OK, so I'm wetting my collective feet (all two of them) in the Tx side of the world, with a comms/telecom application, no less.
I'm playing with the OP25 project, which is an open-source initiative to produce tools to deal with the so-called "Project 25" digital radio standard that is emerging as the way lots of public-service people talk to each other on their radios. The TX side of the project is still fairly primitive--they have a hand-coded flow-graph that implements an APCO-25 4-level FSK modulator, using an *audio* sink, and then carefully plugging the audio into the guts of a physical radio, right at the FM modulator (for those of you who were around in the old amateur packet radio days, that's the way we used to do 9600BPS packet--bypass the usual FM audio filtering, and plug directly into the FM modulator). So, I decided to adapt what they'd done on the audio side to driving a USRP directly (with an appropriate RF card, naturally). The audio version of their modulator is fairly straightforward. It takes two-bit sequences from a packed-byte stream, and uses them to produce symbols from a simple [1.0, 3.0, -1.0, -3.0] chunks-to-simples conversion. From there, the symbols are run through an RRC filter, then an FM preemphasis filter, then a gain multiplier block, and thence to the audio sink. I have constructed a flow-graph in GRC that I think does pretty-much the same thing, only instead of dumping to the audio port, it has an FM modulator block, and then goes to a USRP sink, with appropriate interpolation specified. My GRC flow-graph is here: http://www.sbrac.org/files/fm4_usrp_modulator.grc APCO-25 runs at a nominal symbol rate of 4800sps, to give 9600BPS, and occupies a nominal 12.5KHz of bandwidth. The rest of it involves the usual suspects of FEC coding, packetizing, etc, etc. I'm just interested in getting the modulator correct at this stage, since the rest of the "goo" is already partially addressed in the existing OP25 work. I'm not sure that my parameters for the RRC are correct, for one, and exactly what to set the "sensitivity" parameter to in the the FM modulator (I imagine it's similar to setting the deviation control on an analog FM modulator). Also, I've copied the signal processing chain almost directly from the version of the modulator that dumps to the audio port, so perhaps there are shortcuts I can take to produce a nice APCO-25 4-level FSK signal that are "cheaper". -- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
