need some explaination

Sylvain Munaut 246tnt at
Wed Jul 8 08:49:53 UTC 2020


> this line is working fine
./ --args airspy -a 927 -s 6000000 -B L -f 1553968750 -g 19
> -b 40
> then after
> ./gmr1_rx 4 /tmp/arfcn_927.cfile
> wireshark is giving all informations
> I have "immediate assignment on ARFCN 928 929 930 931
> I tried to do the same with 928 929 930 931
> ./ --args airspy -a 929 -s 6000000 -B L -f 1554031250 -g 25
> -b 40
> with 1 Gb of datas, I can retrieve only 2 frames in wireshark
> I'm sure there is traffic on these 4 channels

So to follow assignments to dedicated channels, you need to use the `live`
branch of the code and use gmr1_rv_live binary.

Basically you can't decode _just_ the traffic channel (i.e. the 928 929 930
931 ARFCNs).  You need to first lock to the BCCH ( 927 ) and then follow
the assignment so you have the proper alignment etc ...
Which means you need to be receiving both channels at the same time, using
multiple `-a` arguments to

And then you use the gmr1_rx_live binary which supports processing several
channels in parallel and feed it all the data at once using something like :

gmr1_rx_live 4 927:/tmp/arfcn_927.cfile 928:/tmp/arfcn_928.cfile ...

Each of the argument after the '4' (which is the sample-per-symbol) is in
the form ARFCN:FILE   where ARFCN is the ARFCN number and FILE is the path
where to find the data corresponding to that ARFCN number.

And then the code will be able to find the BCCH lock to it and when it sees
an assignement command, it will look if it has a data file available for
that ARFCN and if it does, it will follow the assignement and start
decoding the dedicated channel as well.

> Is "immediate assignment " means TCH3 and voice ?

Not all Immediate Assignement means TCH3/voice, they can be control
channels or packet data or whatever else ... it just means a phone was
assigned a dedicated channel, you need to look at what type and what mode
of channel it is to know what it's going to carry.

> I tried all branches on git hub with same result
> What is usage of ? and arguments

So this script doesn't really "do" anything. All it does is take the
filename of a file recorded by the osmocom_fft program (which uses a
filename pattern that includes the center frequency and sample rate) and
then it prints the commands you can use to process that recording. Mostly
it's going to be two commands, the first one that will take the .cfile
recording and split it into a bunch of ARFCN channels and then another
command that will process all those ARFCN channels.

But as I said, it won't run those commands, it just prints them, you need
to cut&paste them and run them and/or adapt them to your needs.

> What is usage of gmr1_ambe_decode and gmr1_rach_gen

* gmr1_rach_gen generate examples RACH bursts, it's used for internal
testing of the c-band monitoring stuff in gnuradio
* gmr1_ambe_decode decodes TCH3 data payloads into PCM audio


     Sylvain Munaut
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