Hi Marcus, Thank you for kind reply. Actually I want to calculate the sum-Signal to sum-Noise ratio, as you have referred Parseval to calculate it.
· But the problem is I don't know how to estimate the noise power of the receive signal. · If I want to change the SNR, can I add a Noise Source to the USRP Source to change it? Many thanks, Yan From: Discuss-gnuradio [mailto:discuss-gnuradio-bounces+eexyh22=nottingham.ac...@gnu.org] On Behalf Of Marcus Müller Sent: 29 April 2016 19:20 To: discuss-gnuradio@gnu.org Subject: Re: [Discuss-gnuradio] How to test and change SNR in GNURadio by B210 Dear Yan, 2.Can I use 'MPSK USRP Estimator' to test SNR to test SNR in this situation? OFDM containing BPSK symbols is not a PSK modulation in time domain; so you're using the wrong estimator as is. Therefore: 1.why the SNR I get is nearly 0(about 10^-3)? If I increase the noise amplitude, the SNR increase slightly(about 10^-1), I think it will decrease as noise increase. Because you're sending something that to the wrong receiver should look like wideband, not-quite-white noise. More energy from a white noise source seems to accidentally increase the SNR estimate. 3.How can I read the SNR information by using 'MPSK USRP Estimator prob'? Not at all, if you want to use OFDM. Generally, this is a very specific estimator for a very specific problem. In multi-carrier system, definining a single SNR is challenging theoretically - and questionable in effect, too. What's the bandwidth of your signal if you've got unoccupied carriers? What's the signal power? Do you look at noise and signal power for the whole system as one, or do you consider the individual carriers individually? If individually, is overall SNR the minimum subcarrier SNR, a sum-Signal to sum-Noise ratio (for OFDM, Parseval would make that simple), or is it maybe some non-linear combination that might make more sense when considering the overall system? And: What is the meaning of SNR here, and what do you intend to use it for? Wouldn't [$\frac{\text E_b}{N_0}$] make more sense? If it does, what is [$N_0$] in the constellation domain (ie. after reversing the OFDM modulation)? Of course, you can OFDM-demodulate (read: DFT) the receiver signal and put it through the MPSK SNR estimator; but that would mean that you'll have to do all the mathematical derivation of how that SNR relates to its input. Notice that if the noise your channel adds isn't really white for all DFT bins (it's probably not, because OFDM in practice isn't infinitely steep at the edges, and due to some effects the subcarriers might not be perfectly orthogonal, either, adding self-interference), you might end up with more noise in some subcarriers than others. BER-over-SNR curves tend to be nonlinear, so this might pose a problem for the interpretation of the SNR as tool to estimate the BER. I haven't actually measured that effect, and to be honest, I don't think it's going to be terrible. What's more important, however, is that you use OFDM exactly when your channel is non-flat; that's because dividing your overall bandwidth into smaller subchannels allows for easier per-subchannel equalization. Now, that means that the signal power will vary between different subcarriers in a situation where you'd prefer OFDM over single-carrier systems - and that makes it necessary to estimate the signal power per subcarrier, which means you will very likely have different SNRs for different subcarriers. Best regards, Marcus On 29.04.2016 19:02, Yan Huang wrote: Hi all, I'm using USRP B210 to employ spectrum sensing, I want to plot ROC curve of the sensing output signal. So I need to test and change the SNR of the receive signal. The attached picture is the flowgraph I use. I choose 'MPSK USRP Estimator' to test SNR since my tx signal is BPSK. The problem is: 1.why the SNR I get is nearly 0(about 10^-3)? If I increase the noise amplitude, the SNR increase slightly(about 10^-1), I think it will decrease as noise increase. 2.Can I use 'MPSK USRP Estimator' to test SNR to test SNR in this situation? 3.How can I read the SNR information by using 'MPSK USRP Estimator prob'? Many thanks in advance. Kind regards, Yan This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. 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