uhd: Correct timing alignment in 8-PSK and GMSK downlink bursts

Delay the EDGE downlink bursts by one symbol in order to match GMSK
pulse shaping group delay. The difference in group delay arises from
the dual pulse filter combination of the GMSK Laurent represenation
whereas 8-PSK uses a single pulse linear filter.

Signed-off-by: Tom Tsou <tom.tsou@ettus.com>

1 files changed, 14 insertions, 5 deletions

diff --git a/Transceiver52M/sigProcLib.cpp b/Transceiver52M/sigProcLib.cpp
index 2182550..6848727 100644
--- a/Transceiver52M/sigProcLib.cpp
+++ b/Transceiver52M/sigProcLib.cpp
@@ -935,26 +935,35 @@ static signalVector *mapEdgeSymbols(const BitVector &bits)
   return symbols;
 }
 
+/*
+ * EDGE 8-PSK rotate and pulse shape
+ *
+ * Delay the EDGE downlink bursts by one symbol in order to match GMSK pulse
+ * shaping group delay. The difference in group delay arises from the dual
+ * pulse filter combination of the GMSK Laurent represenation whereas 8-PSK
+ * uses a single pulse linear filter.
+ */
 static signalVector *shapeEdgeBurst(const signalVector &symbols)
 {
-  size_t nsyms, nsamps = 625;
+  size_t nsyms, nsamps = 625, sps = 4;
   signalVector *burst, *shape;
   signalVector::iterator burst_itr;
 
   nsyms = symbols.size();
 
-  if (nsyms * 4 > nsamps)
+  if (nsyms * sps > nsamps)
     nsyms = 156;
 
   burst = new signalVector(nsamps, GSMPulse4->c0->size());
-  burst_itr = burst->begin();
 
-  for (size_t i = 0; i < nsyms; i++) {
+  /* Delay burst by 1 symbol */
+  burst_itr = burst->begin() + sps;
+  for (size_t i = 0; i < nsyms - 1; i++) {
     float phase = i * 3.0f * M_PI / 8.0f;
     Complex<float> rot = Complex<float>(cos(phase), sin(phase));
 
     *burst_itr = symbols[i] * rot;
-    burst_itr += 4;
+    burst_itr += sps;
   }
 
   /* Single Gaussian pulse approximation shaping */