Identification over Permutation Channels
Abhishek Sarkar, Bikash Kumar Dey
TL;DR
This work analyzes message identification over the $q$-ary uniform permutation channel, where the channel randomly permutes an $n$-length input vector, yielding zero Shannon capacity for reliable communication but allowing exponential (even doubly exponential with feedback) growth in identifiable messages. The authors establish one-shot achievability with $M=2^{ heta(n^{q-1})}$ messages (via set-system constructions) and prove both soft and strong converses that tightly limit the rate of identifiability, including distribution-approximation techniques for strong converses. They extend the results to multi-shot settings and to block-wise noiseless feedback, where identifiability scales as $2^{q^{Rnl}}$ for $R<1$, with a two-phase strategy enabling common randomness and final-block encoding. The analysis relies on five ID-code modification steps, well-separated set-system bounds, and reduction to NL channels, yielding a cohesive picture of the limits of identification on permutation channels and highlighting a dramatic advantage of feedback for identifiability.
Abstract
We study message identification over a q-ary uniform permutation channel, where the transmitted vector is permuted by a permutation chosen uniformly at random. For discrete memoryless channels(DMCs), the number of identifiable messages grows doubly exponentially. Identification capacity, the maximum second-order exponent, is known to be the same as the Shannon capacity of the DMC. Permutation channels support reliable communication of only polynomially many messages. A simple achievability result shows that message sizes growing as 2^{ε_nn^{q-1}} are identifiable for any ε_n\rightarrow0. We prove two converse results. A ``soft'' converse shows that for any R>0, there is no sequence of identification codes with message size growing as 2^{Rn^{q-1}} with a power-law decay (n^{-μ}) of the error probability. We also prove a ``strong" converse showing that for any sequence of identification codes with message size 2^{R_n n^{q-1}}, where R_n\rightarrow\infty, the sum of type I and type II error probabilities approaches at least 1 as n\rightarrow\infty. To prove the soft converse, we use a sequence of steps to construct a new identification code with a simpler structure which relates to a set system, and then use a lower bound on the normalized maximum pairwise intersection of a set system. To prove the strong converse, we use results on approximation of distributions. The achievability and converse results are generalized to the case of coding over multiple blocks. We finally study message identification over a q-ary uniform permutation channel in the presence of causal block-wise feedback from the receiver, where the encoder receives an entire n-length received block after the transmission of the block is complete. We show that in the presence of feedback, the maximum number of identifiable messages grows doubly exponentially, and we present a two-phase achievability scheme.