Forum: GNU Radio Using GMSK Mod to send digital data (e.g. an ASCII string)

9ff3e82619caad90bfb1905a7a952424?d=identicon&s=25 embyte (Guest)
on 2013-07-20 22:41
(Received via mailing list)
Hello list, I would like to transmit some data using gnu-radio. My data
is a binary representation of an ASCII string.

I wrote my own block that builds the frame (header, payload, CRC,
trailer) as an array of 256 bytes (0s or 1s):

gr_make_io_signature(0, 0, 0)
gr_make_io_signature(1, 1, sizeof(char))

char *out = (char *) output_items[0];

[build the frame]

memcpy (out, frame, 256);
noutput_items = 256;

My question are:

1. Still I have to insert a "Packer Encoder"  between my code and the
GMSK mod? If yes, with which parameters? The GMSK mod is configured with
Samples/Sym=50 , BT=0.4.

2. Can someone explain me how to use the bitrate parameter? I mean,
should I "sync" the output of my block with the modulator? If yes, how?

I tried to redirect the transmitted signal to a file, and use a receiver
(that I know working), but without success. The parameters used in the
receiver's GMSK demod are the same configured in the modulator.

Thanks!
D7e274dc9584d108f4f863e265927685?d=identicon&s=25 Adeel Anwar (Guest)
on 2013-07-21 07:53
(Received via mailing list)
>>1. Still I have to insert a "Packer Encoder"  between my code and the
GMSK mod? If yes, with which parameters? The GMSK mod is configured with
Samples/Sym=50 , BT=0.4.
No. U can make custom packet structure and in that case u need not to
use
Packet-Encoder.

>>2. Can someone explain me how to use the bitrate parameter? I mean,
should I "sync" the output of my block with the modulator? If yes, how?
Sync is not required because Signal-Processing blocks in GR dont
understand bitrate/sampling-rate. If u have connected  Transmitter-FG
with
USRP,  then u can determine the "effective bitrate" of transmitter by
calculating it backwards from USRP-sink sampling-rate.

symbol-rate = usrp-sampling-rate / samples-per-symbol
bit-rate = symbol-rate * modulated_bits_per_symbol

-Adeel
9ff3e82619caad90bfb1905a7a952424?d=identicon&s=25 embyte (Guest)
on 2013-07-24 12:34
(Received via mailing list)
Hello Adeel/ML, this is fine, thanks.

Next doubt is the data format as input of the GMSK block.

Should it be as sequence of unsigned char[] bits ("01010101 00000000")
or unsigned char[] bytes ("\x55\x00")? Looking at the Mod code
(gmsk.py), the modulator first does a bytes_to_syms().

Second, in GRC 3.5.6.1 there are 2 GMSK modulator blocks, called:
- GMSK Mod (Samples/Symbols, BT)
- GMSK Modulator (Samples/Symbols, 3dB Time-Bandwith Product, Pulse
Duration)

What's their difference, and what are the meaning for the parameters on
the 2nd (GMKS Modulator)?

Thanks!


Il 21/07/2013 07:52, Adeel Anwar ha scritto:
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