On the Covering Radius of Small Codes Versus Dual Distance

Tietäväinen's upper and lower bounds assert that for block-length-n linear codes with dual distance d, the covering √ radius R is at most (n/2) - ((1/2) - o(1)) dn and typically at least (n/2) - θ((dn log (n/d))
^1/2
). The gap between those bounds on R - (n/2) is a Θ((log (n/d))1/2) factor related to the gap between the worst covering radius given d and the sphere-covering bound. Our focus in this paper is on the case when d = o(n), i.e., when the code size is subexponential and the gap is w(1). We show that up to a constant, the gap can be eliminated by relaxing the covering requirement to allow for missing o(1) fraction of points. Namely, if the dual distance d = o(n), then for sufficiently large d, almost all points can be covered with radius R ≤ (n/2)-Θ((dn log (n/d))
^1/2
). Compared with random linear codes, our bound on R - (n/2) is asymptotically tight up to a factor less than 3. We give applications to dual-BCH codes. The proof builds on the author's previous work on the weight distribution of cosets of linear codes, which we simplify in this paper and extend from codes to probability distributions on {0, 1}n, thus enabling the extension of the earlier result to (d - 1)-wise independent distributions.