How To measure SNR value

[powerpuffgirl]

Hi all
I am doing my project work on ad hoc networks(AODV protocol). I need to measure SNR(Signal to Noise Ratio) value for simulation in NS-2. Can any one tell how i could measure SNR without hardware support.

Thanks for any reply

[Team Leader]

Here are two ways:

1. By taking the signal (S) to be the mean pixel intensity value in a region of interest (ROI) minus any offset, and the noise to be the standard deviation (σ) in pixel intensity in the same ROI. [Another method, similar to this is to use an ROI for signal and another ROI over just air for the noise SD.] SNR is calculated using SNR = 0.655∙S/σ. The 0.655 factor is due to the Rayleigh distribution of the background noise in a magnitude image.

2. If the image homogeneity is not considered to be good, then the SNR may be derived more accurately using the following method. Two images should be acquired by consecutive scans with identical receive and transmitter settings. The images should then be subtracted one from the other, to generate a third pixel-by-pixel difference image. [The only difference between the two original images should be due to noise, provided the image has not suffered from ghosting or any other instability.] So we now have two original images, and a subtracted image.
Using either of the original images the signal (S) is again defined as the mean pixel intensity value in an ROI minus any offset. Take a mean value of up to five ROIs.
The noise is the standard deviation (σnoise) in a ROI on the subtracted image. Again take a mean value from the same ROIs.
The signal to noise ratio is determined using SNR = √2∙S/σnoise, where the factor of √2 arises due to the fact that the standard deviation is derived from the subtraction image and not from the original image.

Source: revisemri.com

[powerpuffgirl]

Here are two ways:

1. By taking the signal (S) to be the mean pixel intensity value in a region of interest (ROI) minus any offset, and the noise to be the standard deviation (σ) in pixel intensity in the same ROI. [Another method, similar to this is to use an ROI for signal and another ROI over just air for the noise SD.] SNR is calculated using SNR = 0.655∙S/σ. The 0.655 factor is due to the Rayleigh distribution of the background noise in a magnitude image.

2. If the image homogeneity is not considered to be good, then the SNR may be derived more accurately using the following method. Two images should be acquired by consecutive scans with identical receive and transmitter settings. The images should then be subtracted one from the other, to generate a third pixel-by-pixel difference image. [The only difference between the two original images should be due to noise, provided the image has not suffered from ghosting or any other instability.] So we now have two original images, and a subtracted image.
Using either of the original images the signal (S) is again defined as the mean pixel intensity value in an ROI minus any offset. Take a mean value of up to five ROIs.
The noise is the standard deviation (σnoise) in a ROI on the subtracted image. Again take a mean value from the same ROIs.
The signal to noise ratio is determined using SNR = √2∙S/σnoise, where the factor of √2 arises due to the fact that the standard deviation is derived from the subtraction image and not from the original image.

Source: revisemri.com

Thanks for ur reply but sorry to say that i dint understand that. Can u explain me in detail. from which grapgh we can find ROI and all.