Wiener Filtering Explained

Theory of Wiener Filtering

Wiener filtering is a fundamental technique in statistical signal processing used to estimate a desired signal from a noisy observation. The objective is to design a linear time-invariant (LTI) filter whose output minimizes the mean-square error (MSE) between the estimated signal and the desired signal.

Signal Model and Notation

• x[n] denote the observed (input) signal
• d[n] denote the desired (reference) signal
• h[n] denote the impulse response (filter coefficients)
• y[n] denote the filter output (estimate of d[n])
• e[n] denote the estimation error

The output of the LTI filter is given by the convolution:

$$ y[n] = \sum_{m=-\infty}^{\infty} h[m]\,x[n-m] $$

Here, h[m] represents the filter coefficient at lag m, and x[n-m] is the input sample delayed by m. The summation combines all weighted past and future samples (for the general case).

The estimation error is defined as:

$$ e[n] = d[n] - y[n] $$

This represents the difference between the desired signal and its estimate.

Mean-Square Error Criterion

The performance of the filter is evaluated using the mean-square error:

$$ \xi = E\{e^2[n]\} = E\{(d[n] - y[n])^2\} $$

Here, E{⋅} denotes the statistical expectation operator. The squaring ensures that both positive and negative errors contribute positively.

The Wiener filtering problem consists of determining the impulse response h[n] that minimizes ξ:

$$ h_{\text{opt}} = \arg \min_h E\{(d[n] - y[n])^2\} $$

Here, arg min denotes the value of h[n] that minimizes the MSE.

Correlation Functions

Autocorrelation Function

$$ \varphi_{xx}[k] = E\{x[n]\,x[n+k]\} $$

Here, k is the lag (time shift). This function measures how similar the signal is to a shifted version of itself.

For wide-sense stationary (WSS) processes, φxx[k] depends only on k. At k = 0, φxx[0] = E{x²[n]} represents the average power of the input signal.

Cross-Correlation Function

$$ \varphi_{xd}[k] = E\{x[n]\,d[n+k]\} $$

This measures how the input signal x[n] is related to the desired signal d[n] at a shift of k.

Wiener–Hopf Equation

$$ \sum_{m=-\infty}^{\infty} h_{\text{opt}}[m]\,\varphi_{xx}[k-m] = \varphi_{xd}[k], \quad \forall k $$

Here, h_opt[m] are the optimal filter coefficients. This equation expresses that the filtered input must match the cross-correlation with the desired signal for all lags k.

Frequency-Domain Representation

$$ H_{\text{opt}}(z) = \frac{\Phi_{xd}(z)}{\Phi_{xx}(z)} $$

• Φxx(z) is the power spectral density (PSD) of x[n]
• Φxd(z) is the cross-power spectral density between x[n] and d[n]

Here, H_opt(z) represents the frequency response of the optimal filter.

Noise Reduction Model

$$ x[n] = s[n] + v[n] $$

• s[n] is the desired signal
• v[n] is additive noise

Assume s[n] and v[n] are uncorrelated, and let d[n] = s[n]. Then:

$$ \Phi_{xx}(z) = \Phi_{ss}(z) + \Phi_{vv}(z), \quad \Phi_{xd}(z) = \Phi_{ss}(z) $$

The optimal Wiener filter becomes:

$$ H_{\text{opt}}(z) = \frac{\Phi_{ss}(z)}{\Phi_{ss}(z) + \Phi_{vv}(z)} $$

At frequencies where the noise power Φ_vv(z) is large, the denominator increases, causing the filter to reduce the gain (attenuation) at those frequencies.

Finite-Length (FIR) Wiener Filter

$$ y[n] = \sum_{k=0}^{N-1} h[k]\,x[n-k] $$

Here, only past and present samples are used, making the filter causal.

Define the coefficient vector and input vector:

$$ H = [h[0], h[1], \dots, h[N-1]]^T $$ $$ X[n] = [x[n], x[n-1], \dots, x[n-N+1]]^T $$

Then the filter output is:

$$ y[n] = H^T X[n] $$

Here, H^T denotes the transpose of H, representing an inner product.

• R = E{X[n] X^T[n]} : autocorrelation matrix of the input
• P = E{X[n] d[n]} : cross-correlation vector

$$ H_{\text{opt}} = R^{-1} P $$

• R^-1: The inverse of the input autocorrelation matrix. It helps the filter isolate the unique parts of the signal.
• P: The cross-correlation vector. It indicates how the filter weights should be adjusted to match the desired output.

Minimum Mean-Square Error

$$ \xi_{\min} = \sigma_d^2 - P^T R^{-1} P $$ $$ \sigma_d^2 = E\{d^2[n]\} $$

Here, d[n] denotes the desired signal, and σ_d² represents its average power. The term P^T R^-1 P represents the amount of the desired signal power that can be recovered (estimated) from the observed input signal x[n].

Applications

• Noise Reduction: Removing background hiss from recordings.
• System Identification: Mapping how an unknown system behaves.
• Channel Equalization: Fixing distorted signals in 4G/5G networks.
• Image Restoration: Removing "blur" from digital photographs.