# Digital Communications Questions and Answers – Shannon Hartley Theorem and Turbo Codes

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Shannon hartley theorem and turbo codes”.

1. The minimum nyquist bandwidth needed for baseband transmission of Rs symbols per second is
a) Rs
b) 2Rs
c) Rs/2
d) Rs2

Explanation: Theoretical minimum nyquist bandwidth needed for the baseband transmission of Rs symbols per second without ISI is Rs/2.

2. The capacity relationship is given by
a) C = W log2 ( 1+S/N )
b) C = 2W log2 ( 1+S/N )
c) C = W log2 ( 1-S/N )
d) C = W log10 ( 1+S/N )

Explanation: The capacity relationship from Shannon-hartley capacity theorem is given by C = W log2 ( 1+S/N ).

3. Which parameter is called as Shannon limit?
a) PB/N0
b) EB/N0
c) EBN0
d) None of the mentioned

Explanation: There exists a limiting value for EB/N0 below which they can be no error free communication at any information rate. This EB/N0 is called as Shannon limit.

4. Entropy is the measure of
a) Amount of information at the output
b) Amount of information that can be transmitted
c) Number of error bits from total number of bits
d) None of the mentioned

Explanation: Entropy is defined as the average amount of information per source output.

5. Equivocation is the
a) Conditional entropy
b) Joint entropy
c) Individual entropy
d) None of the mentioned

Explanation: Shannon uses a correction factor called equivocation to account for uncertainty in the received signal. It is defined as the conditional entropy of the message X given Y.
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6. For a error free channel, conditional probability should be
a) Zero
b) One
c) Equal to joint probability
d) Equal to individual probability

Explanation: For a error free channel conditional probability should be zero, because having received Y there is complete certainty about the message X.

7. Average effective information is obtained by
a) Subtracting equivocation from entropy
c) Ratio of number of error bits by total number of bits
d) None of the mentioned

Explanation: According to Shannon the average effective information is obtained by subtracting the equivocation from the entropy of the source.

8. Turbo codes are
a) Forward error correction codes
b) Backward error correction codes
c) Error detection codes
d) None of the mentioned

Explanation: Turbo codes are a class of high performance forward error correction codes.

9. Components used for generation of turbo codes are
a) Inter leavers
b) Punching pattern
c) Inter leavers & Punching pattern
d) None of the mentioned

Explanation: There are many instances of turbo codes, using different component encoders, input/output ratios, inter leavers, punching patterns.

10. Decoders are connected in series.
a) True
b) False

Explanation: Two elementary decoders are connected in serial connection for decoding the turbo codes.

11. The inter leaver connected between the two decoders is used to
a) Remove error bursts
b) Scatter error bursts
d) None of the mentioned

Explanation: An inter leaver installed between the two decoders connected in series is used to scatter error bursts.

12. In soft decision approach what does -127 mean?
a) Certainly one
b) Certainly zero
c) Very likely zero
d) Very likely one

Explanation: The decoder front end produces an integer for each bit in the data stream. This integer is the measure of how likely it is that the bit 0 or 1 and is called as soft bit. It ranges from -127 to 127. Here -127 represents certainly zero.

13. In soft decision approach 100 means?
a) Certainly one
b) Certainly zero
c) Very likely zero
d) Very likely one

Explanation: The decoder front end produces an integer for each bit in the data stream. This integer is the measure of how likely it is that the bit 0 or 1 and is called as soft bit. It ranges from -127 to 127. Here 100 represents very likely one.

14. In soft decision approach 0 represents
a) Certainly one
b) Certainly zero
c) Very likely zero
d) Could be either zero or one

Explanation: The decoder front end produces an integer for each bit in the data stream. This integer is the measure of how likely it is that the bit 0 or 1 and is called as soft bit. It ranges from -127 to 127. Here 0 represents ‘could be either zero or one’.

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