# Real-time FFT over multiple symbols

**URL:** <https://www.ruby-forum.com/t/real-time-fft-over-multiple-symbols/243718>\
**Category:** GNU Radio\
**Created:** [December 25, 2015, 10:52pm UTC](https://www.ruby-forum.com/t/real-time-fft-over-multiple-symbols/243718 "2015-12-25T22:52:10Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Sadaf\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@Sadaf\_N](https://www.ruby-forum.com/u/Sadaf_N)\
**Post date:** [December 25, 2015, 10:52pm UTC](https://www.ruby-forum.com/t/real-time-fft-over-multiple-symbols/243718/1 "2015-12-25T22:52:10Z")

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Hello  
Firstly, I should say that I am new to GNURadio, and I am still learning  
to work with the basics through Tutorials and Videos.  
I am trying to do FFT over multiple symbols (OFDM, each with 64  
streams).  
My flow graph involves following sequence of blocks:

Random Uniform Source (min: 255, max: 255) -\>  
OFDM Modulation (Mod: QAM 16 (initially tried with QPSK, but that won’t  
guarantee same symbols to be delivered),FFT len: 64, Occupied tones: 48,  
Cyclic Prefix length: 16, Padding: No) -\>  
Keep M in N (64 in 80) -\> Virtual Sink (One Symbol).

As per my understanding, this sequence gives me one OFDM symbol,  
comprising of 64 streams.

To get multiple symbols for FFT operation, I feed 4 such symbols to  
Streams to Mux(4, Lengths as 64, 64, 64, 64) block, with  
first input as One Symbol, second input as One Symbol -\> Skip Head (64),  
third input as One Symbol -\> Skip Head (128), and so on.

As per my understanding, as I am feeding in the same sequence of bytes,  
it should be generating same set of symbols, and then further if we do  
FFT over multiple symbols which are same, we should get peaks, instead  
of continuous FFT graph.  
This works fine if I do just with two symbols, but when I feed in more  
than two, it results in error, which I think is happening because of the  
Skip Head block ( I think that the data doesn’t reach the FFT block  
concurrently, because the graph I get is same when I feed Null source  
and a Symbol source to FFT).  
It would be great if anyone can guide me with it.  
I have attached my .grc file.