DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
2 and 16 MARK QUESTIONS AND ANSWERS
SUBJECT CODE : EC1351
SUBJECT NAME : DIGITAL COMMUNICATION
Prepared by
S.SELVA ANITHA (Lect/ECE)
UNIT I
PULSE MODULATION
1. Define Dirac comb or ideal sampling function. What is its Fourier Transform?
Dirac comb is nothing but a periodic impulse train in which the impulses
are spaced by a time interval of Ts seconds. The equation for the function is given
by
’ Ts W W- n Ts)
n =-’ 7KH)RXULHU7UDQVIRUPRI Ts (t) is given by
’ )> Ts (t)]= f s I- m fs)
m =-’
2. Give the interpolation formula for the reconstruction of the original signal g(t)
from the sequence of sample values {g(n/2W)}.
’ J W J Q:VLQF :W-n)
n =-’
where 2W is the bandwidth
n is the number of samples.
3. State sampling theorem.
· If a finite –energy signal g(t) contains no frequencies higher than W hertz
,it is completely determined by specifying its co=ordinates at a sequence
of points spaced 1/2W seconds apart.
· If a finite energy signal g(t) contains no frequencies higher than W hertz, it
may be completely recovered from its co=ordinates at a sequence of points
spaced 1/2W seconds apart.
4. Define quadrature sampling.
Quadrature sampling is used for uniform sampling of band pass signals
Consider g(t) = gI WFRV IFW– gQ WVLQ IFW
The in-phase component gI(t) and the quadrature component gQ(t) may be
REWDLQHGE\PXOWLSO\LQJWKHEDQGSDVVVLJQDOJ WE\FRV IFWDQGVLQ IFW
respectively and then suppressing the sum-frequency components by means of
appropriate low pass filter. Under the assumption that fc>W,we find that
gI(t)&gQ(t) are both low-pass signals limited to -Wcomponent may be sampled at the rate of 2W samples per second. This type of
sampling is called quadrature sampling.
5. What is aliasing?
The phenomenon of a high-frequency in the spectrum of the original
signal g(t) seemingly taking on the identity of a lower frequency in the spectrum
of the sampled signal g(t) is called aliasing or foldover.
6. Give the expression for aliasing error and the bound for aliasing error.
(m+1/2fs)
’ ( _ >-exp (-M PIst)] œ* IH[S M IWGI_
n =-’
(m-1/2fs)
where E is the aliasing error.
|G(f)| is the amplitude spectrum of the signal g(t).
E < 2 œ_* I_GI
| f | > fs /2
Where E is the bound for aliasing error.
7. What is meant by PCM?
Pulse code modulation (PCM) is a method of signal coding in which the
message signal is sampled, the amplitude of each sample is rounded off to the
nearest one of a finite set of discrete levels and encoded so that both time and
amplitude are represented in discrete form.. This allows the message to be
transmitted by means of a digital waveform.
8. Define quantizing process.
The conversion of analog sample of the signal into digital form is called
quantizing process.
9. What are the two fold effects of quantizing process.
1. The peak-to-peak range of input sample values subdivided into a finite set of
decision levels or decision thresholds
2. The output is assigned a discrete value selected from a finite set of
representation levels are reconstruction values that are aligned with the treads
of the staircase.
10. What is meant by idle channel noise?
Idle channel noise is the coding noise measured at the receiver output with
zero transmitter input.
11. What is meant by prediction error?
The difference between the actual sample of the process at the time of
interest and the predictor output is called a prediction error.
12. Define delta modulation.
Delta modulation is the one-bit version of differential pulse code
modulation.
13. Define adaptive delta modulation.
The performance of a delta modulator can be improved significantly by
making the step size of the modulator assume a time- varying form. In particular,
during a steep segment of the input signal the step size is increased. Conversely,
when the input signal is varying slowly, the step is reduced , In this way, the step
size is adapting to the level of the signal. The resulting method is called adaptive
delta modulation (ADM).
14. Name the types of uniform quantizer?
1. Mid tread type quantizer.
2. Mid riser type quantizer.
15. Define mid tread quantizer?
Origin of the signal lies in the middle of a tread of the staircase.
Output
3a
2a
a Overload level
-3a/2 -a/2
Input
-a
-2a
Peak to peak excursion where a=delta
16. Define mid-riser quantizer?
Origin of the signal lies in the middle of a riser of the staircase
O/p
3a/2
a/2 Over load level
a 2a 3a 4a i/p
17. Define quantization error?
Quantization error is the difference between the output and input values of
quantizer.
18. What you mean by non-uniform quantization?
Step size is not uniform. Non-uniform quantizer is characterized by a step
size that increases as the separation from the origin of the transfer characteristics
is increased. Non-uniform quantization is otherwise called as robust quantization
.
19. Draw the quantization error for the mid tread and mid-rise type of quantizer?
For mid tread type:
Quantization error
a/2
Input
-a/2 a
For mid riser type:
Quantization error
a/2
Input
a
20. What is the disadvantage of uniform quantization over the non-uniform
quantization?
SNR decreases with decrease in input power level at the uniform quantizer
but non-uniform quantization maintains a constant SNR for wide range of input
power levels. This type of quantization is called as robust quantization.
21. What do you mean by companding? Define compander.
The signal is compressed at the transmitter and expanded at the receiver. This
is called as companding. The combination of a compressor and expander is called
a compander.
22. Draw the block diagram of compander? Mention the types of companding?
Block diagram:
Input Compressor uniform quantizer expander o/p signal
Transmitter receiver
Types of companding:
1. μ law companding
2. A law companding
23. What is PAM?
PAM is the pulse amplitude modulation. In pulse amplitude modulation,
the amplitude of a carrier consisting of a periodic train of rectangular pulses is
varied in proportion to sample values of a message signal.
24. What is the need for speech coding at low bit rates?
The use of PCM at the standard rate of 64 Kbps demands a high channel
bandwidth for its transmission ,so for certain applications, bandwidth is at
premium, in which case there is a definite need for speech coding at low bit rates,
while maintaining acceptable fidelity or quality of reproduction.
25. Define ADPCM.
It means adaptive differential pulse code modulation, a combination of
adaptive quantization and adaptive prediction. Adaptive quantization refers to a
quantizer that operates with a time varying step size. The autocorrelation function
and power spectral density of speech signals are time varying functions of the
respective variables. Predictors for such input should be time varying. So adaptive
predictors are used.
UNIT II
BASEBAND TRANSMISSION
26. What is meant by forward and backward estimation?
AQF: Adaptive quantization with forward estimation. Unquantized
samples of the input signal are used to derive the forward estimates.
AQB: Adaptive quantization with backward estimation. Samples of the
quantizer output are used to derive the backward estimates.
APF: Adaptive prediction with forward estimation, in which unquantized
samples of the input signal are used to derive the forward estimates of the
predictor coefficients.
APB: Adaptive prediction with backward estimation, in which Samples of
the quantizer output and the prediction error are used to derive estimates of the
predictor coefficients.
27. What are the limitations of forward estimation with backward estimation?
o Side information
o Buffering
o Delay
28. How are the predictor coefficients determined?
For the adaptation of the predictor coefficients the least mean square
(LMS) algorithm is used.
29. Define adaptive subband coding?
It is a frequency domain coder, in which the speech signal is divided in to
number of subbands and each one is coded separately. It uses non masking
phenomenon in perception for a better speech quality. The noise shaping is done
by the adaptive bit assignment.
30. What are formant frequencies?
In the context of speech production the formant frequencies are the
resonant frequencies of the vocal tract tube. The formants depend on the shape
and dimensions of the vocal tract.
31. What is the bit rate in ASBC?
Nfs= (MN) (fs/M)
Nfs->bit rate
M->number of subbands of equal bandwidths
N->average number of bits
fs/M->sampling rate for each subband
32. Define Adaptive filter?
It is a nonlinear estimator that provides an estimate of some desired
response without requiring knowledge of correlation functions, where the filter
coefficients are data dependent. A popular filtering algorithm is the LMS
algorithm.
33. Define data Signalling Rate.
Data signalling rate is defined as the rate measured in terms bits per
second(b/s) at which data are transmitted.
Data signaling rate Rb=I/Tb
Where Tb=bit duration.
34. Define modulation rate.
It is defined as the rate at which signal level is changed depending
On the nature of the format used to represent the digital data.It is measured in
Bauds or symbols per second.
35. State NRZ unipolar format
In this format binary 0 is represent by no pulse and binary 1 is
Represented by the positive pulse.
36. State NRZ polar format.
Binary 1 is represented by a positive pulse and binary 0 is represented by a
Negative pulse.
37. State NRZ bipolar format.
Binary 0 is reporesented by no pulse and binary one is represented by
the alternative positive and negative pulse.
38. State Manchester format.
Binary 0 Æ The first half bit duration negative pulse and the second half
Bit duration positive pulse.
Binary 1Æ first half bit duration positive pulse and the second half
Bit duration negative pulse.
39. What is an eye pattern?
Eye Pattern is used to study the effect of intersymbol interference.
40. What is the width of the eye?
It defines the time interval over which the received waveform can be
sampled without error from intersymbol interference.
41. What is sensitivity of an eye?
The sensitivity of the system to timing error is determined by the rate of
closure of the eye as the sampling time is varied.
42. What is margin over noise?
The height of the eye opening at a specified sampling time defines the margin
over noise.
43. What is Inter symbol interference?
The transmitted signal will undergo dispersion and gets broadened during
its transmission through the channel. So they happen to collide or overlap with the
adjacent symbols in the transmission. This overlapping is called Inter Symbol
Interference.
44. How eye pattern is obtained?
The eye pattern is obtained by applying the received wave to the vertical
deflection plates of an oscilloscope and to apply a saw tooth wave at the
transmitted symbol rate to the horizontal deflection plate.
UNIT III
PASSBAND TRANSMISSION
45. Properties of matched filter.
The signal to noise ratio of the matched filter depends only upon the
the ratio of the signal energy to the psd of white noise at the filter
input
1) The output signal of a matched filter is proportional to a shifted version
of the auto_correlation function of the input signal to which the filter
is matched.
46. Why do we go for Gram-Schmidt Orthogonalization procedure?
Consider a message signal m. The task of transforming an incoming
message mi=1,2,…..M, into a modulated wave si(t) may be divided into separate
discrete time & continuous time operations. The justification for this separation
lies in the Gram-Schmidt orthogonalization procedure which permits the
representation of any set of M energy signals, {si(t)}, as linear combinations of N
orthonormal basis functions, where N £M .
47. What is matched filter receiver ?
A filter whose impulse response is a time reversed & delayed version of
some signal Æ j (t) then it is said to be matched to Æj (t) correspondingly, the
optimum receiver based on the detector is referred to as the matched filter
receiver.
48. What is maximum likelihood detector.
Maximum likelihood detector computes the metric for each transmitted
message compares them and then decides in favor of maximum. The device for
implementing the decision rule
i.e; set ^m = mi if
In [ fx(x/mk)] is maximum for k=i is called maximum –likelihood detector and the
decision rule is called maximum likelihood.
49. Define antipodal signals.
A pair of sinusoidal signals that differ only in a phase shift of 180 degrees
are referred to as antipodal signals.
50. Explain how QPSK differs from PSK in term of transmission bandwidth and bit
information it carries?
For a given bit rate 1/Tb, a QPSK wave requires half the transmission
bandwidth of the corresponding binary PSK wave. Equivalently for a given
transmission bandwidth, a QPSK wave carries twice as many bits of information
as the corresponding binary PSK wave
51. Give the equation for average probability of symbol error for coherent binary
PSK.
Average probability of signal error,
Pe = 1 / 2 erfcÖ Eb / No
52. Give the signal space characterization of QPSK.
Coordinates i/p dibit 0£t £ T Phase of QPSK signal message points
(radians)
Si 1 Si 2
10
00
01
11
Õ/ 4
3Õ/4
5Õ/4
7Õ/4
ÖE/2
-ÖE/2
-ÖE/2
ÖE/2
-ÖE/2
-ÖE/2
ÖE/2
ÖE/2
53. Define QPSK.
QPSK is Quadriphase –shift keying. In QPSK the phase of the carrier
takes on one of the four equally spaced values Such as Õ/4 , 3Õ/4, 5Õ/4 and
7Õ/4.
54. Define Dibit.
A unique pair of bits is called a dibit. Gray encoded set of dibits 10, 00,
01 & 11
55. Give the transmitted signal of Non-coherent binary FSK.
Si(t) = {Ö 2Eb/Tb Cos(2Õ f i t) ,0 £ t £ Tb
O, elsewhere
fi = nc+ i/ Tb
56. Give the two basic operation of DPSK transmitter.
1. differential encoding of the input binary wave
2. Phase –shift keying hence, the name differential phase shift keying
57. Define deviation ratio in MSK .
The parameter h is defined by
h= Tb(f1-f2)
h is deviation ratio , measured with respect to bit rate 1/Tb.
58. Define MSK signal in interval 0£ t £ Tb.
S(t) = Ö 2Eb/Tb Cos [ 2Õ f 1t + q(0)] for symbol 1
Ö 2Eb/Tb Cos [ 2Õ f 2 t + q(0)] for symbol 0
59. What is nominal carrier frequency in MSK ?
Nominal carrier frequency is the arithmetic mean of the two frequencies f1
and f2 and it is given as
fc = ½ (f1 + f2)
Where f1 is the frequency for symbol –1
f2 is the frequency for symbol – 0
60. What are the three broad types of synchronization ?
1. Carrier synchronization
2. Symbol & Bit synchronization
3. Frame synchronization.
61. What is carrier synchronization ?
The carrier synchronization is required in coherent detection methods to
generate a coherent reference at the receiver. In this method the data bearing
signal is modulated on the carrier in such a way that the power spectrum of the
modulated carrier signal contains a discrete component at the carrier frequency.
62. What are the two methods for carrier synchronization.
1. Carrier synchronization using Mth Power loop
2. Costas loop for carrier synchronization
63. What is called symbol or bit synchronization ?
In a matched filter or correlation receiver, the incoming signal is sampled
at the end of one bit or symbol duration. Therefore the receiver has to know the
instants of time at which a symbol or bit is transmitted. That is the instants at
which a particular bit or symbol status and when it is ended. The estimation of
these times of bit or symbol is called symbol or bit synchronization.
64. What are the two methods of bit and symbol synchronization.
1) Closed loop bit synchronization
2) Early late gate synchronizer
65. What are the disadvantages of closed loop bit synchronization.
1) If there is a long string of 1’s and o’s then y(t) has no zero crossings
and synchronization may be lost.
2) If zero crossing of y(t) are not placed at integer multiples of Tb, the
synchronization suffers from timing Jitter.
66. What is called frame synchronization ?
Depending on bits used for encoding, the word length is defined. Thus
each word container some fixed number of bits. The receiver has to know when a
particular frame status and when its individual message bits status. This type of
synchronization is called frame synchronization.
67. Why synchronization is required ?
The signals from various sources are transmitted on the single channel by
multiplexing. This requires synchronization between transmitter and receiver.
Special synchronization bits are added in the transmitted signal for the purpose.
Synchronization is also required for detectors to recover the digital data properly
from the modulated signal.
UNIT IV
ERROR CONTROL CODING
68. What is linear code ?
A code is linear if the sum of any two code vectors produces another code
vector.
69. What is code rate ?
Code rate is the ratio of message bits (k) and the encoder output bits (n).
It is defined by r (i.e) r= k/N
70. Define code efficiency.
It is the ratio of message bits in a block to the transmitted bits for that
block by the encoder i.e
Message bits in a block
Code efficiency = ----------------------------------
Transmitted bits for the block
71. What is hamming distance?
The hamming distance between two code vectors is equal to the number of
elements in which they differ. For example let the two code vectors be
X=(101) and Y= (110)
These two code vectors differ in second and third bits.
Therefore the hamming distance between x and Y is two.
72. What is meant by systematic & non-systematic code?
In a systematic block code, message bit appear first and then check bits.
In the non-systematic code, message and check bits cannot be identified in the
code vector.
73. How syndrome is calculated in Hamming codes and cyclic codes ?
In Hamming codes the syndrome is calculated as ,
S = YH T
Here Y is the received and HT is the transpose of parity check matrix.
In cyclic code, the syndrome vector polynomial is given as,
S (P) = remainder ( y (p)/ G (P) )
Y(P) is received vector polynomial and G (p) is generator polynomial.
74. What is BCH Code ?
BCH codes are most extensive and powerful error correcting cyclic code.
The decoding of BCH coder is comparatively simpler. For any positive integer
‘m’ and ‘t’ , there exists a BCH code with following parameters :
Block length n = 2 m-1
No. of parity check bits : n-k £ mt
Minimum distance : dmin ³ 2t +1
75. What are the conditions to satisfy the hamming code.
1) No. of Check bits q ³ 3
2) Block length n = 2q –1
3) No of message bits K = n-q
4) Minimum distance dmin =3
76. Define code word & block length.
The encoded block of ‘n’ bits is called code word. The no. of bits ‘n’ after
coding is called block length.
77. Give the parameters of RS codes .
Reed Solomon codes. These are non binary BCH codes.
Block length = n =2m -1 symbols
Message size : k symbols
Parity check size : n-k= 2t symbols
Minimum distance , dmin =2t +1 symbols.
78. Why RS codes are called maximum distance separable codes ?
( n,k) Linear block code for which the minimum distance equals n – k + 1 is
called maximum distance separable codes. For RS code minimum distance equals
n – k + 1 so it is called as maximum distance separable codes.
79. What are Golay codes ?
Golay code is the (23, 12) cyclic code whose generating polynomial is,
G(p) =P11+P9+p7+P6+p5+p+1
This code has a minimum distance of dmin=7. This code can correct upto
3 errors. It is perfect code.
80. What are the advantages of cyclic codes ?
1. Encoders and decoders for cyclic codes are simple
2. Cyclic codes also detect error burst that span many successive bits.
UNIT V
SPREAD SPECTRUM MODULATION
81. Define pseudo-noise (PN) sequence.
A pseudo-noise sequence is defined as a coded sequence of 1s and Os with
certain autocorrelation properties. It is used in spread Spectrum communications.
It is periodic in that a sequence of 1s and 0s repeats itself exactly with a known
period.
82. What does the term catastrophic cyclic code represent ?
‘000’ is not a state of the shift register sequence in PN sequence generator,
since this results in a catastrophic cyclic code i.e once the 000 state is entered, the
shift register sequence cannot leave this state.
83. Define a random binary sequence.
A random binary sequence is a sequence in which the presence of a binary
symbol 1 or 0 is equally probable.
84. State the balance property of random binary sequence.
In each period of a maximum length sequence, the number of 1s is always
one more than the number of 0s. This property is called the balance property.
85. Mention about the run property.
Among the runs of 1s and 0s in each period of a maximum length
sequence, one half the runs of each kind are of length one, one fourth are of length
two, one eighth are of length three, and so or as long as these function represent
meaningful numbers of runs. This property is called the run property.
86. Give the correlation property of random binary sequence.
The autocorrelation function of a maximum length sequence is periodic
and binary valued. This property is called the correlation property.
87. Mention the significance of spread spectrum modulation.
An important attribute of spread-spectrum modulation is that it can
provide protection against externally generated interfering (jamming) signals with
finite power. The jamming signal may consist of a fairly powerful broadband
noise or multitone waveform that is directed at the receiver for the purpose of
disrupting communications. Protection against jamming waveforms is provided
by purposely making the information bearing signal occupy a bandwidth far in
excess of minimum bandwidth necessary to transmit it.
88. What is called processing gain ?
Processing Gain (PG) is defined as the ratio of the bandwidth of spread
message signal to the bandwidth of unspreaded data signal ie).
Processing Gain = BW (spreaded signal)
---------------------------- =
BW (Unspreaded signal)
89. What is called jamming effect ?
In the frequency band of the interest, somebody else transmits the signals
intentionally since these signals the in the frequency band of transmission, they
interface the required signal. Hence it becomes difficult to detect the required
signals. This is called jamming effect.
90. What is Anti jamming ?
With the help of spread spectrum method, the transmitted signals are
spread over the mid frequency band. Hence these signals appear as noise. Then it
becomes difficult for the jammers to send jamming signals. This is called
antijamming.
91. What are the three codes used for the anti jamming application ?
1. Golay code (24, 12)
2. Expurgated Golay (24, 11)
3. Maximum length shift register code.
92. What is called frequency hop spread spectrum ?
In frequency hop spread spectrum, the frequency of the carrier hops
randomly from one frequency to another frequency.
93. What is slow frequency hopping?
If the symbol rate of MFSK is an integer multiple of hop rate (multiple
symbols per hop) then it is called slow frequency hopping
94. What is fast frequency hopping?
If the hop rate is an integer multiple of symbol rate (multiple hops per
symbol) then it is called fast frequency hopping.
95. What are the two function of fast frequency hopping?
1. Spread Jammer over the entire measure of the spectrum of Txed
signal.
2. Retuning the Jamming signal over the frequency band of Txed
signal.
96. What are the features of code Division multiple Accesses?
1. It does not require external synchronization networks.
2. CDMA offers gradual degradation in performance when the no. of
users is increased But it is easy to add new user to the system.
3. If offers an external interference rejection capability.
97. What is called multipath Interference?
The interference caused by the interfacing of the signal form the indirect
path with the signal of direct path is called multipath interference.
98. What is the advantage of a spread spectrum technique?
The main advantage of spread spectrum technique is its ability to reject
interference whether it be the unintentional interference of another user
simultaneously attempting to transmit through the channel (or) the intentional
interference of a hostile transmitter to jam the transmission.
99. What is called frequency hop spread spectrum ?
In frequency hop spread spectrum, the frequency of the carrier hops
randomly from one frequency to another frequency.
100 .What is slow frequency hopping?
If the symbol rate of MFSK is an integer multiple of hop rate (multiple
symbols per hop) then it is called slow frequency hopping
Essay Questions (Part B)
1. Derive the power spectral Density of a Synchronous Data pulse stream
generated by a Binary, Zero mean, Wide Sense Stationary Sequence.
i. Define mean. Auto correlation and wide sense stationary
ii. Derive the expression of (Sa) f
iii. Derive the expression of (Sp) f
iv. Combine the values of (Sa) f and (Sp) f
v. Make the necessary approximations.
2. Derive the power spectral Density of a Synchronous Data pulse stream
generated by a Binary, Zero mean, Cyclostationary Sequence.
i. Define mean. Auto correlation and Cyclostationary
ii. Derive the expression of (Sa) f
iii. Derive the expression of (Sp) f
iv. Combine the values of (Sa) f and (Sp) f
v. Make the necessary approximations.
3. Derive the power spectral Density of a Generalized M-ary Markov Source
i. Define mean. Auto correlation and Markov source
ii. Derive the expression of (Sa) f
iii. Derive the expression of (Sp) f
iv. Combine the values of (Sa) f and (Sp) f
v. Make the necessary approximations.
4. Explain in detail about scalar communication and obtain the probability of Mary
Scalar Receiver starting from the implementation of M-ary Scalar
Receiver.
i. Draw the Block Diagram of M-ary Scalar Receiver.
ii. Do the implementation of M-ary Scalar Receiver.
iii. Find the probability of Error of M-ary Scalar Receiver.
iv. Draw the signal space diagram
5. Explain in detail about vector communication and obtain the probability of
Binary Vector Receiver starting from the implementation of Binary Vector
Receiver.
i. Draw the Block Diagram of Binary Vector Receiver.
ii. Do the implementation of Binary Vector Receiver.
iii. Find the probability of Error of Binary Vector Receiver.
iv. Draw the signal space diagram
6. Explain the construction of Block Code and explain how error syndrome is
calculated
i. Representation of Block Code.
ii. Generator Matrix.
iii. Generation of Codewords.
iv. Generation of Parity Check Matrix.
v. Calculation OF Error Syndrome.
7. Explain in detail about Orthogonal Codes, Biorthogonal Codes and
Transorthogonal Codes
i. Definitions of codes,
ii. Formation of orthogonal codes.
iii. Formation of Biorthogonal Codes.
iv. Formation of Transorthogonal Codes.
8. Explain in detail about Shannon Coding Theorem.
i. Statement of Theorem.
ii. Discussion of theory.
iii. Derivation of the Theorem.
9. What is Spread Spectrum Techniques Explain in detail about Direct Sequence
Spread Spectrum Techniques with necessary diagrams?
i. Concept of Spread Spectrum Techniques
ii. Block Diagram Representation.
iii. Waveform at all stages of the system.
iv. Derivation of processing Gain.
10. What is Frequency Hopping? Explain the different types of frequency hopping
with necessary diagrams.
i. Concept of frequency hopping.
ii. Explanation of slow frequency hopping
iii. Explanation of Fast frequency hopping
iv. Block Diagrams and waveform
11. Explain in detail about Golay Codes, Reed Solomon Codes and BCH Codes.
i. Definition of Golay Codes, Reed Solomon Codes and BCH Codes.
ii. Explanation of Golay Codes
iii Explanation of Reed Solomon Codes
iv Explanation of BCH Codes.
12. Explain in detail about Binary Phase Shift Keying and obtain an expression
for its probability of error.
i. Block Diagram of Transmitter and Receiver.
ii. Explanation of Transmitter and receiver.
iii. Signal Space Diagram
iv. Calculation of Probability of Error
13. Explain in detail about Quadrature Phase Shift Keying and obtain an
expression for its probability of error.
i. Block Diagram of Transmitter and Receiver.
ii. Explanation of Transmitter and receiver.
iii. Signal Space Diagram
iv. Calculation of Probability of Error
14. Explain in detail about Minimum Shift Keying and obtain an expression for its
probability of error.
i. Block Diagram of Transmitter and Receiver.
ii. Explanation of Transmitter and receiver.
iii. Signal Space Diagram0
iv. Calculation of Probability of Error
15. Explain in detail about the optimum demodulation of Digital signals in the
presence if ISI and AWGN. Also, explain about the various equalization
techniques.
i. Concept of ISI and AWGN
ii. Derivation of ISI.
iii. Ideal Solution and Practical Solution.
iv. Types of Equalization.
16. Explain in detail about the operation of Non Coherent Receivers in the
presence of Random Phase Channel and implement the receiver.
i. Concept of Non Coherent Receivers.
ii. Derivation of probability of Error.
iii. Implementation of the Receiver.
iv. Waveforms
17. Explain in detail about the operation of Non Coherent Receivers in the
presence of Random amplitude and phase Channel and implement the
receiver.
a. Concept of Non Coherent Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
18. Explain in detail about the operation of Optimum Receivers in Rayleigh
Channel and implement the receiver.
a. Concept of Raleigh Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
19. Explain in detail about the operation of Optimum Receivers in Rician Channel
and implement the receiver.
a. Concept of Rician Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
20. Explain in detail about the operation of Partially Coherent Receivers in the
presence of Random Phase Channel and implement the receiver.
a. Concept of Partially Coherent Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
21. a. Explain in detail about In phase and Quadrature Modulation systems.
b. With necessary diagrams explain the operation of Quadrature Amplitude
Modulation systems.
a. In phase and Quadrature Modulation systems.
· Block Diagram
· Derivation
· Explanation
b. Quadrature Amplitude Modulation systems.
· Draw the Block Diagram
· Derivation
· Explanation
22. Draw the code tree of a Convolutional code of code rate r=1/2 and Constraint
length of K=3 starting from the state table and state diagram for an encoder
which is commonly used.
a. Draw the state Diagram.
b. Draw the state Table.
c. Draw the code Tree
23. Draw the trellis diagram of a Convolutional code of code rate r=1/2 and
Constraint length of K=3 starting from the state table and state diagram for an
encoder which is commonly used.
a. Draw the state Diagram.
b. Draw the state Table.
c. Draw the trellis diagram
24. Decode the given sequence 11 01 01 10 01 of a convolutional code with a
code rate of r=1/2 and constraint length K=3, using viterbi decoding
algorithm.
a. Draw the state Diagram.
b. Draw the state Table.
c. Draw the code Tree
d. Decode the given sequence using trellis diagram
25. Explain in detail about Continuous Phase Frequency Shift Keying and
obtain an expression for its probability of error.
a. Block Diagram of Transmitter and Receiver.
b. Explanation of Transmitter and receiver.
c. Signal Space Diagram
d. Calculation of Probability of Error
26. Derive the power spectral Density of a Synchronous Data pulse stream
generated by a Binary, non-zero mean, Cyclostationary Sequence.
a. Define mean. Auto correlation and wide sense stationary
b. Derive the expression of (Sa) f
c. Derive the expression of (Sp) f
d. Combine the values of (Sa) f and (Sp) f
e. Make the necessary approximations.
KEYWORDS:DIGITAL COMMUNICATION,DIGITAL COMMUNICATION QUESTION PAPER,ANNA UNIVERSITY QUESTION PAPER,ANNA UNIVERSITY,ANNA UNIVERSITY CHENNAI,ANNA UNIVERSITY COIMBATORE,ANNA UNIVERSITY TRICHY,ANNA UNIVERSITY TIRUNELVELI,ANNA UNIVERSITY MADURAI,ANNA UNIVERSITY SYLLABUS,ANNA-UNIVERSITY RESULTS,ANNA UNIVERSITY DISTANCE EDUCATION,ANNA UNIVERSITY MBA-CENTRE FOR DISTANCE EDUCATION,ANNA UNIVERSITY SCHEDULE OF EXAMINATIONS,ANNA UNIVERSITY ADMISSION,ANNA UNIVERSITY COURSES,ANNA UNIVERSITY ACADEMIC,ANNA UNIVERSITY DEPARTMENTS,ANNA UNIVERSITY RESEARCH,ANNA UNIVERSITY MAIL,ANNA UNIVERSITY QUESTION PAPERS,ANNA UNIVERSITY COUNSELLING DATES,ANNA UNIVERSITY RE-EVALUATION RESULTS
2 and 16 MARK QUESTIONS AND ANSWERS
SUBJECT CODE : EC1351
SUBJECT NAME : DIGITAL COMMUNICATION
Prepared by
S.SELVA ANITHA (Lect/ECE)
UNIT I
PULSE MODULATION
1. Define Dirac comb or ideal sampling function. What is its Fourier Transform?
Dirac comb is nothing but a periodic impulse train in which the impulses
are spaced by a time interval of Ts seconds. The equation for the function is given
by
’ Ts W W- n Ts)
n =-’ 7KH)RXULHU7UDQVIRUPRI Ts (t) is given by
’ )> Ts (t)]= f s I- m fs)
m =-’
2. Give the interpolation formula for the reconstruction of the original signal g(t)
from the sequence of sample values {g(n/2W)}.
’ J W J Q:VLQF :W-n)
n =-’
where 2W is the bandwidth
n is the number of samples.
3. State sampling theorem.
· If a finite –energy signal g(t) contains no frequencies higher than W hertz
,it is completely determined by specifying its co=ordinates at a sequence
of points spaced 1/2W seconds apart.
· If a finite energy signal g(t) contains no frequencies higher than W hertz, it
may be completely recovered from its co=ordinates at a sequence of points
spaced 1/2W seconds apart.
4. Define quadrature sampling.
Quadrature sampling is used for uniform sampling of band pass signals
Consider g(t) = gI WFRV IFW– gQ WVLQ IFW
The in-phase component gI(t) and the quadrature component gQ(t) may be
REWDLQHGE\PXOWLSO\LQJWKHEDQGSDVVVLJQDOJ WE\FRV IFWDQGVLQ IFW
respectively and then suppressing the sum-frequency components by means of
appropriate low pass filter. Under the assumption that fc>W,we find that
gI(t)&gQ(t) are both low-pass signals limited to -W
sampling is called quadrature sampling.
5. What is aliasing?
The phenomenon of a high-frequency in the spectrum of the original
signal g(t) seemingly taking on the identity of a lower frequency in the spectrum
of the sampled signal g(t) is called aliasing or foldover.
6. Give the expression for aliasing error and the bound for aliasing error.
(m+1/2fs)
’ ( _ >-exp (-M PIst)] œ* IH[S M IWGI_
n =-’
(m-1/2fs)
where E is the aliasing error.
|G(f)| is the amplitude spectrum of the signal g(t).
E < 2 œ_* I_GI
| f | > fs /2
Where E is the bound for aliasing error.
7. What is meant by PCM?
Pulse code modulation (PCM) is a method of signal coding in which the
message signal is sampled, the amplitude of each sample is rounded off to the
nearest one of a finite set of discrete levels and encoded so that both time and
amplitude are represented in discrete form.. This allows the message to be
transmitted by means of a digital waveform.
8. Define quantizing process.
The conversion of analog sample of the signal into digital form is called
quantizing process.
9. What are the two fold effects of quantizing process.
1. The peak-to-peak range of input sample values subdivided into a finite set of
decision levels or decision thresholds
2. The output is assigned a discrete value selected from a finite set of
representation levels are reconstruction values that are aligned with the treads
of the staircase.
10. What is meant by idle channel noise?
Idle channel noise is the coding noise measured at the receiver output with
zero transmitter input.
11. What is meant by prediction error?
The difference between the actual sample of the process at the time of
interest and the predictor output is called a prediction error.
12. Define delta modulation.
Delta modulation is the one-bit version of differential pulse code
modulation.
13. Define adaptive delta modulation.
The performance of a delta modulator can be improved significantly by
making the step size of the modulator assume a time- varying form. In particular,
during a steep segment of the input signal the step size is increased. Conversely,
when the input signal is varying slowly, the step is reduced , In this way, the step
size is adapting to the level of the signal. The resulting method is called adaptive
delta modulation (ADM).
14. Name the types of uniform quantizer?
1. Mid tread type quantizer.
2. Mid riser type quantizer.
15. Define mid tread quantizer?
Origin of the signal lies in the middle of a tread of the staircase.
Output
3a
2a
a Overload level
-3a/2 -a/2
Input
-a
-2a
Peak to peak excursion where a=delta
16. Define mid-riser quantizer?
Origin of the signal lies in the middle of a riser of the staircase
O/p
3a/2
a/2 Over load level
a 2a 3a 4a i/p
17. Define quantization error?
Quantization error is the difference between the output and input values of
quantizer.
18. What you mean by non-uniform quantization?
Step size is not uniform. Non-uniform quantizer is characterized by a step
size that increases as the separation from the origin of the transfer characteristics
is increased. Non-uniform quantization is otherwise called as robust quantization
.
19. Draw the quantization error for the mid tread and mid-rise type of quantizer?
For mid tread type:
Quantization error
a/2
Input
-a/2 a
For mid riser type:
Quantization error
a/2
Input
a
20. What is the disadvantage of uniform quantization over the non-uniform
quantization?
SNR decreases with decrease in input power level at the uniform quantizer
but non-uniform quantization maintains a constant SNR for wide range of input
power levels. This type of quantization is called as robust quantization.
21. What do you mean by companding? Define compander.
The signal is compressed at the transmitter and expanded at the receiver. This
is called as companding. The combination of a compressor and expander is called
a compander.
22. Draw the block diagram of compander? Mention the types of companding?
Block diagram:
Input Compressor uniform quantizer expander o/p signal
Transmitter receiver
Types of companding:
1. μ law companding
2. A law companding
23. What is PAM?
PAM is the pulse amplitude modulation. In pulse amplitude modulation,
the amplitude of a carrier consisting of a periodic train of rectangular pulses is
varied in proportion to sample values of a message signal.
24. What is the need for speech coding at low bit rates?
The use of PCM at the standard rate of 64 Kbps demands a high channel
bandwidth for its transmission ,so for certain applications, bandwidth is at
premium, in which case there is a definite need for speech coding at low bit rates,
while maintaining acceptable fidelity or quality of reproduction.
25. Define ADPCM.
It means adaptive differential pulse code modulation, a combination of
adaptive quantization and adaptive prediction. Adaptive quantization refers to a
quantizer that operates with a time varying step size. The autocorrelation function
and power spectral density of speech signals are time varying functions of the
respective variables. Predictors for such input should be time varying. So adaptive
predictors are used.
UNIT II
BASEBAND TRANSMISSION
26. What is meant by forward and backward estimation?
AQF: Adaptive quantization with forward estimation. Unquantized
samples of the input signal are used to derive the forward estimates.
AQB: Adaptive quantization with backward estimation. Samples of the
quantizer output are used to derive the backward estimates.
APF: Adaptive prediction with forward estimation, in which unquantized
samples of the input signal are used to derive the forward estimates of the
predictor coefficients.
APB: Adaptive prediction with backward estimation, in which Samples of
the quantizer output and the prediction error are used to derive estimates of the
predictor coefficients.
27. What are the limitations of forward estimation with backward estimation?
o Side information
o Buffering
o Delay
28. How are the predictor coefficients determined?
For the adaptation of the predictor coefficients the least mean square
(LMS) algorithm is used.
29. Define adaptive subband coding?
It is a frequency domain coder, in which the speech signal is divided in to
number of subbands and each one is coded separately. It uses non masking
phenomenon in perception for a better speech quality. The noise shaping is done
by the adaptive bit assignment.
30. What are formant frequencies?
In the context of speech production the formant frequencies are the
resonant frequencies of the vocal tract tube. The formants depend on the shape
and dimensions of the vocal tract.
31. What is the bit rate in ASBC?
Nfs= (MN) (fs/M)
Nfs->bit rate
M->number of subbands of equal bandwidths
N->average number of bits
fs/M->sampling rate for each subband
32. Define Adaptive filter?
It is a nonlinear estimator that provides an estimate of some desired
response without requiring knowledge of correlation functions, where the filter
coefficients are data dependent. A popular filtering algorithm is the LMS
algorithm.
33. Define data Signalling Rate.
Data signalling rate is defined as the rate measured in terms bits per
second(b/s) at which data are transmitted.
Data signaling rate Rb=I/Tb
Where Tb=bit duration.
34. Define modulation rate.
It is defined as the rate at which signal level is changed depending
On the nature of the format used to represent the digital data.It is measured in
Bauds or symbols per second.
35. State NRZ unipolar format
In this format binary 0 is represent by no pulse and binary 1 is
Represented by the positive pulse.
36. State NRZ polar format.
Binary 1 is represented by a positive pulse and binary 0 is represented by a
Negative pulse.
37. State NRZ bipolar format.
Binary 0 is reporesented by no pulse and binary one is represented by
the alternative positive and negative pulse.
38. State Manchester format.
Binary 0 Æ The first half bit duration negative pulse and the second half
Bit duration positive pulse.
Binary 1Æ first half bit duration positive pulse and the second half
Bit duration negative pulse.
39. What is an eye pattern?
Eye Pattern is used to study the effect of intersymbol interference.
40. What is the width of the eye?
It defines the time interval over which the received waveform can be
sampled without error from intersymbol interference.
41. What is sensitivity of an eye?
The sensitivity of the system to timing error is determined by the rate of
closure of the eye as the sampling time is varied.
42. What is margin over noise?
The height of the eye opening at a specified sampling time defines the margin
over noise.
43. What is Inter symbol interference?
The transmitted signal will undergo dispersion and gets broadened during
its transmission through the channel. So they happen to collide or overlap with the
adjacent symbols in the transmission. This overlapping is called Inter Symbol
Interference.
44. How eye pattern is obtained?
The eye pattern is obtained by applying the received wave to the vertical
deflection plates of an oscilloscope and to apply a saw tooth wave at the
transmitted symbol rate to the horizontal deflection plate.
UNIT III
PASSBAND TRANSMISSION
45. Properties of matched filter.
The signal to noise ratio of the matched filter depends only upon the
the ratio of the signal energy to the psd of white noise at the filter
input
1) The output signal of a matched filter is proportional to a shifted version
of the auto_correlation function of the input signal to which the filter
is matched.
46. Why do we go for Gram-Schmidt Orthogonalization procedure?
Consider a message signal m. The task of transforming an incoming
message mi=1,2,…..M, into a modulated wave si(t) may be divided into separate
discrete time & continuous time operations. The justification for this separation
lies in the Gram-Schmidt orthogonalization procedure which permits the
representation of any set of M energy signals, {si(t)}, as linear combinations of N
orthonormal basis functions, where N £M .
47. What is matched filter receiver ?
A filter whose impulse response is a time reversed & delayed version of
some signal Æ j (t) then it is said to be matched to Æj (t) correspondingly, the
optimum receiver based on the detector is referred to as the matched filter
receiver.
48. What is maximum likelihood detector.
Maximum likelihood detector computes the metric for each transmitted
message compares them and then decides in favor of maximum. The device for
implementing the decision rule
i.e; set ^m = mi if
In [ fx(x/mk)] is maximum for k=i is called maximum –likelihood detector and the
decision rule is called maximum likelihood.
49. Define antipodal signals.
A pair of sinusoidal signals that differ only in a phase shift of 180 degrees
are referred to as antipodal signals.
50. Explain how QPSK differs from PSK in term of transmission bandwidth and bit
information it carries?
For a given bit rate 1/Tb, a QPSK wave requires half the transmission
bandwidth of the corresponding binary PSK wave. Equivalently for a given
transmission bandwidth, a QPSK wave carries twice as many bits of information
as the corresponding binary PSK wave
51. Give the equation for average probability of symbol error for coherent binary
PSK.
Average probability of signal error,
Pe = 1 / 2 erfcÖ Eb / No
52. Give the signal space characterization of QPSK.
Coordinates i/p dibit 0£t £ T Phase of QPSK signal message points
(radians)
Si 1 Si 2
10
00
01
11
Õ/ 4
3Õ/4
5Õ/4
7Õ/4
ÖE/2
-ÖE/2
-ÖE/2
ÖE/2
-ÖE/2
-ÖE/2
ÖE/2
ÖE/2
53. Define QPSK.
QPSK is Quadriphase –shift keying. In QPSK the phase of the carrier
takes on one of the four equally spaced values Such as Õ/4 , 3Õ/4, 5Õ/4 and
7Õ/4.
54. Define Dibit.
A unique pair of bits is called a dibit. Gray encoded set of dibits 10, 00,
01 & 11
55. Give the transmitted signal of Non-coherent binary FSK.
Si(t) = {Ö 2Eb/Tb Cos(2Õ f i t) ,0 £ t £ Tb
O, elsewhere
fi = nc+ i/ Tb
56. Give the two basic operation of DPSK transmitter.
1. differential encoding of the input binary wave
2. Phase –shift keying hence, the name differential phase shift keying
57. Define deviation ratio in MSK .
The parameter h is defined by
h= Tb(f1-f2)
h is deviation ratio , measured with respect to bit rate 1/Tb.
58. Define MSK signal in interval 0£ t £ Tb.
S(t) = Ö 2Eb/Tb Cos [ 2Õ f 1t + q(0)] for symbol 1
Ö 2Eb/Tb Cos [ 2Õ f 2 t + q(0)] for symbol 0
59. What is nominal carrier frequency in MSK ?
Nominal carrier frequency is the arithmetic mean of the two frequencies f1
and f2 and it is given as
fc = ½ (f1 + f2)
Where f1 is the frequency for symbol –1
f2 is the frequency for symbol – 0
60. What are the three broad types of synchronization ?
1. Carrier synchronization
2. Symbol & Bit synchronization
3. Frame synchronization.
61. What is carrier synchronization ?
The carrier synchronization is required in coherent detection methods to
generate a coherent reference at the receiver. In this method the data bearing
signal is modulated on the carrier in such a way that the power spectrum of the
modulated carrier signal contains a discrete component at the carrier frequency.
62. What are the two methods for carrier synchronization.
1. Carrier synchronization using Mth Power loop
2. Costas loop for carrier synchronization
63. What is called symbol or bit synchronization ?
In a matched filter or correlation receiver, the incoming signal is sampled
at the end of one bit or symbol duration. Therefore the receiver has to know the
instants of time at which a symbol or bit is transmitted. That is the instants at
which a particular bit or symbol status and when it is ended. The estimation of
these times of bit or symbol is called symbol or bit synchronization.
64. What are the two methods of bit and symbol synchronization.
1) Closed loop bit synchronization
2) Early late gate synchronizer
65. What are the disadvantages of closed loop bit synchronization.
1) If there is a long string of 1’s and o’s then y(t) has no zero crossings
and synchronization may be lost.
2) If zero crossing of y(t) are not placed at integer multiples of Tb, the
synchronization suffers from timing Jitter.
66. What is called frame synchronization ?
Depending on bits used for encoding, the word length is defined. Thus
each word container some fixed number of bits. The receiver has to know when a
particular frame status and when its individual message bits status. This type of
synchronization is called frame synchronization.
67. Why synchronization is required ?
The signals from various sources are transmitted on the single channel by
multiplexing. This requires synchronization between transmitter and receiver.
Special synchronization bits are added in the transmitted signal for the purpose.
Synchronization is also required for detectors to recover the digital data properly
from the modulated signal.
UNIT IV
ERROR CONTROL CODING
68. What is linear code ?
A code is linear if the sum of any two code vectors produces another code
vector.
69. What is code rate ?
Code rate is the ratio of message bits (k) and the encoder output bits (n).
It is defined by r (i.e) r= k/N
70. Define code efficiency.
It is the ratio of message bits in a block to the transmitted bits for that
block by the encoder i.e
Message bits in a block
Code efficiency = ----------------------------------
Transmitted bits for the block
71. What is hamming distance?
The hamming distance between two code vectors is equal to the number of
elements in which they differ. For example let the two code vectors be
X=(101) and Y= (110)
These two code vectors differ in second and third bits.
Therefore the hamming distance between x and Y is two.
72. What is meant by systematic & non-systematic code?
In a systematic block code, message bit appear first and then check bits.
In the non-systematic code, message and check bits cannot be identified in the
code vector.
73. How syndrome is calculated in Hamming codes and cyclic codes ?
In Hamming codes the syndrome is calculated as ,
S = YH T
Here Y is the received and HT is the transpose of parity check matrix.
In cyclic code, the syndrome vector polynomial is given as,
S (P) = remainder ( y (p)/ G (P) )
Y(P) is received vector polynomial and G (p) is generator polynomial.
74. What is BCH Code ?
BCH codes are most extensive and powerful error correcting cyclic code.
The decoding of BCH coder is comparatively simpler. For any positive integer
‘m’ and ‘t’ , there exists a BCH code with following parameters :
Block length n = 2 m-1
No. of parity check bits : n-k £ mt
Minimum distance : dmin ³ 2t +1
75. What are the conditions to satisfy the hamming code.
1) No. of Check bits q ³ 3
2) Block length n = 2q –1
3) No of message bits K = n-q
4) Minimum distance dmin =3
76. Define code word & block length.
The encoded block of ‘n’ bits is called code word. The no. of bits ‘n’ after
coding is called block length.
77. Give the parameters of RS codes .
Reed Solomon codes. These are non binary BCH codes.
Block length = n =2m -1 symbols
Message size : k symbols
Parity check size : n-k= 2t symbols
Minimum distance , dmin =2t +1 symbols.
78. Why RS codes are called maximum distance separable codes ?
( n,k) Linear block code for which the minimum distance equals n – k + 1 is
called maximum distance separable codes. For RS code minimum distance equals
n – k + 1 so it is called as maximum distance separable codes.
79. What are Golay codes ?
Golay code is the (23, 12) cyclic code whose generating polynomial is,
G(p) =P11+P9+p7+P6+p5+p+1
This code has a minimum distance of dmin=7. This code can correct upto
3 errors. It is perfect code.
80. What are the advantages of cyclic codes ?
1. Encoders and decoders for cyclic codes are simple
2. Cyclic codes also detect error burst that span many successive bits.
UNIT V
SPREAD SPECTRUM MODULATION
81. Define pseudo-noise (PN) sequence.
A pseudo-noise sequence is defined as a coded sequence of 1s and Os with
certain autocorrelation properties. It is used in spread Spectrum communications.
It is periodic in that a sequence of 1s and 0s repeats itself exactly with a known
period.
82. What does the term catastrophic cyclic code represent ?
‘000’ is not a state of the shift register sequence in PN sequence generator,
since this results in a catastrophic cyclic code i.e once the 000 state is entered, the
shift register sequence cannot leave this state.
83. Define a random binary sequence.
A random binary sequence is a sequence in which the presence of a binary
symbol 1 or 0 is equally probable.
84. State the balance property of random binary sequence.
In each period of a maximum length sequence, the number of 1s is always
one more than the number of 0s. This property is called the balance property.
85. Mention about the run property.
Among the runs of 1s and 0s in each period of a maximum length
sequence, one half the runs of each kind are of length one, one fourth are of length
two, one eighth are of length three, and so or as long as these function represent
meaningful numbers of runs. This property is called the run property.
86. Give the correlation property of random binary sequence.
The autocorrelation function of a maximum length sequence is periodic
and binary valued. This property is called the correlation property.
87. Mention the significance of spread spectrum modulation.
An important attribute of spread-spectrum modulation is that it can
provide protection against externally generated interfering (jamming) signals with
finite power. The jamming signal may consist of a fairly powerful broadband
noise or multitone waveform that is directed at the receiver for the purpose of
disrupting communications. Protection against jamming waveforms is provided
by purposely making the information bearing signal occupy a bandwidth far in
excess of minimum bandwidth necessary to transmit it.
88. What is called processing gain ?
Processing Gain (PG) is defined as the ratio of the bandwidth of spread
message signal to the bandwidth of unspreaded data signal ie).
Processing Gain = BW (spreaded signal)
---------------------------- =
BW (Unspreaded signal)
89. What is called jamming effect ?
In the frequency band of the interest, somebody else transmits the signals
intentionally since these signals the in the frequency band of transmission, they
interface the required signal. Hence it becomes difficult to detect the required
signals. This is called jamming effect.
90. What is Anti jamming ?
With the help of spread spectrum method, the transmitted signals are
spread over the mid frequency band. Hence these signals appear as noise. Then it
becomes difficult for the jammers to send jamming signals. This is called
antijamming.
91. What are the three codes used for the anti jamming application ?
1. Golay code (24, 12)
2. Expurgated Golay (24, 11)
3. Maximum length shift register code.
92. What is called frequency hop spread spectrum ?
In frequency hop spread spectrum, the frequency of the carrier hops
randomly from one frequency to another frequency.
93. What is slow frequency hopping?
If the symbol rate of MFSK is an integer multiple of hop rate (multiple
symbols per hop) then it is called slow frequency hopping
94. What is fast frequency hopping?
If the hop rate is an integer multiple of symbol rate (multiple hops per
symbol) then it is called fast frequency hopping.
95. What are the two function of fast frequency hopping?
1. Spread Jammer over the entire measure of the spectrum of Txed
signal.
2. Retuning the Jamming signal over the frequency band of Txed
signal.
96. What are the features of code Division multiple Accesses?
1. It does not require external synchronization networks.
2. CDMA offers gradual degradation in performance when the no. of
users is increased But it is easy to add new user to the system.
3. If offers an external interference rejection capability.
97. What is called multipath Interference?
The interference caused by the interfacing of the signal form the indirect
path with the signal of direct path is called multipath interference.
98. What is the advantage of a spread spectrum technique?
The main advantage of spread spectrum technique is its ability to reject
interference whether it be the unintentional interference of another user
simultaneously attempting to transmit through the channel (or) the intentional
interference of a hostile transmitter to jam the transmission.
99. What is called frequency hop spread spectrum ?
In frequency hop spread spectrum, the frequency of the carrier hops
randomly from one frequency to another frequency.
100 .What is slow frequency hopping?
If the symbol rate of MFSK is an integer multiple of hop rate (multiple
symbols per hop) then it is called slow frequency hopping
Essay Questions (Part B)
1. Derive the power spectral Density of a Synchronous Data pulse stream
generated by a Binary, Zero mean, Wide Sense Stationary Sequence.
i. Define mean. Auto correlation and wide sense stationary
ii. Derive the expression of (Sa) f
iii. Derive the expression of (Sp) f
iv. Combine the values of (Sa) f and (Sp) f
v. Make the necessary approximations.
2. Derive the power spectral Density of a Synchronous Data pulse stream
generated by a Binary, Zero mean, Cyclostationary Sequence.
i. Define mean. Auto correlation and Cyclostationary
ii. Derive the expression of (Sa) f
iii. Derive the expression of (Sp) f
iv. Combine the values of (Sa) f and (Sp) f
v. Make the necessary approximations.
3. Derive the power spectral Density of a Generalized M-ary Markov Source
i. Define mean. Auto correlation and Markov source
ii. Derive the expression of (Sa) f
iii. Derive the expression of (Sp) f
iv. Combine the values of (Sa) f and (Sp) f
v. Make the necessary approximations.
4. Explain in detail about scalar communication and obtain the probability of Mary
Scalar Receiver starting from the implementation of M-ary Scalar
Receiver.
i. Draw the Block Diagram of M-ary Scalar Receiver.
ii. Do the implementation of M-ary Scalar Receiver.
iii. Find the probability of Error of M-ary Scalar Receiver.
iv. Draw the signal space diagram
5. Explain in detail about vector communication and obtain the probability of
Binary Vector Receiver starting from the implementation of Binary Vector
Receiver.
i. Draw the Block Diagram of Binary Vector Receiver.
ii. Do the implementation of Binary Vector Receiver.
iii. Find the probability of Error of Binary Vector Receiver.
iv. Draw the signal space diagram
6. Explain the construction of Block Code and explain how error syndrome is
calculated
i. Representation of Block Code.
ii. Generator Matrix.
iii. Generation of Codewords.
iv. Generation of Parity Check Matrix.
v. Calculation OF Error Syndrome.
7. Explain in detail about Orthogonal Codes, Biorthogonal Codes and
Transorthogonal Codes
i. Definitions of codes,
ii. Formation of orthogonal codes.
iii. Formation of Biorthogonal Codes.
iv. Formation of Transorthogonal Codes.
8. Explain in detail about Shannon Coding Theorem.
i. Statement of Theorem.
ii. Discussion of theory.
iii. Derivation of the Theorem.
9. What is Spread Spectrum Techniques Explain in detail about Direct Sequence
Spread Spectrum Techniques with necessary diagrams?
i. Concept of Spread Spectrum Techniques
ii. Block Diagram Representation.
iii. Waveform at all stages of the system.
iv. Derivation of processing Gain.
10. What is Frequency Hopping? Explain the different types of frequency hopping
with necessary diagrams.
i. Concept of frequency hopping.
ii. Explanation of slow frequency hopping
iii. Explanation of Fast frequency hopping
iv. Block Diagrams and waveform
11. Explain in detail about Golay Codes, Reed Solomon Codes and BCH Codes.
i. Definition of Golay Codes, Reed Solomon Codes and BCH Codes.
ii. Explanation of Golay Codes
iii Explanation of Reed Solomon Codes
iv Explanation of BCH Codes.
12. Explain in detail about Binary Phase Shift Keying and obtain an expression
for its probability of error.
i. Block Diagram of Transmitter and Receiver.
ii. Explanation of Transmitter and receiver.
iii. Signal Space Diagram
iv. Calculation of Probability of Error
13. Explain in detail about Quadrature Phase Shift Keying and obtain an
expression for its probability of error.
i. Block Diagram of Transmitter and Receiver.
ii. Explanation of Transmitter and receiver.
iii. Signal Space Diagram
iv. Calculation of Probability of Error
14. Explain in detail about Minimum Shift Keying and obtain an expression for its
probability of error.
i. Block Diagram of Transmitter and Receiver.
ii. Explanation of Transmitter and receiver.
iii. Signal Space Diagram0
iv. Calculation of Probability of Error
15. Explain in detail about the optimum demodulation of Digital signals in the
presence if ISI and AWGN. Also, explain about the various equalization
techniques.
i. Concept of ISI and AWGN
ii. Derivation of ISI.
iii. Ideal Solution and Practical Solution.
iv. Types of Equalization.
16. Explain in detail about the operation of Non Coherent Receivers in the
presence of Random Phase Channel and implement the receiver.
i. Concept of Non Coherent Receivers.
ii. Derivation of probability of Error.
iii. Implementation of the Receiver.
iv. Waveforms
17. Explain in detail about the operation of Non Coherent Receivers in the
presence of Random amplitude and phase Channel and implement the
receiver.
a. Concept of Non Coherent Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
18. Explain in detail about the operation of Optimum Receivers in Rayleigh
Channel and implement the receiver.
a. Concept of Raleigh Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
19. Explain in detail about the operation of Optimum Receivers in Rician Channel
and implement the receiver.
a. Concept of Rician Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
20. Explain in detail about the operation of Partially Coherent Receivers in the
presence of Random Phase Channel and implement the receiver.
a. Concept of Partially Coherent Receivers.
b. Derivation of probability of Error.
c. Implementation of the Receiver.
d. Waveforms
21. a. Explain in detail about In phase and Quadrature Modulation systems.
b. With necessary diagrams explain the operation of Quadrature Amplitude
Modulation systems.
a. In phase and Quadrature Modulation systems.
· Block Diagram
· Derivation
· Explanation
b. Quadrature Amplitude Modulation systems.
· Draw the Block Diagram
· Derivation
· Explanation
22. Draw the code tree of a Convolutional code of code rate r=1/2 and Constraint
length of K=3 starting from the state table and state diagram for an encoder
which is commonly used.
a. Draw the state Diagram.
b. Draw the state Table.
c. Draw the code Tree
23. Draw the trellis diagram of a Convolutional code of code rate r=1/2 and
Constraint length of K=3 starting from the state table and state diagram for an
encoder which is commonly used.
a. Draw the state Diagram.
b. Draw the state Table.
c. Draw the trellis diagram
24. Decode the given sequence 11 01 01 10 01 of a convolutional code with a
code rate of r=1/2 and constraint length K=3, using viterbi decoding
algorithm.
a. Draw the state Diagram.
b. Draw the state Table.
c. Draw the code Tree
d. Decode the given sequence using trellis diagram
25. Explain in detail about Continuous Phase Frequency Shift Keying and
obtain an expression for its probability of error.
a. Block Diagram of Transmitter and Receiver.
b. Explanation of Transmitter and receiver.
c. Signal Space Diagram
d. Calculation of Probability of Error
26. Derive the power spectral Density of a Synchronous Data pulse stream
generated by a Binary, non-zero mean, Cyclostationary Sequence.
a. Define mean. Auto correlation and wide sense stationary
b. Derive the expression of (Sa) f
c. Derive the expression of (Sp) f
d. Combine the values of (Sa) f and (Sp) f
e. Make the necessary approximations.
KEYWORDS:DIGITAL COMMUNICATION,DIGITAL COMMUNICATION QUESTION PAPER,ANNA UNIVERSITY QUESTION PAPER,ANNA UNIVERSITY,ANNA UNIVERSITY CHENNAI,ANNA UNIVERSITY COIMBATORE,ANNA UNIVERSITY TRICHY,ANNA UNIVERSITY TIRUNELVELI,ANNA UNIVERSITY MADURAI,ANNA UNIVERSITY SYLLABUS,ANNA-UNIVERSITY RESULTS,ANNA UNIVERSITY DISTANCE EDUCATION,ANNA UNIVERSITY MBA-CENTRE FOR DISTANCE EDUCATION,ANNA UNIVERSITY SCHEDULE OF EXAMINATIONS,ANNA UNIVERSITY ADMISSION,ANNA UNIVERSITY COURSES,ANNA UNIVERSITY ACADEMIC,ANNA UNIVERSITY DEPARTMENTS,ANNA UNIVERSITY RESEARCH,ANNA UNIVERSITY MAIL,ANNA UNIVERSITY QUESTION PAPERS,ANNA UNIVERSITY COUNSELLING DATES,ANNA UNIVERSITY RE-EVALUATION RESULTS