Re: [Discuss-gnuradio] Transmit and Receive hopping Sequence in FHSS (using gr-spread module) mismtach

[Ajinkya D Kadam]

Hi Marcus, 

I am not using frequency aligning.

I think your guess about "frequency offset" thing is possible. Since we are never transmitting on -187Khz but we do observe a signal over there. 

The received signal that we observe is just what is being transmitted. We are not applying any demodulation or receiver synthesis.I directly connect the USRPSource --> File Sink.  So my real concern is that why is the receiver not even looking at the transmitter sequence as is ??? 

The goal of the experiment is not to get data through, in which case we would have to deal with further issues. 

Our goal is to observe the transmitter hopping sequence correctly at the receiver. 

ᐧ

On Fri, Aug 26, 2016 at 3:28 PM, Marcus Müller <address@hidden> wrote:

Correct me, but this is 400kS/s – that should really not max out anyone's system, unless the spread blocks are really terribly inefficient (I don't think they are); please check this, though!

If I'm not mistaken, gr-spread doesn't tune the hardware or anything – it just creates a continuous phase stream that changes frequency, in a cyclic manner.

My guess is that the receiver is frequency-offset from the transmitter, and hence, the demodulator "guesses" the wrong frequencies. Would that be possible?

Ajinkya, are you frequency-aligning the two USRPs in any way?

Best regards,

Marcus

On 26.08.2016 21:22, Dave NotTelling wrote:

I have had issues with scheduling transmissions at ~ 10 ms per hop and ending up with transmissions happening on the wrong frequencies.  Usually it's one channel off.  If you slow the system down does everything work properly?  Also, are you pegging out any single core in your system?  I have seen lots of issues with scheduling bursts when the CPU utilization is high or even a single core that's maxed out.

On Fri, Aug 26, 2016 at 3:00 PM, Ajinkya D Kadam <address@hidden> wrote:

Dave, 

Thanks for replying. 

we are hopping every 80ms. 

ᐧ

On Fri, Aug 26, 2016 at 12:32 PM, Dave NotTelling <address@hidden> wrote:

Ajinkya,

     How fast are you hopping?  I've had loads of issues with hopping rapidly and not having the correct frequencies used.

-Dave

On Tue, Aug 23, 2016 at 7:16 PM, Ajinkya D Kadam <address@hidden> wrote:

HI All, 

I am using gr-spread module to transmit a signal using FHSS. When I receive the FHSS signal, the received signal has a completely different hopping sequence as from the transmitted one. I am using USRP N210 for transmission and reception. I am not sure why this is happening. The transmitted sequence is (frequencies in Khz)

    187.5

   -12.5

   162.5

   -62.5

    62.5

   137.5

   -112.5

   -37.5

   112.5

   -162.5

   -137.5

   -87.5

   12.5

   37.5

   87.5

 and the received sequence is 

      37.5

      62.5

      112.5

      -187.5

      12.5

      187.5

     -37.5

      87.5

      162.5

     -87.5

      0

      137.5

     -137.5

     -112.5

      -62.5

      37.5

      62.5

I have created a movie out of the images which plots the IQ, FFT and WaterFall plot for the transmitted signal and the received signal using the "plot_psd_base.py" file in gr-utils. Please have a look at both of them here and here. 

Please see the attached files for the flowgraphs, I am using as the transmitter and receiver.

I have stored the received and transmitted signal to a file. Since these are large files I have uploaded them to google drive here. 

Could someone please help me understand the behavior I am observing ? 

Is this some sort of synchronization issue ? What am I missing ?

Thanks in advance. 

ᐧ

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