In probability and statistics, given two stochastic processes and , the cross-covariance is a function that gives the covariance of one process with the other at pairs of time points. With the usual notation ; for the expectation operator, if the processes have the mean functions and , then the cross-covariance is given by
Cross-covariance is related to the more commonly used cross-correlation of the processes in question.
In the case of two random vectors and , the cross-covariance would be a matrix (often denoted ) with entries Thus the term cross-covariance is used in order to distinguish this concept from the covariance of a random vector , which is understood to be the matrix of covariances between the scalar components of itself.
where and are vectors containing the expected values of and . The vectors and need not have the same dimension, and either might be a scalar value. Any element of the cross-covariance matrix is itself a "cross-covariance".
Example
For example, if and are random vectors, then
is a matrix whose -th entry is .
Properties of cross-covariance matrix
For the cross-covariance matrix, the following basic properties apply:[1]
If and are independent (or somewhat less restrictedly, if every random variable in is uncorrelated with every random variable in ), then
where , and are random vectors, is a random vector, is a vector, is a vector, and are matrices of constants, and is a matrix of zeroes.
If and are complex random vectors, the definition of the cross-covariance matrix is slightly changed. Transposition is replaced by hermitan transposition:
For complex random vectors, another matrix called the pseudo-cross-covariance matrix is defined as follows:
Uncorrelatedness
Two random vectors and are called uncorrelated if their covariance matrix matrix is zero.
Complex random vectors and are called uncorrelated if their covariance matrix and pseudo-covariance matrix is zero, i.e. if .
Cross-covariance of stochastic processes
The definition of cross-covariance of random vector may be generalized to stochastic processes as follows:
Definition
Let and denote stochastic processes. Then the cross-covariance function of the processes is defined by:
By setting (the time lag, or the amount of time by which the signal has been shifted), we may define
.
The autocovariance function of a WSS process is therefore given by:
(Eq.3)
which is equivalent to
.
Cross-covariance of deterministic signals
The cross-covariance is also relevant in signal processing where the cross-covariance between two wide-sense stationaryrandom processes can be estimated by averaging the product of samples measured from one process and samples measured from the other (and its time shifts). The samples included in the average can be an arbitrary subset of all the samples in the signal (e.g., samples within a finite time window or a sub-sampling of one of the signals). For a large number of samples, the average converges to the true covariance.
Cross-covariance may also refer to a "deterministic" cross-covariance between two signals. This consists of summing over all time indices.
For example, for discrete-time signals and the cross-covariance is defined as