types of Digital Modulation Techniques | comparison of digital modulation techniques

types of Digital Modulation Techniques pdf notes  , comparison of digital modulation techniques ppt ?
Q.1.     What are the types of Digital Modulation Techniques? Explain.
Ans.    Basically, digital modulation techniques may be classified into coherent or non-coherent techniques, depending on whether the receiver is equipped with a phase-recovery circuit or not. The phase-recovery circuit ensures that the oscillator supplying the locally generated carrier wave receiver is synchronized to the oscillator supplying the carrier wave used to originally modulate the incoming data stream in the transmitter.
(i)         Coherent Digital Modulation Techniques
            Coherent digital modulation techniques are those techniques which employ coherent detection. In coherent detection, the local carrier generated at the receiver is phase locked with the carrier at the transmitter. Thus, the detection is done by correlating received noisy signal and locally generated carrier. The coherent detection is a synchronous detection.
(ii)        Non-coherent Digital Modulation Techniques
Non-coherent digital modulation techniques are those techniques in which the detection process does not need receiver carrier to be phase locked with transmitter carrier.
The advantage of such type of system is that the system becomes simple. But the drawback of such a system is that the error probability increases.
Q.2.     Explain the principle of Binary Phase Shift Keying (BPSK).
Ans.    In a binary phase shift keying (BPSK), the binary symbols ‘1’ and ‘0’ modulate the phase of the carrier. Let us assume that the carrier is given as,
s(t) = A cos (2pfct)
Here ‘A’ represents peak value of sinusoidal carrier. For the standard 1W load resistor, the power dissipated would be,
P =  A2
or                                                   A =
Now, when the symbol is changed, then the phase of the carrier will also be changed by an amount of 180 degrees (i.e., p radians).
Let us consider, for example,
For symbol ‘1’, we have
s1(t) =   cos (2pfct)
If next symbol is ‘0’, then we have
For symbol ‘0’, we have
s2(t) =  cos ( 2pfct + p)
Now, because cos (q + p) = – cos q, therefore, the last equation can be written as,
s2(t) = cos (2pfct)
With the above equation, we can define BPSK signal combinely as,
s(t) = b(t)  cos (2pfct)
where              b(t) = + 1 when binary ‘1’ is to be transmitted.
– 1 when binary ‘0’ is to he transmitted.
Q.3.     Explain the concept of Non-coherent Binary Amplitude Shift Keying (ASK).
Ans.    In the binary ASK case, the transmitted signal is defined as
s(t) =  cos(2pfct)
Binary ASK signal can also be demodulated non-coherently using envelope detector. This greatly simplifies the design consideration required in synchronous detection. Non-coherent detection schemes do not require a phase-coherent local oscillator. This method involves some form of rectification and low pass filtering at the receiver.
Q.4.     Explain the principle of Differential Phase Shift Keying (DPSK).
Ans.    We can view differential phase-shift keying as the non-coherent version of the PSK. Differential phase shift keying (DPSK) is differentially coherent modulation method. DPSK does not need a synchronous (coherent) carrier at the demodulator. The input sequence of binary bits is modified such that the next bit depends upon the previous bit.

  1. Explain coherent and noncoherent binary modulation techniques.
  2. Explain coherent Binary ASK.
  3. Describe coherent Binary FSK.
  4. Explain the difference between different modulation techniques.
  5. Draw the block diagram of QPSK system and explain its working.
  6. Compare the bandwidth of QPSK system with that of BPSK system.
  7. Represent QPSK signals in the signal space and find distance between them. What is the significance of each?
  8. Write the power spectral density of BPSK and QPSK signals and draw the power spectrum of each.
  9. Explain M-ary FSK system with the help of transmitters and receivers.
  10. Determine the bandwidth required for M-ary FSK system. Draw the geometrical representation of M-ary FSK signals and find out distance between the signals.
  11. Give the block diagram of a coherent signal reception system. Explain the effects of phase error and timing error on the error probability in a BPSK system employing coherent reception giving necessary derivations.
  12. Explain the geometrical representation of Non-orthogonal BFSK.
  13. Derive the expression for the spectrum of BPSK and sketch the same.
  14. How is phase continuity achieved in MSK system?
  15. Explain operation of differentially encoded PSK system. Explain why errors always occur in pairs in this system?
  16. Explain the condition of orthogonally of the two BFSK signals and the signal space diagram for the orthogonal signal and hence derive an expression for the probability of error in synchronous detection of the BFSK signals using matched filters.
  17. Derive an expression for the spectrum of BFSK and sketch it for the condition of orthogonality and minimum bandwidth.
  18. What is the advantage of gray coding of the QPSK system? What is the advantage of differential encoding of the input to the QPSK system?
  19. What type of synchronization is used in QPSK system?
  20. Draw the signal space representation of QPSK and BPSK signals. Show that for the same data rate and same bit energy they have the same bit error rate probability on the same AWGN channel, although one of the signals occupy half the bandwidth.
  21. What is the ambiguity in the decoded output in case of PSK systems? How it is corrected?
  22. Draw the block diagram of DPSK modulator and explain how synchronization problem is avoided for its detection.
  23. Explain the ASK system and derive the relation for error probability of binary ASK.


  1. Sketch the QPSK waveform for the sequence 1 1 0 1 0 1 0 0 1 0, assuming the carrier frequency to be equal to the bit rate.
  2. Suggest a suitable synchronising circuit arrangement for extracting the carrier at the receiver end for coherent detection of binary PSK.
  3. Sketch the waveform of the in-phase and quadrature components of a QPSK signals for the binary sequence 1 1 0 1 0 1 1 0 1 0.
  4. For the sequence given in sketch the QPSK waveform.
  5. Sketch the waveform of the MSK signal for the input binary sequence 1 1 0 0 1 0 0 0 1 0.

Multiple Choice Questions

  1. If optimum coding/decoding is used at the transmitter and receiver, error free data will be recovered at the receiver output, provided Eb/N0 (Eb = Received signal energy/bit and N0/2 power spectral density of the channel noise at receiver input) at the receiver input is larger than

(a)        0 dB                                        (b)        – 1.59 dB
(c)        1.33 dB                                   (d)       2.15 dB

  1. The bit rate of a digital communication system is 34 Mpbs. The modulation scheme is QPSK. The baud rate of the system is

(a)        68 Mpbs                                  (b)        34 Mbps
(c)        17 Mbps                                  (d)       85 Mbps

  1. Comparison of MSK and QPSK schemes show that

(a)        MSK requires less power
(b)        QPSK requires less power
(c)        filtering in simple in MSK
(d)       no comparison

  1. For a BPSK scheme, the bit error probability is given by

(a)                                (b)
(c)                            (d)

  1. For a DPSK scheme, the bit error probability is given by

(a)                                (b)
(c)                                  (d)

  1. For non-coherent FSK, the bit error rate is given by

(a)                              (b)
(c)                             (d)

  1. For coherent FSK system, the bit error probability is given by

(a)                                 (b)
(c)                                            (d)

  1. For choerent ASK scheme, the error probability is given by

(a)        Pe =
(b)        Pe =
(c)        Pe =
(d)       Pe =

  1. BW and MSK ___________ that of QPSK

(a)        higher than
(b)        lower than
(c)        equal to
(d)       less than or equal to

  1. _________ is most affected by noise.

(a)        PSK                                         (b)        ASK
(c)        FSK                                         (d)       DPSK

  1. If the baud rate for a QAM signal is 3000 and a signal element is represented by a tribit, what is the bit rate?

(a)        300                                          (b)        400
(d)       1000                                        (d)       9000

  1. (b) 2. (a)                            3. (a)                            4. (c)
  2. (c) 6. (a)                            7. (b)                            8. (c)
  3. (a) 10. (b)              11. (d)

Leave a Reply

Your email address will not be published. Required fields are marked *