Re: [Discuss-gnuradio] chat app with OFDM -- Asynchronous TX

[Martin Braun]

Nicolas,

this sounds really weird, and I'll admit I don't have an immediate
answer. I used that exact setup at FOSDEM 2 years ago to demonstrate
packet based wireless comms in GNU Radio, and I remember having some
issue with messages getting stuck in some pipeline, but not with the
actual over-the-air communication.

Even with the tx probably being at fault, looking at the rx might reveal
things. In particular, does the detector detect anything? Basically, is
anything happening pre-CRC-check?

M

On 02/11/2016 06:30 AM, Nicolas Cuervo Benavides wrote:
> Hi list!
> 
> I've been playing around with the OFDM TX/RX for a while, and I thought
> it would be interesting to send over the air custom messages and display
> the waveform at the receiver. For that purpose, I took the chat
> sanitizer and receiver block that I came across from [1]. The OFDM
> header modulation resulted helpful for synchronization and tag
> generation at the receiver, which is is continuously running for burst
> samples. However, with the sanitizer in the flowgraph, the TX becomes
> asynchronous, sending data only when an "enter" is hit in the msg text
> entry box. 
> 
> The flowgraph is basically the same OFDM example found
> in gnuradio/gr-digital/examples/ofdm, but with the chat sanitizer and
> async crc as input from the Tx, and a chat receiver (with respective crc
> check) at the end of the demodulator at the Rx. I'm using a sampling
> rate of 500k and as USRP the ettus x310 units  
> 
> With this configuration, I am able to receive only one message,
> corresponding to the first sent from the Tx, but after this one I
> haven't been able to receive more. The following messages seem to be
> lost in the air, but I can see that there is some transmission being
> done as I see the OFDM spectrum in a frequency sink at the receiver. My
> first thought is that it was a synchronization problem at the receiver,
> but then I noticed that If I restart only the Tx several times after
> sending one message each time, the receiver can catch them, but never
> more than one per Tx run, so the header is being received and analyzed.
> Now I'm thinking that the Tx is sending stuff that do not correspond to
> the messages that I'm sending. However I connected a loop back with a,
> lets say, virtual OFDM demodulator at the Tx, and in there I can see my
> messages being propagated. 
> 
> Now I'm running out of ideas of why my messages are not coming through.
> Any ideas on how I can maintain the communication continuously? 
> 
> Thanks in advance!
> 
> Regards,
> 
> Nico
> 
> [1] 
> http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorial_Programming_Topics#533-Example-Chat-Application
> -- 
> Nicolás Cuervo Benavides
> Handy: +49 157 70476855 <tel:%2B49%20157%2070476855>
> Electric and Electronic Engineering department.
> Electronic Engineering
> Universidad Nacional de Colombia
> --
> Student M.Sc. Information and Communication Technology
> Karlsruher Institut für Technologie
> Karlsruhe, Baden Württemberg, Germany 
> 
> 
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