Re: [Discuss-gnuradio] FSK with TX-RX in ISM-band 433 Mhz

[Eric Blossom]

On Tue, Mar 21, 2006 at 02:53:05PM +0100, Luis Simoes wrote:
> Hi all,
> 
> I am a novice in gnuradio and after searching in a lot of forums I put my 
> USRP 
> working and my Gnuradio running. Some FM and AM reception with and without 
> GUI are working fine. Now I have some problems in FSK. I try to receive in 
> ISM 433 MHZ some data packets with a data rate of 38.4 kbaud. The data is 
> manchester encoded. To do this I followed the way in the gnuradio examples 
> (fsk_rx.py) with a channel filter and the simple correlator. To do the 
> correct manchester decoding I changed the correlator block in according to my 
> needs and build the new block howto_manchester_correlator. The difference 
> between my manchester_correlator and the simple correlator is the packit 
> function and the sync bytes (instead of using GRSF_SYNC I use MAN_SYNC= 
> 0x66669999, these are the preamble and syncronization bits).
> Now when I start my application gnuradio starts receiving some data that is 
> written to a file, but my transmitting module is switched off and the data is 
> complete nonsense. When I switch on the transmitter the result is the same. 
> This occurs also when I ran the application with the simple correlator. The 
> seqno where in random order, but there should be nothing because I didn't 
> send anything in the simple_framer format. Now I think the correlators are 
> working well but the problem can be on the radio part.
> My system is working on Linux-kernel 2.6 (Linux 9.3) and the CPU is AMD 
> Athlon 
> XP 2400+
> I have tried to adjust all parameters with no result.
> 
> I am very grateful for any kind of help or advice.
> 
> Luis

Hi Luis,

The examples in gnuradio-examples/python/gmsk2, and the guts in
gnuradio-core/src/python/gnuradio/blksimpl/{gmsk2,gmsk2_pkt}.py are
much better starting points than the "simple correlator" code.  They
implement proper symbol timing tracking algorithms, instead of just
locking up at the header and then drifting from there.

When trying to get a mod/demod working, we usually start with a
simulation into and out of of files, or other sinks and sources.
That way we can look at the output of the transmitter and confirm that
it looks like what we expect.  Then we can sort out the receiver using
known input data.

Hope that helps,
Eric