ADR-037: Multi-Person Pose Detection from Single ESP32 CSI Stream

• Status: Proposed
• Date: 2026-03-02
• Issue: #97
• Deciders: @ruvnet
• Supersedes: None
• Related: ADR-014 (SOTA signal processing), ADR-024 (AETHER re-ID), ADR-029 (multistatic sensing), ADR-036 (RVF training pipeline)

Context

The current signal-derived pose estimation pipeline (derive_pose_from_sensing() in the sensing server) generates at most one skeleton per frame from aggregate CSI features. When multiple people are present, only a single blended skeleton is produced. Live testing with ESP32 hardware confirmed: 2 people in the room yields 1 detected person.

A single ESP32 node provides 1 TX × 1 RX × 56 subcarriers of CSI data per frame. While this is limited spatial resolution compared to camera-based systems, the signal contains composite reflections from all scatterers in the environment. The challenge is decomposing these composite signals into per-person contributions.

Decision

Implement multi-person pose detection in four phases, progressively improving accuracy from heuristic to neural approaches.

Phase 1: Person Count Estimation

Estimate occupancy count from CSI signal statistics without decomposition.

Approach: Eigenvalue analysis of the CSI covariance matrix across subcarriers.

• Compute the 56×56 covariance matrix of CSI amplitudes over a sliding window (e.g., 50 frames / 5 seconds)
• Count eigenvalues above a noise threshold — each significant eigenvalue corresponds to an independent scatterer (person or static object)
• Subtract the static environment baseline (estimated during calibration or from the field model's SVD eigenstructure)
• The residual significant eigenvalue count estimates person count

Accuracy target: > 80% for 0-3 people with single ESP32 node.

Integration point: signal/src/ruvsense/field_model.rs already computes SVD eigenstructure. Extend with a estimate_occupancy() method.

Phase 2: Signal Decomposition

Separate per-person signal contributions using blind source separation.

Approach: Non-negative Matrix Factorization (NMF) on the CSI spectrogram.

• Construct a time-frequency matrix from CSI amplitudes: rows = subcarriers (56), columns = time frames
• Apply NMF with k components (k = estimated person count from Phase 1)
• Each component's frequency profile maps to a person's motion pattern
• NMF is preferred over ICA because CSI amplitudes are non-negative

Alternative: Independent Component Analysis (ICA) on complex CSI (amplitude + phase). More powerful but requires phase calibration (see ruvsense/phase_align.rs).

Integration point: New module signal/src/ruvsense/separation.rs.

Phase 3: Multi-Skeleton Generation

Generate distinct pose skeletons per decomposed component.

Approach: Per-component feature extraction → per-person skeleton synthesis.

• Extract motion features (dominant frequency, energy, spectral centroid) per NMF component
• Map each component to a spatial position using subcarrier phase gradient (Fresnel zone model)
• Generate 17-keypoint COCO skeleton per person with position offset
• Assign person IDs using the existing Kalman tracker (ruvsense/pose_tracker.rs) with AETHER re-ID embeddings (ADR-024)

Integration point: Modify derive_pose_from_sensing() in sensing-server/src/main.rs to return Vec<Person> with length > 1.

Phase 4: Neural Multi-Person Model

Train a dedicated multi-person model using the RVF pipeline (ADR-036).

• Use MM-Fi dataset (ADR-015) multi-person scenarios for training data
• Architecture: shared CSI encoder → person count head + per-person pose heads
• LoRA fine-tuning profile for multi-person specialization
• Inference via the model manager in the sensing server

Accuracy target: [email protected] > 60% for 2-person scenarios.

Consequences

Positive

• Enables room occupancy counting (Phase 1 alone is useful)
• Distinct pose tracking per person enables activity recognition per individual
• Progressive approach — each phase delivers incremental value
• Reuses existing infrastructure (field model SVD, Kalman tracker, AETHER, RVF pipeline)

Negative

• Single ESP32 node has fundamental spatial resolution limits — separating 2 people standing close together (< 0.5m) will be unreliable
• NMF decomposition adds ~5-10ms latency per frame
• Person count estimation will have false positives from large moving objects (pets, fans)
• Phase 4 neural model requires multi-person training data collection

Neutral

• Multi-node multistatic mesh (ADR-029) dramatically improves multi-person separation but is a separate effort
• UI already supports multi-person rendering — no frontend changes needed for the persons[] array

Affected Components

Component	Phase	Change
signal/src/ruvsense/field_model.rs	1	Add estimate_occupancy()
signal/src/ruvsense/separation.rs	2	New module: NMF decomposition
sensing-server/src/main.rs	3	derive_pose_from_sensing() multi-person output
signal/src/ruvsense/pose_tracker.rs	3	Multi-target tracking
nn/	4	Multi-person inference head
train/	4	Multi-person training pipeline

Performance Budget

Operation	Budget	Phase
Person count estimation	< 2ms	1
NMF decomposition (k=3)	< 10ms	2
Multi-skeleton synthesis	< 3ms	3
Neural inference (multi-person)	< 50ms	4
Total pipeline	< 65ms (15 FPS)	All

Alternatives Considered

1. Camera fusion: Use a camera for person detection and WiFi for pose — rejected because the project goal is camera-free sensing.
2. Multiple single-person models: Run N independent pose estimators — rejected because they would produce correlated outputs from the same CSI data.
3. Spatial filtering (beamforming): Use antenna array beamforming to isolate directions — rejected because single ESP32 has only 1 antenna; viable with multistatic mesh (ADR-029).
4. Skip signal-derived, go straight to neural: Train an end-to-end multi-person model — rejected because signal-derived provides faster iteration and interpretability for the early phases.