Improved Model for Building Blockage in Satellite Mobile ...



Wideband Impact of Buildings and Trees on Satellite Mobile Communication Systems

M. S. Al Salameh and M. M. Qasaymeh

Department of Electrical Engineering

Jordan University of Science & Technology

PO Box 3030, Irbid 22110,

Jordan,



Abstract- A propagation model for lossy building with tree attenuation in urban and residential areas is developed for satellite mobile communication systems. This model characterizes the signal transmitted from a medium earth orbit (MEO) satellite when there are buildings and trees in the path of the signal. The analysis is performed using the uniform theory of diffraction (UTD). The tree attenuation is evaluated through the modified exponential decay model (MED). The satellite is assumed to be moving along a circular orbit.

The normalized signal level is computed. Such information is useful in developing the mobile system’s hand-off algorithm. In wide band systems, the delay-spread is dominant because of the inter-symbol interference. For such case, the coherence bandwidth and impulse response were computed.

Key-Words: Satellite, Propagation, Building, Tree attenuation, Diffraction, MEO.

1. Introduction

Signal propagation in land mobile satellite (LMS) communication systems has for the last decade become an essential consideration. Statistical approaches were used in modeling the signal propagation [1]. The input data and computational effort are simple, as the model parameters are fitted to measured data. Due to the lack of physical background, such models however, only apply with good results in environments that are very close to the one they have been inferred from. On the other hand, deterministic models provide high accuracy, but they require actual analytical path profiles and time-consuming computations [2,3]. A combination of both approaches has been developed [4].

For the calculation of the ray contributions in deterministic methods, a combination of geometrical optics (GO) and uniform geometrical theory of diffraction (GTD) is applied [5]. Different research works focused on the effects of building on radio channel in satellite mobile communications [6-8]. In [6], the depolarization effect was considered based on measurements of received signal near the building transmitted from antenna placed on a stationary elevated position. Also, wideband effects of building were studied by including only single diffraction and single reflection [7]. Finally, a propagation model for building blockage in low earth orbit (LEO) satellite mobile communication system was presented [8]. This model assumes perfectly conducting walls of the building, and focuses on how to predict the signal level at the mobile near the building. Effects of trees on propagation paths were discussed in [9,10].

In this paper, the effect of lossy buildings on the signal level from MEO satellite system is examined using high-frequency ray-tracing methods. The analysis is performed using the uniform theory of diffraction (UTD). The tree attenuation is evaluated through the modified exponential decay model (MED) [10]. The signal level at the mobile antenna vs. satellite elevation angle is calculated. In wideband systems, the delay-spread is dominant because of the inter-symbol interference. For such case the coherence bandwidth and impulse response were computed to evaluate the performance.

2. Building and Ground Interference

The geometry of the propagation model is illustrated in Fig. 1. A MEO satellite moving in a circular orbit above the surface of the earth descends behind a row of buildings and trees of height and width hb, wb, ht, wt, respectively, with distance xt between them. An omni directional mobile antenna located at height hm above the ground is at distance xm away from the building, so that all the contributions will have the same gain, also assume hm< ht < hb. The satellite elevation angle ( is measured from the negative x-axis. The satellite transmission frequency is 2.1 GHz.

The building is assumed to be lossy dielectric represented by its permittivity and conductivity. The electric size of both building and tree along the z-axis is assumed large. The incident ray from the satellite might undergo reflection from building and ground surfaces, diffraction from building edges, and attenuation by the tree. These processes are illustrated below.

2.1 Incident Field

For soft (horizontal) polarization, the incident electric field Uzi= Ezi is in the z direction while for hard (vertical) polarization, the incident magnetic field Uzi= Hzi is in the z direction [5], thus the field at a reference point is:

[pic] (1)

where Uo is the amplitude of electric or magnetic field for soft and hard polarizations, respectively, ko is the propagation constant in free space, and p’ is the distance of propagation.

2.2 Reflected Fields

Consider uniform plane waves that are incident on a surface with ((, (, () at an angle ( with the normal of the surface. The reflected fields for soft and hard polarizations are given by:

[pic] (2)

where Γs, Γh are the soft and hard reflection coefficients, respectively, that may be found in [11, 12]. The building has conductivity (b=7 S/m and relative permittivity (rb=15, whereas the ground has conductivity (g=0.005 S/m and relative permittivity (rg=15 [12].

2.3 Diffracted Fields

Consider uniform plane wave Ui that is incident on an edge of lossy dielectric material with ((, (, (). The diffracted field at the observation point is:

[pic] (3)

Where Q is the diffraction point, P is the field point, p is the distance from Q to P, and Ds,h are the soft and hard diffraction coefficients of lossy dielectric wedge [11, 12].

3. Tree Attenuation

The shape of the tree is modeled by a triangle with height ht and base width wt. The distance that the wave passes through the tree is denoted by dt. The distance dt will be one of two cases as shown in Fig. 2. Case 1 when the wave passes through the tree without reaching its base, and Case 2 when the wave enters the tree and reflects from its base.

[pic] (4)

where [pic], h is the height at which the ray meets the center line of the tree, and γ is the angle between the incident ray and the horizontal direction 0 ................
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