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added soft Chase decoding for short BCH codes

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gives about 1.5dB gain while only half the speed of hard decision.
This commit is contained in:
Ahmet Inan 2020-06-08 14:40:53 +02:00
commit 5cf0ff2297
3 changed files with 203 additions and 0 deletions
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78
short_bch_code_decoder.hh
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@ -50,6 +50,84 @@ public:
{
return inp ^ err[(par[inp>>P] ^ inp) & (R-1)];
}
int operator()(const int8_t *code)
{
// maximum likelihood
if (0) {
int word = 0, best = 0;
for (int msg = 0; msg < W; ++msg) {
int sum = 0;
int enc = (msg << P) | par[msg];
int lol = 8 * sizeof(enc) - 1;
for (int i = 0; i < N; ++i)
sum += ((enc << (lol-i)) >> lol) * code[i];
if (sum > best) {
best = sum;
word = enc;
}
}
return word;
}
int cw = 0;
for (int i = 0; i < N; ++i)
cw |= (code[i] < 0) << i;
// hard decision
if (0)
return (*this)(cw);
int word = 0, best = 0;
// flip each bit and see ..
if (0) {
for (int j = 0; j < N; ++j) {
int tmp = 1 << j;
int dec = (*this)(cw ^ tmp);
int lol = 8 * sizeof(dec) - 1;
int sum = 0;
for (int i = 0; i < N; ++i)
sum += ((dec << (lol-i)) >> lol) * code[i];
if (sum > best) {
best = sum;
word = dec;
}
}
}
// Chase algorithm
if (1) {
const int num = 4;
int worst[num] = { 0 };
for (int i = 0; i < N; ++i) {
if (std::abs(code[i]) < std::abs(code[worst[0]])) {
worst[3] = worst[2];
worst[2] = worst[1];
worst[1] = worst[0];
worst[0] = i;
} else if (std::abs(code[i]) < std::abs(code[worst[1]])) {
worst[3] = worst[2];
worst[2] = worst[1];
worst[1] = i;
} else if (std::abs(code[i]) < std::abs(code[worst[2]])) {
worst[3] = worst[2];
worst[2] = i;
} else if (std::abs(code[i]) < std::abs(code[worst[3]])) {
worst[3] = i;
}
}
for (int j = 0; j < (1 << num); ++j) {
int tmp = 0;
for (int i = 0; i < num; ++i)
tmp |= ((j>>i)&1) << worst[i];
int dec = (*this)(cw ^ tmp);
int lol = 8 * sizeof(dec) - 1;
int sum = 0;
for (int i = 0; i < N; ++i)
sum += ((dec << (lol-i)) >> lol) * code[i];
if (sum > best) {
best = sum;
word = dec;
}
}
}
return word;
}
};
}