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A low-cost 24GHz Doppler radar sensor for traffic monitoring implemented in

standard discrete-component technology

Conference Paper · November 2007

DOI: 10.1109/EURAD.2007.4404962 · Source: IEEE Xplore

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 Proceedings of the 4th European Radar Conference

A Low-Cost 24GHz Doppler Radar Sensor for

Traffic Monitoring Implemented in Standard
Discrete-Component Technology
F. Alimenti1, F. Placentino1, A. Battistini2, G. Tasselli2, W. Bernardini2, P. Mezzanotte1,
D. Rascio1, V. Palazzari1, S. Leone1, A. Scarponi1, N. Porzi2, M. Comez2, L. Roselli1
1
Dept. of Electronic and Information Engineering, University of Perugia
via G. Duranti 93, I-06125 Perugia, Italy
alimenti@diei.unipg.it
2
WiS s.r.l.
via A. Vici 14, I-06034 Foligno, Italy
matteo.comez@wis_srl.com

Abstract— This paper deals with both the implementation and industrial costs of the microwave front-end by a proper
the real-life characterization of a low-cost 24GHz Doppler radar selection of technological choices and design options. Finally,
sensor, purposely designed for the traffic monitoring. To reduce an experimental characterization is also reported, describing
industrial costs as much as possible a discrete-components the Doppler radar sensor performances both in velocity-
technology has been adopted for the microwave front-end.
measurement and length-measurements operational modes.
Plastic packaged devices and fiberglass reinforced substrate are
used in such a way as to fit with standard PCB manufacturing The result is a very compact, simple and quite inexpensive
processes and automated assembly procedures. The signal system, showing state-of-the-art performances, so thus being a
manipulation is based on a state-machine algorithm and has been good competitor in the traffic monitoring applications market.
implemented in a 8051 family micro-controller unit. The realized
sensor has a typical output power of 6dBm and mounts a planar
antenna with a 3dB beam-width of ±4.5 degrees. The real-life II. SENSOR DESCRIPTION
measured performances shows a detection range in excess of 300 The realized Doppler radar sensor (see Fig. 1) is based on
meters. the well-know quadrature receiver architecture, necessary to
Index Terms— Doppler radar, road traffic control, microwave detect the versus of the motion, [3].
circuits, velocity measurement. A two antennas configuration, one for the transmitter (TX)
I. INTRODUCTION and one for the receiver (RX) has been adopted in order to
avoid the circulator, thus reducing the overall sensor cost.
The fast growing of highways and roadways traffic Each antenna is a patch array composed by 10x4 elements
congestion is resulting in an increasing number of car
accidents. This calls for a higher traffic control efficiency, featuring a gain of about 13dB and a beam-width of ±4.5
since increased driver safety is getting paramount importance. degrees. These antennas have been designed with the parallel
As a response, the transportation industry has focussed its loaded, step-impedance weighting approach reported in [4].
efforts in developing a number of active and passive systems The continuous 24GHz waveform is generated by a single-
in order to enhance the traffic control, monitoring and chip commercial VCO with an integrated (divide by 16)
optimizing traffic flow in urban and sub-urban areas, thus prescaler. The VCO output power is about +10dBm with a
realizing a sort of intelligent road. phase noise of -70dBc/Hz at 10kHz offset from the carrier. A
Exploiting a Doppler radar sensor is a good compromise Wilkinson power divider (1dB loss) splits such a signal in two
between simplicity, robustness and good performances. It can equal parts, one feeding the transmitting antenna and another
easily measure both vehicles true-ground speed and vehicle the receiving channel. The local oscillator signals needed by
length, thus monitoring a high density traffic road. the I/Q mixers are then derived using a 90 degrees branch-line
The ISM 24GHz frequency band has been devoted, junction (0.5dB loss). Note that only one source is used to
together with other applications, to these services, although derive both transmitted and local oscillator signals: as
the use of lower millimetre-wave band still suffers from high demonstrated in [5] this is the key to detect small Doppler
design and production costs, [1]. shifts even in the presence of phase-noise.
This paper describes both development and characterization The signal reflected back by the target, and containing the
of a 24GHz Doppler radar sensor, purposely conceived for information about the radial velocity of the target itself, is first
traffic monitoring applications, [2]. The following sections amplified by a LNA, then divided in two parts and, finally,
will underline the techniques adopted to minimize the addressed to the RF ports of the down-conversion I/Q mixers.

978-2-87487-004-0 © 2007 EuMA 162 October 2007, Munich Germany

 board for the baseband analog amplification (80dB typical)
and digital processing. An 8051 family MCU manages both
real-time detection algorithm and interfacing procedures.

III. TECHNOLOGIES & DESIGN

In order to reduce the industrial costs of the Doppler sensor
as much as possible, some basic technology choices have been
assumed as constraints during the design process.
As a first point, a discrete-component technology based on
packaged SMT devices, has been adopted in all the active
circuitry of the front-end. In this way an automated pick and
place process can be used for the board assembly, with the
advantages of low production costs.

Fig. 1 Block diagram of the realized 24GHz Doppler radar sensor: the I/Q
mixer is adopted to discriminate the motion direction.

The low-noise amplifier uses a plastic packaged Hetro-

Junction FET and is characterized by a linear gain of 10dB, an
input return loss of -15dB and a noise-figure of 1.5dB. The
(a)
comparison between circuit simulations and measured
performances of the LNA is reported in Fig. 2.

14
S21 simulation
12 S21 experiment
NF simulation
10
amplitude [dB]

2 (b)

0 Fig. 3 Photograph of the realized multi-layer 24GHz PCB: (a) antennas side;
23,0 23,5 24,0 24,5 25,0 (b) microwave front-end side.
frequency [GHz]
The second point is the choice of a fiberglass reinforced
Fig. 2 Small-signal gain and noise figure of the 24GHz low-noise amplifier; microwave substrate, along with a multi-layer technology.
comparison between measurement and circuit simulations. The cost of this kind of substrates, indeed, is less than that of
The I/Q mixers have been designed exploiting a single- PTFE materials, moreover they can be easily inserted in a
balanced configuration based on the microstrip 180 degrees consumer fabrication process, typically tuned for FR4
rat-race junction, [6]. To save substrate area, the two diodes substrates. From the manufacturing point of view, the main
and the relevant radial stubs have been placed within the ring, advantage of fiberglass materials is a high finishing quality of
leading to a very compact layout. A 5.5dB state-of-the-art the drilled-holes and a superior yield in realizing multi-layer
conversion loss has been achieved at 24GHz exploiting low- structures.
barrier Schottky diodes and properly matching both LO and As a result (see Fig. 3) a complete integration between
RF ports. The LO power to optimally driving the mixer is antenna and microwave front-end has been achieved. These
about +1dBm. two circuits share the same ground plane. For each of the two
The IF signals of the I/Q mixers constitute the outputs of antennas the feeding point is connected to the front-end by
the front-end module. These signals enter a mixed-signal means of a via-through transition that crosses the ground

163
plane. The latter has been properly discharged and the size of where α is the angle formed by the line connecting moving
such a ground-plane opening has been exploited as a design object and radar sensor with respect to the road axis. This
parameter to match the via-trough transition. Experimental angle is time dependent since, as the object travels toward the
results of the antenna connected to such a transition show a sensor, the distance x is reduced while the offset S remains
return-loss better than -16dB. constant (see Fig. 4). With the above model, the true object
The final circuit does not needs for any tuning operation, velocity can be retrieved combining (1) and (2):
thus saving the corresponding production costs. It can be c0 (3)
interfaced to both a personal computer via RS232 serial v= ⋅ fD
2 f 0 cosα
connection, or to a radar network by means of a IEEE 485
Assuming a 5% maximum tolerable discrepancy between
bus. The current consumption of the whole sensor (microwave
front-end and mixed-signal processing unit) is about 100mA vρ and v, i.e. cos α = 0.95, one obtains that the radar readout is
at 12V supply. acceptable until x is reduced down to about 3S. In the real
practice S is less than 5m and vρ can be assumed equal to v
IV. RESULTS until the distance between moving object and sensor is greater
than about 15m.
This section describes the results obtained during the real-
A typical velocity measurement result is shown in Fig. 5,
life characterization of the 24GHz Doppler radar sensor in
where the initial time (0s in the graph) correspond to the begin
both velocity-measurement and length-measurement
of the data acquisition, i.e. to the maximum distance between
operational modes. All the presented results have been
moving object and the radar sensor. The car under observation
obtained from the same measurement site: a straight road
was a FIAT Panda and is velocity vary with time in according
segment about 400m long in sub-urban area, with a road cross
with true acceleration and deceleration of the car itself.
at both ends.
A. Velocity-measurement mode
The velocity-measurement mode is the basic operational
mode of the Doppler radar, and is used to determine the
velocity of any objects moving toward the sensor. In this case
the antenna beam is pointed in such a way as to be parallel to
the road direction, as shown in Fig. 4.

Fig. 5 Measured velocity in km/s as a function of time: t=0s indicate the time
instant when the car has been seen by the doppler radar sensor.

The steps in the ascending ramp correspond to the

Fig. 4 Experimental set-up adopted during the real-life characterization of the transition between second to third gear and third to fourth gear
24GHz Doppler radar sensor in the velocity-measurement mode.
and have been correctly detected by the sensor. From the
According to the well-known Doppler effect, the frequency analysis of this figure one can clearly see that the 50km/h
shift measured in tracking mode - fD - is proportional to the velocity limit has been surpassed: this information can be used
radial velocity of the target vρ: to activate a warning display placed along the road.
Once the velocity has been measured with respect to time it
2 f0 (1)
fD = ⋅ vρ is possible to numerically integrate (post-processing) the
c0 motion equation in such a way as to obtain the distance x:
In this equation f0 is the carrier frequency of the continuous t tF
wave transmitted signal and c0 is the velocity of the light in a x(t ) = ∫ v (τ ) ⋅ dτ − ∫ v (τ ) ⋅ dτ (4)
vacuum. For f0 = 24GHz the first factor of (1) assumes the 0 0
constant value of about 160Hz per m/s or, equivalently, 44Hz
per km/h. The radial velocity vρ is related to the true object Fig. 6 is obtained applying (4) to the results in Fig. 5 (final
velocity by elementary trigonometrics: time tF = 25s) and eliminating the temporal variable between
space x(t) and velocity v(t). A maximum radar detection range
v ρ = v ⋅ cos α (2) of about 350m can be estimated from this figure.

164
 2 f 0 sin β (7)
N D = tC ⋅ f D = ⋅l
c0
The relationship (7) can easily be corrected in such a way
as to consider a non-zero antenna beamwidth. This has the
effect of increasing the apparent object length by a term lF:
 θ   θ  (8)
l F = S tan β + 3 dB  − S tan β − 3 dB 
 2   2 
being S the displacement between the radar position and the
motion line (see Fig. 7). As a consequence (7) become:
2 f sin β
ND = 0 ⋅ (l + l F ) (9)
c0
To verify the performances in length-measurement mode,
the sensor has been pointed with an angle β=π/4 with respect
to the road direction. The displacement S of the sensor was
5m, while the antenna footprint lF has been estimated in about
Fig. 6 Measured velocity in km/h as a function of the relative distance
between car and radar sensor: the reduction of the velocity in the proximity of 1.6m assuming a beamwidth of ±4.5 degrees.
the sensor is due to a true deceleration of the car.
TABLE I
B. Length-measurement mode LENGTH-MEASUREMENT EXPERIMENTS

The length-measurement mode is used to determine the ND error

car type length
length of a moving object crossing the antenna beam as [m] ε%
illustrated in Fig. 7. This operational mode is particularly theory measure
useful in traffic statistics since one can both count the number
Volkswagen Golf 4.20 650 665 2.25
of vehicles flowing in a certain road and classifying them (i.e.
distinguish between car, truck, etc.) from their length, [7]. Ford Focus 4.49 680 698 2.57

The resuls of this experiment are quoted in Tab. I, showing

a very good agreement between the number of Doppler pulses
predicted by (9) and those effectively measured by the sensor.
In all the cases the reported error is less than 3%.

V. CONCLUSIONS
A low-cost Doppler radar sensor, purposely designed for
traffic monitoring applications, has been developed. It uses a
discrete-component technology as well as a multi-layer PCB
realized with fiber-glass reinforced microwave substrate. The
sensor has a typical output power of +6dBm and a current
Fig. 7 Experimental set-up adopted during the real-life characterization of the consumption of 100mA at 12V supply (digital unit included).
24GHz Doppler radar sensor in the length-measurement mode. Real-life characterization experiments show a detection range
in excess of 300 meters in the velocity-measurement mode
Let first consider the case of an infinitely narrow antenna and an error less than 3% in the length-measurement mode.
beam, i.e. θ3dB=0. The Doppler frequency shift seen by the
REFERENCES
sensor is related to the set-up pointing angle β:
[1] P. Heide et al., “Coded 24GHz Doppler radar sensors: a new approach to
2 f sin β (5)
fD = 0 ⋅v high-precision vehicle position and ground-speed sensing in railway and
c0 automobile applications,” in Proc. of the IEEE MTT-S, pp. 965-968, 1995.
[2] L. Roselli et al., “A cost driven 24GHz Doppler radar sensor
This frequency is directly proportional to the velocity v of the development for automotive applications,” in Proc. of the 35th European
object. On the other hand, the time tC needed by the same Microwave Conference, pp. 335-338, 2005.
object to cross the antenna beam (slanted line in Fig. 7) is [3] B. Edde, “RADAR: principles, technology and applications”, Prentice
proportional to the length l of the object and inversely Hall, 1992, pp. 492-494.
[4] R.S. Elliot, “Antenna Theory and Design”, Prentice Hall, 1981.
proportional to its velocity: [5] A. D. Droitcour et al., “Range correlation and I/Q performance benefits
l (6) in single-chip silicon Doppler radars for non-contact cardiopulmonary
tC =
v monitoring,” in IEEE Trans. Microwave Theory and Tech., vol. 52, no. 3, pp.
838-848, Mar. 2004.
This means that the product between fD and tC, i.e. the number
[6] S. A. Maas, “Microwave Mixers”, Artech House, 1986.
ND of Doppler pulses at the output of the sensor, is [7] S. J. Park et al., “A novel signal processing technique for vehicle
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