Performance Investigation of Space Diversity for a

28/38 GHz MIMO Antenna

(Applicable to mm-Wave mobile network)

12 1 2 2 3
Hanieh Aliakbari , , Abdolali Abdipour , Alessandra Costanzo , Diego Masotti , Rashid Mirzavand , Pedram
3
Mousavi
i
Electrical Engineering Department, Amirkabir University of Technology Tehran, Iran
2
DEI., University of Bologna, Bologna, Italy
3
1ntelligent Wireless Technology Lab., University of Alberta Edmonton, Canada
hanieh.aliakbari@unibo.it, h.aliakbari@aut.ac.ir

Abstract-A multiple-input-multiple-output ( MIMO ) dual­ particularly in mm-wave band. Linearly polarized may hinder
band antenna system consisting of two elements, each of which its performance in some applications scenarios, where there is
built up of circularly polarized slotted patch antenna for mm­ often misalignment between transmitter and receiver antennas
wave band 5G wireless handheld and portable terminals is
or there is significant loss due to multipath interferences.
described. Space diversity is used to achieve a good independent
Recently a microstrip dual-band mm-wave CP antenna has
channel fading in MIMO systems. In theoretical analysis of
been presented by the authors which is suitable for SG
MIMO systems often assume that each antenna is isolated from
the other elements. It has been shown that the antenna spacing communication [8]. This single feed high efficient antenna has
has a significant effect on both near-field and far-field been designed without any matching circuit to obtain good
performance of each element in two independent working bands impedance matching in lower and upper bands [9].
and should be optimized in millimeter wave compact dual-band The capacity of MIMO systems depends on the correlation
MIMO arrays. However, the main problem is that an optimized of the radio channels between the transmitter and the receiver
distance for lower band may not be suitable for upper band and antenna elements. Space isolation (i.e. space diversity) is one
vice versa. If electromagnetic limitations and interactions of the
of the methods to ensure low correlation between each pair of
antennas are ignored, the MIMO system performance may be
transmitting and receiving antenna elements, in order to
underestimated or overestimated.
achieve a good independent channel fading in MIMO systems.
Index Terms-Diversity, Dual-band, Millimeter-wave, Mobile Conventionally, the correlation between the signals can be
communication, Multiple-input-multiple-output ( MIMO ) . reduced by using large distances (i.e. large fractions of the free
space wavelength) between the antenna elements.

I. INTRODUCTION Nevertheless, in order to reach practical compact MIMO array

configurations on radio enclosure or package, where antennas
Due to the availability of large contiguous chunk of
are close to each other, the near-field degradation and far-field
spectrum, millimeter wave (mm-wave) bands around 30 GHz
distortion can no longer be neglected and should be considered
are attractive options for possibly fifth-generation (SG) cellular
in MIMO array design. Therefore, to account for this effect, the
communication that will emerge between 2020 and 2030 [I].
antenna active pattern (i.e., the pattern produced by the
Furthermore, there is an interest in using multi-antenna
excitation of one antenna when the others are closed on a
technology in shortly coming SG services to enhance wireless
matched load) instead of the isolated antenna pattern has to be
communication performance and to ensure reliable and
employed in the calculation of the correlation coefficients of
efficient transmission/reception of information cellular [2]. In
the transmitter/receiver array.
addition, antenna arrays are physically small at mm-wave
In this paper, combination between CP dual-band structure
frequencies so that large Multiple-Input-Multiple-Output
and MIMO are studied. Because of the high demand of MIMO
(MIMO) systems with enhanced capacity could be realized in a
in SG, the proposed single dual-band CP antenna [8] is used in
small volume and implemented in a mobile phone. The main
the multiport structure and the effect of the antenna spacing on
advantages of MIMO systems in mm-wave bands are higher
the near-field and far-field performance of the antenna is
system capacity, more bit rates, more link reliability, and wider
studied in each working frequency band.
coverage area in a rich electromagnetic scattering environment.
On the other hand, high radiation efficiency and circular II. SINGLE ELEMENT DESIGN
polarization operation (CP) of a MIMO antenna is one of the
The geometry of the proposed dual-band CP MIMO patch
high demands of mobile service [3]-[S]. The enhancements of a
array is illustrated in Fig. 1(a) where the spacing between two
MIMO system capacity have been presented mostly using
consecutive elements is shown by the parameter "d'. The inter­
linearly polarized (LP) antennas [6]-[7]. However, few papers
element spacing (d) is varied between the minimum spaces
have been presented the MIMO system with CP antennas,

978-I-S090-S414-S/16/$31.00 ©20161EEE

41
 Top view system, the wave radiated by one antenna induces a current on
d the other, leading to mutual coupling. The results in terms of
scattering parameters of the array in presence of the other one
are plotted in Figs. 2(a) and 2(b), and are compared with
similar quantities for the standalone element. The reflection
coefficients exhibit resonance frequency shifts up to 2% in the
lower band, and almost no frequency shift is observed in the
upper band. The mutual coupling varies between -4 dB to -32
dB in the lower band and -15 dB to -35 dB in the upper band.
The envelope correlation coefficient (Ecc) is important to
achieve the required diversity gain of the MIMO antenna
system [10]. The Ecc should ideally be computed using the 3-D
radiation pattern. Assuming that the antennas will operate in a
uniform multipath environment, it can be alternatively
->

'\\
calculated by using the scattering parameters [11]. For a two­
'.,
,
- 10
"
antenna system with high radiation efficiencies, Eee can be
I I \ I
estimated by [12]

!- :: >\'!'i·.t, �3E:":::::�::�\"i,.!· ,.;r J

- "lime Domain Simulation
5

'_ 0
(1)

Normalized (GLHCP)
----- Measurmel11
-40
�
r _._._--
i! -1
.......... Normalized (GRJ1CP) 5
and the lower the Eee , the higher the diversity gain (Gdiv) is

- 5 02L
6 --2='=8--=::::
30=-----:
::::> 3"=" 34 :-- --:3""
2---=' 3 8'------,'4 0210
6 =--::': obtained as
FreQuenev (G Hz) (2)
(b)
Fig. I. (a) The geometry of two-port dual-band CP antenna, (b) Performance
The calculated result for the proposed antenna in Fig. 3
of the single antenna versus frequency. shows that the desired operation bands of the proposed antenna
have an Eee less than 0.0 I in each band, which satisfies the
before the contact of the elements to the wavelength at the criterion for enabling the antenna to provide a good level of
lower band (i.e. 28 GHz) in the free space. diversity gain.
The standalone antenna is a composite structure including
the combination of a square patch and L-shaped slots near the
patch edges to achieve two distinct resonant frequencies. The 1 -1'28G"p l
asymmetrical slots also act as a perturbation of the patch which _· _·_ · · I,'GH/2
exciting two orthogonal modes: the proper choice of such ··········i"28GH/2
�
.10

<0
----- )'J8GHZ
asymmetry allowed a 90° phase-shift for CP operation at the
operating bands. By using the slots, the effective current path vr
·15 -. SI50
of the antenna can be increased by folding the patch and hence
the resonance of the patch antenna is able to be shifted down to
·20
a lower frequency. The entire two-band antenna is directly fed
by an off-centered single microstrip line. The chosen topology
consists of a monolayer structure with the bottom ground layer .2�6 28 30 32 34 36 38 40
Frequenc)' (G Hz)
(a)
occupying the back side of the substrate. For fabrication, the
conventional method of optical photolithography was adopted
on an RTlDuroid 5880 with the thickness of 0.254 mm,
dielectric constant of 2.2, and loss tangent of 0.0009 and to test
reproducibility, 10 samples are fabricated. The computed input
reflection coefficient and far-field right handed and left handed
gain exhibit the required dual-band CP behavior, as shown in
Fig. 1(b) [8]. Radiation efficiencies are more that 70% in each
band. The overall size of the dual-band element is 3x4x0.254
3
mm .

Frequcnc), (G H.z)
28 30 32 34 36 38 40
III. MIMO ANTENNA: RESULTS AND DISCUSSION

The diversity characteristic of the antenna is achieved by (b)

the two identical antenna elements are placed, with port one is Fig. 2. The near-field performance of two-port antenna for different antenna
excited and port 2 is terminated in 50 0 load. In a two-antenna spacing (a) Sll (b) S12.

42
 16
0.35 MIMO(28 Gil
1
I
----- MIMO(38 GHz) ,I
•

14
0.3 SISO (28 GHz)
SISO (38 GHz) I
•

--A28GHZ 12
0.25 r
[10
0.2 �"
8
0.15
WiJ" =

. ;; 6
c ..

0.1
..:

4
0.05
,- "
'--

2
-T
-- -
-- -
--,
- ---
- _ __
_ _
_____________ ______
"'

°26 28 30 32 34 36 38 40 0-
2 6 8 12
,

Frequency (GHz)
4 10
Distunce (mm)
Fig. 3. Simulated results for envelope correlation. Fig. 5. Axial ratio of a single antenna.

)'28G1,)3
TABLE I. PROPOSED MIMO ANTENNA CHARACTERISTICS
--
.
�"38Gt/2
Lower bund Upper bund
_._ _.•

.
.. ....... /'28GU/2
Reflection coefficient (dB) ·18.7 ·14.4
-o- S I SO Gain (dB) 7.1 7.2
Axial ratio (dB) 1.7 1.46
·90 90
Correlation Coefficient (dB) l.3397e·5 l.1015e·5
Diversity gain (dB) 9.9999 9.9999

As shown in Fig. 5, the effect of d on AR performance is

reduced for the upper band near "38 GHz and for the lower band
·150 150 near "28 GHz' Therefore, the spacing of antennas is set to be one
·180 wavelength in 28 GHz to reduce the mutual coupling to an
(a) acceptable level in both frequency bands, concurrently. Table 1
presents the characteristic of the MIMO antenna in terms of
reflection coefficient, axial ratio and diversity gain.
The results clearly show that theoretical approaches to
MIMO analysis based on the assumption of the isolated
antenna do not represent the valid description of real-world
compact systems. Indeed, antenna interactions are significant
even when the distance is comparable with the wavelength of
the lower frequency of operation.

IV. CONCLUSION

The main objective of the paper is a thorough study of mm­

wave MIMO approach dedicated to future multi-Gb/s wireless
-180 communication systems. The proposed highly efficient mm­
(b) wave antenna is suitable for MIMO system in order to reduce
Fig. 4. E-plane GRHCP pattern of the standalone patch in the presence of the the envelop correlation coefficient. As in our 5G scenario
other for different inter-element spacing (a) 28 GHz, (b) 38 GHz. dual-band and CP operation in mm-wave is of interest, a dual­
band CP antenna in multi-antenna configuration is used. It has
Fig. 4 (a) and (b) shows the radiation pattern of the passive been demonstrated that mutual coupling deteriorates antenna
MIMO antenna at port 1 at lower and upper band, respectively. input impedance, alters the antenna radiation pattern and axial
The GRHCP at the broad side direction is in the ranges of 3.6-7.1 ratio, in each mm-wave frequency bands and leads to the
dB and 5.5-7.5 dB in the lower and upper bands, respectively. correlation of received signals that ultimately degrades the
Remarkable deformation of the E-plane radiation pattern is diversity gain in a MIMO system. Hence, the MIMO system
observed with respect to the standalone patch in the lower design requires the electromagnetic antenna considerations in
band. The circular polarization characteristic will be affected in addition to the base band signal processing. In our further
two bands by the distance of antennas, as shown in Fig. 5. But work, in order to enhance the gain, the passive mm-wave
the effect in the lower band is more significant at smaller MIMO antenna is integrated with active components and
distances since the patch has the edge radiation mechanism in become active integrated MIMO antenna.
the lower band.

43
 ACKNOWLEDGMENT arrays for MIMO applications," IEEE Antennas Wireless
Propag. Lett. vol. 15,pp. 730-733, 2016.
The authors would like to thank Prof. Vittorio Rizzoli the
[7] 1. Dioum, A. Diallo, S. Farssi and C. Luxey, "A novel compact
dual-band LTE antenna-system for MIMO operation," IEEE
retired professor of University of Bologna, for his inspiration in

Trans. Antennas Propag., vol. 62,no. 4,pp. 2291-2296, 2014.

this project.

[8] H. Aliakbari, A. Abdipour, R. Mirzavand, A. Costanzo, P.

Mousavi, "A single feed dual-band circularly polarized
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