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Photonically synchronized large aperture
radar for autonomous driving
STEFAN PREUSSLER,* FABIAN SCHWARTAU, JOERG SCHOEBEL, AND
THOMAS SCHNEIDER
Institut fuer Hochfrequenztechnik, Technische Universitaet Braunschweig, Schleinitzstr. 22, 38106
Braunschweig, Germany
* stefan.preussler@ihf.tu-bs.de
Abstract: Fully autonomous driving, even under bad weather conditions, can be enabled by
the use of multiple sensor systems including 5D radar imaging. In order to get three
dimensional, high resolution images with Doppler and time tracking of the target, the radar
needs to utilize a large number of transmit/receive modules. For proper beam forming, all of
them demand synchronization. Here a new concept for the optical distribution of radar
signals, comprising low complexity integrated transmitter and receiver chips and a complex
central station, will be introduced. Unavoidable temperature drifts due to environmental
influences were compensated to maintain a continuous electrical output power. Within a
proof-of-concept radar experiment an angular resolution of 1.1° has been achieved.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to significant development efforts and dramatic progress that has been made within the
last decade, self-driving, autonomous vehicles have been getting lots of attention. Among the
many technologies that are used for autonomous vehicles is a combination of sensors and
actuators, sophisticated algorithms, and powerful processors to execute software. In order to
see and interpret what is in front of the autonomous car when going forward, conventionally
cameras and Light Detection and Ranging (LIDAR) systems are used [1]. For close-in
control, such as when parking, lane-changing, or in bumper-to-bumper traffic, the LIDAR
system is not as effective. Additionally, its performance is limited by weather conditions,
such as fog or heavy rain. Therefore, it is supplemented by radars built into the front and rear
bumpers and sides of the vehicle. The operating frequency for these types of radar is usually
77 GHz, which has good RF propagation characteristics, and provides sufficient resolution
[2].
In addition to the precise measurement of target parameters like distance, relative radial
velocity and relative azimuth angle, the target separation turned out to be very important for
high quality object detection, recognition and classification [3]. To enhance the spatial
resolution of a radar system, multiple antennas in multiple-input, multiple-output (MIMO)
configuration can be used, enabling high azimuth and elevation angular resolution at a wide
field of view [4]. This type of radar requires a large number of transmit/receive (T/R)
modules, wherein all of them must be phase and frequency synchronized so that a coherent
transmitting/receiving beam can be constructed. The distribution of these signals through
conventional methods such as coaxial cable or twisted pair not only introduces engineering
complexities and signal loss, but also have limitation of bandwidth, data rate and transmission
distance. Additionally, in electrical powered cars, high electromagnetic fields can result in
interferences and a reduction of the signal to noise ratio. Reliability, maintainability aspects,
and RF leakage over a period of time are other drawbacks of conventional copper based
cables. In contrast, optical systems are immune to electromagnetic interferences and
electromagnetic compatibility issues while providing large bandwidth with increased data rate
[5,6].
Vol. 27, No. 2 | 21 Jan 2019 | OPTICS EXPRESS 1199
#346210
https://doi.org/10.1364/OE.27.001199
Journal © 2019
Received 28 Sep 2018; revised 4 Dec 2018; accepted 5 Dec 2018; published 15 Jan 2019
Although microwave-photonics technology is used in cable networks and sensors
remoting, its application in radar is still being explored [7–11]. Microwave-photonics in radar
has many applications starting from radio frequency front end, moving target indication
filters, to radar signal processing [12,13]. Additionally, advancements in optical
telecommunications have made the technology widely available and fully photonic-based
radar is seen as an evolution to the next generation. In fact, microwave photonic systems with
a performance exceeding that of state-of-the-art electrical radar systems have been shown
[14]. In particular, today’s electronic transceivers cannot achieve the same frequency range
without the use of several parallel architectures, and do not provide an equivalent precision,
especially at high carrier frequencies.
For the last 25 years microwave photonic (MWP) systems have relied almost exclusively
on discrete optoelectronic devices and standard optical fibers and fiber based components,
which makes them not cost-effective enough for cars. However, nowadays, integrated
microwave photonics aims at the incorporation of MWP components and subsystems in
photonic circuits, which is crucial for the implementation of both low-cost and advanced
analog optical front ends. Several major technologies like compound semiconductors (GaAs,
InP), nonlinear crystals (LiNbO3), dielectrics (silica and silicon nitride based waveguides)
and element semiconductor (silicon-on-insulator) are available for the realization [15,16].
However, each technology has its own specific strengths like light generation and detection,
passive routing with low propagation loss, electronic integration, ease in packaging, etc.
Additionally, photonic integrated circuit design poses significant design challenges at the
component and system level. Therefore, the right technology needs to be chosen carefully in
order to meet the high requirements on link gain, noise figure and spurious-free dynamic
range, as an indicator for nonlinearities, for integrated microwave photonic realization.
Additionally, integrated microwave-photonic systems are another roadmap for optically
controlled antenna beams of radars with reconfigurability options [17]. However, the
generation and distribution of RF signals in MIMO radars using a microwave photonic
network, has not been proposed yet.
So far, photonics and optical fibers are hardly used in vehicles. This is mainly due to the
low data rates that occur in a conventional car. In an electrically operated, autonomous
vehicle, however, the resulting data rates even of individual sensors can be in the gigabit
range. Additionally, in autonomous cars the entertainment of the passengers might become
very important. Thus, the very fast processing of data from multiple sensors, internet access
and the distribution of high-bandwidth video and audio content is only possible with high data
rate channels. In electrically driven cars, there are also time-varying, high field strengths,
which require electromagnetic-interference immune data connections [18–20]. With today's
construction and connection technology easy to handle, robust and fully encapsulated optical
connections can be made available, in which the actual optics does not come into contact with
the environment of the vehicle. Due to the high bandwidths of the fibers, even very high
frequencies such as the frequency modulated continuous wave (FMCW) radar signal of 77
GHz can be transmitted almost lossless. At the same time, optical fibers are very light and
flexible. Optical fibers thus represent the ideal transmission medium in future autonomous
electric vehicles.
In this paper, we introduce a new concept for a large aperture MIMO system with optical
distribution of the radar signals. To make the entire system modular, flexible, expandable and
updateable, the individual radar modules have to be as simple as possible, allowing for
smaller size and flexible positioning. Additionally, the same carrier and ramp signal is
distributed via a fiber optic network to each module. The analogue, unprocessed data is send
back to the central station via the same optical fiber network. Furthermore, first
measurements for the radar system are carried out, showing excellent angular resolution.
Vol. 27, No. 2 | 21 Jan 2019 | OPTICS EXPRESS 1200
2. Concept
The basic idea of the proposed radar system is to make the single radar modules as simple as
possible and to shift the whole complexity to a central station, as can be seen in Fig. 1. Within
the central station on the left side the baseband radar signals are generated, received and
processed in the electrical domain. The radar signal itself employs a chirp sequence FMCW
signal, generated by the internal controller or sweep generator. However, other radar signals
might be possible as well. Due to the proposed concept, the modulated signal fm distributed
over the fiber network requires just one eighth of the final radar carrier fc and ramp frequency
fr. With fm = (fc + fr)/8, the bandwidth requirements for the signal generator as well as for the
electro optic modulator are reduced significantly. As will be shown later, the signal is at first
frequency doubled at the photo diode (PD) through difference frequency generation (DFG)
and then electrically multiplied by four within the radar chips. DFG enables the generation of
RF signals with very low phase noise and high stability, which are superior to conventional
RF sources [21–23]. For a 77 GHz radar signal with a 2 GHz ramp sweep, this leads to a
reduced center frequency of fc/8 = 9.625 GHz and a ramp modulation of fr/8 = ± 125 MHz at
the central station.
For general distribution of the signal as well as for DFG at the PD, the signal needs to be
transformed into the optical domain with the help of a Mach-Zehnder modulator (MZM)
driven by an optical source like a distributed feedback laser diode (LD). In order to achieve
proper DFG, the MZM needs to be operated in the carrier suppression mode and requires an
operation point stabilization to maintain full carrier suppression.
Fig. 1. Conceptual drawing of the proposed method including the central station on the left, as
well as the transmitter and receiver chips on the right. The insets show the spectrum of the
transmitted signals for (a) 1/8th of the radar signal with supressed carrier and (b) the received
baseband. LD: laser diode, MZM: Mach-Zehnder modulator, ADC: analog to digital converter,
PC: computer, PD: photo diode.
The modulated optical signal is split into several branches by an optical coupler. The
branch number N depends on the number of transmitter-receiver modules and the signal is
distributed via optical fibers. The exact spatial distribution of the modules is defined by the
design and conditions of the vehicle. Each module can consist of a co-integrated, electric-
photonic chip, as can be seen on the right side in Fig. 1. For a mass market like cars, such a
co-integration can be done with CMOS compatible techniques like silicon-on-insulator (SOI).
Since for the connection between the chips and the optical fibers usually grating couplers are
used, the envisioned fiber network for the distribution of the radar signals consists of standard
single mode fibers. Thermal gradients along the fiber during operation lead to changes in
fiber attenuation as well as length changes, resulting in a delay of the signals. The first
impairment can be compensated easily by optical or electrical amplifiers, and the second one
by algorithms within the central station.
Vol. 27, No. 2 | 21 Jan 2019 | OPTICS EXPRESS 1201
For the back transmission of the analogue signal to the central station, the optical input is
split. In the chip the DFG in a 20 GHz PD converts the 9.625 ± 0.125 GHz optical signal
(inset (a) in Fig. 1) into a 19.25 GHz RF with a 500 MHz (19.25 ± 0.250 GHz) ramp. This
signal is electrically multiplied by four in order to reach the final radar signal at 77 ± 1 GHz.
In principle, an electrical frequency multiplier is a device with a nonlinear transfer function,
which generates higher harmonics and subsequently selects the desired harmonic by filtering.
Conventionally, varactor diodes, step recovery diodes or high power amplifiers are used.
Lately, graphene transistors have been employed for frequency doubling with more than 90%
converting efficiency [24].
Before radiation, the signal is electrically amplified. The received high frequency radar
signal is down converted with an electrical mixer to the baseband. This low frequency signal
is amplified and converted to the optical domain with an integrated modulator (Mod). Again
just low bandwidth electrical and optical components are necessary. The baseband signal is
transmitted back to the central station via a separate optical fiber (inset (b) in Fig. 1). Since
the RF and ramp signal has a much higher frequency than the baseband signal, it can be
simply separated by a low bandwidth photodiode at the central station. Instead of the star
arrangement and a distinguished fiber for up and downlink, different wavelengths in one
single fiber and a ring topology of the chips might be possible.
Back at the central station, the signal is converted to the electrical domain by a PD and
further processed by electronics. Here, a bandwidth in the frequency range of the ramp (500
MHz) is sufficient. Finally, the signals from the different modules are processed by an analog
to digital converter (ADC) and the radar image is generated via a PC.
Although becoming steadily cheaper, today optical devices like EDFA and single
modulators might be too expensive for a cost sensitive market like automotive. However, on
the one hand we believe that with the incorporation in a mass market the prices will fall
drastically. On the other hand, as can be seen from Fig. 1, in the envisioned concept just one
laser, modulator and one single EDFA, together with the low bandwidth photodiodes to
receive the signal, would be required in the central station. The high number of radar modules
are electronic-photonic integrated circuit (EPIC) chips, which are mass-market compatible
and can be produced very cost effective. Additionally, most parts of the central station might
be integrated on cost-effective chips as well.
3. Setup and characterization
In order to test the signal distribution and synchronization as well as the stability of the
system, first proof-of-concept experiments were carried out, where several commercially
available external radar chips are synchronized via optical fibers. The local oscillator (LO)
signal of the commercial chips was 19.25 GHz. Thus, the synchronization signal had twice
the frequency than in the concept, presented in the last section. The functionality of the
proposed transmitter/receiver, including frequency multiplication by four and down
conversion is provided by the used radar chips.
The basic schematic of the proof-of-concept radar system with fiber optic signal
distribution is depicted in Fig. 2. On the right side several radar chips (RC) with multiple
antennas for transmitting (Tx) and receiving (Rx) are indicated. The used radar chip was the
AWR1243 from Texas Instruments with 3 transmit and 4 receive antennas per module.
Multiple chips can be cascaded on a single printed circuit board to improve the target
detection and resolution. Another AWR1243 chip was used as a Master, which provides the
Local Oscillator (LO) signal of 19.25 GHz for the Slaves. In the Slave chips, the radar signal
in the 76-81 GHz band is generated through electrical multiplication by four. The 19.25 GHz
LO from the master already contains the chirp modulation.
Vol. 27, No. 2 | 21 Jan 2019 | OPTICS EXPRESS 1202
Fig. 2. Schematic of the proof-of-concept optically synchronized radar system. Single mode
fiber optic connections are indicated by red lines and electrical connections by black ones. At
the bottom, the link budget with measured optical and electrical output powers is displayed.
The total link gain for the electrical signal from the master output to the RC input is zero,
including the electrical-optical and optical-electrical conversion and all pre and post amplifiers.
LDC: laser diode current controller, LD: laser diode, TEC: temperature controller, µC:
microcontroller, PD: photo diode, EDFA: Erbium doped fiber amplifier, RC: radar chip.
In principle, the optical signal distribution and synchronization system consists of three
main building blocks: an electrical to optical conversion, the distribution by optical single
mode fibers and the back conversion into the electrical signal for each RC. The optical light
source within the E/O block is a conventional distributed feedback laser diode (LD, JDSU
CQF975/208) with a wavelength of 1547.40 nm, comprising narrow linewidth of 2 MHz,
high side mode suppression ratio of 45 dB and a low relative intensity noise of −135 dBc/Hz.
It is equipped with a laser diode driver module (LDC) that operates in constant current mode.
Additionally, a compact and highly integrated temperature controller (TEC) optimized for use
in high performance thermoelectric temperature control applications is applied to the LD. The
output current is directly controlled to eliminate current surges and an adjustable TEC current
limit provides the highest level of TEC protection. The synchronization signal from the
Master chip is transferred into the optical domain with the help of a Mach-Zehnder modulator
(MZM). Input polarization adjustment can be neglected since it is connected with a
polarization maintaining fiber directly to the LD. The used MZM (OptiLab IM-1550-20-TQ)
shows a usable bandwidth up to 20 GHz and a half wave voltage below 5 V.
A characterization of the complete signal distribution and synchronization system
regarding the electrical input power against the electrical output power at the specified
synchronization frequency of 19.25 GHz is depicted in Fig. 3(a). As can be seen, it shows
saturation above an electrical input power of 18 dBm. Since the master chip just provides an
electrical output power of 6 dBm, it needs to be amplified (Mini Circuits ZX60-183A) to
guarantee proper operation.
In order to meet the requirements for automotive environment, all individual components,
connectors and cables are chosen carefully to meet the operational range as specified by −40
to + 125°C. Nevertheless, during operation the heating of the waveguides in the modulator by
the adjacent electrodes as well as changing environmental temperatures, lead to a drift of the
operation point [25]. The higher the applied voltage, the higher the waveguides are heated.
This leads to a length change, which is accompanied with a phase change and results in a
change of the transmission at the output of the MZM. Especially the sideband carrier ratio is
changing, leading to a severe change of the electrical signal after the PD. Therefore, a control
loop was set up to stabilize the operation point for maximum transmission power. Thereby,
the signal is fed through an inline fiber optic power monitor (Oplink ITMS), consisting of a
5% coupler and a low bandwidth photo diode. The received power is monitored by a
microcontroller and maintained to a predefined setpoint. Additionally, the microcontroller
Vol. 27, No. 2 | 21 Jan 2019 | OPTICS EXPRESS 1203
enables volt
a
temperature
s
over time du
r
the operatio
n
diodes, as di
s
with enabled
remaining po
w
Fig.
3
modu
wo/w
The seco
n
split by a pla
n
the different
tens of mete
r
optical ampli
f
shows a flat
g
electrical am
p
of the EDFA
within the pr
o
optical link p
after the PDs.
The last
b
where the si
g
using a phot
o
b
andwidth o
f
characterizati
o
electrical
p
o
w
rises with inc
r
will be a dec
a
The maximu
m
optical input
p
is electrically
a
ge protection
s
tabilization ar
e
r
ing operation
point, there
i
s
played by th
e
stabilization.
A
w
er changes a
r
3
. Electrical cha
r
lator operating p
o
stabilization.
n
d main block
n
ar lightwave
c
Slave module
s
r
s, where the
f
ier (EDFA,
L
g
ain response
p
lifiers and to
a
was set to 17
d
o
of of concep
t
ower can be r
e
b
uilding block
g
nal from the
o
diode (PD)
f
typically 1
9
o
n of the ph
o
w
er of 18 dBm
r
easing optica
l
a
y and saturat
i
m
electrical ou
t
p
ower for a bi
a
amplified (M
i
in order to
a
e
shown in Fi
g
within a fixe
d
i
s a significa
n
e
red trace in
F
A
s can be see
n
r
e subjected to
r
acterization of t
h
o
int stabilization (
is the optical
c
ircuit coupler
s
. The length
o
attenuation c
a
L
iComm OFA
-
around the se
l
a
chieve highe
s
d
Bm. The ove
r
t
experiment
c
e
duced easily,
of the optical
optical ampli
f
as converter.
9
GHz and
a
o
to diodes fo
r
can be seen i
n
l
input power
a
i
on of the elec
t
t
put power (−
4
a
s voltage of
7
i
ni Circuits Z
X
a
void damage
g
. 3(b). The bl
d
temperature
e
n
t drift of the
F
ig. 3(b). The
n
, a constant o
u
the resolution
h
e system (a) an
b), black: temper
a
amplifier and
with a ratio o
f
o
f the fiber in
a
n be neglect
e
-
TCU) is stab
i
l
ected wavele
n
s
t conversion
e
r
all link budg
e
c
an be seen in
if electrical a
m
synchronizati
o
f
ier and splitt
e
The used P
D
a
responsivit
y
r
a modulatio
n
n
Fig. 4(a). As
a
s well as hig
h
t
rical output p
o
4
dBm@19.25
7
V. Before en
t
X
60-183A).
to the MZM
.
l
ack trace sho
w
e
nvironment.
W
electrical out
p
e
green trace s
u
tput power i
s
of the microc
o
n
d the measurem
e
a
ture drift, red/gr
e
splitter modu
l
f
1:4 in order t
o
the vehicle
m
e
d. The outp
u
i
lized via auto
n
gth. Due to t
h
e
fficiency at t
h
e
t for the optic
the bottom o
f
mplifiers wit
h
o
n is the optic
a
e
r is converte
d
D
(Optilab P
D
y
in the ord
e
n
frequency
o
expected, the
h
er bias voltag
e
ower for high
e
GHz) is achie
t
ering the slav
e
.
Measureme
n
w
s the temper
a
W
ithout stabil
i
p
ut power at
t
s
hows the me
a
s
maintained.
T
o
ntroller.
e
nt results of th
e
e
en: output powe
r
l
e. The optica
l
o
distribute th
e
m
ight sum up
t
u
t of the erbi
u
matic light c
o
h
e restricted g
a
h
e PD, the out
p
al and electric
f
Fig. 2. In ge
n
h
a higher gai
n
a
l to electrical
d
back into
R
D
-20) has an
e
r of 0.85
A
o
f 19.25 GH
z
electrical out
p
e
. As can be s
e
e
r op
t
ical inp
u
e
ved for aroun
d
e
radar chips,
t
n
ts of the
a
ture drift
i
zation of
t
he photo
a
surement
T
he small
e
r
l
signal is
e
signal to
t
o several
u
m doped
o
ntrol and
a
in of the
p
ut power
al signals
n
eral, the
n
are used
receiver,
R
F signals
electrical
A
/W. The
z
with an
p
ut power
e
en, there
u
t powers.
d
10 dBm
t
he signal
Vol. 27, No. 2 | 21 Jan 2019 | OPTICS EXPRESS 1204
Fig.
4
frequ
e
Identical
p
are individua
l
amplitude w
e
the use of pr
o
avoid distorti
o
The freq
u
shown in Fig
.
20 GHz. Un
d
shows a max
i
the synchroni
4. Radar sy
s
The complet
e
smaller mod
u
chips and a
n
antennas. In
t
available, wh
local oscillat
o
arrangement
o
b
oth in azim
u
and 12 Tx a
n
azimuthal sc
a
19.25 GHz r
e
the proof-of-
required as
w
the 19.25 G
H
output data
o
processing.
Figure 5
(
distance) wit
h
separated wit
h
the cross cu
t
distance the
h
Doppler of a
t
p
lanned to in
c
modules. Ac
c
with HPBW
4
. Measurement o
f
e
ncy response of t
h
p
erformance o
f
l
ly calibrated
i
e
ight function.
o
per shielding
a
o
n due to com
p
u
ency respons
e
.
4(b). Depen
d
d
er optimum c
o
i
mum output
p
zation freque
n
s
tem
e
radar system
u
les (green pr
i
n
FPGA. Thu
s
t
otal, 5 differe
n
ich can be arr
a
o
r signal, but i
t
o
f the master
u
th and elevati
n
tennas in tot
a
a
n, requiring j
u
e
ference signa
l
concept, othe
r
w
ell, which w
e
z LO signals i
s
o
f the radar c
h
(
right side) s
h
h
two targets
h
an angular s
e
t
through the
h
ardware of th
e
t
arget and trac
k
c
rease the res
o
c
ording to [26,
=
50.8 /
λ
×
f
the photo diode
r
h
e whole signal d
i
f
all receivers
i
i
n a near field
In general, n
o
a
nd grounding
p
ression.
e
of the whol
e
d
ing on the sel
e
o
nditions rega
r
p
ower of −2.2
n
cy of 19.25 G
H
can be seen
i
i
nted circuit b
s
, a module h
n
t modules wi
t
a
nged in diffe
r
t
s Tx and Rx
a
with a perpen
on. Each pair
a
l. For an init
i
u
st two of the
f
l
transmission
.
r
synch
r
oniza
t
e
re transmitted
s
fully integra
t
h
ips is grabbe
d
h
ows the resul
t
at 3.2 m dis
e
paration of 1.
detected patt
e
e
demonstrato
r
k
it over time,
o
lution far bey
o
p. 20] the ha
l
D
, where λ
i
r
esponse for diffe
r
i
stribution and sy
n
i
s not critical,
test range to
o
nlinear effect
. The op
t
ical
d
e
signal distri
b
e
cted compon
e
r
ding optical
a
dBm for lowe
r
H
z.
i
n Fig. 5 on t
h
oards). Each
m
as a total of
t
h different sp
a
r
ent orientatio
n
a
ntennas are n
o
dicular arrang
e
of modules p
r
i
al test just
t
w
f
our optical c
a
.
Since comm
e
t
ion signals
fo
via coaxial c
a
t
ed within the
b
d
by an FPG
A
t
of a two-di
m
tance. The ta
r
1°. The upper
p
e
rn at the tar
g
r
as presented
which is calle
d
o
nd the prese
n
l
f power beam
i
s the wavele
n
r
ent optical input
p
n
chronization syst
e
since all the tr
a
define the di
g
t
s and distorti
o
d
evices are use
b
ution and sy
n
e
nts, there is a
n
a
nd electrical i
n
r
frequencies
a
h
e left side an
d
m
odule carrie
s
8 receive (R
x
a
rse linear ant
e
n
s. One mod
u
o
t used. Figur
e
ement of two
r
ovides a linea
r
w
o modules w
e
a
bles of the rig
e
rcial radar c
h
fo
r the proces
a
bles. The op
t
b
lack box in t
h
A
and transfer
r
m
ensional bea
m
r
gets are clea
r
part on the rig
h
g
et. Besides
a
in Fig. 5 is c
a
d
5D radar im
a
n
ted 1.1° by th
e
width (HPB
W
n
gth and D th
p
owers (a) and th
e
e
m (b).
ansmit/receiv
e
g
ital value of
p
o
ns are minim
i
d in the linear
n
chronization
s
a
n increasing l
o
n
put power, t
h
a
nd around −4
d
consists of i
n
s
two AWR1
2
x
) and 6 tran
s
enna configur
a
u
le (master) de
e
5 (left side)
s
pairs of slave
r sparse array
e
re used to p
e
g
ht side in Fig.
h
ips have bee
n
sing of the
d
t
ical synchron
i
h
e background
.
r
ed to a PC f
o
m
forming (azi
m
r
ly visible an
d
ht side of Fig.
a
zimuth, elev
a
a
pable of mea
s
a
ging. Additio
n
e
incorporatio
n
W
) can be app
r
e width of a
n
e
e
modules
p
hase and
i
zed with
region to
s
ystem is
o
ss above
h
e system
4
dBm for
n
dividual
2
43 radar
s
mit (Tx)
a
tions are
livers the
s
hows the
modules
of 16 Rx
e
rform an
2 for the
n
used for
d
ata were
i
zation of
. The raw
o
r further
m
uth and
d
can be
5 depicts
a
tion and
s
uring the
n
ally, it is
n
of more
r
oximated
n
aperture
Vol. 27, No. 2 | 21 Jan 2019 | OPTICS EXPRESS 1205
radiator. Fol
l
resulting in a
Fig.
5
slave
mast
e
5. Conclusi
o
In conclusio
n
imaging syst
e
p
ossible indi
v
insulator tec
h
that is capabl
system expe
r
implementati
o
are selected
c
nonlinearities
19.25 GHz a
n
compensated.
resolution of
integration a
n
b
ad weather
c
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m where all
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v
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s
h
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i
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s
i
a a fiber opti
c
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s
s-
p
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d
n
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a
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i
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