Composite Fade
Understanding Composite Fade
Composite fade describes what actually happens on a real radio path, where the received signal level is never disturbed by just one effect. A line-of-sight microwave hop carrying backhaul traffic, a satellite feeder link, or a long over-water path all see a mixture of fast and slow fading mechanisms layered on top of the steady free-space loss. The job of the link engineer is to characterize each mechanism statistically, then combine those statistics into a single distribution of fade depth versus probability. That composite distribution answers the practical question that matters for service-level agreements: for what fraction of time will the signal drop below the receiver threshold and cause an outage?
The Contributing Mechanisms
The mechanisms that build up a composite fade differ in speed, frequency dependence, and seasonal behavior. Understanding each one is what allows them to be combined correctly rather than double-counted.
- Multipath fading: Fast, frequency-selective fading caused by atmospheric layering that creates two or more ray paths between the antennas. It dominates the deep-fade statistics below about 10 GHz and follows a Rayleigh distribution in the deep-fade region, where each 10 dB of extra depth is roughly 10 times less likely.
- Rain attenuation: Slow, weather-driven absorption and scattering by raindrops. It is negligible below a few GHz but becomes the dominant limiting term above roughly 10 GHz, with specific attenuation climbing steeply at Ka-band and above.
- Atmospheric and refractive effects: Gaseous absorption from oxygen and water vapor, plus refractive beam bending and ducting governed by the vertical refractivity gradient. These contribute a slowly varying baseline and can also cause defocusing or antenna decoupling.
- Diffraction and obstruction: Partial blockage of the first Fresnel zone by terrain or growth, which adds a fixed or slowly varying loss component.
Combining the Distributions
Below about 10 GHz, multipath and rain rarely produce deep fades at the same instant, so their outage probabilities are added at the threshold level. The total unavailability is the sum of the probability that multipath fade exceeds the margin and the probability that rain fade exceeds it. This addition of outage probabilities, not of decibels, is the heart of composite-fade prediction. Above roughly 15 to 20 GHz the rain term so dominates that the link is sized almost entirely by the rain attenuation exceeded for the target percentage of the worst month or year.
Composite Fade Equations
Ptotal = Pmultipath + Prain + Pother
Multipath outage (deep-fade region, ITU-R P.530 form):
Pmultipath = P0 × 10(−F / 10)
Fade margin:
FM = Prx − Pthreshold (dB)
Availability:
A = (1 − Ptotal) × 100%
Where Ptotal = total fraction of time below threshold; Pmultipath, Prain, Pother = outage probabilities of each mechanism; P0 = multipath occurrence factor for the path (geoclimatic and path-length dependent); F = fade margin in dB; Prx = nominal received level (dBm); Pthreshold = receiver threshold (dBm); A = availability in percent. Example: a 35 dB margin with P0 = 0.5 gives Pmultipath ≈ 0.5 × 10−3.5 ≈ 1.6 × 10−4, near 99.98 percent from multipath alone.
Frequency Bands and Dominant Terms
The table below summarizes which mechanism tends to drive the composite fade and the design approach typically used in each band. Values are representative; actual figures depend on path length, climate region, and antenna configuration.
| Band | Frequency | Dominant Mechanism | Typical Fade Margin | Mitigation |
|---|---|---|---|---|
| L / S | 1 to 4 GHz | Multipath, diffraction | 20 to 30 dB | Path clearance, space diversity |
| C | 4 to 8 GHz | Multipath | 30 to 40 dB | Space / frequency diversity |
| X / Ku | 8 to 18 GHz | Multipath plus rain | 30 to 45 dB | Diversity plus shorter hops |
| K / Ka | 18 to 40 GHz | Rain attenuation | Rain-limited (0.01% rule) | Short hops, adaptive modulation |
| V / E / W | 60 to 90 GHz | Rain plus oxygen absorption | Rain-limited | Very short hops, link power control |
Why It Matters for System Design
Sizing a link to its composite fade rather than to any single mechanism prevents both over-engineering and unexpected outages. Under-margining a path produces dropped traffic during the worst month; over-margining wastes transmit power, antenna size, and tower loading budget. Diversity techniques such as space diversity and frequency diversity attack the multipath term directly, while adaptive coding and modulation trade throughput for extra effective margin during rain events. A correct composite-fade model lets the engineer choose the cheapest combination that still meets the availability objective. For first-pass budgets, see the RF calculators.
Frequently Asked Questions
What is composite fade?
Composite fade is the combined reduction in received signal level on a radio link when more than one fading mechanism acts on the path at the same time. On a terrestrial microwave hop the dominant contributors are multipath fading, rain attenuation, and atmospheric or refractive effects. Rather than designing for each mechanism in isolation, link engineers combine their statistical distributions to predict the total fraction of time the signal drops below the receiver threshold, which sets the required fade margin and the link availability.
How is composite fade different from a single fade mechanism?
A single fade mechanism, such as multipath alone, follows its own probability distribution and dominates over a specific frequency and distance range. Composite fade accounts for the fact that several mechanisms coexist on a real path. Below roughly 10 GHz multipath dominates the deep-fade statistics, while above about 10 GHz rain attenuation becomes the limiting term. Because multipath fades are fast and rain fades are slow and seasonal, their outage probabilities are usually added at the threshold level to estimate the total worst-month or annual unavailability.
How does composite fade set the fade margin?
The fade margin is the difference in decibels between the nominal received signal level and the receiver threshold. Engineers compute the depth that each fade mechanism reaches for the target outage probability, combine those distributions, and size the margin so the composite fade exceeds the margin only for the allowed outage time. A typical short hop below 10 GHz uses 30 to 40 dB of flat fade margin for 99.999 percent availability, while a rain-limited link above 20 GHz may instead be sized by the rain attenuation exceeded for 0.01 percent of the year.