Fade Mechanisms
Microwave fades fall into the following categories:
- Multipath fading
- Obstruction fading
- Fading due to a specular reflection
- Ducting and propagation anomalies
- Rain fading (above ~ 8 GHz)
With the exception of rain fading, microwave fading only depends on the change of the refractive index with height along the path
| Availability | Seconds | Minutes | Hours |
|---|---|---|---|
| 99% | 315360 | 5256 | 87.6 |
| 99.9% | 31536 | 525 | 8.76 |
| 99.99% | 3153 | 52.5 | 0.876 |
| 99.999% | 315 | 5.3 | 0.088 |
Link Performance Definition
The definition of link performance only considers multipath and rain fades. This implies the following assumptions:
- the path has sufficient clearance / fade margin to obviate the possibility of an obstruction fade.
- signal nulls due to a specular reflection are small com-pared to the fade margin or a space diversity configura-tion has been implemented.
- the probability of ducting is unknown.
Microwave link performance
ANSI (North America) All fades durations are considered i.e. the total time that the signal is below the threshold level due to rain and multi-path. The overall link performance is expressed as an annual percentage of the link availability e.g 99.999% (315 sec-onds total outage) in both directions of transmission.
Multipath fades on a link are not correlated. The total time below level due to multipath, is the sum of the outage times in each direction. Rain fades affect both directions of transmission equally. Therefore, the total time below level for rain and multipath is the sum of the multipath outage time in each direction plus the outage time due to rain. European Telecommunications Standards Institute (ETSI)
