On Tue, Feb 21, 2012 at 9:14 AM, Michael Dickens <m...@alum.mit.edu> wrote:
> Hi Florian - Interesting observation about better reception when using > larger bandwidths. I tried it out, and yes indeed they do seem better at 1 > MS/s compared with 250 or 500 kS/s -- meaning that more packets are > received correctly at the higher rate than the lower rates. I didn't try > > 1 MS/s yet, but I will later. I need to review my OFDM to get a better > clue as to why this might be the case. > It's because with the larger bandwidth, the subcarriers, too, have a larger bandwidth. The coarse frequency correction is only set to look at so large an offset based on a number of subcarriers (+/-5 or 10), so now with the same frequency offset, 5 (or 10) carriers is a larger frequency span to check. > Though this is interesting and useful, my problem wasn't the packet error > rate. > > My issue was that the Tx center frequency was 1 MHz high with a center > frequency of 5 GHz [1], no matter which USRP1 I used for Tx, when using GNU > Radio -> UHD. When I wrote my own C++ program to use UHD for doing Tx of a > real sinusoid -- "real" so as to see both the + and - spikes, so as to be > able to determine where the center frequency is -- everything centered > correctly when I set the UHD frequency to 5 GHz [2]. > > I don't have time right now to investigate further. Mostly throwing this > out to see if anyone else had encountered it. - MLD > > [1] 1 MHz off at 5 GHz is 200 ppm (yes?). IIRC a reasonable spec on these > boards is 5-10 ppm. So, > 10x off the spec? I doubt it. My guess is > it's something in GNU Radio that wasn't fixed when the benchmarks were > transitioned from gr-usrp* to UHD. > > [2] The offset was ~1 kHz, or ~0.2 ppm, which is well within hardware spec. I think this is a known problem with the XCVR that has been fixed. Make sure you have the most up-to-date master branch of both the UHD and GNU Radio. Tom > On Feb 20, 2012, at 12:23 PM, Florian Schlembach wrote: > > we encountered a very similar issue and spent already a lot of time on > that. Configuration is the same except we are using an USRP2 though. > > > > We also did not receive anything at the RX side although the spectrum > looked quite good as we have checked that via usrp_fft.py. > > Ultimately, we found out that it was about the sampling rate. We were > able to receive something with a sampling rate (respective bandwidth ) from > 1.5 MSamples/s, better 2 MS/s. > > We are assuming that this might be a decimation rate issue. We facing > the same problems with both a XCVR2450 and an RFX daugherboards. We checked > whether lower sampling rates are working with both daughterboards (with > another SDR implementation called Iris, also uses UHD) and they do! Thus, > this might eventually be a problem of GNU Radio. > > > > Could you check whether you are receiving something with higher sampling > rates? Just invoke your benchmark scripts with a higher bandwidth, e.g. -W > 2M > >
_______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
