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[Discuss-gnuradio] Real-time FFT over multiple symbols

From: Aditya Virmani
Subject: [Discuss-gnuradio] Real-time FFT over multiple symbols
Date: Thu, 31 Dec 2015 15:40:00 -0500


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 (screenshot of that attached), but when I feed in more than two, it results in error ( 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 someone can guide me with it.

I have attached my .grc file.

Attachment: sampled.grc
Description: Binary data

Attachment: FFT over 2 ofdm symbols.tiff
Description: TIFF image

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