Link Budgets
A link budget is a systematic accounting of all gains and losses in a communication system from transmitter to receiver. It determines whether a link will achieve adequate signal-to-noise ratio under specified conditions, and by how much margin.
Link Budget Equation
\(P_r = P_t + G_t - L_{tx} - FSPL - L_{misc} + G_r - L_{rx}\) (dBm)
Then compare received power to the minimum detectable signal (MDS):
\(MDS = kTB + NF + SNR_{min}\) (dBm)
Link margin = \(P_r - MDS\). A positive margin means the link works; typical design margins are 3–20 dB depending on the fade margin required.
Free-Space Path Loss
\(FSPL = 20\log_{10}(d) + 20\log_{10}(f) + 20\log_{10}\!\left(\dfrac{4\pi}{c}\right)\)
At 2.4 GHz, FSPL over 100 m ≈ 80 dB. FSPL grows with distance² and frequency² — doubling distance adds 6 dB; doubling frequency adds 6 dB.
Additional Loss Mechanisms
- Multipath fading — reflections cause constructive/destructive interference. Fast fading margin: 20–30 dB for 99.9% link reliability.
- Rain attenuation — significant above 10 GHz; up to several dB/km in heavy rain.
- Atmospheric absorption — oxygen absorption peak at 60 GHz (15 dB/km) limits this band to short-range applications.
- Polarisation mismatch — misaligned transmit/receive polarisations cause 0–∞ dB loss.
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