OP25j

= P25 Phase 2 / TDMA =

Support for P25 Phase 2 / TDMA AMBE voice decode was added to OP25 in the first quarter of 2014.

This initial release is in the form of several command line (CLI) utility programs which serve to divide the demodulation and decoding process into the following steps:

• Complex signal capture and storage
• Channel selection and decimation
• Demodulation
• TDMA demultiplexing and speech decoding

Example
We use the {{{rtl_sdr}}} app which is included in the rtl-sdr http://sdr.osmocom.org/trac/wiki/rtl-sdr package. {{{
rtl_sdr -s 2400000 -f 388500000 capture.bin
}}}

We don't need the entire 2.4 MHz band, so we select a 96 KHz wide slice of spectrum containing the desired channel

{{{
python arb-resample.py -i capture.bin -s 2400000 -r 96000 -g 10 -c 706300 -o capture-96k.dat
}}}

Demodulate the complex I/Q data to result in a file of symbols (dibits)

{{{
python cqpsk-demod-file.py -i capture-96k.dat -a -6 -r dbit.dat
}}}

Extract and play audio in TDMA slot 1

{{{
tdma_audio.py -i dbit.dat -n 0x4a1 -s 0x4a2 -w 0xbee00 -t 1
}}}

Notes

• When capturing signal data you shouldn't use the exact channel frequency as the capture frequency. This is so as to avoid "DC offset" or "zero IF" interference. A tuning offset of at least 50 KHz is recommended.
• The above tuning offset, plus an additional temperature dependent frequency offset (sometimes known as "PPM") must be compensated when selecting the channel ({{{-f 706300}}} in the example above). The offset frequency is used to tune the desired signal to zero IF. The general problem of obtaining proper tuning in the presence of frequency offset is one of the most common "troubles" reported to the GNU Radio mailing list; it's not an OP25-related issue.
• Also, the CQPSK frequency tuning loop (Costas loop) used in OP25 requires a tuning error less than about 1,200 Hz, to avoid spurious phase-locking to offsets at frequencies of PI/4 away from the desired center frequency.