Re: [Discuss-gnuradio] USRP noise at higher frequencies
> > Hi, > > > > I posted about this before (High frequency OFDM), but could not get a > > reply. I tried my best, but am completely stuck by now and would > > accept any wild guesses. > > > > > > I have two USRPs (TVRX and WBX-TX) connected via cable that transmit > > a OFDM signal (courtesy of gnuradio-examples/python/ofdm, default > > parameters) in between them. I have manually taken care that the > > carrier frequency is centered correctly on the receiver and that the > > amplitudes at both the transmitter and receiver are configured > > optimally. > > > > Now when I transmit said OFDM signal at around 100MHz, everything is > > looking perfectly fine. If I look at the transmitted and received > > signal, I see a perfectly sharp edged OFDM block in the spectrum and > > get 100% of the data from one USRP to the other. Nice. When I switch > > that same configuration into higher areas of the spectrum (400MHz+), > > again adjusting carrier-frequency and amplitudes, I also see the same > > sharp edged blocks. The SNR is a little worse, but still well beyond > > 30dB. This is where my problem lies. For some reason, most of the > > packets are broken or bursts are missing completely. If I switch to > > wider subcarrier distance, the transmission works again - but that > > isn't what I am looking for. > > > > > > 1) What happens at higher frequencies that "invisibly" destroys the > > OFDM block? Phase noise? Impulsive noise? Something to do with the > > sampling rate conversion? Any tips or pointers would be of great > > help. > > > > 2) How could I model a high frequency transmission (including phase > > noise) in GNU Radio, so that I can more easily see the problem? > > > This is due to frequency offset. Oscillator error is in parts per > million (roughly 5 to 10 ppm for a USRP1). The higher the carrier > frequency, the higher the absolute error. At 100 MHz, 10 ppm error is 1 > kHz. At 400 MHz, 10 ppm error is 4 kHz. > > The code you are using only works at a limited frequency error. The > allowable frequency error is proportional to the tone spacing. So when > you go to higher tone spacing it is more tolerant of the higher > frequency error, and that is why it works. > > What you need to do is modify the code to search for the signal over a > wider frequency band. I thought I took care of this by manually centering the spectrum (I do this at every frequency). I do this: 1.Launch the OFDM transmitter at 400MHz 2.Launch a receiver at 400MHz with a GNU Radio fft-sink and write down the constant offset (say 400.021MHz) 3.Launch the OFDM receiver at 400.021MHz So the OFDM signal is centered perfectly, but still I only receive broken packets. Or do you mean something else? Is the narrow band OFDM signal maybe distorted in some fashion and at >400MHz can not be properly recovered anymore? > Or use the wider tone spacing. That is a perfectly valid solution that works 100%, but actually I am just wondering why the narrow band transmission that works fine at <100MHz does not work anymore at >400MHz. -- GMX DSL: Internet-, Telefon- und Handy-Flat ab 19,99 EUR/mtl. Bis zu 150 EUR Startguthaben inklusive! http://portal.gmx.net/de/go/dsl ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] USRP noise at higher frequencies
Hi, I posted about this before (High frequency OFDM), but could not get a reply. I tried my best, but am completely stuck by now and would accept any wild guesses. I have two USRPs (TVRX and WBX-TX) connected via cable that transmit a OFDM signal (courtesy of gnuradio-examples/python/ofdm, default parameters) in between them. I have manually taken care that the carrier frequency is centered correctly on the receiver and that the amplitudes at both the transmitter and receiver are configured optimally. Now when I transmit said OFDM signal at around 100MHz, everything is looking perfectly fine. If I look at the transmitted and received signal, I see a perfectly sharp edged OFDM block in the spectrum and get 100% of the data from one USRP to the other. Nice. When I switch that same configuration into higher areas of the spectrum (400MHz+), again adjusting carrier-frequency and amplitudes, I also see the same sharp edged blocks. The SNR is a little worse, but still well beyond 30dB. This is where my problem lies. For some reason, most of the packets are broken or bursts are missing completely. If I switch to wider subcarrier distance, the transmission works again - but that isn't what I am looking for. 1) What happens at higher frequencies that "invisibly" destroys the OFDM block? Phase noise? Impulsive noise? Something to do with the sampling rate conversion? Any tips or pointers would be of great help. 2) How could I model a high frequency transmission (including phase noise) in GNU Radio, so that I can more easily see the problem? -- GMX DSL: Internet-, Telefon- und Handy-Flat ab 19,99 EUR/mtl. Bis zu 150 EUR Startguthaben inklusive! http://portal.gmx.net/de/go/dsl ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] High frequency OFDM
> > Hi, > > > > I successfully implemented OFDM with two USRPs with LFRX/LFTX at > 1-50MHz, connect by cable. Now I tried switching over to higher frequencies > via > TVRX/WBX. While I receive a perfectly valid looking spectrum with a good SNR, > it does not seem to decode at >200MHz. > > If I increase the sub-carrier spacing of the transmission and thus the > bandwidth, it works again. But why do I have to do that if the signal and > SNR look okay? > > > > What destroys a OFMD signal at high frequencies? Is it phase noise? Is > the USRPs frequency resolution different at higher frequencies? > > > > Any literature/papers on this topic would also be helpful - I am kind of > stuck here. > > It's probably due to the frequency offset. Since the local oscillator > has a certain offset at its nominal frequency, that offset is > multiplied when the signal is multiplied to a higher frequency. So the > higher the frequency, the more you are offset. In the case of the OFDM > signals, you are now farther off frequency than the lock-in range of > the receiver. > > You already found one fix for this, which is making the subcarriers > larger. The other thing to do is look at the frequency offset between > the two devices and try to compensate for it. > > There was a discussion about this a few months ago that you can look > for that explains a way to handle larger offsets better than the look > and tune method, which, while it works, isn't a realistic solution. > > Tom Thanks for the reply! Sadly I already noticed and compensated for the frequency offset you mentioned and manually centered the frequency block that OFDM consists of. I also adjusted the respective levels, so clipping should not be an issue as well. I just wonder why you can't use my OFDM configuration (that works perfectly at low frequencies) at a higher frequency. My two USRPs are connected via cable and the SNR and signal spectrum look good, so it should work, shouldn't it? :( -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01 ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] High frequency OFDM
Hi, I successfully implemented OFDM with two USRPs with LFRX/LFTX at 1-50MHz, connect by cable. Now I tried switching over to higher frequencies via TVRX/WBX. While I receive a perfectly valid looking spectrum with a good SNR, it does not seem to decode at >200MHz. If I increase the sub-carrier spacing of the transmission and thus the bandwidth, it works again. But why do I have to do that if the signal and SNR look okay? What destroys a OFMD signal at high frequencies? Is it phase noise? Is the USRPs frequency resolution different at higher frequencies? Any literature/papers on this topic would also be helpful - I am kind of stuck here. -- GMX DSL: Internet-, Telefon- und Handy-Flat ab 19,99 EUR/mtl. Bis zu 150 EUR Startguthaben inklusive! http://portal.gmx.net/de/go/dsl ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] Freeze when writing to linux fifo pipe
Hi, I am trying to write to a linux pipe/fifo, but sadly this seems to freeze python. I am using Ubuntu 10.04 and gnuradio 3.2.2.dfsg-1ubuntu1. Please try to execute the code and tell me why this makes the app pause. #! /usr/bin/python # Import stuff from gnuradio import gr, gru, eng_notation, optfir from gnuradio.eng_option import eng_option from gnuradio.wxgui import slider, powermate from gnuradio.wxgui import stdgui2, fftsink2, form, scopesink2 import sys import os # Main application class main_app (stdgui2.std_top_block): def __init__(self,frame,panel,vbox,argv): stdgui2.std_top_block.__init__ (self,frame,panel,vbox,argv) print '!!!Cleanup' os.spawnlp(os.P_WAIT, 'rm', 'rm','/tmp/gnutest1.raw') os.spawnlp(os.P_WAIT, 'rm', 'rm','/tmp/gnutest2.raw') print '!!!make a fifo' os.spawnlp(os.P_WAIT, 'mkfifo', 'mkfifo','/tmp/gnutest1.raw') print '!!!make file sink without the fifo' outok = gr.file_sink(gr.sizeof_short,'/tmp/gnutest2.raw') print '!!!make file sink with the fifo (FREEZING)' outcrash = gr.file_sink(gr.sizeof_short, '/tmp/gnutest1.raw') print '!!!everything went better than expected' if __name__ == '__main__': app = stdgui2.stdapp (main_app, "crashtest") app.MainLoop () -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01 ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
