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[fix](trx-ft8): increase FT2 LDPC iterations and improve OSD fallback

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Increase BP/SP iteration count from 30 to 50 to match WSJT-X reference
and give belief propagation more opportunities to converge near-threshold
candidates.

Replace the parity-based OSD-1/OSD-2 fallback (which required LDPC to
have nearly converged) with ft2_osd_lite_decode applied to all five LLR
combination passes. The CRC-based decoder works directly from raw LLRs
without depending on LDPC convergence, searching the 24 least-reliable
systematic bits for up to three bit errors via OSD-3.

Also increase max_candidates in ft2_osd_lite_decode from 12 to 24 for
broader coverage of likely error positions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
This commit is contained in:
Stan Grams
2026-03-15 08:06:19 +01:00
parent 3b28281684
commit eac2efdb35
1 changed files with 14 additions and 19 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/decoders/trx-ft8/src/ft8_wrapper.c
+14 -19
View File
@@ -1074,7 +1074,7 @@ static bool ft2_osd_lite_decode(const float log174[FTX_LDPC_N], ftx_message_t* m
}
qsort(reliabilities, FTX_LDPC_K, sizeof(reliabilities[0]), ft2_cmp_reliability_asc);
const int max_candidates = 12;
const int max_candidates = 24;
const int n = (FTX_LDPC_K < max_candidates) ? FTX_LDPC_K : max_candidates;
uint8_t trial_a91[FTX_LDPC_K_BYTES];
uint8_t best_codeword[FTX_LDPC_N];
@@ -1393,8 +1393,6 @@ static bool ft2_decode_hit(
bool ok = false;
uint8_t cw[FTX_LDPC_N] = { 0 };
int global_best_errors = FTX_LDPC_M;
int global_best_pass = 0;
for (int pass = 0; pass < 5 && !ok; ++pass)
{
float log174[FTX_LDPC_N];
@@ -1406,7 +1404,7 @@ static bool ft2_decode_hit(
uint8_t bp_plain[FTX_LDPC_N];
int bp_errors = FTX_LDPC_M;
bp_decode(log174, 30, bp_plain, &bp_errors);
bp_decode(log174, 50, bp_plain, &bp_errors);
if (bp_errors < nharderror)
{
nharderror = bp_errors;
@@ -1425,7 +1423,7 @@ static bool ft2_decode_hit(
{
uint8_t sp_plain[FTX_LDPC_N];
int sp_errors = FTX_LDPC_M;
ldpc_decode(log174, 30, sp_plain, &sp_errors);
ldpc_decode(log174, 50, sp_plain, &sp_errors);
if (sp_errors < nharderror)
{
nharderror = sp_errors;
@@ -1441,12 +1439,6 @@ static bool ft2_decode_hit(
}
}
if (nharderror < global_best_errors)
{
global_best_errors = nharderror;
global_best_pass = pass;
}
if (pass_diag)
{
pass_diag->ntype[pass] = ntype;
@@ -1455,15 +1447,18 @@ static bool ft2_decode_hit(
}
}
// OSD-1 / OSD-2: when BP/SP leave only a few parity errors, try flipping
// the least-reliable bits to bridge the gap.
if (!ok && global_best_errors <= 8)
// CRC-based OSD: try flipping 1/2/3 of the least-reliable systematic bits.
// Works directly from LLRs without depending on LDPC convergence.
if (!ok)
{
float osd_log174[FTX_LDPC_N];
memcpy(osd_log174, llr_passes[global_best_pass], sizeof(osd_log174));
ft2_normalize_log174(osd_log174);
if (ft2_osd_decode(cw, osd_log174, global_best_errors) && ft2_unpack_message(cw, message))
ok = true;
for (int pass = 0; pass < 5 && !ok; ++pass)
{
float osd_log174[FTX_LDPC_N];
memcpy(osd_log174, llr_passes[pass], sizeof(osd_log174));
ft2_normalize_log174(osd_log174);
if (ft2_osd_lite_decode(osd_log174, message))
ok = true;
}
}
if (!ok && fail_stage)