Abstract We focus on two aspects of cyclic orbit codes: invariants under equivalence and quasi-optimality. Regarding the first aspect, we establish a connection between the cyclic orbit code generated by a subspace U of $${{\mathbb {F}}}_{q^n}$$ F q n and the associated linear set $$L_{U\times U}$$ L U × U . Relating the size of the linear set to the number of fractions formed by the elements of U allows us to derive new bounds on the parameters of the cyclic orbit code. In the second part, we study a particular family of (quasi-)optimal cyclic orbit codes. With the aid of these codes we establish the existence of quasi-optimal codes in even-dimensional vector spaces over finite fields of any characteristic. Finally, for the particular code family we determine the automorphism groups in various general linear group, depending on the assumed ground field, and their orbits under the Galois group over the prime field.

Abstract. We study the direct sum of [Formula: see text]-matroids by way of their cyclic flats. Using that the rank function of a [Formula: see text]-matroid is fully determined by the cyclic flats and their ranks, we show that the cyclic flats of the direct sum of two [Formula: see text]-matroids are exactly all the direct sums of the cyclic flats of the two summands. This simplifies the rank function of the direct sum significantly. A [Formula: see text]-matroid is called irreducible if it cannot be written as a (nontrivial) direct sum. We provide a characterization of irreducibility in terms of the cyclic flats and show that every [Formula: see text]-matroid can be decomposed into a direct sum of irreducible [Formula: see text]-matroids, which are unique up to equivalence.

We consider (symmetric, non-degenerate) bilinear spaces over a finite field and investigate the properties of their$\ell $-complementary subspaces, i.e., the subspaces that intersect their dual in dimension$\ell $. This concept generalizes that of a totally isotropic subspace and, in the context of coding theory, specializes to the notions of self-orthogonal, self-dual and linear-complementary-dual (LCD) codes. In this paper, we focus on the enumerative and asymptotic combinatorics of all these objects, giving formulas for their numbers and describing their typical behavior (rather than the behavior of a single object). For example, we give a closed formula for the average weight distribution of an$\ell $-complementary code in the Hamming metric, generalizing a result by Pless and Sloane on the aggregate weight enumerator of binary self-dual codes. Our results also show that self-orthogonal codes, despite being very sparse in the set of codes of the same dimension over a large field, asymptotically behave quite similarly to a typical, not necessarily self-orthogonal, code. In particular, we prove that most self-orthogonal codes are MDS over a large field by computing the asymptotic proportion of the non-MDS ones for growing field size.

This paper investigates the theory of sum-rank-metric codes for which the individual matrix blocks may have different sizes. Various bounds on the cardinality of a code are derived, along with their asymptotic extensions. The duality theory of sum-rank-metric codes is also explored, showing that MSRD codes (the sum-rank analogue of MDS codes) dualize to MSRD codes only if all matrix blocks have the same number of columns. In the latter case, duality considerations lead to an upper bound on the number of blocks for MSRD codes. The paper also contains various constructions of sum-rank-metric codes for variable block sizes, illustrating the possible behaviours of these objects with respect to bounds, existence, and duality properties.

This paper investigates the construction of rankmetric codes with specified Ferrers diagram shapes. These codes play a role in the multilevel construction for subspace codes. A conjecture from 2009 provides an upper bound for the dimension of a rank-metric code with given specified Ferrers diagram shape and rank distance. While the conjecture in its generality is wide open, several cases have been established in the literature. This paper contributes further cases of Ferrers diagrams and ranks for which the conjecture holds true. In addition, the proportion of maximal Ferrers diagram codes within the space of all rank-metric codes with the same shape and dimension is investigated. Special attention is being paid to MRD codes. It is shown that for growing field size the limiting proportion depends highly on the Ferrers diagram. For instance, for [m × 2]-MRD codes with rank 2 this limiting proportion is close to 1/e.

We consider data transmission over a network where each edge is an erasure channel and the inner nodes transmit the random linear combinations of their incoming information. We distinguish two channel models in this setting: the row and the column erasure channel model. For both models, we investigate spread codes and determine their symbol erasure correction capability and the probability of decoding success. We also compare the performance of spread codes to other known codes suitable for those models. Furthermore, we explain how to decode these codes in the two channel models and compare the decoding complexities. The results show that depending on the application and the to-be-optimized aspect, any combination of codes and channel models can be the best choice.

This paper investigates properties of realizations of linear or group codes on general graphs that lead to local reducibility. Trimness and properness are dual properties of constraint codes. A linear or group realization with a constraint code that is not both trim and proper is locally reducible. A linear or group realization on a finite cycle-free graph is minimal if and only if every local constraint code is trim and proper. A realization is called observable if there is a one-to-one correspondence between codewords and configurations, and controllable if it has independent constraints. A linear or group realization is observable if and only if its dual is controllable. A simple counting test for controllability is given. An unobservable or uncontrollable realization is locally reducible. Parity-check realizations are controllable if and only if they have independent parity checks. In an uncontrollable tail-biting trellis realization, the behavior partitions into disconnected sub-behaviors, but this property does not hold for nontrellis realizations. On a general graph, the support of an unobservable configuration is a generalized cycle.

This paper investigates tail-biting trellis realizations for linear block codes. Intrinsic trellis properties are used to characterize irreducibility on given intervals of the time axis. It proves beneficial to always consider the trellis and its dual simultaneously. A major role is played by trellis properties that amount to observability and controllability of trellis fragments of various lengths. For fragments of length less than the minimum span length of the code it is shown that fragment observability and fragment controllability are equivalent to irreducibility. For reducible trellises, a constructive reduction procedure is presented. The considerations also lead to a characterization for when the dual of a trellis allows a product factorization into elementary (“atomic”) trellises.

This paper is concerned with the local reducibility properties of linear realizations of codes on finite graphs. Trimness and properness are dual properties of constraint codes. A linear realization is locally reducible if any constraint code is not both trim and proper. On a finite cycle-free graph, a linear realization is minimal if and only if every constraint code is both trim and proper. A linear realization is called observable if it is one-to-one, and controllable if all constraints are independent. Observability and controllability are dual properties. An unobservable or uncontrollable realization is locally reducible. A parity-check realization is uncontrollable if and only if it has redundant parity checks. A tail-biting trellis realization is uncontrollable if and only if its trajectories partition into disconnected subrealizations. General graphical realizations do not share this property.

It is shown that a trellis realization can be locally reduced if it is not state-trim, branch-trim, proper, observable, and controllable. These conditions are not sufficient for local irreducibility. Making use of notions that amount to “almost unobservability/uncontrollability”, a necessary and sufficient criterion of local irreducibility for tail-biting trellises is presented.

This paper investigates the constructions of tail-biting trellises for linear block codes as introduced by Koetter and Vardy (2003) and Nori and Shankar (2006). For a given code, the sets of characteristic generators are defined slightly more generally than by Koetter and Vardy. In particular, they are not uniquely determined by the code. The effect of the choice of characteristic generators on the resulting product trellises, called KV-trellises, is discussed in detail. It is shown that each KV-trellis is a span-based BCJR-trellis and that the latter are always nonmergeable. Finally, a duality conjecture posed by Koetter and Vardy is addressed by making use of a dualization technique of BCJR-trellises. The conjecture is proven for minimal trellises.

This paper focuses on dualizing tail-biting trellises, particularly KV trellises. These trellises are based on characteristic generators, as introduced by Koetter-Vardy (2003), and may be regarded as a natural generalization of minimal conventional trellises, even though they are not necessarily minimal. Two dualization techniques will be investigated: the local dualization, introduced by Forney (2001) for general normal graphs, and a linear-algebra-based dualization tailored to the specific class of tail-biting Bahl-Cocke-Jelinek-Raviv (BCJR) trellises, introduced by Nori-Shankar (2006). It turns out that, in general, the BCJR dual is a subtrellis of the local dual, while for KV trellises these two coincide. Furthermore, making use of both the BCJR construction and the local dualization, it will be shown that for each complete set of characteristic generators of a code there exists a complete set of characteristic generators of the dual code such that their resulting KV trellises are dual to each other if paired suitably. This proves a stronger version of a conjecture formulated by Koetter-Vardy.

We consider the constructions of tail-biting trellises for linear codes introduced by Koetter/Vardy and Nori/Shankar. We will show that each one-to-one product trellis can be merged to a BCJR-trellis defined in a slightly stronger sense than in and that each trellis that originates from the characteristic matrix defined in is a BCJR-trellis. Furthermore, BCJR-trellises are always nonmergeable. Finally, we will consider a certain duality conjecture of Koetter/Vardy and show that it holds true for minimal trellises.

A MacWilliams identity for convolutional codes will be established. It makes use of the weight adjacency matrices of the code and its dual, based on state space realizations (the controller canonical form) of the codes in question. The MacWilliams identity applies to various notions of duality appearing in the literature on convolutional coding theory.

The adjacency matrix associated with a convolutional code collects in a detailed manner information about the weight distribution of the code. A MacWilliams identity conjecture, stating that the adjacency matrix of a code fully determines the adjacency matrix of the dual code, will be formulated, and an explicit formula for the transformation will be stated. The formula involves the MacWilliams matrix known from complete weight enumerators of block codes. The conjecture will be proven for the class of convolutional codes where either the code itself or its dual does not have Forney indices bigger than one. For the general case, the conjecture is backed up by many examples, and a weaker version will be established.

The weight adjacency matrix associated with a convolutional code collects in a detailed manner information about the weight distribution of the code. We will formulate a MacWilliams Identity Conjecture stating that the adjacency matrix of a code fully determines the adjacency matrix of the dual code. An explicit formula for the transformation will be stated as well; it involves the MacWilliams matrix known from complete weight enumerators of block codes. The conjecture will be proven for the class of convolutional codes where either the code or its dual is a unit memory code. For the general case the conjecture is backed up by many examples, and a weaker version will be established.

Maximum-distance separable (MDS) convolutional codes have the property that their free distance is maximal among all codes of the same rate and the same degree. In this paper, a class of MDS convolutional codes is introduced whose column distances reach the generalized Singleton bound at the earliest possible instant. Such codes are called strongly-MDS convolutional codes. They also have a maximum or near-maximum distance profile. The extended row distances of these codes will also be discussed briefly.

In this paper we introduce a specific class of cyclic convolutional codes. The construction is based on Reed-Solomon block codes. The algebraic parameters as well as the distance of these codes are determined. This shows that some of these codes are optimal or near optimal

This paper describes the powerful theory of cyclic block codes, which suggests studying the impact of cyclicity for convolutional codes. The convolutional codes, which are invariant under the cyclic shift, have overall constraint length zero, and hence called as block codes. Cyclic Convolutional codes are constructed with large free distance and generator polynomial to easily determine the dimension and prove dual of a CCC is cyclic.

Maximum-distance separable (MDS) convolutional codes are characterized through the property that the free distance attains the generalized singleton bound. The existence of MDS convolutional codes was established by two of the authors by using methods from algebraic geometry. This correspondence provides an elementary construction of MDS convolutional codes for each rate k/n and each degree /spl delta/. The construction is based on a well-known connection between quasi-cyclic codes and convolutional codes.