IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 1107

A Compact Planar MIMO Antenna Systemof Four Elements With Similar Radiation

Characteristics and Isolation StructureHui Li, Student Member, IEEE, Jiang Xiong, Student Member, IEEE, and Sailing He, Senior Member, IEEE

Abstract—A compact planar multiple-input–multiple-output(MIMO) antenna system of four elements with similar radiationcharacteristics is proposed for the whole 2.4-GHz WLAN band.It consists of two proximity-coupled fed microstrip square ringpatch antennas and two � microstrip slot antennas of the samelinear polarization. These two types of antennas are printed ondifferent sides of the substrate to reduce mutual coupling. With anovel isolation structure etched on the ground plane of the FR4substrate, high port isolation (below �� dB) and good MIMOperformance are achieved. The overall lateral size of the MIMOsystem is only � �� � �� , and good impedance matching(��� �� dB) is achieved across the operating band for all theantenna elements. Full spherical radiation patterns are measuredfor the MIMO system, showing similar radiation characteristics,and the gains are above 2.3 dB across the operating band.

Index Terms—Compact antenna, isolation technology, multiple-input–multiple-output (MIMO) systems, wireless LAN.

I. INTRODUCTION

I N MODERN wireless communication systems, highdata rate is required over band-limited channels. Mul-

tiple-input–multiple-output (MIMO) systems that utilizemultiple antennas to increase channel capacity without sacri-ficing additional spectrum or transmitted power have receiveda growing amount of interest in recent years. Up to date,most MIMO antenna systems with more than two antennasare three-dimensional rather than planar [1], [2]. In practice,low-profile planar antennas are more preferred so that antennaradiators can be easily integrated with other printed circuitboard (PCB) components in portable devices. In addition, it issometimes desirable that all the elements have simultaneouslygood impedance matching, similar radiation patterns, and thesame polarization. However, due to the integration of severalclosely packed antennas on a single PCB with very limitedspace, mutual coupling between elements is severe, and this

Manuscript received July 14, 2009; revised September 10, 2009. First pub-lished October 13, 2009; current version published October 27, 2009. This workwas supported in part by VINNOVA for “IMT advanced and beyond.”

H. Li, and J. Xiong are with the Center for Optical and Electromagnetic Re-search, Zhejiang University, Hangzhou 310027, China (e-mail: lihui@coer.zju.edu.cn; xiongjiang@coer.zju.edu.cn).

S. He is with the Center for Optical and Electromagnetic Research, ZhejiangUniversity, Hangzhou 310027, China, and also with the Division of Electromag-netic Engineering, School of Electrical Engineering, 100 44 Stockholm, Sweden(e-mail: sailing@kth.se).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2009.2034110

leads to rather poor antenna and system performance (low totalefficiency and channel gain).

Many techniques have been proposed for diminishing themutual couplings of antennas [3]–[11]. In [3], a compactdecoupling network for enhancing the port isolation betweentwo closely spaced antennas is proposed. Low mutual couplingcan also be achieved through specific ground structure [4], [5],the neutralization technique [6], [7], electromagnetic band-gap(EBG) filters [8], and coupling elements [9]. This researchworks on planar MIMO antenna structures focus on the isola-tion improvement for two-element systems. When the numberof antenna elements increases, mutual couplings become morecomplicated and high degree of isolation is much more difficultto achieve. Guterman et al. have proposed a MIMO systemfor WLAN application with six radiating elements [10], butits lateral size is a bit large. More recently, a compact planarMIMO antenna system with four radiating elements has beenproposed in [11]. However, the measured isolation betweenantennas is only dB, not low enough by the well-acceptedstandards (lower than dB).

In this letter, a compact planar MIMO antenna system of fourelements with high port isolation is proposed. Two types of an-tenna elements printed on different sides of the substrate areused in the structure for better isolation performance. In ad-dition, a new isolation structure composed of a series of slitsetched in the ground plane is proposed. It has further improvedthe mutual coupling by 10 dB. In Section II, the geometry andparameters of the structure are described. The proposed iso-lation structure is analyzed, and its mechanism for improvingthe isolation is explained in Section III. The measured antennasystem performances, including matching, port isolation, andMIMO performance, are given in Section IV. Finally, some con-clusions are given in Section V.

II. ANTENNA DESIGN

The geometry and parameters of the proposed four-port an-tenna system are shown in Fig. 1. The antenna system consists oftwo types of radiating elements. One is the quarter-wavelengthmicrostrip slot antenna [1] (antennas 2 and 3 in Fig. 1), and theother is the proximity-fed square ring patch antenna (antennas 1and 4 in Fig. 1). The slot antenna is etched on the ground plane(the bottom side of the substrate) and simply fed by a microstripline. Its total length is 18.75 mm. The microstrip square ringpatch, with a lateral size of only , is more compactthan the conventional rectangular patch. It is proximity-fedby a T-shaped strip on the FR4 substrate (with permittivity of

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1108 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

Fig. 1. Geometry of the compact MIMO antenna system with four elements:(a) side view, (b) top view, (c) bottom view. (� � ���� mm, � � �� mm,� � ��mm� � �mm,� � ����mm,� � ��mm,� � �����mm,�� � � mm, � � � mm).

4.4). This feeding structure is simple and easy for impedancematching. Good impedance matching with the 50- microstripline can be simply achieved by adjusting the height of the ringpatch above the feeding strip [ in Fig. 1(a)]. Another reasonfor choosing the T-shaped strip is for better cross polarization.The electric current of opposite direction along the two arms ofthe T-shaped microstrip feed induces symmetric current distri-bution on the shorter edge of the ring. Therefore, the electricfield along x-direction in far-field range can be partly canceled,and a pure y-direction polarization with low cross polarizationis expected.

There are two sources of mutual coupling: the inherent edgecoupling between antenna elements and current flowing on theshared ground. Here, we make the neighboring antennas on dif-ferent sides of the substrate to avoid inherent edge coupling be-tween them. The current of the patch is mainly distributed onthe upper ring patch, while the current of the slot antennas ismainly on the ground plane. This way, mutual coupling betweenantenna elements can be reduced. In order to curb the electriccurrent flowing on the shared ground, we etched a series of slotsin the ground as an isolation structure between the slot antennaand the ring patch antenna [see Fig. 1(c)]. Specific analysis ofthis structure will be described in the following section.

III. PROPOSED ISOLATION STRUCTURE

The proposed isolation structure consists of several slits (withlength and width ) interleaved with strips, which can beconsidered as a bandstop filter. To demonstrate the bandstop ef-fect of the isolation structure, similar to the method conductedin [4], a structure composed of seven slits on the FR4 substratewith the same dimensions as in our four-element antenna system[see Fig. 1(c)] is examined. The isolation structure is connected

Fig. 2. Frequency responses of the isolation structure connected with two mi-crostrip lines.

with two transmission lines to test its reflection and transmissioncharacteristics. The isolation structure and its simulated S-pa-rameters are shown in Fig. 2. According to the results of S11and S21, one sees that most of the energy is reflected back toport 1 and little is transmitted to the other port across the WLANband (2.4–2.485 GHz). Thus, little electric current can be cou-pled from one antenna to the others, resulting in an improvedisolation. It is noted that the periphery (except the open end atthe edge of the ground plane) of the slits are all connected withmetal, and this makes sure that the structure shown in the inset ofFig. 2 is an exact extraction of that used in our antenna system.In Fig. 2, a stop band from 1.95 to 2.85 GHz is observed. Com-pared to the ground plane structure in [4], the proposed structurereduces the number of slits by half, thus the structure occupiesless space and has less influence on the antenna performance.

To better illustrate the working mechanism of the proposedisolation structure, electric current distributions on the groundplane of the antenna system with and without the isolation struc-ture are shown in Fig. 3(a) and (b), respectively. In this compar-ison, antenna 2 (one of the two slot antennas) is excited, andother antennas are terminated with the standard 50- matchingloads. In Fig. 3, one sees that, without the isolation structure,strong current is coupled from the excited slot antenna to theground below the patch and also to the other slot antenna. In thepresence of the isolation structure in Fig. 3(b), the electric cur-rent is trapped around the series of slits and cannot flow to otherneighboring radiators. This greatly helps to reduce the mutualcoupling between antennas. This isolation structure is especiallyuseful in cases where strong current on the ground plane is themain cause of mutual coupling.

With this isolation structure, the isolation of the proposed an-tenna system can be improved by 10 dB without increasing in-terspace between radiators. To specify the effectiveness of theisolation structure in the ground, the simulated S-parameters ofthe antennas with and without the isolation structure are shownin Fig. 4. Considering the symmetry of the diagonally placedradiators (antennas 1 and 4 are the same microstrip square ringpatch antennas, and antennas 2 and 3 are the same quarter-

LI et al.: A COMPACT PLANAR MIMO ANTENNA SYSTEM OF FOUR ELEMENTS WITH SIMILAR RADIATION CHARACTERISTICS AND ISOLATION STRUCTURE 1109

Fig. 3. Magnitude of current flow normalized to the maximum value on theground. (a) The antenna system without isolation structure. (b) The proposedantenna system.

wavelength slot antennas) and their locations on the PCB, somecurves overlap with each other, and consequently we only needto show six curves in the figure. In Fig. 4(a), the simulated iso-lations are below dB, which is already better than the pre-viously reported dB in [11]. Furthermore, with our isolationstructure, the isolation performance shown in Fig. 4(b) is im-proved to be below dB. Such level of mutual coupling israther low for most current MIMO systems.

The etched slits of the isolation structure do not affect the an-tenna system much except for two minor influences. One is thatthe bandwidth of the proposed antenna is slightly reduced, yetit still well satisfies the WLAN band requirements. The reduc-tion of bandwidth is mainly because part of the current flowsto the series of slits. The other is that the physical length (and in Fig. 1) of the slot antenna should be adjusted duringthe process of parameter optimization; otherwise, its operatingfrequency will shift a little due to the existence of the isolationstructure.

IV. ANTENNAS PERFORMANCE

The proposed four-port antenna system is measured with atwo-port Advantest R3765C Network Analyzer. The measuredS-parameters are shown in Fig. 5, with a fabricated prototypeshown in the inset. The relative position of the antennas is in-dicated in the figure (see also Fig. 1(b) for the top view of theantenna structure). The mutual couplings between any two ra-diators are obtained in such a way that these two radiators areconnected with the vector network analyzer and the other tworadiators are terminated with the standard 50- matching loads.Due to the symmetry of the proposed structure and for the clarityof the figure, only six characteristic curves (represented by S11,S22, S12, S13, S14, and S23) are shown in Fig. 5 (as mentionedin the previous section). The measured results show that the an-tenna system covers the band of 2.4–2.5 GHz, with S11 below

dB. Due to fabrication tolerance, the resonant frequencyof the slot antennas shifts slightly toward higher frequency. Itshows that very good isolation (below dB between any twoports) is achieved, even better than that given by the simulation.This is due to insertion loss and material loss of FR4.

The full spherical radiation patterns are measured in Lenovo’sanechoic chamber and are shown in Fig. 6 with a top view. Four

Fig. 4. Simulated S-parameters of the antenna system (a) without the proposedisolation structure and (b) with the isolation structure.

Fig. 5. Measured S-parameters of the proposed antenna system. (only six rep-resentative curves are shown due to the symmetry of the structure).

antennas show similar broadside radiation patterns. The slightdifference is due to different antenna types and locations on the

1110 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

Fig. 6. Measured full spherical radiation patterns at 2.45 GHz for (a) antenna 1,(b) antenna 2, (c) antenna 3, and (d) antenna 4.

Fig. 7. Computed envelope correlation coefficients between antennas.

ground, which is beneficial to the isolation between antenna el-ements. The measured gains of patch antennas and slot antennasare above 2.3 and 2.8 dB, respectively, across the WLAN band.

The set of correlation among signals received by the targetantennas is a critical factor for a MIMO system. Envelope cor-relation coefficients are computed from full spherical E-field ra-diation patterns and are shown in Fig. 7. One sees that the enve-lope correlation coefficient is always below 0.022, indicating agood MIMO performance of the proposed antenna system.

V. CONCLUSION

A compact planar MIMO antenna system of four elementshas been designed and analyzed in this letter. Two types of an-tenna elements printed on different sides of the substrate are

used in the structure to achieve good isolation. To further reducethe mutual coupling, a series of slits are etched in the ground, en-hancing the isolation level by 10 dB. This isolation structure canbe used where current on the ground is the main cause of mu-tual coupling. All four antennas can cover the WLAN band andshow similar radiation characteristics, including patterns andpolarization. The measured isolation is lower than dB. Fullspherical radiation patterns are measured, and the peak gainsare 2.84 and 3.52 dB for a square ring patch antenna and a slotantenna, respectively, indicating both good antenna and MIMOperformance.

ACKNOWLEDGMENT

The authors sincerely appreciate the help of the Lenovo Com-pany, Shanghai, China, for providing full spherical radiationmeasurement.

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