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range of 3.1–10.6 GHz is proposed. The wideband isolation can be achieved through a tree-like structure on the ground plane. The effectiveness of the tree-like ...
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

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Ultrawideband MIMO/Diversity Antennas With a Tree-Like Structure to Enhance Wideband Isolation Shuai Zhang, Student Member, IEEE, Zhinong Ying, Senior Member, IEEE, Jiang Xiong, Student Member, IEEE, and Sailing He, Senior Member, IEEE

Abstract—A compact printed ultrawideband (UWB) multipleinput–multiple-output (MIMO)/diversity antenna system (of two elements) with a size of 35 40 mm2 operating at a frequency range of 3.1–10.6 GHz is proposed. The wideband isolation can be achieved through a tree-like structure on the ground plane. The effectiveness of the tree-like structure is analyzed. Measured S-parameters show that the isolation is better than 16 dB ( 20 dB in most of the band) across the UWB of 3.1–10.6 GHz. The radiation patterns, gain, and envelope correlation coefficient are also measured. The proposed antenna is suitable for some portable MIMO/ diversity applications. Index Terms—Diversity, isolation, multiple-input–multiple-output (MIMO), ultrawideband (UWB) antennas.

I. INTRODUCTION

U

LTRAWIDEBAND (UWB) has been expanding rapidly as a promising technology for radar applications, localizations and date communications, etc. The Federal Communications Commission (FCC) has authorized the unlicensed use of the 3.1–10.6-GHz spectrum for UWB applications [1]. Different from conventional wireless technology, UWB has the advantage of low operating power level, immunity from other system interference or noise, and extremely high data rate. Multiple-input–multiple-output (MIMO) has been considered as an efficient way to increase the channel capacity without the need for additional power or spectrum in rich scattering environments [2]–[4]. MIMO UWB systems were studied in [5], which can further increase the channel capacity as compared to conventional MIMO systems for narrowband applications. To combat the multipath fading problem in an indoor UWB wireless communication system, an UWB diversity antenna system is a promising candidate. Both MIMO and diversity UWB systems require an increase of the isolation among antennas. At the same time, the MIMO/diversity UWB antennas

Manuscript received October 03, 2009; revised November 02, 2009. First published November 24, 2009; current version published December 08, 2009. This work was supported in part by VINNOVA (Sweden) for “IMT advanced and beyond.” S. Zhang and J. Xiong are with the Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou 310058, China. Z. Ying is with Sony Ericsson Mobile Communications AB, 221 83 Lund, Sweden. S. He is with the Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou 310058, China, and also with the Division of Electromagnetic Engineering, School of Electrical Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden (e-mail: [email protected]). 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.2009.2037027

should be compact enough for portable devices. Therefore, compact MIMO/diversity UWB antenna systems with low mutual coupling among antenna elements are desirable for practical portable MIMO/diversity UWB applications. Many studies have been made to reduce the size of the antennas and increase the isolation and impedance bandwidths for MIMO/diversity UWB antenna systems [6]–[9]. In [6], a UWB diversity monopole antenna has been designed, and a T-shaped reflector has been inserted between the two radiators to reduce the mutual coupling. The antenna can operate from 2.3–7.7 GHz. Another diversity UWB antenna has been proposed in [7], and the impedance bandwidth is 3.1–5.8 GHz. The isolation can be enhanced through a slot etched on a T-shaped reflector. In [8], a diversity antenna for PDA application has been proposed, and it covers the band of 2.27–10.2 GHz. Three stubs have been introduced to weaken the mutual coupling. However, in order to ensure an acceptable isolation, the size of the antennas in [6]–[8] are still relatively large. Recently, an UWB diversity antenna with a small size has been presented in [9]. The proposed antenna has a size of 37 45 mm , and a good isolation has been achieved. However, the operating frequency of the antenna in [9] is only from 3.1 to 5 GHz (lower UWB) and cannot cover the whole band of 3.1–10.6 GHz. Low mutual coupling is also difficult to achieve in the whole UWB with the decoupling structure in [9]. In this letter, we propose a compact UWB MIMO/diversity antenna system. The designed antenna system has a small size of 35 40 mm and can cover the whole UWB (3.1–10.6 GHz). Through a tree-like structure, the measured isolation better than 16 dB in the whole UWB ( 20 dB in most of the band) can be achieved. II. ANTENNA CONFIGURATION The prototype and geometry of the proposed UWB MIMO/ diversity antenna system is shown in Figs. 1 and 2, respectively. The antenna system is printed on the dielectric substrate (Taconic ORCER RF-35) with a printed circuit board (PCB) thickness of 0.8 mm and relative permittivity of 3.5. The loss tangent of the substrate is 0.0018. The distance between the two radiators is only 3 mm. Each radiator is fed with a 50- microstrip line. The wideband isolation between the two UWB radiators can be efficiently enhanced by a tree-like structure that extends from the ground plane. The effectiveness of this treelike structure will be shown and analyzed in the next section. mm and There is a small notched block with height mm at the bottom of the tree-like structure [see width Fig. 2(b)]. This notched block is used to adjust the impedance bandwidth. The shape of the radiators in this MIMO/diversity

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Fig. 1. Prototype of the proposed UWB MIMO/diversity antenna system: (a) top view, (b) back view.

Fig. 3. Simulated S-parameters when the total number of the branches varies: (a) S and S , (b) S and S .

remains the same (3 mm; so that the antenna system keeps compact). III. THE PROPOSED TREE-LIKE STRUCTURE

Fig. 2. Geometry of the proposed UWB MIMO/diversity antenna system: (a) top view, (b) back view.

system is similar to that in [9]. However, there are two major differences for the radiating parts. One is that an L-shaped slot is etched on the radiator to enhance the impedance bandwidth by providing an additional resonance determined by the length of the slot. The other is that compared to the geometry in [9], the hypotenuses of the two triangular radiators have been separated from each other as much as possible (to reduce the mutual coupling), but the shortest distance between the two radiators

In this section, computer-aided design (CAD) with commercial software CST is carried out in order to study the effectiveness of the proposed tree-like structure. A wideband isolation can be achieved with the tree-like structure mainly due to two mechanisms. One is that branch 1 [see Fig. 2(a)] can be viewed as a reflector (see [6] and [9]), which can reduce the wideband mutual coupling through separating the radiation patterns of the two radiators. In this letter, we increase the number of the branches, and the mutual coupling can be further weakened. The other mechanism is that as the number of the branches of the tree increases, more resonances will be introduced, each of which is provided by a monopole of one branch or a quarter-wavelength slot formed by two neighboring branches. With some properly chosen length and position of each branch, the wideband isolation can be efficiently enhanced as the resonances are in different frequency ranges. In Fig. 3(b), one sees the improvement of the isolation across the operating band, as the total number of the branches increases from one to three. Our simulation results show that branches 1,

ZHANG et al.: UWB MIMO/DIVERSITY ANTENNAS

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Fig. 5. Measured and simulated S-parameters of the proposed UWB MIMO/ diversity antenna system.

Fig. 4. Surface current distributions with Port 1 excitation: (a) 4, (b) 7, and (c) 10 GHz.

2, and 3 mainly affect low, middle and high frequencies, respectively. This is very useful when designing the tree-like structure for other antenna systems. To further investigate the effect of the tree-like structure, the surface current distributions at 4, 7, and 10 GHz are shown in Fig. 4. The case of an antenna system without the tree-like structure in between is also shown in Fig. 4 for comparison. When Port 1 is excited, the current from the radiator tending to couple to Port 2 has been blocked by the tree-like structure and cannot flow to the other radiator through the common ground plane at all these frequencies. The effect is the same when Port 2 is excited. It is shown that the proposed tree-like structure can efficiently enhance the isolation between the radiators across the whole UWB. IV. RESULTS AND DISCUSSIONS A. Impedance Performance The measured (with Anritsu 37369D Vector Network Analyzer) and simulated S-parameters for the proposed UWB

Fig. 6. Measured radiation patterns: (a) x-z plane, (b) y -z plane, (c) x-y plane.

MIMO/diversity antenna system are shown in Fig. 5. According to the measured results, the impedance bandwidth dB can cover the whole UWB of 3.1–10.6 GHz, and an isolation of more than 16 dB (more than 20 dB in most of the band) in the whole UWB is achieved. B. Radiation Performance The radiation patterns of our proposed antenna system are measured at 4, 7, and 10 GHz and shown in Fig. 6. During the measurements, only Port 1 is excited, while Port 2 is terminated with a 50- load. One sees in Fig. 6 that the radiation patterns of the designed antenna are relatively stable across the UWB. In addition, as our proposed antenna is symmetrical and the tolerance in fabrication is within 0.03 mm, when Port 2 is excited

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tree-like structure. The diversity gain (1%) has also been calculated and is larger than 9.95 dB across the whole UWB of 3.1–10.6 GHz. These indicate that our designed antenna has a good MIMO/diversity performance. Additionally, the time-domain performance of our proposed antennas is not given in this letter, but one can find the methods of analysis in, e.g., [9] and [11]. V. CONCLUSION

Fig. 7. Measured antenna gain.

with Port 1 terminated, the patterns in the - plane are similar to those in Fig. 6(b), but the patterns in the - and - planes are mirror transformations (about - plane; such a difference is useful for pattern diversity) of those in Fig. 6(a) and (c). The antenna gain has also been measured and plotted in Fig. 7 in the UWB (according to the standard given by the FCC) from 3.1 to 10.6 GHz. Due to the symmetry of the structure and the small fabrication tolerance, the gains of the two radiators are almost the same. Only the gain for Port 1 is presented, and during the measurements, only Port 1 is excited, while Port 2 is terminated with a 50- load. One sees that the variation of the antenna gain across the UWB is almost within 3.1 dBi. C. Diversity Performance The envelope correlation coefficient is a critical parameter for evaluating the MIMO/diversity performance. For a two-port system in uniform propagation environment, the envelope correlation coefficient can be simply calculated by [10] (1) The envelope correlation coefficient for our proposed UWB MIMO/diversity antenna, obtained from the measured S-parameters and (1), is below 0.01 across the whole UWB of 3.1–10.6 GHz. The correlation at low-frequency range, which is usually much higher, is still very low in our proposed antenna system. These benefits from the good impedance matching and the high isolation are due to the effectiveness of the designed

A compact UWB MIMO/diversity antenna system has been proposed. The wideband isolation can be achieved through an appropriately designed tree-like structure. The effectiveness of the tree-like structure has been shown. The obtained impedance bandwidth of our designed antenna can cover the whole UWB of 3.1–10.6 GHz. Within the UWB, the measured isolation is better than 16 dB in the whole band and 20 dB in most of the band. The radiation patterns, antenna gain, and envelope correlation coefficient have also been measured. The measurement results have shown that the proposed antenna system is very suitable for portable MIMO/diversity applications. REFERENCES [1] FCC, “First report and order,” 2002. [2] R. D. Murch and K. B. Letaief, “Antenna systems for broadband wireless access,” IEEE Commun. Mag., vol. 40, no. 4, pp. 76–83, Apr. 2002. [3] G. Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Labs Tech. J., vol. 1, no. 2, pp. 41–59, 1996. [4] J. Wallace, M. Jensen, A. Swindlehurst, and B. Jeffs, “Experimental characterization of the MIMO wireless channel: Data acquisition and analysis,” IEEE Trans. Wireless Commun., vol. 2, no. 2, pp. 335–343, Mar. 2003. [5] V. Tran and A. Sibille, “Spatial multiplexing in UWB MIMO communications,” Electron. Lett., vol. 42, no. 16, pp. 931–932, 2006. [6] K. L. Wong, S. W. Su, and Y. L. Kuo, “A printed ultra-wideband diversity monopole antenna,” Microw. Opt. Technol. Lett., vol. 38, no. 4, pp. 257–259, 2003. [7] L. Liu, H. P. Zhao, T. S. P. See, and Z. N. Chen, “A printed ultrawideband diversity antenna,” in Proc. Int. Conf. Ultra-Wideband, Sep. 2006, pp. 351–356. [8] S. Hong, K. Chung, J. Lee, S. Jung, S. S. Lee, and J. Choi, “Design of a diversity antenna with stubs for UWB applications,” Microw. Opt. Technol. Lett., vol. 50, no. 5, pp. 1352–1356, 2008. [9] T. S. P. See and Z. N. Chen, “An ultrawideband diversity antenna,” IEEE Trans. Antennas Propag., vol. 57, no. 6, pp. 1597–1605, Jun. 2009. [10] S. Blanch, J. Romeu, and I. Corbella, “Exact representation of antenna system diversity performance from input parameter description,” Electron. Lett., vol. 39, no. 9, pp. 705–707, May 2003. [11] Y. D. Dong, W. Hong, Z. Q. Kuai, and J. X. Chen, “Analysis of planar ultrawideband antennas with on-ground slot band-notched structures,” IEEE Trans. Antennas Propag., vol. 57, no. 7, pp. 1886–1893, Jul. 2009.