Azimuth Angle Resolution Improvement Technique
with Neural Network
Hyungju Kim, Sungjin You, Byung Jang Jeong, Woojin Byun
Radio & Satellite Research Division
ETRI
Daejeon, Korea
kimhyungju@etri.re.kr, sjyou@etri.re.kr, bjjeong@etri.re.kr, wjbyun@etri.re.kr
Abstract— This paper introduces a method to improve the
azimuth angle resolution using MIMO-FMCW radar. When
using a MIMO-FMCW radar, a 2D radar image composed of a
range axis and an azimuth axis can be obtained. The range
resolution is determined by the bandwidth, and the azimuth
resolution is determined by the length of the virtual antenna
array and the number of virtual antenna elements. To improve
the azimuth angle resolution while avoiding aliasing, in this
paper, the virtual antenna was placed wider with non-uniform
spacing. Then, deep learning technique was applied to reduce
the side lobe effect. The proposed method was verified through
experiments using simulation signals and emulation signals
based on measurements.
Keywords—MIMO-FMCW radar, automotive radar, azimuth
angle resolution, neural network, deep learning.
I. INTRODUCTION
An automotive radar is one of the most important sensors
for the detection and recognition of objects around a vehicle
in an autonomous driving system, along with a camera and a
lidar [1-3]. A radar is a radio wave sensor that transmits a
signal toward a target, receives a signal reflected by the target,
and measures its delay time to obtain a distance from the radar
to the target. In order to acquire a 2-D radar image composed
of a range axis and an azimuth axis, a technique of obtaining
a distance to a target and a technique of obtaining an azimuth
angle of a target are required.
In this paper, FMCW technique is used to measure the
distance of the target, and MIMO technique is used to obtain
the azimuth angle of the target. In MIMO FMCW radar, the
distance resolution is determined by bandwidth, pulse length,
FMCW frequency slope, ADC sampling rate, etc. The
azimuth angle resolution is determined by the number of Tx
and Rx antennas, antenna spacing, etc. When all available
bandwidths are used in the 77-81 GHz band, then the distance
resolution can meet up to 3.75 cm. However, in the case of the
azimuth angle resolution, since the number of antennas cannot
be freely increased without restriction to improve the
resolution, a study on the DOA algorithm for improving the
azimuth angle resolution is required.
In this paper, the signal model of MIMO FMCW radar is
introduced in section 2, and then the method for improving the
azimuth angle resolution is described in section 3. Section 4
describes the verification process of the proposed method.
II. MIMO-FMCW
RADAR
In the case of an FMCW radar, a triangular wave or a
sawtooth wave is generally used. In this paper, a sawtooth type
signal is used. After transmitting the sawtooth signal toward
the target, mixing the transmitted signal and the signal
reflected by the target can obtain a beat signal, and the
frequency of the beat signal is determined by the distance of
the target. Hence, the frequency domain obtained by Fourier
transform of the beat signal becomes the range domain of the
target. The 2D target scattering signal model using the MIMO
FMCW radar is expressed as follows.
( , )=
In (1), K is a total number of scatterer, T
c
is a pulse duration,
r
k
and
k
is range and azimuth angle at kth scatterer,
respectively.
The target scattering signal model expressed in (1) has an
advantage of easily obtaining range and azimuth angle using
a 2-D Fourier transform. However, the scattering signal model
of the target expressed in equation (1) is assuming that the
target does not move during one sawtooth period and the total
length of the antenna array is short enough that the target is
observed with the same azimuth angle in each antenna
element. Thus, when extracting target range and azimuth
angle information from the target scattering signal model of
equation (1), it should be noted that it is an observation
environment that satisfies the previous assumptions, and a
new target scattering signal model is required in an
observation environment outside these assumptions. In the
case of an automotive radar, the period of the sawtooth wave
is determined in consideration of the moving speed of the
vehicle, and a target is observed in sufficiently far away from
radar with a small antenna array. Therefore, the signal model
of equation (1) can be used because the signal is measured in
a situation where these assumptions are satisfied. Range
resolution and azimuth angle resolution are expressed as
follows [4].
=
=
In equation (3), is the wavelength of the radar signal, d
is the separation distance between antenna elements.
Since the range resolution is determined by the bandwidth,
it is possible to achieve a high resolution distance resolution
by using a wide bandwidth. However, in the case of azimuth
angle resolution, 120 antenna elements are required to reach
1° azimuth angle resolution [5]. When an array antenna
consisting of a large number of antenna elements is actually
constructed to secure azimuth angle resolution, the designer
faces a design problem of a module controlling a large number
of antennas, and a calibration problem of phase error and loss
caused by the antenna line. Therefore, in this paper, MIMO
technique is used to form multiple virtual antennas with fewer
Tx and Rx antennas to maximize azimuth angle resolution.
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