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 <[email protected]> wrote: > missed off the filename: ofdm_receiver.py > > :) > > > > On 17 June 2010 10:27, Mike Jameson <[email protected]> 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 <[email protected]> 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 <[email protected]>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 <[email protected]> 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 >>>>> [email protected] >>>>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>>> >>>> >>>> >>>> _______________________________________________ >>>> Discuss-gnuradio mailing list >>>> [email protected] >>>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>> >>>> >>> >>> _______________________________________________ >>> Discuss-gnuradio mailing list >>> [email protected] >>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>> >>> >> >
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