1. Introduction to Communications
Prof. R. C. T. Lee
National Chi Nan University

2. Fourier Transform

3. What does the above signal contain?

4. A Music Signal

6. What does the above signal contain?
For human voice signals, Fourier Transform shows that the significant frequency components are under 3k Hertz.
This in turn means that the wavelength of the signal is around meters.
Again, this means that the antenna has to be exceedingly long.

7. What does the above signal contain?
This means that the human voice can not be transmitted wirelessly because of the problem about the required length of the antenna. This leads to the concept of modulation.

8. (a) Rectangular pulse in time. (b) The FT of x(t).

9. This means that a smaller width of the
impulse will imply the existence of
higher frequencies.
Smaller width also means higher bit rate.
This is why a transmission line transmitting
a large number of pulses in one second
is called the broadband system.

10. Modulation
The main effect of modulation is to increase the frequency of the signals significantly such that a wireless transmitting of these signals can be done.

11. Modulation
Modulation: The letter is put onto a horse by the person who wants to deliver the letter.
Demodulation: The letter is taken down from the horse by the person who receives the letter.

12. Amplitude Modulation
m(t): signal to be transmitted (baseband signal, usually of relatively low frequency)
c(t): the carrier wave signal (usually of relatively high frequency)

13. For AM broadcast, fc is in the range of 500k to 1.6
Amplitude Modulation
For AM broadcast, fc is in the range of 500k to 1.6
mega Hertz

14. Amplitude Modulation (a) Baseband signal m(t) (b) AM wave for
all t
(c) AM wave for
for some t

15. Demodulation
Envelop detector

16. Demodulation
(a) The message signal (b) The AM signal (c) The demodulated signal

17. Demodulation
(a) Spectrum of baseband signal (b) Spectrum of AM wave

18. Double Sideband (DSB)

19. Double Sideband (DSB) The RC-ckt can no longer be used to demodulate
(a) The message signal (b) The DSB-SC modulated signal

20. Double Sideband (DSB) Spectrum of the message signal
(b) Spectrum of the DSB-SC modulated signal

21. Double Sideband (DSB)
Demodulation

22. Double Sideband (DSB)
Coherent detection of DSB-SC modulated signal

23. Double Sideband (DSB)
Illustrating the spectrum of the demodulation process

24. Frequency Modulation
We change frequency, instead of amplitude.

25. Frequency Modulation Sinusoidal message signal (b) Carrier wave
(c) Frequency-modulated signal

26. Frequency Modulation
Demodulation: After differentiation

27. Frequency Modulation
Block diagram of a slope detector for FM demodulation

28. Frequency Modulation
The envelope of v(t) is

29. Digital Modulation
Baseband Pulse Transmission

30. Digital Modulation
Amplitude Shift Keying

31. Digital Modulation
Phase Shift Keying
Demodulation

32. Digital Modulation Coherent detector for BPSK Threshold x
Say “1” if x>
decision device
Say “0” if x<
Coherent detector for BPSK

33. Digital Modulation (a) Transmitted signal (b) The signal
(c) The output of the correlator and the corresponding sampling points

34. Digital Modulation
Frequency Shift Keying

35. Digital Modulation
FSK

36. Digital Modulation
FSK
Demodulation

37. Digital Modulation
FSK
if 1 is transmitted, .
if 0 is transmitted, .

38. Digital Modulation
FSK

39. Digital Modulation
FSK
Coherent detector for FSK system

40. Digital Modulation
FSK

41. Multiple Access Communication
Frequency Division Multiple Access(FDMA)

42. Multiple Access Communication
Spectrum of an FDMA system

43. Multiple Access Communication
Time Division Multiple Access(TDMA)

44. Multiple Access Communication
Code Division Multiple Access(CDMA)

45. Multiple Access Communication
CDMA : Two users : 1 and 2.
User i transmits to send 1 and to send 0.

46. Multiple Access Communication
CDMA
To find what (i=1,2) is, we multiply r(t) by
and integrate

47. Multiple Access Communication
CDMA

48. Multiple Access Communication
CDMA

49. Multiple Access Communication
K-User CDMA System

50. Multiple Access Communication
Carrier Sense Multiple Access (CSMA)

51. Electromagnetic Waves (Maxwell Equations)
Solution:

52. Electromagnetic Waves (Maxwell Equations)
A wave at t=0

53. Electromagnetic Waves (Maxwell Equations)
A wave at t=t

54. Electromagnetic Waves (Maxwell Equations)
The Electric and Magnetic Fields in a Plane Electromagnetic Wave

55. Electromagnetic Waves (Maxwell Equations)
Hertz,
f = 1800 mega Hertz (mobile phone),

56. Transmission Lines
A Co-Axial Cable Transmission Line.

57. Transmission Lines
Twin-Strip Parallel Plate Transmission Line.

58. Transmission Lines
An Equivalent Circuit of a Lossless Transmission Line.

59. Transmission Lines

61. The Waves on a Transmission Line.

63. Without great mathematicians such as Fourier, Maxwell and so on, there can be no modern communication technology.
Even the ADSL technology is based upon the Fourier transform.