Added decoding and error correction on the NID fields: NAC and DUID. Making use of the IT++ library for BCH encoding/decoding.

check_nid.cpp

@@ -0,0 +1,109 @@
+#include <itpp/itcomm.h>
+
+#include "check_nid.h"
+
+
+
+using namespace itpp;
+
+// Ideas taken from http://op25.osmocom.org/trac/wiki.png/browser/op25/gr-op25/lib/decoder_ff_impl.cc
+// See also p25_training_guide.pdf page 48.
+// See also tia-102-baaa-a-project_25-fdma-common_air_interface.pdf page 40.
+// BCH encoder/decoder implementation from IT++. GNU GPL 3 license.
+
+itpp::BCH bch(63, 16, 11, "6 3 3 1 1 4 1 3 6 7 2 3 5 4 5 3", true);
+
+/** Convenience class to calculate the parity of the DUID values. Keeps a table with the expected outcomes
+ * for fast lookup.
+ */
+class ParityTable
+{
+private:
+    unsigned char table[16];
+
+    unsigned char get_index(unsigned char x, unsigned char y)
+    {
+        return (x<<2) + y;
+    }
+
+public:
+    ParityTable()
+    {
+        for (unsigned int i=0; i<sizeof(table); i++) {
+            table[i] = 0;
+        }
+        table[get_index(1,1)] = 1;
+        table[get_index(2,2)] = 1;
+    }
+
+    unsigned char get_value(unsigned char x, unsigned char y)
+    {
+        return table[get_index(x,y)];
+    }
+
+} parity_table;
+
+/** Checks a NID value, returns the fixed NAC, DUID and also an indication if it failed to decode the NID.
+ *
+ *  \param bch_code Input. An array to the 63 bytes, each containing one bit of the NID.
+ *  \param new_nac Output. An address where to store the calculated NAC value after error correction. Should
+ *                 be large enough to accommodate for an integer.
+ *  \param new_duid Output. An address where to store the calculated DUID value after error correction. Should
+ *                  be able to accommodate 3 chars.
+ *  \param parity Input. The parity value read.
+ *  \return 0 if there were errors processing the NID.
+ */
+int check_NID(char* bch_code, int* new_nac, char* new_duid, unsigned char parity)
+{
+    int result;
+
+    // Fill up with the input given
+    bvec input(63);
+    for(unsigned int i=0; i<63; i++) {
+        input[i] = bch_code[i];
+    }
+
+    // Decode it
+    bvec decoded = bch.decode(input);
+
+    // Check if the decoded output is zero
+    bool zero = true;
+    for (int i=0; i<decoded.length(); i++) {
+        if (decoded[i] != 0) {
+            zero = false;
+            break;
+        }
+    }
+
+    if (zero) {
+        // Decode failed
+        result = 0;
+    } else {
+        // Encode it again
+        bvec encoded = bch.encode(decoded);
+
+        // Take the fixed NAC from the encoded output. It's a 12 bit number starting from position 47.
+        // Just convert the output from a binary sequence to an integer.
+        int nac = 0;
+        for (int i=47; i<47+12; i++) {
+            nac <<= 1;
+            nac |= (int)encoded[i];
+        }
+        *new_nac = nac;
+
+        // Take the fixed DUID from the encoded output. 4 bit value starting at position 59.
+        unsigned char x = (((int)encoded[59])<<1) + ((int)encoded[60]);
+        unsigned char y = (((int)encoded[61])<<1) + ((int)encoded[62]);
+        new_duid[0] = x + '0';
+        new_duid[1] = y + '0';
+        new_duid[2] = 0;
+
+        // Check the parity
+        unsigned char expected_parity = parity_table.get_value(x,y);
+
+        // Returns false (0) if they don't match
+        result = (expected_parity == parity);
+    }
+
+    return result;
+}