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2290 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY2015 [13] H.-J. Stöckmann, Quantum Chaos: An Introduction. Cambridge, U.K.: Cambridge Univ. Press, 2006. [14] L. R. Arnaut and G. Gradoni, “Probability distributions of the quality factor of a mode-stirred reverberation chamber,” IEEE Trans. Electromagn. Compat., vol. 55, no. 1, pp. 35–44, Feb. 2013. [15] F. Monsef, “Why a reverberation chamber works at low modal overlap,” IEEE Trans. Electromagn. Compat., vol. 54, no. 6, pp. 1314–1317, Dec. 2012. [16] L. R. Arnaut, “Mode-stirred reverberation chambers: A paradigm for spatio-temporal complexity in dynamic electromagnetic environments,” Wave Motion, vol. 51, no. 4, pp. 673–684, Jun. 2014. [17] L. R.Arnaut, “Measurement uncertainty in reverberation chambers–I. Sample statistics,” Natl. Phys. Lab., Teddington, U.K., NPL Report TQE 2, 2nd ed., pp. 1–136, Dec. 2008. [18] L. R.Arnaut, M. I. Andries, J. Sol, and P. Besnier, “Evaluation method for the probability distribution of the quality factor of reverberation chambers,” IEEE Trans. Antennas Propag., vol. 62, no. 8, pp. 4199–4208, Aug. 2014. [19] L. R. Arnaut, “Compound exponential distributions for undermoded reverberation chambers,” IEEE Trans. Electromagn. Compat., vol. 44, no. 3, pp. 442–457, Aug. 2002. [20] L. R. Arnaut, “Evaluation of the NPL untuned stadium reverberation chamber using mechanical and electronic stirring techniques,” Natl. Phys. Lab., Teddington, U.K., NPL Report CEM 11, Aug. 1998. [21] L. Bernadó, T. Zemen, F. Tufvesson, A. F. Molisch, and C. F. Mecklenbräuker, “Delay and Doppler spreads of nonstation- ary vehicular channels for safety-relevant scenarios,” IEEE Trans. Veh. Technol., vol. 63, no. 1, pp. 82–93, Jan. 2014. [22] Y. Rissafi, L. Talbi, and M. Ghaddar, “Experimental characterization of an UWB propagation channel in underground mines,” IEEE Trans. Antennas Propag., vol. 60, no. 1, pp. 240–246, Jan. 2012. [23] T. J. Willink, “Observation-based time-varying MIMO channel model,” IEEE Trans. Veh. Technol., vol. 59, no. 1, pp. 3–15, Jan. 2010. [24] L. R. Arnaut, “Statistics of the quality factor of a rectangular reverberation chamber,” IEEE Trans. Electromagn. Compat., vol. 45, no. 1, pp. 61–76, Feb. 2003. A Wideband Compact WLAN/WiMAX MIMO Antenna Based on Dipole With V-shaped Ground Branch Han Wang, Longsheng Liu, Zhijun Zhang, Yue Li, and Zhenghe Feng Abstract—A wideband printed dipole with V-shaped ground branches is proposed, which is designed for multiple-input multiple-output (MIMO) antennas. It is based on a dipole with an integrated balun, and V-shaped ground branches are introduced to improve the impedance matching. The bandwidth of this element reaches 62.3% (2.30–4.40 GHz), which covers all three WiMAX bands (2.30, 2.50, and 3.30 GHz) and the 2.40 GHz WLAN band. Based on this element, a quad-element MIMO antenna is designed and fabricated. By reusing the V-shaped ground structure between adja- cent elements, the size of this quad-element antenna is only 0.31λ × 0.31λ × 0.01λ. Meanwhile, a bandwidth of 60.6% (2.30–4.30 GHz) is achieved, in which the S 11 < -10 dB, S 12 < -10 dB, and S 13 < -13 dB. Directional radiation patterns with 2.1 dBi average gain are attained, which are very stable throughout this band. This antenna system can be suitable for multielement MIMO devices such as wireless routers and adapters. Index Terms—Multiple-input multiple-output (MIMO), V-shaped dipole, wireless local area network (WLAN). I. I NTRODUCTION During the last decades, the growing demand for high-speed wireless data access has promoted the development of broadband wireless access techniques such as wireless local area network (WLAN) and world interoperability for microwave access (WiMAX). More spectrum resources have been allocated, and multiple-input-multiple- output (MIMO) technology has widely been deployed in these systems to further improve the spectrum efficiency. It cooperates with multiple independent spatial streams simultaneously to increase the channel capacity, and these spatial streams are sent from an M-element array to an N -element array, with which the MIMO system is called an M × N MIMO system. In mature wireless applications that are based on 802.11n (WLAN) and 802.16e (WiMAX), two spatial streams (M,N ≤ 2) are typically deployed, and extensive studies have been performed on the dual-element MIMO antenna design. Elements with compact size and wideband characteristics have been proposed in [1]–[3], and low-mutual coupling is achieved with various methods such as metamaterial-based isolator [4], decoupling network [5], and parasitic element [6]. However, with a growing demand for higher transmitting rates, new wireless standards appeared, such as 802.11ac (WLAN) [7] and 802.11 m (WiMAX) [8] that support from four to eight spatial streams. MIMO systems with 4 × 4 up to 8 × 8 configurations have gradually Manuscript received July 30, 2014; revised February 02, 2015; accepted February 08, 2015. Date of publication February 24, 2015; date of current version May 01, 2015. This work was supported in part by the National Basic Research Program of China under Contract 2013CB329002, in part by the National High Technology Research and Development Program of China (863 Program) under Contract 2011AA010202, in part by the National Natural Science Foundation of China under Contract 61271135, in part by the National Science and Technology Major Project of the Ministry of Science and Technology of China 2013ZX03003008-002, in part by the China Postdoctoral Science Foundation funded project 2013M530046. The authors are with the State Key Laboratory of Microwave and Communications, Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing, 100084, China (e-mail: zjzh@ tsinghua.edu.cn). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2015.2405091 0018-926X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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