Technical Deep Dive: Understanding Wi-Fi CSI

What is Channel State Information?

Channel State Information (CSI) describes how a wireless signal propagates from a transmitter to a receiver. Unlike simple RSSI (signal strength), CSI provides fine-grained information about the wireless channel.

The Physics Behind CSI

When Wi-Fi signals travel through space, they:

1. Reflect off walls and objects
2. Diffract around edges
3. Scatter from rough surfaces
4. Get absorbed by materials

This creates a unique "fingerprint" of the environment at the receiver.

Signal Path Visualization:

TX ─────────────────────────────► RX (Direct Path)
    ╲                           ╱
     ╲    ┌─────────┐         ╱
      ╲───│  Human  │────────╱  (Reflected Path)
          │  Body   │
          └─────────┘

How Human Movement Affects CSI

When a person moves in the environment:

• Their body acts as a reflector/absorber
• Signal paths change dynamically
• CSI values fluctuate in characteristic patterns

Key insight: Different activities (walking, sitting, falling) create distinct CSI patterns that can be classified using machine learning.

CSI Data Structure

Each CSI packet contains data for multiple OFDM subcarriers:

# CSI data format (simplified)
csi_data = {
    'timestamp': 1705xxx,
    'rssi': -45,
    'subcarriers': [
        {'index': 0, 'amplitude': 23.5, 'phase': 1.23},
        {'index': 1, 'amplitude': 21.2, 'phase': 0.87},
        # ... 52 subcarriers total
    ]
}

Why CSI for Fall Detection?

Falls create a unique signature:

• Sudden velocity change - rapid CSI fluctuation
• Impact signature - characteristic spike pattern
• Post-fall stillness - CSI stabilizes in new configuration

This makes falls distinguishable from normal activities like sitting down quickly.