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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