# Wireless & Mobile Communications Questions & Answers – Block Codes and Finite Fields

This set of Wireless & Mobile Communications Multiple Choice Questions & Answers (MCQs) focuses on “Block Codes and Finite Fields”.

1. In block codes, parity bits are ___________ to block of messages.
b) Subtracted
c) Multiplied
d) Divided

Explanation: In block codes, parity bits are added to the block of message bits to make codewords or code blocks. They enable a limited number of errors to be detected and corrected without retransmission.

2. How many redundant bits are added in block codes for k information bits and n code bits?
a) n+k
b) n-k
c) k2
d) n2

Explanation: In block encoder, k information bits are encoded into n code bits. A total of n-k redundant bits are added to the k information bits for the purpose of detecting and correcting errors.

3. For (n,k) block codes, rate of the code is defined as __________
a) n2/k
b) k2/n
c) n/k
d) k/n

Explanation: The block code is referred to as an (n,k) code, and the rate of the code is defined as R=n/k. It is equal to the rate of information divided by the raw channel rate.

4. The ability of the block code to correct errors is a function of __________
a) Number of parity bits
b) Number of information bits
c) Number of code bits
d) Code distance

Explanation: The ability of a block code to correct errors is a function of the code distance. Block codes can be used to improve the performance of communication systems when other means of improvement are impractical.

5. The weight of code is given by number of _______
a) Non-zero elements in the codeword
b) Zero elements in the codeword
c) Total elements in the codeword
d) Elements in parity bits

Explanation: The weight of a code is given by number of non-zero elements in the codeword. For a binary code, the weight is basically the number of ones in the codeword.
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6. Which of the following is not a property of block code?
a) Linearity
b) Systematic
c) Cyclic
d) Non linearity

Explanation: Block codes are linear, systematic and cyclic in nature. Encoding and decoding techniques make use of mathematical constructs known as finite fields.

7. In systematic codes, parity bits are appended at the __________
a) Beginning
b) End
c) End
d) Odd places

Explanation: A systematic code is one in which the parity bits are appended to the end of the information bits. For an (n,k) code, the first k bits are identical to the information bits, and the remaining (n-k) bits of each code word are linear combinations of k information bits.

8. Which of the following is not an example of block code?
a) Hamming code
b) Cyclic code
c) Convolution code
d) BCH codes

Explanation: Hamming codes, cyclic codes and BCH codes are the example of block codes. Convolution code does not come in the category of block code. Some other examples of block codes are Reed Solomon codes, Golay codes and Hadamard codes.

9. Which of the following code is a class of non-binary BCH?
a) Hamming code
c) Golay code
d) Reed Solomon codes

Explanation: The most important and most common class of non binary is the family of codes known as Reed Solomon codes. BCH codes are among the most popular block codes that exist for a wide range of rates, achieve significant coding gains.

10. Which of the following linear codes achieve largest possible minimum distance?
a) Hamming code
c) Golay code
d) Reed Solomon codes

Explanation: RS codes achieve the largest possible minimum distance, dmin of any linear code. They are non-binary codes which are capable of correcting errors that appears in bursts.

11. CDPD stands for ___________
a) Cellular Digital Packet Data
b) Cellular Decoded Packet Data
c) Cellular Demodulated Packet Data
d) Cellular Decoded Plane Data

Explanation: CDPD (Cellular Digital Packet Data) is a wide area mobile data service which uses unused bandwidth. It was mostly used in AMPS phones. Reed Solomon code in US CDPD uses m=6 bits per code symbol.

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