Spatial Availability in Satellite-To-Indoor Broadcasting Communications

SPATIAL AVAILABILITY IN SATELLITE-TO-INDOOR BROADCASTING COMMUNICATIONS

Nuan Song1, Giovanni Del Galdo1, Marko Milojevic´1, Martin Haardt1, Albert Heuberger2

1Communications Research Laboratory, Ilmenau University of Technology, P.O. Box 100565, 98684 Ilmenau, Germany, {nuan.song, giovanni.delgaldo, marko.milojevic, martin.haardt}@tu-ilmenau.de,

2Fraunhofer Institute Integrierte Schaltungen IIS, Am Wolfsmantel 33, 91058 Erlangen, Germany, [email protected]

ABSTRACT polarization diversity. With high power broadcasting satellites becoming techni- To analyse the satellite-to-indoor channel, either empirical cally feasible, satellite-to-indoor propagation has drawn in- measurements can be performed or deterministic channel creasing interest. This contribution investigates the spatial modeling methods such as ray-tracing can be used. In order availability of satellite links inside rooms in particular in an to achieve a good reception quality, it is crucial to know office environment. The static satellite-to-indoor channels the propagation conditions inside a room. In the litera- are obtained by a 3D ray tracing engine and by a geometry- ture, only a few measurements are available dealing with based channel modeling tool. Different antenna configura- the performance of different receive antennas strategies, es- tions are considered at the receiver: single as well as multi- pecially with respect to polarization diversity. Measurement ple antennas with different polarimetric beam patterns. The results in [1] show that the received electromagnetic field is results show that the additional antennas placed at the re- strongly diffused in the interior of a building. Therefore the ceiver reduce the spatial variability of the channel, leading received power does not depend on either the gain or the to much lower transmit power that is required in order to polarization of different types of antennas. At frequencies obtain the same availability. If the power is kept the same, close to those assigned to the future Galileo navigation sys- the availability increases significantly. tem (1.1 GHz - 1.6 GHz), it was confirmed in [2] through both statistical and ray tracing channel modeling based on 1. INTRODUCTION the measurements that wall attenuation influences the indoor coverage the most. The MAESTRO project [3] car- In future mobile satellite communications, broadcasting, ried out the satellite-to-indoor measurements in Erlangen and navigation systems, the knowledge of the satellite-to- and Athens by using the Worldspace Afristar satellite (48 indoor propagation conditions is becoming more and more dBW) in the L-band. From the local Cumulative Distribu- important with the increasing needs for the delivery of tion Function (CDF) of the building penetration loss it was multimedia content such as audio or video to the termi- concluded that the percentage of positions where a satellite nal, for example, in Satellite Digital Multimedia Broadcast signal can be received lies in the range of 30 % up to 80 % (SDMB) systems. Satellite broadcast systems are of high with corresponding link margins of 10 dB to 16 dB. With interest for future mobile communications because of high the help of simulations based on a 3D ray tracing tool it is transmission power and large radio link margins. For in- shown in [4] that the polarization state of the propagation stance, the Effective Isotropic Radiated Power (EIRP) of the wave changed significantly inside the room. satellite planned for the future SDMB service will be rather Performing the measurements is very time-consuming and high, in the range of 72 dBW and higher, and existing newer expensive. Moreover, measurement data always represents satellites like the XM Radio satellites offer link margins of a specific scenario and its propagation conditions which up to 18 dB, which makes the reception from satellite to limit the flexibility of measurements-based channel mod- indoor possible. Moreover, since in mobile reception only eling. Therefore, in our work we apply deterministic chan- time diversity (using long interleavers) has been employed nel modeling based on the 3D ray-tracing WinProp simula- so far, there is still some room for improvement in stationary tion tool developed by AWE Communications [5] and the or portable reception (e.g., inside rooms) by using spatial or IlmProp, a flexible geometry-based MIMO channel modeling tool developed at Ilmenau University of Technology [6], as well as other objects such as desks and cupboards can be which supports different receive antenna patterns. modeled with planar elements that have an arbitrary number This paper is organized as follows: First, we briefly de- of corners and individual material properties. Doors and scribe the features and functions of the WinProp followed windows can be created as subdivisions inside walls. The with the proposal of an office environment scenario and the proposed office scenario consists of a closed room (of size creation of the indoor database, i.e., its geometrical mod- 5.6m × 6.4m × 3m) with windows, doors, and some fur- eling. Then by introducing the IlmProp, we describe the niture such as tables, closets, and chairs. Table 1 describes satellite-to-indoor channel model considering polarization the structure and material parameters [7] of the room. An and receive antenna patterns. Finally, we compare the re- overview of the indoor scenario is shown in Figure 2. sulting power values at the receiver using different antenna Table 1. Material properties of indoor scenario configurations with respect to correlation and evaluate the Object Material Reflection Diffraction spatial availability of signals inside the room. (Thickness) Loss (dB) Loss (dB) Wall Brick (30cm) 9.52 22 Roof/Floor Concrete (30cm) 7.51 21 2. THE SATELLITE-TO-INDOOR CHANNEL Window Glass (5mm) 7.53 21 MODEL Windowsill Clinker (20cm) 9.94 22 Door Wood (15cm) 15.09 24 For satellite broadcast systems, the reception quality is of Table/Closet Wood (5cm) 11.43 22 Chair Wood (10cm) 15.09 24 prime interest especially in indoor scenarios. Investigations of the improvement in reception quality focus on signal availability and coverage given different antenna configura- 4 4 tions considering polarization issues. In our work the chan- 1 2 nel fading properties are assessed by using the WinProp [5] 2 to obtain the propagation paths, and the IlmProp [6] to re- 1 trieve the channels for different antennas. Figure 1 shows the satellite-to-indoor channel modeling framework and the 1 spatial availability analysis. 3 3

6 5

Fig. 2. Office layout (1-chair; 2-desk; 3-closet; 4-window; 5-door; 6-wall) Path Parameters 2.1.2. Satellite-to-Indoor Realization IPlm rop The calculations of the satellite-to-indoor scenario were car- ried out using 3D ray tracing in WinProp. For the propagation paths, transmission, reflection, and diffraction are taken Channel Coefficients into account. In [8], [9], and [10], WinProphas already been validated with the help of measurements. The elevation of geostationary satellites in the area of Eu- rope lies in the range of 33 ◦ and 45 ◦. In this satellite-to- indoor scenario, the elevation is set to 40 ◦. A circular polarized satellite signal (EIRP assumed to be 48 dBW from Fig. 1. Block diagram of our analysis the satellite) with carrier frequency 2.0 GHz is received by a stationary receiver through a window in a closed room. The signals undergo frequency flat fading. The described indoor scenario was used to evaluate the field 2.1. The Ray Tracing Engine strength coverage. The satellite antenna was simulated by 2.1.1. Indoor Scenario Definition a vertical omni-directional antenna set far enough from the room to obtain the approximately planar wavefronts. Fig- An indoor database was generated with WinProp [5]. It uses ure 3 shows the predicted receive power with resolution a full 3D vector data base to describe the scenario. Furniture 8 cm at a height of 1 meter and also ray paths. the multipath propagation. The signal in the area that is

ReceivePower very close to the window includes a strong LOS compo- dBW -145 nent, which determines the total ﬁeld strength as well as -150 the polarization state. However, the diffuse property of the -155 -160 Non Line Of Sight (NLOS) ﬁeld may provide more room -165 for the improvements in reception quality by using low-cost -170 antennas and certain receive diversity schemes which will -175 -180 be discussed in the following section. -185 -190 3. SIMULATION RESULTS AND DISCUSSIONS

The aim of the simulations is to assess the possible improvements of the system coverage when different receive Fig. 3. Receive power prediction (1-meter height and 8-cm antenna configurations are used in satellite-to-indoor com- resolution) with ray paths simulated by WinProp munications. Of primary interest is the NLOS indoor region where exists high variance of the received power. In fact, in 2.2. Geometry-Based Channel Modeling LOS regions the received power is relatively constant and has much higher values than in the NLOS regions (typically The IlmProp [6] developed at TU Ilmenau is a flexible 10 dB higher). This implies that in LOS communications geometry-based propagation model for wireless communi- we cannot gain much from neither polarization nor spatial cations. After the satellite-to-indoor scenario is defined and diversity, and the coverage is determined merely by the av- modeled by the Winprop, the path strength and spatial infor- erage received power. In practical cases, if the availability mation about each path obtained from WinProp are further in NLOS regions is higher than 60 %, then in LOS region processed as scatterer-related parameters. Each path is char- the availability reaches values close to 100%. Therefore, acterized by the positions of the points of interaction and by we restrict our simualtions to investigate diversity in NLOS complex reflection coefficients which determine the attenu- regions. ation. All these parameters are then input into the IlmProp The simulation results presented here are obtained in the which gives us the flexibility to apply different receive an- following order. As explained in Figure 1, the WinProp has tenna patterns at different positions in order to investigate been used to obtain the information of the receive points receive diversity. with resolution 8 cm in the room. For each point, the Ilm- Prop uses this information to produce the channel impulse 2.3. Polarization and Different Antenna Configurations responses in a window of size 2λ × 2λ by using 16 × 16 Uniform Rectangular Array (URA), resulting in 256 sub- Due to the reflection or diffraction by objects in the in- channels. In this way we were able to test the performance door environment, the transmitted signal often arrives at of the following receive antenna configurations: the receiver with more than one path, causing the multipath propagation. If the reflecting objects are oriented such • one vertically polarized dipole antenna. that they are not aligned with the polarization of the inci- • one circularly polarized antenna. dent wave, the reflected wave will experience a polarization • two vertically polarized dipoles with different offsets. shift. Random diffraction will also change the polarization • two horizontally polarized dipoles with different off- of the waves. Since the total signal at the receiver will be a sets. sum of all the multipath signals, the resultant field strength • one vertically and one horizontally polarized dipole an- is comprised of these multipath waves with different polar- tennas placed at the same position. izations. The beam patterns of the antennas are shown in Figure 4. The polarization at the receiver is our main interest because it determines the choice of antennas and the diversity schemes. From the literature [1] and [4] it can be concluded 3.1. Spatial Correlation that in the satellite-to-indoor scenario the electromagnetic First we investigated the spatial correlation between anten- field will be diffuse and the circular polarization will not nas. The complex correlation coefficient ρ of two complex be preserved in the far field away from the window. There- processes H1 and H2 is defined as: fore, in the IlmProp simulation, a random polarization state is given to each path, with the exception of the LOS, which ρ = ∗ ∗ is set to Left Hand Circular Polarization (LHCP). In the fol- H1H − H1 H E[ 2 ] E[ ]E[ 2 ] (1) H H∗ − H H∗ H H∗ − H H∗ lowing discussion, we focus our attention on the effect of q(E[ 1 1 ] E[ 1]E[ 1 ])(E[ 2 2 ] E[ 2]E[ 2 ]) Antenna Patterns nas placed near each other. Anyway, if we took antenna 90 120 60 coupling into account the correlation coefficient would still be low since the Cross-Polarization Ratio (XPR) has usually 150 30 high values as presented in [11]. This result suggests that the use of one vertically and one horizontally polarized antenna placed at the same position can significantly increase 180 0 the coverage of the system. −20

210 −10 330 3.2. Spatial Availability 0 240 300 10 270 The effective carrier-to-noise ratio C/N is the difference eﬀ Circularly Polarized Antenna between the achieved and the reference : Vertical Dipole C/N C/Nref

C/Neff =C/N − C/Nref . (2) Fig. 4. Antenna patterns vs. elevation for a fixed azimuth. The channel fades in the NLOS region where the coverage The beam patterns are symmetric with respect to the vertical should also be achieved. Therefore, the reference is defined axis and the values are expressed in dBi. at the point just outside the window, i.e., in a pure LOS regime. If the loss due to the penetration is higher than this where E denotes the expectation operator, H1 and H2 ob- reference C/Nref , no availability (coverage) is achieved. If tained from IlmProp simulations are the channels of the first we denote the attenuation of the signal inside the room com- ∗ and second antenna respectively, and denotes complex pared to the signal level just outside the window as PLpen conjugation. then the availability A at that point is calculated as: In Figure 5 we show the magnitude of the correlation co- 1, if C/N ≥ PL A= eff pen (3) efficient for different distances between the antennas. The 0, otherwise. notation ’V-V’ stands for the correlation between two vertically polarized dipole antennas, ’H-H’ for the correlation In this contribution we concentrate on the spatial availability between two horizontally polarized dipole antennas, ’V-H’ assuming that the received power is constant in time. The for the correlation between one vertically and one horizon- temporal changes due to the movement of people inside the tally polarized dipole antenna, and ’C-C’ for the correlation room have not been considered. Results on temporal fading between two circularly polarized antennas. can be found in [12] and [13]. The spatial service availability averaged over the whole NLOS region is shown in Fig- ure 6: ’V’ stands for a system with one vertically polarized

V−V receive dipole antenna(the same results are obtained for one 1 H−H horizontally polarized receive dipole antenna), ’V-V’ for a V−H system with two vertically polarized dipole antennas a quar- 0.8 C−C ter of a wavelength apart, ’V-H’ for system with one vertically and one horizontally polarized dipole antenna placed 0.6 | ρ | at the same position, and ’C’ for one circularly polarized an-

0.4 tenna. Maximum Ratio Combining (MRC) is assumed for the simulations with two antennas. 0.2 As shown in Figure 6(a), availability of 30 % is achieved

with a C/Neff between 17 and 21 dB, while 80 % with 0 0 0.5 1 1.5 2 C/Neff of 22 to 28 dB (depending on the receive anten- Antenna distance in λ nas). Usually we are interested in higher availabilities. If we zoom the plot as shown in Figure 6(b) and we look at Fig. 5. Spatial correlation coefficient the results at 99 % availability then a significant difference in the performance can be observed. The best performance The correlation between two antennas of the same kind de- is given by the combination of one vertically and one hori- creases with distance, as expected. On the other hand, one zontally polarized antenna. It outperforms one antenna sys- vertically and one horizontally polarized antenna are com- tem of approximately 12.5 dB. In other words the satellite- pletely uncorrelated even if they are placed at the same po- to-indoor system with one receive vertical dipole antenna sition. Please note that we did not take into account antenna needs 12.5 dB more transmit power to achieve the same coupling. This effect increases the correlation coefficient availability as the ’V-H’ receive antenna. The gain of a ’V- between vertically and horizontally polarized dipole anten- H’ antenna in comparison to one circular polarized antenna 1 1 V V 0.9 V−V 0.98 V−V 0.8 V−H 0.96 V−H C C 0.7 0.94 abscissa abscissa 0.6 0.92 ≤ ≤ 0.5 0.9

0.4 0.88

0.3 0.86 Probability for x Probability for x 0.2 0.84

0.1 0.82

0 0 10 20 30 40 50 20 25 30 35 40 45 C/N [dB] C/N [dB] eff eff (a) Original (b) Zoom

Fig. 6. Spatial availability at 99 % availability is approximately 6.5 dB. Table 2 shows the spatial availability and the corresponding link margins 0.08 for different recievers. Figure 6 and Table 2 can also be 0.07 used if the user wants to determine the coverage inside the 0.06 room at certain given link margin. Here we assume that the antennas are static. In case of mobile receive antennas 0.05 the interleaving and the error correction will lead to higher 0.04 PDF values of the availability in all antenna conﬁgurations, re- 0.03 ducing the difference between them. The improvement of a two antenna receiver over a one antenna system is due to 0.02 the exploitation of spatial diversity and, when available, of 0.01 polarization diversity. 0 0 10 20 30 40 P − P [dB] Table 2. Link margins for 90 % and 50 % availabilities V H Receive Link Margins [dB] Antennas Availability 90 % Availability 50 % Fig. 7. Probability density function of the difference be- ’V’ 31.5 23.3 tween the received power at one vertically and one horizon- ’C’ 27.5 21.4 ’V-V’ 26.5 20.2 tally polarized antenna placed at the same position ’V-H’ 25.0 19.9

In Figure 7 the probability density function of the difference between the received powers on one vertically and one horizontally polarized antenna is shown. It can be observed that the two powers differ signiﬁcantly quite often. This trans- lates into a high spatial diversity, which is exploited by the MRC scheme.

Let (C/Neff )0 be the C/N needed to achieve the spatial availability of 83 % for one vertically polarized received dipole antenna setup. In Figure 8 we show the availability in space for the defined indoor scenario NLOS region and a ’V-H’ receive antennas. The availability for each point plot- ted has been computed by considering 256 points within a window of size 2λ × 2λ. It can be concluded that the availability is quite random in the NLOS region, and does not Fig. 8. Availability as a function of the area position in depend on the distance from the window. The reason is that NLOS regions for V-H scheme (refered to the office layout the rich scattering provides spatial Rayleigh fading and a in Figure 2) few meters difference between the points near the window and the ones far away is not significant with respect to the 5. REFERENCES path loss given by the 36, 000 km which are between the satellite and earth. [1] B. G. Molnár, I. Frigyes, Z. Bodnár, Z. Herczku, Zs. Next, we assume that the user is willing to change the po- Kormányos, J. Bérces, I. Papp, and L. Juhász, “A detailed experimental study of the LEO satellite to indoor channel sition of the receiver in order to improve the signal qual- characteristics,” International Journal of Wireless Infor- ity. In Figure 9 we show the availability under this assump- mation Networks, April 1999. tion for different receive antenna configurations and different number of antenna positions. The constraint is that the [2] F. Pérez-Fontán, B. Sanmart´ın, A. Steingaß, A. Lehner, J. Selva, E. Kubista, and B. Arbesser-Rastburg, “Mea- user always stays in the NLOS region. The availability is surements and modeling of the satellite-to-indoor channel for Galileo,” in European Navigation Conference GNSS

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