# Digital Communications Questions and Answers – Properties of Convolution Codes and Reed-Solomon Codes

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This set of Digital Communication Multiple Choice Questions & Answers focuses on “Properties of Convolution Codes and Reed-Solomon Codes”.

1. Which distance is related to the error correcting capability of the code?
a) Maximum distance
b) Minimum distance
c) Maximum & Minimum distance
d) None of the mentioned

Explanation: Minimum distance between all pair of code words is related to the error correcting capability of the code.

2. A catastrophic error is an event whereby ______ number of code symbol error causes _______ number of decoded data error bits.
a) Finite, finite
b) Finite, infinite
c) Infinite, finite
d) Infinite, infinite

Explanation: A catastrophic error is defined as an event whereby a finite number of code symbols error causes an infinite number of decoded data bit errors.

3. A code should be selected such that it should
a) Not have catastrophic error propagation
b) Have minimum free distance
c) All of the mentioned
d) None of the mentioned

Explanation: Generally code’s are selected based on the free distance property. The criteria for selecting code is that it should not have catastrophic error propagation and it should have maximum free distance.

4. The error correcting capability of a code scheme increases as the
a) Number of channel symbols per information bit increases
b) Bandwidth increases
c) Information per bit increases
d) All of the mentioned

Explanation: The error correcting capability of a code scheme increases as the number of channel symbols n per information bit k increases or the rate k/n decreases. As n increases bandwidth and complexity also increases.

5. Which has a limited resolution?
a) Hard decision decoder
b) Soft decision decoder
c) Hard & Soft decision decoder
d) None of the mentioned

Explanation: The soft decision algorithm has limited resolution than hard decision algorithm and thus it cannot use hamming distance metric.

6. Which parameter is used in soft decision algorithm?
a) Euclidean distance
b) Euclidean distance squared
c) Euclidean distance & distance squared
d) None of the mentioned

Explanation: As soft decision decoding algorithm has limited resolution it cannot use hamming distance metric. Instead it uses Euclidean and monotonic metric such as Euclidean distance squared.

7. Which factor is desirable?
a) Minimum correlation
b) Maximum correlation
c) Maximum distance
d) None of the mentioned

Explanation: While using Euclidean distance squared metric, maximum correlation is desirable rather than minimum distance.

8. Soft decision results in
a) Increase in complexity
b) Decrease in storage
c) Increase in complexity & Decrease in storage
d) None of the mentioned

Explanation: Soft decisions require more storage and it also increases the complexity. For this reason soft decisions are preferred less when compared to hard decisions.

9. A feedback decoder makes hard decision.
a) True
b) False

Explanation: The feedback decoder makes hard decision on data bits.

10. The look ahead length L is given by
a) m+1
b) m-1
c) (m+1)/2
d) 1-m

Explanation: Look ahead length L is given by m+1 where m is the preselected positive integer.

11. On increasing L
a) Coding gain increases
b) Complexity increases
c) Coding gain & Complexity increases
d) None of the mentioned

Explanation: On increasing the look ahead length the coding gain increases and also increases the decoder implementation complexity.

12. Reed Solomon codes are
a) Non binary
b) Cyclic
c) Non binary & Cyclic
d) None of the mentioned

Explanation: Reed Solomon codes are non binary cyclic codes with symbols made up of m bit sequences where m is any positive integer with value greater than 2.

13. The minimum distance for Reed Solomon code is given by
a) n+k+1
b) n-k+1
c) 1-n-k
d) 1+n-k

Explanation: Reed Solomon code achieves the largest possible code minimum distance for any linear code and it is given by n-k+1.

14. R-S codes have
a) High code rate
b) Low redundancy
c) High code rate & Low redundancy
d) None of the mentioned

Explanation: Reed Solomon codes have high code rate and low redundancy. The complexity of a high speed implementation increases with redundancy.

15. Greater the redundancy greater will be the error correcting capability.
a) True
b) False

Explanation: In coding, the improving mechanism, the greater the redundancy the greater will be the error correcting capability of the code.

16. A primitive element are those which yield ______ when raised to higher order exponents.
a) All zero elements
b) Non zero elements
c) Unity elements
d) None of the mentioned

Explanation: In a primitive polynomial at least one of its roots will be primitive element. A primitive elements are those which yield non zero elements when raised to a higher order exponent.

17. If the inversion was performed correctly, then the multiplication of the original matrix with the inverted matrix should yield identity matrix. This is known as
a) Identity theorem
b) Safety theorem
c) Multiplication theorem
d) Inversion theorem 