# ch3 - 1 EE670 Information Theory and Coding Class home page...

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2 EE670 Information Theory and Coding Class home page: http://koala.ece.stevens-tech.edu/~utureli/EE670/ Class information, assignments and other links. Text Elements of Information Theory, Thomas Cover and Joy A Thomas, Wiley, 1991. Entropy, Data Compression, Rate Distortion Theory Ch. 2- 3 HW Due Feb 3: 2.5,2.11,2.19,2.23,3.3
3 Entropy Entropy (uncertainty, self information) of a discrete R.V. X: i x i i x p p x p x p x p x p X H 1 log )) ( log( ) ( ) ( 1 log ) ( ) ( = = = In the logarithm, when base 2 is used: information measured in bits (for discrete R.Vs) When base e is used: information measured in nats (for continuous R.V.s)

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4 Entropy Properties Shift and Scale Invariant Depends on the probability of outcomes not on the values of outcomes Semi positive H(X) >=0 since log (p(x)) <= 0 when p(x) <= 1 Other: Maximum entropy for R.V. is when outcomes equally likely Source coding will provide a justification for the definition of entropy as the uncertainty of a R.V.
5 Joint and Conditional Entropy Joint Entropy Examples: 1. Two fair coin tosses, P(X=x)=1/2, P(Y=y)=1/2 Independent P(X=x,Y=y)=1/2x1/2=1/4 H(X,Y)= 4 x ¼ log 4 = 4 x ¼ * 2 = 2 bits! 2. If X, Y independent P(x,y)=P(x)P(y) H(X,Y)=H(X)+H(Y) = = ) , ( 1 log ) , ( 1 log ) , ( ) , ( ) , ( , y x p E y x p y x p Y X H y x p y x

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6 Independent RVs X, Y ) ( ) ( ) , ( ) ( ) ( ) ( ) ( ) , ( ) ( ) ( ) ( ) ( ) , ( ) ( 1 log ) ( ) ( ) ( 1 log ) ( ) ( ) , ( ) ( 1 log ) ( ) ( ) ( 1 log ) ( ) ( ) , ( ) ( ) ( 1 log ) ( ) ( ) , ( 1 log ) , ( 1 log ) , ( ) , ( , , , , Y H X H Y X H x p Y H y p X H Y X H Y H x p X H y p Y X H y p y p x p x p x p y p Y X H y p y p x p x p y p x p Y X H y p x p y p x p y x p y x p y x p Y X H x y x y xy x x y y x y x y x y x + = + = + = + = + = = =
7 Joint Entropy The joint entropy H(X,Y) can also be expressed as: [ ] [ ] [ ] [ ] ) ( ) ( )] ( log [ ) ( log ) , ( )) ( log( ) ( (log ) ( ) ( log ) , ( log ) , ( Y H X H y p E x p E Y X H y p x p E y p x p E y x p E Y X H + = + = + = = =

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8 Conditional probability and Bayes’ Rule The knowledge that a certain event occurred can change the probability that another event will occur. P(x|y) denotes the conditional probability that x will happen given that y has happened. Bayes’ rule states that: The complete probability formula states that P(A) = P(A|B)P(B) + P(A| ¬ B)P( ¬ B) or in the more general case Note : P(A) = α P(A|B) + (1- α )P(A| ¬ B) mirrors our intuition that the unconditional probability of an event is somewhere between it’s conditional probability based on two opposing assumptions.
9 Information Theory: Entropy We want a fundamental measure that will allow us to

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