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List:       gnuradio-discuss
Subject:    Re: [Discuss-gnuradio] OFDM packets
From:       Tom Rondeau <trondeau () vt ! edu>
Date:       2008-02-29 15:43:12
Message-ID: 47C82810.7020501 () vt ! edu
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Crespi Floriana wrote:
> Scrive Eric Blossom <eb@comsec.com>:
>
>   
>> On Thu, Feb 28, 2008 at 12:51:18PM +0100, Crespi Floriana wrote:
>>     
>>> Hi,
>>> I have a problem with OFDM system.
>>> I transmit OFDM signal at 2.4 GHz, but I receive half of the total packets.
>>> Moreover, only half of the packets received are correct.
>>> CRC controller is perfect but i want a ber estimate, so I change the
>>> payload (binary file) and I  receive check bit by bit.
>>> The result of my alteration test is similar than CRC test.
>>> I have a lot of frequency offset, therefore I change manually the frequency
>>> receiver.
>>> I use revision 6845 and RFX 2400.
>>> Example with crc:
>>> The final packet I receive:
>>> ok: False      pktno 4602   n_rcvd 1300       n_right 83
>>>
>>> Why can't i get all the packets?
>>> Could you help me??
>>>
>>> I'm testing OFDM system without USRP and none of the packets are lost.
>>>
>>> Thanks a lot.
>>>
>>>       
>> Hi Crespi,
>>
>> You have neglected to tell us virtually anything about your problem.
>> Could you please tell us:
>>
>>   which version of GNU Radio?  trunk, tarball (which one), etc?
>>   what operating system, distribution and architecture?
>>   the command line you issued
>>   any modifications you may have made to distributed code?
>>   anything else that might be relevant
>>
>>
>> Thanks,
>> Eric
>>
>>
>>     
>
>
>
> Hi, thank you for your reply.
> I use:
> * Linux K-Ubuntu 7.04
> *Gnuradio Trunk 6845
> The command are :
> --TX--
> sudo python benchmark_ofdm_tx.py -f 2.442e9 -T A --tx-amplitude 10000
> --RX--> sudo python benchmark_ofdm_rx.py -f 2.4419899e9 -T A
> (occuped tones, size packet and fft are default value)
>   

That's a very high amplitude. The PAPR of OFDM will hit you at this 
amplitude. Stay between 1000 and 4000.

> Important: If I don't change Rx frequency it can't receive!!
>   

OFDM is notoriously sensitive to frequency offset. With the RFX2400 
boards, the frequency deviation is likely just outside the range we allow.

Tom


> I change the payload. I upload file to the code and I calculate its lenght.
> Number packets= (lenght file/ size packets)+1
> Before my modification I tested ofdm system and the result is similar then after
> modification.
> The trial simulation (without USRP and antennas) is perfect. I Receive all
> packets and they are correct.
> I tested with coax for estimate channel problem but the result is similar to
> real channel. I thought the problem was the channel but this experiment reject
> my theory.
> I don't find the problem because my alteration code don't change the core code.
>
>
> This is my result. final packet
> ===================================================================================
> Trial simulation
> ok: True         pktno: 4609     n_rcvd: 4610    n_right: 4610
>
> bit right 14750243
> bit wrong  0.0
> ===================================================================================
> All correct
>
> ===================================================================================
> Coax
>
> ok: False 	 pktno: 4000 	 n_rcvd: 1300 	 n_right: 830
>
> bit right 2109817
> bit wrong 2050183
> ===================================================================================
>
>
>
>
> Thank you a lot for your help.
> _________________________________________________________________
>
>
>
>
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> Discuss-gnuradio@gnu.org
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
>   




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