PathlossWiki - New pages [en]

NLCD

2022-10-11T17:14:47Z

Chrisg: Created page with "You can use the download tool to access these files: FetchSRTM - Download tool for finding and downloading SRTM and NED terrain data."

You can use the download tool to access these files:

FetchSRTM - Download tool for finding and downloading SRTM and NED terrain data.

Refractivity

2020-02-21T18:38:28Z

Cassieg:

The refractive index (n) of air is defined as:

n = c/v

[[File:equationtest.wmf]]

where:
*c is the velocity of light in a vacuum.
*v is the velocity of light in air.

A typical value of n is 1.000301. For convenience the refractive index is expressed as refractivity (N)

N = (n-1) x 10^6

which results in a typical value of 301 Refractivity is defined by the equation:

N = 77.6 x (P/T) + 3.732 x 10^5 x (e/T^2)

where:
*P is the atmospheric pressure.
*T is the absolute temperature.
*e is the water vapor pressure.
*N varies with elevation. In a well mixed troposphere, the variation is exponential; however, in the range from 1 to 2 kilometers above sea level, the variation is approximately linear. This is referred to as a constant refractivity gradient. The refractivity gradient determines the microwave beam-bending.

Handling the variations of refractivity with elevation is the major challenge in the design of high reliability microwave links

Basic Link Design Steps

2020-02-21T18:30:38Z

Cassieg:

# Generate the terrain profile.
# Path clearance. Determine the antenna heights required for free space line of sight. This step may fail if the heights are impractical.
# Enter the radio, antenna and transmission line specifications. The receive signal level and fade margin are avail-able at this point.
# Calculate the availability due multipath fading.
# Calculate the availability due to high intensity rain.
# Determine if path is susceptible to obstruction fading. This step may require an increase in antenna heights.
# Determine if the path geometry supports a specular reflection.
# Determine if the path is susceptible to ducting.

Fade Mechanisms

2020-02-21T18:19:12Z

Chrisg:

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

{| class="wikitable" style="display: inline-table;"
|+
|-
! 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)

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)

Point to Point Microwave Link Design

2020-02-21T17:42:37Z

Cassieg:

*[[Fade Mechanisms]]

*[[Basic Link Design Steps]]

*[[Refractivity]]