB signal generation: H i ( f ) = Vestigial shaping filter Synchronous detection

19. 19 VSB (3): Example The carrier frequency is 20 kHz. Baseband signal bandwidth is 6 kHz. H i (f) is shown if fig (a). Determine H (f). Solution:

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