Here is the list of chapters on the Digital Communications Questions and Answers MCQ UNIT 1 – subject covering 100+ topics. You can practice the MCQs chapter by chapter starting from the 1st chapter to other chapters.

## Digital Communications Chapters

- Signals and Spectra
- Formatting and Baseband Modulation
- Baseband Demodulation and Detection
- Bandpass Modulation and Detection
- Communication Link Analysis
- Channel Coding
- Modulation and Coding Trade-off
- Synchronization
- Multiplexing and Multiple Access
- Spread Spectrum Techniques
- Encryption and Decryption
- Fading Channels
- Digital Communication
- Error Detection & Channel Coding
- Source Coding

## 1. Signals and Spectra

The section contains multiple choice questions and answers on various aspects of signals and spectra which include signal classification, digital communication signal processing, autocorrelation, concepts of noise, process types and information on baseband systems.

- Digital Communication Processing
- Signal Classification
- Spectral Density and Autocorrelation
- Random Signals
- Linear Systems Transmission
- Noise – 1
- Noise – 2
- Digital Communication
- Digital Communication System – 1
- Digital Communication System – 2
- Process Types
- Signals

## Digital Communication Signal Processing

**Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on Digital Communication Signal Processing**

- Which circuit is called as regenerative repeaters?

a) Analog circuits

b) Digital circuits

c) Amplifiers

d) A/D converters

**Answer: b**

Explanation: The main advantage of digital communication is that the signals can be reproduced easily. Thus digital circuits are called as regenerative repeaters.

- What are the advantages of digital circuits?

a) Less noise

b) Less interference

c) More flexible

d) All of the mentioned

**Answer: d**

Explanation: Digital circuits are less subject to noise, distortion and interference as it works on digital pulses and also the pulses can be regenerated.

- How many different combinations can be made from a n bit value?

a) 2(n+1)

b) 2(n)

c) 2(n)+1

d) None of the mentioned

**Answer: b**

Explanation: 2(n) different combinations can be made from n bit value. For example, from 2 bit value 22 different combinations-00,01,10,11 can be made.

- How many bytes does a gigabyte have?

a) 1 million bytes

b) 10 million bytes

c) 1 billion bytes

d) 10 billion bytes

**Answer: c**

Explanation: One gigabyte has 1 billion bytes.

- What is the ASCII value of space?

a) 32

b) 48

c) 96

d) 65

**Answer: a**

Explanation: The ASCII value of space is 32 and ASCII value of 0 is 48.

- Which block or device does the data compression?

a) Channel encoder

b) Source encoder

c) Modulator

d) None of the mentioned

**Answer: b**

Explanation: Source encoder converts the digital or analog signal to a sequence of binary digits. This process is called as source encoding or compression.

- What is the code rate?

a) k/n

b) n/k

c) All of the mentioned

d) None of the mentioned

**Answer: a**

Explanation: Here n is the total bits of sequence and k bits are mapped. Amount of redundancy introduced is given by n/k and its reciprocal is the code rate.

- Pulse shaping is done by which block or system?

a) Encoder

b) Baseband modulator

c) Pulse code modulator

d) Demodulator

**Answer: c**

Explanation: Pulse code modulator does filtering process to build pulses that occupy more than one bit time.

- Equalizer is used for?

a) Filtering

b) Diminish distortion

c) All of the mentioned

d) None of the mentioned

**Answer: c**

Explanation: Equalizer is used as a filtering option and also diminishes or reduces the distortion.

- Source coding block is used for?

a) Compressing

b) Digitizing

c) A/D conversion

d) All of the mentioned

**Answer: d**

Explanation: Source encoding does all these processes-compression, digitizing the signal and performs analog to digital conversion.

- Which measurement considers phase as an important parameter?

a) Coherent

b) Non-coherent

c) All of the mentioned

d) None of the mentioned

A**nswer: a**

Explanation: Coherent measurement considers phase as an important parameter.

- The size of the alphabet M in symbol is calculated as?

a) 2(k+1)

b) 2k

c) 2(k-1)

d) 1+2k

**Answer: b**

Explanation: The size of the alphabet is calculated using 2^k where k is the number of bits in the symbol.

## Digital Communications Questions and Answers – Classification of Signals

- What is the period of a signal x(t)?

a) T

b) 2T

c) T/2

d) None of the mentioned

**Answer: a**

Explanation: A signal is said to be periodic if the duration of one complete cycle is T and it repeats itself after this duration.

- Which of the given signals are periodic?

a) x(t) = 4 cos(5πt)

b) x(t) = u(t) – 1/2

c) x(t) = 4u(t) + 2sin(3t)

d) x[n] = 2sin(3n)

**Answer:a**

Explanation: T=2π / 5π = 2/5, periodic with period 2/5.

- Check whether the signal is periodic or not?

x(t) = cos(4t) + 2sin(8t)

a) Periodic with period π/2

b) Periodic with period 2

c) Periodic with period 2/π

d) Not periodic

**Answer: a**

Explanation: T1=2π /4 T2=2π /8, T1/T2 = 2 T=T1 or 2T2, T=π /2.

- Find the periodicity of the following signal. x(t)=cos((2π/7)t)sin((3π/5)t)

a) 30

b) 7

c) 35

d) 5/3

**Answer: c**

Explanation: T1=2π/2π/7 = 7, T2=2π/3π/5=5/3, T1/T2=5/21, T=21T1 or 5T2, T=35.

- Find the fundamental period of 1+sin^2 (3π/5)n.

a) 10/3

b) 5

c) 3π/5

d) None of the mentioned

**Answer: b**

Explanation: sin2(3π/5)n = (1-cos(6π/5)n)/2 = 1/2-1/2cos(6π/5)n ; period = 6π/(5*2π) = k/N = 3/5 ; N=5.

- Which signal is called as energy signal?

a) Finite energy and zero power

b) Finite energy and non-zero power

c) Infinite energy and zero power

d) Infinite energy and non-zero power

**Answer: a**

Explanation: A signal is said to be an energy signal if it has finite energy and zero average power.

- Which signal is said to be power signal?

a) Infinite power and zero energy

b) Infinite power and non-zero energy

c) Finite power and infinite energy

d) Finite power and zero energy

Answer: c

Explanation: A signal is said to be power signal if it has finite power and infinite energy.

- Determine the periodicity and also find whether it is a power or energy signal?

e(-at)u(t)

a) Periodic and energy signal

b) Non periodic and energy signal

c) Periodic and power signal

d) Periodic and energy signal

Answer: b

Explanation: This is non periodic as it does not repeat itself after any period. And on calculating we will get energy as 1/2a which is finite. Thus it is an energy signal.

- Continuous Impulse signal is a power or energy signal?

a) Power signal

b) Energy signal

c) Both power and energy

d) Neither power nor energy signal

Answer: d

Explanation: On calculating using the power and energy calculation formula we will arrive at it is neither power nor energy signal.

- Discrete impulse signal is a power or energy signal?

a) Power signal

b) Energy signal

c) Both power and energy signal

d) Neither power or energy signal

Answer: b

Explanation: On calculating using energy formula we will get the energy of this signal as 1 which is a finite value. Thus it is a energy signal.

- A unit impulse function has?

a) Large amplitude

b) Zero pulse width

c) Unity weight

d) All of the mentioned

Answer: d

Explanation: An impulse function has infinite or large amplitude, zero pulse width and unity weight which is concentrated at zero.

## Spectral Density and Autocorrelation

- Power spectral density function is a?

a) Real and even function

b) Non negative function

c) Periodic

d) All of the mentioned

Answer: d

Explanation: A power signal is periodic signal and its function is a real even and non negative function as per the definition.

- Energy spectral density defines

a) Signal energy per unit area

b) Signal energy per unit bandwidth

c) Signal power per unit area

d) Signal power per unit bandwidth

Answer: b

Explanation: Energy spectral density gives the signal energy equal to the area under the waveform energy spectral density versus frequency curve.

- Power spectrum describes distribution of
under frequency domain.*_*

a) Mean

b) Variance

c) Gaussian

d) None of the mentioned

Answer: b

Explanation: Power spectrum gives the distribution of variance of a signal in the frequency domain, sampled into spectral components.

- How can power spectral density of non periodic signal be calculated?

a) By integrating

b) By truncating

c) By converting to periodic

d) None of the mentioned

Answer: b

Explanation: A power signal usually is a periodic signal. But power spectral density of non periodic signal can be calculated by truncating it and observing it in the range of (-T/2,T/2).

- What is Wiener-Khinchin theorem?

a) Spectral density and auto-covariance makes a fourier transform pair

b) Spectral density and auto-correlatioon makes a fourier tranform pair

c) Spectral density and variance makes a fourier tranform pair

d) None of the mentioned

Answer: b

Explanation: According to the theorem spectral density of a signal x(t) and auto correlation function makes a fourier tranform pair.

- According to Parseval’s theorem the energy spectral density curve is equal to?

a) Area under magnitude of the signal

b) Area under square of the magnitude of the signal

c) Area under square root of magnitude of the signal

d) None of the mentioned

Answer: b

Explanation: According to Parseval/s theorem the energy spectral density function can be given as equal to the square of the magnitude of the signal x(t).

- Spectogram is the graph plotted against?

a) Frequency domain

b) Time domain

c) Frequency & Time domain

d) None of the mentioned

Answer: b

Explanation: Spectral density function is plotted against frequency domain and spectogram is a graph which is plotted against time domain.

- Autocorrelation is a function which matches

a) Two same signals

b) Two different signal

c) One signal with its delayed version

d) None of the mentioned

Answer: c

Explanation: Autocorrelation is a function that matches a signal with its delayed version.

- Autocorrelation is a function of

a) Time

b) Frequency

c) Time difference

d) Frequency difference

Answer: c

Explanation: Autocorrelation is a function of time difference as it matches the signal with its delayed version.

- Autocorrelation is maximum at
*_*

a) Unity

b) Origin

c) Infinite point

d) None of the mentioned

Answer: b

Explanation: According to its properties autocorrelation is maximum at origin.

- Autocorrelation function of periodic signal is equal to
*_*

a) Energy of the signal

b) Power of the signal

c) Its area in frequency domain

d) None of the mentioned

Answer: b

Explanation: Autocorrelation function of a real valued signal is equal to the energy of the signal and auto-correlation function of the periodic signal is equal to the average power of the signal.

- Autocorrelation is a
function.*_*

a) Real and even

b) Real and odd

c) Complex and even

d) Complex and odd

Answer: a

Explanation: According to properties of autocorrelation function it a even function when the frequency value f is real.

- Autocorrelation function of white noise will have?

a) Strong peak

b) Infinite peak

c) Weak peak

d) None of the mentioned

Answer: a

Explanation: Autocorrelation function curve of continuous time white noise signal has a strong peak.

## Digital Communications Questions and Answers Random Signals

- Random variables give relationship between
**_**

a) Two random events

b) Probability of occurence of two random events

c) Random event and a real number

d) Random event and its probability of occurrence

View Answer

Answer: c

Explanation: A random variable gives a functional relationship between a random event and a real number.

- The distribution function of random variable is

a) P(X less than or equal to x)

b) P(X greater than or equal to x)

c) P(X less than x)

d) P(X greater than x)

Answer: a

Explanation: The distribution function of a random variable is the probability that the value taken by the random variable is less than or equal to the real number x.

- The distribution function of -(infinity) and (infinity) is
**_**

a) 0 and 1

b) 1 and 0

c) Both 0

d) Both 1

View Answer

Answer: a

Explanation: F(minus infinity) is 0 and F(infinity) is 1.

- The value of the probability density function of random variable is

a) Positive function

b) Negative function

c) Zero

d) One

Answer: a

Explanation: The probability density function is always greater than 0. It is a non negative function with the area of 1.

- Which gives the measure of randomness of the random variable?

a) Mean

b) Variance

c) Standard variance

d) Pdf

Answer: b

Explanation: Variance gives the randomness of the random variable. It is the difference between the mean square value and square of the mean.

- Random process is a function of
**__**

a) Random event and time

b) Random event and frequency

c) Random event and real number

d) None of the mentioned

Answer: a

Explanation: Random process is a function of two variables: a random event and its time of occurrence.

- A random process is called as stationary in strict sense if

a) Its statistics vary with shift in time origin

b) Its statistics does not vary with shift in time origin

c) Its autocorrelation vary with shift in time

d) Its autocorrelation does not vary with shift in time

Answer: a

Explanation: A random process is defined to be stationary in a strict sense if its statistics varies with a shift in time origin.

- For a stationary process, autocorrelation function depends on

a) Time

b) Time difference

c) Does not depend on time

d) None of the mentioned

Answer: b

Explanation: Autocorrelation function depends on the time difference between t1 and t2.

- The autocorrelation function is maximum at

a) Origin

b) Infinity

c) Origin & Infinity

d) None of the mentioned

Answer: a

Explanation: On substituting different values in the formula for autocorrelation function it wil be maximum at the origin.

- Standard deviation is
**__**

a) Rms value of dc

b) Rms value or ac

c) Either ac or dc

d) Neither dc nor ac

Answer: b

Explanation: The standard deviation of a random variable gives the rms value of an ac component.

- The average power of white noise is

a) Zero

b) Unity

c) Infinity

d) Between zero and one

Answer: c

Explanation: The average power of white noise is infinity because its bandwidth is infinite.

- White noise has
variance.*_ mean and __*

a) Zero and zero

b) Finite and zero

c) Zero and finite

d) One and zero

Answer: c

Explanation: White noise is a zero mean function with infinite average power, finite variance and infinite bandwidth.

## Baseband Systems and Signal Transmission through Linear Systems

- The process of data conversion along with formatting the data is called as
**__**

a) Formatting

b) Modulation

c) Source coding

d) Amplifying

**Answer: c**

Explanation: The process of converting source information to a digital signal and also formatting it is called as source coding.

- Formatting is the process which includes

a) Pulse code modulation

b) Sampling

c) Quantization

d) All of the mentioned

**Answer: d**

Explanation: Formatting is the process in which the source information is converted to digital signals using methods like sampling, pulse code modulation etc.

- Analog information is converted to digital data using

a) Sampling

b) Quantization

c) Coding

d) All of the mentioned

**Answer: d**

Explanation: Textual information, analog information and digital information undergoes different processes before converting to binary or digital data. The method used for converting analog information are the given methods.

- The process that transforms text into binary digits is called as
*_*

a) Binary coding

b) Data coding

c) Character coding

d) Sampling

**Answer: c**

Explanation: According to the definition the process of converting textual data to binary digits is called as character coding.

- For the number of bits, k=1 and number of symbols, M=2 the system is called as

a) Unary

b) Binary

c) Quaternary

d) None of the mentioned

**Answer: b**

Explanation: For k number of bits, the number of symbols are M=2k. For k=1 and M=2 the system is called as binary coding.

- Perform the bit stream partitioning and find the 8-ary waveform for the word ‘THINK’.

a) 1204443464

b) 4643444021

c) 1240443464

d) none of the mentioned

**Answer: a**

Explanation: “THINK”

ASCII value: T-001010 ; H-000100; I-100100; N-011100; K-110100

8-ary symbols(split into 3 bits): 1 2 0 4 4 4 3 4 6 4.

- Find the 32-ary waveform for the word ‘THINK’.

a) 514172520

b) 202517415

c) 541172520

d) None of the mentioned

**Answer: a**

Explanation: “THINK”

ASCII value: T-001010; H-000100; I-100100; N-011100; K-110100

32-ary symbols(split into 5 bits): 5 1 4 17 25 20.

- Find the 8-ary waveform for the word ‘HAPPY’.

a) 4040020246

b) 0440020246

c) 6420200440

d) None of the mentioned

**Answer: b**

Explanation: “HAPPY”

ASCII value: H-000100; A-100000; P-000010; P-000010; Y-100110

8-ary symbols: 0 4 4 0 0 2 0 2 4 6.

- Find the 32-ary waveform for the word ‘HAPPY’.

a) 280856

b) 658082

c) 288056

d) None of the mentioned

**Answer: a**

Explanation: “HAPPY”

ASCII value: H-000100; A-100000; P-000010; P-000010; Y-100110

32-ary symbols: 2 8 0 8 5 6.

- What are the characteristics of an ideal transmission line?

a) Different amplitude

b) No distortion

c) Time delay

d) All of the mentioned

**Answer: d**

Explanation: The output from the ideal transmission line has some time delay different amplitude but it should not have distortion it should have the same shape as the input.

- The point at which the output signal power has fallen to 0.707 of its peak value is called as
**_**

a) 3db point

b) Half power point

c) 3db & Half power point

d) None of the mentioned

**Answer: c**

Explanation: The frequency at which the output signal power falls to half the peak value is called as half power point or 3db point.

- The ratio of the filter bandwidth at -60db and -6db amplitude response point is called as
**_**

a) Half point factor

b) Normalized factor

c) Shape factor

d) None of the mentioned

**Answer: c**

Explanation: Shape factor is a measure of how well realizable filter approximates to ideal filter. It is the ratio of filter bandwidths -60db and -6db amplitude points.

- As the order of the filter increases, the cost
**_ and complexity _**

a) Increases, increases

b) Increases, decreases

c) Decreases, increases

d) Decreases, decreases

**Answer: a**

Explanation: According to the design of the filter as the order increases the complexity and cost of the filter also increases.

- Which filter has maximum flatness?

a) Chebyshev filter

b) Butter-worth filter

c) High pass filter

d) Normalized filter

**Answer: b**

Explanation: Butter-worth filter as the best approximation to ideal filter and has the maximum flatness in the filter pass-band.

- The measure of the width of the main lobe is called as
*__*

a) Null to null bandwidth

b) Half power bandwidth

c) Normalized bandwidth

d) Absolute bandwidth

**Answer: a**

Explanation: The main lobe consists of the maximum signal power. The measure of the width of the main lobe is called as null to null bandwidth.

### Digital Communication MCQ UNIT 2

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