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Chamber Array Antenna Layout for

Compact OTA Measurements

Mohammad Poordaraee1and Andr´

es Alay´

on Glazunov1,2

1Department of Electrical Engineering, University of Twente, Enschede, The Netherlands.

2Department of Electrical Engineering, Chalmers University of Technology, Gothenburg, Sweden.

Abstract—An optimized irregular planar array antenna lay-

out with uniform excitation of antenna elements is presented

for Random-LOS OTA (Random Line-Of-Sight Over-The-Air)

characterization setups. A plane wave is synthesized within a

cylindrical 3D test zone at 2.7GHz, which can be scaled to

mmWave frequencies. The obtained thinned array achieves a 52%

reduction of the number of elements and a 45% aperture size

as compared to a uniform fully populated planar array with an

inter-element distance of 0.93λ, which is the optimum distance

through [λ/2, λ]based on the presented cost function at this

paper . The obtained maximum phase deviation and the maxi-

mum ﬁeld amplitude deviation from the average ﬁeld distribution

in the 3D test zone of the proposed optimized chamber array

antenna layout are approximately 6.4◦and 3.9dB, respectively.

The numerical computation of the radiation pattern of a 10 ×10

element uniform planar array antenna as AUT (Antenna Under

Test) placed within the test zone was performed too. Results show

a rather good agreement between the radiation pattern to that

obtained assuming far-ﬁeld conditions.

I. INTRODUCTION

The Over-The-Air (OTA) performance evaluation of wire-

less devices and antenna systems is in high demand since

conducted measurements standards are becoming obsolete.

Massive multiple-input multiple-output (MIMO) antenna sys-

tems are in focus due to their large physical sizes, e.g., at

sub-6GHz bands. Compact OTA measurement setups are

desirable to keep costs down. For example, in a Random Line-

Of-Sight (Random LOS) OTA setup, far-ﬁeld-like conditions

ought to be emulated within the test zone [1], [2]. The

objective is to synthesize a plane wave within the test zone,

e.g., of cylindrical shape, in the near-ﬁeld of the chamber

array antenna. The emulated plane wave ﬁeld can be used

to experimentally verify the performance of massive MIMO

systems or, in general, any antenna systems [3], [4], [5],

[6]. In [7] a numerical method was proposed to synthesize

a plane wave in the near ﬁeld, while in [8], [9], the fully-

populated 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. In this paper, we

consider the joint minimization of the phase and the intensity

ﬂuctuations of a vertically polarized wave ﬁeld within the test

zone by means of the GA. We further compute the radiation

pattern of an AUT with 10×10 elements in the test zone of the

proposed optimized irregular planar array (OIPA) antenna and

Fig. 1. Planar array layouts for compact OTA measurement setup.

compare it with the numerical computation of the theoretical

radiation pattern of the considered AUT.

II. CO MPAC T OTA SETUP AND IRREGULAR PLANAR

AR RAY LAYO UT D ESIGN PROC ED URE

Fig. 1 shows a schematic representation of a Random-LOS

OTA setup emulating a plane wave ﬁeld in a cylindrical test

zone of width 2Rand height Hin an anechoic chamber. The

AUT, e.g., a base station or an array antenna may be placed

on a turn table at a distance Dfrom the chamber antenna in a

shielded anechoic or semi-anechoic chamber. The performance

of the AUT could then be evaluated by rotating the platform

which axis coincides with the axis of the cylindrical test

zone. The center of the planar chamber array antenna and

the center of the imaginary cylinder are aligned as shown in

Fig. 1. The center of the chamber array is selected as center of

the Cartesian coordinate system to perform the optimization

procedure explained in the following.

Two Figures of Merit (FoM) are considered for character-

izing the uniformity of the plane wave. The ﬁrst FoM is used

to evaluate the maximum ﬁeld intensity ratio and is given by

EF I Rmax = max {|EF I R|} ,(1)

where max {.}yields the maximum and parameter EF I R is

the Electrical Field Intensity Ratio in dB scale and deﬁned as

follows

EF I R = 20 log |Ez|

Eavg .(2)

where |Ez|is the magnitude of the z-component of the

electrical ﬁeld at any point within the test zone and Eavg =

MEAN {|Ez|} is the average value of the magnitude of the

z-component of the electrical ﬁeld. The average is taken over

the points lying in the central parallel plane within test zone,

as shown in Fig. 1. As mentioned above, the antenna ele-

ments of the chamber array are vertically polarized, thus only

the z-component of the electrical ﬁeld is considered. Other

components are negligible compared to the z-component. The

second FoM evaluates the phase ﬂuctuations as the maximum

absolute deviation from the mean value on the central parallel

plane to the chamber array deﬁned as

∆φmax = max {| − k(D−x)−MEAN {φ},|} .(3)

where k= 2π/λ is the wave number, λis the free space

wavelength at the operation frequency, φ= arg {Ez}is the

phase of the complex amplitude of z-component of the E-ﬁeld

at a point within the zone. The x-coordinate of each point

within test zone is used to compute −k(D−x)to compensate

the phase difference between the central parallel plane and

the other parallel planes within the test zone. MEAN {φ}is

the average phase on the central parallel plane. For an ideal

plane wave, the phase and magnitude ﬂuctuations of the E-

Field shall be zero. Therefore, the optimization problem aims

at minimizing the linear combination of the EF I Rmax and

∆φmax within the cylindrical test zone.

min

d,nj

αEF I Rmax + ∆φmax (4a)

s.t. λ/2≤d < λ, (4b)

nj= 0,1, j = 1 . . . Nelem,(4c)

where one of the optimization parameters is the inter-element

distance d, which is set to be the same in both the vertical

and the horizontal dimensions of the chamber array (zand

yaxes in Fig. 1). The constraint λ/2≤d≤λis used to

prevent mutual coupling effects as well as the appearance

of grating lobes. The other optimization parameter is the

weighting coefﬁcient njtaking binary values 1or 0depending

on whether an antenna element is present or not in the

array layout, respectively. The index jdenotes the number

of an antenna element and Nelem denotes the total number

of antenna elements of the fully populated array. An element

with zero weighting coefﬁcient nj= 0 indicates that it can

be removed from the planar array which leads to reduction

of chamber antenna cost and complexity. The parameter αis

ﬁxed coefﬁcient in order to establish a balance between the

impacts of EF I Rmax and ∆φmax in the cost function and is

chosen empirically.

(a) (b)

Fig. 2. Phase and ﬁeld intensity distribution over the central parallel plane

as shown in Fig. 1 at x= 3.5m a) φnorm[°], b) EF I R [dB].

(a) (b)

(c) (d)

Fig. 3. Phase and ﬁeld intensity distribution over a square 5×5m2area on

the x-yplane at z= 0 m a) ∆φnorm , [°], b) EF I R, [dB]. Phase and ﬁeld

intensity distribution over circular test zone area with R= 0.5m on the x-y

plane at z= 0 m as shown in Fig. 1, c) φnorm, [°] and d) E F IR, [dB] .

III. SIMULATION RES ULTS

The simulations are specialized to synthesize a plane wave

ﬁeld within the 3D test zone, i.e., an imaginary cylinder with

dimensions R= 0.5m and H= 1 m. As a starting point, a

fully populated uniform planar array with Nelem = 23 ×23 =

529 isotropic antenna elements and inter-element distances

d=λat f= 2.7GHz is assumed [8].

In order to consider the phase ﬂuctuations within the volume

of the test zone, ∆φmax and EF I Rmax are computed through-

out the test zone. Furthermore, to balance the contributions of

the phase and amplitude variations in (4), α= 1.4was chosen.

Indeed, α= 1.4means that 1dB increase in EF I Rmax

changes the value of the cost function to a similar amount as

a1.4◦increase in ∆φmax. Hence, the inﬂuence of phase and

intensity variation of the electrical ﬁeld on the cost function

could be tuned by α. It should be stressed that αwill impact

TABLE I. Field uniformity and Aperture size (Ap. size) comparison.

FPPA Fully Populated Planar Array, OIPA Optimized Irregular Planar Array,

Ref.[10]

FoM ∆φmax EF I Rmax Nelem Ap. size d/λ

FPPA 23.5°3.9dB 529 86% ×23λ×23λ0.93

OIPA 6.38°3.9dB 255 48% ×23λ×23λ0.69

Ref.[10] 46.5°4.8dB 291 35% ×23λ×23λ0.59

(a)

(b)

Fig. 4. Plane wave uniformity evaluation on planes parallel to z-yas function

of x, (a) φmax and (b) σdB.

the optimized conﬁguration.

The OIPA antenna conﬁguration optimized by GA is shown

in Fig. 1. The optimized parameters for the optimized irregular

planar array are d= 0.0763 m = 0.69λwith Nelem = 255.

The colored circles show the positions populated by the

antenna elements, while the white circles where an antenna

element is absent as a result of the optimization. The op-

timization, produces an array with a 52% reduction of the

antenna elements and a 45% reduction of array aperture. The

phase and amplitude distributions of the E-ﬁeld on the central

parallel (z-y) plane at x= 3.5m are plotted in Figs. 2 (a) and

(b). Fig. 3 shows the E-Field distribution on the perpendicular

plane to the chamber array plane at z= 0 (x-yplane). Planar

test zone areas are outlined in Figs. 3 (a) and (b) by circles.

For the sake of comparison we also considered two addi-

tional 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-yparallel plane within the test zone as function of

(a)

(b) (c)

Fig. 5. Computed radiation pattern of an uniform planar array with 10 ×10

elements and symmetric inter-element space of λ/2in both directions, within

the test zones of FPPA, OIPA and Ref. [10] arrays. a) full radiation pattern

b) magniﬁed ﬁrst null c) magniﬁed ﬁrst side lobe.

x[m], evaluated as ∆φmax and EF I Rmax 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. In terms of EF I Rmax on

parallel planes, FPPA and OIPA have better performance in

comparison with [10] to implement more uniform amplitude

although the OIPA provides a layout implementation that is

simpler and more cost-effective. A summary of the FoMs

of the wave ﬁeld uniformity (plane wave) for all of three

layouts is given in Table I. An OTA setup serves the purpose

of characterizing a device’s performance. Therefore we also

compare the performance of the aforementioned array layouts

by calculating the radiation pattern of an AUT in the test zone.

For this purpose we consider an uniform planar array with

10 ×10 elements with inter-element distance of λ/2in both

dimensions. The comparison of the main beam level, the side

lobe levels and nulls is obtaining by computing the radiation

pattern of the AUT. The results obtained assuming different

OTA array layouts are compared with the computed radiation

pattern of the theoretical (ideal) reference case (Ref. pattern)

as shown in Fig. 5. To better highlight the differences, the

ﬁrst null and side lobe level are shown separately in Figs. 5

(b) and (c), respectively. As can be seen from Fig. 5 (b) the

OIPA antenna “reproduces” the null more precisely than the

FPPA and the layout in Ref.[10]. FPPA, OIPA and Ref.[10]

have 21.2dB, 1.58 dB and 29.26 dB difference in comparison

with theoretical pattern in absolute value of the null shown in

Fig. 5 (b), respectively. For the absolute peak value of the ﬁrst

side lobe level as shown in Fig. 5 (c) there are 2.47 dB, 0.7

dB and 0.7dB difference between FPPA, OIPA , Ref.[10] and

value of the theoretical pattern, respectively. Hence, the OIPA

antenna is an improvement as compared with the previously

presented result.

IV. CONCLUSIONS

In this paper we compare various layouts of planar arrays

used to synthesize a plane wave ﬁeld within a 3D cylindrical

test zone for compact OTA applications. It is concluded that

among the considered layouts and optimization criteria the

irregular planar array with uniform excitation of elements

where minimization of phase and amplitude ﬂuctuations has

been done over the 3D volume gives the better results as

compared to the other methods and the resulting array an-

tennas. The obtained array antenna has 52% less antenna than

the fully populated counterpart. The difference between the

absolute value of the ﬁrst null of the radiation pattern obtained

from the optimized irregular planar array and the theoretical

pattern is 1.58 dB. On the other hand the same difference for

considered fully populated planar array antenna 21.2dB. The

corresponding differences between the absolute value of the

ﬁrst side lobe levels are 0.7dB and 2.4dB, respectively. This

result shows that a Random-LOS OTA measurement setup

with a chamber array which layout has been optimized over

a 3D test volume can be used to measure an antenna’s (or a

device’s) radiation pattern with good accuracy.

V. ACKNOWL ED GMENT S

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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