15-10-2016, 02:38 PM
PREDICTION OF RAIN ATTENUATION AND COMPARISON OF DIFFERENT MODELS AT VIZAG AND HYDERABAD REGION
1459233794-PREDICTIONOFRAINATTENUATIONANDCOMPARISONOFDIFFERENTMODELSATVIZAGANDHYDERABADREGION.docx (Size: 203.81 KB / Downloads: 5)
ABSTRACT: Satellite communication systems operating at and above Ku band frequencies must overcome the problem of propagation impairments which includes fading due to rain, clouds, snow, gases, and amplitude scintillations, for obtaining the required performance. Among all impairments, rain is the predominant factor that affects the signal most and the mechanism involved is absorption and scattering of the signal energy. The estimation of attenuation on the slant path to a satellite is essential to the process of establishing a margin in the link budget that ensures the required availability of the link is met.
Rain fade is the dominant factor in path loss variation above 10GHz, and can have an effect below that frequency at low elevations. Rain fades vary with frequency, location, polarisation and rainfall rate. Several attenuation models have been developed to overcome this problem.Here, in this project the following models are compared: ITU-R model, DAH model, Excell model, Karasawa model, Leitao-Watson Model.
INTRODUCTION:
Increased need for the usage of higher frequencies band came with the increased demand for bandwidth, availability of wide range spectrum. At higher frequencies, especially at 10GHz, the rain is the main cause for degradation of signal. Operating at higher frequencies results in many advantages for the telecommunication system like: large band width, small device size and increased frequency reuse. Absorption and scattering of radio waves take place with the rain drops which leads to signal attenuation and also reduction in system reliability and availability.
Inorder to make correct prediction of rain attenuation on propagation paths, planning microwave and terrestrial line of sight system links plays a very important role. In tropical regions, measured rain attenuation data is not sufficient to estimate the link. So studies were still needed to be pursued leading to new prediction model. So, it is needed to perform experiments to study the effect of rainfall and impact on the INSAT services. In this study, rain attenuation models are compared within two regions hyderabad and vizag.
EFFECT OF WAVE PROPAGATION DUE TO RAIN
Fog attenuation plays major role in infrared and optical bands whereas rain attenuation dominates in mill metric wave region. Due to changes in lower atmosphere microwave propagation effects a lot. This is due to the presence of hydrometeors like rain, fog, water vapor, and oxygen in radio wave path can produce an extremely significant effect in energy absorption. With increase in rain rate, attenuation in radio link communication is increased, which shows adverse effect at microwave and mill metric frequencies, since EM waves are most effected by scattering and absorption phenomena. Rayleigh and Mie scattering are the main cause of attenuation at higher altitude of the atmosphere.
With respect to these hydrometeors, the path loss due to rain attenuation has been recognized as major obstacles in design of microwave communication link operating at frequencies above 10 GHz.
RAIN ATTENUATION PREDICTION MODELS
ITU-R MODEL:
This model gives estimations of slant path rain attenuation at any location for frequencies less than 55 GHz. On the basis of the data from ITU data bank and by using a satellite with elevation angles from 6º to 82.5º, this model was level (R0.01) to find the attenuation.
The rain attenuation is given by the equation:
Ap=A0.01(p/0.01)^(-(0.655+0.033 ln(p)-0.0451 ln(A0.01)-β(1-p)sinѳ))
For p<1%, Ө>25º and |φ|<36º, β=-0.05 { |φ|-36º} and A0.01=γLE
DAH MODEL:
This prediction model is similar to ITU-R model. It is used for frequencies from 4 to 35 GHz. Inhomogeniety in rain in both horizontal and vertical directions are taken for prediction. Here, it is applicable for the percentage probability from 0.001% to 10%.
The rain attenuation is given by:
Ap=A0.01( p/0.01)^(-(0.655+0.0331 ln(p)-0.045 ln(A0.01)-z(1-p)sinӨ))
EXCELL MODEL:
The basic idea of this proposed model is to describe the precipitation environment by means of “synthetic isolated cells.” These cells are with exponential profile of the rain inside and the rotational symmetry is derived by the study of radar rain maps.
The rain attenuation is given by:
Ap={K*(2^α) for p≤ Pp
Ap=K*(Rp^α) for p>Pp
Here, Pp is the time percentage of exceeding the rain intensity value of 2mm/hr.
Total rain attenuation due to rain,
A_t=k*R^∝*L_s+A_p
LEITAO-WATSON MODEL:
This model is based on the structure of storms that are observed on dual-polarization radar. This is interpreted using scattering theory. This model is based on the frequencies from 10 to 30 GHz.
The rain attenuation at horizontal projection L, of slant path Ls is given by:
A=CA(R,L)*R*Ls,
here CA is path to path conversion factor
CA(R,L)=(1/Ls)∫_0^L▒γ(x) dx)/(R(x=0))