Wireless & Mobile Communications Multiple Choice Questions on “Convolutional Codes”.
1. Block codes can achieve a larger coding gain than convolution coding.
a) True
b) False
Answer: b
Clarification: Convolution code can achieve a larger coding gain that can be achieved using a block coding with the same complexity. Their mapping is highly structured, enabling a decoding method considerably different from block codes.
2. Which of the following indicates the number of input bits that the current output is dependent upon?
a) Constraint length
b) Code length
c) Search window
d) Information rate
Answer: a
Clarification: Constraint length determines the number of input data bits that the current output is dependent upon. The constraint length determines how powerful and complex the code is.
3. Which of the following is not a way to represent convolution code?
a) State diagram
b) Trellis diagram
c) Tree diagram
d) Linear matrix
Answer: d
Clarification: Linear matrix is not a way to represent convolution code. Various ways of representing convolution codes are generator matrix, generator polynomial, logic tables, state diagram, tree diagram and trellis diagram.
4. Which of the following is not an algorithm for decoding convolution codes?
a) Viterbi algorithm
b) Stack algorithm
c) Fano’s sequential coding
d) Ant colony optimization
Answer: d
Clarification: There are a number of techniques for decoding convolution codes. The most important of these methods is Viterbi algorithm. Other decoding algorithms for convolutional codes are Fano’s sequential coding, stack algorithm and feedback coding.
5. Viterbi algorithm performs ____________ decoding of convolutional codes.
a) Maximum likelihood
b) Maximum a posteriori
c) Minimum square
d) Minimum mean square
Answer: a
Clarification: Viterbi algorithm performs maximum likelihood decoding of convolutional codes. The algorithm was first developed by A.J. Viterbi. It is one of the most important algorithm used for decoding convolutional codes.
6. Fano’s algorithm searches all the paths of trellis diagram at same time to find the most probable path.
a) True
b) False
Answer: b
Clarification: Fano’s algorithm searches for the most probable path through the trellis diagram by examining one path at a time. The error rate performance of Fano’s algorithm is comparable to Viterbi’s algorithm.
7. Which of the following is not an advantage of Fano’s algorithm in comparison to Viterbi’s algorithm?
a) Less storage
b) Large constraint length
c) Error rate
d) Small delays
Answer: d
Clarification: In comparison to Viterbi decoding, sequential decoding has a significantly larger delay. In advantage over Viterbi decoding is that it requires less storage, and thus codes with larger constraint lengths can be employed.
8. In comparison to stack algorithm, Fano’s algorithm is simpler.
a) True
b) False
Answer: b
Clarification: In comparison to Fano’s algorithm, the stack algorithm is computationally simpler since there is no retracting over the same path. But stack algorithm requires more storage than Fano’s algorithm.
9. Which of the following is not an error correction and detection code?
a) Block code
b) Convolutional codes
c) Passive codes
d) Turbo codes
Answer: c
Clarification: There are three basic types of error correction and detection codes. They are block codes, convolutional codes and turbo codes. A channel coder operates on digital message data by encoding the source information into a code sequence.
10. Which decoding method involves the evaluation by means of Fano’s algorithm?
a) Maximum Likelihood Decoding
b) Sequential Decoding
c) Maximum a priori
d) Minimum mean square
Answer: b
Clarification: Fano’s algorithm involves sequential decoding. It searches for the most probable path through the trellis by examining one path at a time.
11. In Viterbi’s algorithm, the selected paths are regarded as __________
a) Survivors
b) Defenders
c) Destroyers
d) Carriers
Answer: a
Clarification: In Viterbi’s algorithm, the selected paths are regarded as survivors. The path thus defined is unique and corresponds to the decoded output.
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