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Re: [Discuss-gnuradio] High packet error and reception problems with XC

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] High packet error and reception problems with XCVR 2450
Date: Tue, 16 Jun 2009 08:21:47 -0700

On Tue, Jun 16, 2009 at 7:31 AM, udadidd<address@hidden> wrote:

> 3) benchmark_tx/rx do not receive any packets though i give a transmit
> amplitude of 30000 and rx-gain of 75. The transmission seems to be sending
> 666 packets where as i receive only total of around 60 packets on the
> receiver end with ok=FALSE Always!! I get a 100% error rate.

Not commenting on the rest, but with respect to power and gain settings:

More transmit power is not always better.  More receiver gain is not
always better.  The above settings are almost guaranteed to fail.

You should use usrp_fft.py on the receiver side and leave it at
mid-gain.  Then use benchmark_tx.py and increase or decrease the
transmit amplitude, while monitoring the receive spectrum, until you
get the widest separation between the noise floor and the modulated
signal spectrum.  In general, the RFX2400 and other RFX boards start
to go into gain compression at a transmit amplitude of about 10-15%
DAC full-scale, which with the USRP1, is about 3000-5000.  Beyond
this, transmitting non-constant envelope signals will result in
significant distortion.  For a bench top transmission channel, the
RFX2400 can generate far more power than is needed to get essentially
error-free reception.

Once you have a transmit power that corresponds to the "best looking"
spectrum in usrp_fft.py on the receiver, you can then adjust the
receiver gain to further improve the signal-to-noise ratio.  Too low a
gain doesn't amplify the signal enough, so you see it drop into the
noise floor.  Too high a gain starts to increase the noise floor
without further increasing the signal level, due to non-linearities in
the receiver causing distortion.  So again, there will be a "just
right" spot.  At this point, you may be able to go back to the
transmitter and further improve things now that the receiver is
operating with the better gain setting.

Once you have a good pair of transmit amplitude and receiver gain for
your particular setup, then you can start running the benchmark_rx.py
script with the same gain setting to start receiving the packets being
sent by the transmitter.


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