What is the knife edge diffraction model and when do I use it for path loss estimation?
Knife Edge Diffraction
Knife-edge diffraction occurs when an electromagnetic wave encounters a sharp edge obstacle. The wave bends around the edge, allowing some signal to reach the shadow region behind the obstacle. The amount of bending decreases with frequency (shorter wavelength = less diffraction = deeper shadow). This is why mmWave signals are strongly blocked by buildings and terrain while lower-frequency signals can partially propagate around obstacles.
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
When does this model apply?
Use the knife-edge model for: single sharp obstructions (building rooftop, ridge line), preliminary link analysis (quick estimate of diffraction loss), and terrain where the obstacle profile approximates a thin barrier. Do not use for: rounded hills (use rounded earth diffraction), dense urban environments (use ray tracing), or vegetation (use ITU-R P.833).
How accurate is the single knife-edge model?
For a truly sharp obstacle (building corner, metal wall): accurate within 2-3 dB. For rounded terrain: overestimates the diffraction loss by 5-10 dB because the smooth surface diffracts better than a sharp edge. For multiple obstacles: the single knife-edge model applied to the dominant obstruction gives a quick estimate, but multiple edge diffraction methods are more accurate.
Does diffraction work at mmWave?
Very poorly. At 60 GHz: even small obstructions (ν = 2) cause 22 dB diffraction loss, and building corner diffraction exceeds 30 dB. This is why 5G mmWave planning focuses on line-of-sight coverage and uses reflections from building surfaces (specular reflection) rather than diffraction for coverage extension.