Osmo-tetra » History » Revision 17
Revision 16 (laforge, 02/21/2016 08:01 AM) → Revision 17/21 (laforge, 02/21/2016 09:59 AM)
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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
{{graphviz_link()
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" ];
}
}}
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.
!osmo-tetra-demod.png! [[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.
