Codes for the Z-channel

This paper is a collection of results on combinatorial properties of codes for the Z-channel. A Z-channel with error fraction $\tau$ takes as input a length-$n$ binary codeword and injects in an adversarial manner up to $n\tau$ asymmetric errors, i.e., errors that only zero out bits but do not flip $0$'s to $1$'s. It is known that the largest $(L-1)$-list-decodable code for the Z-channel with error fraction $\tau$ has exponential size (in $n$) if $\tau$ is less than a critical value that we call the $(L-1)$-list-decoding Plotkin point and has constant size if $\tau$ is larger than the threshold. The $(L-1)$-list-decoding Plotkin point is known to be $L^{-\frac{1}{L-1}} - L^{-\frac{L}{L-1}}$, which equals $1/4$ for unique-decoding with $L-1=1$. In this paper, we derive various results for the size of the largest codes above and below the list-decoding Plotkin point. In particular, we show that the largest $(L-1)$-list-decodable code $\epsilon$-above the Plotkin point, {for any given sufficiently small positive constant $\epsilon>0$,} has size $\Theta_L(\epsilon^{-3/2})$ for any $L-1\ge1$. We also devise upper and lower bounds on the exponential size of codes below the list-decoding Plotkin point.