### 数学代写|编码理论代写Coding theory代考|Cyclic Codes over Finite Fields

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## 数学代写|编码理论代写Coding theory代考|Notation and Introduction

A brief introduction to cyclic codes over finite fields was given in Section 1.12. The objective of this chapter is to introduce several important families of cyclic codes over finite fields. We will follow the notation of Chapter 1 as closely as possible.

By an $[n, \kappa, d]{q}$ code, we mean a linear code over $\mathbb{F}{q}$ with length $n$, dimension $\kappa$ and minimum distance $d$. Notice that the minimum distance of a linear code is equal to the minimum nonzero weight of the code. By the parameters of a linear code, we mean its length, dimension and minimum distance. An $[n, \kappa, d]{q}$ code is said to be distance-optimal (respectively dimension-optimal) if there is no $[n, \kappa, d+1]{q}$ (respectively $[n, \kappa+1, d]{q}$ ) code. By the best known parameters of $[n, \kappa]$ linear codes over $\mathbb{F}{q}$ we mean an $[n, \kappa, d]_{q}$ code with the largest known $d$ reported in the tables of linear codes maintained at [845].

In this chapter, we deal with cyclic codes of length $n$ over $F_{q}$ and always assume that $\operatorname{gcd}(n, q)=1$. Under this assumption, $x^{n}-1$ has no repeated factors over $\mathbb{F}{q}$. Denote by $C{i}$ the $q$-cyclotomic coset modulo $n$ that contains $i$ for $0 \leq i \leq n-1$. Put $m=$ ord $n(q)$, and let $\gamma$ be a generator of $F_{q^{m}}^{*}:=F_{q^{m}} \backslash{0}$. Define $\left.\alpha=\gamma^{m}-1\right) / n$. Then $\alpha$ is a primitive $n^{\text {th }}$ root of unity. The canonical factorization of $x^{n}-1$ over $\mathbb{F}{q}$ is given by $$x^{n}-1=M{\alpha^{i} 0}(x) M_{\alpha^{i} 1}(x) \cdots M_{\alpha^{i} t}(x)$$

where $i_{0}, i_{1}, \ldots, i_{t}$ are representatives of the $q$-cyclotomic cosets modulo $n$, and
$$M_{\alpha^{i j}}(x)=\prod_{h \in C_{i j}}\left(x-\alpha^{h}\right),$$
which is the minimal polynomial of $\alpha^{i_{j}}$ over $\mathbb{F}{q}$ and is irreducible over $\mathbb{F}{q}$.
Throughout this chapter, we define $\mathcal{R}{(n, q)}=\mathbb{F}{q}[x] /\left\langle x^{n}-1\right\rangle$ and use $\operatorname{Tr}{q}{ }^{m} / q$ to denote the trace function from $F{q^{m}}$ to $F_{q}$ defined by $\operatorname{Tr}{q^{m} / q}(x)=\sum{j=0}^{m-1} x^{q^{j}}$. The ring of integers modulo $n$ is denoted by $\mathbb{Z}_{n}={0,1, \ldots, n-1}$.

Cyclic codes form an important subclass of linear codes over finite fields. Their algebraic structure is richer. Because of their cyclic structure, they are closely related to number theory. In addition, they have efficient encoding and decoding algorithms and are the most studied linear codes. In fact, most of the important families of linear codes are either cyclic codes or extended cyclic codes.

## 数学代写|编码理论代写Coding theory代考|Subfield Subcodes

Let $\mathcal{C}$ be an $[n, \kappa]{q^{t}}$ code. The subfield subcode $\left.\mathcal{C}\right|{F_{q}}$ of $\mathcal{C}$ with respect to $\mathbb{F}{q}$ is the set of codewords in $\mathcal{C}$ each of whose components is in $\mathbb{F}{q}$. Since $\mathcal{C}$ is linear over $\mathbb{F}{q^{\pm}},\left.\mathcal{C}\right|{E_{q}}$ is a linear code over $\mathbb{F}_{q}$.

The dimension, denoted $\kappa_{q}$, of the subfield subcode $\left.\mathcal{C}\right|{P{q}}$ may not have an elementary relation with that of the code $\mathcal{C}$. However, we have the following lower and upper bounds on $\kappa_{q}$.

Theorem 2.2.1 Let $\mathcal{C}$ be an $[n, \kappa]{q^{t}}$ code. Then $\left.\mathcal{C}\right|{\mathbb{F}{q}}$ is an $\left[n, \kappa{q}\right]$ code over $\mathbb{F}{q}$, where $\kappa \geq \kappa{q} \geq n-t(n-\kappa)$. If $\mathcal{C}$ has a basis of codewords in $\mathbb{F}{q}^{n}$, then this is also a basis of $\left.\mathcal{C}\right|{\mathbb{F}{q}}$ and $\left.\mathcal{C}\right|{F_{q}}$ has dimension $\kappa$.

Example 2.2.2 The Hamming code $\mathcal{H}{3,2^{2}}$ over $\mathbb{F}{2^{2}}$ has parameters $[21,18,3]{4}$. The subfield subcode $\left.\mathcal{H}{3,2^{2}}\right|{\mathbb{R}{2}}$ is a $[21,16,3]{2}$ code with parity check matrix $$\left[\begin{array}{lllllllllllllllllllll} 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 1 \ 0 & 1 & 0 & 0 & 1 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 \ 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 & 0 & 1 & 1 & 0 \ 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 \ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 0 & 0 & 0 & 0 \end{array}\right] .$$ In this case, $n=21, \kappa=18$, and $n-t(n-\kappa)=15$. Hence $\kappa{q}=16$, which is very close to $n-t(n-\kappa)=15$

The following is called Delsarte’s Theorem, which exhibits a dual relation between subfield subcodes and trace codes. This theorem is very useful in the design and analysis of linear codes.
Theorem 2.2.3 (Delsarte) Let $\mathcal{C}$ be a linear code of length $n$ over $\mathbb{F}{q^{\pm}}$. Then $$\left(\left.\mathcal{C}\right|{E_{q}}\right)^{\perp}=\operatorname{Tr}{q^{t} / q}\left(\mathcal{C}^{\perp}\right),$$ where $\operatorname{Tr}{q^{\pm} / q}\left(\mathcal{C}^{\perp}\right)=\left{\left(\operatorname{Tr}{q^{\pm} / q}\left(v{1}\right), \ldots, \operatorname{Tr}{q^{+} / q}\left(v{n}\right)\right) \mid\left(v_{1}, \ldots, v_{n}\right) \in \mathcal{C}^{\perp}\right}$.
Theorems $2.2 .1$ and 2.2.3 work for all linear codes, including cyclic codes. Their proofs could be found in $[1008$, Section $3.8]$. We shall need them later.

## 数学代写|编码理论代写Coding theory代考|Fundamental Constructions of Cyclic Codes

In Section $1.12$, it was shown that every cyclic code of length $n$ over $\mathbb{F}{q}$ can be generated by a generator polynomial $g(x) \in \mathbb{F}{q}[x]$. The objective of this section is to describe several other fundamental constructions of cyclic codes over finite fields. By a fundamental construction, we mean a construction method that can produce every cyclic code over any finite field.

An element $e$ in a commutative ring $\mathcal{R}$ is called an idempotent if $e^{2}=e$. The ring $\mathcal{R}{(n, q)}$ has in general quite a number of idempotents. Besides its generator polynomial, many other polynomials can generate a cyclic code $\mathcal{C}$. Let $\mathcal{C}$ be a cyclic code over $\mathbb{F}{q}$ with generator polynomial $g(x)$. It is easily seen that a polynomial $f(x) \in \mathbb{F}_{q}[x]$ generates $\mathcal{C}$ if and only if $\operatorname{gcd}\left(f(x), x^{n}-1\right)=g(x)$.

If an idempotent $e(x) \in \mathcal{R}{(n, q)}$ generates a cyclic code $\mathcal{C}$, it is then unique in this ring and called the generating idempotent. Given the generator polynomial of a cyclic code, one can compute its generating idempotent with the following theorem [1008, Theorem 4.3.3]. Theorem 2.3.1 Let $\mathcal{C}$ be a cyclic code of length $n$ over $\mathbb{F}{q}$ with generator polynomial $g(x)$. Let $h(x)=\left(x^{n}-1\right) / g(x)$. Then $\operatorname{gcd}(g(x), h(x))=1$, as it was assumed that $\operatorname{gcd}(n, q)=1$. Employing the Extended Euclidean Algorithm, one computes two polynomials a $(x) \in \mathbb{F}{q}[x]$ and $b(x) \in \mathbb{F}{q}[x]$ such that $1=a(x) g(x)+b(x) h(x)$. Then $e(x)=a(x) g(x) \bmod \left(x^{n}-1\right)$ is the generating idempotent of $\mathcal{C}$.

The polynomial $h(x)$ in Theorem $2.3 .1$ is called the parity check polynomial of $\mathcal{C}$. Given the generating idempotent of a cyclic code, one obtains the generator polynomial of this code as follows $[1008$, Theorem $4.3 .3]$.

Theorem 2.3.2 Let $\mathcal{C}$ be a cyclic code over $\mathbb{F}{q}$ with generating idempotent $e(x)$. Then the generator polynomial of $\mathcal{C}$ is given by $g(x)=\operatorname{gcd}\left(e(x), x^{n}-1\right)$, which is computed in $\mathbb{F}{q}[x]$.
Example 2.3.3 The cyclic code $\mathcal{C}$ of length 11 over $\mathbb{F}_{3}$ with generator polynomial $g(x)=$ $x^{5}+x^{4}+2 x^{3}+x^{2}+2$ has parameters $[11,6,5]$ and parity check polynomial $h(x)=x^{6}+$ $2 x^{5}+2 x^{4}+2 x^{3}+x^{2}+1$

Let $a(x)=2 x^{5}+x^{4}+x^{2}$ and $b(x)=x^{4}+x^{3}+1$. It is then easily verified that $1=$ $a(x) g(x)+b(x) h(x)$. Hence
$$e(x)=a(x) g(x) \bmod \left(x^{11}-1\right)=2 x^{10}+2 x^{8}+2 x^{7}+2 x^{6}+2 x^{2}$$
which is the generating idempotent of $\mathcal{C}$. On the other hand, we have $g(x)=\operatorname{gcd}\left(e(x), x^{11}-\right.$ 1).

A generator matrix of a cyclic code can be derived from its generating idempotent as follows $[1008$, Theorem $4.3 .6]$.

## 数学代写|编码理论代写Coding theory代考|Notation and Introduction

1.12 节简要介绍了有限域上的循环码。本章的目的是介绍有限域上几个重要的循环码族。我们将尽可能地遵循第 1 章的符号。

Xn−1=米一个一世0(X)米一个一世1(X)⋯米一个一世吨(X)

## 数学代写|编码理论代写Coding theory代考|Subfield Subcodes

[100110011001111001101 010010110011010011001 001100110011001100110 000001111000000001111 000000000111111110000].在这种情况下，n=21,ķ=18， 和n−吨(n−ķ)=15. 因此ķq=16, 非常接近n−吨(n−ķ)=15

(C|和q)⊥=Tr⁡q吨/q(C⊥),在哪里\operatorname{Tr}{q^{\pm} / q}\left(\mathcal{C}^{\perp}\right)=\left{\left(\operatorname{Tr}{q^{\pm} / q}\left(v{1}\right), \ldots, \operatorname{Tr}{q^{+} / q}\left(v{n}\right)\right) \mid\left(v_ {1}, \ldots, v_{n}\right) \in \mathcal{C}^{\perp}\right}\operatorname{Tr}{q^{\pm} / q}\left(\mathcal{C}^{\perp}\right)=\left{\left(\operatorname{Tr}{q^{\pm} / q}\left(v{1}\right), \ldots, \operatorname{Tr}{q^{+} / q}\left(v{n}\right)\right) \mid\left(v_ {1}, \ldots, v_{n}\right) \in \mathcal{C}^{\perp}\right}.

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