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Re: [Discuss-gnuradio] BER & constellation plot for OFDM transmit/receiv


From: Martin Braun
Subject: Re: [Discuss-gnuradio] BER & constellation plot for OFDM transmit/receive
Date: Thu, 21 Apr 2016 11:01:12 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

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>> 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
>         >
>         >
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