Transceiver: Fix clipping detection.

There are two primary changes in this commit:

1) Return values of detect functions changed form bool to int to actually pass
the return value from the inner function and notify higher levels about clipping.
Previously the information was lost due to conversion to bool.

2) Clipping level is not the final verdict now. We still try to demod a burst
and mark it as clipped only if the level is above the clipping level AND we can't
demod it. The reasoning for this is that in real life we want to do as much as
possible to demod the burst, because we want to get as much from our dynamic
range as possible. So a little bit of clipping is fine and is expected. We just
don't want too much of it to break our demod.

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

1 files changed, 31 insertions, 15 deletions

diff --git a/Transceiver52M/sigProcLib.cpp b/Transceiver52M/sigProcLib.cpp
index 9d8410b..42b4e6e 100644
--- a/Transceiver52M/sigProcLib.cpp
+++ b/Transceiver52M/sigProcLib.cpp
@@ -28,6 +28,7 @@
 
 #include "sigProcLib.h"
 #include "GSMCommon.h"
+#include "Logger.h"
 
 extern "C" {
 #include "convolve.h"
@@ -1372,16 +1373,17 @@ static int detectBurst(signalVector &burst,
   return 1;
 }
 
-static int detectClipping(signalVector &burst, float thresh)
+static float maxAmplitude(signalVector &burst)
 {
-	for (size_t i = 0; i < burst.size(); i++) {
-		if (fabs(burst[i].real()) > thresh)
-			return 1;
-		if (fabs(burst[i].imag()) > thresh)
-			return 1;
-	}
-
-	return 0;
+    float max = 0.0;
+    for (size_t i = 0; i < burst.size(); i++) {
+        if (fabs(burst[i].real()) > max)
+            max = fabs(burst[i].real());
+        if (fabs(burst[i].imag()) > max)
+            max = fabs(burst[i].imag());
+    }
+
+    return max;
 }
 
 /* 
@@ -1399,6 +1401,7 @@ int detectRACHBurst(signalVector &rxBurst,
 		    float *toa)
 {
   int rc, start, target, head, tail, len;
+  bool clipping = false;
   float _toa;
   complex _amp;
   signalVector *corr;
@@ -1407,8 +1410,14 @@ int detectRACHBurst(signalVector &rxBurst,
   if ((sps != 1) && (sps != 4))
     return -SIGERR_UNSUPPORTED;
 
-  if (detectClipping(rxBurst, CLIP_THRESH))
-    return -SIGERR_CLIP;
+  // Detect potential clipping
+  // We still may be able to demod the burst, so we'll give it a try
+  // and only report clipping if we can't demod.
+  float maxAmpl = maxAmplitude(rxBurst);
+  if (maxAmpl > CLIP_THRESH) {
+    LOG(DEBUG) << "max burst amplitude: " << maxAmpl << " is above the clipping threshold: " << CLIP_THRESH << std::endl;
+    clipping = true;
+  }
 
   target = 8 + 40;
   head = 4;
@@ -1430,7 +1439,7 @@ int detectRACHBurst(signalVector &rxBurst,
       *amp = 0.0f;
     if (toa)
       *toa = 0.0f;
-    return 0;
+    return clipping?-SIGERR_CLIP:SIGERR_NONE;
   }
 
   /* Subtract forward search bits from delay */
@@ -1455,6 +1464,7 @@ int analyzeTrafficBurst(signalVector &rxBurst, unsigned tsc, float thresh,
                         bool chan_req, signalVector **chan, float *chan_offset)
 {
   int rc, start, target, head, tail, len;
+  bool clipping = false;
   complex _amp;
   float _toa;
   signalVector *corr;
@@ -1463,8 +1473,14 @@ int analyzeTrafficBurst(signalVector &rxBurst, unsigned tsc, float thresh,
   if ((tsc < 0) || (tsc > 7) || ((sps != 1) && (sps != 4)))
     return -SIGERR_UNSUPPORTED;
 
-  if (detectClipping(rxBurst, CLIP_THRESH))
-    return -SIGERR_CLIP;
+  // Detect potential clipping
+  // We still may be able to demod the burst, so we'll give it a try
+  // and only report clipping if we can't demod.
+  float maxAmpl = maxAmplitude(rxBurst);
+  if (maxAmpl > CLIP_THRESH) {
+    LOG(DEBUG) << "max burst amplitude: " << maxAmpl << " is above the clipping threshold: " << CLIP_THRESH << std::endl;
+    clipping = true;
+  }
 
   target = 3 + 58 + 16 + 5;
   head = 4;
@@ -1486,7 +1502,7 @@ int analyzeTrafficBurst(signalVector &rxBurst, unsigned tsc, float thresh,
       *amp = 0.0f;
     if (toa)
       *toa = 0.0f;
-    return 0;
+    return clipping?-SIGERR_CLIP:SIGERR_NONE;
   }
 
   /* Subtract forward search bits from delay */