Implement Golay (24, 12, 8) encoder/decoder

include/satnogs/CMakeLists.txt

@@ -74,5 +74,6 @@ install(FILES
     lrpt_sync.h
     lrpt_decoder.h
     frame_acquisition.h
-    shift_reg.h DESTINATION include/satnogs
+    shift_reg.h
+    golay24.h DESTINATION include/satnogs
 )

include/satnogs/golay24.h

@@ -0,0 +1,66 @@
+/* -*- c++ -*- */
+/*
+ * gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
+ *
+ *  Copyright (C) 2016, 2017, 2018
+ *  Libre Space Foundation <http://librespacefoundation.org/>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef INCLUDED_SATNOGS_GOLAY24_H
+#define INCLUDED_SATNOGS_GOLAY24_H
+
+#include <satnogs/api.h>
+
+#include <cstdint>
+#include <vector>
+
+namespace gr
+{
+namespace satnogs
+{
+
+/*!
+ * \brief A binary Golay (24,12,8) encoder and decoder.
+ *
+ * A binary Golay (24,12,8) encoder and decoder. The implementation uses LUT
+ * based on the process described in the book:
+ *
+ * Morelos-Zaragoza, Robert H. The art of error correcting coding.
+ * John Wiley & Sons, 2006.
+ *
+ */
+class SATNOGS_API golay24
+{
+public:
+  golay24 ();
+  ~golay24 ();
+
+  uint32_t
+  encode12(uint16_t in);
+
+  bool
+  decode24(uint16_t *out, uint32_t in);
+private:
+  std::vector<uint32_t>          d_H;
+  std::vector<uint32_t>          d_X;
+
+};
+
+} // namespace satnogs
+} // namespace gr
+
+#endif /* INCLUDED_SATNOGS_GOLAY24_H */
+

lib/CMakeLists.txt

@@ -73,7 +73,8 @@ list(APPEND satnogs_sources
     convolutional_deinterleaver.cc
     lrpt_decoder_impl.cc
     frame_acquisition_impl.cc
-    shift_reg.cc)
+    shift_reg.cc
+    golay24.cc)
 
 if(${INCLUDE_DEBUG_BLOCKS})
 	list(APPEND satnogs_sources ${satnogs_debug_sources})

lib/golay24.cc

@@ -0,0 +1,168 @@
+/* -*- c++ -*- */
+/*
+ * gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
+ *
+ *  Copyright (C) 2016, 2017, 2018
+ *  Libre Space Foundation <http://librespacefoundation.org/>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <gnuradio/io_signature.h>
+#include <satnogs/golay24.h>
+
+#include <satnogs/utils.h>
+
+namespace gr
+{
+namespace satnogs
+{
+
+golay24::golay24 ()
+{
+
+  d_H = { 0x7ff, 0xee2, 0xdc5, 0xb8b, 0xf16, 0xe2d, 0xc5b, 0x8b7, 0x96e, 0xadc,
+      0xdb8, 0xb71 };
+  d_X = {0x800, 0x400, 0x200, 0x100, 0x080, 0x040, 0x020, 0x010, 0x008, 0x004,
+      0x002, 0x001};
+}
+
+golay24::~golay24 ()
+{
+}
+
+/**
+ * Encodes a 12-bit message
+ * @param in the input 12-bit message. The message should be placed at the
+ * 12 LS bits
+ * @return the coded 24-bit message. The message is placed at the 24 LS bits
+ */
+uint32_t
+golay24::encode12 (uint16_t in)
+{
+  uint32_t c[2] =
+    { 0x0, 0x0 };
+  c[0] = in & 0xFFF;
+  for (size_t i = 0; i < 12; i++) {
+    uint32_t tmp = 0;
+    for (size_t j = 0; j < 12; j++) {
+      tmp ^= (((c[0] & d_H[i]) >> j) & 0x01);
+    }
+    c[1] = (c[1] << 1) ^ tmp;
+  }
+  return ((c[0] & 0xFFF) << 12) | (c[1] & 0xFFF);
+}
+
+static inline uint32_t
+ham_dist(uint32_t x)
+{
+  return bit_count(x & 0xFFF);
+}
+
+/**
+ * Decodes a single Golay (24, 12, 8) codeword
+ *
+ * @param out the 12-bit decoded message. The message is placed at the 12 LS bits
+ * @param in the coded 24 bit code word. The message should be placed at the
+ * 24 LS bits
+ * @return true if the decoding was successful, false in case the error correction
+ * could not be performed
+ */
+bool
+golay24::decode24(uint16_t *out, const uint32_t in)
+{
+  uint32_t e[2] = {0x0, 0x0};
+  uint32_t r[2] = {0x0, 0x0};
+  uint32_t c[2] = {0x0, 0x0};
+  uint32_t syndrome = 0x0;
+  bool found = false;
+  uint32_t col = 0;
+  uint32_t q = 0;
+
+  r[0] = (in >> 12) & 0xFFF;
+  r[1] = in & 0xFFF;
+
+  for(size_t j = 0; j < 12; j++) {
+    uint32_t tmp = 0x0;
+    for(size_t i = 0; i < 12; i++) {
+      tmp ^= (((d_X[j] & r[0]) >> i) & 0x1);
+    }
+
+    for(size_t i = 0; i < 12; i++) {
+      tmp ^= (((d_H[j] & r[1]) >> i) & 0x1);
+    }
+    syndrome = (syndrome << 1) ^ tmp;
+  }
+
+  if(ham_dist(syndrome) <= 3) {
+    e[0] = syndrome;
+    e[1] = 0x0;
+  }
+  else{
+    do {
+      if (ham_dist (syndrome ^ d_H[col]) <= 2) {
+        e[0] = syndrome ^ d_H[col];
+        e[1] = d_X[col];
+        found = 1;
+      }
+      col++;
+    }
+    while ((col < 12) && !found);
+
+    if ((col == 12) && !found) {
+      for (size_t j = 0; j < 12; j++) {
+        uint32_t tmp = 0x0;
+        for (size_t i = 0; i < 12; i++) {
+          tmp ^= (((d_H[j] & syndrome) >> i) & 0x1);
+        }
+        q = (q << 1) ^ tmp;
+      }
+    }
+
+    if (ham_dist (q) <= 3) {
+      e[0] = 0;
+      e[1] = q;
+    }
+    else {
+      col = 0;
+      found = 0;
+      do {
+        if (ham_dist (q ^ d_H[col]) <= 2) {
+          e[0] = d_X[col];
+          e[1] = q ^ d_H[col];
+          found = 1;
+        }
+        col++;
+      }
+      while ((col < 12) && !found);
+
+      if (col == 12 && !found) {
+        return false;
+      }
+    }
+  }
+  c[0] = r[0] ^ e[0];
+  c[1] = r[1] ^ e[1];
+  /* Return the message part only, not the parity */
+  *out = c[0];
+  return true;
+}
+
+} /* namespace satnogs */
+} /* namespace gr */
+