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
