# find the dependence ratio

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son on 29 Aug 2014
hi, i have a graph above. Now i want to comment in this graph. For example, if the channels increase 4 times, the OSNR decrease 'x' times. What function should i use to find 'x'?

Salaheddin Hosseinzadeh on 29 Aug 2014
Dear Son,
You can make a function if you have a series of measurements.
You can compare results with a base signal, and then find a equation for it using cftool
Lets say channel(ch) = 2 then OSNR reduced 1
ch = 3, ONSR = 2
ch = 4, OSNR = 3
and so on
What you need to do is then go to the cftool
cftool
then assign the x and y data
x is the ch
y is the OSNR
Find a fit for it, whichever method you prefer.
Then you have an equation or relation between ch and OSNR which allows you to predict and tell the dependency of CH and OSNR.
Hope this helps
Good Luck!
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son on 29 Aug 2014
hello could u please explain the result:
Linear model Poly2:
f(x) = p1*x^2 + p2*x + p3
Coefficients (with 95% confidence bounds):
p1 = -0.1474 (-0.3859, 0.09112)
p2 = 5.23 (0.5256, 9.934)
p3 = 5.431 (-11.6, 22.47)
Goodness of fit:
SSE: 0.3463
R-square: 0.9994
RMSE: 0.5884
Salaheddin Hosseinzadeh on 29 Aug 2014
That's good Son,
So you managed to find a polynomial fit with max degree of 2
Now you have the equation which is
f(X) = (-14.74)X^2 + 5.23X + 5.431
Where x is I guess number of channels

Salaheddin Hosseinzadeh on 29 Aug 2014
Hi Son,
I guess you want to compare between your signals to see how close the are! At least that what I understood from your unclear question.
To see this you need to use Xcorr I believe to see the correlation between your signals.
To find the corss correlation in MATLAB you can either use crosscorr function or xcorr
doc crosscorr
doc xcorr
type those above in MATLAB to see the matlab documentation about them, they are straightforward to use.
If this is not what you're looking for just let us know.
Good Luck.
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Salaheddin Hosseinzadeh on 29 Aug 2014
Then what is the problem?
Find the max or mean of one graph and compare it to the min or max of the other graph!
By comparing I mean either division or subtraction, depending on what's your scale? linear or logarithmic. Which yours is in dB.
son on 29 Aug 2014
yeah, but i want to find a function to show this dependence. is it possible?
if i know that function, i can find the OSNR decrease for the number of channels increase 2.5 WITHOUT plot a graph.