(no subject)

Hi
I am trying to establish a simple link between two USRPs. My flow graph
is
File_Source->PSK_MOD(BPSK)->USRP_sink
USRP_Source->PSK_Demod->pack_k_bits->File_Sink.
The received data is not matching with the source data. I removed the
USRPs
and directly connected the PSK_MOD to PSK_DEMOD in flow gragh and
compared
the data. First 50 bits were in error but rest of the data were exactly
the
same after demodulation. I tried to add packet encoder in the
transmitter
side before modulation and packet decoder after demodulation and ended
with
nothing at the file sink. My question is why those first 50 bits are in
error? and why packet encoder doesnt work when the data is transfered
over
the air? Any help would be appreciated
Thanks in advance

Warm Regards

hello SHRIHARSHA,
If your simulation looks good, try and see what’s different about the
incoming signal on air 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.
The packet decoder will work pefectly if the input signal of the
demodulator
is good enough. The sample rate is also an issue, you should find the
best
value for your application.
Zakaria


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Hi Shriharsha,

As Zakaria already pointed out, since real channels are imperfect,
packet losses must occur.
The demod will need some samples to lock on to your rx signal, so losses
at start are to be expected.

I can only point you to some reference:
there is a GNU Radio Companion file in
gnuradio/gr-digital/examples/demod/ called dpsk_loopback.grc . That
should work.
You can insert a channel model in between mod and demod, and see what
happens. Increase channel influences (noise power, etc.) until they
match or exceed
what you’d expect from your real-world channel.
Later, split the flowgraph in halves and connect them to your USRP
sink/sources (don’t forget to remove the throttle at that point).

Greetings,
Marcus

PS: For the future, when asking a question, please try to apply the
hints from
http://gnuradio.org/redmine/projects/gnuradio/wiki/ReportingErrors .
This will make it much easier to help you :slight_smile:

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