On Wed, Feb 10, 2010 at 11:40 AM, bin zan <[email protected]> wrote: > Hi Tom, > Can you help me answer the following questions? > 1. Does that mean, the data has not be divided into sync'd > segments until ofdm_sampler.cc?
I'm not sure what "that" is you are talking about (please reply inline and don't top post when responding), but I think I get what you are asking. Yes, the sampler separates the stream into vectors that are fft_length in size. > 2. Why d_timeout_max=1000? Pre-defined preambles only appear > every 1000 FFT length of data? 1000 was just an arbitrary number. We wanted to make sure that it would stop sampling after a while if no new packets are seen. It just has to be long enough to make sure the timeout/reset doesn't happen during the reception of one full packet. > 3. Only data after state_frame will go through FFT process, others > will be dropped (including CP)? Yes, the sampler removes the CP. It's purpose is served so it is no longer necessary. > 4. According to your powerpoint OFDM implementation in GNU radio, > FFT happens before preamble correlation, which file actually do the preamble > correlation? This correlation is done in the ofdm_frame_acq (part of ofdm_receiver). We use the preamble in the frequency domain to figure out the subcarrier offset (how many bins away from DC the signal is) and then create the equalizer from it. The output of this block is just the occupied tones post equalizer. Tom > On Wed, Feb 10, 2010 at 12:36 PM, Tom Rondeau <[email protected]> > wrote: >> >> On Wed, Feb 10, 2010 at 8:10 AM, bin zan <[email protected]> wrote: >> > Hi, >> > I just wonder why in gr_ofdm_sampler.cc, the consume_each for >> > STATE_NO_SIG and STATE_PREAMBLE are different. >> > consume_each(index - d_fft_length + 1); >> > consume_each(index-d_fft_length); >> > >> > Both suppose to leave one fft length, right? >> > Can any one explain it? >> > >> > Thanks, >> > Bi >> >> It's just like the comments say, in STATE_NO_SIG, we consume 1 less >> because we need to leave behind a full fft_length of items to test for >> the preamble. When we have the preamble in STATE_PREAMBLE, we consume >> everything including that one so that the next input block is the >> start of the packets. >> >> FYI: Matt and I are working on the OFDM stuff this week. We're seeing >> some issues that we need to work out, so things we thought were right >> could be wrong and will hopefully be fixed. >> >> Tom _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
