RF Link Budget — Complete Reference
A link budget is an accounting of all gains and losses from the transmitter power amplifier output to the receiver demodulator input. It determines whether the received signal power will be sufficient to achieve a required signal-to-noise ratio (SNR) with an adequate fade margin. Every wireless system — Wi-Fi, LoRa, 5G, satellite, microwave backhaul — is designed against a link budget.
The Full Link Budget Equation
Cascaded link budget
P
RX = P
TX − L
TX + G
TX − FSPL − L
atm − L
rain − L
misc + G
RX − L
RX
Noise floor (dBm) = −174 + 10·log
10(B) + NF
SNR (dB) = P
RX − N
floor
Link margin (dB) = SNR − SNR
required − Fade
required
PTX = TX power (dBm) · GTX/RX = antenna gains (dBi) · L = losses (dB) ·
B = noise bandwidth (Hz) · NF = receiver noise figure (dB)
Understanding Each Term
- EIRP (Effective Isotropic Radiated Power) = PTX + GTX − LTX. This is the power the transmit antenna appears to radiate toward an isotropic reference.
- Free-space path loss is the theoretical minimum loss — FSPL = 20·log(4πdf/c). It grows with both distance and frequency.
- Atmospheric absorption is typically <0.5 dB/km below 10 GHz, but rises sharply at 60 GHz (oxygen absorption ~15 dB/km) and in heavy rain at millimetre-wave frequencies.
- Noise floor is the thermal noise power in the receiver bandwidth. At 290 K it is −174 dBm/Hz — add 10·log(bandwidth) and the noise figure of the receiver chain.
- SNR is the ratio of signal to noise at the receiver input. Higher SNR enables higher-order modulation and more data throughput.
- Fade margin accounts for multipath fading, atmospheric scintillation, rain attenuation, and other time-varying effects. 10 dB is minimum; 20–30 dB for critical links.
- Link margin is the margin remaining above what is needed. Positive = link closes. Negative = link fails.
Required SNR by Modulation
| Modulation |
Typical SNR (BER 10⁻⁶) |
Spectral Efficiency |
Application |
| LoRa SF12 | −20 dB | 0.02 bps/Hz | LPWAN IoT — extreme range |
| LoRa SF7 | −7.5 dB | 0.15 bps/Hz | LPWAN IoT — shorter range |
| BPSK | 10.6 dB | 1 bps/Hz | Satellite, GNSS, telemetry |
| QPSK | 10.6 dB | 2 bps/Hz | Wi-Fi MCS0, LTE, satellite |
| 16-QAM | 17.5 dB | 4 bps/Hz | Wi-Fi MCS4, LTE MCS9 |
| 64-QAM | 24.1 dB | 6 bps/Hz | Wi-Fi MCS7, cable, DOCSIS |
| 256-QAM | 30.8 dB | 8 bps/Hz | Wi-Fi 802.11ac/ax, 5G NR |
| 1024-QAM | 37.6 dB | 10 bps/Hz | Wi-Fi 6E, 5G mmWave |
Typical Link Budget Examples
| System |
EIRP |
FSPL budget |
Noise floor |
Typical range |
| LoRa SF12, 868 MHz |
+16 dBm |
155 dB |
−143 dBm |
10–50 km outdoor |
| Wi-Fi 802.11n, 2.4 GHz |
+23 dBm |
96 dB |
−87 dBm |
50–300 m outdoor |
| 5G NR FR1, 3.5 GHz |
+38 dBm |
138 dB |
−95 dBm |
200–1000 m urban |
| P2P 5.8 GHz bridge |
+50 dBm |
148 dB |
−87 dBm |
5–30 km LOS |
| Microwave backhaul, 11 GHz |
+61 dBm |
175 dB |
−90 dBm |
10–50 km LOS |
Design sequence: (1) Fix frequency and modulation → determines required SNR. (2) Calculate noise floor from bandwidth and NF. (3) Set minimum received power = noise floor + SNRreq + fade margin. (4) Calculate path loss budget = EIRP − PRX,min + GRX − LRX. (5) Max range is the distance where FSPL equals the path loss budget minus environmental margins.