Random modulation: Difference between revisions

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{{Short description|Concept in signal processing}}

In the theories of [[modulation]] and of [[stochastic process]]es, '''random modulation''' is the creation of a new signal from two other signals by the process of [[quadrature amplitude modulation]]. In particular, the two signals are considered as being [[random process]]es. For applications, the two original signals need have a limited frequency range, and these are used to modulate a third sinusoidal ([[carrier) signal]] whose frequency is above the range of frequencies contained in the original signals.

==Details==

The random modulation procedure starts with two stochastic [[Baseband_signalBaseband signal#Baseband_signalBaseband signal|baseband signals]], <math>x_c(t)</math> and <math>x_s(t)</math>, whose [[frequency spectrum]] is non-zero only for <math>f \in [-B/2,B/2]</math>. It applies [[quadrature modulation]] to combine these with a carrier frequency <math>f_0</math> (with <math>f_0 > B/2</math>) to form the signal <math>x(t)</math> given by

:<math>x(t)=x_c(t)\cos(2 \pi f_0 t)-x_s(t)\sin(2 \pi f_0 t)= \Re \left \{ \underline{x}(t)e^{j 2 \pi f_0 t}\right \} ,</math>

where <math>\underline{x}(t)</math> is the [[Baseband_signalBaseband signal#Equivalent_baseband_signalEquivalent baseband signal|equivalent baseband representation]] of the modulated signal <math>x(t)</math>

:<math>\underline{x}(t)=x_c(t)+j x_s(t).</math>

In the following it is assumed that <math>x_c(t)</math> and <math>x_s(t)</math> are two real jointly [[Wide_sense_stationaryWide sense stationary#Weak_or_wideWeak or wide-sense_stationaritysense stationarity|wide sense stationary]] processes. It can be shown{{citation needed|date=August 2011}} that the new signal <math>x(t)</math> is wide sense stationary [[iff|if and only if]] <math>\underline{x}(t)</math> is circular complex, i.e. if and only if <math>x_c(t)</math> and <math>x_s(t)</math> are such that

:<math>R_{x_c x_c}(\tau)=R_{x_s x_s}(\tau) \qquad \text{and }\qquad R_{x_c x_s}(\tau)=-R_{x_s x_c}(\tau).</math>

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== Bibliography ==

*{{en}}{{cite book |title=Probability, random variables and stochastic processes |last1= Papoulis|first1= Athanasios|authorlink1author-link1= Athanasios Papoulis|first2=S. Unnikrishna|last2= Pillai |year= 2002|publisher= McGraw-Hill Higher Education|edition= 4th|chapter=Random walks and other applications|pages=463–473}}

*{{it}}{{cite book |title=Segnali, Processi Aleatori, Stima |last1= Scarano|first1= Gaetano|year= 2009|publisher= Centro Stampa d'Ateneo|language=it}}

*{{Cite journal | last1 = Papoulis | first1 = A. | doi = 10.1109/TASSP.1983.1164046 | title = Random modulation: A review | journal = IEEE Transactions on Acoustics, Speech, and Signal Processing | volume = 31 | pages = 96–105| year = 1983 }}

*{{cite doi|10.1109/TASSP.1983.1164046}}

{{DEFAULTSORT:Random Modulation}}

[[Category:SignalStatistical signal processing]]

[[Category:Time series analysis]]