This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Convolution encoding and decoding”.
1. The measure of the amount of redundancy is given by
a) Code size
b) Code weight
c) Code rate
d) Minimum distance
View Answer
Explanation: The rate of the code gives the measure of the amount of redundancy. The rate is given by the ratio of number of data bits that form an input to a block encoder to the total number of bits.
2. The number of k bit shift over which a single information bit influences the encoder output is given by
a) Code rate
b) Constraint length
c) Code length
d) Code weight
View Answer
Explanation: The constraint length represents the number of k bit shifts over which a single information bit influences the encoder output.
3. The method used for representing convolution encoder are
a) Connection pictorial
b) State diagram
c) Tree diagram
d) All of the mentioned
View Answer
Explanation: Several methods are used for representing convolution encoder such as connection pictorial, connection vector or polynomials, state diagram, tree diagram and trellis diagram.
4. Periodic truncation is done by
a) Appending zero to end of the input data sequence
b) Appending zero to the beginning of the input data sequence
c) Appending one’s to end of the data sequence
d) Appending one’s to beginning of the data sequence
View Answer
Explanation: Convolution code are forced into a block structure by periodic truncation which is done by appending zero to the end of the input data sequence, for the purpose of clearing or flushing the encoding shift register of the data bits.
5. Finite state machines have
a) Past memories also
b) Only present and future memories
c) Only future memories
d) Only present memories
View Answer
Explanation: A convolution encoder belongs to a class of device called as finite state machines which are those machines that have a memory of past signals.
6. How many number of transitions can be made at each bit time?
a) One
b) Two
c) Three
d) Half
View Answer
Explanation: An a consequence of shifting-in-one bit at a time there are only two possible transitions that the register can make at each bit time.
7. In trellis diagram, the number of nodes ______ at successive branching.
a) Increases by 1
b) Doubles
c) Triples
d) None of the mentioned
View Answer
Explanation: As per the algorithm, in the trellis diagram at successive branching the number of nodes doubles.
8. Example for convolution encoder state diagram is
a) Tree diagram
b) Trellis diagram
c) Tree & Trellis diagram
d) None of the mentioned
View Answer
Explanation: Tree diagram and trellis diagram are both examples for convolution encoding state diagram.
9. In maximum likelihood decoding technique, the likelihood function compares the
a) Joint probabilities
b) Individual probabilities
c) Conditional probabilities
d) None of the mentioned
View Answer
Explanation: If all input messages are equally likely a decoder that achieves the minimum probability of error is that which compares the conditional probabilities also called the likelihood function.
10. In maximum likelihood detector the error probability is
a) Maximum
b) Minimum
c) Zero
d) None of the mentioned
View Answer
Explanation: The maximum likelihood detector is an optimal detector which minimizes the error probability.
11. For a binary code, the maximum number of possible sequence made up of L branch words is
a) L2
b) 2L
c) 2L
d) L/2
View Answer
Explanation: In a binary code, all possible different code word sequences that can be made up of L branch words are 2L.
12. If the quantization level of a demodulator output is ________ then it is called as soft decision decoding.
a) Equal to 2
b) More than 2
c) Less than 2
d) None of the mentioned
View Answer
Explanation: If the quantization level of a demodulator output is 2 it is called as hard decision decoding and if it is greater than 2 it is called as soft decision decoding.
13. Soft decision decoding requires
a) Less memory
b) More memory
c) More speed
d) None of the mentioned
View Answer
Explanation: The price paid for soft decision decoding is an increase in required memory size at the decoder.
14. A binary symmetric channel is a
a) Discrete memoryless channel
b) Continuous memoryless channel
c) Discrete memory channel
d) Continuous memory channel
View Answer
Explanation: A binary symmetric channel is a discrete memoryless channel that has binary input and output alphabets and symmetric transition probabilities.
15. Binary symmetric channel is hard decision channel.
a) True
b) False
View Answer
Explanation: A binary symmetric channel is an example for hard decision channel which means that even though continuous values may be received by the demodulator the BSC only allows firm decisions.
16. Gaussian channel is a hard decision channel.
a) True
b) False
View Answer
Explanation: The quantized Gaussian channel is referred to as the soft decision channel.
17. Branch word synchronization is the process of determining the
a) Beginning of received sequence
b) Beginning of transmitted sequence
c) End of received sequence
d) End of transmitted sequence
View Answer
Explanation: Branch word synchronization is the process of determining the beginning of a branch word in the received sequence. Such synchronization can take place without adding new information to the transmitted symbols.
Sanfoundry Global Education & Learning Series – Digital Communications.
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