Automotive Radar Signal Processing
The latest versions of upper class automobiles (e.g. Mercedes S-Class) are equipped with the first serial radar sensors. Abbreviations like ACC (Adaptive Cruise Control) refer to systems which determine distance and speed of the preceding cars by several radar sensors located in the front bumper and then adjust the own speed accordingly.
These systems are presently offered only as comfort systems. Thus braking due to a slower car in front is made only with 20% of the maximum possible brake power. The algorithms used are not yet reliable enough for application as security system which is able to recognize the end of a traffic-jam and to start full braking. Just imagine the case of wrongly interpreting a fix object besides the roadway as an obstacle on the road. The resulting unnecessary full brake would cause a crash of the car coming behind with utmost probability.
A great problem of radar signal processing is the dependency of the radar echo on the form of the target. Plane metal surfaces like the rear of a truck react like a mirror to radar signals. If it is hit vertically it might cause a very strong echo, but if the angle of incidence varies slightly, a great part of the radar energy will be reflected into a completely different direction. Therefore it is very difficult to classify targets only with the radar echo received; for example in order to distinguish between a motor cycle, a compactl car or a truck.
The research activities at the Chair of System Theory and Signal Processing concentrate on multiple topics of future radar sensor systems, such as:
Design of waveforms for radar systems
Baseband processing and parameter estimation (e.g. Direction of Arrival estimation)
Tracking and Classification of detected reflektions.
Especially performance and reliability are in the focus of our work to facilitate the use of radar systems for safety systems.
Automotive Radar History
First experiments in the field of automotive radar took place already in the late 50's. In the 70's, more or less
intensive radar developments started at microwave frequencies.
The activities of the last decades were concentrated mainly on developments at 17 GHz, 24 GHz, 35
GHz, 49 GHz, 60 GHz, and 77 GHz. Even from the early beginning in automotive radar the key driver of all these
investigations has been the idea of collision avoidance; this idea has spent enormous motivation for many engineers
all over the world to develop smart vehicular radar units. During this quite long period a lot of know-how
has been gained in the field of microwaves and in radar signal processing. Accompanied by the remarkable progress
in semiconductor microwave sources (esp. Gunn sources and GaAs MMICs) and in available computing power of microcontrollers and digital signal processing units, the commercialization of automotive radar became
feasible in the 90's , ACC was commercialized for the first time in Japan in 1995. Whereas Lidar-ACC has been favored esp. in Japan, European and US companies have been focused mainly on radar based ACC. In 1999, Mercedes introduced the 77 GHz "Distronic" into the S class, followed by other premium models equipped optionally with an
ACC, such as BMW 7 series, Jaguar, ACC will become an option in the new BMW 3 series and in the new VW Passat, both with start of production in 2005. Whereas European car manufacturers offer 77 GHz systems only for ACC systems so far, their Japanese competitors Honda and Toyota already introduced an active brake assist for collision mitigation (additionally to ACC) in 2003 based on 77 GHz long range radar (LRR) technology. In contrast to the only smooth deceleration capability of an ACC system (because ACC is only marketed as a comfort feature), the active brake assist provides much higher braking forces for deceleration, when a threatening situation is identified and the driver starts braking, but maybe not as strong as it would be necessary to avoid a crash.
The next step in this evolutionary process will be to gain some more milliseconds in advance for reaction and for automatic activation of suitable protection measures. Bosch names this system "Predictive Safety System (PSS)", which will have mainly three stages. The first one (PSS1, to be introduced in 2005) is a preset of the brake system. As soon as a threat will be identified by the 77 GHz LRR, the brake system will be pre-filled, but his won't be noticed by the driver. But when the driver pushes the brake pedal in such a situation, maximum braking forces will be available without any latency.
Short range radar (SRR) sensors for passenger cars will be mounted first in premium class models for recrash ensing, ACC support, parking assistance, and blind spot surveillance. Preferred microwave technology is 24 GHz in ultra wideband (UWB) operation with high range resolution in the range of cm.
Automotive Radar Equipment
The following automotive manufacturers are known to be including automotive radar
devices on vehicles: Daimler-Benz, BMW, Jaguar, Nissan, Toyota, Honda, Volvo and
Ford. Fujitsu, an electronic component manufacturer, is known to be producing
semiconductor devices specifically for automotive radar systems.
A typical automotive radar is mounted behind the front grille of a vehicle at a height of less
than 1 metre, where it is able to interrogate the road ahead and the adjacent traffic lanes
forward of the vehicle's location. Using this radar an activated intelligent cruise control
system within a vehicle adjusts the vehicle's road speed in response to a slower vehicle in a
merging lane, or when following a vehicle in the same lane, in order to maintain the
driver's selected minimum separation distance behind the other vehicle. The intelligent
cruise control system would also sound an alarm and disengage itself if closing speed or
separation distance to an object exceeded safe margins.
SP 4/01 - 2 - April 2001
The following technical characteristics, drawn from detailed technical information (some
of which is proprietary) supplied by the Australian automotive industry, summarise the key
radiofrequency aspects of the automotive radar component of the intelligent cruise control
systems that the industry is keen to make available in this country:
System type: pulse
Frequency Band: 76 – 77 GHz
Emission bandwidth: up to 500 MHz
EIRP (max): up to 20 Watts
Average EIRP: < 10 mW
Antenna beamwidth (max): 4° elevation, 15° azimuth
Frequency Bands for Automotive Radar
Frequency Band(s) Supported Organisation
76 - 77 GHz CEPT (Europe)
76 - 77 GHz ETSI (Europe)
46.7 - 46.9 GHz, 76 - 77 GHz FCC (USA)
60 - 61 GHz, 76 - 77 GHz ITU
60 - 61 GHz, 76 - 77 GHz MPT (Japan)
Table 1 - Frequency Bands supporting Automotive Radar
Propagation Characteristics in the 76 - 77 GHz Band
Radiowave propagation in the EHF range (30 – 300 GHz) is generally line-of-sight; any
obstruction in the optical path is indicative of considerable attenuation in the radiowave
path. With the exception of reflection, line-of-sight conditions are generally required for
successful propagation of an EHF signal.
Within various parts of the EHF spectrum, signals are attenuated by absorption by
atmospheric gases. Absorption by oxygen and water vapour accounts for much of this
attenuation. ITU Recommendation ITU-R P.676-4 [6] provides procedures for calculating
gaseous attenuation for dry air and water vapour. According to ITU-R P.676-4, the
combined attenuation for air and water vapour is 0.3 – 0.5 dB/km within the band
76 - 77 GHz. The range of a typical automotive radar system is 150 metres, or 300 metres
round-trip: attenuation by air and water vapour will account for about 0.09 – 0.15 dB of
signal attenuation. It is reasonable to ignore this loss and apply a free-space propagation
model to this application, ie:
Path Loss = 32.5 + 20 log(fMHz) + 20 log(dkm)
Example: At 76.5 GHz, and for a distance of 100 metres, path loss is 110 dB.
Note: Much of the path loss is attributed to the very high frequency used - over a given
distance a signal at 77 GHz will experience 60dB, or 1 million times, more attenuation that
a signal at 77 MHz.
Christian Argenis Umaña Zambrano
C.i 17678077
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