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MIMO communications: A key to gigabit wireless, Proc IEEE 92 (2004), 198 –218. I.E. Telatar, Capacity of multi-antenna Gaussian channels, Euro Trans Telecommun 10 (1999), 585–595. D. Gesbert, M. Shafi, D. Shiu, P.J. Smith, and A. Najuib, From theory to practice: An overview of MIMO space-time coded wireless systems, IEEE J Sel Areas Commun 21 (2003), 281–302. A.G. Burr, Capacity bounds and estimates for the finite scatterers MIMO wireless channel, IEEE J Sel Areas Commun 21 (2003), 812– 818. A. Goldsmith, S.A. Jafar, N. Jindal, and S. Vishwanath, Capacity limits of MIMO channels, IEEE J Sel Areas Commun 21 (2003), 684 –702. T. Svantesson and A. Ranheim, Mutual coupling effects on the capacity of multi-element antenna systems, Proc ICASSP Conf 4 (2001), 2485–2488. J.W. Wallace and M.A. Jensen, The capacity of MIMO wireless systems with mutual coupling, VTC Conf, Vancouver, Canada, 2002, pp. 696 –700. J. Wallace and M. Jensen, Mutual coupling in MIMO wireless systems: A rigorous network theory analysis, IEEE Trans Wireless Commun 3 (2004), 1317–1325. P.N. Fletcher, M. Dean, and A.R. Nix, Mutual coupling in multielement array antennas and its influence on MIMO channel capacity, IEEE Electron Lett 39 (2003), 342–344. V. Jungnickel, V. Pohl, and C.V. Helmolt, Capacity of MIMO systems with closely spaced antennas, IEEE Commun Lett 7 (2003), 361–363. M.E. Bialkowski, S. Durrani, K. Bialkowski, and P. Uthansakul, Understanding and analyzing the performance of MIMO systems from the microwave perspective, IEEE Int Microwave Conf, California, 2005, pp. 2251–2254. S. Durrani and M.E. Bialkowski, Effect of mutual coupling on the interference rejection capabilities of linear and circular arrays in CDMA systems, IEEE Trans Antennas Propagat 52 (2004), 1130 –1134. C.A. Balanis, Antenna theory: Analysis and design, 2nd ed., Wiley, New York, 1997. B. Clerckx, D.V. Janvier, C. Oestges, and L. Vandendorpe, Mutual coupling effects on the channel capacity and the space–time processing of MIMO communication systems, Proc ICC Conf 4 (2003), 2638 –2642. T. Svantesson, Modeling and estimation of mutual coupling in a uniform linear array of dipoles, ICASSP Conf, 1999, pp. 2961–2964. P. Uthansakul, M.E. Bialkowski, S. Durrani, K. Bialkowski, and A. Postula, Effect of line of sight propagation on capacity of an indoor MIMO system, IEEE Int Antennas Propagat Symp, Washington, DC, 2005, pp. 707–710. P. Uthansakul and M.E. Bialkowski, Investigations into MIMO capacity in a mixed LOS/NLOS environment, Asia-Pacific Microwave Conf, Suzhou, China, 2005, pp. 2778 –2781.

© 2006 Wiley Periodicals, Inc.

THREE-ANTENNA MIMO SYSTEM FOR WLAN OPERATION IN A PDA PHONE Kin-Lu Wong,1 Chih-Hua Chang,1 Brian Chen,2 and Sam Yang2 1 Department of Electrical Engineering National Sun Yat-Sen University Kaohsiung 804, Taiwan 2 Research & Development Division V Compal Communications, Inc. Taipei 105, Taiwan Received 13 December 2005 ABSTRACT: A multiple input multiple output (MIMO) system using three EMC (electromagnetic compatible) chip antennas in a personal digital assistant (PDA) phone is demonstrated. The three EMC chip an-

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tennas are mounted at three corners of the system ground plane of the PDA phone and all generate a wide bandwidth covering the wireless local area network (WLAN) operation in the 2.4-GHz band (2400 –2484 MHz). By adding a T-shaped shorted strip in the proposed MIMO antenna system, large improvements in the isolation (S12, S13, and S23 all less than ⫺20 dB) between any two antennas of the MIMO system are achieved. Detailed effects of the T-shaped shorted strip on the isolation improvement in the proposed MIMO antenna system are analyzed. Radiation characteristics of the three antennas are also studied. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1238 –1242, 2006; Published online in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/mop.21665 Key words: antennas; MIMO antennas; WLAN antennas; EMC chip antennas 1. INTRODUCTION

By using a MIMO system with multiple antennas, a much higher channel capacity over that of the traditional wireless system with a single antenna can be obtained [1]. However, for multiple antennas in mobile devices such as a PDA phone or a smart phone for MIMO operation, good isolation between any two antennas of the MIMO system embedded in the mobile device may not be achieved. This is mainly because the available spaces inside the mobile device for employing the antennas are usually very limited, and this behavior may lead to unacceptable MIMO operation for practical applications. To overcome the problem, in this paper we propose a promising three-antenna MIMO system embedded in a PDA phone for WLAN operation in the 2.4-GHz band (2400 –2484 MHz) [2]. In this study, the EMC chip antennas [3–5] used in the proposed MIMO system are arranged to be at the corners of the system ground plane of the mobile device. In this case, since nearby electronic components inside the mobile device can be placed in close proximity to or in direct contact with the employed antennas, a more compact integration of the associated components inside the mobile device can be obtained. As for achieving improved isolation between any two antennas of the proposed MIMO system, a T-shaped shorted strip is introduced in the proposed design. Detailed experimental and simulation results of the proposed three-antenna MIMO system are presented and analyzed. 2. PROPOSED THREE-ANTENNA MIMO SYSTEM

Figure 1(a) shows the configuration of the proposed three-antenna MIMO system with a T-shaped shorted strip for WLAN operation in a PDA phone. The three antennas used in the study are of the same dimensions, and the detailed dimensions of the metal pattern of the antenna unfolded into a planar structure are shown in Figure 1(b). Note that the three antennas are EMC chip antennas [3–5] with a foam base of 18 ⫻ 10 ⫻ 4 mm3, and the antennas are implemented by bending the planar metal plate shown in Figure 1(b), which is obtained by line-cutting a single metal plate (a 0.2-mm-thick copper plate was used in the study), and then mounting it onto the foam base of the antenna. The EMC chip antennas are mainly comprised of a shorted spiral radiating strip, two side ground walls of size 18 ⫻ 4 mm2 and 10 ⫻ 4 mm2, and an antenna ground portion of size 18 ⫻ 10 mm2. Note that the shorted spiral radiating strip has a mean length of about 32 mm, corresponding to about a quarter-wavelength of the frequency at 2442 MHz. In this case, the EMC chip antenna can generate a wide resonant mode covering the 2.4-GHz band for WLAN operation (2400 – 24834 MHz). For testing the antennas in this study, the 50⍀ mini coaxial line is used. The central feeding pin and grounding sheath of the mini coaxial line are connected to the feeding point (point A

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Figure 2 Measured S-parameters (S 11 , S 22 , S 33 , S 12 , S 23 , S 13 ) for the three antennas in the proposed MIMO system with the T-shaped shorted strip. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

plane. In this case the isolation between any two of antennas 1 to 3 can be greatly enhanced. Detailed results of the excited surfacecurrent distributions in the system ground plane with and without the presence of the T-shaped shorted strip will be analyzed in the next section. 3. RESULTS AND DISCUSSION

Figure 1 (a) Configuration of the proposed three-antenna MIMO system with a T-shaped shorted strip for WLAN operation in a PDA phone; (b) detailed dimensions of the metal pattern of the EMC chip antenna unfolded into a planar structure in the proposed MIMO system. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley. com]

in the figure) and the grounding point (point B) with a feed gap of 1 mm. Also note that, as opposed to from the study in [3, 4] in which the antenna ground portion is in direct contact with the system ground plane, the antenna ground portions of the three antennas in this study are perpendicular to the system ground plane and face the interior of the mobile device. In this case, the side ground wall of size 18 ⫻ 4 mm2 of each antenna is in direct contact with the system ground plane. This arrangement makes the three antennas occupy a minimum board space of the system ground plane of the PDA phone. Also note that the three antennas are mounted at three corners of the system ground plane, with antennas 1 and 2 at the top edge of the system ground plane and antenna 3 at the opposite edge of the system ground plane. Antennas 1 and 2 are also arranged in a back-to-back configuration, that is, the shorting strips of antennas 1 and 2 face each other, which is expected to result in an enhanced isolation between antennas [6 –9]. Antenna 3 is mounted at the opposite edge facing antenna 1. With this arrangement, it is found that isolation (S 12 , S 13 , S 23 ) between any two of antennas 1 to 3 of less than ⫺20 dB, is difficult to obtain. Since it has been demonstrated that the isolation between two internal antennas is strongly related to the excited surfacecurrent distributions in the system ground plane of the mobile device [10], we introduce a T-shaped shorted strip along the side edge of the system ground plane between antennas 2 and 3 to modify the surface-current distributions in the system ground

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The proposed three-antenna MIMO system shown in Figure 1 was constructed and studied. Figure 2 shows the measured S-parameters (S 11 , S 22 , S 33 , S 12 , S 23 , S 13 ) for the three antennas. From the results of S 11 , S 22 , and S 33 , the obtained ⫺10-dB impedance bandwidths of the three antennas all cover the 2.4-GHz WLAN band. Note that the three antennas are of the same dimensions, and thus the variations in the measured S 11 , S 22 , and S 33 are due to their different relative locations in the system ground plane. For the isolation (S 12 , S 23 , S 13 ) between any two of the three antennas, they are all well below ⫺20 dB for frequencies over the 2.4-GHz WLAN band. Figure 3 shows the comparison of the isolation results for the

Figure 3 Measured isolation (S 12 , S 23 , S 13 ) for the three antennas with and without the presence of the T-shaped shorted strip. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley. com]

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three antennas with and without the presence of the T-shaped shorted strip. The curves with solid marks are for the case without the T-shaped shorted strip, while those with open marks are for the case with the T-shaped shorted strip. From the results, it is observed that the isolation between any two of the three antennas is improved. Especially, for S 23 and S 13 , the isolation is greatly improved. Over the 2.4-GHz band, the maximum improvement in the isolation is larger than 10 dB. The isolation as a function of the length L of the T-shaped shorted strip was also studied. A typical case of the measured S 23 as a function of L is shown in Figure 4. Three different lengths of L ⫽ 24, 26, and 32 mm are presented. It is seen that for L ⫽ 26

and 32 mm (about 0.211 and 0.26 wavelength at 2442 MHz, respectively), large improvement in the measured S 23 is obtained. On the other hand, for L ⫽ 24 mm (about 0.195 wavelength at 2442 MHz), the improvement in the isolation is relatively small. These results indicate that the length L has a large effect on the isolation improvement, and large isolation improvement can be obtained only when the length L is larger than at least 0.2 wavelength of the desired center operating frequency (2442 MHz in this study). To analyze in more detail the effect of the T-shaped shorted strip, the excited surface-current distributions in the system ground plane at 2442 MHz for the proposed MIMO system with and without the T-shaped shorted strip of length L ⫽ 32 mm are studied. The surface current distributions are obtained using the Ansoft simulation software High-Frequency Structure Simulator (HFSS) [11]. In Figure 5, the simulated surface-current distributions for the case of antenna 1 excited at 2442 MHz are shown. In this case, antennas 2 and 3 are terminated to a 50⍀ load. From the results, it is clearly seen that the excited surface current distributions near antenna 3 are much smaller for the case with the T-shaped shorted strip than for the case without the T-shaped shorted strip. The presence of the T-shaped shorted strip effectively modifies the surface current distributions in the system ground plane, which in turn leads to large improvements in the isolation between the antennas in the proposed MIMO system. In this case, the isolation S 13 is expected to be greatly improved, which agrees with the results shown in Figure 3. The results for the case of antenna 2 excited at 2442 MHz, with antennas 1 and 3 terminated to 50⍀ load, are shown in Figure 6. Again, it is seen that the excited surface-current distributions near

Figure 5 Simulated surface-current distributions at 2442 MHz for the three-antenna MIMO system with and without the T-shaped shorted strip (antenna 1 excited at 2442 MHz). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

Figure 6 Simulated surface-current distributions at 2442 MHz for the three-antenna MIMO system with and without the T-shaped shorted strip (antenna 2 excited at 2442 MHz). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

Figure 4 Measured S 23 as a function of the length L of the T-shaped shorted strip. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

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Figure 9 Measured radiation patterns for antenna 2 at 2442 MHz for the proposed MIMO system studied in Fig. 2 (antenna 2 is excited with antennas 1 and 3 terminated to a 50⍀ load). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

Figure 7 Simulated surface-current distributions at 2442 MHz for the three-antenna MIMO system with and without the T-shaped shorted strip (antenna 3 excited at 2442 MHz). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

antenna 3 are greatly decreased, due to the presence of the Tshaped shorted strip. Some decreases in the surface-current distributions near antenna 1 are also seen. The obtained results explain the improvement in the measured S 23 and S 12 in Figure 3. Figure 7 shows the case of antenna 3 excited at 2442 MHz, with antennas 1 and 2 terminated to a 50⍀ load. Similarly, it is seen that the excited surface-current distributions near antenna 2 are greatly decreased. Some decreases in the surface-current distributions near

Figure 8 Measured radiation patterns for antenna 1 at 2442 MHz for the proposed MIMO system studied in Fig. 2 (antenna 1 is excited with antennas 2 and 3 terminated to a 50⍀ load). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

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antenna 1 are also seen. This result explains the large improvements in the isolation S 13 and S 23 observed in Figure 3. The radiation characteristics of antennas 1 to 3 in the proposed MIMO system are studied in Figures 8 –10. In Figure 8, measured radiation patterns of antenna 1 are plotted. The results in three principal planes are shown, and E ␪ and E ␾ components are seen to be comparable in the three principal planes. Similar results are seen for the radiation patterns plotted in Figure 9 for antenna 2 and also in Figure 10 for antenna 3. With comparable E ␪ and E ␾ components, the radiation characteristics of antennas 1 to 3 are advantageous for WLAN operation. This is because the wavepropagation environments are usually complex for WLAN operation, especially for indoor WLAN operation. For frequencies over the 2.4-GHz band (2400 –2484 MHz), radiation patterns similar to those in Figures 8 –10 are obtained, and stable measured peak antenna gain of about 2.5 dBi is also obtained. 4. CONCLUSION

A three-antenna MIMO system for application in a PDA phone has been proposed. EMC chip antennas are used in the proposed MIMO system and a T-shaped shorted strip is added for isolation

Figure 10 Measured radiation patterns for antenna 3 at 2442 MHz for the proposed MIMO system studied in Fig. 2 (antenna 3 is excited with antennas 1 and 2 terminated to a 50⍀ load). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com]

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improvements. The proposed three-antenna MIMO system has been constructed and studied. The obtained bandwidths of the three antennas all cover the 2.4-GHz band for WLAN operation. In addition, although the available spaces inside the PDA phone for employing the antennas are very limited, good isolation between any two of the three antennas (all less than ⫺20 dB) has been obtained. The improvements in the isolation have been explained from the excited surface-current distributions in the system ground plane. Good radiation characteristics of the three antennas have also been obtained.

REFERENCES 1. Multiple-input multiple-output, Wikipedia, http://en.wikipedia.org/ wiki/MIMO. 2. K.L. Wong, Planar antennas for wireless communications, Wiley, New York, 2003, ch. 5. 3. K.L. Wong and C.H. Chang, An EMC foam-base chip antenna for WLAN operation, Microwave Opt Technol Lett 47 (2005), 80 – 82. 4. K.L. Wong and C.H. Chang, WLAN chip antenna mountable above the system ground plane of a mobile device, IEEE Trans Antennas Propagat 53 (2005), 3496 –3499. 5. C.M. Su, K.L. Wong, C.L. Tang, and S.H. Yeh, EMC internal patch antenna for UMTS operation in a mobile device, IEEE Trans Antennas Propagat 53 (2005), 3836 –3839. 6. K.L. Wong, Y.Y. Chen, S.W. Su, and Y.L. Kuo, Diversity dual-band planar inverted-F antenna for WLAN operation, Microwave Opt Technol Lett 38 (2003), 223–225. 7. K.L. Wong, A.C. Chen, and Y.L. Kuo, Diversity metal-plate planar inverted-F antenna for WLAN operation, Electron Lett 39 (2003), 590 –591. 8. K.L. Wong, J.H. Chou, C.L. Tang, and S.H. Yeh, Integrated internal GSM/DCS and WLAN antennas with optimized isolation for a PDA phone, Microwave Opt Technol Lett 46 (2005), 323–326. 9. K.L. Wong and J.H. Chou, Integrated 2.4- and 5-GHz WLAN antennas with two isolated feeds for dual-module application, Microwave Opt Technol Lett 47 (2005), 263–265. 10. K.L. Wong, J.H. Chou, S.W. Su, and C.M. Su, Isolation between GSM/DCS and WLAN antennas in a PDA phone, Microwave Opt Technol Lett 45 (2005), 347–352. 11. Ansoft Corporation, HFSS, http://www.ansoft.com/products/hf/hfss. © 2006 Wiley Periodicals, Inc.

1. INTRODUCTION

Wireless devices for future applications are required to provide image, speech, and data simultaneously. This demand spurs the need for antennas covering an ultra-wide bandwidth. Planar antennas are mostly suitable for these applications due to their compact size and impedance bandwidth that is continuously widened by using varying techniques. A simple square planar monopole is shown to have a ratio impedance bandwidth of 2:1, which has been greatly enhanced by using an offset feeding point, a semi-circular base [1], a shorting-post and beveling technique [2], a technique with double or three feeds [3], and so forth. In [4], a printed elliptic patch juxtaposed with a ground pattern in a single substrate is presented, where the impedance bandwidth is enhanced by using a new impedance-matching technique of cutting a notch in the ground pattern opposite the microstrip line. In [5], a CPW-fed planar T-shaped monopole antenna fed with a trapeziform ground plane is introduced, which has achieved a ratio impedance bandwidth of 6.3:1. In this paper, we analyze the impedance bandwidth of the antenna proposed in [5] using different characteristics impedances of the CPW feeder, and enhance its bandwidth notably by selecting an optimum characteristics impedance and using a linearly tapered CPW feeder. Both the theoretical and experimental results are presented and discussed. 2. ANTENNA DESIGN

The geometry of the proposed UWB printed monopole antenna is shown in Figure 1. Both the rectangular monopole of length b and width a and the trapeziform ground plane with top width D min , bottom width D max , and height H are etched on the same side of a RO4350 substrate with thickness h ⫽ 1.524 mm and relative permittivity ␧ r ⫽ 3.48. The distance between the rectangular monopole and the top side of trapeziform ground plane is t. A CPW feeder in the middle of the trapeziform ground plane is used to feed the monopole. By appropriately choosing the abovementioned parameters, the total input impedance may be adjusted to have slight relation with frequency, since the total input impedance can be viewed as consisting of the rectangular monopole’s selfimpedance, the trapeziform ground plane’s self-impedance, and the mutual-impedance between them. The proposed UWB planar monopole is designed by using the simulation software CST Mi-

TAPERED CPW-FED PRINTED MONOPOLE ANTENNA Xian-Ling Liang, Shun-Shi Zhong, and Wei Wang School of Communication and Information Engineering Shanghai University Shanghai 200072, China Received 5 January 2006 ABSTRACT: A compact coplanar rectangular monopole antenna fed by a tapered coplanar waveguide (CPW) feeder in the middle of a trapeziform ground plane is introduced. By using the tapered CPW feeder, the impedance bandwidth of the antenna is enhanced by a factor of about 1.7 times compared with a 50⍀ CPW feeder. The measured ratio impedance bandwidth reaches about 10.7:1, covering frequencies from 0.76 to 8.15 GHz. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1242–1244, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21666 Key words: rectangular monopole antenna; tapered coplanar waveguide; trapeziform ground plane

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Figure 1 Geometry of the proposed antenna

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