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# Gauss-Markov Theorem and Wiener Filtering

Module by: Clayton Scott, Robert Nowak. E-mail the authors

Let xx and yy be jointly Gaussian distributed: ( x y )𝒩( m x m y )( R xx R xy R yx R yy ) x y m x m y R xx R xy R yx R yy Then the conditional distribution of y y given x x is y | x 𝒩 m y + R yx R xx -1(x m x )Q y | x m y R yx R xx x m x Q where Q= R yy R yx R xx -1 R xy Q R yy R yx R xx R xy

We know that the conditional mean y ^= m y + R yx R xx -1(x m x ) y m y R yx R xx x m x is the best estimate of yy give xx in a (mean) squared error sense.

## Example 1

x=y+W x y W where y𝒩0 R yy y 0 R yy and W𝒩0 R WW W 0 R WW and yy and WW are independent. ( x y )𝒩( 0 0 )( R yy + R WW R yy R yy R yy ) x y 0 0 R yy R WW R yy R yy R yy y | x 𝒩 R yx R xx -1x R yy R yx R xx -1 R xy y | x R yx R xx x R yy R yx R xx R xy y ^= R yy R yy + R WW -1x=Hx y R yy R yy R WW x H x where H H is the Wiener filter. Minimum MSE estimator of yy given xx. R yx = R yy R yx R yy and R xx = R yy + R WW R xx R yy R WW .

## Direct Optimization

x=y+W x y W y ^=Gx y G x H=argminGE(yGx)T(yGx) H G y G x y G x where E(yGx)T(yGx) y G x y G x is the MSE.

MS=E(yGx)T(yGx)=Etr(yGx)(yGx)T=trE(yGx)(yGx)T MS y G x y G x tr y G x y G x tr y G x y G x
(1)
Minimizing MSE is equivalent to minimizing
ε2=E(yGx)(yGx)T= R yy G R xy R yx GT+G R xx GT ε 2 y G x y G x R yy G R xy R yx G G R xx G
(2)
Taking the derivative with repsect to GG ε2 G =2 R yx +2G R xx =0 G ε 2 -2 R yx 2 G R xx 0 This implies
H= R yx R xx -1= R yy R yy + R WW -1 H R yx R xx R yy R yy R WW
(3)

## Orthogonality Condition

The optimal Wiener filter H= R yy R yy + R WW -1 H R yy R yy R WW satisfies the following condition E(y y ^) y ^T=0 y y y 0

E(y y ^) y ^T=E(yxTHTHxxTHT)= R yx HTH R xx HT= R yy HTH( R yy + R WW )HT= R yy R yy + R WW -1 R yy R yy R yy + R WW -1( R yy + R WW ) R yy + R WW -1 R yy =0 y y y y x H H x x H R yx H H R xx H R yy H H R yy R WW H R yy R yy R WW R yy R yy R yy R WW R yy R WW R yy R WW R yy 0
(4)

## The Classical Wiener Filter

We want to find Hω H ω that minimizes the MSE
ε2=E yt ^yt2= R ŷ ŷ 02× R y ŷ 0+ R yy 0 ε 2 y t y t 2 R ŷ ŷ 0 2 R y ŷ 0 R yy 0
(5)
where R yy τ=Eytyt+τ R yy τ y t y t τ . We can express the MSE in the frequency domain by noting that R yy 0=12π S yy ωd ω R yy 0 1 2 ω S yy ω where S yy ω S yy ω is the power spectrum of yt y t .

### Recall:

R yy τ R yy τ and S yy ω S yy ω are FT pairs: S yy ω= R yy τe(iωτ)d τ S yy ω τ R yy τ ω τ R yy τ=12π S yy ωeiωτd ω R yy τ 1 2 ω S yy ω ω τ

## Random Signal Response of Linear Systems

yt ^=huxtud u y t u h u x t u
E yt ^=huExtud u = m x hud u y t u h u x t u m x u h u
(6)
since Extu x t u is a constant. In particular, if xt x t is zero-mean then so is yt ^ y t .

## Autocorrelation of Output Process

R ŷ ŷ τ=E yt ^ yt+τ ^=Ehsxtsd s huxt+τud u =Extsxt+τuhshud s d u = R xx τ+suhshud s d u = R xx τ*hτ*hτ R ŷ ŷ τ y t y t τ s h s x t s u h u x t τ u u s x t s x t τ u h s h u u s R xx τ s u h s h u R xx τ h τ h τ
(7)

## Power Spectrum

S ŷ ŷ ω= S xx ωHω¯Hω= S xx ωHω2 S ŷ ŷ ω S xx ω H ω H ω S xx ω H ω 2
(8)

## Cross-Correlation and Cross-Spectrum

R ŷ x τ=E yt ^xt+τ=Ehsxtsd s xt+τ=hs R xx τ+sd s = R xx τ*hτ R ŷ x τ y t x t τ s h s x t s x t τ s h s R xx τ s R xx τ h τ
(9)
This implies S ŷ x ω= S xx ωHω¯ S ŷ x ω S xx ω H ω R ŷ ŷ 0=12πHω2 S xx ωd ω R ŷ ŷ 0 1 2 ω H ω 2 S xx ω R y ŷ 0=12πHω¯ S yx ωd ω R y ŷ 0 1 2 ω H ω S yx ω S xx ω= S yy ω+ S ww ω S xx ω S yy ω S ww ω S yx ω= S yy ω S yx ω S yy ω since yt y t and wt w t are independent. Thus, the expression for the MSE becomes ε2=12πHω2( S yy ω+ S ww ω)2Hω¯ S yy ω+ S yy ωd ω ε 2 1 2 ω H ω 2 S yy ω S ww ω 2 H ω S yy ω S yy ω ε2 ε 2 is minimized by minimizing the integrand each frequency ωω. This implies Hω( S yy ω+ S ww ω)= S yy ω H ω S yy ω S ww ω S yy ω Hω= S yy ω S yy ω+ S ww ω H ω S yy ω S yy ω S ww ω

## Comparison

### Signal Vector (discrete-time)

H= R yy R yy + R ww -1 H R yy R yy R ww Here H H is a matrix, R yy R yy is the signal "power", and R ww R ww is the noise "power".

### Classical (continuous-time)

Hω= S yy ω S yy ω+ S ww ω= S yy ω S yy ω+ S ww ω-1 H ω S yy ω S yy ω S ww ω S yy ω S yy ω S ww ω
(10)
Which means that the Wiener Filter is defined as the ratio of signal power to the sum of signal power and noise power.

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