[USRP-users] Random errors while transmitting certain BPSK/QPSK types on x310s

Marcus Müller marcus.mueller at ettus.com
Fri Jul 7 13:37:33 EDT 2017


Hi Michael,

point is that having a magnitude too close to 1 can lead to problems in
the FPGA side of DSP – not in GNU Radio.

Best regards,

Marcus


On 07/07/2017 07:20 PM, Michael Carosino via USRP-users wrote:
> Hi Marcus, 
>
> you are correct, reducing the magnitude did seem to fix the issue. I
> am wondering though, the gnuradio constellation modulator hier block
> also seems to suffer from the same issue and doesn't have any options
> for reduced magnitude output - do most people just throw in a
> downstream block to reduce the magnitude? Also, is there any reference
> that speaks a bit about the scaling issues that can happen with the
> USRP's or is that the just type of thing you learn on the fly?
>
> Thanks,
> Mike
>  
>
>     Date: Thu, 06 Jul 2017 21:43:54 -0400
>     From: "Marcus D. Leech" <mleech at ripnet.com <mailto:mleech at ripnet.com>>
>     To: usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>
>     Subject: Re: [USRP-users] Random errors while transmitting certain
>             BPSK/QPSK types on x310s
>     Message-ID: <595EE75A.4070805 at ripnet.com
>     <mailto:595EE75A.4070805 at ripnet.com>>
>     Content-Type: text/plain; charset="windows-1252"; Format="flowed"
>
>     On 07/06/2017 09:36 PM, Michael Carosino via USRP-users wrote:
>     > A quick update to this question with more info. I did some further
>     > analysis by capturing the received I/Q data from the USRP Source
>     block
>     > when transmitting the BPSK that works without errors (symbols are
>     > 0.707+0.707j, -0.707-0.707j) and also when using the BPSK that gives
>     > errors (symbols are +1/-1). You can see in the attached image
>     there is
>     > quite a bit of small magnitude anomalies for the second image (also
>     > errors are probably occurring a bit more frequently than I had
>     > previously estimated).
>     >
>     > To me this points to the issue being either something to do with the
>     > USRP or possibly with the TX chain blocks.
>     With a baseband magnitude near 1, scaling
>     issues/filtering/interpolation
>     issues can combine to produce distortions.
>     I usually use a baseband magnitude no greater than 0.9 or so.
>     >
>     > On Thu, Jul 6, 2017 at 4:37 PM, Michael Carosino
>     <m.carosino at gmail.com <mailto:m.carosino at gmail.com>
>     > <mailto:m.carosino at gmail.com <mailto:m.carosino at gmail.com>>> wrote:
>     >
>     >     Hi all,
>     >
>     >     running Gnuradio 3.7.10.2 and UHD 4.0.0 rfnoc-devel latest
>     commit
>     >     (tried earlier versions too). I've got a simple tx/rx flowgraph
>     >     going on. The simple description is:
>     >
>     >     Random input data -> Pack 1 Bit->Chunks to
>     Symbols->Interpolating
>     >     FIR Filter->USRP Sink
>     >
>     >     USRP Source-> Polyphase Clock Sync -> Costas Loop->
>     Constellation
>     >     Receiver->Unpack 1 Bit
>     >
>     >
>     >     I'm transmitting on RF-B TX/RX and receiving on RF-B RX. The
>     >     system works almost perfectly, except that there are single bit
>     >     errors occurring (not many, maybe every couple of seconds at
>     500k
>     >     samp rate).
>     >
>     >     Now here's the real strange thing, these errors are ONLY present
>     >     if running real BPSK (-1,+1), imaginary BPSK (+j,-j), or rotated
>     >     QPSK/QAM (+1j,-1j,+1,-1)
>     >
>     >     If I use a BPSK having symbols with real and complex parts like
>     >     (-.707-.707j, .707+.707j) or QPSK (+/- .707+/- .707j) the errors
>     >     are NOT present.
>     >
>     >     A couple more notes:
>     >     Happens if using different x310s or the same for tx/rx.
>     >     Happens even if I try to add a small complex value before
>     sending
>     >     data to the USRP.
>     >     Error always happens as a single bit error (not bursty).
>     >
>     >     Attached are the constellation plots out of the polyphase clock
>     >     sync (PFB) and the costas loop. My guess is the issue is
>     either at
>     >     the USRP or the Polyphase Clock Sync block.
>     >
>     >     Anyone seen something like this before? I'll probably start
>     diving
>     >     in the polyphase clock sync code to figure what's going on.
>     >
>     >     Thanks,
>     >     Mike
>     >
>     >
>     >
>     >
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