[Patch-gnuradio] Re: gr_mpsk_receiver_cc.cc fix for qpsk demodulation

[Ben Green]

On Mon, Jun 8, 2009 at 2:11 PM, Ben Green <address@hidden> wrote:

In gr_mpsk_receiver_cc.cc, the qpsk-optimized function for calculating phase error makes guesses about the constellation point based on constellation ( 1 + 1j), (-1 + 1j), (-1 -1j), and (1 - 1j). However, the rest of the receiver assumes a constellation of (1 + 0j), (0 + 1j), (-1 + 0j), and (0 - 1j). Currently the BER is terrible for qpsk demod, and with much noise at all it fails completely. This patch fixes the function so that it is consistent with the rest of the block, and works much better now.

I apologize for file type issues, here is the patch in plaintext:

--- gr_mpsk_receiver_cc.cc    2009-05-29 11:15:17.000000000 -0500
+++ gr_mpsk_receiver_cc.cc    2009-06-08 13:18:37.478473275 -0500

@@ -124,8 +124,21 @@
 float
 gr_mpsk_receiver_cc::phase_error_detector_qpsk(gr_complex sample) const
 {
-  float phase_error = -((sample.real()>0 ? 1.0 : -1.0) * sample.imag() -
-            (sample.imag()>0 ? 1.0 : -1.0) * sample.real());

+  
+  float phase_error = 0;
+  if(fabsf(sample.real()) > fabsf(sample.imag())) {
+    if(sample.real() > 0)
+      phase_error = sample.imag();
+    else
+      phase_error = -sample.imag();
+  }
+  else {
+    if(sample.imag() > 0)
+      phase_error = -sample.real();
+    else
+      phase_error = sample.real();
+  }
+
   return -phase_error;
 }