What Is a Link Budget?

A link budget is a calculation that quantitatively assesses whether a communications link will perform successfully. It provides a detailed analysis of the power budget, accounting for the gains and losses at each stage of the transmission path. A link budget calculates the link margin \(LM\), given as

\(LM = P_{rx} - RS\), where

\(P_{rx}\) is received power in dBm, and
\(RS\) is receiver sensitivity in dBm.

Receiver sensitivity is the minimum received power that will allow successful communication to occur. The goal of any wireless design is to ensure a positive link margin.

Link budgets play a crucial role in designing and analyzing the performance of terrestrial and satellite communications systems. Using MATLAB^® to create link budgets for your wireless system design, you can quickly assess system performance

The Link Budget Equation

The link budget equation is the fundamental equation used in link budget calculations. It calculates the received signal power by accounting for all the gains and losses in the communication link. The link budget equation can be expressed as follows:

\(P_{rx} = P_{tx} + G_{tx} – L_{tx} – L_{fs} – L_{prop} + G_{rx} – L_{rx}\), where

\(P_{rx}\) is received power in dBm,
\(P_{tx}\) is transmitter power amplifier output power in dBm,
\(G_{tx}\) is transmitter antenna gain in dBi,
\(L_{tx}\) is transmitter losses (e.g., cable, connectors, radome) in dB,
\(L_{fs}\) is free space path loss in dB,
\(L_{prop}\) is other propagation losses (e.g., antenna mispointing, fading margin, polarization mismatch, interference, atmospheric environment) in dB,
\(G_{rx}\) is receiver antenna gain in dBi, and
\(L_{rx}\) is receiver losses (e.g., cable, connectors, radome, demodulator loss) in dB.

You can use the fspl function in MATLAB to calculate the free space path loss \(L_{fs}\). You can also use the fogpl, gaspl, and rainpl functions to use International Telecommunications Union (ITU) models to calculate propagation losses from fog, gas, and rain, respectively. Also, you can use the cranerainpl function to calculate propagation losses based on the Crane rain model.

Link Budgets in MATLAB

Since satellite communications systems are typically power limited, link budgets are more important for them than for terrestrial systems, which are typically interference limited.

Satellite Communications Toolbox includes the Satellite Link Budget Analyzer app, which provides a user-friendly interface for performing comprehensive link budget analysis for satellite communication systems. This app simplifies the process by guiding you through the necessary parameters and calculations, making it easier to design and analyze satellite communication links. It also enables availability analysis with the ITU P.618 propagation loss model and allows you to perform qualitative sensitivity analysis through the creation of custom visualizations.

If you want to incorporate RF effects such as S-parameters, impedance mismatches, and nonlinearities into your link budget, you can use the RF Budget Analyzer app, which offers both analysis and simulation capabilities.

Importance of Link Budgets

Link budgets are essential for several reasons:

1. System Design: Link budgets guide the design and dimensioning of communication systems, ensuring that the transmitted power, antenna gains, and receiver sensitivity are appropriately chosen for reliable communication.
2. Performance Evaluation: By accounting for all gains and losses, link budgets provide a quantitative assessment of the system’s performance, helping engineers estimate the achievable data rates, coverage range, and signal quality.
3. Regulatory Compliance: Link budgets help ensure compliance with regulatory standards, such as maximum permissible transmit power levels, interference limits, and signal quality requirements.