Guest Editorial: IEEE AWPL Special Cluster on Terminal ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

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Guest Editorial: IEEE AWPL Special Cluster on Terminal Antenna Systems for 4G and Beyond

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N THE past, mobile terminals were mainly used for voice calls and text messaging. However, wireless communication systems are nowadays essential for the acquisition and exchange of information. Over the past few years, the world has seen tremendous growth in the use of smartphones for dataintensive applications such as Internet access, online gaming, and video streaming. The latest figures from the International Telecommunication Union (ITU) [1] show that up to 2/3 of Internet users rely on mobile broadband service for Internet access, and the numbers are still growing. Therefore, from the terminal antenna perspective, the evolution toward the 4G and beyond mobile communication systems raises new challenges in terms of quality of performance. The requirements for terminal antennas were relatively relaxed in the 1990s and the 2000s, in response to the trend of decreasing terminal sizes. In contrast, antennas for today’s smart terminals must cope with ever-increasing system requirements, both in terms of information volume and signaling complexity to support the high data rates needed by multimedia-rich applications. In fact, antennas are now key components of such terminals, as random and time-varying wireless environments call for innovative solutions with reliable and seamless connection to the mobile networks. These antennas and their associated RF circuitry must be embedded in an unobtrusive way inside the handsets, in which the real state is extremely precious. In view of the stringent requirements on system performance, traditional design approaches may no longer be adequate, and multidisciplinary strategies encompassing electromagnetic field theory, radio engineering, signal processing, and fabrication technologies are becoming increasingly important. The introduction of multiple-input–multiple-output (MIMO) technology in mobile terminals can be seen as an important first step to meet the demand for increasing data rates (i.e., up to 1 Gb/s for 4G networks such as LTE-Advanced). This is because MIMO incurs no additional expense on either frequency spectrum or transmit power. Along with MIMO, many other antenna technologies and innovations are being developed for terminals to support advanced services under challenging operating conditions. It is the intention of this Special Cluster to collect high-quality works that are representative of current technological advances on terminal antenna systems. A. Advances in Terminal Antenna Design Although novel design strategies have appeared, traditional antenna element design still has its place in the development of

Digital Object Identifier 10.1109/LAWP.2014.2301025

antennas for mobile communications terminals. Hence, the first section of the cluster is dedicated to advances in conventional approaches to terminal antenna design. In the case of multiantenna terminals with diversity and MIMO capacity performance, it is important to be able to decouple the signals in the different antenna elements. Some interesting examples using neutralization lines are presented in [2] and [3]. The paper from Chattha et al. proposes an alternative decoupling approach by implementing polarization diversity in a handset, through the use of a planar inverted-F antenna (PIFA) with two different ports. The decoupling between the ports is performed by etching some slots in the ground plane. This technique had already been tested in previous works, for increasing the bandwidth of the antenna, improving the efficiency [4], or decoupling the effect of the ground plane [5]. In terminal antenna designs, the importance of antenna element miniaturization is still evident today. Indeed, modern antenna terminals include many different radio systems, and they have to integrate the corresponding radiating elements [6]. Therefore, physically small antennas will always be required, which will imply accepting some tradeoffs in terms of antenna performance [7]–[9]. However, in all cases, the main requirement is related to the thickness of the terminal. Modern antenna solutions need to be embedded, favoring planar structures [10], [11] that are either printed directly on the printed circuit board (PCB) of the handset or have a reduced height over it. In some cases, the use of lumped circuit elements will be necessary to overcome the problems caused by the small size of the antenna elements, to increase the number of frequency bands [12], and even to compensate for the effect of the human user [13]. This idea is further developed here by Caporal del Barrio et al., who propose a combination of highly isolated, narrowband tunable antennas and a tunable front end to overcome the challenge of bandwidth in LTE terminals. This architecture leads to highly efficient terminals with low power consumption. Lin et al., on the other hand, propose a multilayer meander-line antenna with very small size, which can operate in three LTE bands. Although metamaterials have found many interesting applications, their large electrical sizes have so far prevented them from being applied to terminal antenna design. Nevertheless, metamaterial-inspired antenna structures [14] that involve loading conventional antenna structure with metamaterial unit cells have been found to offer many attractive properties, including efficient miniaturization and multiband resonances. In this context, Ibrahim and Safwat generalize the design methodology for a multiband microstrip-fed planar monopole antenna

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loaded with composite right/left-handed (CRLH) unit cells, based on their earlier triband design. The multiband structure offers omnidirectional radiation pattern, which is desirable for terminal application. A pentaband design is presented to validate the design concept. In the remaining two papers in this section by Li et al., zerothorder resonators (ZORs) are used to provide either enhanced multiband performance to a terminal antenna, by improving the impedance bandwidth, or low mutual coupling between the resonators in an MIMO terminal. B. Emerging Topics on Terminal Antenna Design Novel design approaches and applications of terminal antennas offer a glimpse into exciting future possibilities, revitalizing a topic that may otherwise be perceived as mature and stagnant. One promising new research topic in terminal antenna systems is to apply the Theory of Characteristic Modes [15] to antenna design, which has become a topic of extensive research [16]–[23]. This possibility was inspired by Vainikainen et al. [24], [25], who were the first to highlight the important role of the chassis in terminal antenna design. In this context, Miers et al. propose to exploit the correlation between the characteristic currents and near fields of different modes with high modal significance to design a single feed that offers enhanced bandwidth and multiband resonances. Using this technique, a dual-band MIMO prototype (818–896 and 1841–2067 MHz) with average total efficiencies of above 1.5 dB and envelope correlation coefficient of below 0.1 is achieved in measurement. On the other hand, Martens et al. focus on achieving a broadband antenna that covers most of the LTE-Advanced spectrum using a capacitive element coupled to the PCB. In addition, dual-antennan MIMO operation is enabled at the 2.6-GHz band by a second port formed with the out-of-phase feeding of two inductive coupling elements (slots). This configuration is easy to integrate in an LTE terminal, and it allows for good isolation between the antenna ports, which facilitates good MIMO performance. Another exciting development in the field of mobile terminals is the appearance of new devices for new applications. In a world with ubiquitous communications, the now classic mobile phones have to coexist with new terminals of different shapes, sizes, and materials, foreseen for a myriad of applications. Here, some examples are highlighted. First, Cihangir et al. present a feasibility study of how to integrate several antennas in an eyewear device, as required for wearable computers with optical head-mounted displays. This trend has been led by Google, with some prototypes of the Google Glass already presented in early 2013 and a market introduction planned for 2014, with Wi-Fi 802.11b/g capability [26]. In this paper, the authors investigate how to incorporate additional communications standards and the effect it would have on the human user in terms of specific absorption rate (SAR). Further trends in mobile computing and communications will also require integrating mobile devices into clothing. In fact, the use of textile materials in antenna design has been extensively


studied [27], [28]. In this context, conductive threads can be woven or embroidered into textile material to form PIFAs, as suggested by Ivsic et al. In this case, the density and pattern of the threads are important. C. User Effects and Performance Characterization The final section of this cluster considers the more practical terminal antenna issues of antenna size–performance tradeoff, user effects, and over-the-air performance characterization, which are crucial to ensure the best possible performance of terminal antennas in their true operating environments. Abdullah Al-Hadi and Tian study the impact of the space (volume and location) available for the antenna in a handset on the performance of the system and extend the results to the case of multiantenna terminals. By optimizing the position and volume of the radiating elements, an improvement can be achieved in terms of impedance matching, decorrelation, and MIMO performance. Although the concept of beamspace MIMO has been introduced for some years, it has only been recently integrated into mobile terminals [29]. Yousefbeiki et al. investigate the influence of the user in terminal antennas that apply beamspace MIMO. Simulation results confirm that typical human body interaction scenarios do not significantly disturb the orthogonality of the beamspace MIMO basis, which ensures that the technique will still work effectively. Going beyond quantifying user effects, Vasilev et al. examine the potential benefits of applying adaptive impedance matching (AIM) to counteract performance degradation in MIMO terminals due to user proximity. Extensive field measurements involving a dual-band MIMO antenna prototype with and without real tuners demonstrate that AIM can provide significant performance gains, even when tuner losses are taken into account. Over-the-air (OTA) testing of MIMO terminals facilitates the evaluation of fully integrated terminals on the market, and appropriate test methodologies are being developed within the standardization bodies of 3GPP and CTIA. Fan et al. present a study on one candidate OTA test setup, based on channel emulation using multiprobes in anechoic chamber. The letter shows two techniques to obtain the weights as well as angular locations for the OTA probes for accurate reconstruction of the spatial correlation from standardized channel models. Simulation examples show that accurate spatial correlation can be reproduced with a small number of probes in a flexible setup. It is our sincere hope the letters collected in the Special Cluster will serve to attract greater interest in the field, as well as to provide a useful reference to active researchers in the field. We are confident that the exciting and rapid development of terminal antenna systems will help to ensure that wireless communications continue to revolutionize modern society, leading to vast improvements in the quality of life globally. Finally, the Guest Editors would like to thank the authors and the reviewers for adhering to the tight deadlines of this Special Cluster. We thank the outgoing and incoming IEEE ANTENNAS


AND WIRELESS PROPAGATION LETTERS Editors-in-Chief, Prof. Gianluca Lazzi and Prof. Yang Hao, respectively, as well as the editorial staff, Ms. Claire Sideri and Ms. Camille Ventura, for their tireless support in the preparation of this Special Cluster. We also acknowledge the role of two European Cooperation in Science and Technology (COST) Actions, i.e., IC1102 “Versatile, Integrated and Signal aware Technologies for Antennas” (VISTA) and IC1004 “Cooperative Radio Communications for Green Smart Environments,” in inspiring the creation of this Special Cluster based on their activities in this active research topic. The editors want also to dedicate this cluster to the memory of Prof. Pertti Vainikainen, one of the key contributors to the development of mobile terminal antennas.

BUON KIONG LAU, Guest Editor Lund University 221 00 Lund, Sweden (e-mail: [email protected]) MARIA MARTÍNEZ-VÁZQUEZ, Guest Editor IMST GmbH 47475 Kamp-Lintfort, Germany (e-mail: [email protected])

REFERENCES [1] ITU, Geneva, Switzerland, “Global ICT developments, 2001–2013,” 2013 [Online]. Available: http://www.itu.int/ict/statistics [2] A. Diallo, C. Luxey, P. Le Thuc, R. Staraj, and G. Kossiavas, “Study and, reduction of the mutual coupling between two mobile phone PIFAs, operating in the DCS1800 and UMTS bands,” IEEE Trans. Antennas, Propag., vol. 54, no. 11, pp. 3063–3074, Nov. 2006. [3] A. Diallo, C. Luxey, P. Le Thuc, R. Staraj, G. Kossiavas, M Franzen, and P.-S. Kildal, “MIMO performance of enhanced UMTS four-antenna structures for mobile phones in the presence of the user’s head,” in Proc. IEEE Int. Symp. Antennas Propag., Honolulu, HI, USA, 2007, pp. 2853–2856. [4] S.-J. Lee, J.-M. Seo, C.-W. Park, and Y.-S. Kim, “Radiation efficiency improvement for dual-mode mobile phone using a slot on a ground plane,” Electron. Lett., vol. 47, no. 19, pp. 1063–1065, 2011. [5] Y. Lu, Y. Huang, H. T. Chattha, and Y. Shen, “Technique for minimising the effects of ground plane on planar ultra-wideband monopole antennas,” Microw. Antennas Propag., vol. 6, no. 5, pp. 510–518, 2012. [6] P. Vainikainen, J. Holopainen, C. Icheln, O. Kivekäs, M. Kyrö, M. Mustonen, S. Ranvier, R. Valkonen, and J. Villanen, “More than 20 antenna elements in future mobile phones, threat or opportunity?,” in Proc. 3rd EuCAP, Berlin, Germany, Mar. 23–27, 2009, pp. 2940–2943. [7] A. K. Skrivervik, J.-F. Zürcher, O. Staub, and J. R. Mosig, “PCS antenna design: the challenge of miniaturization,” IEEE Antennas Propag. Mag., vol. 43, no. 4, pp. 12–27, Aug. 2001. [8] R. C. Hansen, “Fundamental limitations in antennas,” Proc. IEEE, vol. 69, no. 2, pp. 170–182, Feb. 1981. [9] Handbook on Small Antennas: Design, Technologies and Applications,, L. Jofre, M. Martínez-Vázquez, R. Serrano, and G. Roqueta, Eds. Brussels, Belgium: EurAAP, 2012.

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[10] Analysis, Design, and Measurement of Small and Low Profile Antennas, K. Hirasawa and M. Haneishi, Eds. Norwood, MA, USA: Artech House, 1992, ch. 5. [11] K.-L. Wong, Planar Antennas for Wireless Communications. Hoboken, NJ, USA: Wiley, 2003. [12] K. R. Boyle and P. J. Massey, “Nine-band antenna system for mobile phones,” Electron. Lett., vol. 42, no. 5, pp. 265–266, Mar. 2006. [13] K. R. Boyle, E. Spits, M. A. de Jongh, S. Sato, T. Bakker, and A. van Bezooijen, “A self-contained adaptive antenna tuner for mobile phones featuring a self-learning calibration procedure,” in Proc. 6th EuCAP, Prague, Czech Republic, Apr. 2012, pp. 1804–1808. [14] A. Erentok and R. Ziolkowski, “Metamaterial-inspired efficient electrically small antennas,” IEEE Trans. Antennas Propag., vol. 56, no. 3, pp. 691–707, Mar. 2008. [15] R. F. Harrington and J. R. Mautz, “Theory of characteristic modes for conducting bodies,” IEEE Trans. Antennas Propag., vol. AP-19, no. 5, pp. 622–628, Sep. 1971. [16] E. Antonino-Daviu, M. Cabedo-Fabres, and M. Ferrando-Bataller, “Modal analysis and design of band-notched UWB planar monopole antennas,” IEEE Trans. Antennas Propag., vol. 58, no. 5, pp. 1457–1467, May 2010. [17] J. Eichler, P. Hazdra, M. Capek, T. Korinek, and P. Hamouz, “Design of a dual-band orthogonally polarized L-probe-fed fractal patch antenna using modal methods,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 1389–1392, 2011. [18] A. Araghi and G. Dadashzadeh, “Oriented design of an antenna for MIMO applications using theory of characteristic modes,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 1040–1043, 2012. [19] H. Li, Y. Tan, B. K. Lau, Z. Ying, and S. He, “Characteristic mode based tradeoff analysis of antenna-chassis interactions for multiple antenna terminals,” IEEE Trans. Antennas Propag., vol. 60, no. 2, pp. 490–502, Feb. 2012. [20] H. Li, B. K. Lau, Z. Ying, and S. He, “Decoupling of multiple antennas in terminals with chassis excitation using polarization diversity, angle diversity and current control,” IEEE Trans. Antennas Propag., vol. 60, no. 12, pp. 5947–5957, Dec. 2012. [21] J. J. Adams and J. T. Bernhard, “Broadband equivalent circuit models for antenna impedances and fields using characteristic modes,” IEEE Trans. Antennas Propag., vol. 61, no. 8, pp. 3985–3994, Aug. 2013. [22] K. K. Kishor and S. V. Hum, “A two-port chassis-mode MIMO antenna,” IEEE Antennas Wireless Propag. Lett., vol. 12, pp. 690–693, 2013. [23] H. Li, Z. Miers, and B. K. Lau, “Design of orthogonal MIMO handset antennas based on characteristic mode manipulation at frequency bands below 1 GHz,” IEEE Trans. Antennas Propag., accepted for publication. [24] P. Vainikainen, J. Ollikainen, O. Kivekäs, and K. Kelander, “Resonator-based analysis of the combination of mobile handset antenna and chassis,” IEEE Trans. Antennas Propag., vol. 50, no. 10, pp. 1433–1444, Oct. 2002. [25] J. Villanen, J. Ollikainen, O. Kivekäs, and P. Vainikainen, “Coupling element based mobile terminal antenna structures,” IEEE Trans. Antennas Propag., vol. 54, no. 7, pp. 2142–2153, Jul. 2006. [26] Google, Mountain View, CA, USA, “Tech specs,” [Online]. Available: https://support.google.com/glass/answer/3064128?hl=en&ref_topic=3063354 [27] Antennas and Propagation for Body-Centric Wireless Communications, P. S. Hall and Y. Hao, Eds. Norwood, MA, USA: Artech House, 2006. [28] N. H. M. Rais, P. J. Soh, F. Malek, S. Ahmad, N. B. M. Hashim, and P. S. Hall, “A review of wearable antenna,” in Proc. Loughborough Antennas Propag. Conf., Loughborough, U.K., 2009, pp. 225–228. [29] J. Perruisseau-Carrier, O. N. Alrabadi, and A. Kalis, “Implementation of a reconfigurable parasitic antenna for beam-space BPSK transmissions,” in Proc. EuMC, Paris, France, Sep. 2010, pp. 644–647.

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Buon Kiong Lau (S’00–M’03–SM’07) received the B.E. degree (with honors) from the University of Western Australia, Perth, Australia, in 1998, and the Ph.D. degree from Curtin University of Technology, Perth, Australia, in 2003, both in electrical engineering. During 2000 to 2001, he worked as a Research Engineer with Ericsson Research, Kista, Sweden. From 2003 to 2004, he was a Guest Research Fellow with the Department of Signal Processing, Blekinge Institute of Technology, Karlskrona, Sweden. Since 2004, he has been with the Department of Electrical and Information Technology, Lund University, Lund, Sweden, where he is now an Associate Professor. He has been a Visiting Researcher with the Department of Applied Mathematics, Hong Kong Polytechnic University, Hong Kong; the Laboratory for Information and Decision Systems, Massachusetts Institute of Technology, Cambridge, MA, USA; and Takada Laboratory, Tokyo Institute of Technology, Tokyo, Japan. His main research interests include various aspects of multiple antenna systems, particularly the interplay between antennas, propagation channels, and signal processing. Dr. Lau is an Associate Editor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION and a Guest Editor of the 2012 Special Issue on MIMO Technology for the same journal. From 2007 to 2010, he was a Co-Chair of Subworking Group 2.2 on “Compact Antenna Systems for Terminals” (CAST) within EU COST Action 2100. Since 2011, he has been a Swedish National Delegate and the Chair of Subworking Group 1.1 on “Antenna System Aspects” within COST IC1004. Since 2012, he has been the Regional Delegate of the European Association on Antennas and Propagation (EurAAP) for Region 6 (Iceland, Norway, and Sweden). He is also a Member of the Education Committee within the IEEE Antennas and Propagation Society (AP-S), where he also serves as the Coordinator for the 2013–2015 IEEE AP-S Student Design Contests.

Marta Martínez-Vázquez (M’06–SM’09) was born in Santiago de Compostela, Spain, in 1973. She received the M.Sc. and Ph.D. degrees in telecommunication engineering from the Universidad Politécnica de Valencia, Valencia, Spain, in 1997 and 2003, respectively. In 1999, she was granted a fellowship from the Pedro Barrié de la Maza Foundation for postgraduate research with IMST GmbH, Kamp-Lintfort, Germany. In 2000, she joined the Antennas and EM Modelling Department of IMST as a full-time staff member. She works now there as a Senior Engineer and Project Manager. She has over 50 publications, including books, book chapters, journal and conference papers, and patents. She is also the co-organizer of the course “Antenna Project Management” of the European School of Antennas (ESoA) and is involved in other courses of the same. Her research interests include the design and applications of antennas for mobile communications, planar arrays, sensors, and RF systems. Dr. Martínez-Vázquez is the Chair of the COST IC1102 Action “Versatile, Integrated and Signalaware Technologies for Antennas (VISTA). Previously, she was a member of the Executive Board of the Antennas Centre of Excellence (ACE) Network of Excellence and the leader of its small antennas activity, and the Vice-Chair of the COST IC0603 Action “Antenna Sensors and Systems for Information Society Technologies.” She was also involved as an expert in the COST Action 260 “Smart Antennas” and as a working group leader in COST 284 “Innovative Antennas for Emerging Terrestrial & Space-Based Applications.” She is a member of the Board of Directors of the European Association of Antennas and Propagation (EurAAP), the Technical Advisory Panel for the Antennas and Propagation Professional Network of IET, and the IEEE AP-S Committees on “New Technology Directions” and “Education.” She is also a former member of the Administrative Committee of the IEEE Antennas and Propagation Society. She has also served as a Distinguished Lecturer for the IEEE AP-S. She is one of the Editors of the IEEE Antennas and Propagation Magazine, an Associate Editor of the IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, and a member of the Editorial Board of Radioengineering. She was awarded the 2004 Best Ph.D. award of the Universidad Politécnica de Valencia for her dissertation on small multiband antennas for handheld terminals, and the 2013 IEEE AP-S Lot Shafai Mid-Career Distinguished Achievement Award “for contribution to the development of antenna systems for practical applications from UHF to mm-waves and giving visibility to women engineers.”