Fused FEC: sub-block integrity + Reed-Solomon + per-layer UEP

[josephnef]

What

A cross-layer fused FEC stack for the long-range video downlink. Today three error-correction resources never cooperate: the PHY-MCS inner code, an application-layer erasure code, and DEVOURER_RX_KEEP_CORRUPTED (kept FCS-failed frames). This wires them together: instead of discarding a CRC-failed frame as one whole-frame erasure, it salvages the frame's CRC-valid sub-blocks — turning the chip's all-or-nothing FCS into fine-grained per-symbol erasures the outer code already knows how to recover from.

Full architecture, design rationale, and results: docs/fused-fec.md.

Contents (tools/precoder/, all numpy-light Python)

• fec_subblock.py — the sub-block-integrity (SBI) layer: fixed-size, individually CRC-guarded sub-blocks (one sub-block == one outer-code symbol), with stream_id multiplexing. Fixed-size on purpose so a corrupted length byte can't desync neighbours.
• stream_fec_rs.py — Reed-Solomon outer scheme (systematic GF(2⁸) Vandermonde, Rizzo/zfec construction), added as a third scheme alongside RaptorQ/RLC; MDS, best for small low-latency blocks.
• svc_uep_fec.py — per-SVC-layer FEC-rate UEP (HEVC NAL → temporal layer → RS redundancy); the application-FEC half complementing the MCS ladder in svc_tx.h.
• fused_fec_link.py + fused_fec_tx/rx.py — chip-path sender/receiver + CLIs; the RX runs baseline and SBI decoders in lockstep and reports the gain.
• fec_fusion_sim.py — offline simulation quantifying the SBI gain and sizing the sub-blocks (no hardware).

tests/: fused_fec_onair.sh (chip↔chip on-air harness), sdr_interferer.py (USRP B210 calibrated co-channel source — a reproducible SNR knob without an attenuator). txdemo/stream_tx_demo/main.cpp: DEVOURER_TX_PWR_OVERRIDE.

Results

• 22 new unit tests; full precoder suite 130/130 green.
• On-air, real silicon (8812 → 8821, MCS7, 15 s): +129 RS blocks recovered that the drop-whole-frame baseline lost (~13 % lift in delivered blocks).

Notes

• The SDR-RX path (USRP B210 + a gr-ieee802-11 fork) is a separate experiment in sister repos; this PR is the chip-path + the shared FEC stack. The SDR over-air gain is currently decoder-limited (hard-decision Viterbi) — documented as future work in docs/fused-fec.md.
• No change to the WiFiDriver core RX/TX behaviour beyond the additive DEVOURER_TX_PWR_OVERRIDE knob.

🤖 Generated with Claude Code