Measurements of single tree attenuation for vehicular satellite communications at X-band

Lars Erling Bråten, Vegard Arneson Norwegian Defence Research Establishment (FFI), NO-2027 Kjeller, Norway, lars-erling.braten@ffi.no, vegard.arneson@ffi.no

Abstract—The objective of this study is to obtain empirical experience with the expected signal degradation due to roadside trees for vehicular satellite communications at X-band. Broadband measurements of single tree attenuation were carried out at six locations at the campus of FFI, Kjeller, Norway. The excess vegetation attenuation was measured, utilising a network analyser, between 7.25 and 8 GHz during dry summer conditions. The species investigated were maple, spruce, lime, pine and birch. The elevation angle resembled typical elevation towards geostationary satellites. Reference measurements were performed besides the trees to calibrate the equipment, enabling extraction of the time, and frequency dependent attenuation variations. The signal component propagation through the trees was found to dominate the received power. The effect on attenuation from the main trunk obstructing the path was compared to measurement through branches only, and found to be significantly higher. The measured attenuation was compared with one existing model, showing a relatively large spread around predicted values. The attenuation (in dB) closely resembled a lognormal distribution. The observed attenuation values are expected to cause degraded communication quality during vegetation shadowing for vehicular satellite communication located close to trees.

Index Terms—vehicular satellite communications, X-band, vegetation attenuation, measurement.

I. INTRODUCTION Vehicular satellite communications, or satellite

communications on the move (SOTM), is receiving considerable interest for provisioning of relatively broadband connections while in motion. The X-band spectrum between 7.25 GHz and 8.4 GHz is considered in this article, especially interesting for governmental utilization. As a preparation for a larger experiment, it was decided to measure single tree attenuation at FFIs campus at Kjeller, Norway (59.8oN, 11.0oE). At this latitude a typical elevation angle towards geostationary satellites is slightly above 20 degrees. Hence, a vehicle with a roof mounted tracking antenna is expected to experience shadowing due to roadside trees quite frequently. The main objective of the current study is to obtain empirical experience on typical attenuation values one might expect when single trees obstruct the path with geometry similar to what one may expect from roadside trees at X-band. This is of importance to determine the expected transfer quality of a communication system with limited power margin to overcome fast fluctuation vegetation attenuation. Of main interest here is a vehicle with a single tracking antenna communicating via a geostationary satellite.

Models for satellite slant path attenuation due to woodland are described in ITU Rec. P-833 [1]. The method includes frequency scaling of the attenuation through vegetation based on the elevation angle and the path length trough vegetation. The recommendation also includes a calculation method for single tree attenuation based ground reflected components, propagation through the tree and diffracted components. The probability of experiencing vegetation along the path, and an empirical roadside tree shadowing model, are described in ITU Rec. P.681 [2]. The models are based on a number of measurements, including for example [3]-[6]. To the authors knowledge, limited works are reported for the frequency range of interest in the current paper.

A description of the measurement set-up and analysis is given in Section II. Section III contains measurement results and comparison with one selected model. A discussion on system performance degradation is given in Section IV, followed by conclusions in Section V.

II. EXPERIMENT AND METHODOLOGY Broadband measurements of single tree attenuation were

taken at the premises of FFI to get an indication of the expected attenuation levels, and possible degradation due to frequency selective fading, for vehicular satellite communications. The measurements were taken over the frequency range 7.25 to 8.0 GHz utilizing horn antennas, with elevation angle close to the expected elevation towards geostationary satellites. The measurement geometry is shown in Fig. 1. The height of the receive antenna is denoted by h1, and the height of the transmit antenna by h2. The horizontal lengths d1 and d2 are the distances from the receive and transmit antennas to the trunk of the tree. The path length through vegetation is denoted by dv. Information regarding the different measurement locations is given in Tables I and III.

Fig. 1. Measurement geometry.

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TABLE I. MEASUREMENTS

Loc. Species No Path d1 d2 h1 h2

1 Maple 1 Branches 1.8 16.3 2.3 10 22 2 Trunk 3 Branches 4.9 16.3 2.3 10 20 4 Branches

2 Spruce 1 Trunk 3.2 6.9 2.3 6.9 24 2 Branches 3 Trunk 5.2 6.9 2.3 6.9 20 4 Branches

3 Lime 1 Trunk 3.0 9.2 2.3 6.3 18 2 Branches 3 Trunk 7.0 9.2 0.6 6.3 19

4 Pine 1 Trunk 2.0 8.0 2.3 12.3 44 2 Branches 3 Trunk 7.5 8.0 0.6 12.3 37 4 Branches

5 Birch 1 Trunk 3.0 11.7 2.3 8.3 22 2 Branches 3 Trunk 8.0 11.7 2.3 8.3 17 4 Branches

6 Birch 1 Trunk 3.0 12.5 2.3 8.7 22 2 Branches 3 Trunk 7.5 12.5 2.3 9.7 17 4 Branches

All distances are in meters, and the elevation angle in degrees. The measurements were all taken with dry vegetation, with the deciduous tree in leaf.

A. Antennas Two horn antennas of the type DRG 4010/A from ARA

Technologies were utilised. The usable frequency range is from 4.0 to 10.0 GHz, and the gain varies between 11.0 and 17.0 dB. The 3 dB beamwidth is between 35 and 20 degrees depending on the frequency. The antenna has an aperture of 13.0x9.4 cm. The estimated gain is about 15 dBi in the frequency range of interest. With the set-up shown in Fig. 2, the transmitted signal will be horizontal linear polarised through the tree. The antennas were lined up by visually aiming the antennas towards each other, and the receive antenna adjusted to maximise the received power.

B. Network analyser and amplifiers The antennas were connected to a vector network analyser of the type ZVC from Rohde & Schwarz. The analyser measured the transfer function at 201 points linearly separated in frequency with a resolution bandwidth of 3 kHz. The sweep time is about 111 ms, resulting in a sampling rate of 9 samples per second at each frequency. The network analyser transmits a sweep signal over a chosen frequency range, resolution and sweep time. The network analyser is controlled from a PC through a GPIB interface, utilising a Matlab script to set up the instrument and logging magnitude and phase results. Two amplifiers are used to increase the dynamic range. One between the ZVC output and transmitting antenna with a

Fig. 2. Picture of the horn antennas.

nominal max gain of 27 dB, and one after the receiving antenna of the type AVANTEK SA81-0500 with a gain of approximately 34 dB. Before measurement start, the set-up was calibrated. It was observed that the external amplifiers gain, as function of frequency, drifted with time from turning them on. This is due to temperature dependent gain, and is one of the main sources of accuracy degradation. The drift was in the order of ±1 dB. With the external amplifiers the set-up tolerated about 80 dB total attenuation before noise started to degrade the results. A somewhat similar set-up is described in [7].

C. Data processing and extraction of excess attenuation For each measurement, two nearby antenna locations were

utilised. A reference line-of sight (LOS) measurement were taken close to the tree, but were the path did not traverse the tree. The LOS results for location 1, measurement no. 1 is shown in Fig. 3. The stability of the channel during the measurement is visible in Fig. 3a. The time averaged attenuation shown in Fig. 3b does show some frequency dependence, but reflects a channel without any major reflectors. The measured attenuation through vegetation at the same location is shown in Fig 4a, and the time averaged attenuation in Fig 4b. The main attenuation variation in Fig. 4a is along the time axis. The time averaged attenuation as function of frequency exhibits limited frequency variations with somewhat less variations compared to the LOS case, see Fig. 4b. The reference LOS measurement was utilized to extract the excess attenuation due to vegetation. A constant attenuation, based on time and frequency averaged LOS attenuation, was subtracted from the original tree attenuation

a)

b)

Fig. 3. LOS reference measurement, location 1, number 1 a) Measured time and frequency variation, b) Time averaged frequency dependent attenuation.

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a)

b)

Fig. 4. Measurement orignal attenuation through tree, location 1, number 1, a) Measured time and frequency variation, b) Time averaged attenuation.

to obtain excess attenuation. The resulting excess attenuation is displayed in Fig. 5, where a) displays the time and frequency variation, and b) displays the time dependent, frequency averaged, attenuation. The time variations in the attenuation are assumed to be caused by wind moving the leaves and branches. The normalized power delay profile (PDP) is shown in Fig. 6a, based on the inverse discrete Fourier transform of Fig. 5a,

a)

b) Fig. 5. Excess vegetation attenuation, location 1, number 1, a) Time and frequency variation, b) Time variable - frequency averaged attenuation.

a)

b)

Fig. 6. Measurement power delay profile, location 1, number 1. a) averaged and normalised impulse response / power delay profile.

converting the measured 201 frequencies to excess distance with 0.4 m spacing. For most of the measured PDPs, there are limited time variations. The resulting time averaged PDP is displayed in Fig. 6b. The PDP for location 1, measurement number 1, indicates that the majority of received power is due to the component(s) travelling through the tree. Diffracted components and components reflected from the vicinity of the tree are weak compared to the through component(s).

III. RESULTS

In this section the complete set of measurements is analysed to identify the major propagation degradation factors applicable to a stationary SOTM terminal with a nearby single tree obstructing the LOS path.

A. Power delay profiles

The investigated PDP for one location was displayed in the previous section. The time averaged PDPs for all measurements are shown in Fig. 7. In all cases, the signal

Fig. 7. Time averaged PDPs for all measurements.

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TABLE II. AVERAGE PDP AS FUNCTION OF EXCESS DISTANCE

Excess dist. (m) 0.4 0.8 1.2 1.6 2.0 2.2 2.4 PDP (dB) -7.4 -11.2 -12.3 -13.9 -14.6 -15.3 -15.8

propagation through the tree is the dominating component. Location 5, no. 3 has significant delayed components with 3 dB attenuation at 0.4 m excess distance. Similarly, location 6, no. 3 has a 5 dB attenuated component at 1.6 m excess distance. The average PDP for the shortest distances are displayed in Table II. At the shortest excess distance of 0.4 m, the average PDP is reduces by 7.4 dB compared to the first arriving component. In the following analysis we consider the frequency averaged, time varying excess attenuation, resulting in the assumption of a frequency flat channel.

B. Comparison with a selected model

The model selected for comparison with measured results was designed for satellite slant paths in woodland. This model was selected mainly because of its simplicity, although it is noted that [1] also contains a model for single tree attenuation taking into account reflected and diffracted signal components. The required variables, such as tree height and width, leaf dimensions etc., were unfortunately not available. The attenuation model for satellite slant paths in woodland is given by [1]:

( ) dBECvdBAfL += θ (1)

where f is the frequency in MHz, dv is the vegetation depth, is the elevation in degrees and A, B, C, D, E and G are empirical constants. The estimated excess attenuation in woodland is given in Table III as L (dB), together with path length through vegetation, dv. The estimated attenuation values L are based on calculated values for each measured frequency, averaged linearly over the set of measurement frequencies to obtain comparable results to the measured values M (dB). The actual vegetation distance along the path was not directly measured. Thus, dv is based on the measured vegetation horizontal width, adjusted by taking into account the elevation angle. Relative large deviations are observed between measured and estimated vegetation attenuation, see also Fig. 8. In most cases where the trunk obstructed the path, both the mean and median attenuation exceed the model. The root-mean-square error between the model and the average attenuation is 10.1 dB and 7.7 dB for the median. The relatively large variability indicates that a statistical model might better describe the results.

C. Statistical analysis The first order statistical distributions of vegetation attenuation were extracted from the time series of the excess attenuation. The accumulated complementary cumulative distribution functions (CCDFs) for all 6 locations are shown in Fig. 9. At a 0.1 probability, excess attenuation of 25.2, 30.8 and 29.8 dBs where measured for branches only, trunk paths and all measurements, respectively. The distributions indicate separate ranges of attenuation for

TABLE III. MODEL AND STATISTICAL RESULTS

Loc. No Path m s dv M L

1

1 B 9.29 1.00 2.22 0.09 6.3 9.4 15.0 2 T 14.34 0.90 2.66 0.06 8.4 14.3 16.2 3 B 26.30 5.48 3.25 0.20 11.6 31.3 17.5 4 B 22.53 4.02 3.10 0.16 11.6 26.5 17.5

2

1 T 30.87 1.80 3.43 0.05 6.7 32.9 15.3 2 B 13.18 0.44 2.58 0.03 6.6 13.2 15.2 3 T 29.99 2.89 3.40 0.09 6.5 33.3 15.1 4 B 13.38 0.91 2.59 0.07 6.4 13.5 15.1

3 1 T 19.78 1.00 2.98 0.05 5.3 19.9 14.4 2 B 12.22 2.07 2.49 0.16 6.6 13.1 15.2 3 T 22.52 3.90 3.10 0.15 5.3 27.8 14.4

4

1 T 15.61 0.30 2.75 0.02 5.3 15.6 14.4 2 B 25.77 1.97 3.25 0.07 5.4 26.6 14.5 3 T 20.32 0.87 3.01 0.04 5.0 20.4 14.2 4 B 6.65 0.72 1.89 0.10 5.1 6.7 14.3

5

1 T 22.00 2.52 3.09 0.10 6.6 23.5 15.2 2 B 13.40 2.35 2.58 0.16 6.5 14.5 15.1 3 T 29.01 5.80 3.35 0.20 6.4 33.6 15.1 4 B 20.14 4.88 2.98 0.21 6.8 25.9 15.3

6

1 T 18.83 0.61 2.93 0.03 5.9 18.9 14.8 2 B 11.98 0.54 2.48 0.04 8.7 12.0 16.3 3 T 20.93 2.23 3.04 0.09 9.4 22.6 16.6 4 B 14.38 0.46 2.67 0.03 9.1 14.4 16.5

Fig. 8. Measured and modelled vegetation attenuation.

the different locations. The accumulated histograms for each location, including both trunk and branch paths, are shown in Fig 10. Tests for each of the 23 time series of attenuation (in dB) indicate a reasonable fit to the lognormal distribution with parameters and . These parameters, as well as the means (m) and standard deviations (s) of attenuation (in dB), are given in Table III. The results include both paths through branches and trunks, and represent a roughly equal mixture of

Fig. 9. Measurement distribution of excess vegetation attenuation.

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Fig. 10. Measurement total PDF of excess vegetation attenuation.

the two types of paths. The difference between the mean values M and m are due to averaging of linear and logarithmic values, respectively. Similar findings with respect to the distribution of excess vegetation attenuation are reported in e.g. [7], where also the signal attenuation was found to increase with decreasing distance between receive antenna and the vegetation.

IV. DISCUSSION Link budget power margins are required to investigate the

severity of communication degradation when encountering single tree obstacles. X-band SOTM terminals often have parabolic antennas with diameter of about 0.5 m. The maximum equivalent isotropically radiated power (EIRP) of such a terminal is typically about 40 dBW, and G/T is about 8 dB/K. Table IV shows example power requirements for supporting a 1 Mbps link between a terminal and a hub, with either 13 m or a 3.8 m antenna diameter. The corresponding G/T are 34.5 dB/K and 23.8 dB/K. Binary phase shift keying with ½ rate turbo coding is assumed, with required total Eb/N0 of 2.1 dB and a 1 dB implementation margin. The satellite has one 4x6 degree regional beam (RB) and one 3x3 degree spot beam (SB) with G/T towards the terminal of 1.3 dB/K and 5.4 dB/K.

As seen in Table IV, most of the observed attenuation levels exceed the terminal maximum return uplink EIRP margin in the return (RET) direction. This also applies when the terminal is located in the spot beam, communicating with a large HUB in the regional beam, where the terminal EIRP margin is about 6 dB. In the forward (FWD) direction, it would require a considerable amount of power from the satellite to compensate for the attenuation levels.

TABLE IV. EXAMPLE LINK BUDGET

Beam RB SB HUB in RB - SB Direction FWD RET FWD RET FWD RET

Hub ant. dia. (m) 13 13 3.8 3.8 13 13 Uplink

Pow. (W) 0.84 6.66 2.99 5.64 0.27 2.75 EIRP (dBW) 56.9 37.6 51.8 36.9 52.1 33.8 Attn. (dB) 203.5 203.5 202.8 202.8 203.5 202.8 Eb/N0 (dB) 23.8 4.0 23.0 8.1 19.0 5.0

Downlink Pow. (W) 2.58 0.03 0.83 0.03 0.84 0.01 EIRP (dBW) 29.9 10.6 29.2 14.3 29.3 7.5 Attn. (dB) 202.7 202.7 201.8 201.8 201.8 202.6 Eb/N0 (dB) 3.3 11.1 3.3 4.9 3.4 8.0

I. CONCLUSIONS The objective of this study is to obtain empirical experience

with expected signal propagation degradation due to roadside trees for vehicular satellite communications at X-band. Broadband measurements of single tree attenuation were carried out at six locations at the campus of FFI, Kjeller, Norway. The excess vegetation attenuation was measured between 7.25 and 8 GHz utilising a network analyser, during dry summer conditions.

The species investigated were maple, spruce, lime, pine and birch. The elevation angle resembled typical elevation towards geostationary satellites. Reference measurements were performed besides the trees, to calibrate the equipment, enabling extraction of the time and frequency dependent variation in excess vegetation attenuation.

The signal component propagation through the trees was found to dominate the received power. The effect of attenuation from the main trunk obstructing the path was compared with measurement through branches only, and found to be significantly higher. The measured attenuation was compared with one existing model, showing a relatively large spread around predicted values. The attenuation (in dB) closely resembled a lognormal distribution. The measured X-band excess attenuation values due to closely located vegetation obstacles may represent a significant challenge with respect to maintaining communications for SOTM terminals.

ACKNOWLEDGMENT The measurements were carried out by the student Kristian Bjørke with support from Bjørn Skeie and Jostein Sander.

REFERENCES [1] ITU-R Rec. P.833-7, ”Attenuation in vegetation,” Geneva, 2012. [2] ITU-R Rec. P.681-7, “Propagation data required for the design of Earth-

space land mobile telecommunication systems,” Geneva, 2009. [3] W. J. Vogel and J. Goldhirsh, “Earth-satellite tree attenuation at 20

GHz: Foliage effects," Electron. Lett., Vol. 29, No. 18, pp. 1640-1641, 1993.

[4] F. Teschl, M. Schonhuber, F. Perez-Fontan, V. Hovinen and R. Prieto-Cerdeira, “Slant path attenuation in vegetation at Ku- and C-band,” In Proc. 5th European Conf. Ant. and Prop. (EUCAP), Rome, pp. 3734 – 3738, 2011.

[5] N. C. Rogers et al., ”A Generic Model of 1-60 GHz Radio Propagation through Vegetation - Final Report,” QINETIQ/KI/COM/CR020196/1.0, May 2002.

[6] Basari, K. Saito, M. Takahashi and K. Ito, “Field Measurement on Simple Vehicle-Mounted Antenna System Using a Geostationary Satellite,” IEEE Trans. Veh. Techn., vol. 59, No. 9, Nov. 2010.

[7] Yu Tishchenko et al., “Vegetation Effects on Passive Microwave Measurements,” In Proc. Recent Advances in Space Technologies, pp. 289 - 293, 14-16 June, 2007

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