ESP32 Antenna Array and WiFi Signal Visualization

Introduction

• ESP32 antenna array: Capable of visualizing WiFi signals.
• Example: Unmodified smartphone generating WiFi traffic visualized as a glowing green blob.

WiFi Signal Visualization

• Direct and Reflected Paths:
  • Metallic walls act as mirrors for radio signals.
  • Reflected paths exhibit a yellowish tint due to path delay.
  • Blocking direct line of sight shows only the reflected path.
• Angle of Incidence:
  • Reflection visibility depends on the angle and location.
• Non-metallic Walls:
  • Attenuate signals, allowing device visibility through walls.
• Directional Antennas:
  • Yagi antennas focus power into narrow beams, acting like RF flashlights.
  • Pointing at ESP32 array causes overexposure in webcam image.

Passive Target Tracking

• Reflective Materials:
  • Bright spots in the image for reflective materials like tinfoil.
  • Shadows indicate non-reflective targets.
• Passive Radar System:
  • Exploits existing WiFi signals for tracking.

Outdoor Signal Propagation

• Path Delays: Indicated by color, shorter paths in purple/blue and longer paths in red.

Technical Explanation

• Electromagnetic Wave Emission:
  • Transmitter as point source, ESP32 observes wavefronts from the source and reflector.
• Phase Coherence and Synchronization:
  • Synchronization of receiver chains requires frequency and phase coherence.
  • Phase locked loop (PLL) introduces phase uncertainty.
  • Phase reference packets help compensate for phase offsets.

Channel State Information

• Channel State Information (CSI):
  • Provides phase and amplitude measurements for subcarriers.
  • Visual changes observed when transmitter moves.

Indoor Navigation and Localization

• Array Processing Algorithms:
  • Determine angle of arrival, generate WiFi images.
  • Triangulation and TDoA with multiple arrays enhance accuracy.
• Self-Supervised Channel Charting:
  • Neural networks determine geometry of environments.
  • Single array can locate transmitters behind obstacles.

Conclusion

• Large Arrays:
  • Offer more accurate estimates and higher resolution.
• Technical Resources: Further information and datasets available on project website.