Telecommunications Industry Association (TIA)
Document Number: TR45.WGMS-161214-373
DOCUMENT SUBMITTED TO:
TR45 Working Group on Microwave Systems
SOURCE: Micronet Communications, Inc.
CONTACT: Fredrich Lamprecht, [email protected]
TITLE: Recommendations and additional Issues for Discussion under Chapter 8
PROJECT NUMBER (PN): TBD
DISTRIBUTION:
TR45 WORKING GROUP ON MICROWAVE SYSTEMS
INTENDED PURPOSE OF DOCUMENT:
__ FOR INCORPORATION INTO TIA PUBLICATION final text
__ FOR INFORMATION preliminary working document
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ABSTRACT:
This document seeks to offer an alternative recommendation of clearance criteria for microwave links that
operate between 2GHz and 13GHz. The predicted outcome is that it would render lower minimum antenna
height required compared to the clearance criteria currently in use.
Improved re-use of microwave spectrum should be a positive fall-out in the long run from being able to use
lower antenna heights.
For many years (approximately 20 years or more) the ITU-R1 has been using a distance based definition
of minimum K values under sub-refractive conditions in temperate and tropical climates. (It is to be noted
that most part of the contiguous US can probably be considered temperate and / or tropical)
Refer to the following graph for the ITU-R’s definition of Minimum K values vs Path length:
Graph 1: ITU-R’s definition of minimum K value vs path length for temperate /
tropical areas
In its current format, Chapter 8 (Multipath Fading) recommends consideration of clearance objectives for
different climate areas as per the following table:
Table 1: Recommended antenna clearance criteria as in Chapter 8 (Multipath Fading)
It is probably safe to say that temperate and tropical climates can be associated to a greater or lesser
degree with climate areas in Table 1 above that excludes the “Good” category. The incumbent clearance
criteria for such climate areas when under sub-refractive fading, can be probably be summed up roughly
as either 0% F1 clearance when K = 0.5 or 30% F1 clearance when K = 0.66.
For microwave links that operate between 2GHz and 13GHz, sub-refractive fading becomes the controlling
criteria that determines minimum antenna height required when the path length starts to exceed the 13 mile
1
ITU-R P.530-15
mark more or less. In other words, the chosen minimum K value becomes significant for microwave paths
that operate between 2GHz and 13GHz of 13+ miles in length.
Calculations were done to compare the effect on minimum antenna height required under sub-refractive
conditions by using the two incumbent recommendations for poor climate areas against using the ITU
method (assuming a similar 30% F1 clearance needed). The comparison was done considering link
distances that range from 12.5 to 50 miles. Note as well that the minimum antenna height required in all
three cases (ITU method + two incumbent methods) was taken as the maximum of 100% F1 clearance
under Kmedian = 1.33 conditions and the respective Kminimum definition under consideration.
The results can be graphed as follows:
Minimum Antenna height required vs Path length
900
Minimum antenna height required (ft)
800
700
600
500
400
300
200
100
0
10
15
20
25
30
35
40
Path length (miles)
F0 = 6,175 MHz
Kmin = 0.5
F1 = 0%
F0 = 6,175 MHz
Kmin = 0.66
F1 = 30%
Graph 2: Comparison results of the minimum antenna height required vs path length
F0 = 6,175 MHz
Kmin (ITU)
F1 = 30%
45
50
Excess antenna height required (over ITU method) vs Path length
350
Excess antenna height (ft)
300
250
200
150
100
50
0
12.5
15
17.5
20
22.5
25
27.5
30
32.5
35
37.5
40
42.5
45
47.5
50
Path length (miles)
F0 = 6,175 MHz
Kmin = 0.66
F1 = 30%
F0 = 6,175 MHz
Kmin = 0.5
F1 = 0%
Graph 3: Alterative presentation of the results to show the excess antenna height required (over the ITU method) for different path
distances
(Graph 3 is simply an alternative presentation of the results already given in Graph 2)
For example, for a 30 mile link operating over perfect flat terrain in the lower 6GHz band, the ITU method
results in minimum antenna height savings of 38 ft and 87 ft respectively compared to the incumbent subrefractive clearance criteria for poor climates.
For a 50 mile link operating over perfect flat terrain in the lower 6GHz band, the ITU method results in
minimum antenna height savings of 162 ft and 333 ft respectively compared to the incumbent sub-refractive
clearance criteria for poor climates.
Conclusions:
Antenna height, feeder length and potential tower height savings can become significant as path length
increases when using the ITU distance based definition of Minimum K.
More significantly however, and in lieu of the ever increasing use of microwave spectrum, a definition of K
values under sub-refractive fading conditions that will result in reduced minimum antenna height required,
should assist in minimizing overshoot interference potential and in turn, maximize the re-use potential of
frequencies for microwave links between 2 to 13 GHz links and over 13 miles long.
Recommendation:
If consensus can be reached that the ITU-R’s method may be adopted, then the following change is
recommended regarding minimum antenna clearance criteria for non-diversity links that operate in the
range 2 to 13GHz:
a) That the incumbent clearance criteria recommendations of Table 1 be retained for those who may
feel more comfortable with its more conservative approach (and/or its traditional and historic use).
b) That an alternative option be introduced as follows:
Minimum antenna height may be calculated as the greater of the antenna height required for 100%
F1 clearance at Median K = 1.33 and the antenna height required at a Minimum K value (in
accordance to Graph 1) that would result in calculated diffraction loss less than or equal to 25% of
the link’s thermal fade margin.
In tabular form, the alternative option can therefore be summarized as follows:
Climate Area
ALL
(contiguous US)
Alternative method
1.0 F1, K = 4/3
≤ 25% diffraction loss of TFM, K value as per Graph 1
Table 2: Proposed alternative clearance criteria option for non-diversity links that operate in the range 2 to 13 GHz.
Space diversity configured microwave links may be treated as follows:
Climate Area
SD path
Main
ALL
(contiguous US)
Diversity (to mitigate against a
specular reflection)
Diversity (to mitigate against
multipath propagation)
Alternative method
1.0 F1, K = 4/3
≤ 25% diffraction loss of TFM, K as per Graph 1
0.6 F1, K = 4/3
≤ 25% diffraction loss of TFM, K as per Graph 1
0.6 F1, K = 4/3
Table 3: Proposed alternative clearance criteria option for space diversity links that operate in the range 2 to 13 GHz.
It is proposed that this alternative option be made applicable the contiguous US irrespective of the
climate area categories as listed in Table 1.
[A quick comparison between the proposed alternative option (assuming 30% F1 clearance at a
distance based minimum K value) and the clearance criteria recommendations for “Good” and
“Very poor” climate areas as per Table 1, gave the following results (Positive values indicate the
incumbent method requiring a higher minimum antenna height; negative values indicate the opposite):
Comparison against the “Good” climate area criteria of Table 1:
Comparison with the “Very poor” climate area criteria of Table 1:
]