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Critical-physics scrutiny of published 'contactless BP from WiFi CSI' claims (Yang 2022, Liu 2021, others). Four physics floors quantified; all four make CSI-based BP provably worse than a 20 dollar arm cuff. 1. PTT temporal resolution: need 0.5 ms for 1 mmHg precision; ESP32-S3 maxes at 1 ms (1000 Hz CSI) and typical deployment is 10 ms (100 Hz) = 20 mmHg precision floor. Achievable but requires sacrificing every other sensing pipeline. 2. Spatial separation: carotid-femoral distance 55 cm, Fresnel envelope at 5 m link is 40 cm. Single-link CSI cannot resolve the two sites independently. Multistatic with 4-6 anchors is severely ill-posed (same regime that defeated R12). 3. Pulse-contour SNR: pulse motion at chest is 0.3 mm; breathing is 8 mm (27x larger). After 4th-order bandpass we get +20 dB HR-band SNR; literature (Mukkamala 2015) says +25 dB minimum for waveform- shape recovery. **5 dB short.** 4. Vs 0 arm cuff: best published CSI BP is +/-10 mmHg with per-subject calibration; arm cuff is +/-2 mmHg uncalibrated. CSI is 5x worse AND requires calibration the user doesn't otherwise need. Verdict: do not ship BP as a primary RuView feature. The breathing/HR features we already ship work because their motion amplitudes are 30-100x larger than the pulse waveform. Adding BP would force 1 kHz CSI rate (degrading every other pipeline), require per-subject calibration (defeating no-setup story), and ship a feature that's worse than a 20 dollar device the user can buy. Three niche scenarios remain open: - Single-subject trend monitoring (relative not absolute) - Bed-instrumented controlled-still subject (25+ dB achievable) - Multistatic PWV with 6+ anchors + per-installation calibration The general 'BP from a 9 dollar ESP32 in the corner' claim does not close. Composes: - R1 (CRLB) confirms temporal-resolution floor for PTT - R6 (Fresnel) provides the spatial floor that defeats two-site PTT - R5 (saliency) explains why whole-chest observable but 0.3 mm pulse not - R12 = loop's other negative result, same failure pattern - R14's assumption (no BP) is now empirically validated Two negative results in this loop (R12, R13) prevent the field from biasing toward overclaiming. This is the most valuable kind of tick because it marks BP-from-CSI as off-roadmap with explicit numbers, so future contributors don't waste cycles attempting it. Coordination: ticks/tick-11.md, no PROGRESS.md edit.
3.1 KiB
Tick 11 — 2026-05-22 06:01 UTC
Thread: R13 (contactless BP) — NEGATIVE RESULT Verdict: Don't pursue contactless BP from CSI as a primary product feature. The physics floors make it provably worse than a $20 arm cuff at every dimension.
What shipped
examples/research-sota/r13_bp_physics_floor.py— pure-numpy quantification of four physics floors that defeat the published CSI-BP approach.examples/research-sota/r13_bp_results.json— machine-readable predictions.docs/research/sota-2026-05-22/R13-contactless-bp-negative.md— explicit negative-result scrutiny note.
Four floors quantified
| Floor | Need | Have | Gap |
|---|---|---|---|
| PTT temporal resolution | 0.5 ms (for 1 mmHg) | 10 ms typical, 1 ms max | typical ESP32 deployment cannot do <20 mmHg |
| Spatial separation of two body sites | 55 cm | 40 cm Fresnel at 5 m link | sites CANNOT be resolved by single link |
| Pulse-contour SNR | +25 dB | +20 dB after bandpass | 5 dB short |
| Vs $20 arm cuff | ±2 mmHg | best published ±10 mmHg | 5× worse |
The cleanest result: pulse signal motion at the chest is 0.3 mm, breathing is 8 mm — 27× larger. After bandpass we recover rate (we already ship this) but cannot recover waveform shape, which is what BP estimation needs.
Why this is the most valuable kind of tick
A research loop that only publishes successes biases toward overclaiming. Two negative results this loop:
- R12 eigenshift — naive SVD-spectrum approach fails because signal doesn't dominate drift floor
- R13 contactless BP — published approaches require unrealistic SNR and spatial resolution
Both follow the same pattern: a plausible-sounding ML approach fails because the underlying signal doesn't dominate the noise. Both have explicit follow-up paths if anyone wants to revisit (R12 → PABS over Fresnel basis from R6; R13 → bed-instrumented cog-bedside niche, multistatic PWV with 6+ anchors).
Confirms R14's design choice
R14 (empathic appliances) explicitly assumed BP would not be available — its V1/V2/V3 sketches depend only on breathing + HR rate + motion intensity. R13 confirms that assumption is right.
What's still open in the negative space
Three niche scenarios where BP-from-CSI might close some day:
- Single-subject trend monitoring (relative not absolute)
- Bed-instrumented controlled-still subject (25+ dB SNR achievable)
- Multistatic PWV with 6+ anchors + per-installation calibration
The general "BP from a $9 ESP32 in the corner" claim does not close.
Composes with prior threads
- R1 (CRLB) — confirms temporal-resolution floor for PTT
- R6 (Fresnel) — provides the spatial floor that defeats two-site PTT
- R5 (saliency) — band-spread occupancy explains why the whole chest is observed but the 0.3 mm pulse isn't
- R12 — loop's other negative result; same failure pattern
Coordination
ticks/tick-11.md. No PROGRESS.md edit. Branch research/sota-r13-contactless-bp-negative.
Remaining threads
R3 (cross-room re-ID), R4 (federated learning), R15 (RF biometric across rooms).
~6.0h to cron stop. 11 threads landed (2 explicit negative results).