Hey Marcus this was very very informative.This offset was really killing me every time when I was running the OFDM example.I think I need to order some GPSDO :)
Marcus D. Leech wrote: > > On 09/09/2011 07:26 PM, Tuan (Johnny) Ta wrote: >> As far as I know there's no open source code for an OFDM transceiver >> available. I was trying to build one half a year back but wasn't >> successful before I had to move on to something else. The >> benchmark_ofdm code will give you a simplex OFDM system. Ie you can >> run the transmitter on 1 USRP and receiver on another. >> >> Ie. run this on 1 USRP >> ./benchmark_ofdm_tx.py -f 2.412G >> >> And this on the other >> ./benchmark_ofdm_rx.py -f 2.412G >> >> The value of the frequency depends on the daughterboards you're using. >> If you're using USRP1 make sure the decimation rate is 1/2 of the >> interpolation rate as the ADC is 2 times faster than the DAC on the >> USRP1 (or the other way around, you should chek that). > The DAC on the USRP1 runs at twice the rate of the ADC. > >> >> Watch out for the frequency offset, it killed the system for me. If >> the above doesn't work, run the transmitter on 1 USRP and usrp_fft.py >> on the other. Check the center frequency of the FFT plot and manually >> adjust the receiver center frequency. I used the RFX2400 boards and >> the offset for me was ~ 40kHz. >> >> > Frequency offset comes up a lot on this list. It's usually in the > context of someone who has up to this point in their DSP/SDR "career" > only been dealing with baseband signals inside a simulation > environment--and environment that doesn't always adequately reflect > what you'll experience in real-world systems, and real-world channels. > > RF synthesizers are only as good as their reference clock. The > reference clocks on most garden variety RF platforms are usually of > good-to-excellent quality. But they may still have residual errors > of a few 10s of PPM. So that means for every MHz of frequency, > the absolute, actual frequency could be "off" by a few 10s of Hz. > Multiply that up to typical channel frequencies for many experiments > in the modern communications domain of 1 to 3GHz or even higher, and > you can easily end up with 10s of Khz of absolute frequency offset, > and this applies to both the transmitter and receiver. > > In typical cellular phone systems like LTE, and GSM and the like, the > base-station transmitters typically have really good reference clocks-- > good to a few PPB--a local rubidium clock, or a GPSDO. The the > hand-helds typically have cheap local reference clocks, in order to meet > the grueling BOM cost requirements of typical consumer electronics. > > What that means is that the demodulation chain needs some mechanism to > deal with frequency offset, and provide feedback to "center" > the baseband signal--either by tweaking the RX hardware, or shifting > the baseband signal in software. But the example code that's floating > around is typically *not* a *complete* system in this regard. In > some sense, much of it was designed to work in the "fantasy" land of > the simulation environment, and may not work that well in the real > world. In some OFDM systems, for example, I understand that there > is often a "pilot" carrier against which one can correlate some kind > of sequence, and once you've found the most-strongly-correlated > "bin" in the OFDM "comb", you can use that to estimate the frequency > offset relative to the transmitter. Examples and simulations may or > may not have that covered. College-level programs in DSP and SDR may > or may not discuss that important "real world" detail. > > Physics, it turns out, is a harsh mistress... > > > > > -- > Marcus Leech > 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 > > ----- Sumit Kr. Research Assistant Communication Research center IIIT Hyderabad India -- View this message in context: http://old.nabble.com/OFDM-Implementation-tp32380874p32503942.html Sent from the GnuRadio mailing list archive at Nabble.com. _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
