Running GMSK mod/demod test


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 28496, 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!



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?