Forum: GNU Radio False Packets for benchmark TX/RX

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On 04/12/12 07:47 PM, Yahya Ezzeldin wrote:
>
> Inline image 2
> ok = False  pktno =  230  n_rcvd =    6  n_right =    0
>
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The usual reason for this is frequency-offset between RX and TX.  In the
'real world' this is a constant problem,
  which is why complete, market-ready, radio-based digital comms systems
have a frequency locking mechanism
  in the receiver.

Try offseting the RX frequency in small amounts in either direction, and
see if you can get good packets.

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I am using RFX900 (FLEX 900) and the VERT 900 Antennas

Best Regards,
--
Yahya Ezzeldin
 <http://www.ieee.org>

On 12/04/2012 08:39 PM, Yahya Ezzeldin wrote:
> I am using RFX900 (FLEX 900) and the VERT 900 Antennas
>
> Best Regards,
> --
> Yahya Ezzeldin
>
The RFX900 doesn't have analog TX gain setting -- you have to use the
baseband magnitude exclusively to control RF output gain.
   Try dropping the amplitude to 0.4, and also adjust RX gain and RX
center frequency.

On Tue, Dec 4, 2012 at 8:51 PM, Marcus D. Leech <mleech@ripnet.com> 
wrote:

> baseband magnitude exclusively to control RF output gain.
>
Actually, the signal in the screen shot looks pretty ugly. I'd recommend
reducing the amount of power at the receiver. GMSK is tolerant of
nonlinearities, but there is a limit.

Tom

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On Wed, Dec 5, 2012 at 2:23 AM, Yahya Ezzeldin 
<yahyaezzeldin@gmail.com>wrote:

>
> made most of them be accepted as True (pass the CRC32 check if I understand
> correctly). What is the explanation for this ?
>

Often, the reason higher rates work better is that the relative 
frequency
offset is lower. You said that you couldn't find a frequency offset that
helped with your 250 kbps signal, so I'm not sure if this is really the
right answer for you. But that's generally the case.


> 2) Increasing the samples per symbol further improved the reception, why
> is that ?
>

Off the top of my head here... GMSK actually introduces ISI, but how 
that
happens is based on the shaping filter used, which we determine by the
number of sps. Increasing this is probably making a better shaped 
signal.


> 3) The two FFT screenshots at 250k and 1M are as follows. How does the
> difference between them solve the nonlinearity problem, Tom ?
>

Both of these looked good. I'm not sure it was really nonlinearities, 
that
was just a guess. The original signal you showed us had a strange bulge 
in
left half of the signal. These signals here look symmetric and properly
shaped.

Tom

Thank you Tom,

I have been experimenting with the working setting that I have now.

Why do other modulations like bpsk, qpsk fail while gmsk works perfectly 
?

Best Regards,
--
Yahya Ezzeldin

On Thu, Dec 6, 2012 at 10:44 AM, Yahya Ezzeldin 
<yahyaezzeldin@gmail.com>wrote:

> Thank you Tom,
>
> I have been experimenting with the working setting that I have now.
>
> Why do other modulations like bpsk, qpsk fail while gmsk works perfectly ?
>

That's a very open-ended question and can't be answered. There are so 
many
physical realities to getting signals to work, so it's likely not just 
one
thing (and certainly won't be as easy as it sounds when you read a 
textbook
on the difference in BER performance of modulation schemes).

One thing to keep in mind, though, is that you don't want to use BPSK or
QPSK. Use DBPSK and DQPSK instead. There is no mechanism for resolving 
the
phase ambiguity at the receivers. So when running these signals, make 
sure
that they are differentially encoded (use the -v option to get a verbose
output of the parameters).

Tom