tetra

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162 discussions

Re: [Tetra] TETRA Demodulation
by Harald Welte 05 Jan '11

05 Jan '11

(we have a mailinglist and we should use it... and write in english for sylvain's benefit) On Tue, Jan 04, 2011 at 08:16:12PM +0000, Dieter Spaar wrote: > Kleine Korrektur: > > Das Muster im vorherigen Bild ist natuerlich drei Bursts mit > "Frequency Correction Field" und dann einer ohne, das koennte > eventuell etwas mit dem Frame bestehend aus vier Slots zu > tun haben. The tdma multiplex of TETRA is unfortunately much more complicated than in GSM. I only really understood it earlier today while reading a textbook, from the specs it is very difficult to understand. The Frequency Correction Field is only present in the sync bursts (SB), which in turn are only (required to be) sent in Frame number 18 , but not on every multiframe. Interestingly, the SB is also present in _every_ timeslot, but not in every Frame 18 of every TS. The formula for the Timeslot Number is "4-(MN+1)mod4", it is indicated in Table 9.25 in Chapter 9.5.1

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[Tetra] code for the tetra MAC layer
by Harald Welte 05 Jan '11

05 Jan '11

Hi, I've started to implement the TETRA TMO MAC layer, i.e. the part that takes MAC blocks from higher layers and applies CRC, FEC, interleaving and scrambling before finally handing the bits to the DQPSK-modulator. The code can now generate a (hopefully valid) to-be-modulated bit pattern for a 'continuous downlink synchronization burst', i.e. what corresponds to the 'SCH+FCCH+BCCH' in GSM. You can find the code (including a small README) at: gitosis@git.osmocom.org:tetra-misc.git in the 'src' directory. If you don't yet have git access, feel free to mail me your sshv2 key. What's missing: * CRC16-CCITT to be computed over odd number of bits like 60 or 124 (all common implementations assume you encode integral # of bytes) [zecke] * decoding: * decoder for the RM(30,14) code [zecke] * soft-input/soft-decision viterbi for the RCPC code * synchronizer, able to detect synchro and normal training sequence [laforge] Once those tasks are completed, we can not only generate simulated burst bit patterns, but also decode them - which should be a good start for the time when Hopscotch' demodulator will spit out some real received bitstream. Regards, Harald -- - Harald Welte <laforge(a)gnumonks.org> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)

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