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Re: [Discuss-gnuradio] OFMD TX and RX

From: Mike Jameson
Subject: Re: [Discuss-gnuradio] OFMD TX and RX
Date: Tue, 15 Jun 2010 10:53:14 +0100

Hi Chris,
The default settings do drop alot of packets as you are seeing.
These are some of the things I've adjusted which happen to have brought my packet reception rate up to 99% using 1 USRP with a Basic RX and TX joined with a 10db attenuator:
- For benchmark_tx.py and benchmark_rx.py add '-m qpsk' to the command line for qpsk instead of bpsk.
- For benchmark_tx.py add '-s 4' to the command line to reduce the packet size sent.
- For benchmark_tx.py add '-d 1' to the command line to reduce the rate and change the benchmark_tx.py script to sleep for 0.01 seconds instead of 1.
- For benchmark_tx.py add '--tx-amplitude=3000' to the command line to boost the default output power.
- In transmit_path.py pad_for_usrp=true and msgq_limit=1

On 15 June 2010 10:20, Chris_Spain <address@hidden> wrote:

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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