ADR-113: Multistatic anchor placement strategy

Status: Proposed · Date: 2026-05-22 · Author: SOTA research loop tick-31 · Amends: ADR-029 (RuvSense multistatic sensing mode)

Context

ADR-029 (RuvSense multistatic) introduced multi-anchor CSI sensing but did not specify how many anchors, where to place them, or how zones depend on the target cog. The SOTA research loop (2026-05-22) produced 9 ticks in the R6 family that quantitatively answer these questions:

• R6 / R6.1: Fresnel forward model (single + multi-scatterer)
• R6.2: 2D placement search
• R6.2.1: 3D placement (ceiling-only fails)
• R6.2.2: 2D N-anchor saturation (knee at N=5)
• R6.2.2.1: 3D N-anchor (2D knee doesn't hold)
• R6.2.3: chest-centric zones (+27 pp gain for vital signs)
• R6.2.4: 3D + chest composition (knee at N=6, no ceiling)
• R6.2.5: multi-subject union (N=5 hits 100% for 1-4 occupants)

This ADR consolidates the findings into a single placement specification, parameterised by dimension × zone-mode × occupant-count × cog.

Decision

Adopt the 4-axis placement decision matrix below as the binding RuView installation specification.

Decision matrix

Key rules (extracted from R6 family)

1. Ceiling-only mounting always fails (R6.2.1): both antennas at ceiling height produce a Fresnel envelope sitting AT ceiling, never reaching floor-level targets. Always include at least one low-anchor.
2. Vertical link diversity wins in 3D (R6.2.1): diagonal-in-z links (e.g. 0.8 m → 1.5 m) tilt the ellipsoid through multiple elevations.
3. Anchor heights should match target zone heights (R6.2.4): chest-centric zones at z=0.3-1.5 don't benefit from ceiling (z=2.4) anchors. Full-body coverage does.
4. Chest-centric beats body-centric for vital signs (R6.2.3): +27 pp coverage gain at N=5 from smaller, occupant-specific zones.
5. Multi-subject union is the right target for households (R6.2.5): single-subject placement loses 29 pp when extended to 4 occupants; multi-subject-optimised placement keeps 100%.
6. N=5 is the consumer recommendation (R6.2.2 + R6.2.5): the 2D chest-centric multi-subject knee. Beyond N=5, marginal gains are <1 pp.
7. Avoid placing target zones on the LOS line (R6.1): path-delta is 2nd-order in offset for on-LOS scatterers; breathing motion barely changes path length. Real installations need subjects OFF the LOS.

CLI specification (productisation)

The R6.2 CLI tool surfaced through the family ticks:

wifi-densepose plan-antennas
    --room W H [Z]                        # 2D or 3D
    --target NAME X Y W H [DX DY DZ]      # repeatable
    --target-mode {body, chest}           # R6.2.3
    --freq-ghz F                          # 2.4, 5.0, 6.0
    --n-anchors N                         # auto-saturate if omitted
    --restarts K                          # 4 default
    --cog COG_NAME                        # auto-select target-mode + N

Total LOC for productisation: ~100 LOC on top of the R6.2.5 reference implementation.

MCP surface (per ADR-104)

ruview_placement_recommend(
    room: {width, depth, ceiling?},
    targets: [{name, position, size}],
    cog: str  // auto-configures target-mode + N
) -> {
    anchors: [{x, y, z, height_category}],
    expected_coverage: float,
    placement_rationale: str
}

Alternatives considered

A. Keep ADR-029 silent on placement

Status: rejected. Without explicit guidance, installations choose placement arbitrarily; R6.2 measured 93× spread between optimal and median placement. Silence is a 93× implicit loss.

B. Always recommend N=5 + body-centric

Status: rejected. The 2D body-centric N=5 recommendation under-promises for vital-signs (chest-centric is better) and over-promises for 3D body-centric (97% → 49% in honest 3D, per R6.2.2.1).

C. Always recommend N=8

Status: rejected. R6.2.2.1 showed the 3D saturation curve never has a clean knee; bumping to N=8 gets 65% coverage at body-centric, but the chest-centric N=6 alternative hits 82% with fewer hardware units. Per-cog decision is the right granularity.

D. Recommend per-cog without dimension awareness

Status: rejected. R6.2.1 + R6.2.2.1 surface that the 2D recommendation systematically under-promises 3D realities. The dimension axis must be explicit.

Threat model

Placement strategy is not a security-critical decision in itself; coverage gaps create functional risk, not adversarial risk. The 4-axis matrix ensures:

Consequences

Positive

1. Single canonical placement spec for installers, replacing tribal knowledge with a numbers-backed decision matrix.
2. Per-cog optimization without overlapping with within-cog tuning (target zones, sensitivity thresholds).
3. CLI tool unblocks self-service installation — customers can run wifi-densepose plan-antennas in 2 minutes and get a placement diagram.
4. MCP tool unblocks AI-agent-driven deployment — empathic appliance integration partners can call ruview_placement_recommend programmatically.
5. R7 mincut adversarial defence is automatically satisfied for all multi-feature cogs (which need N ≥ 4 anyway).

Negative

1. The matrix is one geometry deep — 5×5 m bedroom benchmarks. Larger rooms / oddly-shaped rooms need separate benchmarks; the matrix should be extended over time.
2. Per-cog matrix entries require periodic re-validation when cogs change architecture.
3. Adds installer-time complexity — choosing the right matrix row requires knowing the cog's category. The CLI's --cog flag absorbs this.
4. Multi-cog deployments need union-of-matrix-rows logic, currently catalogued for future work.
5. 3D body-centric still under-performs (65% N=8) — no architectural fix; chest-centric is the workaround for vital-signs, but pose-estimation in 3D may need a different approach.

What this ADR DOES NOT cover

1. Production validation on real hardware — all matrix values are synthetic-physics derived. Bench validation on COM5 ESP32-S3 is the next step.
2. Time-varying placement — the matrix assumes fixed anchors; mobile anchors (e.g. on a Roomba) are a different regime.
3. Multi-room placement — within-room only; cross-room sensing needs separate analysis.
4. Per-room-shape benchmarking — only 5×5 m bedroom + 4×6 m living-room-class tested.
5. Per-frequency matrix variation — all rows are 2.4 GHz; 5 GHz and 6 GHz have different envelope widths and may shift the optimum.

Bridge to existing ADRs

• ADR-029 (RuvSense multistatic) — directly amends: ADR-029's deferred "anchor placement" specification is now this matrix.
• ADR-079 / ADR-101 (pose tracker): depends on accurate pose extraction; ADR-113's anchor count guarantees N ≥ 5 for pose cogs, which gives the pose tracker enough multistatic coverage.
• ADR-100 (cog packaging): cogs are signed with ADR-100; placement decisions are independent.
• ADR-103 (cog-person-count): 2D body-centric N=4 entry maps to this cog.
• ADR-104 (ruview-mcp + ruview-cli): ruview_placement_recommend becomes a new MCP tool.
• ADR-105 / ADR-106 / ADR-107: federation operates on signed cog outputs; placement quality affects federation gradient quality (better placement → faster ε convergence).
• ADR-108 / ADR-109: PQC chain protects placement-recommendation outputs in transit.

Per-cog target-mode auto-selection

The --cog flag in the CLI looks up the cog category and maps to matrix row:

Connection to research-loop threads

• R5 (saliency) — explains why placement maximising Fresnel coverage gives band-spread saliency.
• R6 / R6.1 (forward model) — physical foundation.
• R6.2 family (9 ticks) — the entire R6.2 family feeds this ADR.
• R7 (mincut) — N ≥ 4 satisfied for all multi-feature cogs.
• R10 (foliage) — wildlife corridor placement is a 1D linear variant; future R6.2.6 could specialise.
• R11 (maritime) — cabin placement is in the matrix.
• R12 PABS / R12.1 — placement coverage = intrusion-detection sensitivity.
• R14 (empathic appliances) — V1 lighting (chest-mode N=5) + V2 HVAC (mixed) + V3 attention (chest-mode) covered.
• R15 (RF biometric) — per-primitive saliency may need a future placement axis.

Honest scope

• Synthetic physics derivation — all matrix values come from numpy simulations, not bench measurements. Real-world deployment may shift values by ±5-15%.
• Single room-geometry baseline — 5×5 m + 4×6 m. The matrix should grow over time to cover hallways, large living rooms, factory floors.
• 5 cm pose-tracker noise — assumed in R12.1; degraded pose tracking may invalidate some recommendations.
• Free-space propagation — no multipath modelling; real rooms add 5-15% coverage.
• No furniture occlusion — sofas, walls, wardrobes ignored.
• Greedy + 4-restart search — global optimum may be 1-2 pp higher.

Implementation plan

Total ~260 LOC. Combined with R6.2 productisation (~100 LOC), placement-strategy budget is ~360 LOC.

Decision-making record

• 2026-05-22 10:06 UTC — drafted by SOTA research loop tick-31 consolidating 9 R6-family ticks. Status: Proposed.
• Pending: ADR-029 author (this is an amendment), production-validator (matrix needs bench validation), MCP/CLI maintainer (CLI surface extension).

What this ADR closes

The multistatic placement question that ADR-029 left open. After this ADR, ADR-029 + ADR-113 + the R6.2 CLI form a coherent multistatic sensing specification with quantified expected coverage per cog and dimension.

This is the 9th ADR the SOTA loop has produced (counting ADR-105 → ADR-109 + ADR-113), and the last one focused on a research-loop output. Future ADRs (ADR-110/111/112) are operational, not research-driven.

Closing observation

The R6 family produced 9 ticks of physics + simulation, each adding 1-2 axes to the placement question. ADR-113 collapses all 9 into a single decision matrix that a non-physicist installer can use. The loop's most ship-relevant integrative output.