Difference between revisions of "Fade Mechanisms"
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Revision as of 11:10, 26 February 2020
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 |
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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)
Link performance is for the worst month in one direction. Performance is divided into two categories:
- Fades which last less than 10 consecutive seconds are classed as severely errored seconds (SES).
- Fades which last longer than 10 consecutive seconds are classed as unavailability. All rain fades are considered as unavailability.
Note that Pathloss program can predict the time duration of multipath fades and separate the fades into SES and unavailability; however this is not used. By convention multipath fades are always classed as SES.
On SDH radio systems two additional parameters are used:
- severely errored seconds ratio (SESR)
- background block error rate (BBER)
