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Chapter 8 Communication Systems

Chapter 8 Communication Systems. He Chun (SCIE,UESTC) cerelia@uestc.edu.cn;Room:KB244A. Communication Channel. Source Signals. Modulated Signals. Modulator. Introduction. A communication system. Be best suited for transmitting. The general process of embedding an information

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Chapter 8 Communication Systems

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  1. Chapter 8Communication Systems He Chun (SCIE,UESTC) cerelia@uestc.edu.cn;Room:KB244A

  2. Communication Channel Source Signals Modulated Signals Modulator Introduction • A communication system Be best suited for transmitting

  3. The general process of embedding an information -bearing signal into a second signal. Modulation : Extracting the information-bearing signal. Demodulation : Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) Modulation : Amplitude Modulation(AM) Modulating Signal Modulated Signal Carrier Signal

  4. Access Methods Base Station Forward link Reverse link Mobile Station Mobile Station Mobile Station Mobile Station 通信抗干扰技术国家重点实验室

  5. Frequency C C f 2 B B f 1 A A f 0 Time Multiplexing and Demultiplexing:FDMA

  6. Frequency f 0 C B A C B A C B A C B A A Time B C Multiplexing and Demultiplexing:TDMA

  7. Radio Spectrum Base-band Spectrum B Code A B Frequency Code A Code B A C C B C B B A B A A C A A B Time Multiplexing and Demultiplexing:CDMA spread spectrum Sender Receiver

  8. Summary of Multiple Access FDMA power TDMA frequency time power CDMA frequency time power frequency time

  9. Sinusoidal Amplitude Modulation • AM/w·c-------AM/with carrier • AM/s·c-------AM/suppressed carrier • DSB----Double side-band • SSB----Single side-band

  10. Amplitude Modulation(AM) Modulating system model: 8.1 Complex Exponential and Sinusoidal Amplitude Modulation Modulating Signal Modulated Signal Carrier Signal

  11. (1) Modulation Theory 8.1.1 Amplitude Modulation with Complex Exponential Carrier Exponential carrier: For convenience, let c=0, so Output signal(modulated signal):

  12. (2) Implementation

  13. 8.1.2 Amplitude Modulation with Sinusoidal signal For convenience, choose c=0, so

  14. Double Side Band

  15. 8.2.1 Synchronous demodulation (1) Demodulation process 8.2 Demodulation for Sinusoidal AM

  16. In time domain: In frequency domain: Expected signal:

  17. (2) Synchronous problem

  18. Time domain: The output of lowpass filter: Ideal output: x(t) • When ,it is referred to as synchronous demodulation. • When ? ?

  19. 8.2.2 Asynchronous demodulation Amplitude-modulated signal: A = 0 ⇒ DSB/SC A > 0 ⇒ DSB/WC Time Domain Frequency Domain

  20. Asynchronous demodulator In order for it to function properly, the envelop function must be positive definite, i.e. A + x(t) > 0.

  21. The two basic assumptions required for asynchronous demodulation are: • A+x(t) is positive; • x(t) vary slowly compared to wc. • Advantages of asynchronous demodulation: • Simpler in design and implementation. • Disadvantages of asynchronous demodulation: • Requires an extra transmitting power [Acoswct]2 to make sure A+x(t)>0

  22. BPF BPF BPF 8.3 Frequency-Division Multiplexing All the channels can share the same physical medium.

  23. Demultiplexing and Demodulation • Channels must not overlap ⇒ Bandwidth Allocation. • It is difficult (and expensive) to design a highly • selective bandpass filter with a tunable center frequency. • Solution — the SuperheterodyneReceivers.

  24. LPF Communication Channel LPF Quadrature Multiplexing

  25. Upper sideband Upper sideband Lower sideband 8.4 Single-Sideband Sinusoidal Amplitude Modulation Double Sideband Modulation (DSB)

  26. Upper sideband Upper sideband Lower sideband Retain the upper sideband Retain the lower sideband Single Sideband Modulation (SSB)

  27. Single Sideband Modulation

  28. Figure 8.21 SSB system using a 90o phase-shift network. 90ophase-shift

  29. Lower Sideband

  30. Chapter 8 Communication Systems HW8: 8.1 8.3 8.8 8.22 8.28

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