# How to calculate BER in different modulations with OFDM using GNURadio Companion **URL:** \ **Category:** GNU Radio\ **Created:** [August 31, 2015, 1:29pm UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186 "2015-08-31T13:29:20Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\ **Post date:** [August 31, 2015, 1:29pm UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186/1 "2015-08-31T13:29:20Z")
Hello, I’m doing my College final project about gnuradio companion. Specifically I have to calculate the BER with OFDM using different modulations to the subcarriers. After that, I have to do a graphic with the BER numbers. The configuration I use is in the image attached. When I’m using GRC to calculate BER I always have the same problem. First I try to simulate without introducing any noise and get a BER data. Then I try to introduce AWGN noise and the BER data is the same. Clearly I’m making a mistake, but I’m not able to identify what it is. In view of the configuration used, someone could tell me which are my mistakes? Also, I would like to know how to configure the block Channel Model correctly. Thank you all. ---
**Author:** ![palollano91](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/3da27b/32.png) [@palollano91](https://www.ruby-forum.com/u/palollano91)\ **Post date:** [August 31, 2015, 1:30pm UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186/2 "2015-08-31T13:30:30Z")
The configuration to which I refer in the previous email is the corresponding to the image attached 2015-08-31 13:18 GMT+02:00 ANTONIO TAMAYO [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![palollano91](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/3da27b/32.png) [@palollano91](https://www.ruby-forum.com/u/palollano91)\ **Post date:** [August 31, 2015, 2:42pm UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186/3 "2015-08-31T14:42:56Z")
Hi Antonio, The problem might be due to delay introduced by the processing blocks. Your OFDM modulator and demodulator don’t work instantaneously, they take some time to do their processing and forward their outputs to the next block in line. Therefore, there is a delay between the output of the OFDM demod block and the original signal. What you need to do is add a delay block between your Random source and the BER block and set the delay properly. When you’re not adding any noise, your BER should be 0. So you can try to find the delay value this way. Then add noise and see how your BER evolves. I hope this helps. Regards, Jawad 2015-08-31 13:21 GMT+02:00 ANTONIO TAMAYO [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![palollano91](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/3da27b/32.png) [@palollano91](https://www.ruby-forum.com/u/palollano91)\ **Post date:** [September 2, 2015, 8:08am UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186/4 "2015-09-02T08:08:48Z")
Hello Antonio, Please take the habit of replying to the list. People might get into the same problems or someone might have some knowledge to share. Also, try to provide a flowgraph (.grc file) to make testing easier for the people helping you out. That said, I did some testing and it looks like I was wrong. The delay isn’t the problem here (at least not on my machine). But I think I figured out the problem. It looks like you provided a Payload length of 0 in your OFDM modulator block. If I set the payload length to 100 and then the random source Num. samples to the same value, the BER goes to 0 without adding delay. And you can see the BER moving when you change the delay. There is one problem though, it doesn’t seem to work when the payload length and the Num. samples are not set to the same value. Regards, Jawad 2015-09-01 13:50 GMT+02:00 ANTONIO TAMAYO [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![palollano91](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/3da27b/32.png) [@palollano91](https://www.ruby-forum.com/u/palollano91)\ **Post date:** [September 2, 2015, 12:15pm UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186/5 "2015-09-02T12:15:54Z")
Hello Jawad, I’ve made the changes you said. As you say, when I assign the Payload parameter to the number of samples of the block Random Source without introducing Delay, I get a BER value equal to 0. But what I need to introduce a noise source so that when I increase the power noise, also increase the BER. This doesn’t happen making the changes that you say, when I introduce a noise source and the noise power is increasing the value of BER remains 0. This is incorrect. As you can see in the spectrum received, the signal contains errors that they aren’t appreciated by the block Error Rate. (SistemasOFDM\_prueba.grc) Also, I need to send an image and calculate the BER in the same way as with the random vector. When I use the block File Source and calculate the BER, I have the same problem, the BER remains constant at 0.51 and although the noise power increases, the BER remains unchanged. (SistemaOFDM\_Pruebas1.grc) The image that I send in the previos example is called “imagen” Thank you. Regards, Antonio 2015-09-01 15:04 GMT+02:00 Jawad S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![palollano91](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/3da27b/32.png) [@palollano91](https://www.ruby-forum.com/u/palollano91)\ **Post date:** [September 2, 2015, 2:15pm UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186/6 "2015-09-02T14:15:04Z")
Hi Antonio, I tried to open your GRC file. This must have been with a pretty old version of GNU Radio (which version are you using?). I strongly recommend getting a newer version of GNU Radio, because especially the OFDM code has improved a lot. By the way, a BER of about ~0.5 means that as many bits are right as there are wrong bits, which means that you get absolutely no information through (if there were more wrong than right bits, you could just toggle the bits and get more information); this means that the input and output streams are not related, or have an offset relative to each other that doesn’t get corrected; increase the BER window size (you don’t need the delay if you’ve got a bigger window). Best regards, Marcus ---
**Author:** ![palollano91](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/3da27b/32.png) [@palollano91](https://www.ruby-forum.com/u/palollano91)\ **Post date:** [September 5, 2015, 11:53am UTC](https://www.ruby-forum.com/t/how-to-calculate-ber-in-different-modulations-with-ofdm-using-gnuradio-companion/243186/7 "2015-09-05T11:53:59Z")
Hello Jawad, I’ve made the changes you said. As you say, when I assign the Payload parameter to the number of samples of the block Random Source without introducing Delay, I get a BER value equal to 0. But what I need to introduce a noise source so that when I increase the power noise, also increase the BER. This doesn’t happen making the changes that you say, when I introduce a noise source and the noise power is increasing the value of BER remains 0. This is incorrect. As you can see in the spectrum received, the signal contains errors that they aren’t appreciated by the block Error Rate. (SistemasOFDM\_prueba.grc) Also, I need to send an image and calculate the BER in the same way as with the random vector. When I use the block File Source and calculate the BER, I have the same problem, the BER remains constant at 0.51 and although the noise power increases, the BER remains unchanged. (SistemaOFDM\_Pruebas1.grc) The image that I send in the previos example is called “imagen” Thank you. Regards, Antonio 2015-09-01 15:04 GMT+02:00 Jawad S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):