Optimum Linear Codes With Support-Constrained Generator Matrices Over Small Fields

We consider the problem of designing optimal linear codes (in terms of having the largest minimum distance) subject to a support constraint on the generator matrix. We show that the largest minimum distance can be achieved by a subcode of a Reed-Solomon code of small field size and with the same minimum distance. In particular, if the code has length n, and maximum minimum distance d (over all generator matrices with the given support), then an optimal code exists for any field size q >= 2n-d. As a by-product of this result, we settle the GM-MDS conjecture in the affirmative.