# lect9 - Error Correcting Codes Combinatorics Algorithms and...

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Unformatted text preview: Error Correcting Codes: Combinatorics, Algorithms and Applications (Fall 2007) Lecture 9: Converse of Shannon’s Capacity Theorem September 17, 2007 Lecturer: Atri Rudra Scribe: Thanh-Nhan Nguyen & Atri Rudra In the last lecture, we stated Shannon’s capacity theorem for the BSC, which we restate here: Theorem 0.1. Let ≤ p < 1 / 2 be a real number. For every < ε ≤ 1 / 2- p , the following statements are true for large enough integer n : (i) There exists a real δ > , an encoding function E : { , 1 } k → { , 1 } n , and a decoding function D : { , 1 } n → { , 1 } k , where k ≤ b (1- H ( p + ε )) n c such that the following holds for every m ∈ { , 1 } k : Pr noise e of BSC p [ D ( E ( m ) + e ) 6 = m ] ≤ 2- δn . (ii) If k ≥ d (1- H ( p ) + ε ) n e then for every encoding and decoding functions E : { , 1 } k → { , 1 } n and D : { , 1 } n → { , 1 } k the following is true for some m ∈ { , 1 } k : Pr noise e of BSC p [ D ( E ( m ) + e ) 6 = m ] ≥ 1 / 2 . In today’s lecture, we will prove part (ii) of Theorem 0.1. 1 Preliminaries Before we begin with the proof we will need a few results, which we discuss first. 1.1 Chernoff Bound Chernoff bound states a bound on the tail of a certain distribution that will be useful for us. Here we state the version of the Chernoff bound that we will need. Proposition 1.1. For i = 1 , ··· , n , let X i be a binary random variable that takes a value of 1 with probability p and a value of with probability 1- p . Then the following bounds are true: (i) Pr [ ∑ n i =1 X i ≥ (1 + ε...
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lect9 - Error Correcting Codes Combinatorics Algorithms and...

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