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4 - Expectation and averages

Published online by Cambridge University Press:  05 June 2012

Robert M. Gray
Affiliation:
Stanford University, California
Lee D. Davisson
Affiliation:
University of Maryland, College Park
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Summary

Averages

In engineering practice we are often interested in the average behavior of measurements on random processes. The goal of this chapter is to link the two distinct types of averages that are used – long-term time averages taken by calculations on an actual physical realization of a random process and averages calculated theoretically by probabilistic calculus at some given instant of time, averages that are called expectations. As we shall see, both computations often (but by no means always) give the same answer. Such results are called laws of large numbers or ergodic theorems.

At first glance from a conceptual point of view, it seems unlikely that long-term time averages and instantaneous probabilistic averages would be the same. If we take a long-term time average of a particular realization of the random process, say {X(t, ω0); tT}, we are averaging for a particular ω which we cannot know or choose; we do not use probability in any way and we are ignoring what happens with other values of ω. Here the averages are computed by summing the sequence or integrating the waveform over t while ω0 stays fixed. If, on the other hand, we take an instantaneous probabilistic average, say at the time t0, we are taking a probabilistic average and summing or integrating over ω for the random variable X(t0, ω).

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Publisher: Cambridge University Press
Print publication year: 2004

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  • Expectation and averages
  • Robert M. Gray, Stanford University, California, Lee D. Davisson, University of Maryland, College Park
  • Book: An Introduction to Statistical Signal Processing
  • Online publication: 05 June 2012
  • Chapter DOI: https://doi.org/10.1017/CBO9780511801372.006
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  • Expectation and averages
  • Robert M. Gray, Stanford University, California, Lee D. Davisson, University of Maryland, College Park
  • Book: An Introduction to Statistical Signal Processing
  • Online publication: 05 June 2012
  • Chapter DOI: https://doi.org/10.1017/CBO9780511801372.006
Available formats
×

Send book to Google Drive

To send content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about sending content to Google Drive.

  • Expectation and averages
  • Robert M. Gray, Stanford University, California, Lee D. Davisson, University of Maryland, College Park
  • Book: An Introduction to Statistical Signal Processing
  • Online publication: 05 June 2012
  • Chapter DOI: https://doi.org/10.1017/CBO9780511801372.006
Available formats
×