Tick 18 — 2026-05-22 07:24 UTC

Thread: R6.1 (multi-scatterer additive Fresnel forward model) Verdict: Working 6-scatterer body model. Discovers a 4.7 dB multi-scatterer penalty that matches R13's 5-dB-shortfall finding — gives R13 a physical origin and unblocks R12's PABS revision path.

What shipped

• examples/research-sota/r6_1_multiscatterer.py — pure-numpy multi-scatterer Fresnel forward model with 6 body-part scatterers + breathing motion.
• examples/research-sota/r6_1_multiscatterer_results.json — machine-readable predictions.
• docs/research/sota-2026-05-22/R6_1-multiscatterer-forward-model.md — research note.

Headline finding

5 m link, 2.4 GHz, subject 25 cm off LOS, 30-second breathing time-series:

Configuration	Breathing SNR (best subcarrier)
Single-scatterer ideal (R6)	+23.7 dB
Multi-scatterer realistic (R6.1, 6 parts)	+19.0 dB
Multi-scatterer penalty	+4.7 dB

This 4.7 dB penalty is the gap between R6's idealised physics and realistic deployment — and it matches R13's 5 dB shortfall to within 0.3 dB, suggesting R13's "we are 5 dB short of pulse-contour recovery" finding has a physical origin in the static body parts, not just measurement noise.

Per-body-part energy contribution

• Chest: 27.6% of total CSI energy (highest reflectivity, 5× per-limb value)
• Each limb / head: 1.1% each
• The chest IS the breathing signal; limbs are confound, not signal

Architectural implications

1. Chest-centric placement targeting (R6.2.3) — current R6.2 treats body as single point; should target chest specifically.
2. Mask limbs in vital_signs pipeline — pose pipeline (ADR-079, ADR-101) already extracts limb positions; vital_signs just doesn't use them.
3. R14 V3 re-scope — attention-respecting conversational appliance needs +25 dB pulse-contour recovery, which R6.1 says is unachievable. V3 should depend only on breathing rate stability, not pattern shape.

R12's PABS revision unblocked

R12 (NEGATIVE eigenshift) suggested PABS over Fresnel basis as the revision. R6.1 IS the explicit A(voxel) forward operator that PABS needs. R12 + R6.1 = tractable structure-detection implementation.

Why this is a satisfying integration

• R6 = bound (idealised single-scatterer)
• R6.1 = floor (realistic multi-scatterer)
• R13 = the actual failure mode (5 dB short)

The three threads now have a coherent physics story: pulse-contour recovery is bound below by what R6.1 leaves achievable, which is 4.7 dB worse than the R6 idealised limit, which is enough to make R13's contour recovery infeasible.

On-LOS placement is degenerate

First simulation run had subject at y=0 (exactly on LOS), giving SNR of -60 dB (essentially undetectable). Path-delta is 2nd-order in offset for on-LOS scatterers, so breathing in y direction barely changes path. Lesson surfaced: real installations need subject OFF the LOS line, not on it. The off-LOS placement (25 cm) gives the +19 dB number.

This is a non-obvious deployment requirement that R6.2 placement search should respect — don't place antennas such that the primary target zone sits on the LOS line.

Composes with prior threads

• R5: subcarrier selection prefers reliable, not high-SNR
• R6: provides the per-scatterer building block
• R6.2 / R6.2.2 / R6.2.3 (future): chest-centric placement
• R7: residual-against-forward-model gives tighter adversarial detection
• R12 NEGATIVE: PABS A operator now unblocked
• R13 NEGATIVE: 5-dB gap has physical origin
• R14: V3 needs rescope to rate-only

Honest scope

• 6 scatterers is 1st-order; 50-100 voxel body would be better
• Reflectivity ratios are guesses (RCS measurements at 2.4 GHz on real humans would refine)
• Static body assumption (limbs do micro-move during breathing)
• 2D top-down (3D would add vertical structure)
• No multipath (room reflections add scatterers; model is general enough to include them)

Coordination

ticks/tick-18.md. No PROGRESS.md edit. Branch research/sota-r6.1-multiscatterer-fresnel.

Remaining work

• R3 follow-up: physics-informed env_sig prediction (uses R6 + room map → zero-shot cross-room)
• R6.2.1: 3D ceiling/floor placement
• R6.2.3: chest-centric / pose-trajectory-aware target zones (now strongly motivated by R6.1)
• R12 PABS implementation: forward operator now available
• ADR-107: cross-installation federation w/ secure aggregation

~4.6h to cron stop. 18 ticks landed. Loop has covered R1-R15 + 2 ADRs + 3 deferred follow-ups (R6.2, R6.2.2, R6.1).