Csr2011 june17 09_30_yekhanin

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Csr2011 june17 09_30_yekhanin

  1. 1. Locally Decodable Codes<br />Sergey Yekhanin<br />Microsoft Research<br />
  2. 2. Data storage<br /><ul><li>Store data reliably
  3. 3. Keep it readily available for users</li></li></ul><li>Data storage: Replication<br /><ul><li>Store data reliably
  4. 4. Keep it readily available for users
  5. 5. Very large overhead
  6. 6. Moderate reliability
  7. 7. Local recovery:</li></ul> Loose one machine, access one<br />
  8. 8. Data storage: Erasure coding<br /><ul><li>Store data reliably
  9. 9. Keep it readily available for users</li></ul>…<br /><ul><li>Low overhead
  10. 10. High reliability
  11. 11. No local recovery:</li></ul> Loose one machine, access k<br />…<br />…<br />k data chunks<br />n-k parity chunks<br />Need: Erasure codes with local decoding<br />
  12. 12. Local decoding: example<br />E(X)<br />X1<br />X2<br />X3<br />X<br />X1<br />X2<br />X3<br />X1X2<br />X1X3<br />X2X3<br />X1X2X3<br /><ul><li>Tolerates 3 erasures
  13. 13. After 3 erasures, any information bit can recovered with locality 2
  14. 14. After 3 erasures, any parity bit can be recovered with locality 2</li></li></ul><li>Local decoding: example<br />E(X)<br />X1<br />X2<br />X3<br />X<br />X1<br />X2<br />X3<br />X1X2<br />X1X3<br />X2X3<br />X1X2X3<br /><ul><li>Tolerates 3 erasures
  15. 15. After 3 erasures, any information bit can recovered with locality 2
  16. 16. After 3 erasures, any parity bit can be recovered with locality 2</li></li></ul><li>Locally Decodable Codes<br />Definition : A code is called - locally decodable if for all and all values of the symbol can be recovered from accessing only symbols of , even after an arbitrary 10% of coordinates of are erased. <br />k long message<br />Decoder reads only r symbols<br />Adversarial erasures<br />n long codeword<br />
  17. 17. Parameters<br />Ideally:<br />High rate: close to . or <br />Strong locality: Very small Constant. <br />One cannot minimize and simultaneously. There is a trade-off.<br />
  18. 18. Parameters<br />Ideally:<br />High rate: close to . or <br />Strong locality: Very small Constant. <br />Potential applications for data transmission / storage.<br />Applications in complexity theory / cryptography.<br />
  19. 19. Early constructions: Reed Muller codes<br />Parameters:<br />The code consists of evaluations of all degree polynomials in variables over a finite field <br />High rate: No locality at rates above 0.5<br />Locality at rate <br />Strong locality: for constant <br />
  20. 20. State of the art: codes<br />High rate: [KSY10]<br />Multiplicity codes: Locality at rate <br />Strong locality: [Y08, R07, KY09,E09, DGY10, BET10a, IS10, CFL+10,BET10b,SY]<br /> Matching vector codes: for constant<br /> for <br />
  21. 21. State of the art: lower bounds[KT,KdW,W,W]<br />High rate: [KSY10]<br />Multiplicity codes: Locality at rate <br />Strong locality: [Y08, R07, E09, DGY10, BET10a, IS10, CFL+10,BET10b,SY11]<br /> Matching vector codes: for constant<br /> for <br />Locality lower bound:<br />Length lower bound:<br />
  22. 22. State of the art: constructions<br />Matching vector codes<br />Reed Muller codes<br />Multiplicity codes<br />
  23. 23. Plan<br />Reed Muller codes<br />Multiplicity codes<br />Matching vector codes<br />
  24. 24. Reed Muller codes<br />Parameters: <br />Code: Evaluations of degree polynomials over <br />Set: <br />Polynomial yields a codeword:<br />Parameters:<br />
  25. 25. Reed Muller codes: local decoding<br />Key observation: Restriction of a codeword to an affine line yields an evaluation of a univariate polynomial of degree <br />To recover the value at <br />Pick an affine line through with not too many erasures.<br />Do polynomial interpolation.<br /><ul><li> Locally decodable code: Decoder reads random locations.</li></li></ul><li>Multiplicity codes<br />
  26. 26. Multiplicity codes<br />Parameters: <br />Code: Evaluations of degree polynomials over <br /> and their partial derivatives.<br />Set:<br />Polynomial yields a codeword: <br />Parameters:<br />
  27. 27. Multiplicity codes: local decoding<br />Fact: Derivatives of in two independent directions determine the derivatives in all directions.<br />Key observation: Restriction of a codeword to an affine line yields an evaluation of a univariate polynomial of degree <br />
  28. 28. Multiplicity codes: local decoding<br />To recover the value at <br />Pick a line through . Reconstruct<br />Pick another line through . Reconstruct<br />Polynomials and determine<br /><ul><li> Increasing multiplicity yields higher rate.
  29. 29. Increasing the dimension yields smaller query complexity.</li></li></ul><li>RM codes vs. Multiplicity codes<br />
  30. 30. Matching vector codes<br />
  31. 31. Matching vectors<br />Definition: Let <br /> We say that form a matching family if :<br />For all<br />For all <br />Core theorem: A matching vector family of size yields an query code of length <br />
  32. 32. MV codes: Encoding<br />Let contain a multiplicative subgroup of size <br />Given a matching family<br />A message: <br />Consider a polynomial in the ring: <br /><ul><li> Encoding is the evaluation of over </li></li></ul><li>Multiplicity codes: local decoding<br />Concept: For a multiplicative line through in direction<br />Key observation:evaluation of is a evaluation of a univariate polynomial whose term determines<br />To recover <br />Pick a multiplicative line <br />Do polynomial interpolation<br />
  33. 33. RM codes vs. Multiplicity codes<br />
  34. 34. Summary<br />Despite progress, the true trade-off between codeword length and locality is still a mystery.<br />Are there codes of positive rate with ?<br />Are there codes of polynomial length and ?<br />A technical question: what is the size of the largest family of subsets of such that<br />For all modulo six; <br />For all modulo six. <br />

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