digital modulation techniques

1. DIGITAL COMMUNICTIONS
UNIT-2
DIGITAL MODULATION TECHNIQUES
ASK,FSK,PSK
Presented By:
M.KAIVALYA
Assistant Professor
Department of E.C.E.
GIET Engineering CollegeLecture Details:
DIGITAL COMMUNICATIONS
ECE, III B.Tech I sem

2. INTRODUCTION
•Digital band-pass modulation techniques
–Amplitude-shift keying
–Phase-shift keying
–Frequency-shift keying
•Receivers
–Coherent detection
•The receiver is synchronized to the transmitter with respect to carrier
phases
–Non-coherent detection
•The practical advantage of reduced complexity but at the cost of -degraded
performance

3. Amplitude Shift Keying (ASK)
 Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary
data in the form of variations in the amplitude of a signal.
 The amplitude of the resultant output depends upon the input data whether it should be a
zero level or a variation of positive and negative, depending upon the carrier frequency.
 One binary digit is represented by the presence of a carrier, the other binary digit is
represented by the absence of carrier, frequency is remains fixed.
 Following is the diagram for ASK modulated waveform along with its input.

4. Amplitude Shift Keying (ASK)

5. Amplitude Shift Keying (ASK)
Let the carrier be
Where A represents peak value of sinusoidal carrier. In the standard 1Ω resistance, the power
dissipated will be
S(t)= 2𝑝 cos(2π𝑓0t)
Similarly for ASK let the carrier be S(t)= 2𝑝 cos(2π𝑓0t) (to transmit ‘1’)
similarly S(t)=0 (to transmit ‘0’)
That means no signal is transmitted. S(t) contains some complete cycles of carrier frequency f.
thus , symbol 1----> pulse is transmitted
symbol o----> no pulse is transmitted
Thus ASK waveform look like an ON-OFF of the signal. Hence it is also called ON-OFF Keying(OOK)

6. Generation of ASK
 Any modulated signal has a high frequency carrier. The binary signal when ASK is modulated,
gives a zero value for LOW input and gives the carrier output for HIGH input.
ASK modulator:
 The ASK modulator block diagram comprises of the carrier signal generator, the binary
sequence from the message signal and the band-limited filter.
 Following is the block diagram of the ASK Modulator.

7. Generation of ASK
From the above fig.
S(t)= 2𝑝 cos(2π𝑓0t)
Let re-arrange the above equation as
S(t)=b(t) 𝑃𝑇b
2
𝑇𝑏
cos(2π𝑓0t)
Where Tb is the bit time (or) bit duration
 Let us assume
2
𝑇𝑏
cos(2π𝑓0t) =φ1(t)
s(t)=b(t) 𝑃𝑇b φ1(t)
 The bit energy Eb is defined in terms of power ‘p’ and bit duration Tb as
Eb=pTb

8. Generation of ASK
Then the equation s(t) becomes
S(t)=± 𝐸b φ1(t)
Now in binary ASK system follows symbol 1 & 0 are represented as follows
S(t) = 𝐸b φ1(t), 0≤t≤Tb for symbol ‘1’
= 0 , 0≤t≤Tb for symbol ‘0’

9. Detection of ASK
ASK Demodulator:
There are two types of ASK Demodulation techniques. They are −
 Asynchronous ASK Demodulation/detection
 Synchronous ASK Demodulation/detection
• The clock frequency at the transmitter when matches with the clock frequency at the
receiver, it is known as a Synchronous method, as the frequency gets synchronized.
Otherwise, it is known as Asynchronous.

10. Detection of ASK
Asynchronous ASK Demodulator
The Asynchronous ASK detector consists of a half-wave rectifier, a low pass filter, and a
comparator. Following is the block diagram for the same.

11. Detection of ASK
 The modulated ASK signal is given to the half-wave rectifier, which delivers a positive half
output. The low pass filter suppresses the higher frequencies and gives an envelope detected
output from which the comparator delivers a digital output.
Synchronous ASK Demodulator:
 Synchronous ASK detector consists of a Square law detector, low pass filter, a comparator,
and a voltage limiter. Following is the block diagram for the same.

12. Detection of ASK
 The ASK modulated input signal is given to the Square law detector. A square law detector is
one whose output voltage is proportional to the square of the amplitude modulated input
voltage. The low pass filter minimizes the higher frequencies. The comparator and the
voltage limiter help to get a clean digital output.
 The simplest way is to use an envelope detector, exploiting the non constant-envelope
property of the BASK signal

13. Encoding Techniques
There are some different types of encoding techniques to
encode the input binary sequence. They are
 Unipolar NRZ
 Polar NRZ
 Unipolar RZ
 Bipolar RZ
 Split Phase Manchester

15. Phase shift keying(PSK)
 The phase of the output signal gets shifted depending upon the input. These are mainly of
two types, namely BPSK and QPSK, according to the number of phase shifts.
 The other one is DPSK which changes the phase according to the previous value.
 Phase Shift Keying (PSK) is the digital modulation technique in which the phase of the carrier
signal is changed by varying the sine and cosine inputs at a particular time.
 PSK technique is widely used for wireless LANs, bio-metric, contactless operations, along with
RFID and Bluetooth communications.

16. Phase shift keying(PSK)
Principle of BPSK:
 In BPSK binary symbol ‘1’ and ‘0’ modulate the phase of the carrier, Let the
carrier be
 Consider for example

17. Phase shift keying(PSK)
Then we can write the above equation as
With the above equation we can define BPSK signal combinely as
Here b(t)=+1, when binary ‘1’ is to be transmitted
b(t)=‘-1’ when binary ‘0’ is to be transmitted

18. Generation of BPSK
BPSK Modulator:
 The block diagram of Binary Phase Shift Keying consists of the balance modulator which has
the carrier sine wave as one input and the binary sequence as the other input.
 Following is the diagrammatic representation.

19. Generation of BPSK
 The modulation of BPSK is done using a balance modulator, which multiplies the two signals
applied at the input. For a zero binary input, the phase will be 0° and for a high input, the
phase reversal is of 180°.
 Following is the diagrammatic representation of BPSK Modulated output wave along with its
given input.

20. Detection of BPSK
BPSK Demodulator:
 The transmitted BPSK signal is

21. Detection of BPSK
Square Law Device:
 At the output of the square law device, the signal will be
 We know that

22. Detection of BPSK
Band pass filter:
 The signal is then passed through a bandpass filter whose pass band is centered around 2f0
band pass filter removes the DC level of ½ and at its output we get
Frequency Divider:
 The output of a frequency divider we get a carrier signal whose frequency is f0 i.e
Synchronous demodulator:
 It multiplies the input signal and the recovered carrier, therefore the output of the multiplier
we get

23. Detection of BPSK
Bit synchronizer and Integrator:
 The integrator integrates the signal over one bit period
 The bit synchronizer take care of starting and ending times of a bit
 At the end of bit duration Tb, the bit synchronizer closes switch s2 temporarily.
 This connects the output of an integrator to decision device.
 It is equivalent to sampling the output of integrator

24. Detection of BPSK

25. Detection of BPSK

26. Detection of BPSK

27. Spectrum of BPSK signals

28. Spectrum of BPSK signals

29. Spectrum of BPSK signals

30. Spectrum of BPSK signals

31. Bandwidth of BPSK

32. Frequency Shift Keying(FSK)
 In BFSK, the frequency of a sinusoidal carrier is shifted according to the binary
symbol. i.e the frequency of a sinusoidal is shifted between two discrete values.
 This means that we have two different frequency signals according to the binary
symbols
 Hence there is increase or decrease in frequency by ‘w’
 The above equations can be combinely written as

33. Frequency Shift Keying(FSK)
 Thus when signal ‘1’ is to be transmitted, the carrier frequency will be
 If symbol ‘0’ is to be transmitted, the carrier frequency will be
Then they are represented as fH and fL

34. Generation of FSKFSK Modulator:
 The FSK modulator block diagram comprises of two oscillators with a clock and the input
binary sequence. Following is its block diagram.

35. Generation of FSK

36. Coherent Detection of FSK
 The block diagram of Synchronous FSK detector consists of two mixers with local oscillator
circuits, two band pass filters and a decision circuit. Following is the diagrammatic
representation.

37. Spectrum of BFSK
 The BFSK signal s(t) may be written as
s(t)= 2𝑝. pH(t). Cos(2πfH(t))+ 2𝑝. pL(t). Cos(2πfL(t)) ----------(1)
Let us compare this equation with BPSK equation which is written as
sBPSK(t)=b(t). 2𝑝. cos (2πf0t) -----------(2)
 It may be noted that this equation is identical to BFSK equation. In BPSK equation b(t) is polar
signal where as in BFSK, the similar coefficients pH(t) or pL(t) are unipolar.
Hence let us convert these coefficients in polar form.
pH(t)=
1
2
+
1
2
pH’(t) ------(3)
pL(t)=
1
2
+
1
2
pL’(t) ------(4)

38. Spectrum of BFSKSubstitute the values in the 1st equation
S(t)= 2𝑝.[
1
2
+
1
2
pH’(t)] cos (2πfH(t)+ 2𝑝. [
1
2
+
1
2
pL’(t)] cos (2πfL(t)]
=
𝑝
2
cos (2πfH(t) +
𝑝
2
cos (2πfL(t) +
𝑝
2
pH’(t) cos (2πfH(t) +
𝑝
2
pL’(t) cos (2πfL(t)
 The first term represents the single frequency impulse at fH, the second term represents the
pulse at fL ,those are constant amplitudes.
 The last two terms are similar to BPSK equation, here pH’(t) & pL’(t) are equivalent to b(t).
 Those last two terms in eq. 5 produce the spectrum which are similar to that of BPSK, one
spectrum is located at fH and other at fL. . Therefore we can write the power spectral density
of BFSK as
----(5)

39. Spectrum of BFSK

40. Non Coherent Detection of FSK
 The block diagram of Asynchronous FSK detector consists of two band pass filters, two
envelope detectors, and a decision circuit. Following is the diagrammatic representation.

41. Non Coherent detection of FSK
 The FSK signal is passed through the two Band Pass Filters BPFs tuned to Space and Mark
frequencies.
 The output from these two BPFs look like ASK signal, which is given to the envelope detector.
 The signal in each envelope detector is modulated asynchronously.
 The decision circuit chooses which output is more likely and selects it from any one of the
envelope detectors. It also re-shapes the waveform to a rectangular one.

42. Differential Phase Shift Keying(DPSK)
 DPSK is the non coherent version of PSK
 DPSK does not need a synchronous carrier at the demodulator.
 The input sequence of binary bits is modified such that the next bit depends upon
the previous bit.
 Therefore in receiver the previous received bits are used to detect the present bit.

43. Generation of DPSK
The input sequence is d(t), output sequence is b(t) and b(t-Tb) is the previous output
delayed by one bit period.
Depending up on the values of d(t) & b(t-Tb) exclusive OR gate generates the output
sequence b(t)

44. Generation of DPSKExample:
Encoding a data decoding a data

45. DPSK Wave Forms

46. DPSK receiver
 The received DPSK signal is applied to one of the input of multiplier.
 The delayed version of received DPSK signal by the time interval Tb is applied as
second input to the multiplier.
 By comparing the integrator output with a decision level, the decision device can
reconstruct the binary sequence by assigning a symbol 0 for –ve output and a
symbol 1 for +ve output.

47. Bandwidth of DPSK

48. Advantages and Disadvantages

49. Differentially Encoded PSK(DEPSK)

50. DEPSK
 If b(t)= b(t-Tb) then output of the EX-OR gate will be 0 i.e., d(t) =0
 If b(t)= b(t-Tb) then output of the EX-OR gate will be 1 i.e., d(t) =1
Errors in DEPSK system:
 In DEPSK the error will always occur in pairs i.e., single error
b(t)= 0 1 1 0 1 1 0 0 b’(t)= 0 1 1 1 1 1 0 0
b(t-Tb)= 0 1 1 0 1 1 0 0 b’(t-Tb)= 0 1 1 1 1 1 0 0
---------------------- ----------------------
d(t)= 1 0 1 1 0 1 0 d(t)= 1 0 0 0 0 1 0
Two errors

51. Advantages and Disadvantages
Advantage:
 The probability of error is lessthan the probability of error for DPSK because DPSK
uses the non coherent type demodulator technique
Disadvantges:
 Demodulator is very complex
 The errors are occur in pairs, one error in b(t) will give rise to two errors in d(t)

52. Quadrature Phase Shift Keying(QPSK)
 The Quadrature Phase Shift Keying QPSK is a variation of BPSK, and it is also
a Double Side Band Suppressed Carrier DSBSC modulation scheme, which sends
two bits of digital information at a time, called as bigits.
 Instead of the conversion of digital bits into a series of digital stream, it
Converts them into bit pairs. This decreases the data bit rate to half, which allows
space for the other users.
S1 S2 S3 S4
2Tb 2Tb  2Tb2Tb
01 1000 11

53. QPSK
 Four different phase states in one symbol period
 Two bits of information are transmitted in each symbol
 Twice the bandwidth efficiency of the BPSK
 Phase: 0, π/2, π, 3π/2 → possible phase values
 Symbol: 00 01 11 10

54. QPSK
Alternative Representation:
Phase Data
45 degrees Binary 00
135 degrees Binary 01
225 degrees Binary 11
315 degrees Binary 10

55. QPSK
Mathematical representation of QPSK:
VQPSK(t)= 2𝑝. cos[wct+(2m+1)π/4], m=0,1,2,3,…
 The QPSK system of modulation is called as four state PSK
or 4-psk.
s1(t)= 2𝑝. cos[wct+π/4]
s2(t)= 2𝑝. cos[wct+3π/4]
s3(t)= 2𝑝. cos[wct+5π/4]
s4(t)= 2𝑝. cos[wct+7π/4]

56. QPSK
QPSK Transmitter:

57. QPSK

58. QPSK1 2 3 4 5 6 7 8

59. QPSK
Hence VQPSK= 2𝑝. bo(t) sin wct+ 2𝑝. be(t) cos wct.
Symbol bo(t) be(t) VQPSK Quadrant
00 -1 -1 − 2𝑝. sin wct - 2𝑝. cos wct II
01 -1 +1 - 2𝑝. sin wct + 2𝑝. cos wct I
11 +1 +1 + 2𝑝. sin wct + 2𝑝. cos wct IV
10 +1 -1 - 2𝑝. sin wct + 2𝑝. cos wct III

60. QPSK
Phasor diagram of QPSK:

61. QPSK Receiver

62. QPSK Receiver
Bandwidth of QPSK:

63. Quadrature Amplitude Shift Keying(QASK)
 We have seen in the preceeding sections that the correct detection of the signal
depends up on the separation between the signal points in the space
 In case of PSK system all points lie on the circumference of the circle, this is
because PSK signal has constant amplitude throughput.
 If the amplitude of the signal is also varied then the points will lie inside the circle
also on the signal space diagram, this further increases the noise immunity of the
system.
 Such systems involves phase as well as amplitude shift keying, it is called
quadrature amplitude shift keying or simply QASK, it is also called quadrature
amplitude modulation (QAM) .

64. QASK/QAM
Geometrical representation of QASK:
 Assuming that we are transmitting a symbol consisting of 4 bits.
i.e., N=4 => M=24
=16 different possible symbols.
 In the geometric representation of fig. each signal point is equally distant from its nearest
neighbours, this distance is d=2a.

65. QASK/QAM
 From the above fig. we can determine the energy associated with a signal by
considering 4 signals in the 1st quadrant.
From fig.S1=(a,3a)
S2=(3a,3a)
S3=(3a,a)
S4=(a,a)
(from the formula average power(x,y)=𝑥2 + 𝑦2)

66. QASK/QAM
Types of QAM:
Depending on the number of bits per message, the QAM signals ,may be
classified as
Name of scheme Bits per symbol No. of symbols
4 QAM 2 22 = 4
8 QAM 3 23=8
16 QAM 4 24=16
32 QAM 5 25=32
64 QAM 6 26=64

67. QASK/QAM
QASK transmitter:

68. QASK/QAM
The QASK signal can be mathematically represented as

69. QASK/QAM

70. QASK/QAM
 The bit stream b(t) is applied to serial to a parallel converter operating on a clock
which has a period of TS seconds.
 The bits b(t) are stored by the converter and presented in the parallel form,
the four bit symbol is
 Out of these four bits , the first two bits are applied to a D/A converter and the
other two bits are applied to the second D/A converter.
 The output of the first D/A converter is Ae(t) which is used to modulate the carrier
Where as the output of second D/A converter is Ao(t) is used to
modulate the carrier in the balanced modulator
 then balanced modulator outputs are added together to get QASK output signal

71. QASK/QAM
QASK receiver:

72. M-ary(multi level) signaling scheme

73. M-ary(multi level) signaling scheme

74. M-ary(multi level) signaling scheme

75. M-ary(multi level) signaling scheme

76. M-ary ASK

77. M-ary ASK

78. M-ary ASK

79. M-ary ASK

80. M-ary FSK

81. M-ary FSK

82. M-ary FSK

83. M-ary FSK

84. M-ary PSK

85. M-ary PSK

86. M-ary PSK

87. M-ary PSK

88. M-ary PSK

89. Assaignment quetions
1. Explain with neat block diagram the generation and receiver of
BPSK and also explain about PSD of BPSK
2. Explain BFSK modulation in detail, and draw signal space
representation and PSD of BFSK
3. Explain the generation and detection of QPSK signals with the
help of block diagram and also draw output waveforms of QPSK.
4. The binary sequence 1100100010 is applied to DPSK transmitter,
(i) Sketch the resulting waveform at the transmitter output
(ii) Applying this waveform to the DPSK receiver, show that
original data is reconstructed in the received output.