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An Evaluation of Approaches for Modeling of Terrestrial, HAPand
Satellite Systems Performance during Rain Events
S. Zvanovec, L. Subrt, and P. PechacDepartment of
Electromagnetic Field, Czech Technical University in Prague, Czech
Republic
Abstract— This paper presents the results from site-specific
simulations of route diversityperformance in case of High Altitude
Platform system, terrestrial system and satellite systemduring
specific rain events. Based on the radar rainfall database, route
diversity dependenceson altitude of transmitter were studied using
the simulation tool. The main goal of the paperis to adopt concept
for HAP systems from investigations of both terrestrial
point-to-multipointsystems and satellite systems operating in
millimeter frequency bands.
1. INTRODUCTION
The proper choice of the modeling approaches for electromagnetic
wave propagation within themillimeter wave band is crucial. This is
particularly due to the high cost of millimeter waveequipments
associated with transmitting power. Considerably high attenuation
can be causedby interaction of transmitted waves with rain drops.
There can be found several approaches toenumeration and
compensation of signal fades caused by rain [1, 2]. Investigations
of millimeterwave propagation aspects have been conducted out
within the frame of European internationalprojects such as CRABS
[3] and COST Action 280 [4] and COST Action 297 [5].
Nevertheless,due to the variability of systems configurations,
these models can be used only for investigationof alone
terrestrial, satellite or High Altitude Platforms (HAPs) links.
Spatial distribution of rainattenuation observed by users in the
area 50 km×50 km from transmitter placed either at terrestrialmast
(10 m above ground) or at HAP (deployed on quasi-stationary
unmanned vehicles/airships inthe stratosphere at the altitude of 20
km) or at LEO satellite (altitude of 320 km) is depicted inFigure
1. These results clearly demonstrate, how particular systems are
able to connect with usersbehind a rain event.
One of the rain fade mitigating approaches used in millimeter
waveband introduces a routediversity [2] — i.e., two joint links to
one HAP station from two different ground localities — duringa
storm, when a rain cell moves across the service area, any outage
of a terminal can be avoided whenit is able to connect to two
different HAP stations. Nevertheless [2], involves only the
parametersgiven based on the measurements of Earth-space links
within particular geographical localities.The paper brings a
comparative study of modeling approaches for propagation of
millimeter waveswithin rain events and their adaption to HAP
systems. The rain database from the period of2002–2005 was utilized
including 250 km × 250 km rain scans from Czech meteoradars (rain
ratedistributions with 1 km grid resolution and 1 minute time
steps). Particular results of HAP systemsimulations performed at
the frequency of 48 GHz will be discussed. Two evaluation methods
weretested based on simulation results: the first following
terrestrial approaches [6] and the second onefrom the other side
utilizing a satellite approach [2].
(a) (b) (c) (d)
Figure 1: (a) rain rate distribution in [mm/h] and corresponding
rain attenuation in [dB] for coverage from(b) terrestrial
transmitter; (c) HAP; (d) satellite.
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2. SYSTEM PERFORMANCE COMPARISON
Analyze of dependences of rain spatial parameters on system
performance for both terrestrial andHAP systems was elaborately
performed in [7] with the result — the rain spatial classification
forthe evaluation of rain influences on terrestrial systems can be
with particular corrections adaptedin the case of stratospheric
systems. Let us now consider a dependence of transmitter
altitude.At first, performance of two joint links was analyzed for
terrestrial, HAP and satellite systems.Common transmitting
frequency of 48GHz was assumed (the frequency allocated worldwide
toHAP systems [8]). Significant rain events recorded by meteoradars
in the Czech Republic on 13thof July 2002 was chosen from the rain
database [9]. In Figure 2, an illustrative example fromperformed
analyses — diversity gain dependences on angular separations of
route diversity links(the main link length 20 km and the diversity
link length 40 km) are depicted. As it can be seen,utilization of
route diverzity is very efficient in case of terrestrial systems.
In case of HAP systems,diverzity gain concept can be utilized for
statistically shorter time a year (see example of results inFigure
2(b)). Nevertheless, it was observed that the HAP system with a
particular route diversityscheme is able to efficiently combat the
rain attenuation.
Contrary to terrestrial and HAP systems, links of satellite
systems cross due to the high ele-vations rainy layer only within
near proximity of user (note the average rain height determinedfor
Europe is of 3.36 km [10]). This has impact on used route diversity
scheme, where diversitygains are substantially smaller (see Figure
2(c)). In next step, a diversity gain was determinedbased on base
station altitude. Example for approximately 5 hour rain event is
shown in Figure 3.There can be clearly demonstrated iterative
approaches from terrestrial and satellite models toHAP systems.
From simulations of 3 year period (years 2002–2004), it was derived
that terres-trial models can be applied with very slight deviances
in results for transmitter up to altitudes of1–10 kilometers. For
higher altitudes route diversity expressed by diversity gain
becomes differen-tiate (in most cases rapidly decreases) for more
than 3 dB. This follows gradually with increasingaltitude.
Surprisingly, it was observed HAP systems propagation dependences
statistical behavior
0 100 200 300 400 500 6000
10
20
30
40
50
60
70
80
90
100
time [min]
div
ers
ity g
ain
[dB
]
10 deg
20 deg
45 deg
60 deg
90 deg
0 100 200 300 400 500 6000
0.5
1
1.5
2
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time [min]
div
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ain
[dB
]
10 deg
20 deg
45 deg
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90 deg
0 100 200 300 400 500 6000
10
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time [min]
div
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ain
[dB
]
10 deg
20 deg
45 deg
60 deg
90 deg
(a) (b) (c)
Figure 2: Example of time dependence of diverzity gain for (a)
terrestrial system; (b) HAP system; (c)satellite system.
Figure 3: Dependence of diversity gain on altitude.
PIERS ONLINE, VOL. 6, NO. 8, 2010 718
has higher correlation with terrestrial link statistics
(correlation coefficient 0.85) than with satellitelink statistics
(correlation coefficient 0.14).
3. CONCLUSIONS
Rain represents one of the main limitations of millimeter wave
band systems regardless altitudeof transmitter. In the paper, a
comparison of route diversity utilization for terrestrial, HAP
andsatellite links was accomplished based on actual rainfall radar
data. HAP link statistics proved tobe more related to terrestrial
ones than to satellite statistics. Based on the results discussed
in thispaper, detailed relationships on altitudes of transmitter
will be subsequently derived.
ACKNOWLEDGMENT
The simulations and presentation of results were supported by
Czech Science Foundation grant102/08/P346. The research is a part
of the activities of the Department of Electromagnetic Fieldof the
Czech Technical University in Prague in the frame of the research
project No. OC09075 ofthe Ministry of Education, Youth and Sports
of the Czech Republic.
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