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From: | Jingyi Sun |
Subject: | Re: [Discuss-gnuradio] BER & constellation plot for OFDM transmit/receive |
Date: | Fri, 22 Apr 2016 01:02:40 -0400 |
What do you mean by "hard-decision" equalizer, and "drop them down"?Thanks so much!!On Thu, Apr 21, 2016 at 2:01 PM, Martin Braun <address@hidden> wrote:That's because the equalizer used is a hard-decision equalizer. You can
write your own equalizers and drop them down for better results; that's
something I've been meaning to do for a while, but haven't found the
time yet.
Cheers,
M
On 04/20/2016 09:56 PM, Jingyi Sun wrote:
> Hope pictures below give more context. Has anyone seen this happen before?
>
> To reword the part of our question about constellation plots a little,
> does anyone know when the constellation block would output just four
> single points?
>
> The problem we’re currently facing is that the constellation plot does
> not even really show up -/there’s not even a giant blob to indicate
> noise/. Instead, we get only 4 points, one in each of the four locations
> you’d expect for a QPSK signal - but only those 4 points./It almost
> feels like the constellation plot stops triggering after these first
> four points/, and that’s why we don’t receive anymore data. *We’re using
> a free trigger on the positive slope, centered at 0, *which I feel
> should trigger at least something even if it’s just noise. We have also
> tried other trigger methods, which output either 4 points, 5 points (1
> additional point in the center), or just 1 center point.
>
>
>
>
>
>
> On Mon, Apr 18, 2016 at 5:01 PM, Jingyi Sun <address@hidden
> <mailto:address@hidden>> wrote:
>
> We are not compensating for any lost packets, although we would be
> aware of them thanks to the packet number. Could you tell us if
> there’s a block to do this, or would we need a custom block?
>
>
> Also, could you tell us why the constellation diagram is not
> displaying properly?
>
>
> Thanks,
>
> Jenny
>
>
>
> On Mon, Apr 18, 2016 at 1:26 PM, Martin Braun
> > address@hidden <mailto:address@hidden>> <address@hidden <mailto:address@hidden>> wrote:
>
> Are you comparing the correct packets? E.g., if packets get
> lost, do you
> take that into account?
>
> M
>
> On 04/16/2016 02:38 PM, Jingyi Sun wrote:
> > Hi everyone,
> >
> > We are working on an experiment for a conference paper deadline in two
> > weeks, and need to transmit and receive OFDM packets and want to study
> > the constellation diagram and BER.
> >
> > I put together a flow graph consisting of an *OFDM transmitter
> block*
> > and an *unpacked OFDM receiver* based on the online example
> rx_ofdm.grc.
> > Here's how I'm trying to measure constellation diagram and BER:
> >
> > * I inserted a QT constellation sink right before the
> constellation
> > decoder on the payload IQ stream, but it does not seem to output
> > anything meaningful. The plot just shows single, clean points, which
> > I am pretty sure does not correspond to real data. I suspect that
> > the plots are not triggering properly, but am not sure.
> >
> > * For BER, we tried several different configurations, and
> they mostly
> > give BER = 0.5 (i.e. random). Our leading theory is that we're not
> > comparing the data at the correct points in the flow graph. Any
> > suggestions as to what the BER inputs should be would be helpful.
> >
> > We've been running some diagnostics that seem to eliminate our
> > communication channel as the problem:
> >
> > * We are transmitting the data over-the-air at 915 MHz using
> > two omnidirectional antennas, placed roughly 1 meter apart. The
> > output spectra at the transmitter output and receiver input are
> > attached - all signals are comfortably above the noise floor.
> > * From the tag debug output, we see that the OFDM packet
> headers are
> > being received. For example, we can see when the packets are
> > received, the packet numbers, as well as the channel estimation tap
> > values. We take this to mean that we are receiving data
> > successfully, and that our difficulties regarding BER and
> > constellation diagram are something we're executing incorrectly in
> > the software.
> >
> >
> > The relevant annotated GRC block diagrams are attached.
> >
> > Thanks so much,
> > Jenny
> >
> >
> > _______________________________________________
> > Discuss-gnuradio mailing list
> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> >
>
>
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