# Re: anyone successful in running tx\_ofdm and rx\_ofdm grc files? **URL:** \ **Category:** GNU Radio\ **Created:** [May 21, 2013, 7:57pm UTC](https://www.ruby-forum.com/t/re-anyone-successful-in-running-tx-ofdm-and-rx-ofdm-grc-files/234964 "2013-05-21T19:57:47Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [May 21, 2013, 7:57pm UTC](https://www.ruby-forum.com/t/re-anyone-successful-in-running-tx-ofdm-and-rx-ofdm-grc-files/234964/1 "2013-05-21T19:57:47Z")
Martin, I think my question would have been clearer if i had said that when I talk of symbol rate I do not define a symbol to be the OFDM symbol of length fft\_length+CP\_length, but its fft\_length+CP\_length components. So for me symbol rate is symbol\_rate = (fft\_length+CP\_length) x OFDM\_symbol\_rate The grc examples are defining this as the sample\_rate, which implies that they use one sample for each symbol (my definition). In any case, from your answer I understand that the S&C operates on ONE sample per symbol (my definition). * * * I still have trouble getting anything out of the S&C synchronizer: The frequency offset output responds as expected when i introduce a freq error but the “detect” output does not give anything out even with noiseless input… I have uploaded the follwoing grc files [http://web.eecs.umich.edu/~anastas/gnuradio/tx\_ofdm2.grc](http://web.eecs.umich.edu/~anastas/gnuradio/tx_ofdm2.grc) [http://web.eecs.umich.edu/~anastas/gnuradio/rx\_ofdm2.grc](http://web.eecs.umich.edu/~anastas/gnuradio/rx_ofdm2.grc) which, illustrate my point. Any thoughts? thanks for the help, Achilleas On Sun, May 19, 2013 at 11:45:42PM -0400, Achilleas A. wrote: > I am having trouble setting some parameters and getting anything out of it… > > In particular > > in ofdm\_rx > > 1. is the processing done on an 1 sample per symbol basis at the Scmidl&Cox ? I don’t understand the question. The correlator has no info about OFDM symbol boundaries. > In that case the input sample rate has to be set to the symbol rate? > (what is called samp\_rate in ofdm\_tx) This is the input sample rate. The bit rate depends on the number of carriers used, the length of the CP and the modulation type on the payload. The OFDM symbol rate is the sample rate / (fft\_len + cp\_len). ---
**Author:** ![Achilleas\_A](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/74df32/32.png) [@Achilleas\_A](https://www.ruby-forum.com/u/Achilleas_A)\ **Post date:** [May 23, 2013, 7:37pm UTC](https://www.ruby-forum.com/t/re-anyone-successful-in-running-tx-ofdm-and-rx-ofdm-grc-files/234964/2 "2013-05-23T19:37:19Z")
On Tue, May 21, 2013 at 01:55:57PM -0400, Achilleas A. wrote: > Martin, > > I think my question would have been clearer if i had said that when I > talk of symbol rate I do not define a symbol to be the OFDM symbol of > length > fft\_length+CP\_length, but its fft\_length+CP\_length components. OK, that works. > * * * > > I still have trouble getting anything out of the S&C synchronizer: > The frequency offset output responds as expected when i introduce a > freq error but the “detect” output does not give anything out > even with noiseless input… The synchronizer works fine, at least it does here. Try writing the output into a file, and then view it with gr\_plot\_char (or whatever you use). You will see a train of spikes at the distance of the length of the packets. The flow graph itself randomly stalls, though. I’m currently working on the reason, but it has to do with the large abuse of the scheduler. MB – Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin B. Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT – University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association