The list size of random linear and LDPC codes at capacity
Shashwat Silas
We show that for every prime power $q$ and $ρ\in(0,1-\frac1q)$ there exists $\varepsilon_0(q, ρ) > 0$ such that for all positive $\varepsilon < \varepsilon_0(q, ρ)$, a random linear code over $\mathbb{F}_q$ of rate $1-h_q(ρ)-\varepsilon$ has list size $\lfloor h_q(ρ)/\varepsilon\rfloor$ or $\lfloor h_q(ρ)/\varepsilon\rfloor+1$ with probability $1-o(1)$. Before this work, the list size was known only up to a constant factor for $q\ge3$, and for $q=2$ it was known to be either $\lfloor h_2(ρ)/\varepsilon\rfloor-1$, $\lfloor h_2(ρ)/\varepsilon\rfloor$ or $\lfloor h_2(ρ)/\varepsilon\rfloor+1$. We also show that the same two-point concentration holds for the list size of random LDPC codes. This follows from a general theorem establishing that random LDPC codes and random linear codes have the same threshold rate for distance, list decoding, list recovery from errors and any other \emph{convex} property, which answers a question of Mosheiff, Resch, Ron-Zewi, Silas and Wootters.
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