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365 lines
13 KiB
C++
365 lines
13 KiB
C++
///////////////////////////////////////////////////////////////////////////////////
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// Copyright (C) 2020 Edouard Griffiths, F4EXB //
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// //
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// Inspired by: https://github.com/myriadrf/LoRa-SDR //
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// //
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// This program is free software; you can redistribute it and/or modify //
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// it under the terms of the GNU General Public License as published by //
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// the Free Software Foundation as version 3 of the License, or //
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// (at your option) any later version. //
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// //
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// This program is distributed in the hope that it will be useful, //
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// but WITHOUT ANY WARRANTY; without even the implied warranty of //
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the //
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// GNU General Public License V3 for more details. //
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// //
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// You should have received a copy of the GNU General Public License //
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// along with this program. If not, see <http://www.gnu.org/licenses/>. //
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///////////////////////////////////////////////////////////////////////////////////
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#include "chirpchatdemoddecoderlora.h"
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void ChirpChatDemodDecoderLoRa::decodeHeader(
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const std::vector<unsigned short>& inSymbols,
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unsigned int nbSymbolBits,
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bool& hasCRC,
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unsigned int& nbParityBits,
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unsigned int& packetLength,
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int& headerParityStatus,
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bool& headerCRCStatus
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)
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{
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// with header (H: header 8-bit codeword P: payload-8 bit codeword):
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// nbSymbolBits = 5 |H|H|H|H|H| codewords => 8 symbols (always) : static headerSymbols = 8
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// nbSymbolBits = 7 |H|H|H|H|H|P|P|
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// without header (P: payload 8-bit codeword):
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// nbSymbolBits = 5 |P|P|P|P|P| codewords => 8 symbols (always)
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// nbSymbolBits = 7 |P|P|P|P|P|P|P|
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// Actual header is always represented with 5 8-bit codewords : static headerCodewords = 5
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// These 8-bit codewords are encoded with Hamming(4,8) FEC : static headerParityBits = 4
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std::vector<uint16_t> symbols(headerSymbols);
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std::copy(inSymbols.begin(), inSymbols.begin() + headerSymbols, symbols.begin());
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//gray encode
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for (auto &sym : symbols) {
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sym = binaryToGray16(sym);
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}
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std::vector<uint8_t> codewords(nbSymbolBits);
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// Header symbols de-interleave thus headerSymbols (8) symbols into nbSymbolBits (5..12) codewords using header FEC (4/8)
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diagonalDeinterleaveSx(symbols.data(), headerSymbols, codewords.data(), nbSymbolBits, headerParityBits);
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// whitening does not apply to the header codewords
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Sx1272ComputeWhiteningLfsr(codewords.data() + headerCodewords, nbSymbolBits - headerCodewords, 0, headerParityBits);
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bool error = false;
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bool bad = false;
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uint8_t bytes[3];
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// decode actual header inside 8-bit codewords header with 4/8 FEC (5 first codewords)
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bytes[0] = decodeHamming84sx(codewords[1], error, bad) & 0xf;
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bytes[0] |= decodeHamming84sx(codewords[0], error, bad) << 4; // length
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bytes[1] = decodeHamming84sx(codewords[2], error, bad) & 0xf; // coding rate and crc enable
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bytes[2] = decodeHamming84sx(codewords[4], error, bad) & 0xf;
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bytes[2] |= decodeHamming84sx(codewords[3], error, bad) << 4; // checksum
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bytes[2] ^= headerChecksum(bytes);
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if (bad)
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{
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headerParityStatus = (int) ParityError;
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}
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else
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{
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if (error) {
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headerParityStatus = (int) ParityCorrected;
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} else {
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headerParityStatus = (int) ParityOK;
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}
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if (bytes[2] != 0) {
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headerCRCStatus = false;
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} else {
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headerCRCStatus = true;
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}
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}
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hasCRC = (bytes[1] & 1) != 0;
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nbParityBits = (bytes[1] >> 1) & 0x7;
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packetLength = bytes[0];
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}
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void ChirpChatDemodDecoderLoRa::decodeBytes(
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QByteArray& inBytes,
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const std::vector<unsigned short>& inSymbols,
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unsigned int nbSymbolBits,
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bool hasHeader,
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bool& hasCRC,
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unsigned int& nbParityBits,
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unsigned int& packetLength,
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bool& earlyEOM,
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int& headerParityStatus,
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bool& headerCRCStatus,
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int& payloadParityStatus,
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bool& payloadCRCStatus
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)
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{
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// need at least a header (8 symbols of 8 bit codewords) whether an actual header is sent or not
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if (inSymbols.size() < headerSymbols)
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{
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qDebug("ChirpChatDemodDecoderLoRa::decodeBytes: need at least %u symbols for header", headerSymbols);
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earlyEOM = true;
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return;
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}
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else
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{
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earlyEOM = false;
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}
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if (hasHeader)
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{
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decodeHeader(
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inSymbols,
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nbSymbolBits,
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hasCRC,
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nbParityBits,
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packetLength,
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headerParityStatus,
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headerCRCStatus
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);
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}
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qDebug("ChirpChatDemodDecoderLoRa::decodeBytes: crc: %s nbParityBits: %u packetLength: %u",
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hasCRC ? "on": "off", nbParityBits, packetLength);
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if (nbParityBits > 4)
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{
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qDebug("ChirpChatDemodDecoderLoRa::decodeBytes: invalid parity bits in header: %u", nbParityBits);
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return;
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}
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const unsigned int numSymbols = roundUp(inSymbols.size(), 4 + nbParityBits);
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const unsigned int numCodewords = (numSymbols / (4 + nbParityBits))*nbSymbolBits;
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std::vector<uint16_t> symbols(numSymbols);
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std::copy(inSymbols.begin(), inSymbols.end(), symbols.begin());
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//gray encode, when SF > PPM, depad the LSBs with rounding
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for (auto &sym : symbols) {
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sym = binaryToGray16(sym);
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}
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std::vector<uint8_t> codewords(numCodewords);
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// deinterleave / dewhiten the symbols into codewords
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unsigned int sOfs = 0;
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unsigned int cOfs = 0;
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// the first headerSymbols (8 symbols) are coded with 4/8 FEC (thus 8 bit codewords) whether an actual header is present or not
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// this corresponds to nbSymbolBits codewords (therefore LoRa imposes nbSymbolBits >= headerCodewords (5 codewords) this is controlled externally)
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if (nbParityBits != 4) // different FEC between header symbols and the rest of the packet
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{
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// Header symbols de-interleave thus headerSymbols (8) symbols into nbSymbolBits (5..12) codewords using header FEC (4/8)
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diagonalDeinterleaveSx(symbols.data(), headerSymbols, codewords.data(), nbSymbolBits, headerParityBits);
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if (hasHeader) { // whitening does not apply to the header codewords
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Sx1272ComputeWhiteningLfsr(codewords.data() + headerCodewords, nbSymbolBits - headerCodewords, 0, headerParityBits);
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} else {
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Sx1272ComputeWhiteningLfsr(codewords.data(), nbSymbolBits, 0, headerParityBits);
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}
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cOfs += nbSymbolBits; // nbSymbolBits codewords in header
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sOfs += headerSymbols; // headerSymbols symbols in header
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if (numSymbols - sOfs > 0) // remaining payload symbols after header symbols using their own FEC (4/5..4/7)
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{
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diagonalDeinterleaveSx(symbols.data() + sOfs, numSymbols - sOfs, codewords.data() + cOfs, nbSymbolBits, nbParityBits);
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if (hasHeader) {
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Sx1272ComputeWhiteningLfsr(codewords.data() + cOfs, numCodewords - cOfs, nbSymbolBits - headerCodewords, nbParityBits);
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} else {
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Sx1272ComputeWhiteningLfsr(codewords.data() + cOfs, numCodewords - cOfs, nbSymbolBits, nbParityBits);
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}
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}
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}
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else // uniform 4/8 FEC for all the packet
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{
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// De-interleave the whole packet thus numSymbols into nbSymbolBits (5..12) codewords using packet FEC (4/8)
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diagonalDeinterleaveSx(symbols.data(), numSymbols, codewords.data(), nbSymbolBits, nbParityBits);
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if (hasHeader) { // whitening does not apply to the header codewords
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Sx1272ComputeWhiteningLfsr(codewords.data() + headerCodewords, numCodewords - headerCodewords, 0, nbParityBits);
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} else {
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Sx1272ComputeWhiteningLfsr(codewords.data(), numCodewords, 0, nbParityBits);
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}
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}
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// Now we have nbSymbolBits 8-bit codewords (4/8 FEC) possibly containing the actual header followed by the rest of payload codewords with their own FEC (4/5..4/8)
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std::vector<uint8_t> bytes((codewords.size()+1) / 2);
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unsigned int dOfs = 0;
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cOfs = 0;
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unsigned int dataLength = packetLength + 3 + (hasCRC ? 2 : 0); // include header and CRC
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if (hasHeader)
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{
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cOfs = headerCodewords;
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dOfs = 6;
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}
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else
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{
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cOfs = 0;
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dOfs = 0;
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}
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if (dataLength > bytes.size())
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{
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qDebug("ChirpChatDemodDecoderLoRa::decodeBytes: not enough data %lu vs %u", bytes.size(), dataLength);
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earlyEOM = true;
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return;
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}
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// decode the rest of the payload inside 8-bit codewords header with 4/8 FEC
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bool error = false;
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bool bad = false;
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for (; cOfs < nbSymbolBits; cOfs++, dOfs++)
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{
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if (dOfs % 2 == 1) {
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bytes[dOfs/2] |= decodeHamming84sx(codewords[cOfs], error, bad) << 4;
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} else {
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bytes[dOfs/2] = decodeHamming84sx(codewords[cOfs], error, bad) & 0xf;
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}
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}
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if (dOfs % 2 == 1) // decode the start of the payload codewords with their own FEC when not on an even boundary
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{
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if (nbParityBits == 1) {
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bytes[dOfs/2] |= checkParity54(codewords[cOfs++], error) << 4;
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} else if (nbParityBits == 2) {
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bytes[dOfs/2] |= checkParity64(codewords[cOfs++], error) << 4;
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} else if (nbParityBits == 3){
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bytes[dOfs/2] |= decodeHamming74sx(codewords[cOfs++], error) << 4;
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} else if (nbParityBits == 4){
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bytes[dOfs/2] |= decodeHamming84sx(codewords[cOfs++], error, bad) << 4;
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} else {
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bytes[dOfs/2] |= codewords[cOfs++] << 4;
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}
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dOfs++;
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}
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dOfs /= 2;
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// decode the rest of the payload codewords with their own FEC
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if (nbParityBits == 1)
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{
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for (unsigned int i = dOfs; i < dataLength; i++)
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{
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bytes[i] = checkParity54(codewords[cOfs++],error);
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bytes[i] |= checkParity54(codewords[cOfs++], error) << 4;
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}
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}
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else if (nbParityBits == 2)
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{
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for (unsigned int i = dOfs; i < dataLength; i++)
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{
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bytes[i] = checkParity64(codewords[cOfs++], error);
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bytes[i] |= checkParity64(codewords[cOfs++],error) << 4;
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}
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}
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else if (nbParityBits == 3)
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{
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for (unsigned int i = dOfs; i < dataLength; i++)
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{
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bytes[i] = decodeHamming74sx(codewords[cOfs++], error) & 0xf;
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bytes[i] |= decodeHamming74sx(codewords[cOfs++], error) << 4;
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}
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}
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else if (nbParityBits == 4)
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{
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for (unsigned int i = dOfs; i < dataLength; i++)
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{
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bytes[i] = decodeHamming84sx(codewords[cOfs++], error, bad) & 0xf;
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bytes[i] |= decodeHamming84sx(codewords[cOfs++], error, bad) << 4;
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}
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}
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else
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{
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for (unsigned int i = dOfs; i < dataLength; i++)
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{
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bytes[i] = codewords[cOfs++] & 0xf;
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bytes[i] |= codewords[cOfs++] << 4;
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}
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}
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if (bad) {
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payloadParityStatus = (int) ParityError;
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} else if (error) {
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payloadParityStatus = (int) ParityCorrected;
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} else {
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payloadParityStatus = (int) ParityOK;
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}
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// finalization:
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// adjust offsets dpending on header and CRC presence
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// compute and verify payload CRC if present
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if (hasHeader)
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{
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dOfs = 3; // skip header
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dataLength -= 3; // remove header
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if (hasCRC) // always compute crc if present skipping the header
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{
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uint16_t crc = sx1272DataChecksum(bytes.data() + dOfs, packetLength);
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uint16_t packetCRC = bytes[dOfs + packetLength] | (bytes[dOfs + packetLength + 1] << 8);
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if (crc != packetCRC) {
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payloadCRCStatus = false;
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} else {
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payloadCRCStatus = true;
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}
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}
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}
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else
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{
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dOfs = 0; // no header to skip
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if (hasCRC)
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{
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uint16_t crc = sx1272DataChecksum(bytes.data(), packetLength);
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uint16_t packetCRC = bytes[packetLength] | (bytes[packetLength + 1] << 8);
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if (crc != packetCRC) {
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payloadCRCStatus = false;
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} else {
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payloadCRCStatus = true;
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}
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}
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}
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inBytes.resize(dataLength);
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std::copy(bytes.data() + dOfs, bytes.data() + dOfs + dataLength, inBytes.data());
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}
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void ChirpChatDemodDecoderLoRa::getCodingMetrics(
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unsigned int nbSymbolBits,
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unsigned int nbParityBits,
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unsigned int packetLength,
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bool hasHeader,
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bool hasCRC,
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unsigned int& numSymbols,
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unsigned int& numCodewords
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)
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{
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numCodewords = roundUp((packetLength + (hasCRC ? 2 : 0))*2 + (hasHeader ? headerCodewords : 0), nbSymbolBits); // uses payload + CRC for encoding size
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numSymbols = headerSymbols + (numCodewords / nbSymbolBits - 1) * (4 + nbParityBits); // header is always coded with 8 bits and yields exactly 8 symbols (headerSymbols)
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}
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