Papers › Small Width, Low Distortions: Quantized Random Embeddings of Low-complexity Sets

Small Width, Low Distortions: Quantized Random Embeddings of Low-complexity Sets

23 Apr 2015arXiv:1504.06170links table onlyarchive 2025-07-28

Laurent Jacques

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Under which conditions and with which distortions can we preserve the pairwise-distances of low-complexity vectors, e.g., for structured sets such as the set of sparse vectors or the one of low-rank matrices, when these are mapped in a finite set of vectors? This work addresses this general question through the specific use of a quantized and dithered random linear mapping which combines, in the following order, a sub-Gaussian random projection in ℝᴹ of vectors in ℝᴺ, a random translation, or "dither", of the projected vectors and a uniform scalar quantizer of resolution δ>0 applied componentwise. Thanks to this quantized mapping we are first able to show that, with high probability, an embedding of a bounded set 𝒦 ⊂ℝᴺ in δℤᴹ can be achieved when distances in the quantized and in the original domains are measured with the ℓ₁- and ℓ₂-norm, respectively, and provided the number of quantized observations M is large before the square of the "Gaussian mean width" of 𝒦. In this case, we show that the embedding is actually "quasi-isometric" and only suffers of both multiplicative and additive distortions whose magnitudes decrease as M^(-1/5) for general sets, and as M^(-1/2) for structured set, when M increases. Second, when one is only interested in characterizing the maximal distance separating two elements of 𝒦 mapped to the same quantized vector, i.e., the "consistency width" of the mapping, we show that for a similar number of measurements and with high probability this width decays as M^(-1/4) for general sets and as 1/M for structured ones when M increases. Finally, as an important aspect of our work, we also establish how the non-Gaussianity of the mapping impacts the class of vectors that can be embedded or whose consistency width provably decays when M increases.

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