Hey guys, Please pardon my ignorance, I am trying to learn everything I need for DSP / OFDM on the fly. But I have noticed something with this OFDM gnuradio example. If I take the example and run it directly into the usrp without changing any variables, I get a 350 KHz wide signal out. Now the sample rate is only 100k, the occupied carriers are -26 to 26 (zero omitted) with pm 21 and pm 7 as pilot carriers. The fft length is 64.
If the bandwidth is directly related to the sampling rate how am I getting 3x the bandwidth at low sample rates. For some further comparison I changed only the sample rate to a few other values, here is what I observed: Sample Rate : Observed Bandwidth 100k : 350 KHz 50k : 350 KHz 1M : 840 KHz 20M : 17 MHz The 1M and 20M rates make sense but I don't understand what is happening with the 100k and 50k rates. Thank you for the help. ----------------------------- Jacob Knoles On Wed, May 9, 2018 at 6:22 PM Marcus D. Leech <mle...@ripnet.com> wrote: > On 05/09/2018 07:57 PM, Jacob Knoles wrote: > > Thanks for the quick reply guys. > > Marcus the Re-sampling option makes perfect sense, and I believe, in > theory, since I am writing data to a file for later use I could interpolate > it just before writing then read it out at the usrp sample rate, right? > > Yes. > > > Ian, very interesting suggestion. I will have to give it a try. Thanks for > the input. And since I am doing all of the heavy processing prior to tx'ing > I don't image this change will create too great of a burden on the CPU. As > for reading from the file, I am just creating a small data set which gets > loaded into memory and repeated. > > Thanks!! > ----------------------------- > Jacob Knoles > > > > On Wed, May 9, 2018 at 4:21 PM Ian Buckley via USRP-users < > usrp-users@lists.ettus.com> wrote: > >> >> > On May 9, 2018, at 4:07 PM, Marcus D. Leech via USRP-users < >> usrp-users@lists.ettus.com> wrote: >> > >> > On 05/09/2018 06:53 PM, Jacob Knoles via USRP-users wrote: >> >> Hello All, >> >> >> >> I am trying to generate OFDM signals of various bandwidths using the >> X300 (UBX-160), particularly 20/40/80 and 160 MHz bandwidths. >> >> I have used the gnuradio ofdm_tx.grc example file to generate a data >> file which I then feed into the USRP an monitor on a spectrum analyzer. >> >> >> >> To quickly note, I do not care about the data being transferred, it >> will not be received or demodulated in any way and is simply an interfering >> signal. >> >> >> >> At this time I can produce a 20 MHz wide OFDM signal as well as a 100 >> MHz wide signal (?) but the 40/80 MHz signals are rounded and look more >> like an 802.11b signal. >> >> >> >> I have noted a message from the X300 that the requested sample rates >> (40/80 MS respectively) cannot be achieved due to the 200/x ratio being odd. >> >> >> >> So my question is this, how do I decouple the USRP's sample rate with >> the bandwidth of the signal I am trying to produce? >> >> To put it another way, I produce a data file at 40 MS/s rate then run >> it on the X300 at 100 MS/s and I get a 100 MHz wide signal instead of the >> 40 MHz I want. >> >> >> >> Thanks for the help. >> >> ----------------------------- >> >> Jacob Knoles >> >> >> > You would need to interpolate it up to the desired rate. UHD has no >> way of knowing that your samples represent data sampled at 40Msps, so when >> you >> > pull it out of your file at 100Msps, it will get presented as if it >> were 100Msps data. >> > >> > You'll need to use some DSP code, or Gnu Radio to up-sample your sample >> file. >> > >> ….or perhaps generate it off line using a non 2^n Fourier transform size >> that targets the USRP sample rate…for example instead of 64 bins @ 40MHz, >> 80 bins @ 50MHz, >> With zero data in the extra outlying bins (as you would have anyway in >> other bins). Might get interesting getting high bitrates out of a file, but >> equally, high bitrate M:N sample rate conversion will also be tricky for CPU >> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> > >
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