7 research outputs found
Zero-Rate Thresholds and New Capacity Bounds for List-Decoding and List-Recovery
In this work we consider the list-decodability and list-recoverability of arbitrary q-ary codes, for all integer values of q ? 2. A code is called (p,L)_q-list-decodable if every radius pn Hamming ball contains less than L codewords; (p,?,L)_q-list-recoverability is a generalization where we place radius pn Hamming balls on every point of a combinatorial rectangle with side length ? and again stipulate that there be less than L codewords.
Our main contribution is to precisely calculate the maximum value of p for which there exist infinite families of positive rate (p,?,L)_q-list-recoverable codes, the quantity we call the zero-rate threshold. Denoting this value by p_*, we in fact show that codes correcting a p_*+? fraction of errors must have size O_?(1), i.e., independent of n. Such a result is typically referred to as a "Plotkin bound." To complement this, a standard random code with expurgation construction shows that there exist positive rate codes correcting a p_*-? fraction of errors. We also follow a classical proof template (typically attributed to Elias and Bassalygo) to derive from the zero-rate threshold other tradeoffs between rate and decoding radius for list-decoding and list-recovery.
Technically, proving the Plotkin bound boils down to demonstrating the Schur convexity of a certain function defined on the q-simplex as well as the convexity of a univariate function derived from it. We remark that an earlier argument claimed similar results for q-ary list-decoding; however, we point out that this earlier proof is flawed
Generalized List Decoding
This paper concerns itself with the question of list decoding for general
adversarial channels, e.g., bit-flip () channels, erasure
channels, (-) channels, channels, real adder
channels, noisy typewriter channels, etc. We precisely characterize when
exponential-sized (or positive rate) -list decodable codes (where the
list size is a universal constant) exist for such channels. Our criterion
asserts that:
"For any given general adversarial channel, it is possible to construct
positive rate -list decodable codes if and only if the set of completely
positive tensors of order- with admissible marginals is not entirely
contained in the order- confusability set associated to the channel."
The sufficiency is shown via random code construction (combined with
expurgation or time-sharing). The necessity is shown by
1. extracting equicoupled subcodes (generalization of equidistant code) from
any large code sequence using hypergraph Ramsey's theorem, and
2. significantly extending the classic Plotkin bound in coding theory to list
decoding for general channels using duality between the completely positive
tensor cone and the copositive tensor cone. In the proof, we also obtain a new
fact regarding asymmetry of joint distributions, which be may of independent
interest.
Other results include
1. List decoding capacity with asymptotically large for general
adversarial channels;
2. A tight list size bound for most constant composition codes
(generalization of constant weight codes);
3. Rederivation and demystification of Blinovsky's [Bli86] characterization
of the list decoding Plotkin points (threshold at which large codes are
impossible);
4. Evaluation of general bounds ([WBBJ]) for unique decoding in the error
correction code setting
Tight Bounds on List-Decodable and List-Recoverable Zero-Rate Codes
In this work, we consider the list-decodability and list-recoverability of
codes in the zero-rate regime. Briefly, a code is
-list-recoverable if for all tuples of input lists
with each and the number of
codewords such that for at most
choices of is less than ; list-decoding is the special case of
. In recent work by Resch, Yuan and Zhang~(ICALP~2023) the zero-rate
threshold for list-recovery was determined for all parameters: that is, the
work explicitly computes with the property that for all
(a) there exist infinite families positive-rate
-list-recoverable codes, and (b) any
-list-recoverable code has rate . In fact, in the
latter case the code has constant size, independent on . However, the
constant size in their work is quite large in , at least
.
Our contribution in this work is to show that for all choices of and
with , any -list-recoverable code must
have size , and furthermore this upper bound is
complemented by a matching lower bound . This
greatly generalizes work by Alon, Bukh and Polyanskiy~(IEEE Trans.\ Inf.\
Theory~2018) which focused only on the case of binary alphabet (and thus
necessarily only list-decoding). We remark that we can in fact recover the same
result for and even , as obtained by Alon, Bukh and Polyanskiy: we
thus strictly generalize their work.Comment: Abstract shortened to meet the arXiv requiremen
Multiple Packing: Lower and Upper Bounds
We study the problem of high-dimensional multiple packing in Euclidean space.
Multiple packing is a natural generalization of sphere packing and is defined
as follows. Let and . A multiple packing is a
set of points in such that any point in lies in the intersection of at most balls of radius around points in . We study the multiple packing
problem for both bounded point sets whose points have norm at most
for some constant and unbounded point sets whose points are allowed to be
anywhere in . Given a well-known connection with coding theory,
multiple packings can be viewed as the Euclidean analog of list-decodable
codes, which are well-studied for finite fields. In this paper, we derive
various bounds on the largest possible density of a multiple packing in both
bounded and unbounded settings. A related notion called average-radius multiple
packing is also studied. Some of our lower bounds exactly pin down the
asymptotics of certain ensembles of average-radius list-decodable codes, e.g.,
(expurgated) Gaussian codes and (expurgated) spherical codes. In particular,
our lower bound obtained from spherical codes is the best known lower bound on
the optimal multiple packing density and is the first lower bound that
approaches the known large limit under the average-radius notion of
multiple packing. To derive these results, we apply tools from high-dimensional
geometry and large deviation theory.Comment: The paper arXiv:2107.05161 has been split into three parts with new
results added and significant revision. This paper is one of the three parts.
The other two are arXiv:2211.04408 and arXiv:2211.0440
LIPIcs, Volume 261, ICALP 2023, Complete Volume
LIPIcs, Volume 261, ICALP 2023, Complete Volum
List-decodable zero-rate codes
We consider list decoding in the zero-rate regime for two cases: the binary alphabet and the spherical codes in Euclidean space. Specifically, we study the maximal τ ϵ [0,1] for which there exists an arrangement of M balls of relative Hamming radius τ in the binary hypercube (of arbitrary dimension) with the property that no point of the latter is covered by L or more of them. As M → ∞ the maximal τ decreases to a well-known critical value T[subscript L]. In this paper, we prove several results on the rate of this convergence. For the binary case, we show that the rate is Θ (M-¹) when L is even, thus extending the classical results of Plotkin and Levenshtein for L=2. For L=3 , the rate is shown to be Θ (M -(2/3) ). For the similar question about spherical codes, we prove the rate is Ω (M-¹) and O([mathematical figure; see resource]). ©201
List-decodable zero-rate codes for the Z-channel
This paper studies combinatorial properties of codes for the Z-channel. A Z-channel with error fraction τ takes as input a length-n binary codeword and injects in an adversarial manner up to nτ 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 τ has exponential (in n) size if τ is less than a critical value that we call the Plotkin point and has constant size if τ is larger than the threshold. The (L−1)-list-decoding Plotkin point is known to be L−1L−1−L−LL−1. In this paper, we show that the largest (L−1)-list-decodable code ε-above the Plotkin point has size Θ L (ε −3/2 ) for any L − 1 ≥ 1