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

[Martin Braun]

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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