Terrestrial Microwave Backhaul Design

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 ( $\lambda \approx 3\text{--}5\text{ cm}$ ) 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 ( $\lambda \approx 3.5\text{ mm}$ ), 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 $n$ -th Fresnel Zone at point $d_1$ from the transmitter:

F_n = 17.32 \sqrt{\frac{n \cdot d_1 \cdot d_2}{f \cdot D}}

Where $d_1, d_2$ are distances to ends (km), $D$ is total distance (km), and $f$ 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.

Link Optimal (60% Clearance Met)

Center Obstacle Height60m

Ground (0m)Tower Height (120m)

Blockage: 0% of lower Fresnel Zone radius.
Requirement: < 40% (The 60% Clearance Rule)

SNR (Signal/Noise)

32 dB

ACM Profile

1024-QAM

Link Capacity

1000 Mbps

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 ( $\lambda$ ) 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 ( $\gamma_R$ )

\gamma_R = k \cdot R^\alpha \quad \text{[dB/km]}

Where $R$ is the rain rate in mm/hr. The coefficients $k$ and $\alpha$ 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 ( $n$ ) 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.

K = 4/3Standard Atmosphere

The beam bends slightly with the Earth's curve.

K < 1Sub-Refraction

The beam bends away from Earth, risking "earthing" the signal.

K = ∞Ducting

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_{rx} = P_{tx} + G_{tx} + G_{rx} - L_{fs} - L_a - L_m

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: $P_{rx} - \text{Sensitivity}$

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.

L_{fs} = 20 \log_{10}(d) + 20 \log_{10}(f) + 32.44

(Where $d$ is in km and $f$ 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.

Frequently Asked Questions

Engineering Knowledge Expansion

Physics

Atmospheric Absorption and Rain Fade Modeling

Microwave signals in the 6–80 GHz range experience attenuation from atmospheric gases and precipitation that fundamentally limits link availability. Oxygen absorption peaks at 60 GHz (approximately 16 dB/km) and 120 GHz, while water vapor has a broader absorption band centered around 22 GHz. For common backhaul bands, the dry-air attenuation at 18 GHz is approximately 0.02 dB/km, increasing to 0.2 dB/km at 38 GHz and 1.5 dB/km at 70 GHz. These values dictate the maximum hop length: at 18 GHz, a 40 km hop is feasible; at 70 GHz, the practical limit is approximately 4 km before atmospheric absorption consumes the entire link budget.

Rain fade introduces a statistical dimension to link budgeting. The ITU-R P.838 model calculates specific attenuation due to rain as:

\gamma_R = k R^\alpha

Where $R$ is the rain rate in mm/h, and $k$ and $\alpha$ are frequency and polarization-dependent coefficients from ITU-R P.838.

For a 38 GHz link with vertical polarization, $k \approx 0.23$ and $\alpha \approx 1.0$ . At a rain rate of 30 mm/h (heavy rain in most temperate regions), the specific attenuation is approximately 7 dB/km. A 5 km hop at 38 GHz would experience 35 dB of rain fade, which exceeds the typical fade margin of 20–25 dB designed into most backhaul links. This is why high-frequency microwave links require either very short hop distances oradaptive modulation — the modem automatically drops from 256 QAM (6 bits/symbol) to QPSK (2 bits/symbol) during rain events, sacrificing throughput to maintain connectivity. The availability target for carrier-grade backhaul is 99.999% (5.26 minutes of outage per year), requiring the link budget to close against the rain rate corresponding to the 0.001% worst-month statistic for the deployment region.

Diversity Techniques and Multi-Path Mitigation

Multi-path propagation — where the signal arrives at the receiver via multiple paths due to atmospheric refraction, ground reflection, or scattering — is the dominant impairment in terrestrial microwave links. The received signal is the vector sum of the direct path and one or more reflected paths. If the reflected path arrives with opposite phase, it causes destructive fading, potentially reducing the signal power by 20–30 dB within a fraction of a second. This flat fading event can cause complete link loss if the fade depth exceeds the fade margin.

Space diversity is the primary mitigation technique for multi-path fading. Two receive antennas are mounted on the same tower, vertically separated by approximately 10–15 meters (typically 100–200 wavelengths at the operating frequency). The received signals from both antennas are combined using a maximum ratio combining (MRC) algorithm in the modem. Because the multi-path nulls are spatially localized — the destructive interference pattern creates a standing wave with nulls spaced by approximately $\lambda h / d$ , where $h$ is the antenna height and $d$ is the path length — the probability that both antennas are simultaneously in a null is reduced by a factor of 10–100 compared to a single antenna. The diversity gain achieved is typically 5–10 dB of improvement in fade margin.

Frequency Diversity and Adaptive Equalization

In extreme multi-path environments (long over-water paths or paths over highly reflective terrain), space diversity alone may be insufficient. Frequency diversity transmits the same data on two different frequencies (typically with 10% separation). If one frequency experiences a fade null, the other likely will not. This doubles spectrum usage but provides essentially infinite fade margin. An alternative is adaptive time-domain equalization in the modem, where a decision-feedback equalizer (DFE) with 16–64 taps can synthesize the inverse channel response, canceling multi-path echoes with delays up to several symbol periods. For 256 QAM at 56 MHz symbol rate, a 32-tap DFE can cancel echoes up to 0.57 μs — sufficient for the vast majority of terrestrial paths.

The deployment of diversity systems requires careful engineering of the waveguide or coax feeds from each antenna to the modem. The differential phase delay between the two receive paths must be calibrated — a 1 cm difference in cable length at 38 GHz corresponds to approximately 45 degrees of phase error, which can reduce the MRC combiner efficiency from the theoretical 3 dB to near zero. Calibration is performed by transmitting a known pilot tone and adjusting the phase shifter in the diversity combiner until the combined signal power is maximized.