ECPS 2005 Conference, March 2005, BREST, FRANCE

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2 STUDY OF AUTOMOTIVE RADAR SYSTEMS PROPAGATION CHANNEL IN THE GHZ FREQUENCY BAND: COMPARISONS BETWEEN SIMULATION AND MEASUREMENTS C. Brousseau, J. Hilairet, L. Le Coq, A. Bourdillon IETR - Institut d' Electronique et de Télécommunications de Rennes UMR CNRS 6164, Université de Rennes RENNES Cedex (France) Christian.Brousseau@univ-rennes1.fr SHORT ABSTRACT Characteristics of the propagation channel fitted beneath a vehicle, in the GHz frequency band, are measured as a function of distance between transmitter and receiver. Attenuation, impulse response and coherence bandwidth are determined and compared to results obtained using a ray-tracing model. Keywords: Channel characterization, propagation, millimetre wavelengths, radar, automotive application, ray-tracing model. 1 INTRODUCTION In the next few years, Autonomous Intelligent Cruise Control (AICC) systems will be integrated to more and more vehicles, in order to control the distance between vehicles and to inform vehicle of a new situation (accident, etc.). These RF systems are expensive and installing such devices on front, back or side of a vehicle may quickly become prohibitive. For this reason, it might be interesting to use a front device system with an electronic preview mirror [1], which would communicate to following vehicles (figure 1). Fig. 1. The pre-view mirror concept [1]. In this case, it is necessary to characterize the propagation channel between the transmitter, fitted beneath the front bumper of the vehicle and a receiver fitted in the same location on the following one, and to compare the measurements to simulated results obtained with a propagation model. 2 MEASUREMENT METHOD The transmission channel parameters are measured by an HP8510 vector network analyser. The transmitting and receiving antennas are rectangular aperture horns placed in horizontal polarisation (figure 2).

3 The network analyser performs a comparison between transmitted and received signals measuring the gain and phase of the propagation channel. Then, the impulse response and the coherence bandwidth of the channel can be determined. Fig. 2. Experimental principle. 3 CHANNEL MODELLING A simple approach to the propagation channel modelling is to consider the reflexion on a perfect flat ground (figure 3). Fig. 3. Propagation channel modelling.

4 This ideal situation is well known, and a good approximation can be obtained [2]. The received electric field E is given by: 2. E E0. 1 i.exp i i where E0 is the received electric field in free space, i and i, the modulus and the phase of Fresnel reflexion coefficient, and i, the difference between direct and reflected paths. The effect of a second reflexion on the underside of transmitting vehicle can be neglected because the antenna is closer to the underside than to the road. Small additional fluctuations of received power (max. of 1 db) are observed on a distance equal to double the car length. Also, in real driving conditions, ground isn t perfect, but rough. In these frequency bands, the average irregularity height H for bituminized road is close to the wavelength. The electromagnetic field can be scattered in many other directions (figure 4). Fig. 4. Rough surface effect on wave propagation. In order to take road irregularities into account, the phase difference due to two different wave trajectories can be calculated. If this phase difference is negligible (i.e. smaller than ), the surface could be considered as perfectly plane, without scattering. In order to establish whether the roughness is negligible, the Rayleigh criterion can be used to determine the critical asperity height Hlim [3]: H lim 8.cos where is the incidence angle, which can be express as a function of distance between transmitter and receiver, and is the wavelength. The figure 5 shows an example of variation of critical asperity height as a function of distance between transmitter and receiver for different vehicle ground clearance (GC), at a frequency of 77 GHz. For vehicles with a high ground clearance (for example around 30 cm for a bus) and more than 5 meters behind the transmitter, Hlim is larger than the average asperity height of a bituminized road (around 5 mm). For a majority of vehicle more than few meters behind the transmitter, a typical bituminized road can be considered as perfectly plane.

5 GC=11cm GC=17.5cm GC=27.5cm Crital height H lim (mm) E/R Distance (m) Fig. 5. Critical asperity height as a function of distance between transmitter and receiver for different vehicle ground clearance (GC). Finally, EHF waves interact with atmospheric molecules, particularly oxygen and water vapour, causing power attenuation during propagation (figure 6) [3, 4]. Attenuation GHz GHz Oxygen 0,12 db/km 0,04 db/km Water vapour 0,3 db/km 0,45 db/km Drizzle (0,25 mm/h) 0,3 db/km 0,36 db/km Lashing rain (5 mm/h) 3 db/km 3,4 db/km Thunderstorm (100 mm/h) 30 db/km 32 db/km Fig. 6. Usual attenuation of atmospheric gas in the EHF band. In the propagation model, the attenuation can be consider as constant in the frequency band and equal to 0.12 db/km (oxygen) and 0.3 db/km (water vapour). 4 OUTDOOR EXPERIMENT 4.1 Car to car transmission The complex transfer function has been measured in horizontal polarisation, under a small car (Peugeot 206) on a distance up to 50 meters (figure 7). The main characteristics of this vehicle are: Width : 165 cm, Length: 383,5 cm, Ground clearance height: around 10 cm.

6 Fig. 7. Outdoor experimental setup. The antennas are placed at a height of 7.5 cm. In order to align the emitter and the receiver, two laser beams on each side of the transmitter and targets on the receiver, have been installed. An example of magnitude and phase of the transfer function measured in the GHz frequency band is presented in figure 8. Fig. 8. Magnitude and phase of the transfer function of the channel for a transmitter / receiver distance of meters GHz frequency band.

7 The corresponding impulse response is presented in figure 9. Line of sight propagation produces the main peak at a distance of meters. Figure 10 shows a comparison between measured and simulated attenuation at a frequency of 77 GHz, as a function of distance. Fig. 9. Impulse response of the channel for a transmitter / receiver distance of meters (Peugeot 206) GHz frequency band. Fig. 10. Attenuation as a function of distance between transmitter and receiver f = 77 GHz. Fading effects are observed near the rear of vehicle. For larger distances, the channel attenuation corresponds to free space propagation. The frequency correlation function has been determined from measurements (figure 11) and then used to estimate the coherence bandwidth: Around 140 MHz for a coherence factor of 90 %, Around 500 MHz at 70 % (-3 db), Around 950 MHz at 50 %.

8 Fig. 11. Frequency correlation at a distance of meters between transmitter and receiver GHz frequency band. 4.2 Truck to truck transmission In this case, the main characteristics of the vehicle are: Ground clearance height: 30 cm, Antenna heights: 27.5 cm. Figure 12 shows the simulated results of the variation of attenuation as a function of distance, at a frequency of 77 GHz,. Fig. 12. Attenuation as a function of distance between transmitter and receiver Truck to truck transmission f = 77 GHz.

9 Fading effects are observed for longer distances (maximum 40 meters) than car-to-car transmission. For upper distances, propagation is equivalent to line of sight transmission. In figure 13, the frequency correlation function is represented as a function of distance and frequency shift. For distance greater than 40 meters, the coherence bandwidth is very important suitable to high bit rate digital communication. Fig. 13. Frequency correlation at a distance of meters between transmitter and receiver Truck to truck transmission GHz frequency band. 5 CONCLUSION The transfer function has been measured in the frequency domain in the GHz band, for propagation under vehicle. Fading effects have been measured for distance close to the rear of vehicle. For larger distances, the channel attenuation corresponds to free space propagation with a coherence bandwidth suitable to high bit rate digital communication. A simple propagation model taking specular reflection on the ground into account, gives results in agreement with the measurements. 6 REFERENCES [1] M. Heddebaut, J. Rioult, M. Cuvelier, S. Ambellouis, M. Saint Venant, A. Rivenq, Technical Evaluation of an Electronic Millimeter Wave Pre-View Mirror, IEEE Vehicular Technology Conference, Boston, USA, 2000, pp [2] J.R. Wait, Electromagnetic Waves in Stratified Media, Vol.3, Pergamon Press, 1962, pp [3] A. ISHIMARU, Wave propagation and scattering in Random Media, Vol.2, Academic Press, 1978, pp [4] L. Boithias, «Propagation des ondes radioélectriques dans l environnement terrestre», Dunod, 1983.