OsmocomTETRA

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h1. Osmocom TETRA MAC/PHY layer experimentation code

This code aims to implement the sending and receiving part of the

TETRA MAC/PHY layer.

If you read the ETSI EN 300 392-2 (TETRA V+D Air Interface), you will

find this code implementing the parts between the MAC-blocks (called

type-1 bits) and the bits that go to the DQPSK-modulator (type-5 bits).

It is most useful to look at Figure 8.5, 8.6, 9.3 and 19.12 of the

abovementioned specification in conjunction with this program.

h2. Big picture

<pre>

<code class="graphviz">

digraph G {

  graph [ rankdir = LR ];

  bits_file2 -> tetra_rx [ label = "read" ];

  tetra_rx -> console [ label = "stdout" ];

  tetra_rx -> wireshark [ label = "GSMTAP" ];

  float_file2 -> float_to_bits [ label = "read" ];

  float_to_bits -> bits_file1 [ label = "write" ];

  USRP -> tetra_demod [ label = "USB" ];

  tetra_demod -> float_file1 [ label = "write" ];

  bits_file1 [ shape=box label="file.bits" ];

  bits_file2 [ shape=box label="file.bits" ];

  float_file1 [ shape=box label="file.float" ];

  float_file2 [ shape=box label="file.float" ];

  tetra_rx [ label="tetra-rx" ];

  tetra_demod [ label="tetra-demod.py" ];

}

</code></pre>

h2. Source Code

The source code is available via read-only git access at

<pre>

git clone git://git.osmocom.org/osmo-tetra.git

</code></pre>

You can also browse the source code at http://cgit.osmocom.org/

You will need "libosmocore":http://bb.osmocom.org/trac/wiki/libosmocore to link.

h2. Mailing List

There is a public mailing list regarding development of this project, you can

visit the subscription page at https://lists.osmocom.org/mailman/listinfo/tetra

This list is *for discussion between software developers* who intend to improve the

Osmocom TETRA software.  It is not a forum for individuals asking how they can tap

into police radio (which is encrypted anyway).

h2. FAQ

We now have a [[FAQ]] (Frequently asked Questions) page!

h2. Demodulator

<pre>

******** contains a gnuradio based pi4/DQPSK demodulator, courtesy of KA1RBI

<pre>

******** call demodulator on a 'cfile' containing complex baseband samples

<pre>

******** use demodulator in realtime with a USRP1 SDR

<pre>

******** use demodulator in realtime with a USRP2 SDR

<pre>

<pre>

******** use demodulator in realtime with a [[Funcube_Dongle]]. Please use the "qthid":https://github.com/csete/qthid application to tune the dongle and adjust its gain/filter parameters for best reception result. This demodulator may also be used with other Softrock-type receivers by downconverting the intermediate frequency of a radio scanner to the complex baseband.

The output of the demodulator is a file containing one float value for each symbol,

containing the phase shift (in units of pi/4) relative to the previous symbol.

You can use the "float_to_bits" program to convert the float values to unpacked

bits, i.e. 1-bit-per-byte

h2. PHY/MAC layer

h3. library code

Specifically, it implements:

<pre>

******** CRC16-CCITT (currently defunct/broken as we need it for

          non-octet-aligned bitfields)

<pre>

******** 16-state Rate-Compatible Punctured Convolutional (RCPC) coder

<pre>

******** Block interleaving (over a single block only)

<pre>

******** (30, 14) Reed-Muller code for the ACCH (broadcast block of

          each downlink burst)

<pre>

******** Scrambling

<pre>

******** Convolutional decoder for signalling and voice channels

<pre>

******** Routines to encode continuous normal and sync bursts

<pre>

h3. Receiver Program

<pre>

stream of unpacked bits, i.e. 1-bit-per-byte.

h3. Transmitter Program

<pre>

burst (SB), contining:

******** a SYNC-PDU as block 1

******** a ACCESS-ASSIGN PDU as broadcast block

******** a SYSINFO-PDU as block 2

Scrambling is set to 0 (no scrambling) for all elements of the burst.

It does not actually modulate and/or transmit yet.

h2. Quick example

assuming you have generated a file samples.cfile at a sample rate of 195.312kHz (100MHz/512 == USRP2 at decimation 512)

<pre>

./src/demod/python/tetra-demod.py -i /tmp/samples.cfile -o /tmp/out.float -s 195312 -c 0

./src/float_to_bits /tmp/out.float /tmp/out.bits

./src/tetra-rx /tmp/out.bits

</code></pre>

Also, you may use pipes to glue the three programs running in different terminals together to achieve real time operation.

<pre>

mkfifo /tmp/out.float

mkfifo /tmp/out.bits

./src/demod/python/fcdp-tetra_demod.py -D hw:1,0 -o /tmp/out.float

...

</code></pre>

The most user friendly way is the script osmosdr-tetra_demod_fft.py which is based on "gr-osmosdr":http://sdr.osmocom.org/trac/wiki/GrOsmoSDR and supports various radio hardware (OsmoSDR, RTLSDR, FCD, UHD) as well as IQ file input.

* Adjust the center frequency (-f) and gain (-g) according to your needs.

* Use left click in Wideband Spectrum window to roughly select a TETRA carrier.

* In Wideband Spectrum you may also tune by 1/4 of the bandwidth at once by clicking on the rightmost/leftmost spectrum side.

* Use left click in Channel Spectrum window to fine tune the carrier by clicking on the left or right side of the spectrum.

[[Image(osmo-tetra-demod.png,25%)]]

For live capture call:

<pre>

src$ ./demod/python/osmosdr-tetra_demod_fft.py -o /dev/stdout | ./float_to_bits /dev/stdin /dev/stdout | ./tetra-rx /dev/stdin

</code></pre>

You may specify gr-osmosdr device arguments by using the --args commandline option.

To use a gnuradio .cfile as input:

<pre>

src$ ./demod/python/osmosdr-tetra_demod_fft.py -a "file=/path/to/tetra_sps1024e3.cfile,rate=1024e3,repeat=true,throttle=true" -o /dev/stdout | ./float_to_bits /dev/stdin /dev/stdout | ./tetra-rx /dev/stdin

</code></pre>

Note the mandatory rate argument and optional repeat & throttle arguments.