Forum: GNU Radio Running GMSK mod/demod test

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Florian Schlembach (Guest)
on 2013-08-12 13:06
(Received via mailing list)
Attachment: gmsk_test.grc (10 KB)
I trying to run a GMSK mod/demod test under GRC. My objective is to
create a single GSM-like burst and to store it in a file. The waveform
should have defined bits (by now I assume them to be random).

Thus I created a flowgraph that looks like this

Random Source (fixed # of samples, no repeat) -> Packet Encoder -> GMSK
Mod (->File Sink gmsk_tx) -> GMSK Demod -> Packet Decoder -> File Sink
(integer of bits_out)

1. If I keep increasing the number of bits in Random Source, I have
noticed that the number generated complex samples (I read them into
MATLAB using read_complex_binary) stays constant at 8496 complex samples
until 255 modulated bits. From 255 to 383 it is 2*8496, from 384 to ...
it is 3*8496. Are these the bursts? Why are the bound to integers of

2. When forming a loopback, reading the modulated signal and
demodulating it, the bits_out are not demodulated when reading the file
bits_out via read_int_binary. Whats wrong with the flowgraph?

I attached the respective .grc file. I appreciate any help!

Florian Schlembach (Guest)
on 2013-08-13 15:31
(Received via mailing list)
To put it in simple words, I am wondering why the GMSK-Mod blocks always
outputs 8192 bits - no matter when I input 50 bits or 255 bits.

Is there a GMSK-burst-like modulation happening?
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