Cyclic code

Let $\backslash mathcal\{C\}$ be a linear code over a finite field (also called Galois field) $GF(q)$ of block length $n$. $\backslash mathcal\{C\}$ is called a cyclic code if, for every codeword $c=(c\_1,\backslash ldots,c\_n)$ from $\backslash mathcal\{C\}$, the word $(c\_n,c\_1,\backslash ldots,c\_\{n-1\})$ in $GF(q)^n$ obtained by a cyclic right shift of components is again a codeword. Because one cyclic right shift is equal to $n-1$ cyclic left shifts, a cyclic code may also be defined via cyclic left shifts. Therefore, the linear code $\backslash mathcal\{C\}$ is cyclic precisely when it is invariant under all cyclic shifts.

Cyclic codes have some additional structural constraint on the codes. They are based on Galois fields and because of their structural properties they are very useful for error controls. Their structure is strongly related to Galois fields because of which the encoding and decoding algorithms for cyclic codes are computationally efficient.

Cyclic codes can be linked to ideals in certain rings. Let $R\; =\; A[x]\; /\; (x^n-1)$ be a quotient of a polynomial ring over the finite field $A\; =\; GF(q)$. Identify the elements of the cyclic code $\backslash mathcal\{C\}$ with polynomials in $R$ such that

$(\; c\_0,\; \backslash ldots,\; c\_\{n-1\}\; )$ maps to the polynomial

$c\_0\; +\; c\_1x\; +\backslash cdots+c\_\{n-1\}\; x^\{n-1\}$: thus multiplication by $x$ corresponds to a cyclic shift. Then $\backslash mathcal\{C\}$ is an ideal in $R$, and hence principal, since $R$ is a principal ideal ring. The ideal is generated by the unique monic element in $\backslash mathcal\{C\}$ of minimum degree, the generator polynomial $g$.{{harvnb|Van Lint|1998|p=76}}

This must be a divisor of $x^n-1$. It follows that every cyclic code is a polynomial code.

If the generator polynomial $g$ has degree $d$ then the rank of the code $\backslash mathcal\{C\}$ is $n-d$.

If $\backslash mathcal\{C\}$ is a cyclic code, the dual code $\backslash mathcal\{C\}^\backslash perp$ is also a cyclic code. The generator polynomial $h(x)$ for $\backslash mathcal\{C\}^\backslash perp$ is also called the parity-check polynomial or simply check polynomial for $\backslash mathcal\{C\}$. It can also be shown that $g(x)h^*(x)\; =\; x^n\; -\; 1$, where $h^*(x)$ denotes the reciprocal polynomial of $h(x)$.{{harvnb|Ryan|Lin|2009|pp=108-109}}

The idempotent of $\backslash mathcal\{C\}$ is a codeword $e$ such that $e^2\; =\; e$ (that is, $e$ is an idempotent element of $\backslash mathcal\{C\}$) and $e$ is an identity for the code, that is $e\; \backslash cdot\; c\; =\; c$ for every codeword $c$. If $n$ and $q$ are coprime such a word always exists and is unique;{{harvnb|Van Lint|1998|p=80}} it is a generator of the code.

An irreducible code is a cyclic code in which the code, as an ideal is irreducible, i.e. is minimal in $R$, so that its check polynomial is an irreducible polynomial.

For example, if $A=\backslash mathbb\{F\}\_2$ and $n=3$, the set of codewords contained in the cyclic code generated by $(1,1,0)$ is precisely

$$\backslash \{(0,0,0),\; (1,1,0),\; (0,1,1),\; (1,0,1)\backslash \}.$$

This code corresponds to the ideal in $\backslash mathbb\{F\}\_2[x]/(x^3-1)$ generated by $(1+x)$.

The polynomial $(1+x)$ is irreducible in the polynomial ring, and hence the code is an irreducible code.

The idempotent of this code is the polynomial $x\; +\; x^2$, corresponding to the codeword $(0,1,1)$.

Trivial examples of cyclic codes are $A^n$ itself and the code containing only the zero codeword. These correspond to generators $1$ and $x^n-1$ respectively: these two polynomials must always be factors of $x^n-1$.

Over $GF(2)$ the parity bit code, consisting of all words of even weight, corresponds to generator $x+1$. Again over $GF(2)$ this must always be a factor of $x^n-1$.

Many types of commonly used error-correcting codes can be represented as cyclic codes, including BCH codes, Reed-Solomon codes, and some classes of low-density parity-check codes defined from finite geometries.{{harvnb|Ryan|Lin|2009|loc=chpt. 10}}

Cyclic codes can be used to correct errors, like Hamming codes as cyclic codes can be used for correcting single error. Likewise, they are also used to correct double errors and burst errors. All types of error corrections are covered briefly in the further subsections.

The (7,4) Hamming code has a generator polynomial $g(x)\; =\; x^3\; +\; x\; +1$. This polynomial has a zero in Galois extension field $GF(8)$ at the primitive element $\backslash alpha$, and all codewords satisfy $\backslash mathcal\{C\}(\backslash alpha)=0$. Cyclic codes can also be used to correct double errors over the field $GF(2)$. Blocklength will be $n$ equal to $2^m\; -1$ and primitive elements $\backslash alpha$ and $\backslash alpha^3$ as zeros in the $GF(2^m)$ because we are considering the case of two errors here, so each will represent one error.

The received word is a polynomial of degree $n\; -\; 1$ given as

$v(x)\; =\; a(x)g(x)\; +\; e(x)$

where $e(x)$ can have at most two nonzero coefficients corresponding to 2 errors.

We define the syndrome polynomial, $S(x)$ as the remainder of polynomial $v(x)$ when divided by the generator polynomial $g(x)$ i.e.

$S(x)\; \backslash equiv\; v(x)\; \backslash equiv\; (a(x)g(x)\; +\; e(x))\; \backslash equiv\; e(x)\; \backslash mod\; g(x)$ as $(a(x)g(x))\backslash equiv\; 0\backslash mod\; g(x)$.

Let the field elements $X\_1$ and $X\_2$ be the two error location numbers. If only one error occurs then $X\_2$ is equal to zero and if none occurs both are zero.

Let $S\_1\; =\; \{v\}(\backslash alpha)$ and $S\_3\; =\; \{v\}(\backslash alpha^3)$.

These field elements are called "syndromes". Now because $g(x)$ is zero at primitive elements $\backslash alpha$ and $\backslash alpha^3$, so we can write $S\_1\; =\; e(\backslash alpha)$ and $S\_3\; =\; e(\backslash alpha^3)$. If say two errors occur, then

$S\_1\; =\; \backslash alpha^\{i\}\; +\; \backslash alpha^\{i\text{'}\}$ and

$S\_3\; =\; \backslash alpha^\{3i\}\; +\; \backslash alpha^\{3i\text{'}\}$.

And these two can be considered as two pair of equations in $GF(2^m)$ with two unknowns and hence we can write

$S\_1\; =\; X\_1\; +\; X\_2$ and

$S\_3\; =\; (X\_1)^3\; +\; (X\_2)^3$.

Hence if the two pair of nonlinear equations can be solved cyclic codes can used to correct two errors.

The Hamming(7,4) code may be written as a cyclic code over GF(2) with generator $1+x+x^3$. In fact, any binary Hamming code of the form Ham(r, 2) is equivalent to a cyclic code,{{harvnb|Hill|1988|pp=159–160}} and any Hamming code of the form Ham(r,q) with r and q-1 relatively prime is also equivalent to a cyclic code.{{harvnb|Blahut|2003|loc=Theorem 5.5.1}} Given a Hamming code of the form Ham(r,2) with $r\; \backslash ge\; 3$, the set of even codewords forms a cyclic $[2^\{r\}-1,2^\{r\}-r-2,4]$-code.{{harvnb|Hill|1988|pp=162–163}}

A code whose minimum distance is at least 3, have a check matrix all of whose columns are distinct and non zero. If a check matrix for a binary code has $m$ rows, then each column is an $m$-bit binary number. There are $2^m-1$ possible columns. Therefore, if a check matrix of a binary code with $d\_\{min\}$ at least 3 has $m$ rows, then it can only have $2^m-1$ columns, not more than that. This defines a $(2^m-1,\; 2^m-1-m)$ code, called Hamming code.

It is easy to define Hamming codes for large alphabets of size $q$. We need to define one $H$ matrix with linearly independent columns. For any word of size $q$ there will be columns who are multiples of each other. So, to get linear independence all non zero $m$-tuples with one as a top most non zero element will be chosen as columns. Then two columns will never be linearly dependent because three columns could be linearly dependent with the minimum distance of the code as 3.

So, there are $(q^m-1)/(q-1)$ nonzero columns with one as top most non zero element. Therefore, a Hamming code is a $[(q^m-1)/(q-1),\; (q^m-1)/(q-1)-m]$ code.

Now, for cyclic codes, Let $\backslash alpha$ be primitive element in $GF(q^m)$, and let $\backslash beta\; =\; \backslash alpha^\{q-1\}$. Then $\backslash beta^\{(q^m-1)/(q-1)\}\; =\; 1$ and thus $\backslash beta$ is a zero of the polynomial $x^\{(q^m-1)/(q-1)\}\; -\; 1$ and is a generator polynomial for the cyclic code of block length $n\; =\; (q^m-1)/(q-1)$.

But for $q\; =\; 2$, $\backslash alpha\; =\; \backslash beta$. And the received word is a polynomial of degree $n\; -\; 1$ given as

$v(x)\; =\; a(x)g(x)\; +\; e(x)$

where, $e(x)\; =\; 0$ or $x^i$ where $i$ represents the error locations.

But we can also use $\backslash alpha^i$ as an element of $GF(2^m)$ to index error location. Because $g(\backslash alpha)\; =\; 0$, we have $v(\backslash alpha)\; =\; \backslash alpha^i$ and all powers of $\backslash alpha$ from $0$ to $2^m-2$ are distinct. Therefore, we can easily determine error location $i$ from $\backslash alpha^i$ unless $v(\backslash alpha)\; =\; 0$ which represents no error. So, a Hamming code is a single error correcting code over $GF(2)$ with $n\; =\; 2^m-1$ and $k\; =\; n\; -\; m$.

From Hamming distance concept, a code with minimum distance $2t\; +\; 1$ can correct any $t$ errors. But in many channels error pattern is not very arbitrary, it occurs within very short segment of the message. Such kind of errors are called burst errors. So, for correcting such errors we will get a more efficient code of higher rate because of the less constraints. Cyclic codes are used for correcting burst error. In fact, cyclic codes can also correct cyclic burst errors along with burst errors. Cyclic burst errors are defined as

A cyclic burst of length $t$ is a vector whose nonzero components are among $t$ (cyclically) consecutive components, the first and the last of which are nonzero.

In polynomial form cyclic burst of length $t$ can be described as $e(x)=x^ib(x)\backslash mod\; (x^n\; -1)$ with $b(x)$ as a polynomial of degree $t\; -\; 1$ with nonzero coefficient $b\_0$. Here $b(x)$ defines the pattern and $x^i$ defines the starting point of error. Length of the pattern is given by deg$b(x)\; +\; 1$. The syndrome polynomial is unique for each pattern and is given by

$s(x)\; =\; e(x)\backslash mod\; g(x)$

A linear block code that corrects all burst errors of length $t$ or less must have at least $2t$ check symbols. Proof: Because any linear code that can correct burst pattern of length $t$ or less cannot have a burst of length $2t$ or less as a codeword because if it did then a burst of length $t$ could change the codeword to burst pattern of length $t$, which also could be obtained by making a burst error of length $t$ in all zero codeword. Now, any two vectors that are non zero in the first $2t$ components must be from different co-sets of an array to avoid their difference being a codeword of bursts of length $2t$. Therefore, number of such co-sets are equal to number of such vectors which are $q^\{2t\}$. Hence at least $q^\{2t\}$ co-sets and hence at least $2t$ check symbol.

This property is also known as Rieger bound and it is similar to the Singleton bound for random error correcting.

{{See also|Burst error-correcting code#Fire codes}}

In 1959, Philip FireP. Fire, E, P. (1959). A class of multiple-error-correcting binary codes for non-independent errors. Sylvania Reconnaissance Systems Laboratory, Mountain View, CA, Rept. RSL-E-2, 1959. presented a construction of cyclic codes generated by a product of a binomial and a primitive polynomial. The binomial has the form $x^c+1$ for some positive odd integer $c$.Wei Zhou, Shu Lin, Khaled Abdel-Ghaffar. Burst or random error correction based on Fire and BCH codes. ITA 2014: 1-5 2013. Fire code is a cyclic burst error correcting code over $GF(q)$ with the generator polynomial

$g(x)=(x^\{2t-1\}-1)p(x)$

where $p(x)$ is a prime polynomial with degree $m$ not smaller than $t$ and $p(x)$ does not divide $x^\{2t-1\}-1$. Block length of the fire code is the smallest integer $n$ such that $g(x)$ divides

$x^n-1$.

A fire code can correct all burst errors of length t or less if no two bursts $b(x)$ and $x^jb\text{'}(x)$ appear in the same co-set. This can be proved by contradiction. Suppose there are two distinct nonzero bursts $b(x)$ and $x^jb\text{'}(x)$ of length $t$ or less and are in the same co-set of the code. So, their difference is a codeword. As the difference is a multiple of $g(x)$ it is also a multiple of $x^\{2t-1\}-1$. Therefore,

$b(x)\; =\; x^jb\text{'}(x)\; \backslash mod\; (x^\{2t-1\}-1)$.

This shows that $j$ is a multiple of $2t\; -\; 1$, So

$b(x)\; =\; x^\{l(2t-1)\}b\text{'}(x)$

for some $l$. Now, as $l(2t-1)$ is less than $t$ and $l$ is less than $q^m\; -\; 1$ so $(x^\{l(2t\; -\; 1)\}\; -\; 1)b(x)$ is a codeword. Therefore,

$(x^\{l(2t\; -\; 1)\}\; -\; 1)b(x)\; =\; a(x)(x^\{2t-1\}-\; 1)p(x)$.

Since $b(x)$ degree is less than degree of $p(x)$,$p(x)$ cannot divide $b(x)$. If $l$ is not zero, then $p(x)$ also cannot divide $x^\{l(2t-1)\}\; -\; 1$ as $l$ is less than $q^m-1$ and by definition of $m$, $p(x)$ divides $x^\{l(2t-1)\}\; -\; 1$ for no $l$ smaller than $q^m-1$. Therefore $l$ and $j$ equals to zero. That means both that both the bursts are same, contrary to assumption.

Fire codes are the best single burst correcting codes with high rate and they are constructed analytically. They are of very high rate and when $m$ and $t$ are equal, redundancy is least and is equal to $3t\; -\; 1$. By using multiple fire codes longer burst errors can also be corrected.

For error detection cyclic codes are widely used and are called $t\; -\; 1$ cyclic redundancy codes.

Applications of Fourier transform are widespread in signal processing. But their applications are not limited to the complex fields only; Fourier transforms also exist in the Galois field $GF(q)$. Cyclic codes using Fourier transform can be described in a setting closer to the signal processing.

Fourier transform over finite fields

The discrete Fourier transform of vector $v\; =\; v\_0,\; v\_1,\; ....,\; v\_\{n-1\}$ is given by a vector $V\; =\; V\_0,\; V\_1,.....,\; V\_\{n-1\}$ where,

$V\_k$ = $\backslash Sigma\_\{i=0\}^\{n-1\}\; e^\{-j2\backslash pi\; n^\{-1\}ik\}v\_i$ where,

$k\; =\; 0,.....,\; n-1$

where exp($-j2\backslash pi\; /n$) is an $n$th root of unity. Similarly in the finite field $n$th root of unity is element $\backslash omega$ of order $n$. Therefore

If $v\; =\; (v\_0,\; v\_1,\; ....,\; v\_\{n-1\})$ is a vector over $GF(q)$, and $\backslash omega$ be an element of $GF(q)$ of order $n$, then Fourier transform of the vector $v$ is the vector $V\; =\; (V\_0,\; V\_1,.....,\; V\_\{n-1\})$ and components are given by

$V\_j$ = $\backslash Sigma\_\{i=0\}^\{n-1\}\backslash omega^\{ij\}\; v\_i$ where,

$k\; =\; 0,.....,\; n-1$

Here $i$ is time index, $j$ is frequency and $V$ is the spectrum. One important difference between Fourier transform in complex field and Galois field is that complex field $\backslash omega$ exists for every value of $n$ while in Galois field $\backslash omega$ exists only if $n$ divides $q\; -\; 1$. In case of extension fields, there will be a Fourier transform in the extension field $GF(q^m)$ if $n$ divides $q^m\; -\; 1$ for some $m$.

In Galois field time domain vector $v$ is over the field $GF(q)$ but the spectrum $V$ may be over the extension field $GF(q^m)$.

Any codeword of cyclic code of blocklength $n$ can be represented by a polynomial $c(x)$ of degree at most $n\; -\; 1$. Its encoder can be written as $c(x)\; =\; a(x)g(x)$. Therefore, in frequency domain encoder can be written as $C\_j\; =\; A\_jG\_j$. Here codeword spectrum $C\_j$ has a value in $GF(q^m)$ but all the components in the time domain are from $GF(q)$. As the data spectrum $A\_j$ is arbitrary, the role of $G\_j$ is to specify those $j$ where $C\_j$ will be zero.

Thus, cyclic codes can also be defined as

Given a set of spectral indices, $A\; =\; (j\_1,....,\; j\_\{n-k\})$, whose elements are called check frequencies, the cyclic code $C$ is the set of words over $GF(q)$ whose spectrum is zero in the components indexed by $j\_1,...,\; j\_\{n-k\}$. Any such spectrum $C$ will have components of the form $A\_jG\_j$.

So, cyclic codes are vectors in the field $GF(q)$ and the spectrum given by its inverse fourier transform is over the field $GF(q^m)$ and are constrained to be zero at certain components. But every spectrum in the field $GF(q^m)$ and zero at certain components may not have inverse transforms with components in the field $GF(q)$. Such spectrum can not be used as cyclic codes.

Following are the few bounds on the spectrum of cyclic codes.

If $n$ be a factor of $(q^m\; -\; 1)$ for some $m$. The only vector in $GF(q)^n$ of weight $d\; -\; 1$ or less that has $d\; -\; 1$ consecutive components of its spectrum equal to zero is all-zero vector.

If $n$ be a factor of $(q^m\; -\; 1)$ for some $m$, and $b$ an integer that is coprime with $n$. The only vector $v$ in $GF(q)^n$ of weight $d\; -\; 1$ or less whose spectral

components $V\_j$ equal zero for $j\; =\; \backslash ell\_1\; +\; \backslash ell\_2b\; (\backslash mod\; n)$, where $\backslash ell\_1\; =\; 0,....,\; d\; -\; s\; -\; 1$ and $\backslash ell\_2\; =\; 0,....,\; s\; -\; 1$, is the all zero vector.

If $n$ be a factor of $q^m\; -\; 1$ for some $m$ and $GCD(n,b)\; =\; 1$. The only vector in

$GF(q)^n$ of weight $d\; -\; 1$ or less whose spectral components $V\_j$ equal to zero for $j\; =\; l\_1\; +\; l\_2b\; (\backslash mod\; n)$, where $l\_1\; =\; 0,...,\; d\; -\; s\; -\; 2$ and $l\_2$ takes at least $s\; +\; 1$ values in the range $0,....,\; d\; -\; 2$, is the all-zero vector.

When the prime $l$ is a quadratic residue modulo the prime $p$ there is a quadratic residue code which is a cyclic code of length $p$, dimension $(p+1)/2$ and minimum weight at least $\backslash sqrt\{p\}$ over $GF(l)$.

A constacyclic code is a linear code with the property that for some constant $\backslash lambda$ if $(c\_1,\; c\_2,\; \backslash dots,\; c\_n)$ is a codeword, then so is $(\backslash lambda\; c\_n,\; c\_1,\; \backslash dots,\; c\_\{n-1\})$. A negacyclic code is a constacyclic code with $\backslash lambda\; =\; -1$.{{harvnb|Van Lint|1998|p=75}}

A quasi-cyclic code (QC code) has the property that for some $s$ dividing $n$, any cyclic shift of a codeword by $s$ places is again a codeword. That is, for some constant $s$, if $(c\_0,\; c\_1,\; \backslash dots,\; c\_\{n-1\})$ is a codeword, then so is $(c\_\{-s\},\; c\_\{1-s\},\; \backslash dots,\; c\_\{n\; -\; s\; -\; 1\})$, where all subscripts are reduced mod $n$.{{harvnb|MacWilliams|Sloane|1977|p=506}} Such a code is known as an $s$-QC code. A double circulant code is a quasi-cyclic code of even length with $s\; =\; 2$.

An $(n,k)$ linear code is called a shortened cyclic code if it can be obtained by deleting $b$ positions from an $(n+b,\; k+b)$ cyclic code. Code of this form are not generally cyclic.{{harvnb|Ryan|Lin|2009|p=110}}

In shortened codes, information symbols are deleted to obtain a desired block length smaller than the original block length. While deleting the first $b$ symbols is a common approach, in principle any set of information symbols can be deleted. Any cyclic code can be converted to a quasi-cyclic code by dropping every $b$-th symbol, where $b$ is a factor of $n$. If the dropped symbols are not check symbols, this cyclic code is also a shortened cyclic code.

Quasi-twisted codes (QT codes) combine the properties of constacyclic and quasi-cyclic codes, with the shift occurring by $s$ places and with a multiplier of $\backslash lambda$. That is, for some constants $\backslash lambda$ and $s$, if $(c\_0,\; c\_1,\; \backslash dots,\; c\_\{n-1\})$ is a codeword, then so is $(\backslash lambda\; c\_\{-s\},\; c\_\{1-s\},\; \backslash dots,\; c\_\{n\; -\; s\; -\; 1\})$, where all subscripts are reduced mod $n$.{{cite journal |last1=Aydin |first1=Nuh |last2=Halilović |first2=Ajdin |title=A generalization of quasi-twisted codes: multi-twisted codes |journal=Finite Fields and Their Applications |date=2017 |volume=45 |pages=96–106|doi=10.1016/j.ffa.2016.12.002 |s2cid=7694655 |doi-access=free |arxiv=1701.01044 }} Multi-twisted codes are further generalizations of QT codes, which join multiple QT codes end to end.{{cite journal |last1=Aydin |first1=Nuh |last2=Siap |first2=Irfan |last3=K. Ray-Chaudhuri |first3=Dijen |title=The Structure of 1-Generator Quasi-Twisted Codes and New Linear Codes |journal=Designs, Codes and Cryptography |date=2001 |volume=24 |issue=3 |pages=313–326|doi=10.1023/A:1011283523000 |s2cid=17376783 }}

• BCH code
• Binary Golay code
• Cyclic redundancy check
• Eugene Prange
• Reed–Muller code
• Ternary Golay code

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{{PlanetMath attribution|id=6978|title=cyclic code}}

Category:Coding theory

Category:Finite fields