On 31/01/12 08:28 AM, Florian Schlembach wrote: >> But fiddling with gain values is often useful; even if you've already >> done that I recommend trying again, by reducing tx-amplitude and the >> actual gain values, shifting the terminals around (perhaps they're too >> close?). >> > We have now found out that we need a sampling rate of at least 2Msps > which means we have to set the bandwidth to at least 2MHz (I read sth > about that the USRPs have problems with higher decimation rates): > > ./benchmark_tx.py -f 2.400G -A TX/RX --tx-amplitude=0.2 -v -W 2M > ./benchmark_rx.py -f 2.400G -A TX/RX --rx-gain=35 -v -W 2M > > > The OFDM (bpsk) example is now working and all packets seems to be > transmitted. Unfortunately, not all of the packets could be > demodulated correctly as they are marked as "ok: False" - lets say a > quarter of them. This would yield a really bad performance in terms of > a reliable transmission. We also played around with the distance and > the alignment of the omni antennas but ultimately, we could not get > rid of the false packets. > > Have you encountered a similar bad performance? Have you also > encountered such a strange behavior regarding the lower sampling rate? > What else could we try? > > ______________________________________________ > Frequency offset is the usual cause of such problems.
The master oscillators on a USRP vary between about 10PPM and 20PPM, which at higher frequencies means on offset of several kHz. A narrow-band signal suffers much more from frequency offset issues that a wideband one--the frequency error constitutes a larger fraction of the overall signal. Frequency offset errors are normal in any radio communications system--since master oscillator frequencies are *never* perfect. Real-world systems typically have an FLL or AFT somewhere in the receive chain to compensate. This is why the last IF filter on a narrowband FM receiver is usually wider than would be suggested by the modulation bandwidth, and why there's usually some kind of AFT feedback. -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
