# Kerdock and Preparata codes

A most famous relation in coding theory is a discrete avatar of the Fourier transform which relates the weight enumerator $W _ {C}$ of a linear code $C$ and the weight enumerator of its dual $C ^ \perp$ with respect to the standard scalar product, namely

$$W _ {C ^ \perp } ( x,y ) = { \frac{1}{\left | C \right | } } W _ {C} ( x + y,x - y ) .$$

(Cf. MacWilliams identities.) A simple example of a dual pair of codes is the Hamming code $H _ {m}$, a (big) $[ n = 2 ^ {m} ,n - m - 1,4 ]$ code whose dual is the (small) first-order Reed–Muller code $R _ {m}$. The matrix

$$\left ( \begin{array}{ccccccc} 1 & 1 & 1 & 1 & 1 &\cdots & 1 \\ 0 & 1 &\alpha &\alpha ^ {2} &\alpha ^ {3} &\cdots &\alpha ^ {n - 1 } \\ \end{array} \right ) ,$$

where $n = 2 ^ {m} - 1$ and $\alpha$ is a root of ${\overline{f} } ( x )$, is both a generator matrix for $R _ {m}$ and a parity check matrix for $H _ {m}$. The small code $R _ {m}$ has the simple weight distribution

$$W _ {R _ {m} } ( x,y ) = x ^ {n} + y ^ {n} + ( 2n - 2 ) ( xy ) ^ {n/2 } .$$

The MacWilliams formula now gives an explicit expression for $W _ {H _ {m} }$ which would be cumbersome to obtain directly.

There is no Fourier transform without Abelian groups and therefore there is no MacWilliams formula for non-linear codes. The discovery first of the $( 16,256,6 )$ Nordstrom–Robinson code in 1967, non-linear and still formally self-dual for the MacWilliams relation, followed in 1968 and 1972 [a8] by the discovery of two infinite families of non-linear codes, the Preparata codes and Kerdock codes, respectively, whose weight enumerators are MacWilliams dual of each other, were until [a4] an unexplained phenomenon. The unsuccessful efforts of many distinguished researchers on this notoriously difficult problem [a8] led one of them to declare [a5] that it was "merely a coincidence" .

A well-known trick in modulation theory to address the $4$-PSK constellation consists of using a (very simple) case of the Gray mapping. This is a mapping from $\mathbf Z _ {4}$ to ${ \mathop{\rm GF} } ( 2 ) ^ {2}$ defined by

$$0 \rightarrow 00, 1 \rightarrow 01, 2 \rightarrow 11, 3 \rightarrow 10,$$

and extended to a mapping from $\mathbf Z _ {4} ^ {n}$ to ${ \mathop{\rm GF} } ( 2 ) ^ {2n }$ in the natural way. The key property is that the mapping $\phi$ from $\mathbf Z _ {4} ^ {n}$ equipped with the Lee distance to ${ \mathop{\rm GF} } ( 2 ) ^ {2n }$ equipped with the Hamming distance is an isometric mapping of metric spaces. The trace parametrization of quaternary $M$- sequences can be used to show that the Kerdock code as defined in [a8], p. 1107, is essentially the cyclic code associated to construction $A$ of [a1], p. 458, and the construction of [a9]. In particular, if $\xi$ denotes a primitive root of $n$ in a suitable Galois ring, [a7], then the matrix

$$\left ( \begin{array}{ccccccc} 1 & 1 & 1 & 1 & 1 &\cdots & 1 \\ 0 & 1 &\xi &\xi ^ {2} &\xi ^ {3} &\cdots &\xi ^ {n - 1 } \\ \end{array} \right )$$

( $n = 2 ^ {m} - 1$, with $m \geq 3$ odd) is both a generator matrix for the Kerdock code and a parity check matrix for the "Preparata" code. One sees that the Kerdock and Preparata codes are quaternary analogues of the first-order Reed–Muller code and of the extended Hamming code, respectively. The Goethals and Delsarte–Goethals codes, which are also a quaternary dual pair, are related in a less simple manner to three error-correcting BCH codes.

Besides MacWilliams duality, the $\mathbf Z _ {4}$ structure also bears influence on the decoding (cf. [a4], Fig. 2) and on the coset structure of the Preparata code, which gives rise to a new distance-regular graph of diameter $4$[a4].

The article [a4], which solves a twenty-year-old riddle, was awarded the 1995 IEEE Information Theory award for best paper. Geometrical repercussions can be found in [a3], [a6].

How to Cite This Entry:
Kerdock and Preparata codes. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Kerdock_and_Preparata_codes&oldid=52384
This article was adapted from an original article by P. SolÃ© (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article