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Unformatted text preview: ECE 6010 Lecture 9 Linear Minimum MeanSquare Error Filtering Background Recall that for random variable X and Y with finite variance, the MSE E [( X h ( Y )) 2 ] is minimized by h ( Y ) = E [ X  Y ] . That is, the best estimate of X using a measured value of Y is to find the conditional average of X . One aspect of this estimate is that: The error is orthogonal to the data. More precisely, the error X E [ X  Y ] is orthogonal to Y and to every function of Y : E [( X E [ X  Y ]) g ( Y )] = 0 for all measurable functions g . We will assume that E [ g 2 ( Y )] < . We want to show that h minimizes E [( X h ( Y )) 2 ] if and only if E [( X h ( Y )) g ( Y )] = (orthogonality), for all measurable g such that E [ g 2 ( Y )] < . E [( X E [ X  Y ]) g ( Y )] = E [ E [( X E [ X  Y ])  Y ] g ( Y )] = E [( E [ X  Y ] E [ X  Y ]) g ( Y )] = 0 . Conversely, suppose for some g , E [( X h ( Y )) g ( Y )] 6 = 0 . Consider the estimate h ( Y ) = h ( Y ) + g ( Y ) , where = E [( X h ( Y )) g ( Y )] E [ g 2 ( Y )] . Then E [( X h ( Y )) 2 ] = E [( X h ( Y )) 2 ] ( E [( X h ( Y )) g ( Y )]) 2 E [ g 2 ( Y )] < E [( X h ( Y )) 2 ] . Suppose now we are given two random processes { X t } and { Y t } that are statistically related (that is, not independent). Suppose, to begin, that T = R . Suppose we observe Y over the interval [ a, b ] , and based on the information gained we want to estimate X t for some fixed t as a function of { Y t , a t b } . That is, we form X t = f ( { Y , a b } ) for some functional f mapping the function to real numbers. If t < b : We say that the operation of the function is smoothing . If t = b : We way that the operation of the function is filtering . If t > b : We way that the operation of the function is prediction . The error in the estimate is X t X t . The meansquared error is E [( X t X t ) 2 ] . Fact (built on our previous intuition): The MSE E [( X t X t ) 2 ] is minimized by the conditional expectation X ( t ) = E [ X t  Y , a b ] . Furthermore, the orthogonality principle applies: X t E [ X t  Y , a b ] is orthogonal to every function of { Y , a b } . While we know the theoretical result, it is difficult in general to compute the desired conditional expectation....
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This note was uploaded on 03/01/2012 for the course ECE 6010 taught by Professor Stites,m during the Spring '08 term at Utah State University.
 Spring '08
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