Hermitian self-dual abelian codes in a group ring Fq2[G], where Fq2is a finite field of order q2and G is a finite abelian group, are studied. Using the well-known discrete Fourier transform decomposition for a semisimple group ring, a characterization of Hermitian self-dual abelian codes in Fq2[G] is given, together with an alternative proof of necessary and sufficient conditions for the existence of such a code in Fq2[G], i.e., there exists a Hermitian self-dual abelian code in Fq2[G] if and only if the order of G is even and q = 2lfor some positive integer l. Later on, the study is further restricted to the case where F22l[G] is a principal ideal group ring, or equivalently, G ≅ A⊕Z2k with 2 ≠ |A|. Based on the characterization obtained, the number of Hermitian self-dual abelian codes in F22l [A⊕Z2k] can be determined easily. When A is cyclic, this result answers an open problem of Jia et al. concerning Hermitian self-dual cyclic codes. In many cases, F22l [A⊕Z2k] contains a unique Hermitian self-dual abelian code. The criteria for such cases are determined in terms of l and the order of A. Finally, the distribution of finite abelian groups A such that a unique Hermitian self-dual abelian code exists in F22l [A ⊕ Z2] is established, together with the distribution of odd integers m such that a unique Hermitian self-dual cyclic code of length 2 m over F22l exists.

Asymmetric quantum error-correcting codes (AQCs) may offer some advantage over their symmetric counterparts by providing better error-correction for the more frequent error types. The well-known CSS construction of$q$-ary AQCs is extended by removing the$ \BBF _{q}$-linearity requirement as well as the limitation on the type of inner product used. The proposed constructions are called CSS-like constructions and utilize pairs of nested subfield linear codes under one of the Euclidean, trace Euclidean, Hermitian, and trace Hermitian inner products. After establishing some theoretical foundations, best-performing CSS-like AQCs are constructed. Combining some constructions of nested pairs of classical codes and linear programming, many optimal and good pure$q$-ary CSS-like codes for$q \in \{ 2,3,4,5,7,8,9\}$up to reasonable lengths are found. In many instances, removing the$ \BBF _{q}$-linearity and using alternative inner products give us pure AQCs with improved parameters than relying solely on the standard CSS construction.

We study abelian codes in principal ideal group algebras (PIGAs). We first give an algebraic characterization of abelian codes in any group algebra and provide some general results. For abelian codes in a PIGA, which can be viewed as cyclic codes over a semisimple group algebra, it is shown that every abelian code in a PIGA admits generator and check elements. These are analogous to the generator and parity-check polynomials of cyclic codes. A characterization and an enumeration of Euclidean self-dual and Euclidean self-orthogonal abelian codes in a PIGA are given, which generalize recent analogous results for self-dual cyclic codes. In addition, the structures of reversible and complementary dual abelian codes in a PIGA are established, again extending results on reversible and complementary dual cyclic codes. Finally, asymptotic properties of abelian codes in a PIGA are studied. An upper bound for the minimum distance of abelian codes in a non-semisimple PIGA is given in terms of the minimum distance of abelian codes in semisimple group algebras. Abelian codes in a non-semisimple PIGA are then shown to be asymptotically bad, similar to the case of repeated-root cyclic codes.