Link Budget
Understanding Link Budgets
A link budget is the most fundamental analysis tool in wireless system design. It determines whether a proposed system will work: can the receiver detect the transmitted signal after it has traveled through the channel? Every wireless system design begins with a link budget calculation.
Link Budget Equation
P_received = EIRP - FSPL - L_atm - L_misc + G_rx (all in dB/dBm).
Link Margin
Link margin = P_received - P_sensitivity. Positive margin means the link works. Typical design margins are 3-10 dB for clear-sky conditions, and additional fade margin for rain, multipath, and interference.
Key Terms
- EIRP: Effective Isotropic Radiated Power = P_tx + G_tx - L_cable (dBm + dBi).
- FSPL: Free-Space Path Loss, the dominant loss term.
- Atmospheric loss: Rain attenuation, water vapor, oxygen absorption.
- System temperature: Determines receiver noise floor: N = kTB.
P_rx = P_tx + G_tx - L_tx - FSPL - L_atm + G_rx - L_rx
Link margin:
M = P_rx - P_sensitivity (dB)
Required margin:
Clear-sky: 3-6 dB
Rain fade (Ku): +6-10 dB
Rain fade (Ka): +10-20 dB
Multipath: +5-15 dB
Frequently Asked Questions
What is a link budget?
A link budget is a calculation that sums all gains (transmit power, antenna gain) and losses (path loss, cable loss, atmospheric absorption) in a wireless link to determine the received power level. The difference between received power and receiver sensitivity is the link margin.
What is a good link margin?
A minimum of 3-6 dB margin is needed for a reliable link in clear conditions. For satellite links, rain fade margin of 6-20 dB is added depending on frequency band and climate zone. For mobile links, 10-15 dB fade margin handles multipath and shadowing.
How do you improve a link budget?
Increase transmit power or antenna gain (higher EIRP), use a lower noise figure receiver, narrow the receiver bandwidth, use a larger receive antenna, reduce cable losses, or use a shorter path distance. Each option has practical and cost constraints.