# Autocorrelation of a signal with gaps

I am trying to find the analytic expression of the autocorrelation function for a signal $X(t)$ which is defined as follows.

• $X(t = 0) = 1$
• At random times jumps to $X(t) = 0$ happen
• The jump probability in each infinitesimal time-step is $\Gamma dt$
• After the jump the signal stays at $0$ for time $\tau$
• Then the signal jumps back to $1$

Do you know how or even if it is possible to calculate the autocorrelation function of such a signal?

The signal should describe the following scenario:

Imagine you have a house with a window. In front of the window there is a railway where trains can pass. Each train takes a fixed time $\tau$ to pass the window and trains arrive at random times. Assume that I am pointing a laser across the railway trough the window. If I can see the laser $X(t) = 1$ else the laser is hidden by the train and $X(t) = 0$. I want to calculate the autocorrelation function of such a signal.

• Is $\tau$ fixed and deterministic? – Matt L. Jun 11 '14 at 9:26
• To little informations about your signal to give specific answer, but you might find this useful: Random Rectangular Pulse Train ACF – jojek Jun 11 '14 at 9:35
• @physicsGuy: Most importantly please answer Matt's question - it's crucial for understanding. – jojek Jun 11 '14 at 9:54
• Apart from all the other matters discussed in the comments, a very important issue is "What is meant by At random times jumps to...?" There is no such thing as a random number, or a positive random number, unless the distribution is specified: the common response that "random means that all values are equally likely to be chosen" does not work. For example, in the link pointed out by @jojek, the random time is uniformly distributed on $[-T/2,T/2]$. For the problem posed here, "random means $U(0,T)$ gives vastly different results than "random means $\text{Exponential}(\lambda)$". – Dilip Sarwate Jun 11 '14 at 11:42
• It might be fruitful to consider your signal as a convolution of a boxcar (rectangular) filter with a signal of Dirac delta pulses. The time between delta pulses can be no less than $\tau$ – Mark Borgerding Jun 11 '14 at 13:07