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OsmocomTETRA » History » Version 5

laforge, 02/19/2016 10:52 PM
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= Osmocom TETRA MAC/PHY layer experimentation code =
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This code aims to implement the sending and receiving part of the
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TETRA MAC/PHY layer.
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If you read the ETSI EN 300 392-2 (TETRA V+D Air Interface), you will
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find this code implementing the parts between the MAC-blocks (called
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type-1 bits) and the bits that go to the DQPSK-modulator (type-5 bits).
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It is most useful to look at Figure 8.5, 8.6, 9.3 and 19.12 of the
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abovementioned specification in conjunction with this program.
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== Source Code ==
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The source code is available via read-only git access at
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{{{
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        git clone git://git.osmocom.org/osmo-tetra.git
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}}}
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You can also browse the source code at http://cgit.osmocom.org/
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You will need [http://bb.osmocom.org/trac/wiki/libosmocore libosmocore] to link.
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== Mailing List ==
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There is a public mailing list regarding development of this project, you can
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visit the subscription page at https://lists.osmocom.org/mailman/listinfo/tetra
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This list is '''for discussion between software developers''' who intend to improve the
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Osmocom TETRA software.  It is not a forum for individuals asking how they can tap
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into police radio (which is encrypted anyway).
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== Demodulator ==
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src/demod/python/cpsk.py
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        * contains a gnuradio based pi4/DQPSK demodulator, courtesy of KA1RBI
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src/demod/python/tetra-demod.py
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        * call demodulator on a 'cfile' containing complex baseband samples
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src/demod/python/usrp1-tetra_demod.py
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        * use demodulator in realtime with a USRP1 SDR
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src/demod/python/usrp2-tetra_demod.py
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        * use demodulator in realtime with a USRP2 SDR
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The output of the demodulator is a file containing one float value for each symbol,
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containing the phase shift (in units of pi/4) relative to the previous symbol.
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You can use the "float_to_bits" program to convert the float values to unpacked
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bits, i.e. 1-bit-per-byte
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== PHY/MAC layer ==
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=== library code ===
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Specifically, it implements:
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lower_mac/crc_simple.[ch]
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        * CRC16-CCITT (currently defunct/broken as we need it for
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          non-octet-aligned bitfields)
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lower_mac/tetra_conv_enc.[ch]
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        * 16-state Rate-Compatible Punctured Convolutional (RCPC) coder
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lower_mac/tetra_interleave.[ch]
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        * Block interleaving (over a single block only)
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lower_mac/tetra_rm3014.[ch]
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        * (30, 14) Reed-Muller code for the ACCH (broadcast block of
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          each downlink burst)
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lower_mac/tetra_scramb.[ch]
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        * Scrambling
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lower_mac/viterbi*.[ch]
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        * Convolutional decoder for signalling and voice channels
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phy/tetra_burst.[ch]
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        * Routines to encode continuous normal and sync bursts
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phy/tetra_burst_sync.[ch]
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=== Receiver Program ===
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The main receiver program 'tetra-rx' expects an input file containing a
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stream of unpacked bits, i.e. 1-bit-per-byte.
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=== Transmitter Program ===
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The main program conv_enc_test.c generates a single continuous downlinc sync
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burst (SB), contining:
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        * a SYNC-PDU as block 1
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        * a ACCESS-ASSIGN PDU as broadcast block
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        * a SYSINFO-PDU as block 2
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Scrambling is set to 0 (no scrambling) for all elements of the burst.
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It does not actually modulate and/or transmit yet.
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== Quick example ==
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assuming you have generated a file samples.cfile at a sample rate of 195.312kHz (100MHz/512 == USRP2 at decimation 512)
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{{{
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./src/demod/python/tetra-demod.py -i /tmp/samples.cfile -o /tmp/out.float -s 195312 -c 0
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./src/float_to_bits /tmp/out.float /tmp/out.bits
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./src/tetra-rx /tmp/out.bits
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}}}
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