1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
|
#include "ldpc_decoder_generic.h"
#include <cassert>
namespace codings
{
namespace ldpc
{
LDPCDecoderGeneric::LDPCDecoderGeneric(Sparse_matrix pcm) : LDPCDecoder(pcm)
{
int max_deg = 0;
for (size_t rows = 0; rows < pcm.get_n_rows(); rows++)
{
int deg = 0;
std::vector<int> deg_pos;
for (size_t cols = 0; cols < pcm.get_n_cols(); cols++)
if (pcm.at(rows, cols))
deg++;
if (max_deg < deg)
max_deg = deg;
}
d_pcm_num_cn = pcm.get_n_rows();
d_pcm_num_vn = pcm.get_n_cols();
d_pcm_max_cn_degree = max_deg;
d_pcm_num_edges = pcm.get_n_connections();
d_vns = new int16_t[d_pcm_num_vn];
d_vns_to_cn_msgs = new int16_t[d_pcm_max_cn_degree];
d_cn_to_vn_msgs = new int16_t[d_pcm_num_cn * d_pcm_max_cn_degree];
d_abs_msgs = new int16_t[d_pcm_max_cn_degree];
d_vn_addr = new int16_t *[d_pcm_num_edges];
d_row_pos_deg = new int[d_pcm_num_cn * 2];
/* Precompute VN addresses in the VN buffer. Since each row has a different
* degree the VN addresses are compacted. Keeping the offset and the degree
* of each row is therefore required. */
int row_base_idx = 0;
auto mv = pcm;
for (size_t row = 0; row < mv.get_n_rows(); row++)
{
int row_deg = 0;
for (size_t col = 0; col < mv.get_n_cols(); col++)
if (mv.at(row, col))
row_deg++;
assert(row_deg <= d_pcm_max_cn_degree);
d_row_pos_deg[row * 2] = row_base_idx;
d_row_pos_deg[row * 2 + 1] = row_deg;
for (size_t col = 0; col < mv.get_n_cols(); col++)
if (mv.at(row, col))
d_vn_addr[row_base_idx++] = &(d_vns[col]);
}
}
LDPCDecoderGeneric::~LDPCDecoderGeneric()
{
delete[] d_vns;
delete[] d_vns_to_cn_msgs;
delete[] d_abs_msgs;
delete[] d_cn_to_vn_msgs;
delete[] d_vn_addr;
delete[] d_row_pos_deg;
}
int LDPCDecoderGeneric::decode(uint8_t *out, const int8_t *in, int it)
{
int corrections = 0;
/* The length of the input block should correspond to the length of a codeword. */
// if (len != d_pcm->code.n)
// {
// return -1;
// }
/* Copy the input codeword and give lowest LLR value to each punctured
* bit. */
for (int i = 0; i < d_pcm_num_vn; i++)
{
d_vns[i] = (int16_t)in[i];
}
/* Init of CN to VN messages */
for (int i = 0; i < d_pcm_num_cn * d_pcm_max_cn_degree; i++)
{
d_cn_to_vn_msgs[i] = 0;
}
/* Decode step */
while (it--)
{
for (int cn_idx = 0; cn_idx < d_pcm_num_cn; cn_idx++)
{
generic_cn_kernel(cn_idx);
}
}
/* Hard decision */
for (int i = 0; i < d_pcm_num_vn; i++)
{
out[i] = (uint8_t)(d_vns[i] >= 0 ? 1 : 0);
if ((out[i] > 0) != (in[i] > 0))
corrections++;
}
return corrections;
}
void LDPCDecoderGeneric::generic_cn_kernel(int cn_idx)
{
/* Given an indexed CN, gather the messages of all VNs connected
* to that CN and determine a new estimation for each of the VNs. */
cn_row_base = d_row_pos_deg[cn_idx * 2];
cn_deg = d_row_pos_deg[cn_idx * 2 + 1];
cn_offset = d_pcm_max_cn_degree * cn_idx;
for (int vn_idx = 0; vn_idx < cn_deg; vn_idx++)
{
d_vns_to_cn_msgs[vn_idx] =
*d_vn_addr[cn_row_base + vn_idx] - d_cn_to_vn_msgs[cn_offset + vn_idx];
// printf("%d \n", d_vns_to_cn_msgs[vn_idx]);
}
parity = 0;
min1 = UINT8_MAX;
min2 = UINT8_MAX;
if (cn_deg & 0x1)
parity = ~parity;
/* Compute the parity of all soft bits represented by the VNs to CN
* messages, the absolute value of each soft bit, and the overall
* first and second minimums. All these results are used later to
* determine a new estimation sent back to each of the VNs connected
* to the current CN. */
for (int vn_idx = 0; vn_idx < cn_deg; vn_idx++)
{
msg = d_vns_to_cn_msgs[vn_idx];
parity ^= msg;
/* Bit-hack to compute the absolute value of the message */
// abs_mask = msg >> (sizeof(msg) * 8 - 1);
// abs_msg = (int16_t)((msg + abs_mask) ^ abs_mask);
abs_msg = abs(msg);
/* Determine the first and second minimum */
min2 = min2 > abs_msg ? (min1 > abs_msg ? min1 : abs_msg) : min2;
min1 = min1 > abs_msg ? abs_msg : min1;
/* Keep the computed absolute value for later */
d_abs_msgs[vn_idx] = abs_msg;
// printf("%d \n", d_abs_msgs[vn_idx]);
}
/* Compute a new soft bit estimation for each VN respectively and send it
* in a new message.
* Basically, we are updating the value of each adjacent VN of the CN given the values of
* the other VNs. We are computing an error correction value that we add to the
* current value of a VN. The sign of the error correction value has to follow a
* critical property of LDPC codes, that is, the set of all soft bits of VNs
* adjacent to a CN has even parity. Ex: a hard bit should have value '0' if
* if the remaining has even parity, otherwise '1'.
* Concretely, the min-sum variant for belief-propagation works as follows to compute
* a new estimation :
* 1) Take the minimum absolute value among all soft bits of all other VNs
* (the value of the VN to be updated is not taken into account) which
* will become the magnitude of the error correcting value.
* 2) Compute the sign of the error correcting value (which will make a soft
* bit converge toward a negative or positive value and thus to a hard
* '0' or '1' bit)
* 3) Add it to the current soft bit value of a VN */
for (int vn_idx = 0; vn_idx < cn_deg; vn_idx++)
{
equ_min1 = (~(uint16_t(d_abs_msgs[vn_idx] == min1))) + 1; // 0 or -1
min = (min1 & ~equ_min1) | (min2 & equ_min1);
sign = (parity ^ d_vns_to_cn_msgs[vn_idx]);
/* Bit hack in order to multiply by the sign */
sign = (sign >> (sizeof(sign) * 8 - 1));
new_msg = (int16_t)((min + sign) ^ sign);
/* Add error correction value */
to_vn = new_msg + d_vns_to_cn_msgs[vn_idx];
/* Save new soft bit value and CN to VN message */
d_cn_to_vn_msgs[cn_offset + vn_idx] = new_msg;
*d_vn_addr[cn_row_base + vn_idx] = to_vn;
}
}
}
}
|
