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Re: [Discuss-gnuradio] USRP1 cannot transmit/receive a OFDM packet disco

From: Fisheep
Subject: Re: [Discuss-gnuradio] USRP1 cannot transmit/receive a OFDM packet discontinuously sometimes
Date: Tue, 14 Sep 2010 23:26:51 -0700 (PDT)

Hi Tom,

Thanks for your suggestion. I will take your advice and try it out.

But recently I connect two USRP1 with the cable not through the antennas,
hoping to remove the channel effect in this experiment. At the receiver,
using the usrp_fft.py to observe the spectrum of received signal which is
discontinuously transmitted from transmitter. The result shows that
sometimes the spectrum doesn't get rise when transmitter send a packet.Is it
possible that the transmitted packet doesn't transmit from USRP1? If
possible, how do I examine this problem?

Sorry, I'm not very expert in hardware implementation, but I will do my best
to get familiar with USRP. 


Tom Rondeau wrote:
> On Tue, Sep 14, 2010 at 12:52 AM, Fisheep <address@hidden> wrote:
>> Hi,
>> My problem is that I try to discontinuously send "a" OFDM packet by using
>> time.sleep() on USRP1, but fail to successfully receive this OFDM packet
>> at
>> the receiver sometimes.
>> Brief Setting Description:
>> Code : gnuradio-example/python/ofdm/benchmark_ofdm_tx{rx}.py
>> Daughterboard : FLEX900
>> OS : Ubuntu 8.10
>> Tx :
>>    send_pkt(data) -> time.sleep(1) -> send_pkt(data) -> time.sleep(1) ->
>> ...
>> Rx :
>>  ok = False , pktno = 65537, ....
>>  ok = True  , pktno = 1      , ....
>>  ...
>> I have surveyed about this discussion on the forum. Using discontinuous
>> transmission is to ensure the transmitter successly sending a packet and
>> receiver will receive this packet. And I try this scheme on single
>> carrier
>> case like benchmark_tx{rx}.py on digital file, every packet is successful
>> receive at receiver. But when changing to OFDM, not every packet is
>> successful receive.
>> Is it that fft consumes lots of time and causes the transmitter doesn't
>> send
>> this packet?
>> I think this is the main different between single carrier and OFDM.
>> If anyone have any idea about this problem, please let me know.
>> I am deeply appreciative.
>> Fisheep
> Fisheep,
> I don't think anyone has actually tried doing this. I know when Matt
> and I put the system together, we were concerned only with the
> continuous case. We have some improvements to the OFDM pieces that are
> on my todo list, though, and once the continuous case is finished,
> we'll work on the discontinuous. Until then, though, I'm afraid the
> only advice I could give would be completely speculative.
> My first thought would be to see how the synchronization is behaving;
> that's almost certainly where the problem is. We have the
> "gr_plot_ofdm.py" script that helps to visualize what's happening with
> the received symbols (you'll need to turn logging on and have scipy
> and matplotlib installed). Play with that in loopback mode to
> understand what you're seeing, then see what happens with your
> over-the-air tests.
> If you figure out ways to make it better, I'd love to hear it!
> Tom
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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