Re: [Discuss-gnuradio] Carrier Phase Recovery

[David I. Emery]

On Sat, Apr 04, 2009 at 11:16:42PM +0500, Sajjad Sarwar wrote:
> I am doing my UG project on "optimized packet radio" in SDR. Presently, I am
> using costas loop block but i think it's recovering only the frequency of
> local oscillator but not the phase. Please correct me if i am wrong. Now the
> problem is i dont want to use dqpsk because it would cause a 3dB loss of
> power to attain the same BER which would kill the aim of optimization. So
> please give me some solution for "phase recovery" so that i can use simple
> qpsk without differential coding. Thank you.

        A Costas loop WILL  recover phase on a QPSK signal, but with a
pi/2 (90 degree) phase ambiguity meaning there are 4 possible mappings
of the  constellation points to bits (four possible rotations of the
constellation).

        The usual way of handling this is to try all four possible
rotations to see which one provides correct low error rate decoding of
the FEC code used.   Only one will have a zero or low error rate, the
others will be full of errors.   Convolutional or other related inner
FEC is almost universally used with QPSK in actual practice - often with
Vitirbi or other soft decision decode - so this falls out for free.

        It is also possible to look for a specific preamble/sync pattern
in a burst (packet) environment, especially if that pattern is chosen to
be unambiguous across the four possible rotations of the constellation
(eg only one of them produces the correct pattern).

-- 
  Dave Emery N1PRE/AE, address@hidden  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."