From fb6e75789b9a87065814b4caf8c5cad77f253df6 Mon Sep 17 00:00:00 2001
From: Thomas Tsou <tom@tsou.cc>
Date: Mon, 2 Sep 2013 14:39:35 +0800
Subject: 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 and/or input scaling levels.

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

Signed-off-by: Thomas Tsou <tom@tsou.cc>
---
 Transceiver52M/Transceiver.cpp |  4 ++--
 Transceiver52M/sigProcLib.cpp  | 32 ++++++++++++++++++++++++++++++--
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/Transceiver52M/Transceiver.cpp b/Transceiver52M/Transceiver.cpp
index 27e9c12..46c7213 100644
--- a/Transceiver52M/Transceiver.cpp
+++ b/Transceiver52M/Transceiver.cpp
@@ -138,7 +138,7 @@ SoftVector *Transceiver::pullRadioVector(GSM::Time &wTime,
   complex amplitude = 0.0;
   float TOA = 0.0;
   float avgPwr = 0.0;
-
+#ifdef ENERGY_DETECT
   if (!energyDetect(*vectorBurst,20*mSPS,mEnergyThreshold,&avgPwr)) {
      LOG(DEBUG) << "Estimated Energy: " << sqrt(avgPwr) << ", at time " << rxBurst->getTime();
      double framesElapsed = rxBurst->getTime()-prevFalseDetectionTime;
@@ -153,7 +153,7 @@ SoftVector *Transceiver::pullRadioVector(GSM::Time &wTime,
      return NULL;
   }
   LOG(DEBUG) << "Estimated Energy: " << sqrt(avgPwr) << ", at time " << rxBurst->getTime();
-
+#endif
   // run the proper correlator
   bool success = false;
   if (corrType == DriveLoop::TSC) {
diff --git a/Transceiver52M/sigProcLib.cpp b/Transceiver52M/sigProcLib.cpp
index bfcf5a4..b34c580 100644
--- a/Transceiver52M/sigProcLib.cpp
+++ b/Transceiver52M/sigProcLib.cpp
@@ -684,7 +684,26 @@ 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,
@@ -959,12 +978,16 @@ int detectRACHBurst(signalVector &rxBurst,
     return -1;
   }
 
-  _amp = peakDetect(corr, &_toa, NULL);
+   /* Perform fast peak detection (no interpolation) for initial gating */
+  _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();
@@ -984,9 +1007,14 @@ int detectRACHBurst(signalVector &rxBurst,
   if (par < thresh)
     goto notfound;
 
+  /* Run the full peak detection to obtain when we have a burst */
+  _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;
 
-- 
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