Terrestrial Microwave Backhaul
The Physics of 6-80GHz High-Capacity Transport
The Spectral Architecture
Microwave point-to-point links are not a monolithic technology but a spectrum of specialized tools designed for specific geographic and capacity constraints. The selection of a frequency band is the primary engineering decision, dictated by the inverse relationship between bandwidth and propagation distance.
Low-Frequency (6–13 GHz)
Known as Long-Haul bands. These frequencies feature large wavelengths () that are relatively immune to rain droplets.
- Range: Up to 100km+ with high-gain dishes.
- Capacity: Limited by narrow channel spacing (28/56 MHz).
- Use Case: Rural backbone, island hopping, mountainous terrain.
High-Frequency (71–86 GHz)
Known as E-Band. With wavelengths in the millimeter range (), these links offer fiber-like speeds.
- Range: Strictly limited to 2–5km due to rain fade.
- Capacity: 10Gbps to 100Gbps using 2GHz wide channels.
- Use Case: 5G Small Cell backhaul, campus connectivity, metro rings.
The Geometry of Propagation: Fresnel Zones
A common misconception in microwave engineering is that a clear Visual Line of Sight (VLoS) is sufficient. In reality, radio waves do not travel as a laser-thin line; they occupy a concentric ellipsoidal volume between the transmitter and receiver known as the Fresnel Zone.
Radius of the -th Fresnel Zone at point from the transmitter:
Where are distances to ends (km), is total distance (km), and is frequency (GHz).
If an obstacle (like a tree or building) enters the First Fresnel Zone, it causes diffraction. Specifically, the rays reflected from the obstacle may arrive at the receiver 180 degrees out of phase with the direct ray, causing destructive interference.
- The 60% Rule: Engineers mandate that at least 60% of the first Fresnel zone must be clear of any obstruction under all atmospheric conditions.
- Earth Bulge: For links exceeding 15km, the curvature of the Earth acts as a "permanent obstacle" that must be cleared by raising tower heights.
Microwave Fresnel Zone & LoS
Adjust the obstacle height to see how Fresnel zone encroachment impacts Signal-to-Noise Ratio (SNR) and Link Capacity via Adaptive Modulation.
Requirement: < 40% (The 60% Clearance Rule)
SNR (Signal/Noise)
ACM Profile
Link Capacity
Interactive Simulation: Visualizing Fresnel Clearance and Multi-path Reflections
The Rain Fade Forensics
Rain is the primary adversary of high-frequency microwave links. When the wavelength of the signal () approaches the size of a raindrop (approx. 1mm to 5mm), the drop acts as a scattering and absorbing body. This phenomenon is modeled by the ITU-R P.530 recommendation.
Specific Attenuation ()
Where is the rain rate in mm/hr. The coefficients and are frequency-dependent and polarization-dependent. Horizontal polarization usually suffers more attenuation than vertical because raindrops are typically oblate (flattened) due to air resistance as they fall.
"In E-Band, a 100mm/hr tropical storm can introduce over 30dB of loss per kilometer, effectively severing the link if the fade margin is insufficient."
The Laws-Parsons Distribution
Engineers don't just assume "rain"; they use statistical models of droplet size. The Laws-Parsons distribution describes the number of drops of a certain diameter in a cubic meter of air for a given rain rate. This distribution is critical for calculating the cross-sectional scattering area that the radio beam will encounter.
Atmospheric Refraction & The K-Factor
The atmosphere is not a vacuum; its refractive index () decreases with altitude. This causes radio beams to "bend" towards the Earth. To simplify calculations, engineers use the K-Factor to modify the effective radius of the Earth.
The beam bends slightly with the Earth's curve.
The beam bends away from Earth, risking "earthing" the signal.
The beam is trapped in an atmospheric layer, causing massive interference or overshoot.
Link Budget Analysis
The link budget is the definitive balance sheet of an RF link. It determines if the received signal is strong enough to be decoded at a specific Bit Error Rate (BER).
P_tx: Transmitter Power (dBm)
G_tx / G_rx: Antenna Gains (dBi)
L_fs: Free Space Path Loss (dB)
L_a: Atmospheric Absorption (dB)
L_m: Misc Losses (Feeders, Connectors, Radomes)
Fade Margin:
Free Space Path Loss (FSPL) Formula
FSPL represents the "spreading" of the energy as it travels. It is not energy lost to heat, but energy lost because the wavefront expands.
(Where is in km and is in MHz)
Advanced Mitigation: ACM & XPIC
To achieve 99.999% ("five-nines") availability, modern radios rely on real-time hardware intelligence.
Adaptive Coding and Modulation (ACM)
Traditional links were static; if the signal dropped below a threshold, the link died. ACM allows the radio to dynamically change its modulation (e.g., from 4096-QAM down to QPSK) in milliseconds. This trades capacity for link stability during peak rain events.
XPIC (Cross-Polarization Interference Cancellation)
By transmitting two independent streams on the same frequency—one vertically polarized and one horizontally polarized—XPIC effectively doubles the spectral efficiency. The receiver uses advanced DSP to cancel the leakage (XPD) between the two polarizations.
Space Diversity
To combat multipath fading (especially over water), two receiving antennas are mounted at different heights on the tower. The probability that both antennas are in a deep fade "null" at the same time is mathematically negligible.
The Engineering Pipeline
Building a microwave hop is a multi-stage process that combines software modeling with grueling field work.
1. Desktop Planning
Using tools like Pathloss 5 to simulate the link over high-resolution SRTM terrain data. We identify the theoretical tower heights and antenna sizes.
2. TSSR (Technical Site Survey)
Climbing the towers to verify line-of-sight with telescopes/GPS. Identifying existing interference and measuring physical space for dish mounts.
3. TCN (Transmission Coordination)
Submitting the design to the local regulator (e.g., FCC, Ofcom) to obtain a frequency license and ensure we won't interfere with neighboring links.
4. Implementation & Alignment
The most critical step. Teams on both towers use voltmeters or RSSI indicators to "peak" the dishes. A 0.5-degree misalignment can cause a 10dB loss in signal.