On lowest density MDS codes

Let Fq denote the finite field GF (q) and let b be a positive integer. MDS codes over the symbol alphabet Fqb are considered that are linear over Fq and have sparse ("low-density") parity-check and generator matrices over Fq that are systematic over Fqb. Lower bounds are presented on the number of nonzero elements in any systematic parity-check or generator matrix of an Fq-linear MDS code over Fqb, along with upper bounds on the length of any MDS code that attains those lower bounds. A construction is presented that achieves those bounds for certain redundancy values. The building block of the construction is a set of sparse nonsingular matrices over Fq whose pairwise differences are also nonsingular. Bounds and constructions are presented also for the case where the systematic condition on the parity-check and generator matrices is relaxed to be over Fq, rather than over Fqb. © 1999 IEEE.