Hi all, This question might be a little bit off-topic but going through the code on the transmission side, I didn't see CRC method got invoked anywhere. Though I did see that the CRC got removed and checked in the receiving side. I'd be grateful if anyone can give me a pointer.
Thanks, Johnny On Mon, Jun 21, 2010 at 12:00 PM, <discuss-gnuradio-requ...@gnu.org> wrote: > > Hi Rahman, > > Just like you, my USRPs have suddenly started failing the CRC for every > packet. > > By changing the frequency to 1MHZ I'm back to 99% correctly received :) > From > searching the mailing list it looks like it's related to the increased > frequency offset due to aliasing at the higher frequencies. > > Also, there is a setting in ofdm_receiver.py for SYNC. For me, SYNC="ml" > works the best. > > Mike > > > > On 19 June 2010 17:10, Rahman Doost <d.mohamm...@gmail.com> wrote: > > > Thanks Mike for the comment. I did change the bandwidth, but some error > > poped up at runtime for 1.5. Lower values like 0.1 or 0.2 did not change > the > > behavior of the receiver. Still no correct packet. I receive the packets > > though but they fail the CRC check I guess, since I get this "ok:false" > > status on each of them. Any idea? > > > > Thanks, > > Rahman > > > > > > On Thu, Jun 17, 2010 at 5:29 AM, Mike Jameson <mikej...@gmail.com> > wrote: > > > >> missed off the filename: ofdm_receiver.py > >> > >> :) > >> > >> > >> > >> On 17 June 2010 10:27, Mike Jameson <mikej...@gmail.com> wrote: > >> > >>> Hi Rahman, > >>> > >>> Try broadening the bandwidth of the receiver to allow for the frequency > >>> offset between your USRP2s: > >>> > >>> In /usr/lib/python2.6/dist-packages/gnuradio/blks2impl > >>> > >>> on line 67 look for "tb = bw*0.08" > >>> > >>> change this to a higher value such as "tb = bw*1.5" > >>> > >>> This works for me using usrps. > >>> > >>> Mike > >>> > >>> On 16 June 2010 23:13, Rahman Doost <d.mohamm...@gmail.com> wrote: > >>> > >>>> Guys, > >>>> > >>>> I'm working on the same thing, I'm trying to receive something with > OFDM > >>>> example, USRP2 and XCVR boards. But it doesn't work. The transmitter > is > >>>> sending and the rx signal is crystal clear with spectrum analyzer but > no > >>>> packet is caught. > >>>> Could anyone of you send me the command you're using at both > transmitter > >>>> and receiver with benchmarks? I really appreciate it. > >>>> > >>>> Thanks > >>>> Rahman > >>>> > >>>> On Tue, Jun 15, 2010 at 5:53 AM, Mike Jameson <mikej...@gmail.com > >wrote: > >>>> > >>>>> Hi Chris, > >>>>> > >>>>> The default settings do drop alot of packets as you are seeing. > >>>>> > >>>>> These are some of the things I've adjusted which happen to have > brought > >>>>> my packet reception rate up to 99% using 1 USRP with a Basic RX and > >>>>> TX joined with a 10db attenuator: > >>>>> > >>>>> - For benchmark_tx.py and benchmark_rx.py add '-m qpsk' to the > command > >>>>> line for qpsk instead of bpsk. > >>>>> - For benchmark_tx.py add '-s 4' to the command line to reduce the > >>>>> packet size sent. > >>>>> - For benchmark_tx.py add '-d 1' to the command line to reduce the > rate > >>>>> and change the benchmark_tx.py script to sleep for 0.01 seconds > instead of > >>>>> 1. > >>>>> - For benchmark_tx.py add '--tx-amplitude=3000' to the command line > to > >>>>> boost the default output power. > >>>>> - In transmit_path.py pad_for_usrp=true and msgq_limit=1 > >>>>> > >>>>> Cheers, > >>>>> > >>>>> Mike > >>>>> > >>>>> On 15 June 2010 10:20, Chris_Spain <christianper...@gmail.com > >wrote: > >>>>> > >>>>>> > >>>>>> Thank you Fisheep, you confirmed what I was thinking but, I don't > Know > >>>>>> why, > >>>>>> the packets are been received with no errors, I lost packets but the > >>>>>> packets > >>>>>> that I receive are good. > >>>>>> > >>>>>> I don't Know how GNU Radio implementation maps 1024 bytes over 200 > >>>>>> carriers > >>>>>> with a bpsk modulation, and then the transmission it's OK. > >>>>>> > >>>>>> Cheers > >>>>>> > >>>>>> > >>>>>> Fisheep wrote: > >>>>>> > > >>>>>> > Hi, > >>>>>> > > >>>>>> > I'm not sure why you can't receive all the packet correctly, but I > >>>>>> can > >>>>>> > briefly tell you how ofdm implement in GNU Radio. > >>>>>> > > >>>>>> > First, fft_length=512 means it will take 512 subcarriers to do > >>>>>> ifft/fft, > >>>>>> > and occupied_tones=200 means it only take 200 subcarriers to put > >>>>>> your > >>>>>> > modulation symbol. Besides, the 200 subcarriers are set at the > >>>>>> center of > >>>>>> > 512 subcarriers. Then you will see there are the remaining > >>>>>> 312(512-200) > >>>>>> > subcarriers which is zero outside the occupied_tones. > >>>>>> > > >>>>>> > likes this: > >>>>>> > > >>>>>> > xxxxxOOOOOOxxOOOOOOxxxxx > >>>>>> > --------------+----------------> f > >>>>>> > > >>>>>> > O => occupied_tones > >>>>>> > x => remaining subcarriers > >>>>>> > the two x at the center is to avoid DC offset. > >>>>>> > After ifft, the CP is added. > >>>>>> > > >>>>>> > OFDM is sensitive to the bitrate(interpolate/decimate), so set the > >>>>>> bitrate > >>>>>> > carefully. > >>>>>> > > >>>>>> > Hope this will help you little. > >>>>>> > > >>>>>> > Fisheep. > >>>>>> > > >>>>>> > > >>>>>> > > >>>>>> > > >>>>>> > Chris_Spain wrote: > >>>>>> >> > >>>>>> >> Hello everybody, > >>>>>> >> > >>>>>> >> > >>>>>> >> I' am trying to test and understand the OFDM Modulation > implemented > >>>>>> in > >>>>>> >> GNU Radio. I'm using the deafult values, fft_lenth=512, > >>>>>> occupied_tones > >>>>>> >> =200, cp_length= 128 and bpsk modulation, besides, I'm using a > >>>>>> RFX900 > >>>>>> >> Daughterboards on USPR and I'm connecting the TX and RX with a > >>>>>> coaxial > >>>>>> >> cable. > >>>>>> >> > >>>>>> >> I try to send a package of 1024 bytes but I'm not receiving all > >>>>>> packets, > >>>>>> >> always lost the first packet and then is a little bit random, > >>>>>> sometimes I > >>>>>> >> receive and I demodulated the 2º,3º,4º package and lost 5º, or > lost > >>>>>> 3º > >>>>>> >> and recive ok 2 ºand 4º... > >>>>>> >> I don't know what are happening, because I don't know how the > OFDM > >>>>>> >> implementation map the 1024 bytes onto 200 carriers of data. > >>>>>> >> > >>>>>> >> If I'm understanding the OFDM theory, with a bpsk modulation each > >>>>>> carrier > >>>>>> >> are modulated for two bits and my OFDM symbol length are 512+128 > >>>>>> >> (fft_length+cp_length).So, ¿with 2 bits per data carrier (200), > >>>>>> each > >>>>>> >> symbol transport 400 bits, 50 bytes? > >>>>>> >> > >>>>>> >> That can´t be true because I receive some packages with no > errors. > >>>>>> >> Somebody can explain me, How works OFDM Modulation implemented in > >>>>>> GNU > >>>>>> >> Radio? > >>>>>> >> > >>>>>> >> > >>>>>> > > >>>>>> > > >>>>>> > >>>>>> -- > >>>>>> View this message in context: > >>>>>> http://old.nabble.com/OFMD-TX-and-RX-tp28871027p28888978.html > >>>>>> Sent from the GnuRadio mailing list archive at Nabble.com. > >>>>>> > >>>>>> > >>>>>> _______________________________________________ > >>>>>> Discuss-gnuradio mailing list > >>>>>> Discuss-gnuradio@gnu.org > >>>>>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >>>>>> > >>>>> > >>>>> > >>>>> _______________________________________________ > >>>>> Discuss-gnuradio mailing list > >>>>> Discuss-gnuradio@gnu.org > >>>>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >>>>> > >>>>> > >>>> > >>>> _______________________________________________ > >>>> Discuss-gnuradio mailing list > >>>> Discuss-gnuradio@gnu.org > >>>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >>>> > >>>> > >>> > >> > > >
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