SOURCEFORGE » RALF » 2015 Quantitative analysis of RF signals

h3. 1. Definition of inputs and ways to compute the outputs - Theory

To be able to use the USRP, we first need to set it in LabView. We have to configure the device name (RIO0 with the PCI express card), the IQ sampling rate (twice the symbol rate), the carrier frequency and the antenna to use on the USRP. These parameters are really importants because if we put for example a wrong sampling rate, we will not be able to observe a good constellation.

Once we have our signal coming from the USRP, we have to compute the expected results. To observe the spectrum of the signal we just need to put the power spectrum block of LabView with the good parameters. This allows us to compute the power of the signal and the power of the noise. Indeed if we set the carrier frequency of the USRP centered on our signal, we just have to do a peak detection to measure the power of the signal plus the noise. Then we have the power of the noise in taking a carrier frequency completely out the bandwidth of the satellite. Now we have the power of the noise and the power of the signal so we can easily compute the signal to noise ratio (SNR).

Now, let’s have a look to digital results. We want to observe the constellation of the signal and some results like mean error vector magnitude or mean phase error. Our signal analyser does not determine automatically what is the constellation used to transmit the signal, we have to know it before and to enter it in the program as a parameter. Then we can observe the received constellation, the mean error vector magnitude, the mean phase error, and the mean magnitude error. 

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+Figure 1: Measures explanation scheme+