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fix: set unconnected VN->CN messages to +MAX in min-sum

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Prevents magnitude 0 from unconnected columns dominating the
minimum computation and zeroing all CN->VN messages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
cah
2026-02-25 19:06:38 -07:00
parent 5b92587f51
commit 55eb487f5f
1 changed files with 51 additions and 30 deletions
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29
verilog/rtl/ldpc_decoder_core.sv
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@@ -112,13 +112,14 @@ module ldpc_decoder_core #(
// Decoder FSM
// =========================================================================
typedef enum logic [2:0] {
typedef enum logic [3:0] {
IDLE,
INIT, // Initialize beliefs from channel LLRs, zero messages
LAYER_READ, // Read Z beliefs for each of DC columns in current row
CN_UPDATE, // Run min-sum CN update on gathered messages
LAYER_WRITE, // Write updated beliefs and new CN->VN messages
SYNDROME, // Check syndrome after full iteration
SYNDROME_DONE, // Read registered syndrome result
DONE
} state_t;
@@ -167,7 +168,8 @@ module ldpc_decoder_core #(
state_next = LAYER_READ; // next row
end
end
SYNDROME: begin
SYNDROME: state_next = SYNDROME_DONE;
SYNDROME_DONE: begin
if (syndrome_ok && early_term_en)
state_next = DONE;
else if (iter_cnt >= effective_max_iter)
@@ -192,6 +194,7 @@ module ldpc_decoder_core #(
converged <= 1'b0;
iter_used <= '0;
syndrome_weight <= '0;
syndrome_ok <= 1'b0;
end else begin
case (state)
IDLE: begin
@@ -199,6 +202,7 @@ module ldpc_decoder_core #(
row_idx <= '0;
col_idx <= '0;
converged <= 1'b0;
syndrome_ok <= 1'b0;
end
INIT: begin
@@ -221,6 +225,8 @@ module ldpc_decoder_core #(
// VN->CN = belief - old CN->VN message
// (belief already contains the sum of ALL CN->VN messages,
// so subtracting the current row's message gives the extrinsic)
// Skip unconnected columns (H_BASE == -1)
if (H_BASE[row_idx][col_idx] >= 0) begin
for (int z = 0; z < Z; z++) begin
int bit_idx;
int shifted_z;
@@ -234,6 +240,11 @@ module ldpc_decoder_core #(
vn_to_cn[col_idx][z] <= sat_sub(belief_val, old_msg);
end
end else begin
// Unconnected: set to +MAX so magnitude doesn't affect min-sum
for (int z = 0; z < Z; z++)
vn_to_cn[col_idx][z] <= {1'b0, {(Q-1){1'b1}}}; // +31
end
if (col_idx == N_BASE - 1)
col_idx <= '0;
@@ -261,6 +272,8 @@ module ldpc_decoder_core #(
LAYER_WRITE: begin
// Write back: update beliefs and store new CN->VN messages
// Skip unconnected columns (H_BASE == -1)
if (H_BASE[row_idx][col_idx] >= 0) begin
for (int z = 0; z < Z; z++) begin
int bit_idx;
int shifted_z;
@@ -278,6 +291,7 @@ module ldpc_decoder_core #(
// Store new message for next iteration
msg_cn2vn[row_idx][col_idx][z] <= new_msg;
end
end
if (col_idx == N_BASE - 1) begin
col_idx <= '0;
@@ -292,25 +306,32 @@ module ldpc_decoder_core #(
SYNDROME: begin
// Check H * c_hat == 0 (compute syndrome weight)
// Only include connected columns (H_BASE >= 0)
syndrome_cnt = '0;
for (int r = 0; r < M_BASE; r++) begin
for (int z = 0; z < Z; z++) begin
logic parity;
parity = 1'b0;
for (int c = 0; c < N_BASE; c++) begin
if (H_BASE[r][c] >= 0) begin
int shifted_z, bit_idx;
shifted_z = (z + H_BASE[r][c]) % Z;
bit_idx = c * Z + shifted_z;
parity = parity ^ beliefs[bit_idx][Q-1]; // sign bit = hard decision
parity = parity ^ beliefs[bit_idx][Q-1];
end
end
if (parity) syndrome_cnt = syndrome_cnt + 1;
end
end
syndrome_weight <= syndrome_cnt;
syndrome_ok = (syndrome_cnt == 0);
syndrome_ok <= (syndrome_cnt == 0);
iter_cnt <= iter_cnt + 1;
iter_used <= iter_cnt + 1;
end
SYNDROME_DONE: begin
// Check registered syndrome result
if (syndrome_ok) converged <= 1'b1;
end