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Foundations of coding: theory and applications of error-correcting codes

Jiri Adamek

Chapter 10

Cyclic Codes - all with Video Answers

Educators


Chapter Questions

06:13

Problem 1

Verify that the division of a polynomial $a(x)$ by a polynomial $b(x) \neq$ 0 can be performed by the following algorithm:
(a) If $b(x)$ has a degree larger than $a(x)$, then the quotient is 0 , and the remainder is $a(x)$.
(b) If $b(x)$ has a degree smaller or equal to that of $a(x)$, divide the leading coefficient $a_n$ of $a(x)$ by the leading coefficient $b_m$ of $b(x)$. The result $q_{n-m}=a_n \div b_m$ is the coefficient of the quotient at the power $x^{n-m}$.
(c) Put $a(x):=a(x)-q_{n-m} x^{n-m} b(x)$ and perform recursively steps (a), (b), and (c). [Observe that the new polynomial $a(x)$ has a smaller degree than the original one. Thus, the algorithm stops after $n$ repetitions of steps (a), (b), and (c).]

Jonathan Benoit
Jonathan Benoit
Numerade Educator

Problem 2

Prove that polynomial division is unique, i.e., given polynomials $a(x)$ and $b(x) \neq 0$, then whenever
$$
a(x)=b(x) q(x)+r(x)=b(x) q^{\prime}(x)+r^{\prime}(x)
$$
and the degrees of $r(x), r^{\prime}(x)$ are both smaller than that of $b(x)$, then $q(x)=q^{\prime}(x)$ and $r(x)=r^{\prime}(x)$.

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Problem 3

Verify that the binary cyclic code of length 7 with the generator polynomial $g(x)=1+x^2+x^3$ is a Hamming code, and conclude that the code with the generator polynomial $\left(x^7-1\right): g(x)=x^4+x^3+x^2+1$ is the dual (simplex) code (8.2).

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Problem 4

Find all binary cyclic codes of length 5. Find all cyclic codes of length 5 over $\mathbf{Z}_3$.

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Problem 5

Describe the dual code of the binary cyclic code of length 5 with the generator polynomial $g(x)=x^3+2 x^2+3 x+1$.

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Problem 6

In systematic coding, encode the information bits 1101 into a code word of the cyclic binary code of length 7 with the generator polynomial $g(x)=1+x^2+x^3$.

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Problem 7

Describe the Meggitt decoder for the binary code of length 7 with the generator polynomial $g(x)=\left(1+x^2+x^3\right)(x+1)$. Can you decode by error trapping?

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Problem 8

Prove that every double-error-correcting ( $n, k$ )-code with $n \geq 2 k$ can be corrected by error trapping. What about the case $n<2 k$ ?

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Problem 9

What is the smallest length of a binary code with the generator polynomial $g(x)=x^4+x^3+x^2+1$ ? Design an encoder and decoder for the code. How many errors does it correct? Can error-trapping decoding be used?

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Problem 10

Can error-trapping decoding be used for the Golay code? Design an encoder and decoder for that code.

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Problem 11

Find a modification of the er ror-trapping decoding which can decode the double-error-correcting binary code of length 31 with the generator polynomial $g(x)=x^{10}+x^9+x^8+x^6+x^5+x^3+1$.

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Problem 12

Prove that whenever a cyclic code detects an error pattern e, then it also detects all error patterns obtained by cyclic shifts of $e$.

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Problem 13

Design an encoder for the Hamming (15, 11)-code of Example (3) in 10.3 .

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Problem 14

Are the linear binary codes with the following generator matrices cyclic?
$$
\left[\begin{array}{lllllll}
1 & 0 & 1 & 1 & 1 & 0 & 0 \\
1 & 1 & 0 & 1 & 0 & 0 & 0 \\
1 & 1 & 0 & 0 & 1 & 0 & 1 \\
0 & 0 & 1 & 0 & 1 & 1 & 1
\end{array}\right] \quad\left[\begin{array}{lllllll}
1 & 1 & 1 & 1 & 0 & 0 & 0 \\
0 & 1 & 1 & 1 & 1 & 0 & 0 \\
0 & 0 & 1 & 1 & 1 & 1 & 0 \\
0 & 0 & 0 & 1 & 1 & 1 & 1
\end{array}\right]
$$

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Problem 15

If the generator polynomial of a binary code is divisible by $x+1$, verify that all code words have even weight. Does the converse hold?

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Problem 16

What is the necessary and sufficient condition on a generator polynomial of a cyclic code of odd length in order that $111 . .11$ be a code word?

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Problem 17

Describe the generator polynomial of the expurgation of a given cyclic code.

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Problem 18

If $g(x)$ generates a cyclic $(n, k)$-code, verify that $g\left(x^i\right)$ generates a cyclic (in, ik)-code. Describe the code for $g(x)=x+1$ and $g(x)=x^3+x+1$ with $n=7$.

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Problem 19

Verify that the intersection of two cyclic codes of the same length is a cyclic code. What generator polynomial does it have?

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