- Table of contents
- Osmocom TETRA MAC/PHY layer experimentation code
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.
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 libosmocore to link.
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).
Please make sure to read the MailingListRules before posting.
We now have a FAQ (Frequently asked Questions) page!
- contains a gnuradio based pi4/DQPSK demodulator, courtesy of KA1RBI
- call demodulator on a 'cfile' containing complex baseband samples
- use demodulator in realtime with a USRP1 SDR
- use demodulator in realtime with a USRP2 SDR
- use demodulator in realtime with a Funcube_Dongle. Please use the 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
Specifically, it implements:
- CRC16-CCITT (currently defunct/broken as we need it for non-octet-aligned bitfields)
- 16-state Rate-Compatible Punctured Convolutional (RCPC) coder
- Block interleaving (over a single block only)
- (30, 14) Reed-Muller code for the ACCH (broadcast block of each downlink burst)
- Convolutional decoder for signalling and voice channels
- Routines to encode continuous normal and sync bursts
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), containing:
- 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.
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 ...
The most user friendly way is the script osmosdr-tetra_demod_fft.py which is based on gr-osmosdr 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.
For live capture call:
src$ ./demod/python/osmosdr-tetra_demod_fft.py -o /dev/stdout | ./float_to_bits /dev/stdin /dev/stdout | ./tetra-rx /dev/stdin
You may specify gr-osmosdr device arguments by using the --args commandline option.
To use a gnuradio .cfile as input:
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
Note the mandatory rate argument and optional repeat & throttle arguments.