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Re: [Discuss-gnuradio] OFMD TX and RX

From: Fisheep
Subject: Re: [Discuss-gnuradio] OFMD TX and RX
Date: Tue, 15 Jun 2010 01:06:11 -0700 (PDT)


I'm not sure why you can't receive all the packet correctly, but I can
briefly tell you how ofdm implement in GNU Radio.

First, fft_length=512 means it will take 512 subcarriers to do ifft/fft, and
occupied_tones=200 means it only take 200 subcarriers to put your modulation
symbol. Besides, the 200 subcarriers are set at the center of 512
subcarriers. Then you will see there are the remaining 312(512-200)
subcarriers which is zero outside the occupied_tones.

likes this: 

--------------+----------------> f

O => occupied_tones
x => remaining subcarriers
the two x at the center is to avoid DC offset.
After ifft, the CP is added.

OFDM is sensitive to the bitrate(interpolate/decimate), so set the bitrate

Hope this will help you little.


Chris_Spain wrote:
> Hello everybody,
> I' am trying to test and understand the OFDM Modulation implemented in GNU
> Radio. I'm using the deafult values, fft_lenth=512, occupied_tones =200,
> cp_length= 128 and bpsk modulation, besides, I'm using a RFX900
> Daughterboards on USPR and I'm connecting the TX and RX with a coaxial
> cable.
> I try to send a package of 1024 bytes but I'm not receiving all packets,
> always lost the first packet and then is a little bit random, sometimes I
> receive and I demodulated the 2º,3º,4º package and lost 5º, or lost 3º and
> recive ok 2 ºand 4º...
> I don't know what are happening, because I don't know how the OFDM
> implementation map the 1024 bytes onto 200 carriers of data.
> If I'm understanding the OFDM theory, with a bpsk modulation each carrier
> are modulated for two bits and my OFDM symbol length are 512+128
> (fft_length+cp_length).So, ¿with 2 bits per data carrier (200), each
> symbol transport 400 bits, 50 bytes? 
> That can´t be true because I receive some packages with no errors.
> Somebody can explain me, How works OFDM Modulation implemented in GNU
> Radio?

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