Compact Printed MIMO Antenna for UWB Applications - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014

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Compact Printed MIMO Antenna for UWB Applications Jian Ren, Wei Hu, Member, IEEE, Yingzeng Yin, and Rong Fan

Abstract—A compact multiple-input–multiple-output (MIMO) antenna is presented for ultrawideband (UWB) applications. The antenna consists of two open L-shaped slot (LS) antenna elements and a narrow slot on the ground plane. The antenna elements are placed perpendicularly to each other to obtain high isolation, and the narrow slot is added to reduce the mutual coupling of antenna elements in the low frequency band (3-4.5 GHz). The proposed MIMO antenna has a compact size of mm , and the antenna prototype is fabricated and measured. The measured results show that the proposed antenna design achieves an impedance bandwidth of larger than 3.1–10.6 GHz, low mutual coupling of less than 15 dB, and a low envelope correlation coefficient of better than 0.02 across the frequency band, which are suitable for portable UWB applications. slot, multiple-input–multiple-output Index Terms— (MIMO) antenna, open L-shaped slot (LS) antenna, ultrawideband (UWB).

I. INTRODUCTION

I

N RECENT years, ultrawideband (UWB) communication systems have been investigated to meet the demand for high data rate, low cost, and low power. Since the Federal Communications Commission (FCC) allowed 3.1–10.6 GHz unlicensed band for UWB communication, UWB communication has become a hot topic in the wireless communication area [1]. As an important part of the UWB communication systems, UWB antennas have attracted significant research interest in recent years. The challenges of feasible UWB antenna design include wide impedance matching, radiation stability, low profile, compact size, and low cost [2]. Moreover, UWB systems also suffer from multipath fading like other wireless systems. To solve this problem, multiple-input–multiple-output (MIMO) technology is introduced in UWB systems to provide multiplexing gain and diversity gain, making further improvement of the capacity and link quality [3]. Two major challenges are faced in the design process of MIMO antennas for the UWB systems. One is to minimize Manuscript received May 23, 2014; revised July 15, 2014; accepted July 23, 2014. Date of publication July 29, 2014; date of current version August 12, 2014. This work was supported in part by the Fundamental Research Funds for the Central Universities under Grant No. JB140206. J. Ren, W. Hu, and Y. Yin are with the National Key Laboratory of Antennas and Microwave Technology, Xidian University, Xi’an 710071, China (e-mail: [email protected]). R. Fan is with Xi’an Marine Equipment Engineering Research Academy Co., Ltd., Xi’an 710071, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2343454

the antenna elements for the MIMO systems. The other one is to enhance the isolation between the antenna elements. In most cases, the antenna elements should have directional gains. Notice that the methods employed to reduce mutual coupling should have little effect on the wideband impedance matching for the UWB applications. To overcome the challenges mentioned above, many methods have been proposed. These decoupling schemes can be divided into three major categories. The first method is using UWB diversity antennas [4]–[6]. The principle of this method is similar to that of dual-polarized antennas. Due to the orthogonality of gain patterns of the antenna elements, low coupling between the elements can be achieved. Moreover, the MIMO antennas for UWB systems using this method usually have a compact size. The second method employs decoupling structures [7]–[10], such as treelike structure [7], and parasitic meander lines [10]. The antennas adopting this method could obtain a high isolation performance. However, as decoupling structures usually have complex realizing forms, the size of the antenna is hard to realize miniaturization. The third method is used widely, and it can be treated as a hybrid method that combines the former two methods [3], [11]–[14]. By using UWB diversity antennas and decoupling structures between the antenna elements at the same time, compact size and low mutual coupling usually can be achieved. In this letter, a compact MIMO antenna for UWB applications is proposed based on the third method mentioned above. A open slot added between the elements is concurrently used to enhance the isolation of the MIMO antenna in the low working band. The antenna has a compact size of mm , which is smaller than the design in [3], and about 73% and 44% of the designs in [7] and [12], respectively. The design process is described in detail in the following sections. II. ANTENNA DESIGN AND SIMULATED RESULTS A. Antenna Design The geometry of the proposed UWB MIMO antenna, with a small size of mm , is shown in Fig. 1. It is printed on an FR4 substrate with relative permittivity 4.4 and a thickness 0.8 mm. The UWB open L-shaped slot antenna proposed in [15] is used as reference, and the antenna’s dimensions are optimized to get a smaller size. The proposed MIMO antenna consists of two L-shaped slot antenna elements, denoted as LS 1 and LS 2. The two LSs are placed perpendicularly to each other to achieve good isolation between the two antenna elements. The element antenna consists of an L-shaped slot and a rectangular patch that is fed by a 50- microstrip line. To obtain

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Fig. 1. Geometry of the proposed antenna, top view and side view.

TABLE I DESIGN PARAMETERS OF THE PROPOSED ANTENNA SHOWN IN FIG. 1

Fig. 2. (a) Simulated -parameters with and without slot. (b) Current distribution at 3.7 GHz.

the bandwidth enhancement for UWB applications, a T-shaped stub is attached to the rectangular patch, which consists of a horizontal stub and a vertical stub . In order to enhance the isolation between the antenna elements at the low band, a narrow rectangular slot is cut on the left bottom of the ground plane. To obtain the required numerical analysis and proper geometrical parameters, computer simulation using the electromagnetic (EM) simulation tool CST is carried out. The SMA connector was included in the simulated model to improve the simulation precision. The final optimized dimensions of the MIMO antenna are listed in Table I. B. Effects of Narrow Ground Slot The narrow rectangular slot is introduced on the ground to reduce the mutual coupling between two antenna elements at the low frequency band. The slot has a dimension of and a rotated angle of 45 . Fig. 2(a) shows the simulated scattering parameters with and without narrow slot. As can be seen, before adding the slot, the isolation between the antenna elements is low in 3–4.5 GHz. The slot, which resonates at the length of about , makes the current mainly distribute near it at the resonant frequency and leads to the isolation improvement. As shown in Fig. 2, after the slot is added, the value of -parameters is significantly reduced to less than –15 dB at 3–4.5 GHz, satisfying the requirement of typical MIMO/diversity antennas. To further explain the influence of the rectangle slot, Fig. 2(b) compares the surface current distribution with and without the slot at the resonant frequency 3.8 GHz. As it is seen, the current flowing from port 1 to port 2 is blocked by the slot, and when the port 1 is excited, the coupling current on the LS 2 is reduced significantly. The effect is the same as that from port 2 to port 1.

III. RESULTS AND DISCUSSION A. Return Loss and Isolation Between Ports A prototype of the L-shaped-slot MIMO antenna described in Section II is fabricated and measured. The prototype is shown in Fig. 3(a). The bandwidth performance of this proposed antenna is measured by the Anritsu 37269A vector network analyzer. Fig. 3(b) and (c) gives the simulated and measured -parameters of proposed antenna. As indicated in Fig. 3(b), both the LS 1 and LS 2 have bandwidth of more than 2.9–12 GHz for dB and dB. The antenna satisfies the impedance matching requirement for the entire UWB specified by the FCC. The simulated and measured (mutual coupling) between the two input ports are shown in Fig. 3(c). It can be seen that throughout the whole UWB band, the measured isolation is below 15 dB (more than 20 dB during 4.7–10 GHz). As mutual coupling of less than 15 dB is enough for the UWB applications [3], the antenna is suitable for MIMO application across the whole UWB band. B. Radiation Patterns Fig. 4 illustrates the measured 2-D copolarization and crosspolarization radiation patterns ( -, -, and -planes) of the proposed antenna at 3.5, 6, and 10 GHz. Here, the copolarization is defined as the predominant one in the plane. Moreover, in the study, when port 1 or 2 is excited, the other port is terminated with a 50- load. From the results, it can be observed that at the low frequency (3.5 GHz), LS 1 and LS 2 have omnidirectional radiation patterns in the -plane (the -plane of port 1 and the -plane of port 2, respectively), and the radiation patterns in the -plane ( -plane of port 1 and -plane of port 2) is dumbbell-shaped. At the higher frequency of 6 GHz,

REN et al.: COMPACT PRINTED MIMO ANTENNA FOR UWB APPLICATIONS

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Fig. 4. Measured radiation pattern of the proposed antenna at 3.5, 6, and -plane; (b) -plane; (c) -plane. 10 GHz: (a)

Fig. 3. (a) Fabricated prototype antenna. (b) Simulated and measured . (c) Simulated and measured .

and

LS 1 and LS 2 have quasi-omnidirectional radiation patterns in the -plane. However, at a higher frequency of 10 GHz, the radiation patterns in the -plane are less omnidirectional because of the higher-order resonant modes. In addition, from the results, it also can be seen that pattern diversity can be achieved as the patterns for the two ports in the same plane are very different from each other. At 3.5 GHz, for example, the copolarization radiation pattern in the -plane for port 1 is omnidirectional as shown in Fig. 4(b). However, for port 2, the copolarization radiation pattern in the same plane is dumbbell-shaped. These differences enhance the pattern diversity between LS 1 and LS 2. For example, the measured results in Fig. 4(b) show that, in the -plane, LS 1 has a gain of 0 dB

Fig. 5. Measured gain and radiation efficiency of the proposed antenna.

at the angle of 270, but LS 2 has a gain of 20 dB at the same angle. The measured peak gains and radiation efficiency of the antenna with port 1 or 2 excited are shown in Fig. 5. It can be seen that the measured peak gains range from 1.7 to 4.2 dB across the frequency band from 3 to 10.6 GHz, and the radiation efficiency is above 60% across the UWB. The gain is lower than that of [15] as the antenna has a smaller size and gain decrease can be found at the high frequency. This is mainly because the

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REFERENCES

Fig. 6. Simulated and measured ECC.

substrate we used is commercially available and the efficiency of the antenna decreased due to dielectric loss. C. Diversity Performance For the antenna used for MIMO application, the two-port envelope correlation coefficient (ECC) is an important parameter. Recall that for a lossless MIMO antenna, the ECC can be calculated using the method proposed in [16] (1) The simulated and measured ECC curves are plotted in Fig. 6. It shows that both the simulated and measured ECCs are below 0.04 in 3.1–10.6 GHz, which is low enough to ensure good diversity performance for the presented MIMO antenna. IV. CONCLUSION A compact MIMO antenna consisting of two open L-shaped slot elements is presented for UWB applications. To reduce the mutual coupling of antenna elements in the low frequency band (3–4.5 GHz), a narrow slot is added to the ground plane. The antenna prototype is fabricated and measured. The measured results show that the proposed antenna achieves an impedance bandwidth of larger than 3.1–10.6 GHz and low mutual coupling of less than 15 dB through the whole UWB band. The measurements prove that the proposed antenna is a good candidate for UWB applications.

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