Communication Systems 1 - Spring 2024 AM Lectures Group 2.pptx

Communication systems

1. Bandwidth-Efficient Amplitude Modulation Methods In DSB-SC and AM, message of original bandwidth B Hz needs 2B Hz to be transmitted after modulation. (BW Efficiency is 50%) Applications like telephony and TV, where total channel BW is a scarce parameter, BW efficiency of modulation scheme is crucial, and will affect total system capacity. Information in USB and LSB are identical. Three types of Amplitude Modulation enhance BW efficiency over AM and DSB-SC: (a) Quadrature Amplitude Modulation. (QAM) (b) Single Side Band. (SSB) (c) Vestigial Side Band. (VSB)

2. (a) Quadrature Amplitude Modulation (Quadrature Carrier Multiplexing) Utilizes the quadrature null effect to double the BW efficiency ! Note: m 1 (t),m 2 (t) are two different messages signals usually having equal BW. 2   Balanced Product Modulator Balanced Product Modulator

3. Assume that M 1 (f) and M 2 (f) are given by the below spectra: Recall that (from FT properties): So the Fourier transform of the QAM signal could be represented by   3 M 2 (f) f M 1 (f) f M 1 (f-f c ) ` ` f Im(M(f) Re(M(f) - π /2 + π /2 M 1 (f+f c ) M 2 (f-f c )e -j π /2 M 2 ( f+f c ) e +j π /2

4. We usually substitute 3D graph of FT (Re(M(f) and Im (G(f)) vs f ) with two-2D graphs, |G(f)| vs f (magnitude spectrum) and <(G(f)) vs f (phase spectrum). So the magnitude spectrum for QAM signal is shown below 4 2B f c f 2B |S(f)| M 1 (f-f c ) M 2 (f-f c ) -f c M 2 ( f+f c ) M 1 ( f+f c )

5. QAM R x 5 Mixer Mixer V1o (t) V2o (t) V1(t) V2(t)

6. Two signals are transmitted at the same BW of 2B (Spectrum overlap). BW efficiency in QAM is 100%. Exercise : Address the effect of phase-error in receiver carrier on QAM system (co-channel interference or cross-talk). 6

7. (b) Single Side Band (SSB) Modulation To conserve BW i.e. it doubles the BW efficiency of DSB. Recall that DSB results in s(t) with the spectrum shown for a given Since both USB and LSB are identical, we may transmit either USB or LSB without loss of information.

8. Generation of SSB 1. By Filtering (Frequency-Discrimination Method): We first produce a DSB signal then we filter out either LSB or USB. Practically, the message signal must have little or no energy near DC. This condition is satisfied in speech signals f min =300Hz For signals with the shown M(f), the transition band of the BPF must be less than 2f min at f c

9. 9 SSB Generation by filtering

10. 2- Phasing Method for generating SSB: Define the Hilbert Transform of a signal m(t) defined as: , which could be represented as a convolution between m(t) and a system with impulse response 1/ π t, that is Noting that, from Fourier Transform table: Therefore, the Hilbert transform could be considered as passing the signal m(t) through a filter with at transfer function 10  

11. Therefor, M h (f) is define by Now we need reform the SSB spectra using the Hilbert Transform, and then obtain a time-domain representation. From the Fig. 4.14 (next slide), we note the following: 11  

12. 12  

13. Now, we need to express Upper Side Band (USB) and Lower Side Band (LSB) required signals in terms of M + (f) and M - (f). Recall that, from FT modulation property: Therefore, in the time domain: 13

14. 14

15. so, SSB time domain representation is given by Notes: m h (t) is obtained from m(t) by shifting the phase of all frequency components by We multiply s(t) resulting from the phasing method by ½ for equivalence with the filtering method 15 x ~ S SSB (t) cos ω c t x m(t) -90° Hilbert Filter sin ω c t m h (t) USB LSB Σ

16. Demodulation of SSB Coherent detection I required at the R x as in DSB-SC 16 x LPF cos w c t S(t) V (t) V(t)

17. Note: Synchronization errors have a different effect on SSB signals 17 V(f) f 2f c - 2f c V o (f) f c - f c f f c - f c S(f)

18. Effect of phase offset Consider a modulating tone After the LPF, Hence, a phase offset in the R x LO appears as a phase shift for all frequencies. This is phase distortion 18

19. Fortunately, the human ear is insensitive to phase distortion is speech signals (up to 30Hz) Transmission BW of SSB B T =B Effect of a frequency offset A frequency offset causes a spectral shift. For a modulating tone and a frequency offset Exercise: Verify Conclusion: SSB offers twice BW efficiency of DSB but requires more complex transmitters. Also, SSB generation by filtering requires a signal with little energy near DC. 19

20. SSB+C: So under the condition of A>> |m(t)|, SSB+C could be demodulated using Envelope Detector, like AM. However , note that the condition for AM proper reception is that A> m p , where in SSB+C the condition is A>> m(t) so we expect much lower power efficiency. 20

21. (c) Vestigial (asymmetric) Side Band (VSB) Modulation A portion of one sideband and a vestige of the other are transmitted. H i (f) exhibits odd symmetry around f c between f c -f v and f c +f v In that case, coherent demodulation followed by a LPF may be used as in DSB and SSB. 21 x BPF H i (f) cos ω c t m(t) S v (t) f c +f v f c +B f c -f v f c -B has odd symmetry around f c | s DSB (f)| Generation of VSB f c

22. 22

23. Spectra Comparison 23

24. Transmission Bandwidth: B T = B+f v Notes: Used for signals with significant energy near DC eg . TV broadcasting. VSB is a compromise between DSB and SSB in terms of BW efficiency. In TV broadcasting, video signal needs 9MHz if DSB. In VSB, ~6MHz is needed. If H v (f) has no odd symmetry, VSB may still be used, but a filter must be applied at the R x before coherent detection. 24 Spectrum of a TV channel

25. Condition of transmit and receive filters: Exercise: Proof the above requirement on receive filter. Similar to SSB+C, VSB+C could be demodulated using envelope detector under the condition A>>|m(t)| 25   x LPF H o (f) cos ω c t s(t) m(t) Detection of VSB Detection of VSB

26. 26

27. Frequency Division Multiplexing (FDM) Definition: The positioning of several signal spectra in non-overlapping spectral bands using frequency translation for transmission over a common channel. 27

28. FDM may be performed in several stages. The first group of multiplexed signals forms a Basic Group . Several multiplexed basic groups form a Super Group and so on. This is done because of the difficulty in obtaining a high degree of selectivity in BPF’s at high frequency. The bandwidth required is NB for SSB or 2NB for DSB, where N is the number of multiplexed channels and B is the bandwidth of each channel. Note: The above concept of FDM apply to all modulation schemes. 28 ……. Basic Group Super Group

29. FDM in Analog Telephony Systems: 29

30. Synchronization (carrier recovery) in DSB-SC receivers 1- Costas Loop The VCO has a nominal frequency f c (when its input voltage is zero). As the input voltage increases, the VCO frequency increases and vice versa. 30 x LPF LPF LPF x 90° x m(t) cos ω c t VCO Phase discriminator

31. The LPF in the phase discriminator is sufficiently narrow to function as averaging element for m 2 (t). As drifts from zero, the VCO output phase change to bring down to zero. 31

32. Squaring Loop 32 Narrow BPF (2fc) ( ) ^2 Limiter Frequency Divider s(t) DSB-SC signal c(t) recovered carrier

33. Modulation Scheme BW Efficiency Power Efficiency System Complexity (TX & RX) Demodulator type DSB-SC 50 % (2B) 100% moderate Coherent AM 50% (2B) < 33% Low Envelope Detector QAM 100% (2B/2) 100% High Coherent SSB-SC 100% (B) 100% V High Coherent SSB +C 100% (B) <<33% Low Envelope Detector VSB 50<efficiency<100 % ( B+B v ) 100 % moderate Coherent VSB+C 50<efficiency<100 % ( B+B v ) <<33% Low Envelope Detector 33 Comparison Between AM schemes

34. Selection Criteria of a Modulation Scheme: Broadcasting (AM, FM, TV)  Simplify Receiver Complexity, Optimize Transmission BW. Power efficiency is a second priority. Point-To-Point (PTT, Cordless Phones)  Optimize Power Efficiency and overall system complexity. BW efficiency is a second priority. Battery-powered and Energy-sensitive (Satellite Comm.)  Optimize power efficiency. All others are second priority. Cost-sensitive systems  Optimize system complexity. 34

35. Super Heterodyne Receivers Block Diagram for R x 1. The mixer causes a translation of the spectrum of the RF signal. This is called frequency translation, frequency mixing or heterodyning. 35 RF Amp. & Filter X IF Amp. & FIlter AM/FM Detector (LO) Mixer

36. are produced 36 f RF =f c - f RF f LO -f RF - f LO f LO f LO +f RF zero zero

37. 2. If IF filter is centered around f IF = f LO -f RF , it rejects the sum frequency. 3. The RF stage & LO are tuned jointly so that always f LO -f RF = f IF ( f IF is fixed) 4. The advantage of heterodyne receiver is that filtering and amplification occur at fixed frequency (because tunability and selectivity are conflicting objectives). 5. AM radio band goes from 520 to 1610KHz Hence, f LO is chosen to be higher than f RF ,since it is easier to design a tunable LO in the range 1 to 2MHz than in the 0.1 to 1MHz range (This assumes f IF =455KHz) 37

38. Image Frequency: A signal with frequency f image = f LO +f IF will be mixed down to the IF filter band because: The image signal must be attenuated prior to mixing. This is one of the functions of the RF filter (acts as Image Rejection Filter). Example. 38

39. End of Chapter 4 39