Signal Processing - March 2017 - 30
Car 3
(49.5, -1.5°, 0.5°)
1) Apply the beamspace
projection to retain the
useful data corresponding
to target clusters.
2) Perform the superresolution
MUSIC estimation on
data set of lower
dimensions.
50
-20 -10 0
10 20
Azimuthal Angle (°)
Typical Traffic Scenario:
Target Locations
Given in Polar
Coordinates (R, θ, φ)
Output of FFT-Based
Estimation
Range (Meters)
Car 1
Car 2
(50, -1.5°, 0.5°) (50, 1.5°, 0.5°)
Range (Meters)
51
Traffic Sign
(50.5, 0°, 2.5°)
50.5
50
49.5
49
-4 -2.5 -1 0.5 2 3.5
Azimuthal Angle (°)
Beamspace
Superresolution Estimation
Figure 7. Beamspace-based superresolution estimation steps. A 77-GHz FMCW radar ( SNR = 10 dB, BW = 300 MHz, T = 300 ns ) is used. FFT operates
on data size [L, N ] = [16, 512] . The spatial smoothing window size is [L s, N s] = [8, 50], and the beamspace projection reduces data to [L b, N b] = [4, 5] .
Computational complexity is reduced from O (6.4 # 10 7) to O (8 # 10 3) .
complexity of a 2-D superresolution imaging can be reduced
to O (L b N b) 3 . Moreover, 2-D exhaustive search for the target
on a finer grid operates over an area of interest, thereby further
reducing the complexity. The performance of the beamspace
algorithm is demonstrated in Figure 7. More detailed discussion on the complexity analysis and implementation of radar
algorithms can be found in [30].
MIMO radar
MIMO radar systems employ multiple transmitters, multiple
receivers, and multiple waveforms to exploit all available
degrees of freedom [31]. MIMO radars can be classified as
widely separated or colocated. In widely separated MIMO
radar, transmit-receive antennas capture different aspects of
the RCS of a target. In other words, the target appears to be
spatially distributed, providing a different RCS at each antenna element. This RCS diversity can be utilized to improve the
radar performance [32]. On the other hand, with colocated
MIMO radar, the RCS observed by each antenna element is
indistinguishable [10].
Y
Virtual Array
dT
dR
MIMO Array
X
R
(R1, θ1)
First Target
θ
Z
(Rq, θq)
q th Target
Figure 8. Colocated MIMO radar with a virtual array that increases the
angular resolution.
30
Automobiles typically use colocated MIMO radars, which
are compact in size [33]. For proper transmitter spacing, the
colocated MIMO radar can emulate a larger aperture phased
array radar (see Figure 8). This larger array is called a virtual
array. Recall of the range-azimuth estimation problem given in
(8). For the MIMO radar processing, as depicted in Figure 8,
a 1-D receiver array with two transmit antennas is considered.
Let L T and L R denote a number of transmit and receive antenna elements, respectively. Suppose that d T and d R represent
corresponding transmit and receive antenna spacings. Also,
assume that transmit and receive antenna positions in Cartesian
coordinates are given by l T and l R . Hence, the 2-D FMCW
mixer output signal across fast time and aperture is given by
Q -1
d (l T , l R, n) . / a q exp ' j2r ;
q =0
+
2K R q n fc {(l T d T +l R d R) sin i q}
+
c
fs
c
2fc R q
E1 + ~ (l T , l R, n) .
c
(10)
From (10), it is evident that if d T = L R # d R , then MIMO
radar imitates a regular 1-D array radar with single transmit and L T # L R receive antenna elements. This is known
as virtual array representation. Hence, the spatial resolution
of FFT-based target imaging can be improved by the factor of L T . With virtual array representation and substituting
l = l T # L R + l R , the expressions similar to (8) can be obtained
and the estimation algorithms discussed in the sections "Basic
Automotive Radar Estimation Problems" and "Advanced Estimation Techniques" can be applied.
The challenging aspect of MIMO radar is the selection of
waveforms. The waveforms can be made orthogonal in the
frequency, time, or code domain [34], [35]. Consequently, the
matched filter design at the receiver varies, which is necessary
to separate the reflected waveforms originating from different transmitters. From the FMCW radar signal given in (4),
IEEE SIgnal ProcESSIng MagazInE
|
March 2017
|
Table of Contents for the Digital Edition of Signal Processing - March 2017
Signal Processing - March 2017 - Cover1
Signal Processing - March 2017 - Cover2
Signal Processing - March 2017 - 1
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Signal Processing - March 2017 - Cover3
Signal Processing - March 2017 - Cover4
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