The communication complexity of correlation

  • Authors:
  • Prahladh Harsha;Rahul Jain;David McAllester;Jaikumar Radhakrishnan

  • Affiliations:
  • Toyota Technological Institute, Chicago, IL;Centre for Quantum Technologies and Department of Computer Science, National University of Singapore, Singapore, Singapore and University of California, Berkeley, CA and University of Waterloo, Wa ...;Toyota Technological Institute, Chicago, IL;Tata Institute of Fundamental Research, Mumbai, India and Toyota Technological Institute, Chicago, IL

  • Venue:
  • IEEE Transactions on Information Theory
  • Year:
  • 2010

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Abstract

Let X and Y be finite nonempty sets and (X,Y) a pair of random variables taking values in X × Y. We consider communication protocols between two parties, ALICE and BOB, for generating X and Y . ALICE is provided an x ∈ X generated according to the distribution of y, and is required to send a message to BOB in order to enable him to generate y ∈ Y, whose distribution is the same as that of Y|X=x. Both parties have access to a shared random string generated in advance. Let T[X : Y] be the minimum (over all protocols) of the expected number of bits ALICE needs to transmit to achieve this. We show that I[X : Y] ≤ T[X : Y] ≤ I[X : Y] + 2log2(I[X : Y] + 1) + O(1). We also consider the worst case communication required for this problem, where we seek to minimize the average number of bits ALICE must transmit for the worst case x ∈ X. We show that the communication required in this case is related to the capacity C(E) of the channel E, derived from (X,Y), that maps x ∈ X to the distribution of Y|X=x. We also showthat the required communication T(E) satisfies C(E) ≤ T(E) ≤ C(E) + 2log2 (C(E) + 1) + O(1). Using the first result, we derive a direct-sum theorem in communication complexity that substantially improves the previous such result shown by Jain, Radhakrishnan, and Sen [In Proc. 30th International Colloquium of Automata, Languages and Programming (ICALP), ser. Lecture Notes in Computer Science, vol. 2719. 2003, pp. 300-315]. These results are obtained by employing a rejection sampling procedure that relates the relative entropy between two distributions to the communication complexity of generating one distribution from the other.