Equivalence and Characterizations of Linear Rank-Metric Codes Based on Invariants

TL;DR

This paper introduces invariants based on dimension sequences under field automorphisms to classify and distinguish linear rank-metric codes, providing new criteria and bounds for code equivalence and characterization.

Contribution

It develops new invariants for rank-metric codes, derives bounds on code equivalence classes, and characterizes Gabidulin codes using these invariants.

Findings

Dimension sequences serve as invariants for code equivalence.

Bounds on the number of equivalence classes of Gabidulin and twisted Gabidulin codes.

Conditions on code length and field extension degree for code inequivalence.

Abstract

We show that the sequence of dimensions of the linear spaces, generated by a given rank-metric code together with itself under several applications of a field automorphism, is an invariant for the whole equivalence class of the code. The same property is proven for the sequence of dimensions of the intersections of itself under several applications of a field automorphism. These invariants give rise to easily computable criteria to check if two codes are inequivalent. We derive some concrete values and bounds for these dimension sequences for some known families of rank-metric codes, namely Gabidulin and (generalized) twisted Gabidulin codes. We then derive conditions on the length of the codes with respect to the field extension degree, such that codes from different families cannot be equivalent. Furthermore, we derive upper and lower bounds on the number of equivalence classes of Gabidulin codes and twisted Gabidulin codes, improving a result of Schmidt and Zhou for a wider range of parameters. In the end we use the aforementioned sequences to determine a characterization result for Gabidulin codes.