Transceiver52M: Add clipping detection on RACH input

Alert user of overdriven RACH input indicated by a positive
threshold detector result. This indication serves as notification
that the receive RF gain level is too high for the configured
transceiver setup.

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

Transceiver52M/Transceiver.cpp

@@ -332,7 +332,7 @@ SoftVector *Transceiver::pullRadioVector(GSM::Time &wTime,
 				      int &timingOffset)
 {
   bool needDFE = false;
-  bool success = false;
+  int success = 0;
   complex amplitude = 0.0;
   float TOA = 0.0, avg = 0.0;
 
@@ -404,10 +404,21 @@ SoftVector *Transceiver::pullRadioVector(GSM::Time &wTime,
   }
   else {
     // RACH burst
-    if (success = detectRACHBurst(*vectorBurst, 6.0, mSPSRx, &amplitude, &TOA))
+    success = detectRACHBurst(*vectorBurst, 6.0, mSPSRx, &amplitude, &TOA);
+    if (success > 0) {
       channelResponse[timeslot] = NULL;
-    else
+    } else if (success == 0) {
       mNoises.insert(avg);
+    } else {
+      if (success == -SIGERR_CLIP) {
+        LOG(ALERT) << "Clipping detected on RACH input";
+      } else {
+        LOG(ALERT) << "Unhandled RACH error";
+      }
+
+      delete rxBurst;
+      return NULL;
+    }
   }
 
   // demodulate burst

Transceiver52M/sigProcLib.cpp

@@ -30,6 +30,8 @@
 extern "C" {
 #include "convolve.h"
 }
+/* Clipping detection threshold */
+#define CLIP_THRESH     30000.0f
 
 #define TABLESIZE 1024
 
@@ -1241,7 +1243,7 @@ static int detectBurst(signalVector &burst,
   /* Correlate */
   if (!convolve(&burst, sync->sequence, &corr,
                 CUSTOM, start, len, sps, 0)) {
-    return -1;
+    return -SIGERR_INTERNAL;
   }
 
   /* Peak detection - place restrictions at correlation edges */
@@ -1270,6 +1272,18 @@ static int detectBurst(signalVector &burst,
   return 1;
 }
 
+static int detectClipping(signalVector &burst, float thresh)
+{
+	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;
+}
+
 /* 
  * RACH burst detection
  *
@@ -1291,7 +1305,10 @@ int detectRACHBurst(signalVector &rxBurst,
   CorrelationSequence *sync;
 
   if ((sps != 1) && (sps != 4))
-    return -1;
+    return -SIGERR_UNSUPPORTED;
+
+  if (detectClipping(rxBurst, CLIP_THRESH))
+    return -SIGERR_CLIP;
 
   target = 8 + 40;
   head = 4;
@@ -1342,7 +1359,7 @@ int analyzeTrafficBurst(signalVector &rxBurst, unsigned tsc, float thresh,
   CorrelationSequence *sync;
 
   if ((tsc < 0) || (tsc > 7) || ((sps != 1) && (sps != 4)))
-    return -1;
+    return -SIGERR_UNSUPPORTED;
 
   target = 3 + 58 + 16 + 5;
   head = 4;
@@ -1356,7 +1373,7 @@ int analyzeTrafficBurst(signalVector &rxBurst, unsigned tsc, float thresh,
   rc = detectBurst(rxBurst, corr, sync,
                    thresh, sps, &_amp, &_toa, start, len);
   if (rc < 0) {
-    return -1;
+    return -SIGERR_INTERNAL;
   } else if (!rc) {
     if (amp)
       *amp = 0.0f;

Transceiver52M/sigProcLib.h

@@ -36,6 +36,14 @@ enum ConvType {
   UNDEFINED,
 };
 
+enum signalError {
+  SIGERR_NONE,
+  SIGERR_BOUNDS,
+  SIGERR_CLIP,
+  SIGERR_UNSUPPORTED,
+  SIGERR_INTERNAL,
+};
+
 /** the core data structure of the Transceiver */
 class signalVector: public Vector<complex> 
 {