advertisement

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, http://www.just.edu.jo 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 delayspread 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. Consider uniform plane wave Ui that is incident on an edge of lossy dielectric material with (, , ). The diffracted field at the observation point is: U d ( P) D s ,h U i (Q) y Building w1 2.3 Diffracted Fields wb Tree Mobile hb ht hm z xm xt p (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]. Direct ray w2 e jk 0 p x 3. Tree Attenuation wt Fig. 1: Model for building losses and tree attenuation of the signal from MEO satellite. 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: 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. 2 (ht h) tan( ) cos( ) d t ( h, ) ; Case 1 tan( ) cos( )2 sin 2 ( ) (4) wt tan( ) tan( ) d t ( ) ; Case 2 sin( ) tan( ) tan( ) where tan 1 (2 ht / wt ) , 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<γ<90o. Case 1 γ dt (1) U zi (reference) U 0 .e jk0 p 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: E zr s ( ) E zi , H zr h ( ) H zi (2) where Γ , Γ 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]. s h h ht dt Case 2 γ β wt Fig. 2: Triangular tree model. The tree attenuation Lt in dB is estimated through the modified exponential decay (MED) model [10] (5) Lt 0.187 f 0.284 d t 0.588, 200 f 95000 MHz Where dt is in meters. Substituting (4) in (5), Lt will be one of three cases: Lt (h, ) Lt Lt ( ) 1 Case 1 Case 2 elsewhere (6) 4. Formulation of Ray Contributions The ray contributions are: a) Ground reflection then second order diffractions, b) First and second order diffractions, c) First and second order diffractions followed by reflection from the ground, d) Direct wave, e) Ground reflection, f) Ground reflection then diffraction from the building, g) Ground reflection then diffraction from the building then ground reflection, h) Building reflection then ground reflection, i) Building reflection, and j) Ground reflection then building reflection. It is assumed that the mobile will not be inside the tree. The reference ray is the non-attenuated direct component Ei(ref). Thus, to obtain normalized ray contributions we will divide by Ei(ref). Due to space limitations, only the formula for the first ray with soft polarization will be given below. a) Ground reflection then second order diffractions 0<</2 E i (mobile) {gs ( / 2 ).D s (w1 ).D s ( w2 ) e jk0 .( p1 ( hb hm ).sin wb (1cos ) xm . cos ) Lt wb . p1 } (7) where g is the ground reflection coefficient, and s L h, , xtr xm xtM Lt t 1, elsewhere x .( h hm ) h hm h hb t b , tan 1 ( b ) xm xm h hm xtr xt wt / 2, xtM xt . b hb ht p1 (hb hm ) 2 x m 2 Note that Lt in equation (7) is the ratio corresponding to the dB value in equation (5). 5. Wideband Channel Parameters The basic function that characterizes the wideband channel is the impulse response h(t,) [13,14]: h(t , i ) Eio ( wc ) (t i ), i d i / c (8) i where c is the speed of light, di is the length of path i, τi is the time delay along path i, c is the carrier angular frequency, and Eio is the normalized electric field of path i. Applying (8) to the ray contribution in (7), the corresponding impulse response will be: h(t , ) gs ( where 2 ) D s ( w1 ) D s ( w2 ) Lt wb p1 (t ) (9) p1 (hb hm ). sin wb (1 cos ) xm cos / c The coherence bandwidth Bc is a statistical measure of the range of frequencies over which the channel can be considered flat [15]. In other words, coherence bandwidth is the range of frequencies over which two frequency components have a strong potential for amplitude correlation. If the Bc is defined as the bandwidth over which the frequency correlation function is above 0.9, then Bc 1 /(50 rms ) , where τrms is the RMS delay (or delay spread) [13]. In wide band systems, the delay-spread is dominant because of the inter-symbol interference. Thus, a signal with bandwidth larger than Bc is highly affected by the channel. In this case, the channel is said to be frequency selective. 6. Results It was shown in [16] that for midpath-obstacle, there will be 6 dB diffraction loss at the incidence shadow angle (The angle below which, the direct ray from the satellite will not reach the mobile). This result was used to check the validity of the computer programs written to implement the equations presented in this paper. To further check the validity of the formulation in this paper, these programs were used to obtain the results for the LEO satellites [8] where excellent agreement were observed. A typical building in residential environment is chosen to have 14m height and 10 m width. The mobile was at a location 20m away from the building and the antenna height was 3m. A typical building in urban environment is chosen to have 84m height and 20 m width. The mobile was at a location 12m from the building. The tree was at a location 25m from the building, and has 15m height and 1.5 m width. In fact, extensive computations were performed for different building heights, mobile locations, and soft and hard polarizations. Average values of coherence bandwidth are shown in Table 1. From Table 1, coherence bandwidth in residential area is more than that in urban environment, as expected. Table 1: Average coherence bandwidth Bc in MHz in the line of sight (LOS) region Soft Hard polarization polarization Urban 10.78 11.21 Residential 34.17 46.51 The normalized signal level at the mobile antenna in residential and urban environments vs. the elevation angle was computed for soft polarization, Fig. 3 and Fig. 4. It is seen from these figures that the variation of the signal level is faster in urban area as compared with residential area. Moreover, the range of angles over 10 1 Impulse response Normalized field strength (dB) which the signal level is significant is larger in residential area in comparison with urban area. 5 0 -5 -10 -15 -20 0.6 0.4 0.2 -25 -30 0 -35 -40 0 20 40 60 80 100 120 140 160 180 Elevation angle Impulse response 0 -5 -10 -15 -20 50 100 Time in ns 150 200 1 10 5 0 Fig. 5: Impulse response at 120o satellite elevation angle for hard polarization in urban area. Fig. 3: Normalized signal level in typical residential area vs. satellite elevation angle for soft polarization. Normalized field strength (dB) 0.8 0.8 0.6 0.4 0.2 -25 -30 0 -35 -40 0 20 40 60 80 100 120 140 160 180 Elevation angle 0 50 100 Time in ns 150 Fig. 6: Impulse response at 120o satellite elevation angle for hard polarization in residential area. Fig. 4: Normalized signal level in typical urban area vs. satellite elevation angle for soft polarization. The impulse responses for residential and urban environments at 120o satellite elevation angle are shown in Fig. 5 and Fig. 6. It is observed from these figures that the number of significant multipath contributions is larger in the urban case, which implies higher fading level. Note also that for the dimensions chosen for this figure, the tree in the residential case intercepts the direct ray (at the reference 0 ns) while it does not intercept the direct ray in the urban case. This explains why the direct component amplitude is less than unity in the residential case. 7. Conclusion Mobile communications using MEO satellites were considered. The received signal at the earth is the combination of multipath signals. The multipath phenomenon results from diffractions and reflections from the buildings and ground in addition to attenuation of the trees nearby the buildings. The building and ground materials are assumed lossy which required the use of special diffraction and reflection coefficients for lossy structures. The multipath signals were described by formulas for each contribution. Computer programs based on these formulas were written to find the received signal level at the mobile and to compute other parameters for wideband channels. Typical building dimensions in both residential and urban areas were considered. To show the usefulness of the analysis in this paper, various results were presented for signal level, coherence bandwidth, and impulse response. References: [1] C. Loo, ”A statistical model for land mobile satellite link”, IEEE Transactions on Vehicular Technology, Vol. 34, No. 17, pp. 122-127, 1985. [2] H. L. Bertoni, W. Horcharenco, L. R. Maciel and H. H. Xia, “UHF propagation prediction for wireless personal communications”, Proc. IEEE, vol. 82, p. 1333, Sept. 1994. [3] A. G. Kanatas, I. D. Kountouris and G. B. Kostaras "A UTD propagation model in urban microcellular environments ", IEEE Transactions on Vehicular Technology, Vol. 46, No. 1, pp. 185-193, Feb. 1997. [4] S. R. Saunders, C. Tzaras, C. Oestges, and D. Vanhoenacker-Janvier, "Physical-statistical modelling of the land mobile satellite channel", First International Workshop on Radiowave Propagation for SatCom Services at Ku-band and Above, pp. 95-102, The Netherlands, 28-29 Oct. 1998. [5] W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, John Wiley, New York, 1998. [6] S. M. Leach, A. A. Agius and S. R. Saunders "Measurement of the polarization state of satellite to mobile signals in scattering environments", International Mobile Satellite Conference, pp. 134-138, Ottawa, Canada, 16-18 June 1999. [7] C. Oestges, H. Vasseur, and D. VanhoenackerJanvier, "Impact of edge diffraction on the performance of land mobile satellite systems in urban areas", 28th European Microwave Conference, Amsterdam, Netherlands, pp. 357-361, Oct. 1998. [8] P. A. Tirkas, C. M. Wangsvick and C. A. Balanis, “Propagation model for building blokage in satellite mobile communication systems”, IEEE Trans. on Antennas and Propagation, Vol. 46, pp. 991-997, July 1998. [9] W. J. Vogel and G. H. Hagn, ”Effects of trees on slant propagation paths,” International Symposium on Advanced Radio Technology- ISART, Colorado, USA, 8-10 Sept. 1999. [10] T. Sofos, N. Markelos, J.Bitsios, A. Petalas, G. Tsoukos and P. Constantine, “Ray tracing for mobile satellite systems”, IEEE Vehicular Technology Conference- VTC, Amsterdam, The Netherlands, 19-22 Sept. 1999. [11] P. D. Holm, “A new heuristic UTD diffraction coefficient for nonperfectly conducting wedges”, IEEE Trans. on Antennas and Propagation, Vol.48, No. 8, pp. 1211-1219, Aug. 2000. [12] S. Y. Tan and H. S. Tan “A microcellular communications propagation model based on the uniform theory of diffraction and multiple image theory”, IEEE Transactions on Antennas and Propagation, Vol. 44, No.10, pp. 1317-1325, Oct. 1996. [13] T. S. Rappaport, Wireless Communications: Principles and Practice, Prentice-hall, New Jersey, 1996. [14] M. F. Catedre, Cell Planning for Wireless Communication, Artech House, Boston, 1999. [15] S. Tabbane, Handbook of Mobile Radio Networks, Artech House, Boston, 2000. [16] R. E. Collin, Antennas and Radiowave Propagation, McGraw-Hill, New York, 1985.