The concatenated structure of cyclic and Abelian codes

It is shown that a linear two-dimensional cyclic code of length can be factorized into a direct sum of concatenated codes, with cyclic inner and outer codes and, conversely, thai a two-dimensional cyclic code can be constructed in this way. This result is extended, and it appears that the Abelian codes are obtainable by taking a direct sum of several concatenations of cyclic codes. Codes are constructed that are better than any previously known. In particular, low-rate cyclic codes superior to the duals of high-rate BCH codes are constructed.