IEICE Communications Express, Vol.5, No.2, 57–62

Miniaturized six port MIMO
antenna using T-shaped
conductor and reactanceloaded notch antenna
Kazuya Takahashi1, Naoki Honma1a), Kentaro Murata2,and Yoshitaka Tsunekawa1
1

Graduate School of Engineering, Iwate University,

4–3–5 Ueda, Morioka 020–8551, Japan2

Graduate School of Engineering, National Defense Academy,

1–10–20 Hashirimizu, Yokosuka, Kanagawa 239–8686, Japana) honma@iwate-u.ac.jp

Abstract: In this letter, a miniaturized six port Multiple-Input MultipleOutput (MIMO) antenna is presented and evaluated based on simulation. The
proposed antenna consists of three planar inverted-F antennas (PIFA) andthree notch antennas. The combination of these elements yields low mutual
couplings among them since electric ﬁeld directions of them are orthogonal
to each other. Furthermore, PIFA and notch antenna are miniaturized byusing T-shaped conductor and loading reactance, respectively. From theantenna characteristics results, it is found that all mutual couplings weresuppressed less than −10 dB. From the channel capacity characteristics
results, the channel capacity was improved by 10.62 Bits/s/Hz comparedwith six dipole antennas with same aperture width of the proposed antenna.
Keywords: MIMO, miniaturized antenna, decoupling, channel capacity
Classiﬁcation: Antennas and Propagation
References© IEICE 2016
DOI: 10.1587/comex.2015XBL0149
Received October 30, 2015
Accepted December 18, 2015
Published February 16, 2016[1] I. E. Telatar, “Capacity of multi-antenna Gaussian channels,” Tech. Rep., Jun.
1995.
[2] J. B. Andersen and H. H. Rasmussen, “Decoupling and descattering networks
for antennas,” IEEE Trans. Antennas Propag., vol. 24, pp. 841–846, Dec.
1976. DOI:10.1109/TAP.1976.1141437[3] J. W. Wallace and M. A. Jensen, “Mutual coupling in MIMO wireless systems:
a rigorous netowrk theory analysis,” IEEE Trans. Wireless Commun., vol. 3,pp. 1317–1325, Jul. 2004. DOI:10.1109/TWC.2004.830854[4] J. Weber, C. Volmer, K. Blau, R. Stephan, and M. A. Hein, “Miniaturized
antenna arrays using decoupling networks with realistic elements,” IEEETrans. Microw. Theory Techn., vol. 54, no. 6, pp. 2733–2740, Jun. 2006.
DOI:10.1109/TMTT.2006.874874[5] J. C. Coetzee and Y. Yu, “Port decoupling for small arrays by means ofan eigenmode feed network,” IEEE Trans. Antennas Propag., vol. 56, no. 6,
pp. 1587–1593, Jun. 2008. DOI:10.1109/TAP.2008.923301[6] M. D. Migliore, D. Pinchera, and F. Schettino, “Improving channel capacity

57

IEICE Communications Express, Vol.5, No.2, 57–62

using adaptive MIMO antennas,” IEEE Trans. Antennas Propag., vol. 54,
no. 11, pp. 3481–3488, Nov. 2006. DOI:10.1109/TAP.2007.884302[7] N. Honma, K. Nishimori, Y. Takatori, A. Ohta, and K. Tsunekawa, “Proposalof compact three-port MIMO antenna employing modiﬁed inverted F antenna
and notch antennas,” Proc. IEEE Int. Symp. Antennas Propag., pp. 2613–2616,
Jul. 2006. DOI:10.1109/APS.2006.1711136[8] K. Takahashi, N. Honma, K. Murata, and Y. Tsunekawa, “Miniaturized 6-port
MIMO antenna using T-shaped planar inverted-F antennas and capacitor-loaded
notch antennas,” 2014 Asia-Paciﬁc Microwave Conference (APMC2014),Electric Proc. of APMC 2014, TH2B-4, Nov. 2014.
[9] K. Takahashi, N. Honma, K. Murata, and Y. Tsunekawa, “Performance ofminiaturized 6-port MIMO antenna using orthogonal polarization,” 2014
Internationale Symposium on Antennas and Propagation (ISAP 2014), Electric
Proc. of ISAP 2014, WE3C-17, Dec. 2014. DOI:10.1109/ISANP.2014.7026592[10] A. F. Molisch, “A generic model for MIMO wireless propagation channels inmacro- and microcells,” IEEE Trans. Signal Process., vol. 52, no. 1, pp. 61–71, Jan. 2004. DOI:10.1109/TSP.2003.820144

1© IEICE 2016
DOI: 10.1587/comex.2015XBL0149
Received October 30, 2015
Accepted December 18, 2015
Published February 16, 2016

Introduction

In order to achieve high speed wireless communication, Multiple-Input MultipleOutput (MIMO) system is used even in compact mobile communication terminals[1]. However, MIMO antenna performance is deteriorated by the effect of a mutual
coupling between the elements in array. Especially, the high mutual coupling isobserved in a small terminal since the elements are closely packed. To reducethe mutual coupling, various decoupling techniques, such as decoupling networkand parasitic element are used [2, 3]. However, the decoupling network leads tonarrow band property and complicated circuit conﬁguration [4, 5], and the use of
the parasitic element needs implementation of additional antennas [6]. Therefore,
these methods are not suitable for small terminals since these methods require
additional component. On the other hand, the use of the polarization orthogonality
can effectively reduce mutual couplings without such additional components [7].
However, it is difﬁcult to achieve both miniaturization and polarization orthogonalities because polarization orthogonality is easily deteriorated by folding theelement for miniaturization.
In this letter, a miniaturized six port MIMO antenna suitable for small terminalis presented [8, 9]. The proposed antenna consists of three planar inverted-F
antennas (PIFA) and three notch antennas. PIFA and slit length of notch antenna
are miniaturized by using T-shaped conductor and loading reactance, respectively.
The inherently decoupled pair of the antenna elements, i.e. T-shaped PIFA andreactance-loaded notch antenna is used to co-locate multiple antennas within asmall space. In the following section, the proposed antenna conﬁguration isexplained ﬁrst. Next, the proposed antenna characteristics are evaluated based on
the simulation. Also, it is found that channel capacity of the proposed antenna isimproving from comparison with dipole array with same aperture width as theproposed antenna, which has not been well considered in [8, 9].

58

IEICE Communications Express, Vol.5, No.2, 57–62(a) Proposed antenna configuration(b) Reactance-loaded notcha antenna configuration

Fig. 1.

2

Antenna conﬁguration

Proposed antenna conﬁguration

Fig. 1(a) shows the proposed antenna conﬁguration. The proposed antenna consistsof three PIFAs and three notch antennas on the substrate with relative permittivity,r , of 2.2. Center frequency fc is 2.4 GHz. The dimensions of #1 and #4 are sameas those of #2 and #5. PIFA and notch antenna are miniaturized by T-shaped
conductor and loading reactance, respectively. Fig. 1(b) shows reactance-loaded
notch antenna conﬁguration, which is a part of the proposed antenna. C1 and C2
represent capacitances loaded to the notch antennas. The following describes
determination method of loading capacitance.
Fig. 2(a) shows notch antenna model with feeding (#1) and termination ports(#2). Fig. 2(b) shows an equivalent circuit explained by S-parameters model when
the termination port is loaded by a reactance. Sant and Γ represent S-parameter ofthe notch antenna without loading reactance and reﬂection coefﬁcient of loading
reactance, respectively when the reference impedance is Z0 . If an impedance of theloading reactance is Z, the reﬂection coefﬁcient, Γ, is expressed as
¼Z  Z0:
Z þ Z0ð1Þ

Further, Sant is deﬁnded as
Sant ¼© IEICE 2016
DOI: 10.1587/comex.2015XBL0149
Received October 30, 2015
Accepted December 18, 2015
Published February 16, 2016

Sant11Sant12Sant21Sant22!
;

ð2Þ

59

IEICE Communications Express, Vol.5, No.2, 57–62(a) Single element model of reactanceloaded notch antenna

Fig. 2.

(b) Circuit model by S-parameter

Determination of loading reactance

where Sant11 and Sant22 represent reﬂections of #1 and #2, respectively, andSant12 and Sant21 represent transmission characteristics between #1 and #2. Sc isS-parameter when the termination port terminated by Z, and expressed as,Sc ¼ Sant11 þ Sant12 ð1  Sant22 Þ1 Sant21 :ð3Þ

Then, appropriate impedance value and loading position are determined so that Sc
is reduced. Miniaturization of the notch antenna is realized by giving appropriate
impedance value and position of loading. In this conﬁguration, C1 and C2 are
0.51 pF and 0.59 pF, respectively. In the proposed antenna, a pair of T-shaped PIFA
and reactance-loaded notch antenna has inherently a low mutual coupling because
electric ﬁeld directions of the T-shaped PIFA and reactance-loaded notch antenna
are orthogonal at the feed ports, as shown in Fig. 1(a).
3Simulation results

3.1 Antenna characteristics
Fig. 3(a) and 3(b) show the simulated S-parameter of reﬂection and mutual
coupling characteristics, respectively. Note that, the reﬂection and mutual coupling
characteristics for #2 and #5 are omitted due to symmetrical antenna arrangement.
From Fig. 3(a), it can be seen that the reﬂection coefﬁcients of both T-shaped PIFA
and reactance-loaded notch antenna are less than −10 dB at the center frequency.
Furthermore, the fractional bandwidths deﬁned as the frequency ranges where thereﬂection coefﬁcient are less than −10 dB, are 0.94%, 0.94%, 0.83%, 1.78%,1.78%, and 4.06% for #1, #2, #3, #4, #5, and #6, respectively. This result showthat the reactance-loaded notch antennas yield relatively broadband matching. FromFig. 3(b), it can be seen that all mutual couplings are less than −10 dB at the centerfrequency. The highest mutual coupling is jS41 j, and is −10.5 dB at the center
frequency. This is because current distribution around notch antenna is asymmetry,
and the electric ﬁeld orthogonality is deteriorated.

© IEICE 2016
DOI: 10.1587/comex.2015XBL0149
Received October 30, 2015
Accepted December 18, 2015
Published February 16, 2016

3.2 Channel capacity characteristics
Fig. 3(c) shows Cumulative Distribution Function (CDF) of channel capacity. The
MIMO channels are calculated by a geometric based three dimensional scatteringmodel [10]. In this model, path distribution is assumed to be Rayleigh environment,
and polarization of channel rotates randomly at the scatters. In the simulation, twotype of the receiving antennas are used. One is the proposed antenna, and the other60

IEICE Communications Express, Vol.5, No.2, 57–62(a) Reflection characteristics(b) mutual coupling characteristics(c) CDF of the channel capacity(d) CDF of SNR

Fig. 3.

Simulation results

is six element dipole array with same aperture width as the proposed antenna. Asfor Tx arrays, one comprises three vertical and three horizontal polarized dipoles
for proposed antenna, and the other comprises six vertical polarized dipoles for
dipole array. Element spacing of the transmitting antennas is one wavelength,
Signal-to-Noise Ratio (SNR) is set to 20 dB, and the number of trials is set to 1000
for Monte Carlo simulation. The channel capacity is calculated by,C ¼ log2 jI þ HHH =Nt j;ð4Þ

where, I, H, Nt and γ represent an unit matrix, channel matrix, the number oftransmitting antennas, and SNR, respectively. In Fig. 3(c), the solid and dashedlines express the channel capacity of proposed antenna and dipole array, respectively. From the ﬁgure, it is found that the channel capacity of the proposed antenna
is 10.62 bits/s/Hz higher than that of the dipole array. Fig. 3(d) shows the mean
SNR characteristics. The solid and dashed lines in Fig. 3(d) express the mean
SNRs of the proposed antenna and dipole array, respectively. SNR of the proposed
antenna and dipole array are 14.74 dB and 9.03 dB at 50% value, respectively.
Mean spatial correlations of the proposed antenna and dipole array are 0.45 and0.57, respectively. Therefore, it is considered that the improvement of the channel
capacity was brought from both the increase of SNR and improvement of spatial
correlation.
4© IEICE 2016
DOI: 10.1587/comex.2015XBL0149
Received October 30, 2015
Accepted December 18, 2015
Published February 16, 2016

Conclusion

In this letter, the miniaturized six port MIMO antenna suitable for small terminal
has been proposed. The proposed antenna consists of three PIFAs and three notch
antennas, and these elements are miniaturized by using T-shaped conductor and61IEICE Communications Express, Vol.5, No.2, 57–62

loading reactance, respectively. The inherently decoupled pair of the antenna
elements, i.e. T-shaped PIFA and reactance-loaded notch antenna is used toco-locate multiple antennas within a small space. From simulation results, themutual coupling among all elements was suppressed less than −10 dB at thecenter frequency. Also, the channel capacity of the proposed antenna was 10.62
bit/s/Hz higher than that of the dipole array at the 50% value. These results
demonstrate that the proposed design can offer both miniaturization of antenna
element and improvement of the channel capacity.
Acknowledgments
This work is partially supported by the Strategic Information and Communications
R&D Promotion (SCOPE) of the Japanese Ministry of Internal Affairs andCommunications (15654407).

© IEICE 2016
DOI: 10.1587/comex.2015XBL0149
Received October 30, 2015
Accepted December 18, 2015
Published February 16, 201662