540  Infrastructure-Leveraging Sensing

Learning Objectives

After completing this chapter, you will be able to:

  • Understand how to use existing infrastructure as sensors
  • Apply Wi-Fi sensing for presence and gesture detection
  • Use power meters for appliance identification (NILM)
  • Design sensing systems that leverage installed equipment

540.1 Prerequisites

540.2 Infrastructure-Leveraging Sensing: Using What’s Already There

~20 min | Advanced | P06.C08.U05

540.2.1 The Smart Approach: Don’t Deploy New Sensors, Use What Exists

Traditional IoT thinking: “We need temperature data -> Deploy temperature sensors.”

Infrastructure-leveraging thinking: “We need occupancy data -> Use existing Wi-Fi routers.”

This paradigm shift–leveraging existing infrastructure instead of deploying dedicated sensors–can reduce costs by 10-100x while providing area-wide coverage instead of point measurements.

Imagine you want to detect if someone is in a room. Traditional approach:

Deploy Dedicated Sensors ($$$): - Buy PIR motion sensors ($15 each) - Install wiring and power - Calibrate and maintain batteries - Get point measurements (one spot per sensor)

Leverage Infrastructure ($0): - Use existing Wi-Fi router (already there!) - Detect phone/device connections - Get room-wide coverage - No installation, no batteries, no maintenance

The Wi-Fi router becomes a “free” presence sensor. This is infrastructure-leveraging sensing–using existing devices as sensors.

540.3 Core Principle: Indirect Sensing via Existing Infrastructure

Instead of deploying new sensors, extract sensing information from infrastructure already in place:

Infrastructure Traditional Use Sensing Capability What It Measures
Wi-Fi Router Internet connectivity RSSI, CSI, device count Presence, gestures, breathing, occupancy
Power Meter Energy billing Current waveform analysis Appliance identification (NILM)
Cell Tower Mobile calls/data Handoff patterns, signal strength Traffic density, crowd size, movement
Street Light Illumination Current draw, vibration Pedestrian count, vehicle detection
Water Pipe Water delivery Acoustic vibrations Leak detection, flow rate
HVAC Ducts Climate control Airflow patterns Occupancy, room-level activity

Key Insight: Every piece of infrastructure emits signals. We can sense the world by observing how the environment disturbs these signals.

540.4 Comparison: Deploy vs Leverage

Dimension Deploy Dedicated Sensors Infrastructure-Leveraging
Upfront Cost $10-100 per sensor point $0 marginal cost (already installed)
Installation Drilling, wiring, positioning Software-only deployment
Coverage Point measurements (discrete) Area-wide coverage (continuous)
Maintenance Battery replacement, calibration Minimal (infrastructure already maintained)
Privacy Obvious sensors (cameras visible) Less intrusive (passive RF sensing)
Accuracy High (purpose-built) Moderate (indirect inference)
Latency Immediate (direct sensing) May require processing (feature extraction)

When to Choose What: - Deploy sensors when: Accuracy critical, point measurement needed, safety-critical - Leverage infrastructure when: Large-scale deployment, cost-sensitive, retrofit scenario, quick pilot

540.5 Example 1: Wi-Fi Sensing - Detecting Breathing and Gestures

Infrastructure: Standard Wi-Fi router (802.11n/ac/ax)

Sensing Capability: Channel State Information (CSI) captures how Wi-Fi signals propagate through space. Human movement, breathing, even heartbeat disturbs these signals.

What It Measures: - Presence detection: Device count, room occupancy (+/-1 person accuracy) - Gesture recognition: Hand waves, swipes (85-95% accuracy) - Breathing rate: Chest movement modulates signal (+/-2 breaths/min) - Fall detection: Sudden signal disruption pattern

Real Deployment: - University dorms: Detect occupancy without cameras (privacy-preserving) - Elderly care: Fall detection without wearables - Smart homes: Gesture control of lights and appliances

540.6 Example 2: Non-Intrusive Load Monitoring (NILM)

Infrastructure: Smart electricity meter (already installed for billing)

Sensing Capability: Analyze current and voltage waveforms to identify unique “signatures” of individual appliances.

What It Measures: - Appliance state: On/off, standby, active - Energy breakdown: Per-appliance consumption - Usage patterns: When devices are used

How It Works:

Total home power = 150W (baseline)
              -> 1650W (1500W spike = hair dryer turns on)
              -> 1750W (100W added = refrigerator compressor)
              -> 250W (1500W drop = hair dryer off)

Each appliance has unique electrical signatures: - Resistive loads (heaters, toasters): Clean on/off transitions - Motors (refrigerators, fans): Inrush current spike, steady-state hum - Switched-mode power supplies (computers, chargers): High-frequency harmonics

540.7 Example 3: Cellular Signal Analysis

Infrastructure: Cell towers (already everywhere)

Sensing Capability: Analyze signal strength patterns and handoff events to infer crowd density and movement.

What It Measures: - Crowd density: Number of active devices in area - Traffic flow: Movement patterns between cell zones - Event detection: Large gatherings, unusual patterns

540.8 The Three-Layer Model

  1. Infrastructure Layer: Existing devices emitting signals (Wi-Fi, power, cellular)
  2. Disturbance Layer: Environment modulates these signals (human movement, appliance usage)
  3. Analytics Layer: Software extracts sensing information from signal perturbations

540.9 Design Guidelines

TipWhen to Leverage Infrastructure

Good candidates for infrastructure sensing: - Large buildings (retrofit without new wiring) - Privacy-sensitive environments (no cameras needed) - Budget-constrained projects - Quick proof-of-concept deployments - Aggregate measurements (counts, not identity)

Still need dedicated sensors for: - Safety-critical applications (fire, gas) - High-precision measurements (temperature +/-0.1C) - Point-specific data (this exact location) - Real-time response (<100ms) - Regulatory compliance (certified sensors)

540.10 Summary

Key infrastructure-leveraging takeaways:

  1. Existing infrastructure can serve as sensors - Wi-Fi, power, cellular
  2. Software deployment vs hardware installation - Faster, cheaper
  3. Area-wide vs point measurements - Different tradeoffs
  4. Privacy-preserving options - No cameras needed for presence detection
  5. Combine approaches - Infrastructure + dedicated sensors for best results

540.11 What’s Next

Now that you understand infrastructure sensing:

Continue to Calibration Techniques ->