Tx_ofdm and rx_ofdm

GNU Radio
1

Hi:
Now I run the example(tx_ofdm and rx_ofdm) through two usrps.(The
version of my gnuradio is 3.7.4).
1.But in the receive part I receive two many things like this:
INFO: Detected an invalid packet at item 139632
INFO: Parser returned #f
INFO: Detected an invalid packet at item 139680
INFO: Parser returned #f
INFO: Detected an invalid packet at item 139728
INFO: Parser returned #f
INFO: Detected an invalid packet at item 139776
INFO: Parser returned #f
INFO: Detected an invalid packet at item 139824
INFO: Parser returned #f
INFO: Detected an invalid packet at item 139872
INFO: Parser returned #f
INFO: Detected an invalid packet at item 139920
INFO: Parser returned #f
INFO: Detected an invalid packet at item 139968
INFO: Parser returned #f
INFO: Detected an invalid packet at item 140016
INFO: Parser returned #f
INFO: Detected an invalid packet at item 140064
And I search the maillist and I find someone encounter the same problem
like
me.(Re: [Discuss-gnuradio] Fwd: Questions on rx_ofdm example in GR 3.7.1)And
I see"As long as the receive SNR is high enough, the problem does not
show up."And I try to increase the “Multiply Const” to increase the
snr.But it failed.And I try to increase the receive gain.And also
failed.Can someone help me?Thank you very much.
2.And in the transmit part:
UHD Warning:
The hardware does not support the requested TX sample rate:
Target sample rate: 0.100000 MSps
Actual sample rate: 0.195312 MSps
And the device I use is USRP N210.And I set the sample rate is 100k.
Some days ago,someone tell me set the sample rate 100M/even number.And
the 100k seems right,why this UHD warning?Can someone tell me what is
the appropriate sample rate for USRP N210?Thank you very much.
Any advices will be appreciated.Thank you.
Best regards
xianda

2

Hi xianda,

I found out that using a factor of 0.05 in the multiply const works best
for me, with USRPs about 50 - 100 cm appart and rx and tx gains set to
0.

Try using a sample rate of 200k and you shouldn’t get that sample rate
error again.

Regards,
Jawad

3

Hi Jawad:
Thank you so much for your kindly reply.
But the first problem still exist.Can you send me your test document and
I test.Thank you.
And the second problem is solved.Can you tell me why 200k is ok while
100k failed?(they are all 10M/(even number)).Thank you.
Best regards,
xianda

At 2014-05-26 19:42:28,“Jawad S.” [email protected] wrote:

Hi xianda,

I found out that using a factor of 0.05 in the multiply const works best
for me, with USRPs about 50 - 100 cm appart and rx and tx gains set to
0.

Try using a sample rate of 200k and you shouldn’t get that sample rate
error again.

Regards,

Jawad

4

On 26.05.2014 13:03, xianda wrote:

Hi:
Now I run the example(tx_ofdm and rx_ofdm) through two usrps.(The
version of my gnuradio is 3.7.4).
1.But in the receive part I receive two many things like this:
INFO: Detected an invalid packet at item 139632
INFO: Parser returned #f

Xianda,

this questions pops up a lot on this list, and 99% of the time it means
your RF settings are distorting your signal. Try reducing gain and
multiplier, and make sure you get a nice spectrum before you try and
receive (i.e. check it with uhd_fft and make sure it’s rectangular).

UHD Warning:
The hardware does not support the requested TX sample rate:
Target sample rate: 0.100000 MSps
Actual sample rate: 0.195312 MSps
And the device I use is USRP N210.And I set the sample rate is 100k.
Some days ago,someone tell me set the sample rate 100M/even number.And
the 100k seems right,why this UHD warning?Can someone tell me what is
the appropriate sample rate for USRP N210?Thank you very much.

If you want 100K, you need to resample on the host. A stock N210 can go
down to 100e6/512 (i.e. what you’re seeing). But that’s OK, resampling
from 200k to 100k is not too CPU-intensive if you have a decent
hardware.

M

5

First of all, I removed the OFDM receiver part from tx_ofdm and replaced
it
by a USRP sink and added a tag debugger to know when the data was
generated
from the transmitter.

I also removed the OFDM transmitter part from rx_ofdm and replaced it by
a
USRP source.

You can see all of this in the files attached. Change the frequencies
and
sample rates according to your needs.

As long as there is data being sent and received, you shouldn’t see any
of
the warning messages from the receiver.

Regarding the sampling rate problem, I believe there is a minimal sample
rate and you can’t go below it.

Regards,
Jawad

2014-05-26 14:22 GMT+02:00 xianda [email protected]:

6

Hi Martin:
Thank you so much for your help.It can run.Thanks.
Best regards
xianda

7

Hi Jawad:
Thank you so much for your help.Now it can run.Thanks.
Best regards

At 2014-05-26 20:44:13,“Jawad S.” [email protected] wrote:

First of all, I removed the OFDM receiver part from tx_ofdm and replaced
it by a USRP sink and added a tag debugger to know when the data was
generated from the transmitter.

I also removed the OFDM transmitter part from rx_ofdm and replaced it by
a USRP source.

You can see all of this in the files attached. Change the frequencies
and sample rates according to your needs.

As long as there is data being sent and received, you shouldn’t see any
of the warning messages from the receiver.

Regarding the sampling rate problem, I believe there is a minimal sample
rate and you can’t go below it.

Regards,

Jawad

2014-05-26 14:22 GMT+02:00 xianda [email protected]:
Hi Jawad:
Thank you so much for your kindly reply.
But the first problem still exist.Can you send me your test document and
I test.Thank you.
And the second problem is solved.Can you tell me why 200k is ok while
100k failed?(they are all 10M/(even number)).Thank you.
Best regards,
xianda

At 2014-05-26 19:42:28,“Jawad S.” [email protected] wrote:

Hi xianda,

I found out that using a factor of 0.05 in the multiply const works best
for me, with USRPs about 50 - 100 cm appart and rx and tx gains set to
0.

Try using a sample rate of 200k and you shouldn’t get that sample rate
error again.

Regards,

Jawad

8

Hi xianda,

May I know the type of USRP are you using for the above .grc files?
Currently using a ettus USRP with RFX2400MHz daughter board.

I had tried the .grc file attached to one of the thread on this site,
but unable to receive the right data successfully.

In the rx_ofdm.grc file, i had replaced the USRP source with the output
of tx_ofdm.grc (i had written the output of the tx_ofdm to a file sink,
followed by replacing the USRP source with the “received file”)

At the beginning of receiving each data, the packet length of mine is at
200 instead of 96.

I had the error too! Am i suppose to tweak the samp_rate or the sampling
frequency.

INFO: Detected an invalid packet at item 252356
INFO: Parser returned #f
INFO: Detected an invalid packet at item 252402
INFO: Parser returned #f
INFO: Detected an invalid packet at item 252448
INFO: Parser returned #f
INFO: Detected an invalid packet at item 252494
INFO: Parser returned #f
INFO: Detected an invalid packet at item 252540
INFO: Parser returned #f
INFO: Detected an invalid packet at item 252586
INFO: Parser returned #f
INFO: Detected an invalid packet at item 252632
INFO: Parser returned #f

Thank you in advance,