Wireless Notes

Reflection Refraction Diffraction in Wireless Communication

Learn reflection refraction diffraction in radio wave propagation with Snell

When radio waves propagate, they interact with the environment – they reflect off surfaces, refract when the medium changes, and diffract around obstacle edges. Understanding these three phenomena is essential for wireless system design.

🌐 Overview

═══▶	═══▶ (like mirror for light)
	surface
═══▶	(signal reaches shadow zone)
	╲╲╲▶
	obstacle

Mechanism	When it occurs	Object size vs λ	Effect
Reflection	Surface >> λ	Much larger	Signal bounces back
Refraction	Medium boundary	N/A (density change)	Signal bends
Diffraction	Edge/obstacle	Comparable to λ	Signal bends around
Scattering	Object ≈ λ	Similar size	Signal spreads in all directions

🪞 Reflection

Reflection occurs when a radio wave strikes a smooth surface that is much larger than the wavelength. The wave bounces back (like light in a mirror).

Laws of Reflection:

1. Angle of incidence = Angle of reflection (θᵢ = θᵣ) 2. Incident ray, reflected ray, and normal – same plane mein Incident │ Normal │ Reflected ╲ │ │ ╱ θᵢ ╲ │ │ ╱ θᵣ ╲ │ │ ╱ ╲ │ │ ╱ ══════════════╩═╩════════╩╩════════════ Surface θᵢ = θᵣ

Reflection Coefficient (Γ):

Example

Γ = (Reflected signal amplitude) / (Incident signal amplitude) Depends on: • Angle of incidence • Polarization (horizontal vs vertical) • Material properties (permittivity, conductivity) • Frequency |Γ| ranges from 0 (no reflection) to 1 (total reflection)

Types of Reflection:

Surface	Reflection Type	Example
Metal	Strong (	Γ	≈ 1)	Steel building, car body
Glass	Moderate	Windows, glass facades
Concrete	Moderate-Strong	Building walls
Ground (dry)	Moderate	Earth surface
Water	Strong	Lakes, sea, wet roads
Wood	Weak	Wooden structures

Brewster Angle:

Special angle where vertical polarization has zero reflection
Depends on material dielectric constant
For ground: ~15-20° from horizontal

Impact on Wireless:

Multipath: Reflected signals cause fading
Indoor: Signals bounce off walls, ceiling, floor
Urban: Building facades create strong reflections
Design consideration: Metal structures near antenna = strong multipath

🌊 Refraction

Refraction occurs when a radio wave passes from one medium to another (where the propagation speed is different). The direction of the wave changes – this is Snell's Law.

Snell\'s Law:

Example

n₁ × sin(θ₁) = n₂ × sin(θ₂) Where: n₁, n₂ = Refractive indices of media θ₁ = Angle of incidence θ₂ = Angle of refraction For atmosphere: n varies with altitude (density changes)

┌─────────────────────────────────────────────────────────────────────┐ │ REFRACTION IN ATMOSPHERE │ │ │ │ ░░░░░░░░░░ Less dense (upper atmosphere) n₂ < n₁ ░░░░░░ │ │ ─────────────────────────────────────── boundary │ │ ████████████ More dense (lower atmosphere) n₁ ████████ │ │ │ │ Signal bends AWAY from normal (towards ground) │ │ → Extends radio horizon beyond geometric LOS! │ │ │ │ ═══════▶╲ │ │ ╲╲ ← Wave bends downward │ │ ╲╲▶ │ │ │ │ Effect: Radio horizon ≈ 4/3 × geometric horizon │ │ Effective Earth radius factor k ≈ 4/3 │ └─────────────────────────────────────────────────────────────────────┘

Atmospheric Refraction Effects:

Condition	Effect	Hindi
Standard atmosphere	Slightly extends range (k=4/3)	Slightly more range than normal
Sub-refraction	Reduces range	Range decreases
Super-refraction	Greatly extends range	Range increases significantly
Ducting	Signal trapped, extreme range	Signal duct mein trap

LOS Range with Refraction:

Example

d = 4.12 × √(k) × (√h₁ + √h₂) km Standard atmosphere: k = 4/3 d = 4.12 × 1.15 × (√h₁ + √h₂) d ≈ 4.74 × (√h₁ + √h₂) km

🔄 Diffraction

Diffraction occurs when a radio wave passes near the edge of an obstacle. The wave bends around the edge and enters the shadow zone (behind the obstacle).

Huygens\' Principle:

Every point on a wavefront acts as a source of secondary wavelets. These wavelets combine to form the new wavefront – allowing waves to bend around obstacles.

	Obstacle (building/hill)
═══════▶	╱╱╱╱ Signal diffracts
═══════▶	╱╱╱╱╱ around edge
═══════▶	╱╱╱╱╱╱╱
═══════▶	╱╱╱╱╱╱╱╱╱ → Reaches shadow zone!
	╲╲╲╲╲╲╲╲╲
═══════▶
═══════▶	Shadow Zone
	(still receives some signal)

Knife-Edge Diffraction Model:

Diffraction loss depends on Fresnel-Kirchhoff parameter (v)

v = h × √(2(d₁+d₂) / (λ×d₁×d₂))

Where

h = Height of obstacle above direct LOS

d₁ = Distance from TX to obstacle

d₂ = Distance from obstacle to RX

λ = Wavelength

v value	Diffraction Loss	Condition
v < -1	~0 dB (no loss)	LOS clear above obstacle
v = 0	6 dB	Obstacle just touches LOS
v = 1	16 dB	Obstacle well above LOS
v = 2	22 dB	Deeply shadowed
v = 3	26 dB	Very deep shadow

Key Insight:

Lower frequencies diffract more (longer wavelength). That is why AM radio can be heard behind buildings, but 5G mmWave cannot (because the wavelength is very short).

🎯 Fresnel Zones

The Fresnel zone is the elliptical region around the LOS path. 60% of the first Fresnel zone should be clear for reliable communication.

┌─────────────────────────────────────────────────────────────────────┐ │ FRESNEL ZONE │ │ │ │ 1st Fresnel Zone (must be 60%+ clear) │ │ ╱─────────────────────────────╲ │ │ ╱ ╱─────────────────────────╲ ╲ │ │ 📡 TX ──────────────────────────────────── 📡 RX │ │ ╲ ╲─────────────────────────╱ ╱ │ │ ╲─────────────────────────────╱ │ │ 2nd Fresnel Zone │ │ │ │ Radius of 1st Fresnel zone at midpoint: │ │ r₁ = √(λ×d₁×d₂ / (d₁+d₂)) │ │ │ │ At midpoint (d₁=d₂=d/2): │ │ r₁ = √(λ×d/4) = (1/2)√(λ×d) │ └─────────────────────────────────────────────────────────────────────┘

Fresnel Zone Radius Examples:

Frequency	Distance	First Fresnel Zone Radius
900 MHz	1 km	9.1 m
2.4 GHz	1 km	5.6 m
5 GHz	1 km	3.9 m
28 GHz	100 m	0.52 m

In microwave link design, it must be ensured that no obstacle is present in 60% of the first Fresnel zone. Trees, buildings, and terrain all need to be checked.

💡 Practical Implications

For Network Planning:

Mechanism	Design Impact
Reflection	Plan for multipath → use OFDM, MIMO
Refraction	Use 4/3 earth radius for LOS calculations
Diffraction	Coverage possible behind buildings (low freq)
Fresnel zone	Clear LOS path with margin for towers

Frequency Dependence:

├── Good diffraction	coverage behind obstacles
├── Larger Fresnel zone	need taller towers
└── Fewer reflections	less multipath
├── Poor diffraction	shadow zones
├── Smaller Fresnel zone	easier to clear
└── More reflections	rich multipath (good for MIMO!)

📝 Summary

Mechanism	Condition	Formula	Effect
Reflection	Surface >> λ	θᵢ = θᵣ	Multipath, signal copies
Refraction	Medium change	n₁sinθ₁ = n₂sinθ₂	Bending, extended range
Diffraction	Edge ≈ λ	Knife-edge model	Shadow zone coverage
Fresnel Zone	LOS link design	r₁ = √(λd₁d₂/(d₁+d₂))	60% must be clear

❓ FAQ

Q: How does the signal reach behind a building? A: Diffraction! Lower frequency signals bend around the edges of buildings. 4G (700 MHz) penetrates indoors more than 5G mmWave.

Q: Why does WiFi signal become weak through bathroom tiles? A: Behind the tiles there is a metal mesh (for waterproofing) that causes strong reflection (nearly total). The signal bounces back instead of penetrating.

Q: Microwave link mein tree kyun problem hai? A: Trees obstruct the Fresnel zone. Plus seasonal variation – in summer when leaves appear, additional attenuation occurs (5-20 dB at high frequencies).

Continue the lesson path

Study next in this module

4 connected lessons

01Wireless ChannelsRayleigh Fading Model in Wireless CommunicationLearn Rayleigh fading with statistical model, PDF CDF formulas, deep fades, BER degradation over Rayleigh channel, level crossing rate, diversity mitigation, and when Rayleigh applies for engineering students.02Wireless ChannelsRician Fading Model in Wireless CommunicationLearn Rician fading with K-factor definition, PDF formula, comparison with Rayleigh, special cases K=0 and K=infinity, BER improvement, and where Rician fading occurs explained for engineering students.03Wireless ChannelsMultipath Propagation in Wireless CommunicationLearn multipath propagation with delay spread, coherence bandwidth, ISI inter-symbol interference, power delay profile, multipath effects, OFDM MIMO mitigation techniques explained for engineering students.04Wireless ChannelsShadowing Log-Normal Fading in Wireless CommunicationLearn shadowing log-normal fading with distribution model, standard deviation values, shadow margin calculation, coverage probability, correlation distance, and network planning implications for engineering students.

PreviousRayleigh Fading Model in Wireless Communication NextRician Fading Model in Wireless Communication

Wireless Notes

Reflection Refraction Diffraction in Wireless Communication

Learn reflection refraction diffraction in radio wave propagation with Snell

When radio waves propagate, they interact with the environment – they reflect off surfaces, refract when the medium changes, and diffract around obstacle edges. Understanding these three phenomena is essential for wireless system design.

🌐 Overview

═══▶	═══▶ (like mirror for light)
	surface
═══▶	(signal reaches shadow zone)
	╲╲╲▶
	obstacle

Mechanism	When it occurs	Object size vs λ	Effect
Reflection	Surface >> λ	Much larger	Signal bounces back
Refraction	Medium boundary	N/A (density change)	Signal bends
Diffraction	Edge/obstacle	Comparable to λ	Signal bends around
Scattering	Object ≈ λ	Similar size	Signal spreads in all directions

🪞 Reflection

Reflection occurs when a radio wave strikes a smooth surface that is much larger than the wavelength. The wave bounces back (like light in a mirror).

Laws of Reflection:

Reflection Coefficient (Γ):

Example

Types of Reflection:

Surface	Reflection Type	Example
Metal	Strong (	Γ	≈ 1)	Steel building, car body
Glass	Moderate	Windows, glass facades
Concrete	Moderate-Strong	Building walls
Ground (dry)	Moderate	Earth surface
Water	Strong	Lakes, sea, wet roads
Wood	Weak	Wooden structures

Brewster Angle:

Special angle where vertical polarization has zero reflection
Depends on material dielectric constant
For ground: ~15-20° from horizontal

Impact on Wireless:

Multipath: Reflected signals cause fading
Indoor: Signals bounce off walls, ceiling, floor
Urban: Building facades create strong reflections
Design consideration: Metal structures near antenna = strong multipath

🌊 Refraction

Refraction occurs when a radio wave passes from one medium to another (where the propagation speed is different). The direction of the wave changes – this is Snell's Law.

Snell\'s Law:

Example

n₁ × sin(θ₁) = n₂ × sin(θ₂) Where: n₁, n₂ = Refractive indices of media θ₁ = Angle of incidence θ₂ = Angle of refraction For atmosphere: n varies with altitude (density changes)

Atmospheric Refraction Effects:

Condition	Effect	Hindi
Standard atmosphere	Slightly extends range (k=4/3)	Slightly more range than normal
Sub-refraction	Reduces range	Range decreases
Super-refraction	Greatly extends range	Range increases significantly
Ducting	Signal trapped, extreme range	Signal duct mein trap

LOS Range with Refraction:

Example

d = 4.12 × √(k) × (√h₁ + √h₂) km Standard atmosphere: k = 4/3 d = 4.12 × 1.15 × (√h₁ + √h₂) d ≈ 4.74 × (√h₁ + √h₂) km

🔄 Diffraction

Diffraction occurs when a radio wave passes near the edge of an obstacle. The wave bends around the edge and enters the shadow zone (behind the obstacle).

Huygens\' Principle:

Every point on a wavefront acts as a source of secondary wavelets. These wavelets combine to form the new wavefront – allowing waves to bend around obstacles.

	Obstacle (building/hill)
═══════▶	╱╱╱╱ Signal diffracts
═══════▶	╱╱╱╱╱ around edge
═══════▶	╱╱╱╱╱╱╱
═══════▶	╱╱╱╱╱╱╱╱╱ → Reaches shadow zone!
	╲╲╲╲╲╲╲╲╲
═══════▶
═══════▶	Shadow Zone
	(still receives some signal)

Knife-Edge Diffraction Model:

Diffraction loss depends on Fresnel-Kirchhoff parameter (v)

v = h × √(2(d₁+d₂) / (λ×d₁×d₂))

Where

h = Height of obstacle above direct LOS

d₁ = Distance from TX to obstacle

d₂ = Distance from obstacle to RX

λ = Wavelength

v value	Diffraction Loss	Condition
v < -1	~0 dB (no loss)	LOS clear above obstacle
v = 0	6 dB	Obstacle just touches LOS
v = 1	16 dB	Obstacle well above LOS
v = 2	22 dB	Deeply shadowed
v = 3	26 dB	Very deep shadow

Key Insight:

Lower frequencies diffract more (longer wavelength). That is why AM radio can be heard behind buildings, but 5G mmWave cannot (because the wavelength is very short).

🎯 Fresnel Zones

The Fresnel zone is the elliptical region around the LOS path. 60% of the first Fresnel zone should be clear for reliable communication.

Fresnel Zone Radius Examples:

Frequency	Distance	First Fresnel Zone Radius
900 MHz	1 km	9.1 m
2.4 GHz	1 km	5.6 m
5 GHz	1 km	3.9 m
28 GHz	100 m	0.52 m

In microwave link design, it must be ensured that no obstacle is present in 60% of the first Fresnel zone. Trees, buildings, and terrain all need to be checked.

💡 Practical Implications

For Network Planning:

Mechanism	Design Impact
Reflection	Plan for multipath → use OFDM, MIMO
Refraction	Use 4/3 earth radius for LOS calculations
Diffraction	Coverage possible behind buildings (low freq)
Fresnel zone	Clear LOS path with margin for towers

Frequency Dependence:

├── Good diffraction	coverage behind obstacles
├── Larger Fresnel zone	need taller towers
└── Fewer reflections	less multipath
├── Poor diffraction	shadow zones
├── Smaller Fresnel zone	easier to clear
└── More reflections	rich multipath (good for MIMO!)

📝 Summary

Mechanism	Condition	Formula	Effect
Reflection	Surface >> λ	θᵢ = θᵣ	Multipath, signal copies
Refraction	Medium change	n₁sinθ₁ = n₂sinθ₂	Bending, extended range
Diffraction	Edge ≈ λ	Knife-edge model	Shadow zone coverage
Fresnel Zone	LOS link design	r₁ = √(λd₁d₂/(d₁+d₂))	60% must be clear

❓ FAQ

Q: How does the signal reach behind a building? A: Diffraction! Lower frequency signals bend around the edges of buildings. 4G (700 MHz) penetrates indoors more than 5G mmWave.

Continue the lesson path

Study next in this module

4 connected lessons

PreviousRayleigh Fading Model in Wireless Communication NextRician Fading Model in Wireless Communication