What is a link budget?
A link budget is a calculation that tallies all the power gains and losses experienced by a radio signal as it travels from transmitter to receiver in a satellite communication system. The fundamental equation is: Received C/N₀ = EIRP + G/T − FSPL − Losses + Constants, where C/N₀ is carrier-to-noise density, EIRP is the transmitter's effective isotropic radiated power, G/T is the receiver's figure of merit, and FSPL is free-space path loss.
Key parameters
EIRP (dBW): The product of transmit power and antenna gain — how much energy is directed toward the satellite or terminal. A Ku-band VSAT terminal might have an EIRP of 47 dBW.
G/T (dB/K): The receiver antenna gain divided by its system noise temperature. A large GEO gateway dish might achieve G/T of 30 dB/K. Free-space path loss (FSPL): At 14 GHz over 35,786 km (GEO uplink), FSPL ≈ 207 dB — the dominant loss term. Rain fade: At Ka-band in a tropical location, fade exceeding 10 dB for 0.01% of the year must be budgeted. Pointing loss, atmospheric absorption, polarisation mismatch: Smaller but non-negligible contributors.
Closing the link
A link is said to 'close' when the calculated Eb/N₀ (energy per bit to noise density) at the receiver exceeds the required threshold for the chosen modulation and coding scheme, plus a margin (typically 2–3 dB) to handle unexpected losses. DVB-S2X with 32APSK 3/4 requires approximately 14 dB Eb/N₀; a system budgeting 16 dB has a 2 dB margin.
Practical use
Link budgets are the primary tool for satellite system engineers when sizing antennas, specifying amplifier power, selecting modulation and coding schemes, and defining system availability requirements. They are revisited at each design review and validated during in-orbit testing.