How do I account for atmospheric absorption in a link budget at V-band and E-band frequencies?
Atmospheric Absorption
Atmospheric absorption is a fundamental propagation impairment at mmWave and sub-THz frequencies. Unlike free-space path loss (which follows 1/R²), atmospheric absorption follows an exponential decay with distance: the power decreases as exp(-α×R), where α is the specific attenuation in dB/km. For long paths, atmospheric absorption dominates over free-space spreading loss.
The absorption is caused by the quantized rotational energy levels of atmospheric gas molecules. When the electromagnetic wave frequency matches a molecular transition frequency, energy is absorbed and converted to heat. The O2 absorption at 60 GHz is actually a complex of many closely-spaced lines that merge into a broad peak at sea level due to pressure broadening. At higher altitudes (lower pressure), the individual lines are resolved and the absorption decreases.
The ITU-R P.676 model provides the standard calculation method for atmospheric gaseous attenuation. It computes the total specific attenuation from dry air (O2 and N2) and water vapor contributions based on the pressure, temperature, and water vapor density. At sea level with standard atmosphere: total absorption ranges from < 0.01 dB/km at 10 GHz to > 15 dB/km at 60 GHz.
Frequently Asked Questions
Why is E-band popular for backhaul?
The 71-76 GHz and 81-86 GHz bands fall between the O2 peak at 60 GHz and the next H2O peak at 118 GHz. Atmospheric absorption is only 0.3-0.5 dB/km, comparable to Ka-band. Combined with 5 GHz of available bandwidth per direction and high antenna gain from compact dishes, E-band provides multi-Gbps backhaul at 1-3 km range.
Is 60 GHz useless?
No. The high O2 absorption at 60 GHz is an advantage for frequency reuse: signals are naturally attenuated beyond ~100-200m, allowing dense spatial reuse without interference. This makes 60 GHz ideal for in-room wireless (WiGig/802.11ad/ay), kiosk-to-device communication, and short-range sensor links.
How does altitude affect absorption?
Atmospheric absorption decreases with altitude because the gas density decreases. At 3000m altitude: O2 absorption at 60 GHz drops to about 5 dB/km (from 15 dB/km at sea level). At 10,000m (aircraft cruising altitude): absorption is < 1 dB/km. This makes mmWave air-to-air and air-to-satellite links more feasible than ground-level links.