What Is a Link Budget?
How Are Gains and Losses Accounted for Across a Communications Link?
A link budget is an end-to-end accounting of the gains and losses experienced by a communication signal from transmitter to receiver. Like a financial budget, it starts with an available quantity, adds gains, subtracts losses, and determines what remains. The result predicts whether the receiver will obtain enough signal quality for the required service. Link budgets are used for terrestrial radio, microwave, fibre and Free-Space Optical communication, but they are especially visible in Satellite Communications because long paths and limited spacecraft power make every decibel important.
For a simple radio link, the received carrier power can be written in decibels as Pr = Pt + Gt + Gr - Lpath - Lother. Transmitter power and transmit-antenna gain are often combined, after feeder losses, as Effective Isotropic Radiated Power. At the receiving end, antenna gain is related to Effective Aperture. Free-space path loss is Lfs = 20 log10(4 πd/λ), where d is distance and λ is wavelength. All quantities must use consistent units and reference points.
Received power alone does not determine performance. A receiver must distinguish the wanted signal from thermal noise and interference. Noise depends on temperature and bandwidth, while receiver electronics add degradation described by Noise Figure. Satellite receiving systems are often summarized by their G/T Ratio, which combines antenna gain with system noise temperature. The budget may therefore continue from carrier power to Carrier-to-Noise Ratio and Signal-to-Noise Ratio, or to energy per bit divided by noise density, Eb/N0. The chosen Modulation, data rate and error-control coding determine the minimum value needed for acceptable error performance; Coding Gain can reduce that requirement.
The loss side must represent the real path and equipment. It may include cables, connectors, filters, radomes, atmospheric absorption, rain attenuation, Fading, Polarization mismatch and interference. Directional links also need allowances for Antenna Pointing Loss and changes in Beamwidth or gain across the operating band. A satellite system normally has separate uplink and downlink budgets, and a regenerative or bent-pipe payload may require additional accounting through the spacecraft transponder.
A link closes when predicted performance exceeds the required threshold. The difference is link margin, usually stated in decibels. Positive margin provides tolerance for modelling error, component ageing, manufacturing variation and changing conditions. Margin is not a gain that equipment creates, and an arbitrary value should not replace an availability analysis. A fixed terrestrial line-of-sight link, a mobile channel subject to rapid fading, and a Ka-band satellite link affected by rain require different statistical assumptions and outage objectives.
Good practice separates nominal, worst-case and statistical values; identifies whether each figure is a gain or loss; and records its source, units and reference plane. Probabilistic effects should be combined according to the required percentage of time rather than all being forced simultaneously to their individual worst cases. Engineers also guard against double-counting, such as including feeder loss inside EIRP and subtracting it again, or using both Noise Figure and an already adjusted system-noise temperature.
The link budget is also a design tool. If margin is inadequate, engineers can increase transmitter power or antenna gain, improve G/T Ratio, reduce data rate or receiver bandwidth, choose stronger coding or another Modulation, change frequency, or improve tracking. Each option affects cost, mass, power, coverage or capacity. After deployment, measured signal and noise levels can be compared with predictions, turning the budget from a planning spreadsheet into a disciplined model of the system.
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