A 100 GHz FMCW MIMO radar system for 3D
image reconstruction
D. Bleh, M. Rösch, M. Kuri, A. Dyck, A. Tessmann, A. Leuther, S. Wagner, O. Ambacher
Fraunhofer Institute for Applied Solid State Physics (IAF)
Tullastraße 72
D-79108 Freiburg, Germany
Daniela.Bleh@iaf.fraunhofer.de
Abstract—We present a frequency modulated continuous
wave (FMCW) multiple input multiple output (MIMO) radar
demonstrator system operating in the W-band at frequencies
around . It consists of a two dimensional sparse array
together with hardware for signal generation and image
reconstruction that we will describe in more detail. The geometry
of the sparse array was designed with the help of simulations to
the aim of imaging at distances of just a few up to more than 150
meters. The FMCW principle is used to extract range
information. To obtain information in both cross-range
directions a back-propagation algorithm is used and further
explained in this paper. Finally, we will present first
measurements and explain the calibration process.
Keywords—MIMO, FMCW radar, array processing, radar
system, millimeter wave imaging, W-band radar
I.
I
NTRODUCTION
When navigating a helicopter in unknown terrain or unclear
situations especially under adverse visibility conditions, it is
crucial to get information about the landing place to avoid
dangerous episodes. If the wind forces generated by the rotors
swirl particles like sand, dust or snow the occurring cloud of
particles might severely reduce the optical range. This is
known as brown-out or rather white-out for snow. As the
particles typically have diameters in the range of 1μ to
100μ the cloud is not transparent for visible or infrared
waves [1]. For millimeter waves this particle cloud is
transparent and thus offers the possibility to develop imaging
systems to assist the pilot. A landing aid system with a few
antennas giving information on height and lateral movement
was already developed and tested, see e.g. [2].
MIMO systems provide high resolution 3D imaging
capabilities with a 2D sparse array. A MIMO system uses
multiple transmitter elements and multiple receiver elements
and with orthogonal transmitting waveforms it is possible to
get distinct information about every single signal path from any
transmitter to any receiver. Compared to a SIMO system the
number of elements necessary to obtain the same image is
significantly reduced [3]. Thus the hardware effort, cost and the
weight of the system are reduced drastically. FMCW
waveforms for a MIMO radar operating in S- and X-Band were
used e.g. in [12]. Similar imaging systems using stepped
frequency instead of FMCW waveforms based on MIMO were
developed for example at EADS [3] in the wavelength range
from 18 to 40 or at Rohde & Schwarz in the W-
Band [10]. These systems were designed for operation in the
near field, in our applications we aim at larger distances and
operation on mobile platforms and thus need higher output
power, stability and require low noise levels.
II. MIMO
SIGNAL MODEL AND PROCESSING
In this paper, we are considering a MIMO system
containing transmitting elements (Tx) and receiving
elements (Rx).
A. Signal model
The transmitted waveform is a FMCW signal generated by
Direct Digital Synthesis (DDS). For a linear saw tooth profile
the frequency is given by
(
)
=
−
2
+
Δt
,
(1)
with floor operator
∗
, number of frequency steps during
one sweep = /Δ ∈ ℕ, duration of one frequency sweep
, Bandwidth and the carrier frequency
[3,4,5] . For a
single static target the received signal is a delayed version of
the transmitted signal with amplitude 0≤≤1 depending on
its radar cross section. The delay
is equal to the travel time
of the signal from a transmitter at position
,∈[1,…,]
to a target at position and to a receiver at position
,∈
[
1,…,
]
and thus
(
)
=
1
(|
−
|
+
|
−
|)
,
(3)
where is the signal propagation speed. The received
signal is mixed with the transmitted waveform and low-pass
filtered. The resulting signal for one target can be described as
(
)
=
∗
.
(4)
For multiple targets the received signal is the sum of the
signals originating from single targets. If the target is moving a
Doppler shift needs to be taken into account [5].
B. Orthogonal waveforms
To match any measured phase to a pair of one transmitter
and one receiver element it is necessary to use orthogonal
waveforms for all transmitting elements. In our system, we
implemented a time domain multiplexing (TDM) scheme. Only
978-2-87487-045-3 © 2016 EuMA 5–7 Oct 2016, London, UK
Proceedings of the 13th European Radar Conference
37