Link Budget
A link budget is a systematic accounting of all the gains and losses in a radio communications link. In satellite communications it is used to determine whether a signal transmitted from an Earth station to a satellite, or from a satellite to an Earth station, will arrive with sufficient power and quality to support the required service. The term budget is used because the calculation treats the communications path rather like a financial account: transmitter power and antenna gain add to the budget, while path loss, atmospheric loss, pointing loss, feeder loss, and other impairments subtract from it.
A satellite link budget is normally prepared separately for the uplink and downlink. The uplink budget considers the path from the transmitting Earth station to the satellite receiver, while the downlink budget considers the path from the satellite transmitter to the receiving Earth station. In a bent-pipe satellite system, the complete end-to-end performance depends on both parts of the link, as well as the noise and distortion introduced by the satellite transponder.
The starting point for a link budget is usually the transmitted power. This is combined with the transmitting antenna gain and any feeder or waveguide losses to determine the effective isotropic radiated power (EIRP). EIRP is the apparent power that would have to be radiated equally in all directions by an ideal isotropic antenna to produce the same power density in the direction of the satellite or receiving station. A high-gain antenna allows a relatively modest transmitter to produce a large EIRP in the intended direction.
The largest loss in most satellite link budgets is free-space loss (FSL)—also called free-space path loss. This occurs because electromagnetic energy spreads out as it travels. The loss increases with both distance and frequency, which is why geostationary satellite links and high-frequency links require careful attention to antenna gain, transmitter power, and receiver sensitivity. Additional losses may arise from atmospheric gases, rain, clouds, scintillation, polarization mismatch, antenna mispointing, radome loss, and implementation margins. Rain attenuation (weather loss) is especially important at Ku-band, Ka-band, and higher frequencies, while it is usually much less severe at C-band.
On the receiving side, the link budget accounts for receiving antenna gain and receiver noise performance. A common receiving figure of merit is G/T, which is the ratio of antenna gain to system noise temperature. Higher G/T indicates a better receiving system because it combines strong signal collection with low receiver noise. The received carrier power is then compared with the noise power to determine carrier-to-noise ratio (C/N), carrier-to-noise density ratio (C/N₀), or energy-per-bit to noise-density ratio (Eb/N₀), depending on the form of the analysis.
The required value of C/N or Eb/N₀ depends on the modulation, coding, data rate, bandwidth, and target error performance. More robust schemes such as BPSK, QPSK, and strong forward error correction can operate at lower signal-to-noise ratios (SNR), but use spectrum less efficiently. Higher-order modulation schemes can carry more bits per hertz, but require a cleaner link. The link budget therefore connects physical design decisions—such as antenna size and amplifier power—with service requirements such as availability, throughput, and error rate.
Link budgets are also used to estimate link margin. Link margin is the amount by which the predicted link performance exceeds the minimum required performance. A positive margin provides allowance for fading, equipment aging, pointing errors, manufacturing tolerances, and other uncertainties. In commercial satellite systems, the required margin is often tied to availability objectives, such as designing the link to remain usable for 99.9% or 99.99% of the time.
A link budget is therefore one of the central engineering tools in satellite communications. It allows designers to compare frequency bands, antenna sizes, satellite power levels, modulation schemes, coding rates, and service availability before equipment is built or deployed.