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Soft Decoding, Dual BCH Codes, and Better List-Decodable \varepsilon -Biased Codes

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2 Author(s)
Guruswami, V. ; Comput. Sci. Dept., Carnegie Mellon Univ., Pittsburgh, PA, USA ; Rudra, A.

Explicit constructions of binary linear codes that are efficiently list-decodable up to a fraction (1/2 - ε) of errors are given. The codes encode k bits into n = poly(k/ε) bits and are constructible and list-decodable in time polynomial in k and 1/ε (in particular, ε need not be constant and can even be polynomially small in n). These results give the best known polynomial dependence of n on k and 1/ε for such codes. Specifically, they are able to achieve n ≤ Õ(k33+γ) or, if a linear dependence on k is required, n ≤ O(k/ε5+γ) , where γ >; 0 is an arbitrary constant. The best previously known constructive bounds in this setting were n ≤ O(k24) and n ≤ O(k/ε6) . Nonconstructively, a random linear encoding of length n = O(k/ε2) suffices, but no subexponential algorithm is known for list decoding random codes. In addition to being a basic question in coding theory, codes that are list-decodable from a fraction (1/2 - ε) of errors for ε → 0 are important in several complexity theory applications. For example, the construction with near-cubic dependence on ε yields better hardness results for the problem of approximating NP witnesses. Further, the codes constructed have the property that all nonzero codewords have relative Hamming weights in the range (1/2 - ε, 1/2 + ε); this ε-biased property is a fundamental notion in pseudorandomness.

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Information Theory, IEEE Transactions on  (Volume:57 ,  Issue: 2 )