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Chapter 8 Communication Systems

Chapter 8 Communication Systems. 崔琳莉. Transmitter. Channel. Receiver. Destination. Information source. Introduction. A communication system. 调制. 解调. Modulation and Demodulation Modulation: Embedding an information-bearing signal into a second signal

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Chapter 8 Communication Systems

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  1. Chapter 8 Communication Systems 崔琳莉

  2. Transmitter Channel Receiver Destination Information source Introduction • A communication system 调制 解调

  3. Modulation and Demodulation • Modulation: Embedding an information-bearing signal into a second signal • Demodulation: extracting the information-bearing signal • Modulation methods • Amplitude modulation (AM) • Frequency modulation (FM) • Phase modulation (PM)

  4. c(t) x(t) y(t)=x(t)c(t) • Focus here for the most part on Amplitude Modulation (AM) Carrier signal Modulated signal Modulating signal

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

  6. Multiplexing and Demultiplexing • Simultaneous transmission of more than one signal over the same • Methods methods • Time-Division Multiplexing(TDM) • Frequency-Division Multiplexing(FDM)

  7. c(t) x(t) y(t)=x(t)c(t) 8.1 Complex Exponential and Sinusoidal Amplitude Modulation Modulating system model: x(t) --- modulating signal c(t) --- Carrier signal x(t) --- modulated signal

  8. 8.1.1 Amplitude Modulation with Complex Exponential Carrier Carrier frequency (1) Modulation Theory Exponential carrier: For convenience, let c=0, so Output signal(modulated signal):

  9. Re{y(t)} Im{y(t)} (2) Implementation

  10. 8.1.2 AM with Sinusoidal signal For convenience, choose c=0, so

  11. Double Side Band

  12. If wc<wm

  13. 8.2 Demodulation for Sinusoidal AM 8.2.1 Synchronous demodulation (1) Demodulation process

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

  15. (2) Synchronous (同步) problem

  16. Time domain: The output of lowpass filter: Ideal output: x(t) • When qc=fc, it is referred to as synchronous demodulation. • When qc≠fc ?? qc≠fc+p/2 ??

  17. 8.2.2 Asynchronous (异步) demodulation Amplitude-modulated signal: A = 0 ⇒ DSB/SC A > 0 ⇒ DSB/WC Time Domain Frequency Domain

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

  19. 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

  20. Double-Sideband (DSB) and Single-Sideband (SSB) AM Since x(t) and y(t) are real, from conjugate symmetry, both LSB and USB signals carry exactly the same information.

  21. Single Sideband Modulation Can also get SSB/SC or SSB/WC

  22. Frequency-Division Multiplexing (FDM) (Examples: All the radio-station signals) All the channels can share the same physical medium.

  23. FDM in the Frequency-Domain Channel a Channel b Channel c Multiplexed signals

  24. 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 Superheterodyne Receivers. Homework: 8.1 8.3 8.22 8.28

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