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ارتباطات داده (883-40) مدولاسیون های موج پیوسته

ارتباطات داده (883-40) مدولاسیون های موج پیوسته. دانشکده مهندسی کامپیوتر. نیمسال دوّم 93-92 افشین همّت یار. Continuous-Wave Modulations. Introduction Amplitude Modulation Linear Modulation Schemes Frequency Translation Frequency -Division Multiplexing Angle Modulation

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ارتباطات داده (883-40) مدولاسیون های موج پیوسته

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  1. ارتباطات داده (883-40)مدولاسیونهای موجپیوسته دانشکده مهندسی کامپیوتر نیمسال دوّم 93-92 افشین همّتیار

  2. Continuous-Wave Modulations • Introduction • Amplitude Modulation • Linear Modulation Schemes • Frequency Translation • Frequency -Division Multiplexing • Angle Modulation • Frequency Modulation • Nonlinear Effects in FM Systems • Superheterodyne Receiver • Noise in CW Modulation Systems • Noise in Linear Receivers Using Coherent Detection • Noise in AM Receivers Using Envelope Detection • Noise in FM Receivers

  3. Introduction Continuous-Wave Modulation System Transmitter Receiver

  4. Amplitude Modulation (1) Sinusoid Carrier Wave: Amplitude Modulation:

  5. Amplitude Modulation (2) • Oldest method of modulation • Simple in implementation • Wasteful of power • Wasteful of bandwidth

  6. Linear Modulation Schemes (1) • Double Side-Band Suppressed Carrier (DSB-SC) Modulation • Single Side-Band (SSB) Modulation • Vestigial Side-Band (VSB) Modulation

  7. Linear Modulation Schemes (2) DSB-SC Modulation

  8. Linear Modulation Schemes (3) DSB-SC Demodulation

  9. Linear Modulation Schemes (4) SSB • A low-power pilot carrier is transmitted • in addition to the selected sideband. • A highly stable oscillator, tuned to the same frequency as the carrier frequency, is used in the receiver.

  10. Linear Modulation Schemes (5) VSB m(t) >> >> m’(t)

  11. Frequency Translation

  12. Frequency-Division Multiplexing

  13. Angle Modulation Phase Modulation Frequency Modulation

  14. Frequency Modulation (1) Narrowband FM: β << 1 , Wideband FM: β >> 1

  15. Frequency Modulation (2) 1 percent bandwidth

  16. Nonlinearity Effects in FM Systems No sensitive to amplitude nonlinearities but sensitive to phase nonlinearities such as AM-to-PM Conversion

  17. Superheterodyne Receiver • Carrier–frequency tuning • Filtering • Amplification

  18. Noise in CW Modulation Systems (1) • Channel Model: Additive White Gaussian Noise (AWGN) • Receiver Model: Ideal Band-pass filter + Ideal demodulator • Power Spectral Density (PSD) of the Noise: N0/2

  19. Noise in CW Modulation Systems (2) • (SNR)C : Ratio of the average power of the modulated signal to the • average power of channel noise in the message • bandwidth, both measured at the receiver input. • (SNR)O: Ratio of the average power of the demodulated message • signal to the average power of the filtered noise, both • measured at the receiver output. • (Depends on the type of modulation used in the transmitter and the type of the demodulation used in the receiver.) • The figure of merit may equal one, be less than one, or be greater • than one, depending on the type of modulation used. • The higher the figure of merit, the better will the noise • performance of the receiver be.

  20. Noise in CW Modulation Systems (3) • Requirements for comparison between different modulation-demodulation systems are: • The modulated signal transmitted by each system has the same • average power. • The channel noise has the same average power measured in the • message band-width.

  21. Noise in Linear Receivers using Coherent Detection  • Coherent SSB Receiver same as a coherent DSB-SC receiver • Both cases same as transmission without modulation • No trade-off between noise performance and bandwidth

  22. Noise in AM Receivers using Envelope Detector (1)  • The figure of merit of an AM receiver using envelope • detection is always less than unity. • Due to wastage of transmitter power, the noise performance • of a full AM receiver is always inferior to that of a DSB-SC receiver.

  23. Noise in AM Receivers using Envelope Detector (2) • The envelope detector favor strong signals and penalizes weak • signals (weak signal suppression).

  24. Noise in FM Receivers (1)  • Capture effect: The FM receiver locks to the stronger signal. • When the signals are nearly equal strength, the receiver • fluctuates back and forth between them.

  25. Noise in FM Receivers (2) I : Unmodulated carrier II: Sinusoidally modulated carrier Threshold: 10-11 dB

  26. Noise in FM Receivers (3) Threshold Reduction

  27. Noise in FM Receivers (4) Pre-emphasis and De-emphasis Improvement Factor:

  28. Noise in FM Receivers (5) Pre-emphasis and De-emphasis

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