Transceiver52M: Disable energy detector

The adaptive energy threshold gating suffers a near-far problem
at certain gain levels. This is due to exponential threshold
raising, but linear decreases. A large signal level followed by
a period low signal level causes (comparatively) weak signals to
go undetected. Additionally, the algorithm performs differently
at multiple RF gain levels.

This patch switches solely to correlation based gating for burst
detection. The main computational load with this approach is
sub-sample width peak interpolation, which we disable for intial
detection and run after threshold passing.

Signed-off-by: Thomas Tsou <tom@tsou.cc>

1 files changed, 30 insertions, 3 deletions

diff --git a/Transceiver52M/sigProcLib.cpp b/Transceiver52M/sigProcLib.cpp
index a647f84..b0ef21a 100644
--- a/Transceiver52M/sigProcLib.cpp
+++ b/Transceiver52M/sigProcLib.cpp
@@ -653,8 +653,27 @@ complex interpolatePoint(const signalVector &inSig,
   return pVal;
 }
 
-  
- 
+static complex fastPeakDetect(const signalVector &rxBurst, float *index)
+{
+  float val, max = 0.0f;
+  complex amp;
+  int _index = -1;
+
+  for (int i = 0; i < rxBurst.size(); i++) {
+    val = rxBurst[i].norm2();
+    if (val > max) {
+      max = val;
+      _index = i;
+      amp = rxBurst[i];
+    }
+  }
+
+  if (index)
+    *index = (float) _index;
+
+  return amp;
+}
+
 complex peakDetect(const signalVector &rxBurst,
 		   float *peakIndex,
 		   float *avgPwr) 
@@ -928,12 +947,15 @@ int detectRACHBurst(signalVector &rxBurst,
     return -1;
   }
 
-  _amp = peakDetect(corr, &_toa, NULL);
+  _amp = fastPeakDetect(corr, &_toa);
+
+  /* Restrict peak-to-average calculations at the edges */
   if ((_toa < 3) || (_toa > len - 3))
     goto notfound;
 
   peak = corr.begin() + (int) rint(_toa);
 
+  /* Compute peak-to-average ratio. Reject if we don't have enough values */ 
   for (int i = 2 * sps; i <= 5 * sps; i++) {
     if (peak - i >= corr.begin()) {
       avg += (peak - i)->norm2();
@@ -953,9 +975,14 @@ int detectRACHBurst(signalVector &rxBurst,
   if (par < thresh)
     goto notfound;
 
+   /* Run the full peak detection to obtain interpolated values */
+  _amp = peakDetect(corr, &_toa, NULL);
+
   /* Subtract forward tail bits from delay */
   if (toa)
     *toa = _toa - 8 * sps;
+
+  /* Normalize our channel gain */
   if (amp)
     *amp = _amp / gRACHSequence->gain;