Propagation Planning Procedures for LMDS, 1999. 4. Usman, I. S., M. J. Willis, and R. J. Watson, âRoute diversity analysis and modelling for mil- limetre wave ...
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An Evaluation of Approaches for Modeling of Terrestrial, HAP and Satellite Systems Performance during Rain Events S. Zvanovec, L. Subrt, and P. Pechac Department of Electromagnetic Field, Czech Technical University in Prague, Czech Republic
Abstract— This paper presents the results from site-specific simulations of route diversity performance in case of High Altitude Platform system, terrestrial system and satellite system during specific rain events. Based on the radar rainfall database, route diversity dependences on altitude of transmitter were studied using the simulation tool. The main goal of the paper is to adopt concept for HAP systems from investigations of both terrestrial point-to-multipoint systems and satellite systems operating in millimeter frequency bands.
1. INTRODUCTION
The proper choice of the modeling approaches for electromagnetic wave propagation within the millimeter wave band is crucial. This is particularly due to the high cost of millimeter wave equipments associated with transmitting power. Considerably high attenuation can be caused by interaction of transmitted waves with rain drops. There can be found several approaches to enumeration and compensation of signal fades caused by rain [1, 2]. Investigations of millimeter wave propagation aspects have been conducted out within the frame of European international projects 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 investigation of alone terrestrial, satellite or High Altitude Platforms (HAPs) links. Spatial distribution of rain attenuation observed by users in the area 50 km×50 km from transmitter placed either at terrestrial mast (10 m above ground) or at HAP (deployed on quasi-stationary unmanned vehicles/airships in the stratosphere at the altitude of 20 km) or at LEO satellite (altitude of 320 km) is depicted in Figure 1. These results clearly demonstrate, how particular systems are able to connect with users behind a rain event. One of the rain fade mitigating approaches used in millimeter waveband introduces a route diversity [2] — i.e., two joint links to one HAP station from two different ground localities — during a storm, when a rain cell moves across the service area, any outage of a terminal can be avoided when it is able to connect to two different HAP stations. Nevertheless [2], involves only the parameters given 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 waves within rain events and their adaption to HAP systems. The rain database from the period of 2002–2005 was utilized including 250 km × 250 km rain scans from Czech meteoradars (rain rate distributions with 1 km grid resolution and 1 minute time steps). Particular results of HAP system simulations performed at the frequency of 48 GHz will be discussed. Two evaluation methods were tested based on simulation results: the first following terrestrial approaches [6] and the second one from the other side utilizing a satellite approach [2].
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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 and HAP systems was elaborately performed in [7] with the result — the rain spatial classification for the evaluation of rain influences on terrestrial systems can be with particular corrections adapted in 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 48 GHz was assumed (the frequency allocated worldwide to HAP systems [8]). Significant rain events recorded by meteoradars in the Czech Republic on 13th of July 2002 was chosen from the rain database [9]. In Figure 2, an illustrative example from performed 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 in Figure 2(b)). Nevertheless, it was observed that the HAP system with a particular route diversity scheme is able to efficiently combat the rain attenuation. Contrary to terrestrial and HAP systems, links of satellite systems cross due to the high elevations rainy layer only within near proximity of user (note the average rain height determined for Europe is of 3.36 km [10]). This has impact on used route diversity scheme, where diversity gains are substantially smaller (see Figure 2(c)). In next step, a diversity gain was determined based 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 to HAP systems. From simulations of 3 year period (years 2002–2004), it was derived that terrestrial models can be applied with very slight deviances in results for transmitter up to altitudes of 1–10 kilometers. For higher altitudes route diversity expressed by diversity gain becomes differentiate (in most cases rapidly decreases) for more than 3 dB. This follows gradually with increasing altitude. Surprisingly, it was observed HAP systems propagation dependences statistical behavior 100
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Figure 3: Dependence of diversity gain on altitude.
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has higher correlation with terrestrial link statistics (correlation coefficient 0.85) than with satellite link statistics (correlation coefficient 0.14). 3. CONCLUSIONS
Rain represents one of the main limitations of millimeter wave band systems regardless altitude of transmitter. In the paper, a comparison of route diversity utilization for terrestrial, HAP and satellite links was accomplished based on actual rainfall radar data. HAP link statistics proved to be more related to terrestrial ones than to satellite statistics. Based on the results discussed in this paper, detailed relationships on altitudes of transmitter will be subsequently derived. ACKNOWLEDGMENT
The simulations and presentation of results were supported by Czech Science Foundation grant 102/08/P346. The research is a part of the activities of the Department of Electromagnetic Field of the Czech Technical University in Prague in the frame of the research project No. OC09075 of the Ministry of Education, Youth and Sports of the Czech Republic. REFERENCES
1. “Specific attenuation model for rain for use in prediction methods,” International Telecommunications Union, ITU-R Recommendation, 838. 2. “Propagation data and prediction methods required for the design of Earth-space telecommunication systems,” International Telecommunications Union, ITU-R Recommendation, 618-8, 2003. 3. “Report from ACTS project 215 — Cellular radio access for broadband services (CRABS),” Propagation Planning Procedures for LMDS, 1999. 4. Usman, I. S., M. J. Willis, and R. J. Watson, “Route diversity analysis and modelling for millimetre wave point to multi-point systems,” 1st Int. Workshop of COST Action 280, Jul. 2002. 5. “COST297 — HAPCOS high altitude platforms for communications and other services,” [Online], Available: http://www.hapcos.org/. 6. “Propagation data and prediction methods required for the design of terrestrial line-of-sight systems,” International Telecommunications Union, ITU-R Recommendation, 530-11, 2005. 7. Zvanovec, S. and P. Pechac, “Validation of rain spatial classification for high altitude platform systems,” IEEE Trans. on Antennas and Propagation, submitted for publication. 8. “Preferred characteristics of systems in the fixed service using high-altitude platform stations operating in the bands 47.2–47.5 GHz an 47.9–48.2 GHz,” ITU-R Recommendation F.1500, 2000. 9. Zvanovec, S. and P. Pechac, “Rain spatial classification for availability studies of point-tomultipoint systems,” IEEE Trans. on Antennas and Propagation, Vol. 54, No. 12, 3789–3796, 2006. 10. “Rain height model for prediction methods, international telecommunications union,” ITU-R Recommendation P.839-3, 2001.
