5) Link Budget & Propagation

I. The Link Budget
• Every component contributes a dB.
• Determines whether a link is viable.
• Two directions — uplink and downlink.

II. Key Components
• EIRP — Effective Isotropic Radiated Power.
• FSPL — Free Space Path Loss.
• G/T — Receive figure of merit.
• C/N₀ — carrier-to-noise-density ratio.

III. Free Space Path Loss
• FSPL (dB) = 20 log₁₀(4π d / λ).
• Scales as 20 log(d).
• Scales as 20 log(f).

IV. Atmospheric Effects
• Gaseous absorption — water vapour and oxygen.
• Rain fade — dominant above 10 GHz.
• Scintillation — rapid amplitude and phase fluctuations.

V. Quality Metrics
• C/N₀ — carrier to noise density.
• Eb/N₀ = C/N₀ − 10 log₁₀(Rb).
• Compare against required Eb/N₀.
• Link margin = available − required.

VI. Design Practice
• Aim for 3–6 dB clear-sky margin.
• Add rain fade margin — up to 10–15 dB at Ka.
• Uplink power control and ACM adapt to conditions.