RF Budget
Understanding RF Link Budgets
The link budget is the most fundamental system engineering calculation in wireless communications. It determines whether a proposed radio link will work with adequate reliability under specified conditions. Every system design begins with a link budget.
Link Budget Components
- Transmit side: TX power (dBm) - TX cable/connector loss + TX antenna gain (dBi) = EIRP (dBm).
- Propagation: FSPL + atmospheric loss + rain margin + fade margin.
- Receive side: RX antenna gain - RX cable/connector loss = signal at receiver port.
- Margin: Received signal - receiver sensitivity = link margin (dB).
Required Margins
- Clear-sky margin: 3-6 dB for general purpose.
- Rain margin: 5-15 dB at Ka-band, depends on availability requirement.
- Fade margin: 10-30 dB for terrestrial microwave links.
P_rx = P_tx - L_tx + G_tx - FSPL - L_atm + G_rx - L_rx
Link margin:
Margin = P_rx - P_sensitivity
Receiver sensitivity:
P_sens = -174 + 10log(BW) + NF + SNR_min
Example: 10 GHz, 10 km, 1 MHz BW:
P_tx = 30 dBm, G_tx = 30 dBi, G_rx = 30 dBi
FSPL = 132.4 dB, L_tx = L_rx = 2 dB
P_rx = 30 - 2 + 30 - 132.4 + 30 - 2 = -46.4 dBm
Sensitivity = -174 + 60 + 5 + 10 = -99 dBm
Margin = 52.6 dB (excellent)
Frequently Asked Questions
What is a link budget?
A link budget accounts for all gains and losses between transmitter and receiver to determine if the link will work. It adds up transmit power, antenna gains, and subtracts path loss, cable loss, and atmospheric losses. The result is the link margin.
What link margin is needed?
Minimum link margin depends on required reliability. 3-6 dB provides basic operation. 10-15 dB accommodates weather fading. 20-30 dB is used for high-reliability terrestrial microwave links. Satellite links typically require 3-6 dB clear-sky margin plus rain margin.
What if the link budget shows negative margin?
Negative margin means the link cannot close reliably. Options: increase transmit power, use higher-gain antennas, reduce cable losses, use lower-noise receivers, reduce the data rate (narrower bandwidth), or shorten the path distance.