Estimation theory

Estimation theory is a branch of statistics and signal processing that deals with estimating the values of parameters based on measured/empirical data. The parameters describe the physical scenario or object that answers a question posed by the estimator.

For example, it is desired to estimate the proportion of a population of voters who will vote for a particular candidate. That proportion is the unobservable parameter; the estimate is based on a small random sample of voters.

Or, for example, in radar the goal is to estimate the location of objects (airplanes, boats, etc.) by analyzing the received echo and a possible question to be posed is "where are the airplanes?"
To answer where the airplanes are, it is necessary to estimate the distance the airplanes are at from the radar station, which can provide an absolute location if the absolute location of the radar station is known.

In estimation theory, it is assumed that the desired information is embedded into a noisy signal.
Noise adds uncertainty and if there was no uncertainty then there would be no need for estimation.

Fields that use estimation theory

There are numerous fields that require the use of estimation theory.
Some of these fields include (but by no means limited to):

Medicine

* Clinical trials

* Imaging:

** CAT

** EEG

** EKG/ECG

** MRI

** Medical ultrasonography

Opinion polls

Quality control

Radar, sonar

* Localization of objects

Telecommunications

* Channel parameters

* Noise variance

* DC gain (see example below)

Seismology

* Localization of underground oil deposits

Control theory

* Actuator changes with time

Digital image processing

* Position of objects in images (see computer vision)

Digital signal processing

* Parametric (e.g., periodogram, correlogram) spectral analysis

* nonparametric (e.g., MUSIC, Root-MUSIC, ESPRIT) spectral analysis

* Kalman filter

* Wiener filter

Network intrusion detection system

The measured data is likely to be subject to noise or uncertainty and it is through statistical probability that optimal solutions are sought to extract as much information from the data.

Estimation process

The entire purpose of estimation theory is to arrive at an estimator, and preferably an implementable one that could actually be used.
The estimator takes the measured data as input and produces an estimate of the parameters.

It is also preferable to derive an estimator that exhibits optimality.
An optimal estimator would indicate that all available information in the measured data has been extracted, for if there was unused information in the data then the estimator would not be optimal.

These are the general steps to arrive at an estimator:

In order to arrive at a desired estimator for estimating a single or multiple parameters, it is first necessary to determine a model for the system. This model should incorporate the process being modeled as well as points of uncertainty and noise. The model describes the physical scenario in which the parameters apply.

After deciding upon a model, it is helpful to find the limitations placed upon an estimator. This limitation, for example, can be found through the Cramér-Rao inequality.

Next, an estimator needs to be developed or applied if an already known estimator is valid for the model. The estimator needs to be tested against the limitations to determine if it is an optimal estimator (if so, then no other estimator will perform better).

Finally, experiments or simulations can be ran with the estimator to test the performance.

After arriving at an estimator, real data might show that the model used to derive the estimator is incorrect, which may require repeating these steps to find a new estimator.
A non-implementable or unfeasible estimator may need to be scrapped and the process start anew.

In summary, the estimator estimates the parameters of a physical model based on measured data.

Basics

To build a model, several statistical "ingredients" need to be known.
These are needed to ensure the estimator has some mathematical tractability instead of being based on "gut feel."

The first is a set of statistical samples taken from a random vector (RV) of size which can be put into a vector

and their parameters

need to be established with their probability density function (pdf) or probability mass function (pmf)

It is also possible for the parameters themselves to have a probability distribution (e.g., Bayesian statistics) then it is necessary to define the epistemic probability

After the model is formed, the goal is to estimate the parameters, which is commonly denoted where the "hat" indicates the estimate.
One common estimator is the minimum mean squared error (MMSE) estimator that utilizes the error between the estimated parameters and the actual value of the parameters

as the basis for optimality.
This error term is then squared and minimized for the MMSE estimator.

Estimators

This list is some of the more common estimators used:

Maximum likelihood

Cramér-Rao inequality

Minimum mean squared error (MMSE)

Maximum a posteriori (MAP)

Minimum variance unbiased estimator (MVUE)

Best linear unbiased estimator (BLUE)

Particle filter

Markov chain Monte Carlo (MCMC)

Kalman filter

Wiener filter

Example: DC gain in white Gaussian noise

Consider a received discrete signal, , of independent samples that consists of a DC gain with Additive white Gaussian noise with known variance (i.e., ).
Since the variance is known then the only unknown parameter is .

The model for the signal is then
:

Two possible (of many) estimators are:

which is the sample mean

Both of these estimators have a mean of , which can be shown through taking the expected value of each estimator

and
:

At this point, these two estimators would appear to perform the same.
However, the difference between them becomes apparent when comparing the variances.

and
:

It would seem that the sample mean is a better estimator since, as , the variance goes to zero.

Maximum likelihood

Continuing the example using the maximum likelihood estimator, the probability density function (pdf) of the noise for one sample is

and the probability of becomes ( can be thought of a )

By independence, the probability of becomes

Taking the natural logarithm of the pdf

and the maximum likelihood estimator is

Taking the first derivative of the log-likelihood function

and setting it to zero

This results in the maximum likelihood estimator

which is simply the sample mean.
From this example, it was found that the sample mean is the maximum likelihood estimator for samples of AWGN with a fixed, unknown DC gain.

Cramér-Rao lower bounds

To find the Cramér-Rao lower bounds (CRLB) of the sample mean estimator, it is first necessary to find the Fisher information number

and copying from above

Taking the second derivative
:

and finding the negative expected value is trivial since it is now a deterministic constant

Finally, putting the Fisher information into

results in

Comparing this to the variance of the sample mean (determined previously) shows that the sample mean is equal to the Cramér-Rao lower bounds for all values of and .
The sample mean is the minimum variance unbiased estimator (MVUE) in addition to being the maximum likelihood estimator.

This example of DC gain + WGN is a recurring example in Kay's Fundamentals of Statistical Signal Processing.

Books

Fundamentals of Statistical Signal Processing: Estimation Theory by Steven M. Kay (ISBN 0-13-345711-7)

An Introduction to Signal Detection and Estimation by H. Vincent Poor (ISBN 0-38-794173-8)

Detection, Estimation, and Modulation Theory, Part 1 by Harry L. Van Trees (ISBN 0-47-109517-6; website)