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Gauss-Markov Theorem and Wiener Filtering

Module by: Clayton Scott, Robert Nowak

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Let xx and yy be jointly Gaussian distributed: xy∼mxmy 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 ∼my+ R yx R xx -1x−mxQ 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 ̂=my+ R yx R xx -1x−mx 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 y∼0 R yy y 0 R yy and W∼0 R WW W 0 R WW and yy and WW are independent. xy∼00 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 ∼ 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=argminGEy−GxTy−Gx H G y G x y G x where Ey−GxTy−Gx y G x y G x is the MSE.

MS=Ey−GxTy−Gx=Etry−Gxy−GxT=trEy−Gxy−GxT 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=Ey−Gxy−GxT= 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 ∂∂Gε2=-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)

Figure 1: y= y ̂+y− y ̂ y y y y , where y− y ̂ y y is the error. The error and the estimate are statistically orthogonal to each other.
Geometrical Interpretation
Geometrical Interpretation (geoint.png)

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 Ey− y ̂ y ̂T=0 y y y 0

Ey− y ̂ y ̂T=EyxTHT−HxxTHT= R yx HT−H R xx HT= R yy HT−H 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

Figure 2: yt y t is a stationary random signal, wt w t is stationary random noise, and the filter is H H
Figure 2 (block1.png)
We want to find Hω H ω that minimizes the MSE
ε2=E yt ̂−yt2= R ŷ ŷ 0−2 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 ωdω 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-ⅈωτdτ S yy ω τ R yy τ ω τ R yy τ=12π∫-∞∞ S yy ωⅇⅈωτdω R yy τ 1 2 ω S yy ω ω τ

Random Signal Response of Linear Systems

Figure 3: xt x t is a stationary random process and H H is a linear time-invariant system.
Figure 3 (block2.png)
yt ̂=∫-∞∞huxt−udu y t u h u x t u
E yt ̂=∫-∞∞huExt−udu= m x ∫-∞∞hudu y t u h u x t u m x u h u (6)
since Ext−u 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 ̂=E∫hsxt−sds∫huxt+τ−udu=∫∫Ext−sxt+τ−uhshudsdu=∫∫ R xx τ+s−uhshudsdu= R xx τ*h-τ*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 ω∥Hω∥2 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=E∫hsxt−sdsxt+τ=∫hs R xx τ+sds= R xx τ*h-τ 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=12π∫-∞∞∥Hω∥2 S xx ωdω R ŷ ŷ 0 1 2 ω H ω 2 S xx ω R y ŷ 0=12π∫-∞∞Hω¯ S yx ωdω 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=12π∫-∞∞∥Hω∥2 S yy ω+ S ww ω−2Hω¯ S yy ω+ S yy ωdω 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

Example 2

Figure 4
Figure 4 (syy.png)
Figure 5
Figure 5 (sww.png)
Figure 6
Figure 6 (h.png)

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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