Improved robustness of frame type detection, now based on minimum hamming distance between syncwords

openrtx/include/protocols/M17/M17Demodulator.h

@@ -238,14 +238,6 @@ private:
      */
     int8_t quantize(int32_t offset);
 
-    /**
-     * Compute Hamming Distance between two bytes
-     *
-     * @param x: first byte
-     * @param y: second byte
-     */
-    uint8_t hammingDistance(uint8_t x, uint8_t y);
-
     /**
      * Perform a limited search for a syncword using correlation
      *

openrtx/include/protocols/M17/M17FrameDecoder.h

@@ -25,6 +25,7 @@
 #error This header is C++ only!
 #endif
 
+#include <cstdint>
 #include <string>
 #include <array>
 #include "M17LinkSetupFrame.h"
@@ -97,6 +98,17 @@ public:
 
 private:
 
+    /**
+     * Determine frame type by searching which syncword among the standard M17
+     * ones has the minumum hamming distance from the given one. If the hamming
+     * distance exceeds a masimum absolute threshold the frame is declared of
+     * unknown type.
+     *
+     * @param syncWord: frame syncword.
+     * @return frame type based on the given syncword.
+     */
+    M17FrameType getFrameType(const std::array< uint8_t, 2 >& syncWord);
+
     /**
      * Decode Link Setup Frame data and update the internal LSF field with
      * the new frame data.
@@ -129,6 +141,9 @@ private:
     M17LinkSetupFrame lsfFromLich;      ///< LSF assembled from LICH segments.
     M17StreamFrame    streamFrame;      ///< Latest stream dat frame received.
     M17Viterbi        viterbi;          ///< Viterbi decoder.
+
+    ///< Maximum allowed hamming distance when determining the frame type.
+    static constexpr uint8_t MAX_SYNC_HAMM_DISTANCE = 4;
 };
 
 }      // namespace M17

openrtx/include/protocols/M17/M17Utils.h

@@ -39,9 +39,9 @@ namespace M17
  * of bytes. Bits are counted scanning from left to right, thus bit number zero
  * is the leftmost bit of array[0].
  *
- * \param array: byte array.
- * \param pos: bit position inside the array.
- * \return value of the indexed bit, as boolean variable.
+ * @param array: byte array.
+ * @param pos: bit position inside the array.
+ * @return value of the indexed bit, as boolean variable.
  */
 template < size_t N >
 inline bool getBit(const std::array< uint8_t, N >& array, const size_t pos)
@@ -57,9 +57,9 @@ inline bool getBit(const std::array< uint8_t, N >& array, const size_t pos)
  * of bytes. Bits are counted scanning from left to right, thus bit number zero
  * is the leftmost bit of array[0].
  *
- * \param array: byte array.
- * \param pos: bit position inside the array.
- * \param bit: bit value to be set.
+ * @param array: byte array.
+ * @param pos: bit position inside the array.
+ * @param bit: bit value to be set.
  */
 template < size_t N >
 inline void setBit(std::array< uint8_t, N >& array, const size_t pos,
@@ -78,9 +78,9 @@ inline void setBit(std::array< uint8_t, N >& array, const size_t pos,
  * symbols are filled from the least significant bit pair to the most
  * significant bit pair.
  *
- * \param array: byte array.
- * \param pos: symbol position inside the array.
- * \param symbol: symbol to be set, either -3, -1, +1, +3.
+ * @param array: byte array.
+ * @param pos: symbol position inside the array.
+ * @param symbol: symbol to be set, either -3, -1, +1, +3.
  */
 template < size_t N >
 inline void setSymbol(std::array< uint8_t, N >& array, const size_t pos,
@@ -108,6 +108,18 @@ inline void setSymbol(std::array< uint8_t, N >& array, const size_t pos,
     }
 }
 
+/**
+ * Compute the hamming distance between two bytes.
+ *
+ * @param x: first byte.
+ * @param y: second byte.
+ * @return hamming distance between x and y.
+ */
+static inline uint8_t hammingDistance(const uint8_t x, const uint8_t y)
+{
+    return __builtin_popcount(x ^ y);
+}
+
 }      // namespace M17
 
 #endif // M17_UTILS_H

openrtx/src/protocols/M17/M17Demodulator.cpp

@@ -310,11 +310,6 @@ bool M17::M17Demodulator::isLocked()
     return locked;
 }
 
-uint8_t M17Demodulator::hammingDistance(uint8_t x, uint8_t y)
-{
-    return __builtin_popcount(x ^ y);
-}
-
 int32_t M17Demodulator::syncwordSweep(int32_t offset)
 {
     int32_t max_conv = 0, max_index = 0;

openrtx/src/protocols/M17/M17FrameDecoder.cpp

@@ -23,6 +23,7 @@
 #include <M17/M17Interleaver.h>
 #include <M17/M17Decorrelator.h>
 #include <M17/M17CodePuncturing.h>
+#include <M17/M17Utils.h>
 #include <algorithm>
 
 using namespace M17;
@@ -51,30 +52,58 @@ M17FrameType M17FrameDecoder::decodeFrame(const frame_t& frame)
     decorrelate(data);
     deinterleave(data);
 
+    auto type = getFrameType(syncWord);
+
+    switch(type)
+    {
+        case M17FrameType::LINK_SETUP:
+            decodeLSF(data);
+            break;
+
+        case M17FrameType::STREAM:
+            decodeStream(data);
+            break;
+
+        default:
+            break;
+    }
+
+    return type;
+}
+
+M17FrameType M17FrameDecoder::getFrameType(const std::array< uint8_t, 2 >& syncWord)
+{
     // Preamble
-    if((syncWord[0] == 0x77) && (syncWord[1] == 0x77))
+    M17FrameType type   = M17FrameType::PREAMBLE;
+    uint8_t minDistance = hammingDistance(syncWord[0], 0x77)
+                        + hammingDistance(syncWord[1], 0x77);
+
+    // Link setup frame
+    uint8_t hammDistance = hammingDistance(syncWord[0], LSF_SYNC_WORD[0])
+                         + hammingDistance(syncWord[1], LSF_SYNC_WORD[1]);
+    if(hammDistance < minDistance)
     {
-        return M17FrameType::PREAMBLE;
+        type = M17FrameType::LINK_SETUP;
+        minDistance = hammDistance;
     }
 
-    // Link Setup Frame
-    if(syncWord == LSF_SYNC_WORD)
+    // Stream frame
+    hammDistance = hammingDistance(syncWord[0], STREAM_SYNC_WORD[0])
+                 + hammingDistance(syncWord[1], STREAM_SYNC_WORD[1]);
+    if(hammDistance < minDistance)
     {
-        decodeLSF(data);
-        return M17FrameType::LINK_SETUP;
+        type = M17FrameType::STREAM;
+        minDistance = hammDistance;
     }
 
-    // Stream data frame
-    uint8_t hd = __builtin_popcount(syncWord[0] ^ STREAM_SYNC_WORD[0])
-               + __builtin_popcount(syncWord[1] ^ STREAM_SYNC_WORD[1]);
-    if(hd <= 4)
+    // Check value of minimum hamming distance found, if exceeds the allowed
+    // limit consider the frame as of unknown type.
+    if(minDistance > MAX_SYNC_HAMM_DISTANCE)
     {
-        decodeStream(data);
-        return M17FrameType::STREAM;
+        type = M17FrameType::UNKNOWN;
     }
 
-    // If we get here, we received an unknown frame
-    return M17FrameType::UNKNOWN;
+    return type;
 }
 
 void M17FrameDecoder::decodeLSF(const std::array< uint8_t, 46 >& data)