# OFDM example from gr-digital with UHD **URL:** \ **Category:** GNU Radio\ **Created:** [June 25, 2015, 2:23pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464 "2015-06-25T14:23:33Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [June 25, 2015, 2:23pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/1 "2015-06-25T14:23:33Z")
Hello, I was interested to test tx\_ofdm.grc and rx\_ofdm.grc from gr-digital, at the same time using USRP N200 devices. TX OFDM: The changes that I did were, removing throttle block and adding an UHD Sink instead, with center frequency 430 MHz, which is the frequency band for which my department has license to transmit. RX OFDM: In this case I removed all blocks before throttle (included), and placed there an UHD Source at the same center frequency. Flow graphs attached in the files. I monitored the output signal and the received signal. I also used a spectrum analyzer to check the actual signal that is being transmitted. Pics are attached. In the tx signal measured by a spectrum analyzer, we can see that the spectrum is not that we desire. I would like to ask if I am doing something wrong with the configuration. Before transmission there was no signal at that frequency and tx and rx antennas are suitable for the carrier frequency. In addition I tried to transmit a file instead of just random bits. In reception I was not receiving the file. The config was just adding a file sink at the end of the rx block and a file source at the beginning of the tx block. Could you please help me with some tip? ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 25, 2015, 3:08pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/2 "2015-06-25T15:08:56Z")
Hi Gabriele, have you tried adding more gain on the TX side? Also, you will need to increase sampling rate. Right now, you’re using 100kHz, which is 1/1000 of the master clock rate of the N200. If I’m not mistaken, the maximum decimation for the N200 is 512, and hence the minimum rate would be 100MHz/512 = 195.3125kHz. You probably just want to use 200kHz or something similarly practical. I hope that helps! Best regards, Marcus ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 25, 2015, 6:26pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/3 "2015-06-25T18:26:23Z")
On 25.06.2015 06:07, Marcus M. wrote: > Hi Gabriele, > > have you tried adding more gain on the TX side? …or less gain? > Also, you will need to increase sampling rate. Right now, you’re using > 100kHz, which is 1/1000 of the master clock rate of the N200. If I’m not > mistaken, the maximum decimation for the N200 is 512, and hence the > minimum rate would be 100MHz/512 = 195.3125kHz. You probably just want > to use 200kHz or something similarly practical. Or even more, like 1-2 Msps. I haven’t tried a combo of N-Series specifically with this example, but I’ve tried X-Series + B-Series, so I’m confident it’s all OK. However, OFDM is sensitive to distortions, so you need to play around with the amplitude multiplier before tx’ing as well as the gain for your specific setup to get a good transmit spectrum. Since you have a spectrum analyzer, that’s pretty useful for getting those settings right. You might also try playing with the rolloff length and/or adding more empty carriers left and right of your tx spectrum while experimenting with settings. Cheers, Martin ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 26, 2015, 12:54pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/4 "2015-06-26T12:54:41Z")
Thanks Marcus and Martin. I finally increased the sample rate up to 1 Msps and decreased the amplitude multiplier. Now I can see a clear ofdm spectrum with no distortion at all. Did some of you tried file transmission from one end to the other? I did it for GMSK before. Now when it comes ofdm and I open the receiving file while transmitting there is no printed information in the received file. In that configuration I replaced the random source by a file source and a file sink after constellation decoder. Again thanks for your help. It was so useful ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 29, 2015, 9:46pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/5 "2015-06-29T21:46:43Z")
Hi, Gabriele, do you have news about this? I am trying to do the same. And I have another question (if Marcus can help) … what is the modulation in this example (QPSK, QAM)? Sorry for my bad english Thanks – View this message in context: [http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54503.html](http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54503.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 29, 2015, 10:12pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/6 "2015-06-29T22:12:25Z")
Hi Jose, > And I have another question (if Marcus can help) … what is the modulation > in this example (QPSK, QAM)? > You can set that yourself in the OFDM Transmitter Block, separately for > header and payload. > Sorry for my bad english > Your English is really fine, don’t worry. If we don’t understand > something, we will just ask 🙂 Best regards, Marcus ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 26, 2015, 1:28pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/7 "2015-06-26T13:28:27Z")
Hi Gabriele, if there is no output, chances are the receiver side packet detector doesn’t see any valid packets. Make sure tx and rx parameters match, and adjust the RX gain. Make sure no under/overflows happen. There’s quite a lot of things that can go wrong in a real-world transceiver, but these are the most likely to happen in my experience. Greetings, Marcus ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 30, 2015, 8:06pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/8 "2015-06-30T20:06:54Z")
Hi Marcus, Thanks so much for the answer. But like in the screenshots of Gabriele, the blocks OFDM Transmitter e OFDM Receiver are off. And I can’t visualize four symbols (QPSK) or 2 symbols (BPSK) in the output of QT Constellation just a big one (see the screenshot), is this correct? Thanks again. [http://gnuradio.4.n7.nabble.com/file/n54530/ofdm\_tx-\_constellation.ofdm\_tx-\_constellation](http://gnuradio.4.n7.nabble.com/file/n54530/ofdm_tx-_constellation.ofdm_tx-_constellation) – View this message in context: [http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54530.html](http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54530.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 30, 2015, 8:31pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/9 "2015-06-30T20:31:27Z")
I am plotting the signal after the Multiply Const Block … the signal that I will transmitt with USRP. Isn’t correct? [http://gnuradio.4.n7.nabble.com/file/n54534/ofdm\_tx.png](http://gnuradio.4.n7.nabble.com/file/n54534/ofdm_tx.png) – View this message in context: [http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54534.html](http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54534.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 30, 2015, 8:15pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/10 "2015-06-30T20:15:40Z")
On 30.06.2015 10:41, Jose Perez wrote: > Hi Marcus, > > Thanks so much for the answer. But like in the screenshots of Gabriele, the > blocks OFDM Transmitter e OFDM Receiver are off. And I can’t visualize four > symbols (QPSK) or 2 symbols (BPSK) in the output of QT Constellation just a > big one (see the screenshot), is this correct? Are you plotting the output of the IFFT by any chance? M ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 30, 2015, 9:07pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/11 "2015-06-30T21:07:05Z")
On 30.06.2015 11:06, Jose Perez wrote: > I am plotting the signal after the Multiply Const Block … the signal that I > will transmitt with USRP. > Isn’t correct? If what you want to plot is your constellation, then no, it’s not. This is all modulated, pulse shaped and all. Cheers, M ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 30, 2015, 9:37pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/12 "2015-06-30T21:37:28Z")
So … Is it possible to see the Constellation points like QPSK in this flowgraph? Sorry for so many questions! Thanks Martin! – View this message in context: [http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54537.html](http://gnuradio.4.n7.nabble.com/OFDM-example-from-gr-digital-with-UHD-tp54446p54537.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [July 1, 2015, 12:01am UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/13 "2015-07-01T00:01:12Z")
On 30.06.2015 12:12, Jose Perez wrote: > So … Is it possible to see the Constellation points like QPSK in this > flowgraph? > Sorry for so many questions! On TX, you can plot the complex symbols before they go into the carrier allocator. On the receive side, you can do the same after the carrier serializer. This won’t exactly plot 1 OFDM symbol at a time, if that is what you want. Also, in both cases, the output will be pretty boring. On TX, it’s just your constellation; on RX, it’s already equalized, so it’s also just the constellation. M ---
**Author:** ![gabri187](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/e9bcb4/32.png) [@gabri187](https://www.ruby-forum.com/u/gabri187)\ **Post date:** [June 30, 2015, 10:51pm UTC](https://www.ruby-forum.com/t/ofdm-example-from-gr-digital-with-uhd/242464/14 "2015-06-30T22:51:05Z")
Jose, we’re talking about this flow graph, right? [http://gnuradio.4.n7.nabble.com/file/n54534/ofdm\_tx.png](http://gnuradio.4.n7.nabble.com/file/n54534/ofdm_tx.png) In this flow graph, you’re trying to find the individual symbols in time domain; that’s not possible, they only exist clearly distinguishable in frequency. That is why there is an DFT (in this implementation, the FFT) in OFDM! Best regards, Marcus