Data communication

1. 5.1
Chapter 5
Analog Transmission
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2. 5.2
5-1 DIGITAL-TO-ANALOG CONVERSION
• Converting Digital Data to a bandpass analog
signal is called digital to Analog conversion
• Converting a low-pass analog signal to a bandpass
analog signal is called Analog to analog conversion
• Digital-to-analog conversion is the process of
changing one of the characteristics of an analog
signal based on the information in digital data.

3. 5.3
Digital to Analog Conversion
 Digital data needs to be carried on an
analog signal.
 A carrier signal (frequency fc) performs
the function of transporting the digital
data in an analog waveform.
 The analog carrier signal is manipulated
to uniquely identify the digital data
being carried.

4.  By changing the sine wave characteristics
i.e. amplitude , frequency and phase we
can use them to send our digital data
5.4

5. 5.5
Figure 5.1 Digital-to-analog conversion

6. 5.6
Figure 5.2 Types of digital-to-analog conversion

7. 5.7
• Bit rate, N, is the number of bits per
second (bps).
• Baud rate is the number of signal
elements per second (bauds).
• In the analog transmission of digital
data, the signal or baud rate is less
than or equal to the bit rate.
S=Nx1/r bauds
• Where r is the number of data bits per
signal element and N is data rate.
Note

8. 5.8
An analog signal carries 4 bits per signal element. If
1000 signal elements are sent per second, find the bit
rate.
Solution
In this case, r = 4, S = 1000, and N is unknown. We can
find the value of N from
Example 5.1

9. 5.9
Example 5.2
An analog signal has a bit rate of 8000 bps and a baud
rate of 1000 baud. How many data elements are
carried by each signal element? How many signal
elements do we need?
Solution
In this example, S = 1000, N = 8000, and r and L are
unknown. We find first the value of r and then the value
of L.

10. Carrier signal
 In analog transmission the sending device
produces a high frequency signal that acts
as a base for the information signal.
 This base signal is called the carrier signal
or carrier frequency
5.10

11. 5.11
Amplitude Shift Keying (ASK)
 ASK is implemented by changing the
amplitude of a carrier signal to create signal
elements and reflect amplitude levels in the
digital signal.
 Frequency and phase remain constant while
amplitude changes

12. Binary ASK(BASK)
 Normally implemented using only two
levels
 Also referred to as on-off keying(OOK)
 Peak amplitude of one signal level is 0 and
other is the same as the amplitude of the
carrier frequency
5.12

13. 5.13
Figure 5.3 Binary amplitude shift keying

14. 5.14
Figure 5.4 Implementation of binary ASK

15. 5.15

16. 5.16

17. 5.17

18. 5.18
Frequency Shift Keying
 The frequency of the carrier signal is
varied to represent data
 Peak amplitude and phase remains
constatnt

19. Binary FSK
 Consider two carrier frequencies f1 and f2
 F1 for 0 and f2 for 1
5.19

20. 5.20
Figure 5.6 Binary frequency shift keying

21. 5.21

22. 5.22
Implementation :
Coherent and Non Coherent
 In a non-coherent FSK scheme, when
we change from one frequency to the
other, we do not adhere to the current
phase of the signal.
 In coherent FSK, the switch from one
frequency signal to the other only
occurs at the same phase in the signal.

23. 5.23
Multi level FSK
 Similarly to ASK, FSK can use multiple
bits per signal element.
 That means we need to provision for
multiple frequencies, each one to
represent a group of data bits.

24. 5.24
Figure 5.7

25. 5.25
Phase Shift Keyeing
 We vary the phase shift of the carrier
signal to represent digital data.
 PSK is more common
 PSK is much more robust than ASK
noise can change the amlitude easily
rather than ASK

26. Binary PSK
 Only two signal elements one with a
phase of 0 degree and other with a phase
of 180 degree
 PSK is superior to FSK because we do not
need two carrier signals
5.26

27. 5.27
Figure 5.9 Binary phase shift keying

28. 5.28
Figure 5.10 Implementation of BASK (Use polar NRZ signal)

29. Quadrature PSK
 It is called Quadrature PSK or QPSK
because it uses two separate BPSK
mpdulations one is in phase the other
quadrature (out of phase)
 The incoming bits are passed through a
serial to parallel conversion that sends
onebit to one modulator and 2nd bit to
another modulator
5.29

30.  One carrier frequency is phase shifted 90o
from the other - in quadrature.
 The two PSKed signals are then added to
produce one of 4 signal elements. L = 4
here.
5.30

31. 5.31
Figure 5.11 QPSK and its implementation

32. 5.32
Constellation Diagrams
 A constellation diagram helps us to
define the amplitude and phase of a
signal when we are using two carriers,
one in quadrature of the other.
 The X-axis represents the in-phase
carrier and the Y-axis represents
quadrature carrier.

33. 5.33
Figure 5.12 Concept of a constellation diagram

34. 5.34
Example 5.8
Show the constellation diagrams for an ASK (OOK),
BPSK, and QPSK signals.
Solution
Figure 5.13 shows the three constellation diagrams.

35. 5.35
Figure 5.13 Three constellation diagrams

36. 5.36
Quadrature amplitude modulation is a
combination of ASK and PSK.
Note

37. 5.37
Figure 5.14 Constellation diagrams for some QAMs

38. 5.38

39. 5.39

40. 5.40

41. 5.41