Novel Printed MIMO Antenna With Pattern and ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015

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Novel Printed MIMO Antenna With Pattern and Polarization Diversity Jagannath Malik, Student Member, IEEE, Amalendu Patnaik, Senior Member, IEEE, and M. V. Kartikeyan, Senior Member, IEEE

Abstract—In this letter, a novel compact antenna system for diversity/multiple-input–multiple-output (MIMO) application is proposed for WLAN (5.8 GHz) band with good isolation between the two input ports. The novelty of the proposed MIMO antenna system is, though the resonators are physically separated by a distance of ( is the free-space wavelength at 5.8 GHz) equal to the thickness of the substrate (1.524 mm) used for the fabrication of the antenna system, the isolation between them is enough for practical application of the antenna for MIMO. The MIMO system possesses good pattern diversity and polarization diversity with good isolation without the use of any isolation enhancement techniques. Moreover, the concept can be extended to the realization of an MIMO antenna system to operate at other frequencies with suitable scaling and optimization of geometrical parameters of the antenna topology. Index Terms—Antenna diversity, isolation, multiple-input–multiple-output (MIMO), pattern diversity, polarization diversity, wireless LAN.

I. INTRODUCTION

M

ULTIPLE-INPUT–MULTIPLE-OUTPUT (MIMO) system is the answer to the need of higher data rates for advanced futuristic communication. MIMO is the key technology that takes the advantage of multiple antennas at transmitter and/or receiver to increase channel capacity without increasing the spectral bandwidth or transmit power level. The idea is to create parallel resolvable channel for receiving uncorrelated signals at the receiver, and increasing the channel capacity by applying diversity combining approach. Antenna mutual coupling is a key issue of concern and a primary limiting factor for successful MIMO operation [1]. Minimizing the mutual coupling without affecting the performance of each antenna element in an MIMO system, the integration of two antennas operating at the same frequency is quite a challenge under compactness constraint. For the same, novel isolation enhancement techniques, e.g., using decoupling network [2], [3], using neutralization line joining antenna elements [4], using parasitic coupling elements between antenna elements [5], [7], and using ground slot technique [6], are presented. To make the MIMO system further compact by eliminating extra circuitry

Manuscript received October 02, 2014; revised November 12, 2014; accepted November 29, 2014. Date of publication December 12, 2014; date of current version March 02, 2015. The authors are with the Millimeter/THz Wave Laboratory, Department of Electronics and Communication Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2014.2377784

needed to reduce the correlation, pattern and/or polarization diversity is implemented, which utilize orthogonal patterns and orthogonal polarizations as a means to create uncorrelated channels [8]–[13]. In [8], the authors presented a mono-cone antenna loaded with a cup-shaped patch and shorting metallic posts for broadband MIMO operation with pattern diversity. In [10], the authors demonstrated a technique for polarization diversity by placing three linearly polarized printed dipole antennas physically orthogonal to each other. Use of orthogonal feeding to a planar inverted-F-antenna (PIFA) [9] and use of modified shorted bow-tie antenna with two different feeding mechanisms [11] are presented for both pattern and polarization diversity. Exciting and modes simultaneously at overlap frequency in a hybrid-fed circular patch [12] and in a short-circuited ring patch [13] results in excellent port-to-port isolation with combined pattern and polarization diversity. On keen observation of the MIMO antenna structures reported in the above referenced literature, the distance between the individual radiators and the use of a separate isolation technique are the key factors that decide the size of the overall antenna system. The present letter deals with the design of a compact MIMO antenna with decoupled radiators utilizing simultaneous pattern and polarization diversity. One of the radiators is designed for circular polarization, and other one is for linear polarization. The designed MIMO antenna also possesses good pattern diversity due to the orthogonal orientations between the radiation maxima for the radiators. The detailed design methodology is presented, and the simulation and measurement results are discussed to validate the diversity performance of the proposed MIMO antenna. II. ANTENNA DESIGN AND IMPLEMENTATION Design parameters of the proposed MIMO antenna are shown in Fig. 1. The resonators are printed on the opposite sides of the FR4 as the substrate material ( and thickness of 1.524 mm). The dimensions of all the physical parameters of the proposed antenna are given in Table I. The upper radiator is a square patch element with edge fed through a quarter-wave line. The dimensions of the square patch are calculated using transmission line theory for given substrate specifications. The square patch was incorporated with a corner chamfer and slant slot as shown in Fig. 1(a) to get circular polarization. An off-center feed was used to get good axial ratio. Here, it may be noted that, moving the feedline about the center of the patch along the edge, the impedance bandwidth is not much affected, but the axial ratio and 3-dB axial-ratio bandwidth are greatly affected. The final optimized position (2.4 mm from the center

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Fig. 2. Simulated -parameters of the proposed MIMO antenna.

Fig. 3. Measured -parameters of the fabricated MIMO antenna. Fig. 1. Schematic of proposed MIMO antenna. (a) Top view. (b) Bottom view.

TABLE I ANTENNA DESIGN PARAMETER

line of the patch) of the feedline was selected by parametric estimation. Fig. 1(b) shows the bottom radiator of the MIMO antenna system. The bottom radiator is a modified interdigital-type structure and fed by a coplanar slotline. The length of each finger is half a wavelength corresponding to WLAN (5.8 GHz) frequency. The points marked “A” and “B” are the grounding and signal points, respectively, of the discrete port used in simulation. The length of the tapered section is optimized parametrically to achieve optimum impedance matching in the desired band. III. RESULTS AND DISCUSSIONS A. Scattering Parameters CST Microwave Studio ver. 12 was used to design, simulate, and analyze the proposed MIMO antenna. The simulated -parameters are shown in Fig. 2. Isolation of the order of 13 dB between the two radiators over the desired operational band can be marked from the figure. Here, it may be noted that we have not used any isolation enhancement technique as such. The proposed structure was fabricated, and the -parameters

were measured using Agilent PNA series network analyzer to cross-verify the simulated results. Fig. 3 shows the measured

MALIK et al.: NOVEL PRINTED MIMO ANTENNA WITH PATTERN AND POLARIZATION DIVERSITY

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Fig. 4. Simulated parametric analysis of axial ratio for port-1 with variation in (a) chamfer length “ ” and (b) slot length “ .”

Fig. 6. (a) Simulated and measured radiation pattern at 5.8 GHz for (a) port-1 excited and (b) port-2 excited.

scattering parameters. The resonant frequencies of both the radiators were observed to have little right shift toward higher frequency. Still, the measured impedance bandwidth (at 10 dB return loss) covers the desired WLAN band. The port-to-port isolation is found improved in measurement over the expected value from simulation. B. Axial-Ratio Bandwidth

Fig. 5. Simulated 3-D far-field radiation pattern at 5.8 GHz for (a), (b) port-1 excited and (c), (d) port-2 excited.

A parametric analysis was carried out to investigate the circular polarization (CP) performance (axial ratio and CP bandwidth) of the antenna along the broadside direction. Fig. 4(a) shows the effect of varied “ ” on axial ratio, and optimum value of 3.0 mm was chosen. Fig. 4(b) shows the slot length variation “ ” variation on CP performance. The optimum value was chosen as 6.8 mm. The axial ratio was

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for WLAN applications. The present MIMO antenna is compact in the sense that the two radiators are physically separated by a distance (1.524 mm) equal to the thickness of the substrate material. Still, good isolation is ensured between them. The upper resonator is designed to have right-handed circular polarization performance, while the bottom radiator is linearly polarized. The far-field radiation pattern for both radiators is orthogonal to each other, ensuring good pattern diversity performance. The agreement between the simulated and measured results for scattering parameters and radiation patterns are apparent. The design method for the proposed MIMO antenna system is simple and can be extended to design antennas for other frequencies with suitable scaling, parametric analysis, and parametric optimization. Fig. 7. Fabricated antenna and measured gain for both resonators.

measured at three different frequencies in an anechoic chamber. The measured axial ratio at 5.8, 5.9, and 6.0 GHz is 1.92, 2.23, and 2.61, respectively. C. Radiation Patterns and Gain The 3-D view of simulated far-field radiation patterns of the MIMO antenna at 5.8 GHz is shown in Fig. 5. From the figures, it can be seen that, for port-1, the radiation maxima is along the broadside direction ( -axis), while for port-2, it is endfire confirming pattern diversity between the two radiators of the antenna. Fig. 6 shows a comparison of the simulated and measured radiation patterns of the two radiators. The agreement between the two is quite apparent. The simulated radiation efficiency for both the radiators was observed to be .Fig. 7 shows the simulated and measured gains for both the radiators. The envelope correlation coefficient (ECC) is the measure to evaluate the diversity performance of an MIMO antenna. The correct estimate of ECC is ideally calculated from 3-D radiation pattern. Assuming that the MIMO antenna will operate in a uniform multipath rich environment, it can be alternatively calculated by using the scattering parameters. The computed ECC for the proposed MIMO system is well below 0.06 for the desired band, which ensures a good diversity performance. IV. CONCLUSION In this letter, a novel compact MIMO antenna with simultaneous pattern and polarization diversity has been proposed

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