On 04/25/2011 09:15 PM, killol wrote: > Hello all > I am also trying to get OFDM mod and demod blocks already present in > gnuradio distribution to work. > The blocks work when used on the same computer. Once I try to receive on the > other USRP, the OFDM demodulator does not demodulate any signal. > Can anyone please let me know what problem might be there. > > Someone suggested that there might be synchronaztion problem because of > which the receiver frequency drifts and does not demodulate the transmitted > frequency. > > Can someone please help me regarding the same. > > Best, > Killol > > Minor frequency errors are an inevitable part of any radio channel. The impact of these errors is more or less severe, depending on the modulation technique in use.
PLL synthesizers are at the mercy of the crystals that are used to provide a reference clock for them. The crystal oscillators on the USRP family aren't top-of-the-line, but they're quite good--roughly +/-20PPM or so. So, what that means is that for every MHz of center frequency, the actual frequency could be off by +/- 20Hz. For every 1GHz of center frequency, it could be off by as much as +/- 20kHz. For wideband conventional modulation schemes, an error in center frequency of up to +/-20kHz won't make that much difference, because the error frequency is much less than the total bandwidth. But in OFDM, where there a plethora of narrowband frequency bins, offsets can cause real problems. So your receiver has to try to frequency track the transmitter, or you need to use an external reference clock, which on the USRP1, you can't do (unless you use a 3rd-party device like the clocktamer board). I'm not an expert on OFDM, but isn't the existence of a "pilot channel" there to help frequency-track the receiver? -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
