March 2011 - tetra - lists.osmocom.org

PI/4 DQPSK
by ikjtel 22 Mar '11

22 Mar '11

Thanks for including my "cqpsk.py" decoder module in your distribution. That looks to be a very old version and there have been some further discoveries since that time which I wanted to pass along, FWIW... First, the Costas frequency lock loop in GR seems to work much better for PI/4 DQPSK signals if it's patched in phase_error_tracking() : #if ENABLE_COSTAS_CQPSK_HACK if (d_interp_counter & 1) // every other symbol sample = sample * PT_45; // rotate by +45 deg d_interp_counter++; #endif /* ENABLE_COSTAS_CQPSK_HACK */ The effect of adding this hack to the code can be seen in Fig. 10 at my page http://www.lightlink.com/mhp/lsm/ With the patch included, the time to achieve lock is much improved... Second, I've found that a Gardner symbol timing loop works much better in our application than the M&M loop used in GR gr_mpsk_receiver(). Both of these improvements are included in a new GR block which has been optimized for the particular flavor of "PI/4 CQPSK" (LSM). It's in our svn repo - see http://sedition.org.au/op25/wiki/BuildInstructionsPage Look for the block in the repeater/src/lib tree - see repeater_gardner_costas_cc.cc YMMV ;-) [update: Tues. morning] two more things... 1. There is also a new Polyphase Filter Bank receiver which has been added in the latest versions of GNU Radio. It would be interesting to test and compare the performance of that receiver with the current one... 2. Would it be possible to post complex sample files (preferably not encrypted) to permit evaluation of this signal type? Best Regards 73 de KA1RBI Max

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Which hardware for playing out with tetra?
by Fabio Pietrosanti 07 Mar '11

07 Mar '11

Hi all, i have an USRP1 with DRX and RFX900 daughterboard and wanted to play a bit with the basic tetra decoder. Which kind of daughterboard has been successfully tested with the current osmocom tetra? Fabio

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calibration option
by Sim IJskes 02 Mar '11

02 Mar '11

Is there a reason for the calibration option to be an int? A eng_float looks much more logical to me, as it is used the spec a freq. Gr. Sim

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filter width
by Sim IJskes 01 Mar '11

01 Mar '11

When i interpret para 4 of ETSI TS 100 392-15 it looks like the channels are spaced at 25Khz. The default low_pass used in all of the demods is 25Khz. But when i look in the FFT of the complex signal, the 25Khz specified in the cutoff_freq of firdes.low_pass causes a passband of twice the width. Isn't this way to much? Shouldnt the cutoff_freq be 12.5Khz, giving a passband of 25Khz? (Or is there an intentional spectral overlap between adjacent channels?) If this is the cause, should we just change the parameter to filterwidth or channelspacing and use a /2 in the cutoff parm? Gr. Sim

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identifying tetra
by Sim IJskes 01 Mar '11

01 Mar '11

I've identified 4 carriers in the 400Mhz band, with a clear 4 state constellation. Is there an easy way to verify it is indeed tetra? I'm using grc. Gr. Sim

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