Cross-Correlation Function and Cross Power-Spectral Density
Contents
Definition of the cross-correlation function
In many engineering applications, one is interested in a quantitative measure to describe the statistical relatedness between different processes or between their pattern signals.
One such measure is the cross-correlation function (CCF), which is given here under the assumptions of stationarity and ergodicity .
$\text{Definition:}$ For the cross-correlation function of two stationary and ergodic processes with the pattern functions $x(t)$ and $y(t)$ holds:
- $$\varphi_{xy}(\tau)={\rm E} \big[{x(t)\cdot y(t+\tau)}\big]=\lim_{T_{\rm M}\to\infty}\,\frac{1}{T_{\rm M} }\cdot\int^{T_{\rm M}/{\rm 2} }_{-T_{\rm M}/{\rm 2} }x(t)\cdot y(t+\tau)\,\rm d \it t.$$
- The first defining equation characterizes the expected value formation (ensemble averaging),
- while the second equation describes the time averaging over a (as large as possible) measurement period $T_{\rm M}$
.
A comparison with the ACF definition shows many similarities. Setting $y(t) = x(t)$, we get $φ_{xy}(τ) = φ_{xx}(τ)$, i.e., the auto-correlation function, for which, however, in our tutorial we mostly use the simplified notation $φ_x(τ)$ .
$\text{Example 1:}$ We consider a random signal $x(t)$ with triangular ACF $φ_x(τ)$ ⇒ blue curve. This ACF shape results, for example.
- for a binary signal with equal probability bipolar amplitude coefficients $(+1$ resp. $-1)$ and
- for rectangular fundamental momentum.
We consider a second signal $y(t) = \alpha \cdot x (t - t_{\rm 0}),$ which differs from $x(t)$ only by an attenuation factor $(α =0.5)$ and a transit time $(t_0 = 3 \ \rm ms)$ ;
This attenuated and shifted signal has the ACF drawn in red.
- $$\varphi_{y}(\tau) = \alpha^2 \cdot \varphi_{x}(\tau) .$$
The shift around $t_0$ is not seen in the ACF in contrast to the cross correlation function (CCF) (shown in green) for which the following relation holds:
- $$\varphi_{xy}(\tau) = \alpha \cdot \varphi_{x}(\tau- t_{\rm 0}) .$$
Properties of the cross-correlation function
In the following, essential properties of the cross-correlation function are compiled and important differences to the ACF are elaborated.
- The formation of the cross correlation function is not commutative. Rather, there are always two distinct functions, viz.
- $$\varphi_{xy}(\tau)={\rm E} \big[{x(t)\cdot y(t+\tau)}\big]=\lim_{T_{\rm M}\to\infty}\,\frac{1}{T_{\rm M}}\cdot\int^{T_{\rm M}/{\rm 2}}_{-T_{\rm M}/{\rm 2}}x(t)\cdot y(t+\tau)\,\, \rm d \it t,$$
- $$\varphi_{yx}(\tau)={\rm E} \big[{y(t)\cdot x(t+\tau)}\big]=\lim_{T_{\rm M}\to\infty}\,\frac{1}{T_{\rm M}}\cdot\int^{T_{\rm M}/{\rm 2}}_{-T_{\rm M}/{\rm 2}}y(t)\cdot x(t+\tau)\,\, \rm d \it t .$$
- There is a relationship between the two functions $φ_{yx}(τ) = φ_{xy}(-τ)$. In $\text{example 1}$ of the last section, $φ_{yx}(τ)$ would have its maximum at $τ = -3 \ \rm ms$.
- In general, the maximum CCF does not occur at $τ = 0$ $($exception: $y = α - x)$ and the KKF value $φ_{xy}(τ = 0)$ does not have any special, physically interpretable meaning as in the ACF, where this value reflects the process power.
- The magnitude of the KKF is less than or equal to the geometric mean of the two signal powers according to the Cauchy-Schwarz inequality for all $τ$-values:
- $$\varphi_{xy}( \tau) \le \sqrt {\varphi_{x}( \tau = 0) \cdot \varphi_{y}( \tau = 0)}.$$
- In $\text{Example 1}$ on the last page, the equal sign applies:
- $$\varphi_{xy}( \tau = t_{\rm 0}) = \sqrt {\varphi_{x}( \tau = 0) \cdot \varphi_{y}( \tau = 0)} = \alpha \cdot \varphi_{x}( \tau = {\rm 0}) .$$
- If $x(t)$ and $y(t)$ do not contain a common periodic fraction, the limit of the CCF for $τ → ∞$ gives the product of both means:
- $$\lim_{\tau \rightarrow \infty} \varphi _{xy} ( \tau ) = m_x \cdot m_y .$$
- If two signals $x(t)$ and $y(t)$ are uncorrelated, then $φ_{xy}(τ) ≡ 0$, that is, it is $φ_{xy}(τ) = 0$ for all values of $τ$. For example, this assumption is justified in most cases when considering a useful signal and an interfering signal together.
- It should always be noted, however, that the CCF includes only the linear statistical bindings between the signals $x(t)$ and $y(t)$ . Bindings of other types - such as for the case $y(t) = x^2(t)$ - on the other hand, are not taken into account in the CCF formation.
Applications of the cross correlation function
The applications of the cross-correlation function in message systems are many. Here are some examples:
$\text{Example 2:}$ In Amplitude Modulation, but also in BPSK systems (Binary Phase Shift Keying), the so-called synchronous demodulator is very often used for demodulation (resetting the signal to the original frequency range), whereby a carrier signal must also be added at the receiver, and this must be frequency and phase synchronous to the transmitter. If one forms the CCF between the receive signal and the receive carrier signal, the phase synchronous position between the two signals can be recognized by means of the peak of the KKF, and it can be readjusted in case of drifting apart
.
$\text{Example 3:}$ The multiple access method CDMA (Code Division Multiple Access) is used, for example, in the mobile radio standard UMTS . It requires strict phase synchronism, with respect to the added pseudonoise sequences at the transmitter (bandspread) and at the receiver (bandspread) (Bitte um bessere Übersetzung). This synchronization problem is also usually solved using the cross-correlation function.
$\text{Example 4:}$ The cross-correlation function can be used to determine whether or not a known signal $s(t)$ is present in a noisy received signal $r(t) = α - s(t - t_0) + n(t)$ and if so, at what time $t_0$ it occurs. From the calculated value for $t_0$ then, for example, a driving speed can be determined (radar technique). This task can also be solved with the so-called matched filter, which is still described in a later chapter and which has many similarities with the cross-correlation function.
$\text{Example 5:}$ In the so-called Correlation receiver one uses the CCF for signal detection. Here one forms the cross-correlation between the received signal distorted by noise and possibly also by distortions $r(t)$ and all possible transmitted signals $s_i(t)$, where for the running index $i = 1$, ... , $I$ shall hold. Deciding $N$ binary symbols together, then $I = {\rm 2}^N$. One then decides on the symbol sequence with the largest CCF value, achieving the minimum error probability according to the maximum likelihood decision rule.
Kreuzleistungsdichtespektrum
Für manche Anwendungen kann es durchaus vorteilhaft sein, die Korrelation zwischen zwei Zufallssignalen im Frequenzbereich zu beschreiben.
$\text{Definition:}$ Die beiden Kreuzleistungsdichtespektren ${\it Φ}_{xy}(f)$ und ${\it Φ}_{yx}(f)$ ergeben sich aus den dazugehörigen Kreuzkorrelationsfunktionen $\varphi_{xy}({\it \tau})$ bzw. $\varphi_{yx}({\it \tau})$ durch die Fouriertransformation:
- $${\it \Phi}_{xy}(f)=\int^{+\infty}_{-\infty}\varphi_{xy}({\it \tau}) \cdot {\rm e}^{ {\rm -j}\pi f \tau} \rm d \it \tau, $$
- $${\it \Phi}_{yx}(f)=\int^{+\infty}_{-\infty}\varphi_{yx}({\it \tau}) \cdot {\rm e}^{ {\rm -j}\pi f \tau} \rm d \it \tau.$$
Manchmal wird hierfür auch der Begriff spektrale Kreuzleistungsdichte verwendet.
Es gilt hier der gleiche Zusammenhang wie
- zwischen einem deterministischen Signal $x(t)$ und seinem Spektrum $X(f)$ bzw.
- zwischen der Autokorrelationsfunktion ${\it φ}_x(τ)$ eines ergodischen Prozesses $\{x_i(t)\}$ und dem dazugehörigen Leistungsdichtespektrum ${\it Φ}_x(f)$.
Ebenso beschreibt hier die Fourierrücktransformation ⇒ „Zweites Fourierintegral” den Übergang vom Spektralbereich in den Zeitbereich.
$\text{Beispiel 6:}$ Wir nehmen Bezug zum $\text{Beispiel 1}$ mit den beiden „rechteckförmigen Zufallsgrößen” $x(t)$ und $y(t) = α · x(t – t_0)$.
Da die AKF ${\it φ}_x(τ)$ dreieckförmig verläuft, hat – wie im Kapitel Leistungsdichtespektrum beschrieben – das LDS ${\it Φ}_x(f)$ einen ${\rm si}^2$-förmigen Verlauf.
Welche Aussagen können wir allgemein aus dieser Grafik für die Spektralfunktionen ableiten?
- Im zitierten $\text{Beispiel 1}$ haben wir festgestellt, dass sich die Autokorrelationsfunktion ${\it φ}_y(τ)$ von ${\it φ}_x(τ)$ nur um den konstanten Faktor $α^2$ unterscheidet.
- Damit ist klar, dass das Leistungsdichtespektrum ${\it Φ}_y(f)$ von ${\it \Phi}_x(f)$ ebenfalls nur um diesen konstanten Faktor $α^2$ abweicht. Beide Spektralfunktionen sind reell.
- Dagegen besitzt das Kreuzleistungsdichtespektrum einen komplexen Funktionsverlauf:
- $${\it \Phi}_{xy}(f) ={\it \Phi}^\star_{yx}(f)= \alpha \cdot {\it \Phi}_{x}(f) \hspace{0.05cm}\cdot {\rm e}^{- {\rm j } \hspace{0.02cm}\pi f t_0}.$$
Aufgaben zum Kapitel
Aufgabe 4.14: AKF und KKF bei Rechtecksignalen
Aufgabe 4.14Z: Auffinden von Echos