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tsaitgaist, 02/19/2016 10:52 PM
mini-typo fix


PageOutline = 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.

Big picture

{{{
#!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" ];
}
}}}
Source Code
The source code is available via read-only git access at {{{
git clone git://git.osmocom.org/osmo-tetra.git
}}}

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

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

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).

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

{{{src/demod/python/cpsk.py}}} * contains a gnuradio based pi4/DQPSK demodulator, courtesy of KA1RBI {{{src/demod/python/tetra-demod.py}}} * call demodulator on a 'cfile' containing complex baseband samples {{{src/demod/python/usrp1-tetra_demod.py}}} * use demodulator in realtime with a USRP1 SDR {{{src/demod/python/usrp2-tetra_demod.py}}} * use demodulator in realtime with a USRP2 SDR {{{src/demod/python/fcdp-tetra_demod.py}}} {{{src/demod/python/fcdp-tetra_demod_fft.py}}} * use demodulator in realtime with a [wiki:Funcube_Dongle]. Please use the [https://github.com/csete/qthid 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

PHY/MAC layer

=== library code ===

Specifically, it implements:

{{{lower_mac/crc_simple.[ch]}}} * CRC16-CCITT (currently defunct/broken as we need it for
non-octet-aligned bitfields) {{{lower_mac/tetra_conv_enc.[ch]}}} * 16-state Rate-Compatible Punctured Convolutional (RCPC) coder {{{lower_mac/tetra_interleave.[ch]}}} * Block interleaving (over a single block only) {{{lower_mac/tetra_rm3014.[ch]}}} * (30, 14) Reed-Muller code for the ACCH (broadcast block of
each downlink burst) {{{lower_mac/tetra_scramb.[ch]}}} * Scrambling {{{lower_mac/viterbi*.[ch]}}} * Convolutional decoder for signalling and voice channels {{{phy/tetra_burst.[ch]}}} * Routines to encode continuous normal and sync bursts {{{phy/tetra_burst_sync.[ch]}}}

=== Receiver Program ===

The main receiver program {{{tetra-rx}}} expects an input file containing a
stream of unpacked bits, i.e. 1-bit-per-byte.

=== Transmitter Program ===

The main program {{{conv_enc_test.c}}} generates a single continuous downlink sync
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.

Quick example

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

{{{
./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
}}}

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

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

tetramod.png - Tetra carrier as produced by the modulator (42.9 KB) laforge, 06/01/2011 06:03 PM

osmotetrafcdp.png - TETRA carrier received with the FCDP (50.2 KB) horiz0n, 06/12/2011 08:46 PM

osmo-tetra-demod.png (61.7 KB) horiz0n, 06/07/2012 02:09 PM