# Trouble at OFDM modulation FFT waveform

**URL:** <https://www.ruby-forum.com/t/trouble-at-ofdm-modulation-fft-waveform/233491>\
**Category:** GNU Radio\
**Created:** [December 17, 2014, 6:40pm UTC](https://www.ruby-forum.com/t/trouble-at-ofdm-modulation-fft-waveform/233491 "2014-12-17T18:40:34Z")\
**Posts on this page:** 2\
**Page:** 1

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**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:** [December 17, 2014, 6:40pm UTC](https://www.ruby-forum.com/t/trouble-at-ofdm-modulation-fft-waveform/233491/1 "2014-12-17T18:40:34Z")

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<div class="post-metadata">

**Author:** ![gaddam\_yamuna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/779978/32.png) [@gaddam\_yamuna](https://www.ruby-forum.com/u/gaddam_yamuna)\
**Post date:** [December 17, 2014, 6:54pm UTC](https://www.ruby-forum.com/t/trouble-at-ofdm-modulation-fft-waveform/233491/2 "2014-12-17T18:54:41Z")

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Hello Gaddam,

I can’t really make out much in your illustration, but it looks like a  
textbook illustration of the power spectrum density (PSD) of three  
different signals; sadly, you don’t really explain what you’ve tried so  
far, so it’s hard to explain this based on something.

You will have to feed in these three signals into the FFT Plot. You can  
set the number of inputs to three, and you can transform any signal  
between time and frequency domain using the FFT block of GNU Radio.

Assuming you’re new to GNU Radio, please have a look at the Guided  
Tutorials (start with parts 1 and 2), which will explain a lot of things  
very nicely:  
[http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided\_Tutorials](http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorials)

Also note that there is already a fairly well-working OFDM  
implementation in GNU Radio, so although this might be a very  
educational example, you’re kind of re-inventing the wheel. A lot of  
projects that already employ that implementation or use their own exist,  
for example an IEEE802.11 (WLAN) Phy layer and LTE implementations. You  
can build your project atop of these very good references!

Best regards,  
Marcus