Constellation Object

GNU Radio
1

The symbol map feature of the constellation object doesn’t seem to do
anything. In fact, you can leave it blank. The index of the
constellation
points is used to map the complex points to bits. The documentation
leads
one to believe otherwise. Working as intended?

v/r,
Rich

2

On Tue, Nov 4, 2014 at 9:11 PM, Richard B. [email protected]
wrote:

The symbol map feature of the constellation object doesn’t seem to do
anything. In fact, you can leave it blank. The index of the constellation
points is used to map the complex points to bits. The documentation leads
one to believe otherwise. Working as intended?

v/r,
Rich

The symbol map is a list that maps to the constellation points. So
constellation_points[i] holds the symbol defined in symbol_map[i]. An
easy
test to see this working is to take the default Constellation Object in
GRC, which is defined for QPSK, and change the map to [0,1,3,3] and
notice
that the symbol 2 (10b) is missing in the output constellation. Without
a
symbol map defined, it defaults to it being [0, …, i-1]. The
alternative
is to crash or catch that the vector is empty and exit on an error.

There is the Digital Modulation page in the manual that describes these
things. If you have suggestions for how to improve them, please send
them
along:

http://gnuradio.org/doc/doxygen/page_digital.html

Tom

3

On Mon, Nov 10, 2014 at 1:31 PM, Richard B. [email protected]
wrote:

was the index of the constellation point array that was mapping symbols.
Correct me if you experience different.

Rich

Yes, I’ve used the symbol map a lot to change behavior.

A simple way to explore this is to create a test application with a
vector_source → constellation modulator → throttle → qtgui
constellation
sink.

The vector_source can be set to just repeat the same symbol over and
over
again 0 → 0x00, 1 → 0x55, 2 → 0xAA, and 3 → 0xFF. That way, you’ll
only
output one constellation point (assuming QPSK here). Then change your
symbol map table to see how it affects where the constellation point
goes.

Tom

4

Hi Tom,

I read that page before making my post, and that’s what lead me to
conclude the symbol map wasn’t working as intended. I ran a test by
changing the symbol map array [0, 1, 3, 2] vs [1, 0, 3, 2] and noticed
the output didn’t change. Only changing the constellation point order
made a change in the output. [1+1j, -1+1j, -1-1j, -1+1j] vs [-1+1j,
1+1j, -1-1j, -1+1j]

This lead me to conclude the symbol table wasn’t controlling anything,
it was the index of the constellation point array that was mapping
symbols. Correct me if you experience different.

Rich

Sent from my iPad