Forum: GNU Radio blks2impl/dbpsk.py question

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Dimitris S. (Guest)
on 2009-03-05 18:37
(Received via mailing list)
Hi List,
I was reading through the dbpsk.py block in blks2impl, and found
something that doesn't seem to make much sense (to me).
Lines 272-273 read:
        rot = 1
        rotated_const = map(lambda pt: pt * rot,
psk.constellation[arity])
Is this not equivalent to:
        rotated_const = psk.constellation[arity]
??? I mean, it's just multiplying each output item of
psk.constellation with the number 1, or am I mis-reading something?

Thanx for looking at this...

Dimitris S.
"If you think you're too small to make a difference, try sleeping with
a mosquito!" - Amnesty International
Eric B. (Guest)
on 2009-03-05 20:18
(Received via mailing list)
On Thu, Mar 05, 2009 at 05:37:00PM +0100, Dimitris S. wrote:
>
> Thanx for looking at this...

I'm guessing that it's in there because at sometime someone wanted to
rotate the constellation.  (If rot were a suitable complex value, the
multiplication by rot would rotate it.)  rot = 1 means "don't rotate"

Eric
Stefan Brüns (Guest)
on 2009-03-06 18:53
(Received via mailing list)
On Thursday 05 March 2009 19:12:15 Eric B. wrote:
> > psk.constellation with the number 1, or am I mis-reading something?
> >
> > Thanx for looking at this...
>
> I'm guessing that it's in there because at sometime someone wanted to
> rotate the constellation.  (If rot were a suitable complex value, the
> multiplication by rot would rotate it.)  rot = 1 means "don't rotate"

This may be needed based on the type of mpsk decoder in the chain. As
currently there is a differential decoder, sybols will always be on
positions
(theta + i * 2pi/M; i=[1..M]; theta=0). With no differential decoder, it
depends on the carrier tracking loops view of 'zero phase (theta)'. For
the
mspk_receiver_cc, the constellation points will be the same as stated
for the
diff. encoding. In general theta can have any value, and for the qpsk
_encoder_ is set to 45 degrees.

Stefan

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