Wideband tri-port MIMO antenna with good pattern diversity for a MIMO system. This design is very much compact size and directional radiation suited to wideband 3T × 3R MIMO systems, especially for those that pattern have limited space for the antenna system.

Han Wang, Longsheng Liu, Zhijun Zhang and Zhenghe Feng Antenna design: The geometry of the proposed MIMO antenna is shown in Fig. 1. It is fabricated on a triangle-shaped FR-4-based printed circuit board with truncated corners, whose dimensions are shown in Figs. 1b A wideband tri-port multiple-input-multiple-output (MIMO) antenna designed for worldwide interoperability for microwave access and c. On the bottom side of the substrate, three printed dipoles are (WiMAX) and wireless local area network (WLAN) applications is pre- placed rotationally symmetrically around a round-shaped central sented. The proposed antenna is composed of three printed dipoles with ground with an interval of 120°, and are fed with the feeding lines an integrated balun, the size of which is only 0.08λ2. By introducing located on the top side of the substrate. Since both arms of these dipole ground stubs between adjacent elements, impedance matching is elements are connected to the central ground with a λ/4 length strip con- improved and mutual coupling is reduced. A bandwidth of 51.6% ductor, an integrated balun is formed, with which the current on the arms – − − (2.30 3.90 GHz) with S11 < 10 dB and S12, S13 < 10 dB is achieved, is balanced. Meanwhile, the length of the feeding line is also close to λ/4, which can cover the whole three WiMAX bands and the 2.4 GHz thus it acts like an impedance transformer, and a broadband matching is WLAN band. Moreover, directional radiation patterns are achieved in achieved by tuning the width and length of this feeding line. these bands, and the average gain reaches 3.5 dB with up to 21.4 dB front-to-back ratio. The envelope correlation coefficient is below Since the space between elements is quite small, three ground stubs 0.038, which can provide good pattern diversity for a MIMO system. are introduced between the adjacent elements to reduce strong coupling. Meanwhile, these three stubs extend the size of the ground, thus improv- ing the impedance matching at lower frequencies. As a result, the Introduction: Multiple-input-multiple-output (MIMO), as a wireless −10 dB matching band covers from 2.30 to 3.90 GHz, and the mutual technology that can increase the channel capacity effectively, has coupling is below − 10 dB throughout this band. Moreover, these become an essential technique in most wireless standards such as wire- metal stubs also act like reflectors, and three-directional radiation pat- less local area network (WLAN) and worldwide interoperability for terns are achieved with 120° intervals in the azimuth (X–O–Y) plane. microwave access (WiMAX) [1]. Since the channel capacity is directly They are characterised with 3.5 dB average gain and up to 21.4 dB related to the number of spatial streams, the 3T × 3R up to 8T × 8R front-to-back ratio, which can provide very good pattern diversity for MIMO system has gradually been deployed to achieve higher bit rates a multi-elements MIMO system. than a classic 2T × 2R one. This presents new challenges for MIMO antenna designs since the bandwidth and the mutual coupling deteriorate when integrating more elements in one antenna. It is particularly serious for the antenna designs in portable devices at the receiving end, since the elements are closely spaced and the size is limited. In the existing literature, most MIMO antenna designs for multi- elements (N > 2) MIMO systems are of narrowband [2, 3], which is not suitable for WiMAX/WLAN application, which requires wideband coverage from 2.30 to 3.80 GHz. For wideband designs [4, 5], the size is comparatively large, which does not suit the portable devices at the receiving end.

Z 8.9 metal feeding Z Y substrate lines Y Fig. 2 Fabricated prototype X X

FR-4 substrate 41.1 0 WiMAX WiMAX WiMAX (1.6 mm thick) 1.9 feeding 2.30–2.36 GHz 2.50–2.90 GHz 3.30–3.8 GHz 15.3 –5 vias shorting 3.7 vias –10 shorting b vias dipole with Y Z –15 integrated balun 1.9 dipole with X WLAN ground 17.2 dB -parameter, –20

integrated balun S 2.40–2.48 GHz stubs 17.6 –25 simulated return loss S 0.8 11 measured return loss S11 simulated mutual coupling S measured return loss S 6.4 12 12 dipole with 10.5 –30 integrated balun central ground 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 a c frequency, GHz

Fig. 1 Geometry and dimensions (in millimetres) of proposed antenna Fig. 3 Measured and simulated S-parameters a Structure view b Top view Prototype and measured results: To verify the performance of the pro- c Bottom view posed antenna, a prototype was built and measured. Fig. 2 shows its photo image, in which three semi-rigid coaxial cables are connected to In this Letter, a wideband tri-port MIMO antenna designed for perform the measurements. Considering that the radiation elements are WiMAX and WLAN applications is proposed, whose bandwidth rotationally symmetrical and the measured results are similar for all covers the whole three WiMAX bands (2.30–2.36, 2.50–2.90 and three ports, the results of port 1 are provided as follows. 3.30–3.80 GHz) and the 2.4 GHz WLAN band (2.40–2.48 GHz). The S-parameters of the prototype were measured with an Agilent© Printed dipoles with an integrated balun are adopted as the radiation vector network analyser E5071B and the results are shown in Fig. 3. elements, and the size of this tri-port antenna is only 0.08λ2.To It can be observed that good impedance matching and high isolation reduce the mutual coupling between these closely spaced elements, are achieved in both the measured and simulation results. The return ground stubs are introduced and the coupling level is below −10 dB loss (S11) and mutual coupling level (S12) are below −10 dB from in the operating bands. Meanwhile, these stubs improve the impedance 2.30 to 3.90 GHz, which covers the whole three WiMAX bands and matching at lower frequency, with which the bandwidth reaches 51.6% the 2.4 GHz WLAN band. (2.30–3.90 GHz). Moreover, directional radiation patterns with a low The radiation patterns were measured in the ETS anechoic chamber envelope correlation coefficient (ECC) are achieved, which provide AMS8500, and the normalised patterns for 2.35, 2.45, 2.70 and

ELECTRONICS LETTERS 28th August 2014 Vol. 50 No. 18 pp. 1261–1262 3.55 GHz are provided in Fig. 4. These results reflect the radiation expected. It is below 0.038 throughout the band, which means good characteristics of the proposed MIMO antenna in the central frequency pattern diversity is achieved with the proposed antenna. of the three WiMAX bands and the 2.4 GHz WLAN band, and the measured results match the simulation ones well. Directional radiation 6 0.10 patterns can be observed at all frequencies, and a more than 10.1 dB front-to-back ratio is achieved throughout the band. 4 0.08 2 simulated co-pol pattern measured co-pol pattern simulated cross-pol pattern measured cross-pol pattern 0 0.06 0 0 0 0 –2

gain, dB 0.04 –10 –10 300 60 300 60 –4 –20 –20 –6 0.02 –30 –30

–8 simulated gain simulated ECC correlation coefficient envelope 0 –20 –20 measured gain measured ECC 240 120 240 120 –10 –10 –10 X Z 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 0 Y 0 X frequency, GHz X–O–Y plane Z X–O–Z plane Y a Fig. 5 Measured and simulated gain and ECC 0 0 0 0 Conclusion: In this Letter, a wideband tri-port MIMO antenna is pro- –10 –10 posed and fabricated, which is designed for WiMAX and WLAN appli- 300 60 300 60 cations. The size of this antenna is only 0.08λ2, which can be easily –20 –20 integrated into portable devices at receiving ends. Dipoles with an in- –30 –30 tegrated balun are adopted as radiation elements, and three ground

–20 –20 stubs are introduced to lower the mutual coupling. Meanwhile, these stubs extend the ground size, thus improving the impedance matching 240 120 240 120 –10 –10 at lower frequencies. The simulated and measured results verify that X Z 0 Y 0 X the bandwidth of the proposed antenna reaches 51.6% (2.30– X–O–Y plane Z X–O–Z plane Y 3.90 GHz) and the mutual coupling level is below −10 dB, which is b capable of covering the whole three WiMAX bands and the 2.4 GHz 0 0 WLAN band. Moreover, directional radiation patterns with an up to 0 0 21.4 dB front-to-back ratio and 3.5 dB average gain are achieved, and –10 –10 300 60 300 60 the ECC level is 0.038 at maximum. Consequently, this design is –20 –20 suited to be implemented in a 3T × 3R MIMO system with superior characteristics such as compact size, wideband, low mutual coupling –30 –30 and good pattern diversity. –20 –20 240 120 240 120 Acknowledgment: This work was supported by the National Science –10 –10 X Z and Technology Major Project of the Ministry of Science and 0 Y 0 X Technology of China 2013ZX03003008-002. X–O–Y plane Z X–O–Z plane Y c 0 0 © The Institution of Engineering and Technology 2014 0 0 25 June 2014 –10 –10 doi: 10.1049/el.2014.2291 300 300 60 60 One or more of the Figures in this Letter are available in colour online. –20 –20 Han Wang, Longsheng Liu, Zhijun Zhang and Zhenghe Feng (State Key –30 –30 Laboratory of Microwave and Communications, Tsinghua National –20 –20 Laboratory for Information Science and Technology, Tsinghua 240 120 240 120 University, Beijing 100084, People’s Republic of China) –10 –10 X Z E-mail: [email protected] 0 Y 0 X X–O–Y plane Z X–O–Z plane Y References d 1 Li, Q.H., Lin, X.T., Zhang, J.Z., and Roh, W.: ‘Advancement of MIMO Fig. 4 Measured and simulated radiation patterns at different frequencies technology in WiMAX: from IEEE 802.16d/e/j to 802.16 m’, IEEE – a 2.35 GHz Commun. Mag., 2009, 47, (6), pp. 100 107 ‘ b 2.45 GHz 2 Zhang, S., Zetterberg, P., and He, S.: Printed MIMO antenna system of c 2.70 GHz four closely-spaced elements with large bandwidth and high isolation’, d 3.55 GHz Electron. Lett., 2010, 46, (15), pp. 1052–1053 3 Luo, Y., Chu, Q.X., Li, J.F., and Wu, Y.T.: ‘Planar H-shaped directive The gain was also measured in the ETS anechoic chamber, and the antenna and its application in compact MIMO antenna’, IEEE Trans. – results are provided in Fig. 5. It can be observed that the gain is very Antennas Propag., 2013, 61, (9), pp. 4810 4814 stable throughout the band, and the measured result is in good agree- 4 Costa, J.R., Lima, E.B., Medeiros, C.R., and Fernandes, C.A.: ‘Evaluation of a new wideband slot array for MIMO performance ment with the simulation one. The peak gain reaches 3.9 dB in the enhancement in indoor WLANs’, IEEE Trans. Antennas Propag., measurement and the average gain is 3.5 dB. 2011, 59, (4), pp. 1200–1206 5 Nguyen, K.K., Huynh, N.B.P., Quang, N.H., and Chien, D.N.: MIMO performance: To evaluate the MIMO performance of the pro- ‘A compact printed 4 × 4 MIMO-UWB antenna with WLAN band rejec- posed antenna, the ECC was calculated [6] and is shown in Fig. 5.This tion’. IEEE Antennas and Propagation Society Int. Symp., Orlando, FL, value reflects the pattern diversity characteristics of a MIMO antenna, USA, July 2013, pp. 2245–2246 and a lower ECC level means higher diversity in general. Since the radiation 6 Blanch, S., Romeu, J., and Corbella, I.: ‘Exact representation of antenna system diversity performance from input parameter description’, patterns of the proposed antenna are directional and are spaced at intervals – of 120° between adjacent ports, the ECC level is well controlled, as Electron. Lett., 2003, 39, (9), pp. 705 707

ELECTRONICS LETTERS 28th August 2014 Vol. 50 No. 18 pp. 1261–1262