Complex random vector

In probability theory and statistics, a complex random vector is typically a tuple of complex-valued random variables, and generally is a random variable taking values in a vector space over the field of complex numbers. If Z_1,\ldots,Z_n are complex-valued random variables, then the n-tuple \left( Z_1,\ldots,Z_n \right) is a complex random vector. Complex random variables can always be considered as pairs of real random vectors: their real and imaginary parts.

Some concepts of real random vectors have a straightforward generalization to complex random vectors. For example, the definition of the mean of a complex random vector. Other concepts are unique to complex random vectors.

Applications of complex random vectors are found in digital signal processing.

{{Probability fundamentals}}

Definition

A complex random vector \mathbf{Z} = (Z_1,\ldots,Z_n)^T on the probability space (\Omega,\mathcal{F},P) is a function \mathbf{Z} \colon \Omega \rightarrow \mathbb{C}^n such that the vector (\Re{(Z_1)},\Im{(Z_1)},\ldots,\Re{(Z_n)},\Im{(Z_n)})^T is a real random vector on (\Omega,\mathcal{F},P) where \Re{(z)} denotes the real part of z and \Im{(z)} denotes the imaginary part of z.{{cite book |first=Amos |last=Lapidoth |year=2009 |title=A Foundation in Digital Communication |publisher=Cambridge University Press |isbn=978-0-521-19395-5}}{{rp|p. 292}}

Cumulative distribution function

The generalization of the cumulative distribution function from real to complex random variables is not obvious because expressions of the form P(Z \leq 1+3i) make no sense. However expressions of the form P(\Re{(Z)} \leq 1, \Im{(Z)} \leq 3) make sense. Therefore, the cumulative distribution function F_{\mathbf{Z}} : \mathbb{C}^n \mapsto [0,1] of a random vector \mathbf{Z}=(Z_1,...,Z_n)^T is defined as

{{Equation box 1

|indent =

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|equation = {{NumBlk||F_{\mathbf{Z}}(\mathbf{z}) = \operatorname{P}(\Re{(Z_1)} \leq \Re{(z_1)} , \Im{(Z_1)} \leq \Im{(z_1)},\ldots,\Re{(Z_n)} \leq \Re{(z_n)} , \Im{(Z_n)} \leq \Im{(z_n)})|{{EquationRef|Eq.1}}}}

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where \mathbf{z} = (z_1,...,z_n)^T.

Expectation

As in the real case the expectation (also called expected value) of a complex random vector is taken component-wise.{{rp|p. 293}}

{{Equation box 1

|indent =

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|equation = {{NumBlk|| \operatorname{E}[\mathbf{Z}] = (\operatorname{E}[Z_1],\ldots,\operatorname{E}[Z_n])^T |{{EquationRef|Eq.2}}}}

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Covariance matrix and pseudo-covariance matrix

{{see also|Covariance matrix#Complex random vector}}

The covariance matrix (also called second central moment) \operatorname{K}_{\mathbf{Z}\mathbf{Z}} contains the covariances between all pairs of components. The covariance matrix of an n \times 1 random vector is an n \times n matrix whose (i,j)th element is the covariance between the i th and the j th random variables.{{cite book |first=John A. |last=Gubner |year=2006 |title=Probability and Random Processes for Electrical and Computer Engineers |publisher=Cambridge University Press |isbn=978-0-521-86470-1}}{{rp|p.372}} Unlike in the case of real random variables, the covariance between two random variables involves the complex conjugate of one of the two. Thus the covariance matrix is a Hermitian matrix.{{rp|p. 293}}

{{Equation box 1

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|equation = {{NumBlk||

\begin{align}

& \operatorname{K}_{\mathbf{Z}\mathbf{Z}}

=

\operatorname{cov}[\mathbf{Z},\mathbf{Z}]

=

\operatorname{E}[(\mathbf{Z}-\operatorname{E}[\mathbf{Z}]){(\mathbf{Z}-\operatorname{E}[\mathbf{Z}])}^H]

=

\operatorname{E}[\mathbf{Z}\mathbf{Z}^H]-\operatorname{E}[\mathbf{Z}]\operatorname{E}[\mathbf{Z}^H] \\[12pt]

\end{align}

|{{EquationRef|Eq.3}}}}

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:

\operatorname{K}_{\mathbf{Z}\mathbf{Z}}=

\begin{bmatrix}

\mathrm{E}[(Z_1 - \operatorname{E}[Z_1])\overline{(Z_1 - \operatorname{E}[Z_1])}] & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])\overline{(Z_2 - \operatorname{E}[Z_2])}] & \cdots & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])\overline{(Z_n - \operatorname{E}[Z_n])}] \\ \\

\mathrm{E}[(Z_2 - \operatorname{E}[Z_2])\overline{(Z_1 - \operatorname{E}[Z_1])}] & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])\overline{(Z_2 - \operatorname{E}[Z_2])}] & \cdots & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])\overline{(Z_n - \operatorname{E}[Z_n])}] \\ \\

\vdots & \vdots & \ddots & \vdots \\ \\

\mathrm{E}[(Z_n - \operatorname{E}[Z_n])\overline{(Z_1 - \operatorname{E}[Z_1])}] & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])\overline{(Z_2 - \operatorname{E}[Z_2])}] & \cdots & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])\overline{(Z_n - \operatorname{E}[Z_n])}]

\end{bmatrix}

The pseudo-covariance matrix (also called relation matrix) is defined replacing Hermitian transposition by transposition in the definition above.

{{Equation box 1

|indent =

|title=

|equation = {{NumBlk||

\operatorname{J}_{\mathbf{Z}\mathbf{Z}}

=

\operatorname{cov}[\mathbf{Z},\overline{\mathbf{Z}}]

=

\operatorname{E}[(\mathbf{Z}-\operatorname{E}[\mathbf{Z}]){(\mathbf{Z}-\operatorname{E}[\mathbf{Z}])}^T]

=

\operatorname{E}[\mathbf{Z}\mathbf{Z}^T]-\operatorname{E}[\mathbf{Z}]\operatorname{E}[\mathbf{Z}^T]

|{{EquationRef|Eq.4}}}}

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:

\operatorname{J}_{\mathbf{Z}\mathbf{Z}}=

\begin{bmatrix}

\mathrm{E}[(Z_1 - \operatorname{E}[Z_1])(Z_1 - \operatorname{E}[Z_1])] & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])(Z_2 - \operatorname{E}[Z_2])] & \cdots & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])(Z_n - \operatorname{E}[Z_n])] \\ \\

\mathrm{E}[(Z_2 - \operatorname{E}[Z_2])(Z_1 - \operatorname{E}[Z_1])] & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])(Z_2 - \operatorname{E}[Z_2])] & \cdots & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])(Z_n - \operatorname{E}[Z_n])] \\ \\

\vdots & \vdots & \ddots & \vdots \\ \\

\mathrm{E}[(Z_n - \operatorname{E}[Z_n])(Z_1 - \operatorname{E}[Z_1])] & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])(Z_2 - \operatorname{E}[Z_2])] & \cdots & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])(Z_n - \operatorname{E}[Z_n])]

\end{bmatrix}

;Properties

The covariance matrix is a hermitian matrix, i.e.{{rp|p. 293}}

:\operatorname{K}_{\mathbf{Z}\mathbf{Z}}^H = \operatorname{K}_{\mathbf{Z}\mathbf{Z}}.

The pseudo-covariance matrix is a symmetric matrix, i.e.

:\operatorname{J}_{\mathbf{Z}\mathbf{Z}}^T = \operatorname{J}_{\mathbf{Z}\mathbf{Z}}.

The covariance matrix is a positive semidefinite matrix, i.e.

:\mathbf{a}^H \operatorname{K}_{\mathbf{Z}\mathbf{Z}} \mathbf{a} \ge 0 \quad \text{for all } \mathbf{a} \in \mathbb{C}^n.

=Covariance matrices of real and imaginary parts=

{{see also|Complex random variable#Covariance matrix of real and imaginary parts}}

By decomposing the random vector \mathbf{Z} into its real part \mathbf{X} = \Re{(\mathbf{Z})} and imaginary part \mathbf{Y} = \Im{(\mathbf{Z})} (i.e. \mathbf{Z}=\mathbf{X}+i\mathbf{Y}), the pair (\mathbf{X},\mathbf{Y}) has a covariance matrix of the form:

:\begin{bmatrix}

\operatorname{K}_{\mathbf{X}\mathbf{X}} & \operatorname{K}_{\mathbf{X}\mathbf{Y}} \\

\operatorname{K}_{\mathbf{Y}\mathbf{X}} & \operatorname{K}_{\mathbf{Y}\mathbf{Y}}

\end{bmatrix}

The matrices \operatorname{K}_{\mathbf{Z}\mathbf{Z}} and \operatorname{J}_{\mathbf{Z}\mathbf{Z}} can be related to the covariance matrices of \mathbf{X} and \mathbf{Y} via the following expressions:

: \begin{align}

& \operatorname{K}_{\mathbf{X}\mathbf{X}} = \operatorname{E}[(\mathbf{X}-\operatorname{E}[\mathbf{X}])(\mathbf{X}-\operatorname{E}[\mathbf{X}])^\mathrm T] = \tfrac{1}{2}\operatorname{Re}(\operatorname{K}_{\mathbf{Z}\mathbf{Z}} + \operatorname{J}_{\mathbf{Z}\mathbf{Z}}) \\

& \operatorname{K}_{\mathbf{Y}\mathbf{Y}} = \operatorname{E}[(\mathbf{Y}-\operatorname{E}[\mathbf{Y}])(\mathbf{Y}-\operatorname{E}[\mathbf{Y}])^\mathrm T] = \tfrac{1}{2}\operatorname{Re}(\operatorname{K}_{\mathbf{Z}\mathbf{Z}} - \operatorname{J}_{\mathbf{Z}\mathbf{Z}}) \\

& \operatorname{K}_{\mathbf{Y}\mathbf{X}} = \operatorname{E}[(\mathbf{Y}-\operatorname{E}[\mathbf{Y}])(\mathbf{X}-\operatorname{E}[\mathbf{X}])^\mathrm T] = \tfrac{1}{2}\operatorname{Im}(\operatorname{J}_{\mathbf{Z}\mathbf{Z}} + \operatorname{K}_{\mathbf{Z}\mathbf{Z}}) \\

& \operatorname{K}_{\mathbf{X}\mathbf{Y}} = \operatorname{E}[(\mathbf{X}-\operatorname{E}[\mathbf{X}])(\mathbf{Y}-\operatorname{E}[\mathbf{Y}])^\mathrm T] = \tfrac{1}{2}\operatorname{Im}(\operatorname{J}_{\mathbf{Z}\mathbf{Z}} -\operatorname{K}_{\mathbf{Z}\mathbf{Z}}) \\

\end{align}

Conversely:

: \begin{align}

& \operatorname{K}_{\mathbf{Z}\mathbf{Z}} = \operatorname{K}_{\mathbf{X}\mathbf{X}} + \operatorname{K}_{\mathbf{Y}\mathbf{Y}} + i(\operatorname{K}_{\mathbf{Y}\mathbf{X}} - \operatorname{K}_{\mathbf{X}\mathbf{Y}}) \\

& \operatorname{J}_{\mathbf{Z}\mathbf{Z}} = \operatorname{K}_{\mathbf{X}\mathbf{X}} - \operatorname{K}_{\mathbf{Y}\mathbf{Y}} + i(\operatorname{K}_{\mathbf{Y}\mathbf{X}} + \operatorname{K}_{\mathbf{X}\mathbf{Y}})

\end{align}

Cross-covariance matrix and pseudo-cross-covariance matrix

The cross-covariance matrix between two complex random vectors \mathbf{Z},\mathbf{W} is defined as:

{{Equation box 1

|indent =

|title=

|equation = {{NumBlk|| \operatorname{K}_{\mathbf{Z}\mathbf{W}} =

\operatorname{cov}[\mathbf{Z},\mathbf{W}] =

\operatorname{E}[(\mathbf{Z}-\operatorname{E}[\mathbf{Z}]){(\mathbf{W}-\operatorname{E}[\mathbf{W}])}^H] = \operatorname{E}[\mathbf{Z}\mathbf{W}^H]-\operatorname{E}[\mathbf{Z}]\operatorname{E}[\mathbf{W}^H] |{{EquationRef|Eq.5}}}}

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:\operatorname{K}_{\mathbf{Z}\mathbf{W}} =

\begin{bmatrix}

\mathrm{E}[(Z_1 - \operatorname{E}[Z_1])\overline{(W_1 - \operatorname{E}[W_1])}] & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])\overline{(W_2 - \operatorname{E}[W_2])}] & \cdots & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])\overline{(W_n - \operatorname{E}[W_n])}] \\ \\

\mathrm{E}[(Z_2 - \operatorname{E}[Z_2])\overline{(W_1 - \operatorname{E}[W_1])}] & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])\overline{(W_2 - \operatorname{E}[W_2])}] & \cdots & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])\overline{(W_n - \operatorname{E}[W_n])}] \\ \\

\vdots & \vdots & \ddots & \vdots \\ \\

\mathrm{E}[(Z_n - \operatorname{E}[Z_n])\overline{(W_1 - \operatorname{E}[W_1])}] & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])\overline{(W_2 - \operatorname{E}[W_2])}] & \cdots & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])\overline{(W_n - \operatorname{E}[W_n])}]

\end{bmatrix}

And the pseudo-cross-covariance matrix is defined as:

{{Equation box 1

|indent =

|title=

|equation = {{NumBlk|| \operatorname{J}_{\mathbf{Z}\mathbf{W}} = \operatorname{cov}[\mathbf{Z},\overline{\mathbf{W}}] = \operatorname{E}[(\mathbf{Z}-\operatorname{E}[\mathbf{Z}]){(\mathbf{W}-\operatorname{E}[\mathbf{W}])}^T] = \operatorname{E}[\mathbf{Z}\mathbf{W}^T]-\operatorname{E}[\mathbf{Z}]\operatorname{E}[\mathbf{W}^T] |{{EquationRef|Eq.6}}}}

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:\operatorname{J}_{\mathbf{Z}\mathbf{W}} =

\begin{bmatrix}

\mathrm{E}[(Z_1 - \operatorname{E}[Z_1])(W_1 - \operatorname{E}[W_1])] & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])(W_2 - \operatorname{E}[W_2])] & \cdots & \mathrm{E}[(Z_1 - \operatorname{E}[Z_1])(W_n - \operatorname{E}[W_n])] \\ \\

\mathrm{E}[(Z_2 - \operatorname{E}[Z_2])(W_1 - \operatorname{E}[W_1])] & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])(W_2 - \operatorname{E}[W_2])] & \cdots & \mathrm{E}[(Z_2 - \operatorname{E}[Z_2])(W_n - \operatorname{E}[W_n])] \\ \\

\vdots & \vdots & \ddots & \vdots \\ \\

\mathrm{E}[(Z_n - \operatorname{E}[Z_n])(W_1 - \operatorname{E}[W_1])] & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])(W_2 - \operatorname{E}[W_2])] & \cdots & \mathrm{E}[(Z_n - \operatorname{E}[Z_n])(W_n - \operatorname{E}[W_n])]

\end{bmatrix}

Two complex random vectors \mathbf{Z} and \mathbf{W} are called uncorrelated if

:\operatorname{K}_{\mathbf{Z}\mathbf{W}}=\operatorname{J}_{\mathbf{Z}\mathbf{W}}=0.

Independence

{{main|Independence (probability theory)}}

Two complex random vectors \mathbf{Z}=(Z_1,...,Z_m)^T and \mathbf{W}=(W_1,...,W_n)^T are called independent if

{{Equation box 1

|indent =

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|equation = {{NumBlk||F_{\mathbf{Z,W}}(\mathbf{z,w}) = F_{\mathbf{Z}}(\mathbf{z}) \cdot F_{\mathbf{W}}(\mathbf{w}) \quad \text{for all } \mathbf{z},\mathbf{w}|{{EquationRef|Eq.7}}}}

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where F_{\mathbf{Z}}(\mathbf{z}) and F_{\mathbf{W}}(\mathbf{w}) denote the cumulative distribution functions of \mathbf{Z} and \mathbf{W} as defined in {{EquationNote|Eq.1}} and F_{\mathbf{Z,W}}(\mathbf{z,w}) denotes their joint cumulative distribution function. Independence of \mathbf{Z} and \mathbf{W} is often denoted by \mathbf{Z} \perp\!\!\!\perp \mathbf{W}.

Written component-wise, \mathbf{Z} and \mathbf{W} are called independent if

:F_{Z_1,\ldots,Z_m,W_1,\ldots,W_n}(z_1,\ldots,z_m,w_1,\ldots,w_n) = F_{Z_1,\ldots,Z_m}(z_1,\ldots,z_m) \cdot F_{W_1,\ldots,W_n}(w_1,\ldots,w_n) \quad \text{for all } z_1,\ldots,z_m,w_1,\ldots,w_n.

Circular symmetry

A complex random vector \mathbf{Z} is called circularly symmetric if for every deterministic \varphi \in [-\pi,\pi) the distribution of e^{\mathrm i \varphi}\mathbf{Z} equals the distribution of \mathbf{Z} .{{cite book |first=David |last=Tse |year=2005 |title=Fundamentals of Wireless Communication |publisher=Cambridge University Press}}{{rp|pp. 500–501}}

;Properties

  • The expectation of a circularly symmetric complex random vector is either zero or it is not defined.{{rp|p. 500}}
  • The pseudo-covariance matrix of a circularly symmetric complex random vector is zero.{{rp|p. 584}}

Proper complex random vectors

A complex random vector \mathbf{Z} is called proper if the following three conditions are all satisfied:{{rp|p. 293}}

  • \operatorname{E}[\mathbf{Z}] = 0 (zero mean)
  • \operatorname{var}[Z_1] < \infty , \ldots , \operatorname{var}[Z_n] < \infty (all components have finite variance)
  • \operatorname{E}[\mathbf{Z}\mathbf{Z}^T] = 0

Two complex random vectors \mathbf{Z},\mathbf{W} are called jointly proper is the composite random vector (Z_1,Z_2,\ldots,Z_m,W_1,W_2,\ldots,W_n)^T is proper.

;Properties

  • A complex random vector \mathbf{Z} is proper if, and only if, for all (deterministic) vectors \mathbf{c} \in \mathbb{C}^n the complex random variable \mathbf{c}^T \mathbf{Z} is proper.{{rp|p. 293}}
  • Linear transformations of proper complex random vectors are proper, i.e. if \mathbf{Z} is a proper random vectors with n components and A is a deterministic m \times n matrix, then the complex random vector A \mathbf{Z} is also proper.{{rp|p. 295}}
  • Every circularly symmetric complex random vector with finite variance of all its components is proper.{{rp|p. 295}}
  • There are proper complex random vectors that are not circularly symmetric.{{rp|p. 504}}
  • A real random vector is proper if and only if it is constant.
  • Two jointly proper complex random vectors are uncorrelated if and only if their covariance matrix is zero, i.e. if \operatorname{K}_{\mathbf{Z}\mathbf{W}} = 0.

Cauchy-Schwarz inequality

The Cauchy-Schwarz inequality for complex random vectors is

:\left| \operatorname{E}[\mathbf{Z}^H \mathbf{W}] \right|^2 \leq \operatorname{E}[\mathbf{Z}^H \mathbf{Z}] \operatorname{E}[|\mathbf{W}^H \mathbf{W}|].

Characteristic function

The characteristic function of a complex random vector \mathbf{Z} with n components is a function \mathbb{C}^n \to \mathbb{C} defined by:{{rp|p. 295}}

: \varphi_{\mathbf{Z}}(\mathbf{\omega}) = \operatorname{E} \left [ e^{i\Re{(\mathbf{\omega}^H \mathbf{Z})}} \right ] = \operatorname{E} \left [ e^{i( \Re{(\omega_1)}\Re{(Z_1)} + \Im{(\omega_1)}\Im{(Z_1)} + \cdots + \Re{(\omega_n)}\Re{(Z_n)} + \Im{(\omega_n)}\Im{(Z_n)} )} \right ]

See also

References