Microwave Link Engineering: Mitigating Rain Fade in Terrestrial and Satcom Paths In microwave communication links, significant fading and attenuation can occur during rain events (rain fade). Which mitigation methods are most appropriate to solve or reduce this problem in practical systems?

Introduction / Context:
Rain fade is a dominant impairment for microwave and satellite links, especially at frequencies above about 10 GHz. Water droplets absorb and scatter radio waves, causing additional path loss and fast fading. This question asks which techniques best mitigate rain-induced degradation in real deployments.

Given Data / Assumptions:

• Microwave links operate where precipitation can be intense.
• System design may include multiple frequencies, sites, and adaptive strategies.
• Goal is to maintain availability (uptime) and acceptable link margin during rain.

Concept / Approach:

Two robust strategies are path diversity and frequency diversity. Path diversity uses geographically separated routes or space diversity on the same path (two antennas spaced vertically or horizontally) so that deep fades are rarely simultaneous. Frequency diversity uses a second channel at a different frequency, since rain attenuation is strongly frequency dependent; the lower-frequency path often remains serviceable when the higher-frequency path is in deep fade.

Step-by-Step Solution:

Verification / Alternative check:

ITU-R rain models show higher attenuation at higher frequencies; operational networks use dual-frequency and space-diverse links to achieve 99.99%+ availability.

Why Other Options Are Wrong:

Antenna replacement/feed tweaks (A) are maintenance, not targeted mitigation. Amplitude/phase trimming (B) is for calibration, not rain-specific. Polarization shifting and code diversity (C) can help robustness but are not primary cures for the propagation loss. Increasing cable DC current (E) is unrelated and unsafe.

Common Pitfalls:

Relying only on extra transmit power; during heavy rain, dynamic range may be insufficient. Ignoring site diversity spacing guidelines and frequency planning.

Final Answer:

path diversity and frequency diversity