Add cn_mode ('offset'/'normalized') and alpha parameters to
min_sum_cn_update() in ldpc_sim.py and generic_decode() in
ldpc_analysis.py. Normalized mode scales magnitudes by alpha
(default 0.75) instead of subtracting a fixed offset, which
is better suited for low-rate codes.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
- Fixed improved staircase: below-diagonal connections preserve full
parity rank (col7->row0 s3, col1->row4 s15)
- Fixed PEG matrix: staircase backbone with cross-connections,
all parity cols dv>=2, VN degrees [7,3,3,3,2,2,2,2]
- Clean up VN degree display (remove np.int64 wrapper)
- Ran all four analyses with 200 frames per point
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Implements four LDPC code analyses for photon-starved optical channels:
- Rate sweep: compare FER across 1/2, 1/3, 1/4, 1/6, 1/8 IRA staircase codes
- Matrix comparison: original staircase vs improved staircase vs PEG ring
- Quantization sweep: 4-16 bit and float precision impact on FER
- Shannon gap: binary-input Poisson channel capacity limits via binary search
Core infrastructure includes generic IRA staircase builder, GF(2) Gaussian
elimination encoder for non-triangular matrices, parameterized layered
min-sum decoder with variable check degree, and BFS girth computation.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
- Fixed cyclic shift convention (QC-LDPC P_s is left shift, not right)
- Fixed encoder to solve rows sequentially (row 0 first for p1, then 1-6)
- Fixed decoder to only gather connected columns per CN (staircase has dc=2-3)
- Fixed LLR sign convention: positive = bit 0 more likely
- Decoder validates at lam_s >= 4 photons/slot (~90% frame success)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
