Modifying the Parameters of Specific Attenuation Model for Prediction Attenuation Induced by Rain at Ka Band in Xi'an, China Shuhong Gong
#1,
Kai Xie
*2,
Xiaoping Li
*3,
Jiying Huang #4, Lina Zhang
*5,
Houbao Shi #6
# School ofScience, Xidian University No.2 South Taibai Road, Xi 'an, Shannxi, China
'
[email protected] '
[email protected] [email protected]
*School ofElectronical & Mechanical Engineering, Xidian University No.2 South Taibai Road, Xi 'an, Shannxi, China 2
[email protected]
[email protected] [email protected]
Abstract-A simple and effective method for evaluating the parameters of specific attenuation model induced by rain is presented. The parameters at the frequencies from 20GHz to 50GHz are given using the specific attenuation calculated by equivalent permittivity model under the measured raindrop size distribution and the method presented in this paper, which are more applicable in Xi'an, China. The parameters given in this paper are compared with those recommended by ITU-R. And the results
are
discussed
and
analyzed.
The
method
and
the
parameters are significant for more accurately estimating the attenuation by rain.
I. INTRODUCTION The communication work at Ka band will be the mainstream of future wireless communication systems , because of its advantages, for example high capacity, narrow beam, small terminal, stronger anti-jamming capability, good electromagnetic compatibility, small size of facilities etc. [1-41. One of the difficulties of the communication system working atKa band is to estimate and to overcome attenuation induced by troposphere, for example attenuation by cloud, rain, snow, fog and so on. The attenuation by rain is the severest, which even attains tens of decibels, affects stability and validity, and can bring communication interrupt. Attenuation by rain is one prominent problem, which restricts the developing and application ofKa band communication system [1-41. The precision of estimating attenuation by rain depends on accurately evaluating specific attenuation and equivalent path length, and the models of forecasting attenuation by rain are studied by many scholars from different countries, for example discussed in [1]-[11]. Calculating equivalent path length is mainly based on the space distribution of rain cell. The parameters of specific attenuation model for prediction attenuation induced by rain are decided by temperature, characters of rain drop distribution, frequency, link elevation and polarization etc. The parameters given by different
organizations or investigators are distinct, which is primarily caused by the difference of rain drop distribution [I-Ill. So, It is important to study the parameters of specific attenuation model and equivalent path length for estimating and resisting the effects by rain, for instance attenuation, depolarization and additive noise etc.. A simple and effective method for evaluating the parameters of specific attenuation model induced by rain is presented. The parameters at the frequencies from 20GHz to 50GHz are given using the specific attenuation calculated by equivalent permittivity model under the measured raindrop size distribution and the method presented in this paper, which are more applicable in Xi'an, China. The parameters given in this paper are compared with those recommended by ITU-R. And the results are discussed and analysed. The method and the parameters are significant for more accurately estimating the attenuation by rain.
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II. THE MODEL OF CALCULATING SPECIFIC ATTENUATION The model of calculating specific attenuation is written as
(1), which is provided by Olsen etc.[l2I. r
=
b arR r
(dB/km)
(1)
ar and br in (1) are given as
1 ar ="2[ah+aV+(ah-aV)cos2 Bcos2r]
1 br = -[ahbh+avbv+(ahbh-avbv)cos2 Bcos2r] 2ar
(2)
Where ah, av and bh, bv are the coefficients for different polarized wave, subscript h for horizontal polarization and v for vertical polarization. () in (2) is link elevation and T is polarization angle[l2I. a h, av and bh, bv depend on frequency, environment temperature, rain drop distribution and so on, and mainly depend on frequency and rain drop distribution. There
are many different results for ah, av and bh, bv studied by different organizations and scholars, such as ITU-R, Din and Ajayi, IEEE 802.16cc-99/24 etc., which are based on different rain drop distribution [2,13-15]. Fig. I-Fig. 4 are the results of some available parameters.
Fig.l-Figo4 show that the results given by different organizations or different scholars are obviously variant, which is caused by the different rain drop distribution in different countries or different regions. Fig. I-Fig. 4 also say that it is necessary to investigate the model of forecasting rain induced attenuation for more accurately calculating rain attenuation and for providing technology support of resisting attenuation or other effects induced by rain.
0.45 0.4
--+- Din Ajayi I . lTV �Gamma -L-P M-P
0.35 0.3
.
V
1.257 0.2553 0.247 5.073 0.7978 0.7545
0.05 0.0112 0.0112 1.25 0.1646 0.1429 5.252 0.9328 0.9129
�
10
�
8
0.054 0.0103 0.0103 1.249 0.2638 0.2638 5.398 0.4794 0.296 R
H
V
1.243 0.1762 0.1597 5.279 0.9343 0.8292
0.231 0.0579 0.0579 1.25 0.2793 0.2793 0.232 0.0475 0.0438
R
H
V
R
H
V
6.028 0.5696 0.4505
15
0.223 0.0486 0.0486 2.007 0.2931 0.2702 6.097 0.4361 0.2975 R
H
V
2.001 0.1351 0.1309 5.997 0.8291 0.7252
20
25
R(mmlh)
30
35
40
45
50
Fig.7 Comparing the specific attenuation at 50GHz
0.251 0.0269 0.0266 2.02 0.4899 0.4899 6.023 1.0588 0.9895
608
Table II shows that the parameters in Xi'an, Chian are different with that recommended by ITU-R. Fig. 6 and Fig. 7 show that the specific attenuation using the parameters in table II for the same rain rate is observably different. And, the difference is more severer with more higher frequency and more stronger rainfall, For example, Fig. 7 shows about 6dB difference for rain rate of 50mmlh, and about 3dB difference for rain rate of 25mmih, which proves that the difference can not be neglected, and it is significant to estimate the parameters in different countries or different districts. V. CONCLUSIONS
ACKNOWLEDGMENT
This work has been supported by "the Fundamental Research Funds for the Central Universities"
[5]
[6]
[8]
[9] [10] [11] [12] [13] [14]
[15]
REFERENCES
[2]
[4]
[7]
The results in this paper prove that the parameters in Xi'an, China are different with that recommended by ITU-R., the specific attenuation using the parameters in table II for the same rain rate is observably different, and the difference is more severer with more higher frequency and more stronger rainfall, For example, Fig. 7 shows about 6dB difference for rain rate of 50mmih, and about 3dB difference for rain rate of 25mm1h. The results also prove that the difference can not be neglected, and it is significant to estimate the parameters in different countries or different regions. The further working at more frequencies based on more plenty of measured data for improving estimated precision is being done.
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