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

[Nicolas Cuervo Benavides]

Hi Martin,

Thank you for answering me. At the Rx I am not able to see any data after the first message (which is correctly decoded and printed) is received. However I may start thinking that the messages are getting stuck in the tx, which is rather confusing. I checked three things after I start both Tx and Rx, and in both I am watching at the spectrum in frequency/waterfall sinks:

1. The message is ready to send, so I hit 'enter'. I get an 'U' at Tx (which I expect, as I am no longer sending data) and I correctly receive the message in the Rx. However, I do not see the message still in my loopback in the Tx part.

2. I hit "enter" again, the receiver is getting nothing, but now I see the message in my loopback. However, I can't see my OFDM waveform at the transmitter: https://drive.google.com/file/d/0B10fLbRQIxbdOXdzTGI3SGNDNTA/view?usp=sharing

3. I hit "enter" one more time. Again, Rx is not getting anything, but now my waveform is there, which confuses me as the frequency sink is connected at the same point where the USRP sink is, meaning that the signal has been there but is not yet shown. So the flowgraph is stuck: https://drive.google.com/file/d/0B10fLbRQIxbdR1NNdDBsV2xsNkE/view?usp=sharing

Then the behavior is not much different afterwards. I keep hitting enter to see that, at the receiver, I am getting power over the air, but no data is being processed. This waterfall sink is the result of having the "enter" pressed at the Tx part: https://drive.google.com/file/d/0B10fLbRQIxbdSy1iM0FMRHRyT0E/view?usp=sharing

So the Rx is running continuously, and the Tx is sometimes putting something in the air, but that something does not come with each 'enter' press, and is not being decoded as a message. Is this behavior comparable with what you were having years ago?

Regards,

Nico

On Thu, Feb 11, 2016 at 5:52 PM, Martin Braun <address@hidden> wrote:

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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> Discuss-gnuradio mailing list
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Nicolás Cuervo Benavides

Handy: +49 157 70476855

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