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gr-mixalot

gr-mixalot is intended to be a set of GNU Radio blocks/utilities to encode pager messages. Currently it only supports POCSAG numeric and alphanumeric pages. It's also intended to be a fairly clear, short example of basic transmission with GNU Radio.

The POCSAG encoder has been tested against many variants of the Motorola Bravo and Advisor series, as well as the ATI Wireless CCP-6000. Please get in touch if you find a device that doesn't work properly.

Building

Like many other out-of-tree modules, gr-mixalot uses cmake. The only dependencies should be a recent version of GNU Radio (3.6.x or 3.7.0 git should work) and the ITPP library. To use the HackRF sink, you'll also need gr-osmosdr installed.

To build, create a new directory and run:

% cmake PATH-TO-GR-MIXALOT-ROOT
% make

To install (may require root), run from the same directory: % make install

WARNING

POCSAG devices are still in use today. Although it's possible to acquire working pagers from a variety of sources (often for little or no cost), they may be tuned to an active channel. Transmitting over an existing station could cause legitimate users to silently lose messages. It is your responsibility to ensure that this doesn't happen.

Usage

To send a page to a device, you'll need to know:

  • the frequency it's tuned to (often printed on the device)
  • its cap code (often printed on the device)
  • its baud rate (POCSAG is specified for 512, 1200, and 2400 baud)
  • whether it wants alphanumeric or numeric pages

After installing, start gnuradio-companion and open the pocsagtx.grc flowgraph. (HackRF users: try pocsagtx_hackrf.grc.)

Double-click the "Single-Page POCSAG Xmit" block to modify the baud rate, capcode, and message. After saving, double-click the "pagerfreq" Variable block to change the tunning frequency. (This should be specified in Hz). Change the samp_rate and sink parameters as required by your setup. Build and run the flow graph; it should send the page, then exit.

Basic technical walkthrough

POCSAG is plain FSK with a deviation of 4500 Hz around the center frequency.

The pocencode block ("Single-Page POCSAG Xmit" in grc) encodes the message as a POCSAG bitstream. Then, it converts the bitstream into a stream of samples, running at a rate that is an integer multiple of the POCSAG baud rate. (In my example flowgraph, I've used 38400, which is divided by 512, 1200, and 2400 evenly. This seems to work fine.)

Next, the GNU Radio FM block is used in the flowgraph to modulate samples at the intermediate sample rate. Note that the output of pocencode is not a stream of (0, 1) bits, but (1, -1) samples. This is for the benefit of the math inside the FM block.

Finally, the stream is interpolated to the sample rate desired by the sink.
(In my example, there is also a const multiplication to avoid clipping; this may not be necessary for your sink, so feel free to remove it.)

Although this works for me, I wrote this in order to teach myself the basics of GNU Radio and working with signal processing through it; for that reason, the code is not as efficient as it could be. Holler if you see anything seriously wrong or broken, or if you have any suggestions!

Protocol documentation and other references

POCSAG is an extremely simple protocol; the spec is just a few pages long. You can grab it from the ITU here:

http://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.584-2-199711-I!!PDF-E.pdf

Several folks have also written useful pages that clarify or explain certain points of the spec, including:

Current todo list/Coming soon

  • Eliminate dependency on the ITPP library (currently only used for BCH)
  • Golay/GSC encoding
  • Flex support

Credits

gr-mixalot was written by Brandon Creighton cstone@pobox.com. The name is an homage to Sir Mix-a-Lot, who wrote the inspirational track "Beepers".

Suggestions and flames are equally welcome; as are additional pager-protocol documents.

Thanks to the following for testing and general awesomeness, in alphabetical order:

  • arko
  • banasidhe
  • docwho
  • dragorn
  • jeffE
  • the Ninjatel crew; barkode, cnelson, eliot, falconred, far_call, kubla
  • Mike Ossmann
  • pinguino
  • vyrus
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