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Plane Wave Synthesis with Irregular Chamber Planar Antenna Arrays for Compact OTA Measurements

Authors:

Abstract

There is an increasing demand to develop advanced very large antenna systems, especially for the new 5G wireless networks. Over-The-Air (OTA) testing techniques need to meet the challenges arising from the large size and complexity of the massive multi-element array antennas. In the Random-LOS (Line-of-Sight) OTA measurement setup, a plane wave is emulated in the test zone which is also in the near-field of the anechoic chamber antenna used to emulate the far-field from a base station. In this paper, the thinned array antenna concept is applied to synthesize a plane wave within the test zone of the OTA setup. The thinning is performed by means of the Genetic Algorithm. Two planar array antenna configurations are considered for emulating a plane wave for 2D and 3D test zones at 2.7 GHz. The optimized thinned array achieves a 55% reduction of the number of antenna elements. Smaller amplitude and phase fluctuation are observed in the test zone in comparison to other arrays of similar aperture. Consequently, proposed array antennas may be used as chamber antenna in OTA measurement setup test in order to evaluate the performance of MIMO array antennas.
Plane Wave Synthesis with Irregular Chamber Planar
Antenna Arrays for Compact OTA Measurements
Mohammad Poordaraee1, and A. Alayón Glazunov1,2
1 Dept. of Electrical Engineering, University of Twente, Netherlands, m.poordaraee@utwente.nl , a.alayonglazunov@utwente.nl
2 Dept. of Electrical Engineering, Chalmers University of Technology, Sweden, andres.glazunov@chalmers.se
AbstractOver-The-Air (OTA) testing techniques shall
meet cost- and time-efficiency as well as measurement accuracy
constraints stemming from the characterization of massive
multi-element array antennas in 5G systems. The Random-LOS
(Line-of-Sight) OTA measurement setup aims at emulating a
plane wave field in the test zone of the measurement chamber.
The test zone is in the near-field of the chamber antenna used to
emulate the far-field from a base station. In this paper, the
thinned array antenna concept is applied to synthesize the plane
wave field within the test zone. The thinning is performed by
means of the Genetic Algorithm. Two planar array antenna
configurations are considered for emulating a plane wave for 2D
and 3D test zones at 2.7 GHz. The optimized thinned array
achieves a 55% reduction of the number of antenna elements. In
addition, amplitude and phase fluctuations in the test zone have
been considerably reduced in comparison to other similar array
antennas, in this way, improving the measurement accuracy in
Random-LOS OTA measurement setups.
Index TermsOver-The-Air, Planar Array, Chamber
Antenna, 5G systems .
I. INTRODUCTION
In 5G communication systems, Massive MIMO (Multiple-
Input Multiple-Output) array antennas are deployed at the
Base Station (BS). Possibly, also some models of User
Equipment (UE) such as laptops, pads and smart phones will
use array antennas. In this context, the challenging task comes
for Over-The-Air (OTA) performance evaluation of wireless
devices of very large size such as sub-6 GHz Massive
MIMO BSs and also vehicles of various sizes supporting
many different communication standards and deployed in
different propagation environments. Consequently, OTA
testing setups providing a sufficiently large test zone for
performance verification of devices mounted on a car or a
large BS are in high demand [1].
According to the Kildal hypothesis [2], a wireless device
is expected to have good performance in a realistic
propagation environment, in the statistical sense, if it can
achieve good performance both in the Rich Isotropic
Multipath (RIMP) environment and the Random Line-Of-
Sight (Random-LOS) environment [3]. In this paper we focus
on an OTA setup emulating a Random-LOS propagation
scenario, where a linearly polarized plane wave is synthesized
in anechoic or semi-anechoic chambers [1-3].
Synthesizing the Random-LOS environment for OTA
characterization of large antenna systems poses some practical
challenges. First, given the overall large size of the Device
Fig.1. Figure captions will be corrected. Overall configuration of 2D and 3D
test zones. Center of cartesian coordinate system is aligned with the origin of
the array plane.
Under Test (DUT), large measurement chambers are then
required. Second, also massive chamber arrays are required to
test the massive DUTs. All this massiveness may lead to
costly solutions that need to be avoided. Therefore, the main
goal of this paper is to devise compact OTA measurement
setups.
Various plane wave synthesis methods in the near-field of
a planar array for reduced anechoic chamber size have been
proposed, e.g., in [3-6]. The synthetized plane wave emulates
the far-field of a base station [2]. In such a way, an antenna
system can be evaluated in a more compact test zone than in
traditional antenna measurement setups. The same OTA test
setup may be employed for traditional antenna measurements,
e.g., as in a Compact Antenna Test Range (CATR) [7], giving
a great flexibility for testing various propagation scenarios and
applications within the same OTA setup.
As shown in the references given above, employing planar
arrays may reduce the anechoic chamber size but at the
expense of a large number of elements that rapidly grow with
the test zone size making it expensive to implement in
practice. One way to overcome this problem is to use a
reflector antenna [8], [9]. Another way, which is presented in
this paper, is to exclude some of the elements of the planar
array using a thinning optimization procedure resulting in an
irregular planar array. In this work, we were able to show that
a reduction of up to 55% of array elements can be achieved
with the aid of the Genetic Algorithm. Cost can therefore be
reduced while also improving the quality of the test zone as
compared to a fully populated chamber planar array antenna.
However, the reflector antenna solution is still a much cost-
effective solution for Random-LOS OTA setups of very large
DUTs.
II. FOM FOR FIELD UNIFORMITY CHARACTERIZATION
The non-uniformity of the wave field within the test zone
can be systematically evaluated by computing two relevant
Figures of Merit (FoM). The following dispersion
characteristics suggested in [3] are considered here as well.
The power spread is evaluated in dB by the standard deviation
formula [10]
dB
1
5log 1
+

=

, (1)
where is the standard deviation of the normalized power
computed as
 
VAR MEAN
P
P


=


, (2)
where P is in linear units and VAR{P} and MEAN {P} are
the standard deviation and the mean of P, respectively. Since
only the vertical polarization is considered, the power
received by a probe antenna in the test zone is defined as
P=|∑ 𝐸𝑧|2, where Ez is the complex amplitude of the
impinging wave stemming from one antenna element of the
chamber array.
The second FoM is the phase spread computed along a line
parallel to the array plane (see Fig.1) defined as the maximum
phase deviation from the mean
 
 
max max MEAN
 
 =
, (3)
where
is the argument of the plane wave field complex
amplitude Ez.
The analysis presented in this paper is based on the
evaluation of the relative receive power variation in the test
zone
 
norm / MEANP P P=
and the corresponding relative
phase variation
 
norm MEAN
 
=−
.
III. OPTIMIZATION COST FUNCTION
The optimization cost function seeks to minimize the
standard deviation of the normalized power given by (2) based
on the Genetic Algorithm (GA). The standard deviation of the
power in dB, i.e., dB (1), within the desired test zone and the
maximum phase deviation from the mean ∆∅𝑚𝑎𝑥 (3) along the
parallel line are the outputs of the optimization process. The
inter-element distance d (see Fig.1), and a binary decision
variable defining the population of each “node” in the array
antenna, i.e., that a certain antenna element position in the
array layout are the inputs of the optimization fitness function.
The GA implementation in Matlab is used in the
computations.
(a) (b)
Fig. 2. The simulated (a) power and (b) phase within a 5 m 5 m area for 8 ×
23 array.
(a) (b)
Fig. 3. The simulated (a) power and (b) phase within a 5 m 5 m area for 23
× 23 array.
The dB and ∆∅𝑚𝑎𝑥 are then minimized by applying the
GA algorithm for the obtained chamber irregular planar arrays
in the Random LOS setup. A schematic representation of the
OTA setup with a planar array as the chamber array and the
considered test zones are shown in Fig. 1. For instance, a DUT
mounted on a car may be placed on a turntable at a distance D
from chamber array in a shielded anechoic chamber.
Measurements for this case are assumed to take place on a 2D
test zone as shown in Fig. 1. On the other hand, a 3D test zone
is required for performance evaluation of a BS array antenna.
In this case, the array under test with overall width of 2R and
height h may be placed on a rotating platform at a distance of
D from chamber antenna. Consequently, as shown in Fig. 1, a
cylindrical test zone is required for performance evaluation of
the DUT due to the rotation of the DUT around the vertical
(cylinder) axis.
We specialize our analysis to two chamber array
configurations. In the first one, we consider the optimization
of an 8 × 23 planar array with the objective to synthesize a
plane wave within a 2 Dimensional (2D) circular test zone. As
shown in Fig. 1, the 2D test zone is situated on the horizontal
plane perpendicular to the chamber array plane with both
centers aligned. The considered radius of the 2D circular zone
is R = 0.5 m and its center is in distance of D = 3.7 m from
antenna array. In the second configuration a 23 × 23 planar
array is optimized with the objective to synthesize a plane
wave within the 3D cylindrical test zone with height h. In both
of the considered arrays the inter-element distance d is
uniform.
Fig. 4. Array configuration for emulating plane wave within test zone. Colored
squares are active elements while other elements are removed as a result of
the application of the GA.
(a) (b)
Fig. 5. Simulated power and phase spread within the 2D circular test zone.
Fig. 6. Power and phase spread along parallel line.
Table I. Comparison of optimized irregular array and 8 × 23 regular array of
equal aperture size.
FoM
This paper
Ref. [3]
dB
[dB]
Parallel line
0.28
1.02
Circular curve
0.74
0.77
Circular zone
0.28
0.86
max
[]
Parallel line
9.5
13
Number of elements
82
8 × 23=184
The considered thinning procedure minimizes the standard
deviation of the receive power within the 2D circular zone for
the 8 × 23 array. For the 23 × 23 array, the standard deviation
is minimized within the 3D cylindrical zone.
Fig. 7. Array configuration for emulating plane wave within test zone.
Colored squares are active elements while other elements are removed as a
result of the application of the GA.
(a) (b)
Fig. 8. Simulated power and phase spread within the 2D circular test zone.
Fig. 9. Power and phase spread along parallel line.
Table II. Comparison of optimized irregular array and 23 × 23 regular array
of equal aperture size.
FoM
Thinned array
Full array
dB
[dB]
Parallel line
0.51
1.08
Circular curve
1.05
1.83
Circular zone
0.53
1.08
Cylindrical zone
0.52
1.39
max
[]
Parallel line
14
19
Number of elements
291
23 × 23=529
As a result of the optimization procedure, some elements
are removed from the array at the same time as the value of d
is adjusted. During the optimization procedure the amplitudes
and phases of the antenna elements are fixed and equal to each
other. The inter-element distance is limited to λ/2 d λ to
prevent grating lobes and reduce mutual coupling effects,
where λ is the free space wavelength at the considered
operating frequency for the chamber array f = 2.7 GHz.
IV. SIMULATION RESULTS
Fig. 2(a) and (b) show the received power P and phase
as explained above over a 5 m × 5 m area (y-x plane) in front
of the optimized array for 2D zone which contains origins of
the array plane and test zone. Outlined is the 2D circular zone
over which the optimization has been performed.
Corresponding simulations corresponding to the 23 × 23
optimized array for 3D cylindrical zone are shown in Fig. 3.
A. Design of planar array layout for 2D test zone
A uniform planar array antenna with 8 × 23 array elements
is considered here for 2D test zone optimization. The main
ideas behind the chamber array characterization for a Random
LOS OTA setup design are explained in [1], [3] and [8]. A
schematic representation of the array is shown in Fig.1. As a
result of the layout optimization, the optimum array inter-
element spacing is determined as d = 0.7 = 7.7 cm. The
optimized array layout is shown in Fig. 4, where the colored
and white squares show the positions where antenna elements
remained populated, and the positions where they were
removed as a result of the optimization, respectively. As
explained above, no weighting of phases or amplitudes of the
array elements is performed during the optimization for both
optimized array layouts.
Fig. 5(a) and (b) show the receive power P and phase
for the optimized array for the 2D circular test zone defined in
Fig.1. Corresponding results along the parallel line are shown
in Fig. 6. It is worthwhile to note that power spread and phase
spread within the 2D circular test zone are sufficiently low to
conclude that the synthesized field in the test zone is
approximately a plane wave. A summary of the standard
deviation of the power and maximum phase deviation are
presented in the Table I and compared with the chamber array
presented in [3].
From the comparison presented in Table I, it can be
concluded that the GA optimization resulted in 55% and 51%
reduction of the number of antenna elements and antenna
aperture, respectively, and at the same time a considerable
improvement of the accuracy of the synthesized plane wave in
the 2D test zone was achieved.
B. Design of planar array layout for 3D test zone.
In this section, results for uniform planar array antenna
with 23 × 23 array elements are provided for 3D test zone
optimization. Inter-element distance is d = 0.59λ = 6.55 cm
for the optimized configuration. Fig. 7 shows the final
configuration of the thinned chamber planar array. The
colored and white squares show the positions were antenna
elements remained populated, and the ones that removed as a
result of the optimization, respectively.
Fig. 8(a) and (b) show the receive power P and phase
as
explained above within the 2D circular test zone defined in
Fig.1. Corresponding results along the parallel line are shown
in Fig. 9. Further metrics of the plane wave in the cylindrical
3D test zone are presented in the Table II, where results for
the fully populated chamber array are also provided for
comparison. For example, it can be seen that the power
standard deviation 𝜎𝑑𝐵 within the cylindrical test zone is 0.52
dB for the optimized irregular array, whereas for the fully
populated 23 × 23 planar array antenna it is 1.39 dB.
V. CONCLUSIONS
In this paper we investigate the effects of irregular arrays
on the accuracy of plane wave synthesis in the near-field of
the arrays at 2.7 GHz. Two array layouts are considered. In
the first one, the optimization is performed within a 2D
circular test zone, while in the second the optimization is done
within a 3D cylindrical test zone. In both cases, the standard
deviation of power within the desired test zones and the
maximum phase deviation from the mean along the parallel
line are minimized. The optimum value of the inter-element
distance and as well as the population of each “node” of the
array have been obtained by means of the Genetic Algorithm
optimization. Optimization seeks to minimize both the
standard deviation of the received power and the maximum
phase variation within the test zone.
As a result of the optimization it is shown that for the array
targeting the 2D test zone 55% and 51% reduction of number
of antenna elements and antenna aperture can be achieved,
respectively. On the other hand, the array targeting the 3D test
zone, a 45% reduction was achieved for number of array
elements. The optimized planar array improved the standard
deviation from 0.86 dB to 0.28 dB in a circular test zone for
the optimization targeting 2D test zone. For optimization
targeting the 3D test zone, the reduction was from 1.39 dB to
0.53 dB within the 3D cylindrical zone.
Our results show that optimized irregular arrays have less
elements and antenna aperture than the fully populated
uniform planar arrays, which can result in more cost-effective
OTA chamber antennas.
ACKNOWLEDGMENT
This work was supported by the European Union’s H2020:
ITN program for the “mmWave communications in the Built
Environments - WaveComBE” project under the grant no.
766231.
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... In [7] a numerical method was proposed to synthesize a plane wave in the near field, while in [8], [9], the fullypopulated uniform linear and uniform planar array antennas were considered. In [10], thinned planar array antenna layouts were produced by means of the Genetic Algorithm (GA). The optimized thinning minimized the fluctuations of the field intensity, but not the phase fluctuations. ...
... For the sake of comparison we also considered two additional planar array layouts, i.e., the Fully Populated Planar Array (FPPA) with identical inter-element distance of 0.93λ in both directions and the irregular array in [10]. ...
... Figs. 4 (a) and (b) show the plane wave uniformity at each z-y parallel plane within the test zone as function of x [m], evaluated as ∆φ max and EF IR max on each parallel plane, respectively. As can be seen from Fig. 4, Ref. [10] has the largest phase fluctuations, since [10] is not optimized for low variation of phase. Also the proposed array layout has the lowest phase deviation. ...
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