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 n2 − ( 2 − o(1)) √ dn and typically at least n2 − Θ( √ dn log nd ). The gap between those bounds on R − n2 is an Θ( √ log nd ) 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 is at least d ≥ 7, where d = o(n) is odd, then for sufficiently large n, almost all points can be covered with radius R ≤ n2 − √ 1 13 (d− 5)n log n d−1 . Compared to random linear codes, our bound on R − n2 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 above result to (d− 1)-wise independent distributions.