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Re: [Discuss-gnuradio] OFMD TX and RX

From: Chris_Spain
Subject: Re: [Discuss-gnuradio] OFMD TX and RX
Date: Tue, 15 Jun 2010 02:20:32 -0700 (PDT)

Thank you Fisheep, you confirmed what I was thinking but, I don't Know why,
the packets are been received with no errors, I lost packets but the packets
that I receive are good.

I don't Know how GNU Radio implementation maps 1024 bytes over 200 carriers
with a bpsk modulation, and then the transmission it's OK.


Fisheep wrote:
> Hi,
> 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: 
> xxxxxOOOOOOxxOOOOOOxxxxx
> --------------+----------------> 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
> carefully.
> Hope this will help you little.
> Fisheep.
> 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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