Terrestrial Microwave Backhaul

The Spectrum Split: 6GHz vs 80GHz

Microwave engineering is divided by the frequency of operation. Lower frequencies (6-11GHz) are used for Long-Haul links (30km+) due to their resilience to rain fade. Higher frequencies (60-80GHz, E-Band) provide Short-Haul 10Gbps+ capacity but are highly sensitive to oxygen absorption and heavy precipitation.

Clearance and Physics: The Fresnel Zone

Having a visual line-of-sight (LoS) is not enough for a stable microwave link. The radio waves travel in an ellipsoidal volume known as the Fresnel Zone.

• The 60% Rule: At least 60% of the first Fresnel zone must be free of obstacles (buildings, trees, ground). If blocked, the signal will suffer from diffraction and phase cancellation.
• Earth Curvature: For long links (>20km), the bulge of the Earth must be factored into the tower height calculation to maintain clearance.

Atmospheric Absorption

Beyond rain, the atmosphere itself absorbs radio energy at specific frequencies:

• Oxygen Absorption (60GHz): There is a massive peak in attenuation at 60GHz due to molecular oxygen. This makes 60GHz links highly secure (they don't travel far) but strictly limited to short ranges (<1km).
• Water Vapor: High humidity and fog introduce a "steady state" loss that must be modeled in the link budget, especially in coastal regions.

Combating Physics: Diversity Schemes

When a single link is not reliable enough, engineers use Diversity to maintain uptime:

• Space Diversity: Using two receiving antennas at different heights. If one antenna is in a "null" caused by reflection, the other likely isn't.
• Frequency Diversity: Sending the same data on two different frequencies simultaneously. Since atmospheric effects are frequency-dependent, one link may survive while the other fades.

Link Budget Calculation ($P_{rx}$)

The success of a microwave link depends on the Link Budget. The received power ($P_{rx}$) must be greater than the receiver sensitivity ($S_{rx}$) plus a safety buffer called the Fade Margin.

$P_{rx} = P_{tx} + G_{tx} + G_{rx} - L_{fs} - L_{misc}$

Adaptive Modulation (ACM)

Modern microwave radios use Adaptive Coding and Modulation (ACM). In clear weather, the radio may use 4096-QAM for maximum throughput. When rain starts, the radio automatically 'steps down' to QPSK or 16-QAM. The link slows down, but it does not drop.

Polarization & XPIC

To double capacity without using more spectrum, engineers use Vertical and Horizontal polarization on the same frequency. XPIC (Cross-Polarization Interference Cancellation) technology allows the radio to distinguish between the two signals, providing 2x the throughput in the same bandwidth.