Re: [Discuss-gnuradio] QPSK Demodulator

[Mason]

Hi,

   If you're using the gr_mpsk_receiver_cc block, it should still 
expect a rot=1 constellation:

(lines 186-192ish of gnuradio-core/src/lib/general/gr_mpsk_receiver.cc)
void
gr_mpsk_receiver_cc::make_constellation()
{
 for(unsigned int m=0; m < d_M; m++) {
   d_constellation.push_back(gr_expj((M_TWOPI/d_M)*m));
 }
}

d_M is the number of points of PSK modulation, namely 4. This produces a 
(1,0), (0,1), (-1,0), (0,-1) constellation.

If you're using the actual clock_recovery_mm_cc module (I'm not terribly 
familiar with it) it looks to be doing the same thing:

gr_complex
gr_clock_recovery_mm_cc::slicer_45deg (gr_complex sample)
{
 float real= -1, imag = -1;
 if(sample.real() > 0)
   real=1;
 if(sample.imag() > 0)
   imag = 1;
 return gr_complex(real,imag);
}

I would think an improperly rotated constellation would be dealt with as 
a phase offset in the receiver to be dialed out, but you'd obviously 
need your clock recovery and slicer to agree on the constellation. Does 
the clock recovery converge to that 45-degree offset, or does it start 
there and (perhaps over many samples, limited by response time) converge 
to a +?

As an aside, how will you ensure the receiver locks onto the same 
rotation as the transmitter?

--Mason



Pepito31 Pepito31 wrote:
> Hi all,
> I'm working on a qpsk demodulator (not differential), see code below :
>
>
>
>         # Connect & Initialize base class
>         self.connect(self,self.pre_scaler, self.agc, self.costas_loop,
>                          self.rrc_filter, self.clock_recovery)
>
>         self.connect(self.clock_recovery, self.diffdec, self.slicer,
> self.symbol_mapper,self.unpack, self)
>
> I've removed the diff_phasor block in the flowgraph. By default the
> constellation mapper use :
>         # find closest constellation point
>         rot = 1
>         #rot = .707 + .707j
>         rotated_const = map(lambda pt: pt * rot,
> psk.constellation[arity])
>         #print "rotated_const = %s" % rotated_const
>
>         self.slicer = gr.constellation_decoder_cb(rotated_const,
> range(arity))
>
>         if self._gray_code:
>             self.symbol_mapper = gr.map_bb(psk.gray_to_binary[arity])
>         else:
>             self.symbol_mapper = gr.map_bb(psk.ungray_to_binary[arity])
>
> I would like to know if rot has to be set to 1 or 0.707 + 0.707j as I
> have removed the diff_phasor ?
> When I draw the constellation after the clock_recovery block, I see
> graphicaly that the symbol are located around +45°, 135°,225°,-45°.
>
> Best regards,
>
> Pepito
>