> > 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
