Multi-target Detection in FMCW Radar based on Six-Port Technology YANG Jian LIU Fa-Lin Dept. Electronic Engineering and Information Science, Univ. of Science and Technology of China Hefei, Anhui 230027, China E-mail:
[email protected] Abstract: The traditional FMCW radar has its own
traditional heterodyne receiver. While FMCW radar
limitation in multi-target detection. Six-port radar is
based on six-port technology can suppress the false
proposed as a new way to solve this problem because of
alarm easily.
its phase measurement capability. The structure of six-port radar is given and the operating principle of
I. CONVENTIONAL FMCW RADAR AND MULTIPLE
the six-port as direct conversion receiver in FMCW
TARGETS’ FALSE ALARM
radar is discussed. How six-port radar suppresses false
f
alarm is analyzed in this paper. Computer simulations f0
practical.
∆f
Key words: six-port radar technology; direct conversion receiver;
fr
ft
prove that the analysis is correct and the method is
frequency modulated
continuous
wave;
∆f
+
∆f
−
F
2T
T
t
0
multi-target; false alarm.
t
Fig. 1 Frequency Modulated Continuous Wave
Fig.1 shows frequency modulated continuous The six-port network is made up basically of
wave modulated by a triangular signal. f t is the
couplers and diode detectors. It is mostly used to develop reflectometers and network analyzers [1, 2].
frequency of emitted signal whose center frequency
The applications of six-port technology in smart
is f 0 . Modulated cycle of triangular signal is T and
antenna systems [3], for direct millimeter-wave receivers [4] and in position sensing [5] have also
modulated bandwidth is F . f r is the frequency of
been reported. Its most important feature is the capability to perform accurate phase measurements through
simple
scalar
measurements.
the received signal. ∆f
Phase
+
and ∆f
−
represent the
difference between two RF signals is determined
difference frequency of emitted signal and received
from four power measurements. The circuit has no
signal in the rising part and declining part of a
active device, so it can be miniaturized in MMIC
triangular modulated cycle, respectively.
technology [6], which renders it suitable for low cost
The FMCW radar obtains a target’s range R and relative speed v through the measurement of the
and compact size applications. Frequency modulated continuous wave (FMCW)
difference frequency ∆f + and ∆f
is widely used in range/relative speed radar because
−
. If there are
of its simple generation and easy signal processing.
multiple targets, however, the received signals are
However, there is the problem of false alarm in
sum of each target’s received signal; and the
multi-target detection for FMCW radar based on
difference frequencies may result from different
1
0-7803-9433-X/05/$20.00 ©2005 IEEE.
APMC2005 Proceedings
targets. So, it is impossible to get the unique corresponding relation of ∆f + and ∆f
−
modulated by a triangular signal. A part of the emitted signal is coupled to one input port of the
for every
six-port as a reference signal. The received signal is
target. Considering all the possible combinations of n
injected to another input port after low noise
2
targets, we will obtain n pairs of ranges and relative speeds and they contain n 2 − n false targets. Fig. 2 shows two targets condition. In
amplification. Output signals are connected to power
addition to two real targets generated by the rising
to obtain the relative speed and range of the target.
and declining modulated frequency line 1 and 1′ , 2and 2' , we also get two false targets generated by 1and 2 ′ , 2 and1′ .
Or four output signals can go to the analog-to-digital
detectors to get square law detection and then amplified by video amplifiers. Then an ASP is used
(A/D) converter to have digital signal process, also can obtain same results. The frequency counter is used to guarantee a perfect linearity.
V
1
o
•
2
•
o
III.
TARGETS
R
2′
1′
SIX-PORT RADAR CAN DETECT MULTIPLE
Six-port radar transmits signals as fig.1 shows.
Fig. 2 Multi-target detection by FMCW
For six-port’s ability of accurate phase measurement, we can not ignore phase information of the received II. PRINCIPLE OF SIX-PORT RADAR
signal.
According to six-port’s measurement principle, through
power
measurement,
the
phase
In the first half of modulated cycle, the emitted
and
signal can be written as
amplitude of the reflection coefficient can be
s (t ) = sin(2πf 0 t + π
obtained which is defined as the ratio of the received
F 2 t + ϕ0 ) T /2
(1)
After a propagation time ∆t = 2(R − vt ) , the
signal to emitted signal. Six-port as a direct
c
conversion receiver replacing conventional mixer can achieve direct conversion measurement and high
phase difference of received signal and emitted
measurement accuracy. Fig.3 shows the block
signal will be
diagram of the FMCW radar based on six-port
2 f0 R 2 f 0v 8 FR 8 FR 2 − 2π + t +π t −π c c Tc Tc 2 16 FRv 8 Fv 2 8 Fv 2 2 π − π − π t t t Tc Tc 2 Tc 2 ∆θ + = 2π
discriminator [7, 8]. PA
f0
Then difference frequency and difference phase
LNA
Frequency measurement
VCO control
2 f v 4 FR 8 Fv 1 d∆θ + ⋅ =− 0 + − t 2π dt c Tc Tc 8 FRv 8 Fv 2 + − t Tc 2 Tc 2
∆f
R
Sixport
ASP
v
f0 VCO
control
Frequency measurement
Sixport
form of (2) and (3) as follow
DSP
R v
∆f
Fig.3 System block diagram of six-port radar
oscillator
(2)
2 f0R 8 FR 2 (3) −π c Tc 2 Where v
