Impact of Rain Attenuation for Satellite Links at C, Ku, K, Ka and mm Bands in Karachi

Transcription

1 2017, TextRoad Publication ISSN: Journal of Applied Environmental and Biological Sciences Impact of Rain Attenuation for Satellite Links at C, Ku, K, Ka and mm Bands in Karachi Engr. Syed Nauman Ahmed 1, Aamir Zeb Shaikh 2, Shabbar Naqvi 3, Talat Altaf 4 1 Assistant Manager, PTCL, Karachi. Pakistan 2 Assistant Professor, Dept. of Telecommunications Engineering, NED University of Engineering & Technology, Karachi. 3 Dept. of Computer Systems Engineering, Balochistan University of Engineering and Technology, Khuzdar, Pakistan. 4 Professor, Dept. of Electrical Engineering, Sir Syed University of Engineering & Technology, Karachi. Pakistan ABSTRACT Received: July 26, 2016 Accepted: April 23, 2017 The race for higher bandwidth requirement is increasing day by day. This competition is resulting in evolution of newer generations of radios for telecommunications engineering. The requirement of higher bandwidth for end users can be accomplished by dynamic spectrum access (DSA)techniques as well as exploiting RF spectrum higher than 10 GHz i.e. K and mm bands. One of the major challenges to the transmission over these bands is rain attenuation due to smaller wavelength. Hence, counter measures are required to incorporate the issue of rain attenuation in satellite link budget equation. In this paper, ITU-R model is used to evaluate rain attenuation for Karachi city. These values can be used to estimate fade margins that are required to design accurate link budget equations. Estimation of link budgets will also help in exploiting unused RF spectrum at higher bands with greater reliability. KEYWORDS: Elevation angle, rain attenuation, rain fall rate Karachi I. INTRODUCTION 5G communication standard promises various services to end users. These include higher data rates i.e. consensus aggregate data rate to migrate to 1000 times higher from 4G to 5G, improved latency as compared to 4G, lesser cost and energy efficient transmissions[1]. Of these services, data rate is one of the key benefits offered by the next wireless standard. Higher bandwidth requirements by the end-users is resulting in efforts to either opportunistically exploit the available bands (on lower frequency spectrum) through cognitive radio enabled DSA[2] or use higher frequency spectrum including Ka, Ku, V and mm Bands[3][4]. Because, spectrum is free over higher RF spectrum bands. However, the major challenge towards exploiting higher frequency spectrum i.e. over 10 GHz and above includes attenuation due to rain and other environment factors. The major deteriorating factor for operating frequencies > 30 GHz is hydrometeor scattering and hydrometeor absorption for operating frequencies between 10 GHz and 30 GHz. Other factors that contribute towards signal deterioration include cloud attenuation, sky noise, and tropospheric scintillations, inter system interference, hydrometeor absorption and gaseous absorption. Additionally, transmission under low elevation angles also faces higher attenuation due to rain, oxygen and water molecules[5, 6]. The experimental studies show that raindrops play the major role in deteriorating signal quality by attenuating the transmitted signals over the frequencies > 10 GHz. The rain attenuation depends on size, shape and distribution of rain drops, rain temperature, velocity, polarization and rain rate [7]. Additionally, rain attenuation also depends on operating frequency, elevation angle and operating environment of satellite link [8-10]. Thus, prediction and estimation of rain attenuation can help design an improved and better link design for satellite communications over these bands. Additionally, rain drops also produce depolarization of transmitted signals. One of the counter measures used to combat rain attenuation is to exploit site diversity [11]. In this regard, several authors have estimated rain attenuation over different parts of the world. Authors in [7]calculate rain attenuation from C to V Bands for satellite communication links in different tropical and subtropical areas of South Africa. The attenuation is computed using ITU-R model. Additionally, they have also proposed realistic fade margin power for different provinces of South Africa. Authors in [12]estimate the rain attenuation for microwave transmissions in tropical regions of India using exponential model. Furthermore, the simulation results are also compared with ITU R model. Authors [4] predict rain attenuation and rain rate in the form of contour maps using rain fall measurements of past 30 years for Nigeria. The data is collected from coast to arid zones of the country. The rain rate models of the country were developed on the basis of ITU-R models. These models will help satellite communication users to design communication links with higher accuracy and reliability. In this paper, we estimate rain attenuation for Karachi city. The one year data is utilized for the estimation of attenuation. The impact of rain attenuation is calculated for frequencies ranging from C band to mm bands. These estimations will be highly useful to design accurate and reliable satellite communication links from C to mm RF spectrum bands. *Corresponding Author: Aamir Zeb Shaikh, Assistant Professor, Dept. of Telecommunications Engineering, NED University of Engineering & Technology, Karachi. 119

2 Ahmed et al.,2017 The rest of the paper is organized as follows: Computation of Rain attenuation is presented in Section II. Simulation and Numerical results are presented in Section III. Section IV concludes the paper. II. Rain Attenuation on Satellite Bands for Karachi In this section, rain attenuation is computed using ITU model presented in [13]. The computation requires following parameters[7]: f: Operating frequency, GHz θ : Elevation Angle, degrees Φ: Latitude of ground station, degrees h s: Height of ground station above sea level, km Step 1: Determine the slant-length and horizontal projection Using equation (1) and (2) hr hs, forθ > 5 sinθ L 2( r s) S h h, forθ 5 (1) 1/2 2 2( hr hs) sin θ+ + sinθ Re In equation (1) h r represents rain height, h s is the height of earth station antenna, θ is the elevation angle and R e is the radius of earth. Horizontal projection can be expressed as: LG Ls cos( θ) (2) In the equation (2), L G and L S are expressed in km. Step 2:Measure the rain rate, R p in mm/hr. In this paper, rain rate data is taken from measurements done by Pakistan Meteorological Department, Karachi. Pakistan. Step 3: Calculate Specific Attenuation b α ar p db/km (3) In equation (3),a and b are regression coefficients depending upon different polarization schemes. Rp represents rain rate, measured in mm/hr. The regression coefficients of circular polarization i.e. Right hand circular and left hand circular are equal. These coefficients can be related with coefficients of linear polarization schemes as given by equation (4) and (5): ah + av ac 2 (4) ahbh + avbv bc 2ac (5) The values of coefficients for linear polarization schemes under different frequencies can be referred from the table provided in [13]. Step 4:Total rainattenuation can be computed from equation (6) A α L (6) In above equation, A shows total rainattenuation, α is the specific attenuation and L is the effective length of satellite link into rain. Effective path length can be computed using equation (7). L Lsrp (7) r p in equation(7) represents reduction factor. It is function of percent time of the year for which a value has been exceeded. The reduction factors can be referred from the Part of this paper was submitted to first IEEC 2016 Khi, Pakistan. Table 1 [13, 14]:

3 p0.001% Table 1. Reduction Factors r LG 90 p0.01% r L G 180 p0.1% r LG p1% r 1 1 III. SIMULATION AND NUMERICAL RESULTS This section presents the simulation results for the rain attenuation under ITU model for Karachi. Figure 1 depicts the relation of frequency and rain attenuation. In this simulation, it is assumed that the impact of rain on depolarization of transmitted electromagnetic wave (EM) is negligible. Hence, the statistical distribution of the transmitted EM wave is intact over receiver Attenuation vs Frequency Frequency (GHz) Figure 1. Impact of frequency on Rain attenuation The results show the frequency selective behavior of satellite communication channel from 0 to 300 GHz channel. Elevation angle is assumed to be 50 0, height of earth station antenna is 1 km andp 0.1%. This spectrum encompasses all the bands from C to mm band. The figure recommends the transmission of data over frequencies < 3GHz, due to the lower attenuation over this band. Furthermore, the impact of attenuation on different polarization schemes is almost similar.

4 Ahmed et al., x 10-6 Attenuation vs Elevation Angle Elevation Angle ( ) Figure 2.Relation between Elevation angle and Rain Attenuation Figure 2assumes height of earth station antenna 0.5 km, operating frequency 2 GHz and p 0.001%. The observation suggests the preference of higher elevation angles over lower, as can be referred from the plot. For elevation angle < 5 0, rain attenuation is maximum as compared to higher angles. The graph depicts the model of exponential function for all linear and circular polarization schemes Attenuation vs Height of Earth Station Height of Earth Station (Km) Figure 3. Rain attenuation and Height of Earth Station for different polarization schemes 122

5 Attenuation vs Height of Earth Station Height of Earth Station (Km) Figure 3 shows the attenuation component in comparison with height of earth station with elevation angle of 90 0, operating frequency of 110 GHz and p 0.01%. The results clearly suggest the use of higher height of earth station as it produces lower attenuation i.e. almost db for height of 1.4 km for all the linear and circular polarization schemes. Similarly, attenuation due to rain is almost 0.04 db for height of 0.5 km. Additionally, for different height of earth stations, different carrier frequencies and elevation angles, the rain attenuation is also summarized in Table 2. Table 2 Rain Attenuation Measurements for Karachi Height of Earth Station 0.5 Km, Frequency of operation 8 GHz, Elevation Angle 50 ES Height Frequency Elevation Angle Rain Time of Attenuation Polarity (Km) (GHz) ( ) Year (%) (db) Horizontal E Vertical E-06 Circular E-06 Horizontal E Vertical E Circular E-06 Horizontal E Vertical E-06 Circular E-06 Horizontal E-05 1 Vertical E-06 Circular E

6 Ahmed et al.,2017 Height of Earth Station 1 Km, Frequency of operation 40 GHz,Elevation Angle 90 ES Height Frequency Elevation Angle Rain Time of Attenuation Polarity (Km) (GHz) ( ) Year (%) (db) Vertical Vertical Vertical Vertical Height of Earth Station 1.5 Km, Frequency of operation 300 GHz, Elevation Angle 10 ES Height Frequency Elevation Rain Time of Year Attenuation Polarity (Km) (GHz) Angle ( ) (%) (db) Horizontal Vertical Circular Horizontal Vertical Circular Horizontal Vertical Circular Horizontal Vertical Circular IV. CONCLUSION In this paper, rain attenuation for Karachi city is computed using ITU model for both linear i.e. horizontal, vertical and circular polarization schemes. The results suggest the use of higher elevation angles and higher earth station heightsto restrict attenuation losses due to rain. The results will be highly useful for the use of Satellite Communications in RF bands from C to mm. The results can also be used to decide and incorporate appropriate fade mitigation techniques such as site diversity to establish an improved and reliable communications even in the presence of rain. ACKNOWLEDGEMENT The Authors are highly thankful to NED University of Engineering & Technology that provided all the useful resources that were necessary for the completion of this paper. Authors would also like to thank Pakistan Meteorological Department for the provision of the meteorological data that was essential for conducting the research. REFERENCES [1] J. G. Andrews, et al., "What will 5G be?," Selected Areas in Communications, IEEE Journal on, vol. 32, pp , [2] S. Srinivasa and S. A. Jafar, "Cognitive radios for dynamic spectrum access-the throughput potential of cognitive radio: A theoretical perspective," Communications Magazine, IEEE, vol. 45, pp , [3] L. E. Bråten, et al., "One year of 20 GHz satellite measurement data from a nordic maritime environment," in th European Conference on Antennas and Propagation (EuCAP), 2015, pp

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