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https://github.com/lwvmobile/dsd-fme.git
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9e6b3b7978
Added a new "heuristics" module for P25 that improves the estimation of the symbol (dibit) that corresponds to a given analog signal value. Added a BER (bit error reate) estimation for P25.
94 lines
2.4 KiB
C++
94 lines
2.4 KiB
C++
#include <itpp/itcomm.h>
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#include "p25p1_check_nid.h"
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using namespace itpp;
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// Ideas taken from http://op25.osmocom.org/trac/wiki.png/browser/op25/gr-op25/lib/decoder_ff_impl.cc
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// See also p25_training_guide.pdf page 48.
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// See also tia-102-baaa-a-project_25-fdma-common_air_interface.pdf page 40.
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// BCH encoder/decoder implementation from IT++. GNU GPL 3 license.
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itpp::BCH bch(63, 16, 11, "6 3 3 1 1 4 1 3 6 7 2 3 5 4 5 3", true);
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/**
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* Convenience class to calculate the parity of the DUID values. Keeps a table with the expected outcomes
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* for fast lookup.
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*/
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class ParityTable
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{
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private:
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unsigned char table[16];
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unsigned char get_index(unsigned char x, unsigned char y)
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{
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return (x<<2) + y;
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}
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public:
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ParityTable()
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{
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for (unsigned int i=0; i<sizeof(table); i++) {
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table[i] = 0;
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}
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table[get_index(1,1)] = 1;
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table[get_index(2,2)] = 1;
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}
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unsigned char get_value(unsigned char x, unsigned char y)
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{
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return table[get_index(x,y)];
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}
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} parity_table;
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int check_NID(char* bch_code, int* new_nac, char* new_duid, unsigned char parity)
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{
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int result;
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// Fill up with the given input
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bvec input(63);
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for(unsigned int i=0; i<63; i++) {
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input[i] = bch_code[i];
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}
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// Decode it
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bvec decoded, cw_isvalid;
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bool ok = bch.decode(input, decoded, cw_isvalid);
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if (!ok) {
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// Decode failed
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result = 0;
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} else {
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// Take the NAC from the decoded output. It's a 12 bit number starting from position 0.
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// Just convert the output from a binary sequence to an integer.
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int nac = 0;
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for (int i=0; i<12; i++) {
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nac <<= 1;
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nac |= (int)decoded[i];
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}
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*new_nac = nac;
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// Take the fixed DUID from the encoded output. 4 bit value starting at position 12.
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unsigned char new_duid_0 = (((int)decoded[12])<<1) + ((int)decoded[13]);
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unsigned char new_duid_1 = (((int)decoded[14])<<1) + ((int)decoded[15]);
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new_duid[0] = new_duid_0 + '0';
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new_duid[1] = new_duid_1 + '0';
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new_duid[2] = 0; // Null terminate
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// Check the parity
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unsigned char expected_parity = parity_table.get_value(new_duid_0, new_duid_1);
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if (expected_parity != parity) {
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// Ignore, not sure what to do
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//printf("Error in parity detected?");
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}
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result = 1;
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}
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return result;
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}
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