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On air 16_QAM transmission problem with USRP N210

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On air 16_QAM transmission problem with USRP N210

zielalaoui
Hi all,
I am trying to build transmission on air by using USRPs N210. The transmission is good with MPSK modulations but when i try to use 16_QAM i could not receive data (0 bytes at the reception). I tried to adapte transmission power, gain at the tranmitter, gain at receiver, samp_rate but the reception file still always empty. This is my flow graph:
Tx: file source => packet encoder => QAM mod => mult constant = 0.4 => UHD usrp sink
Rx : UHD usrp source => QAM demod => packet decoder => file sink
samp_rate= 2M
samples/symbol = 8
centre frequency=2.4G
Any proposition will be appreciate
Thanks to all
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Re: On air 16_QAM transmission problem with USRP N210

Vanush Vaswani
You'll have to attach the flowgraph.


On Wed, Apr 30, 2014 at 7:22 AM, zielalaoui <[hidden email]> wrote:
Hi all,
I am trying to build transmission on air by using USRPs N210. The
transmission is good with MPSK modulations but when i try to use 16_QAM i
could not receive data (0 bytes at the reception). I tried to adapte
transmission power, gain at the tranmitter, gain at receiver, samp_rate but
the reception file still always empty. This is my flow graph:
Tx: file source => packet encoder => QAM mod => mult constant = 0.4 => UHD
usrp sink
Rx : UHD usrp source => QAM demod => packet decoder => file sink
samp_rate= 2M
samples/symbol = 8
centre frequency=2.4G
Any proposition will be appreciate
Thanks to all



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View this message in context: http://gnuradio.4.n7.nabble.com/On-air-16-QAM-transmission-problem-with-USRP-N210-tp47871.html
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Re: On air 16_QAM transmission problem with USRP N210

Martin Braun-2
In reply to this post by zielalaoui
On 29.04.2014 23:22, zielalaoui wrote:

> Hi all,
> I am trying to build transmission on air by using USRPs N210. The
> transmission is good with MPSK modulations but when i try to use 16_QAM i
> could not receive data (0 bytes at the reception). I tried to adapte
> transmission power, gain at the tranmitter, gain at receiver, samp_rate but
> the reception file still always empty. This is my flow graph:
> Tx: file source => packet encoder => QAM mod => mult constant = 0.4 => UHD
> usrp sink
> Rx : UHD usrp source => QAM demod => packet decoder => file sink
> samp_rate= 2M
> samples/symbol = 8
> centre frequency=2.4G
> Any proposition will be appreciate
> Thanks to all

There are lots of things that can go wrong. At 16-QAM, you'll likely
have some bit error, and you have no FEC to compensate for that.

Martin


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Re: On air 16_QAM transmission problem with USRP N210

Tom Rondeau-2
On Wed, Apr 30, 2014 at 3:22 AM, Martin Braun <[hidden email]> wrote:
On 29.04.2014 23:22, zielalaoui wrote:
Hi all,
I am trying to build transmission on air by using USRPs N210. The
transmission is good with MPSK modulations but when i try to use 16_QAM i
could not receive data (0 bytes at the reception). I tried to adapte
transmission power, gain at the tranmitter, gain at receiver, samp_rate but
the reception file still always empty. This is my flow graph:
Tx: file source => packet encoder => QAM mod => mult constant = 0.4 => UHD
usrp sink
Rx : UHD usrp source => QAM demod => packet decoder => file sink
samp_rate= 2M
samples/symbol = 8
centre frequency=2.4G
Any proposition will be appreciate
Thanks to all

There are lots of things that can go wrong. At 16-QAM, you'll likely have some bit error, and you have no FEC to compensate for that.

Martin

Also synchronization. That QAM Demod you have right after the USRP source block in your email suggests you aren't doing proper timing, freq, and phase recovery.

Tom

 

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Re: On air 16_QAM transmission problem with USRP N210

zielalaoui
Hi all,
For synchronization I did not add blocks because the QAM.py calls synchronization blocks from generic_mod file which already contains AGC, fll, polyphase clock synk, ... I thought that those bolcks are doing the job. So i just think to add FEC block because it is not a part of my flow-graph. Do you thinks that i should add synchronization block even it exists in QAM demod block? or just try to find the best parameters of timing, frequency, and phase in de QAM_demo block?
Thanks a lot for help.
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Re: On air 16_QAM transmission problem with USRP N210

Tom Rondeau-2
On Wed, Apr 30, 2014 at 10:58 AM, zielalaoui <[hidden email]> wrote:
Hi all,
For synchronization I did not add blocks because the QAM.py calls
synchronization blocks from generic_mod file which already contains AGC,
fll, polyphase clock synk, ... I thought that those bolcks are doing the
job. So i just think to add FEC block because it is not a part of my
flow-graph. Do you thinks that i should add synchronization block even it
exists in QAM demod block? or just try to find the best parameters of
timing, frequency, and phase in de QAM_demo block?
Thanks a lot for help.

Yes, synchronization for QAM is difficult and not extremely well-tested in GNU Radio. Have you done this in simulation, yet? That is, make a GRC example with the transmitter and receiver and the add a channel model block to see what happens? Definitely start there before getting ahead of yourself and adding the complexities of the real world.

Tom
 

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Re: On air 16_QAM transmission problem with USRP N210

zielalaoui
Hi Tom,
Yes i've already done the simulation with 16 QAM and even 64 QAM by following your example for MPSK. The simulation was very good even I add noise in the channel model. I had only problem of power, if the transmitting power is high enough the transmission is perfect with no errors. On air, I tried to add timing, frequency, and phase correction block but after running transmitting flow graph the simulation at receiver . what do you think about that?
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Re: On air 16_QAM transmission problem with USRP N210

Tom Rondeau-2
On Wed, Apr 30, 2014 at 12:04 PM, zielalaoui <[hidden email]> wrote:
Hi Tom,
Yes i've already done the simulation with 16 QAM and even 64 QAM by
following your example for MPSK. The simulation was very good even I add
noise in the channel model. I had only problem of power, if the transmitting
power is high enough the transmission is perfect with no errors. On air, I
tried to add timing, frequency, and phase correction block but after running
transmitting flow graph the simulation at receiver . what do you think about
that?

Did you try adjusting the timing and frequency offset in your simulation?

Tom
 

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Re: On air 16_QAM transmission problem with USRP N210

zielalaoui
Yes I did, with channel model like that:
Noise voltage = 200m
frequency offset=50m
epsilon=900m
taps=1
seed=0
And I can receive data.
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Re: On air 16_QAM transmission problem with USRP N210

Tom Rondeau-2
On Wed, Apr 30, 2014 at 1:01 PM, zielalaoui <[hidden email]> wrote:
Yes I did, with channel model like that:
Noise voltage = 200m
frequency offset=50m
epsilon=900m
taps=1
seed=0
And I can receive data.


Ok. A couple of things. First, please keep the rest of the conversation in the mail and bottom post like this. It helps make sure that the thread is readable. Second, I've been trying to coax you into providing more information. When I originally asked if you had simulated the connection, explaining that you have tested with these parameters then would have saved us a lot of time. Please be complete in your explanations.

Now that you've done this part and verified that everything is working in simulation, try and see what's different about the incoming signal as opposed to the simulation. Is the frequency offset maybe too large? Is the received amplitude correct? Try mapping these into your simulation and see where things start to fall apart. At this point, you seem to have the basic setup correct and just need to continue to experiment with the settings.

Tom
 

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Re: On air 16_QAM transmission problem with USRP N210

zielalaoui
Hi to all,
I resolved my problem and I want to share the solution
The problem was the amplitude in the QAM demod block input. I checked the constellation in the transmitter directly before UHD usrp block for being sure the the constellation points are good depending on the samples/symbol value (8 worked good). After that i just try to adapte the amplitude signal in reception by adding gain for to have the same signal which i had in my QAM simulation. Like that the transmission worked good.
Zakaria
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Re: On air 16_QAM transmission problem with USRP N210

Martin Braun-2
On 02.05.2014 16:59, zielalaoui wrote:
> Hi to all,
> I resolved my problem and I want to share the solution
> The problem was the amplitude in the QAM demod block input. I checked the
> constellation in the transmitter directly before UHD usrp block for being
> sure the the constellation points are good depending on the samples/symbol
> value (8 worked good). After that i just try to adapte the amplitude signal
> in reception by adding gain for to have the same signal which i had in my
> QAM simulation. Like that the transmission worked good.
> Zakaria

Zakaria,

generally, your equalizer should be taking care of that. Adjusting gain
is an unreliable method, since the actual attenuation of your signal
over-the-air is a function of many things.

But nice that your experiment is working!

Martin


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Re: On air 16_QAM transmission problem with USRP N210

Tom Rondeau-2
On Fri, May 2, 2014 at 11:08 AM, Martin Braun <[hidden email]> wrote:
On 02.05.2014 16:59, zielalaoui wrote:
Hi to all,
I resolved my problem and I want to share the solution
The problem was the amplitude in the QAM demod block input. I checked the
constellation in the transmitter directly before UHD usrp block for being
sure the the constellation points are good depending on the samples/symbol
value (8 worked good). After that i just try to adapte the amplitude signal
in reception by adding gain for to have the same signal which i had in my
QAM simulation. Like that the transmission worked good.
Zakaria

Zakaria,

generally, your equalizer should be taking care of that. Adjusting gain is an unreliable method, since the actual attenuation of your signal over-the-air is a function of many things.

But nice that your experiment is working!

Martin

Zakaria,

First, great that you got it working!

Martin makes a good point in general. One reason I mentioned the amplitude in my post is because I know that our consetllation objects used in the constellation_receiver has some strange slicing requirements. They expect the constellation to have the symbols at certain points in complex space (instead of, for instance, estimating the maximum energy of the constellation and scaling based on that). Using the LMS_DD equalzier with the same constellation object for the QAM modulation you're using will do what Martin is suggesting.

Tom
 

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