A 5-GHz 4/8-ELEMENT MIMO ANTENNA SYSTEM FOR IEEE 802.11AC DEVICES

Tech. report, Federal Communications Commission, Washington DC, 2002. 2. J.Y. Sze and K.L. Wong, Bandwidth enhancement of a microstripline-fed printed wide-slot antenna, IEEE Trans Antennas Propag 7 (2001), 1020 1024. 3. H.G. Schantz, A brief history of UWB antennas, IEEE Aerospace Electron Syst Mag 4 (2004), 22 26. 4. X. Qing and Z.N. Chen, Monopole-like slot UWB antenna on LTCC, In: Proceedings of the 2008 IEEE international conference on ultra-wideband (ICUWB2008), 2008, pp. 121 124. 5. C.-L. Tsai and C.-L. Yang, Novel compact eye-shaped UWB antennas, IEEE Antennas Wireless Propag Lett 11 (2012), 184 187. 6. M. Bod and M.M.S. Taheri, Compact UWB printed slot antenna with extra bluetooth, GSM, and GPS bands, IEEE Antennas Wireless Propag Lett 11 (2012), 531 534. 7. Y. Sung, UWB monopole antenna with two notched bands based on the folded stepped impedance resonator, IEEE Antennas Wireless Propag Lett 11 (2012), 500 502. 8. S.R. Emadian, C. Ghobadi, J. Nourinia, M. Mirmozafari, and J. 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They are a key enabling component to achieve high data rates for current and future needs of wireless communication services. Therefore, there is an increasing demand for new MIMO antenna systems which are compatible with user terminal devices. In such systems, it is important to have antennas that are compact and can be easily integrated within a small form factor and yet have high isolation and low correlation factors. These requirements make the design of MIMO antenna systems challenging. The 5-GHz band has been assigned for wireless LAN s (WLAN) in the IEEE 802.11ac standard [1]. The standard specifies two-element, four-element, and eight-element MIMO antennas. The minimum bandwidth in this standard has been specified to be 80 MHz. This new standard will provide higher data throughputs than the current IEEE 802.11n (a maximum theoretical data rate of 1.733 Gbps at 80-MHz channel bandwidth and 256-QAM modulation scheme). The two main features for this new standard at the antenna side are the increase in the operating bandwidth and the number of elements in the MIMO antenna system at the user terminal. In Ref. 2, a quad-band two-element MIMO antenna was presented. One of its bands of operation was 5.15 5.35 GHz. It used a k/4 resonator and a combination of C-shaped slot and T- shaped slit with a printed inverted F-antenna (PIFA). The dimensions of a unit element were 11.5 13.5 4mm 3. Two elements were mounted on a ground plane of 50 100 mm 2. The mean effective gain of each antenna element was 2.1 dbi at 5.2 GHz. The antenna reported in Ref. 3 was a frequency reconfigurable two-element MIMO antenna. The antenna elements of the antenna were printed monopoles. PIN diodes were used in the design for the frequency configurability. The antenna had three bands of operation out of which one was in the range of 5.15 5.35-GHz band. The dimensions of the antenna were 80 40 0.8 mm 3. The mean effective gain of the antenna elements in the 5-GHz band was reported to be 1.87 dbi. VC 2013 Wiley Periodicals, Inc. A 5-GHz 4/8-ELEMENT MIMO ANTENNA SYSTEM FOR IEEE 802.11AC DEVICES Mohammad S. Sharawi Department of Electrical Engineering, King Fahd University for Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia; Corresponding author: msharawi@kfupm.edu.sa Received 22 October 2012 ABSTRACT: A highly compact 2 2 (four element) and 2 4 (eight element) multiple-input-multiple-output (MIMO) antenna systems are designed for the IEEE 802.11ac standard. The antennas operate in the 5-GHz band with a minimum effective bandwidth of 80 MHz. The elements of the MIMO antenna system are patch antennas loaded with complementary split-ring resonators. A minimum isolation of 10.5 db and maximum gain of 0.8 dbi are measured. Total size of the MIMO antenna systems is 50 100 0.8 mm 3. VC 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1589 1594, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27611 Key words: multiple-input-multiple-output antennas; complementary split-ring resonator; WLAN; IEEE 802.11ac 1. INTRODUCTION Multiple-input-multiple-output (MIMO) antenna systems have emerged as an integral part of the new 4G wireless standards. Figure 1 Geometry of the 2 4 (eight-element) MIMO antenna system; (a) top side and (b) bottom side DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 1589

Figure 2 Fabricated MIMO antenna systems; (a) four-element and (b) eight-element. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] A dual-band MIMO antenna designed for WLAN was reported in Ref. 4. The MIMO antenna had three-elements each made up of a circular dual-loop antenna. The antenna operated in the 2.4 and 5 GHz bands. The antenna elements were placed on a circular substrate whose diameter was 120 mm. A peak gain of 8.7 dbi was reported in the 5-GHz band. In Ref. 5, a four-element dual-band MIMO antenna was presented. Three different designs based on the element used were presented. Minkowski monopole, Kochi monopole, and PIFA were used as antenna elements in the three different designs. The overall area occupied by each design was 88.7 46.6 mm 2. The antenna operated in the 2.4 and 5-GHz bands. The mean effective gain of the antenna element was 5.62 dbi in the 5-GHz band. A wideband, two-element MIMO antenna designed for mobile phones in the 5-GHz band was reported in Ref. 6. The antenna elements were made up of PIFA. The area Figure 3 Simulated and measured s-parameters for the four-element MIMO antenna system; (a) reflection coefficient and (b) isolation 1590 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 DOI 10.1002/mop

Figure 4 Measured s-parameters for the eight-element MIMO antenna system; (a) reflection coefficient and (b) isolation covered by each element was 5 12 mm 2. The antenna elements were placed on a ground plane whose dimensions were 50 100 1.524 mm 3. The antenna operated in the 4.7 6.2 GHz frequency range. In this article, a compact 2 2 (four-element) and 2 4 (eight-element) MIMO antenna systems are presented. The antennas are designed to operate in the 5-GHz band to comply with the IEEE 802.11ac standard. The antenna elements consist of patch antennas loaded with complementary split-ring resonators (CSRRs) for antenna miniaturization. This is the first work that presents an eight-element MIMO antenna within the standard user terminal size of 50 100 0.8 mm 3. The performance of the proposed MIMO antenna systems (four-element and eight-element) is measured and discussed. The rest of the article is organized as follows. The design of the proposed MIMO antenna systems is outlined in Section 2. Simulation and measurement results are discussed in Section 3. Conclusions are summarized in Section 4. 2. 2 3 2 AND 2 3 4 MIMO ANTENNA SYSTEM DESIGN Figure 1 shows the geometry of the proposed eight-element MIMO antenna system. The four-element design is similar but only occupies half of the top layer of the PCB. The antennas were designed on an FR-4 substrate with dielectric constant of 4.4. The thickness of the substrate was 0.8 mm. The basic antenna element was a rectangular patch with an inset feed matched to 50X. The spacing between the elements was 5 mm. The dimensions of each patch antenna were 11 8mm 2. Such a patch has a resonant frequency of 6.52 GHz when operated as is. A CSRR was etched out from the ground (GND) plane underneath the center of each patch antenna to reduce its resonant frequency. A CSRR is the negative image of a split-ring resonator (SRR). An SRR is made up of two concentric copper rings with slits in each ring. The inner ring resides inside the outer ring with some separation while the slits in each ring are in opposite direction to one another. A CSRR is made by removing copper in the shape of an SRR from the GND plane. The CSRR interacts with the electric field and provides effective negative permittivity around its resonant frequency. The resonant frequency of a CSRR is the same as that of a SRR and it is modeled as a LC tank circuit. The resonant frequency depends on the dimensions of the CSRR. The lumped element model for a SRR and CSRR are derived in Ref. 7 which also give quasianalytical equation for finding the resonant frequency of a SRR and CSRR. Using CSRRs over a patch or underneath, it was also investigated for antenna miniaturization for single antenna applications [8, 9]. A patch antenna can be modeled as a resistor-inductorcapacitor circuit at its resonance. A CSRR acts as an LC circuit. The interaction of the patch antenna and the CSRR results in an equivalent model which lowers the overall resonant frequency of the CSRR-loaded patch antenna. DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 1591

Figure 5 TARC curves for the eight-element MIMO antenna system at angles between 0 and 180 In our proposed design, the dimensions of the CSRR were varied and optimized via simulations to tune the antenna at 5 GHz. The patch elements resonated at 5 GHz when the radius of the outer ring of the CSRR r was 2.5 mm, the width of the rings w were 0.25 mm, the spacing between the rings s was 0.5 mm, and the width of the slit d was 0.5 mm. The feed line was shifted 1.5 mm along the width of patch antenna from its central axis for proper mode excitation. 3. RESULTS AND DISCUSSION The proposed antennas were designed and optimized using HFSS TM. They were then fabricated and measured. The scattering parameters of the MIMO antennas were measured using an Agilent HP8510C network analyzer. The two dimensional (2D) gain measurements of the MIMO antennas were carried out at an outdoor antenna range facility (Oakland University, MI). The dimensions of the CSRR had a profound effect on the resonant frequency of the antenna element (see Fig. 1 for parameter location). It was found that d had no effect on the resonant frequency of the antenna. Therefore, it was kept at 0.5 mm. The resonant frequency of the antenna was found to have an inverse relationship with r. Asr was increased, the resonant frequency of antenna decreased and vice versa. An increase in w and s resulted in an increase in the resonant frequency of antenna and vice versa. A knowledge of these relationships helped in tuning the resonant frequency of the antenna at the desired value. Figure 2 shows the fabricated MIMO antenna systems (fourelement and eight-element). The measured and simulated s-parameters for the four-element MIMO antenna are shown in Figure 3. The resonant frequency was approximately 5.08 GHz. A minimum 6dB bandwidth of 95 MHz was measured and a minimum isolation of 10.6 db was recorded between Elements 1 and 2. Figure 4 shows the measured reflection coefficient curves for the eight-element MIMO antenna system as well as the isolation curves between Element 1 and all others. A minimum bandwidth of 80 MHz was measured and minimum isolation of 10.5 db was obtained between Elements 1 and 2. The evaluation of a multiport antenna system is insufficient using scattering parameters only. For proper broadband characterization of a multiport antenna, the total active reflection coefficient (TARC) should be evaluated [10]. For an N-element MIMO antenna system, TARC is given by, Figure 6 Gain patterns of antenna Elements 1 and 2 for the 4four-element MIMO antenna measured at 5.08 GHz (a) x-z plane and (b) y-z plane. [Circles is co-pol Element 1, solid is co-pol Element 2, dots is cross-pol Element 1, and dashes are cross-pol Element 2] 1592 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 DOI 10.1002/mop

system [11]. The worse case correlation values obtained were between two opposite elements, that is, 1 and 2, 3, and 4, and so forth, with a maximum value of 0.28 at 50% efficiency. This is an acceptable level of correlation within this standard. The correlation coefficient for all other antenna element pairs was less than 0.05. The 2D gain patterns of the MIMO antennas were measured. These measurements were performed at 5.08 GHz. Figure 6(a) shows the normalized gain patterns of antenna Elements 1 and 2 of the four-element MIMO antenna system in the x-y plane, and Figure 6(b) shows the normalized gain patterns of antenna Elements 1 and 2 in x-z plane. Similarly, Figure 7(a) shows the normalized gain patterns of antenna Elements 1 and 2 of the eightelement MIMO antenna system in the x-y plane, and Figure 7(b) shows the normalized gain patterns of antenna Elements 1 and 2 in x-z plane. The maximum gain of the antenna elements in both MIMO antenna systems was 0.8 dbi. It was observed that the gain patterns of the individual patch element were similar to that of a conventional patch antenna with higher back lobes which were due to the CSRR. The low maximum gain value obtained (i.e., 0.8 dbi) was because of antenna miniaturization. 4. CONCLUSION In this article, a highly compact 2 2 and 2 4 MIMO antenna systems were presented for the new IEEE 802.11ac standard. The design was confined within a user terminal device size of 100 50 0.8 mm 3. The elements of the MIMO antennas were patch antennas miniaturized using CSRR loading. The two MIMO antenna systems had an effective operating bandwidth of at least 80 MHz (from TARC calculations) and minimum isolation of 10.5 db at 5.05 GHz. The measured gain patterns of the antennas of the antenna showed a maximum gain of 0.8 dbi. Figure 7 Gain patterns of antenna Element 1 and 2 for the eight-element MIMO antenna measured at 5.08 GHz (a) x-z plane and (b) y-z plane. [Circles is co-pol Element 1, solid is co-pol Element 2, dots is cross-pol Element 1, and dashes are cross-pol Element 2] ACKNOWLEDGMENTS The author would like to acknowledge the support provided by the deanship of scientific research (DSR) at King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia, through project number RG1219. Also, would like to thank Mr. M. U. Khan for his help in the simulations. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P N i¼1 C ¼ jb ij 2 q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P (1) N i¼1 ja ij 2 where a i and b i are the incident and reflected signals, respectively. For the proposed antenna systems, TARC was calculated from the measured s-parameters. The amplitude of all ports was kept at unity while the excitation phases were varied with respect to Port 1. For various combinations of port excitations, TARC curves obtained for the eight-element MIMO antenna system are shown in Figure 5. For all the combinations, a minimum effective bandwidth of 80 MHz was achieved. Similar behavior was observed for the four-element MIMO antenna. The envelope correlation coefficient q gives a measure of how much the antennas are correlated with each other in a MIMO antenna system. A high correlation coefficient degrades the performance of a MIMO antenna system and yields poor use of antenna diversity. The correlation coefficient can be calculated from the measured scattering parameters of the antenna REFERENCES 1. D.A. Hall, Underneath the hood of 802.11ac, Microwave J 54 (2011), 46 52. 2. R.A. Bhatti, J.-H. Choi, and S.-O. Park, Quad-band MIMO antenna array for portable wireless communications terminals, IEEE Antennas Wirel Propag Lett 8 (2009), 129 132. 3. Z.-J. Jin, J.-H. Lim, and T.-Y. Yun, Frequency reconfigurable multiple input multiple-output antenna with high isolation, IEEE Microwave Antennas Propag 6 (2012), 1095 1101. 4. S.-W. Su and C.-T. Lee, Printed, low-cost, dual-polarized dual loop-antenna system for 2.4/5 GHz WLAN access points, In: Proceedings of the 5th European Conference on Antennas and Propagation (EUCAP), 2011, pp.1253 1257. 5. M. Karaboikis, C. Soras, G. Tsachtsiris, and V. Makios, Four-element printed monopole antenna systems for diversity and MTMO terminal devices, In: Proceedings of the 17th international Conference on Applied Electromagnetics and Communications, 2003, pp. 193 197. 6. S. Vergerio, M. Elayachi, J.-P. Rossi, and P. Brachat, Design of multiple antennas at 5 GHz for mobile phone and its MIMO performances, In: International Conference on Electromagnetics in Advanced Applications, (ICEAA 2007), 2007, pp.17 20. DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 1593

VC 7. J.D. Baena, J. Bonache, F. Martin, R. Marques, F. Falcone, T. Lopetegi, M.A.G. Laso, J. Garcia, I. Gil, and M. Sorolla, Equivalent-circuit models for split-ring resonators and complementary split-ring resonators coupled to planar transmission lines, IEEE Trans Microwave Theor Tech 53 (2005), 1451 1461. 8. X. Cheng, et. al. Characterization of microstrip patch antennas on metamaterial substrates loaded with complementary split-ring resonators, Microwave Opt Technol Lett 50 (2008), 2131 2135. 9. Dong, H. Toyao, and T. Itoh, Design and characterization of miniaturized patch antenna loaded with complementary split-ring resonators, IEEE Trans Antennas Propag 60 (2012), 772 785. 10. M. Manteghi and Y. Rahmat-Samii, Multiport characteristics of a wideband cavity backed annular patch antenna for multipolarization operations, IEEE Trans Antennas Propag 53 (2005), 466 474. 11. P. Hallbjorner, The significance of radiation efficiencies when using S-parameters to calculate the received signal correlation from two antennas, IEEE Antennas Wirel Propag Lett 4 (2005), 97 99. 2013 Wiley Periodicals, Inc. LOW-POWER PHOTONIC CONTROL OF A MICROWAVE RING RESONATOR USING BULK ILLUMINATION Mohammad Ali Shirazi-Hosseinidokht and Mani Hossein-Zadeh Center for High Technology Materials, 1313 Goddard, SE, Albuquerque, NM 87106; Corresponding author: mhz@chtm.unm.edu Received 22 October 2012 ABSTRACT: We demonstrate the feasibility of bulk illumination technique for low-power photonic control of RF resonance. Using this technique, the transmitted RF power through a microstripline-ring filter on a junction-less silicon substrate is changed by 11 db with less than 2 mw of interacting optical power. VC 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1594 1599, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27606 Key words: light-controlled RF devices; RF ring resonator; RF-photonics 1. INTRODUCTION Photonic control of RF signal propagation is an important topic that continues to be an active area of research and development in microwave photonics [1 14]. Photonic control has many advantages over conventional electrical control. High degree of electrical isolation between the control signal and the microwave circuit, immunity to parasitic electromagnetic radiation, high power handling, overall weight reduction, high-speed control, and timing precision are among the most important benefits of photonic control. In particular, electrical isolation between the control signal and the microwave structure is crucial for design and fabrication of reconfigurable antennas [15 18] where the radiation pattern and efficiency are affected by the presence of control devices and circuits in the vicinity of antenna pattern [15]. A large variety of techniques, devices, and materials have been explored for designing photonically controlled switches, phase shifters, and attenuators. In almost all these approaches, photonic carrier generation in a semiconductor controls the amplitude and the phase of the RF signal propagating on microstrip or coplanar transmission lines. Free carrier generation in biased and unbiased junctions as well as junction-less regions have been used to control the RF field in discontinuities [8, 11], stubs [9, 13], resonators [6], and terminations [4, 5, 9]. Except few cases where the photosensitive element is added to a transmission line fabricated on a low-loss RF substrate [1, 15, 18], in most proposed structures the RF circuit is fabricated on the photosensitive semiconductor substrate in order to reduce the complexity of the fabrication process and keep the device monolithic. Although compound semiconductors have also been used as the structural materials in these devices [3, 10], implementation of photonically controlled RF devices on silicon substrates is more attractive for monolithic integration of microwave and mm-wave devices using well-developed fabrication processes. Independent of the material and the device structure, in all these approaches and devices a laser wavelength between 600 and 900 nm is used to maximize optical absorption and carrier generation. As a result, the optically affected region has been confined at the surface (due to small optical penetration depth at these wavelengths). In contrast to the previous work, here we explore the potential application of bulk illumination at a longer wavelength (1064 nm) combined with high-q RF resonance to maximize the RF-optical field overlap. Moreover, we use a sidecoupled RF ring resonator to confine the RF field and enhance the interaction of RF field and photogenerated carriers. Note that although previously certain planar resonant structures were used for photonic RF control, the confinement of free carrier on the surface has limited the interaction of the resonant field and the free carriers. In these cases, the presence of free carriers has been mainly modifying the electrical properties at the boundaries of the resonator (effectively tailoring the conductor size). This article describes a photonically controlled RF ring resonator suitable for controlling a variety of silicon-based microwave integrated circuits. By optically controlling the density of free carriers in the regions of the substrate with large resonant RF field, we have demonstrated up to 8 db of transmission loss variation with only 1.9 mw of interacting optical power from a commercial fiber pigtailed laser diode operating at 1064 nm. To our knowledge, this is the largest optical sensitivity of RF transmission ever reported for a passive junction-less photonically controlled microwave device. 2. PHOTONICALLY CONTROLLED RF RING RESONATOR 2.1. Bulk Versus Surface Illumination As photonic free carrier generation controls the RF propagation in most optically controlled components, strong optical absorption is the main criteria for choosing the photoconductive material and the corresponding wavelength. On the other hand to reduce the fabrication cost and complexity, preferably only one type of material is used in the device structure. As a result, lowloss optical waveguides cannot be easily integrated with the device to deliver light directly to the sensitive region, and almost all devices are controlled by top illumination to avoid absorption before reaching the sensitive region. Silicon is one of the most common substrates used in phonically controlled RF devices (mainly because of compatibility with IC fabrication and low fabrication cost). Figure 1(a) shows the absorption depth (d, the depth at which the light intensity drops to 36% of its value at the interface) plotted against wavelength for silicon substrate. Below 900 lm, d is less than 30 lm, and all the photogenerated carriers are effectively confined at the surface (interface between air and silicon). That is why so far the optical control has been mainly achieved by tailoring the conductive structure on top of the semiconductor substrate using laser beam. Here, we examine an alternative approach by choosing a wavelength with an absorption depth larger than the substrate 1594 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 DOI 10.1002/mop