10
FILTERING, LINEAR SYSTEMS, AND ESTIMATION
Ch. 2
Later, we shall pose specific mathematical models for some systems, and
even formally identify the system with the model.
In discussing filtering and related problems, it is implicit that the systems
under consideration are noisy. The noise may arise in a number of ways. For
example, inputs to the system may be unknown and unpredictable except for
their statistical properties, or outputs from the system may be derived with
the aid of a noisy sensor, i.e., one that contributes on a generally random
basis some inaccuracy to the measurement of the system output. Again,
outputs may only be observed via a sensor after transmission over a noisy
channel.
In virtually all the problems we shall discuss here, it will be assumed that
the output measurement process is noisy. On most occasions, the inputs also
will be assumed to be noisy.
Now let us consider exactly what we mean by
jiltering. Suppose there is
some quantity (possibly a vector quantity) associated with the system opera-
tion whose value we would like to know at each instant of time. For the sake
of argument, assume the system in question is a continuous time system, and
the quantity in question is denoted by s(. ).* It may be that this quantity is
not directly measurable, or that it can only be measured with error. In any
case, we shall suppose that noisy measurements Z(.) are available, with Z(. )
not the same as S(.).
The term filtering is used in two senses. First, it is used as a generic term:
filtering is the recovery from Z(.) of s(.), or an approximation to s(.), or
even some information about S(.). In other words, noisy measurements of a
system are used to obtain information about some quantity that is essentially
internal to the system. Second, it is used to distinguish a certain kind of
information processing from two related kinds, smoothing and prediction.
In this
sense, filtering means the recovery at time tof some information about
s(t)using measurements up till time t.The important thing to note is the
triple occurrence of the time argument t.First, we are concerned with obtain-
ing information about S(. ) at time t,i.e., s(t).Second, the information is
available at time t,not at some later time. Third, measurements right up to,
but not after, time t are used. [If information about s(t) is to be available at
time t, then causality rules out the use of measurements taken later than time
t in producing this information.]
An example of the application of filtering in everyday life is in radio
reception. Here the signal of interest is the voice signal. This signal is used to
modulate a high frequency carrier that is transmitted to a radio receiver. The
received signal is inevitably corrupted by noise, and so, when demodulated,
it is filtered to recover as well as possible the original signal.
●Almost without exeeption throughout the book,
x(t) will denote the value taken by
a function at time I, and x(. ) will denote that function. Therefore, x(t) is a number, and
x(. ) an infinite set of pairs, {t, x(t)}, for / ranging over all possible values.