Fix Meshtastic soft FEC payload status

2026-09-15 15:35:32 -04:00 · 2026-08-27 22:37:03 -05:00
parent 262ad92a4b
commit e63a3b792b
2 changed files with 88 additions and 6 deletions
@@ -432,6 +432,8 @@ void MeshtasticDemodDecoderLoRa::decodeBytesSoft(
 {
    payloadCRCStatus = false;
    payloadParityStatus = (int) MeshtasticDemodSettings::ParityUndefined;
+    bool error = false; // set if any payload codeword requires soft FEC correction
+    bool bad = false;   // set if any payload codeword has a structural decode failure

    if (inSymbols.size() < headerSymbols)
    {
@@ -542,7 +544,18 @@ void MeshtasticDemodDecoderLoRa::decodeBytesSoft(
        }
    }

-    auto decodeSoftBlock = [&llrs, spreadFactor](unsigned int symOfs, unsigned int cwLen, unsigned int sfApp, unsigned int crApp, std::vector<uint8_t>& nibbles) {
+    auto decodeSoftBlock = [
+        &llrs,
+        spreadFactor,
+        &error, &bad // accumulated payload FEC status
+    ](
+        unsigned int symOfs,
+        unsigned int cwLen,
+        unsigned int sfApp,
+        unsigned int crApp,
+        unsigned int payloadRowStart, // first row that contributes to payload FEC status
+        std::vector<uint8_t>& nibbles) {
+
        if (sfApp == 0U) {
            return false;
        }
@@ -569,8 +582,22 @@ void MeshtasticDemodDecoderLoRa::decodeBytesSoft(
            }
        }

-        for (unsigned int row = 0; row < sfApp; row++) {
-            nibbles.push_back(decodeCodewordSoft(deinterBin[row], crApp));
+        for (unsigned int row = 0; row < sfApp; row++)
+        {
+            if (row < payloadRowStart)
+            {
+                // Still in header, don't accumulate payload FEC
+                bool ignoredError = false;
+                bool ignoredBad = false;
+                nibbles.push_back(
+                    decodeCodewordSoft(deinterBin[row], crApp, ignoredError, ignoredBad));
+            }
+            else
+            {
+                // Payload row contributes to accumulated payload FEC status
+                nibbles.push_back(
+                    decodeCodewordSoft(deinterBin[row], crApp, error, bad));
+            }
        }

        return true;
@@ -587,7 +614,15 @@ void MeshtasticDemodDecoderLoRa::decodeBytesSoft(
            return;
        }

-        if (!decodeSoftBlock(symOfs, 8U, headerNbSymbolBits, 4U, nibbles)) {
+        const unsigned int payloadRowStart = headerCodewords; // payload starts after the 5 header codewords
+
+        if (!decodeSoftBlock(
+                symOfs,
+                headerSymbols,
+                headerNbSymbolBits,
+                headerParityBits,
+                payloadRowStart,
+                nibbles)) {
            earlyEOM = true;
            return;
        }
@@ -595,9 +630,17 @@ void MeshtasticDemodDecoderLoRa::decodeBytesSoft(
        symOfs += headerSymbols;
    }

+    const unsigned int payloadOnlyRowStart = 0U; // all rows in these blocks are payload
+
    while (symOfs + payloadBlockSymbols <= numSymbols)
    {
-        if (!decodeSoftBlock(symOfs, payloadBlockSymbols, payloadNbSymbolBits, nbParityBits, nibbles)) {
+        if (!decodeSoftBlock(
+                symOfs,
+                payloadBlockSymbols,
+                payloadNbSymbolBits,
+                nbParityBits,
+                payloadOnlyRowStart,
+                nibbles)) {
            earlyEOM = true;
            return;
        }
@@ -628,6 +671,15 @@ void MeshtasticDemodDecoderLoRa::decodeBytesSoft(
        dewhitenPayloadBytes(bytes.data(), packetLength);
    }

+    // Report accumulated payload FEC result independently of CRC
+    if (bad) {
+        payloadParityStatus = (int) MeshtasticDemodSettings::ParityError;
+    } else if (error) {
+        payloadParityStatus = (int) MeshtasticDemodSettings::ParityCorrected;
+    } else {
+        payloadParityStatus = (int) MeshtasticDemodSettings::ParityOK;
+    }
+
    if (hasCRC)
    {
        if ((packetLength >= 2U) && (dataByteLen >= packetLength + 2U))
@@ -274,7 +274,11 @@ private:
        return static_cast<unsigned char>((nibbleBits[0] << 3) | (nibbleBits[1] << 2) | (nibbleBits[2] << 1) | nibbleBits[3]);
    }

-    static inline unsigned char decodeCodewordSoft(const std::vector<float>& codewordLLR, unsigned int crApp)
+    static inline unsigned char decodeCodewordSoft(
+        const std::vector<float>& codewordLLR,
+        unsigned int crApp,
+        bool& error, // set if soft FEC changes a hard decision; caller may accumulate across codewords
+        bool& bad)   // set on structural decode failure; caller may accumulate across codewords
    {
        static const unsigned char cwLUT[16] = {
            0, 23, 45, 58, 78, 89, 99, 116,
@@ -286,10 +290,17 @@ private:
        };

        if ((crApp < 1U) || (crApp > 4U)) {
+            bad = true; // invalid coding rate: structural decode failure
            return 0;
        }

        const unsigned int cwLen = 4U + crApp;
+
+        if (codewordLLR.size() < cwLen) {
+            bad = true; // insufficient LLR data: structural decode failure (later mapped to ParityError)
+            return 0;
+        }
+
        const unsigned char *lut = (crApp == 1U) ? cwLUTCr5 : cwLUT;
        float bestScore = std::numeric_limits<float>::lowest();
        unsigned int bestIdx = 0U;
@@ -312,6 +323,25 @@ private:
            }
        }

+        const unsigned char selectedCW =
+            static_cast<unsigned char>(lut[bestIdx] >> (8U - cwLen));
+
+        for (unsigned int j = 0; j < cwLen; j++)
+        {
+            if (codewordLLR[j] == 0.0f) {
+                continue; // no hard-decision preference for an exactly zero LLR
+            }
+
+            const bool hardBit = codewordLLR[j] > 0.0f;
+            const bool selectedBit =
+                ((selectedCW >> (cwLen - 1U - j)) & 0x1U) != 0U;
+
+            if (hardBit != selectedBit) {
+                error = true; // soft FEC changed one or more raw hard-decision bits
+                break;
+            }
+        }
+
        const unsigned char dataNibbleSoft = static_cast<unsigned char>(cwLUT[bestIdx] >> 4);
        return static_cast<unsigned char>(
            (((dataNibbleSoft & 0x1U) != 0U) << 3) |