Dual-Port MIMO DRA with High Isolation for WiMAX Application

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1 Dual-Port MIMO DRA with High Isolation for WiMAX Application Aftab Ahmad Khan 1, Rizwan Khan 1, Sajid Aqeel 1, Jamal Nasir 1,, Owais 1 1Department of Electrical Engineering COMSATS Institute of Information Technology Abbottabad, Pakistan Faculty of Electrical Engineering, Wireless Communication Centre (WCC), Universiti Teknologi Malaysia, Johor Bahru, Malaysia Corresponding Author: aftabjadoon@ciit.net.pk Abstract A dual-port multiple-input multiple-output (MIMO) antenna system using a dielectric resonator antenna (DRA) is presented in this paper. The dimensions of the antenna are / / / operating at 3.6 GHz and covers WiMAX band ( GHz) entirely. Two symmetric feed lines excite and modes at 3.6 GHz in the dielectric resonator. Impedance bandwidth of 300 MHz at reference level is obtained. In the proposed work high isolation is achieved and compared with other DRAs in literature. MIMO performance measures in terms of envelope correlation coefficient and diversity gain are analyzed. A gain of 5.84 dbi for port 1 and 5.87 dbi for port has been achieved, respectively. Similarly the envelope correlation is 0.04 and diversity gain is 10 db. These results reveal that the proposed antenna is suitable to be used for WiMAX applications. Keywords Dielectric resonator antenna; multiple-input multiple-output; envelop correlation coefficient; diversity gain I. INTRODUCTION Nowadays, IEEE WiMAX standard allows data transmission using multiple broadband frequency ranges. The original 80.16a standard has specified the transmission range from 10 to 66 GHz, but 80.16d standard allowed the lower frequency ranges from to 11 GHz [1]. Various countries have different frequency bands for WiMAX but 80.16d standard use frequency band at 3.6 GHz []. The reason of using low frequency at this specification is to get better coverage due to low signal attenuation. With the advent of 3G and 4G networks, the demand for high data rates has increased significantly. But to increase the transmission speed is a tricky business because of the limitations on the bandwidth and transmitted power. 3G technologies make use of digital modulation techniques to increase the symbol rate, which is another limiting factor on the speed. Signal to noise ratio is used to increase the power level of the signal which ultimately limits the transmission rate. The solution to all these problems is to use MIMO antenna system. MIMO technique results in achieving high data and transmission rate using the bandwidth and transmitted power [3]. Microstrip patch antennas (MPAs) are in practice for MIMO wireless applications but they suffer from metallic losses at higher frequencies. In addition MPAs have low radiation efficiency and bandwidth as they radiate from narrow slot. Dielectric resonator antennas (DRAs) are introduced as a replacement of metallic antennas in various wireless applications [4]. DRAs provide a potential Fig. 1. Geometry of the proposed design (a) 3-D view (b) top view advantage of having light weight, small size, high radiation efficiency, low conductor losses and ease of fabrication and excitation [5]. The concept of MIMO DRA was first demonstrated by Ishimiyaet al. [6-7]. However authors do not provide any detail about design method and mode of excitation. A compact coplanar waveguide (CPW)-Fed DRA for WiMAX application is proposed in [8]. Authors in this work achieved 60 MHz (7%) and 440 MHz (11.6%) impedance bandwidth along with good isolation between two feeding ports. Although this design is compact but still its height is double than this proposed design. A compact MIMO split cylindrical DRA for LTE Femtocell applications in presented in [9]. Authors in this work achieved very good isolations but DRA size is large and two ground planes which make fabrication process more intricate. A dual-port reduced size MIMO DRA in rectangular shape is presented in [10] but the isolation between is feeding port is low. A comparison of size and isolation of DRA in literature has been presented in

2 Table I. Authors in [19] recently presented a dual-polarized MIMO design with a height of 6 mm but isolation achieved is 0dB. The proposed design is simple and easy to fabricate. Two orthogonal modes have been excited using two symmetric feed lines. Upto 6 db isolation has been achieved by placing the feed lines on the optimal position along the DRA. Co and cross-polarization, correlation coefficient and diversity gain are analyzed which are found to be very good for this design to be used for WiMAX applications. The remainder of this paper is organized as follows: Section II presents antenna geometry. Design analysis and modes are discussed in section III. Section IV describes some parametric studies. Result analysis and discussion are presented in section V whereas section VI concludes this paper. Ref. A COMPARISON OF DUAL PORT MIMO DRAS Modes Excited Minimum Isolation (db) Radiation Efficiency (%) Total Size (mm 3 ) [10] TE X 111 / TE Y x90x4 [11] x70x10 [1] TM X 111 TE Y x80x6 This Work TE X 111 / TE Y x50x8 II. ANTENNA GEOMETRY The proposed dual-polarized MIMO rectangular DRA structure is shown in Fig. 1. It consist of a rectangular dielectric resonator (DR) with permittivity of 15 excited through two symmetrical microstrip feed line of equal dimensions. Both the feed lines are extended on top of the dielectric resonator to increase the electrical length for better impedance matching. Length and width of the ground plane and FR-4 substrate is W. Height and permittivity of the substrate is 1.6 mm and 4.6 respectively. Optimized dimensions after parametric analysis of proposed design are listed in Table II. TABLE I. OPTIMIZED DIMENSTIONS OF THE PRPOSED DRA Parameter Values (mm) Parameter Values (mm) h III. DESIGN ANALYSIS MODES Cylindrical and spherical dielectric resonators have rigorous classification of modes but unfortunately it doesn't happen in rectangular shaped DRAs. Modes can be confined and unconfined. Confined modes can be accumulated in the body of revolution. As rectangular shape is not the body of revolution so it can supports only unconfined modes. The lowest order non-confined mode radiates like a magnetic dipole. Dielectric waveguide model (DWM) [13] is used to find the field of lowest order ( ) mode. The field components associated to this mode in the DRA in z- direction are given below [14]; ( ) ( ) ( ) (1) ( ) ( ) ( ) ( ) () ( ) ( ) ( ) (3) ( ) ( ) ( ) (4) ( ) ( ) ( ) (5) 0 (6) where, and are wave numbers along the corresponding directions and is an arbitrary constant. For the dominant mode, and is found by the following transcendental equation. ( 1) (7) In the proposed work two orthogonal modes and have been excited in the DRA. The resonant frequency of mode is where, and Similarly for and + + (8) ( 1) (9) mode, ( 1) (10) where / is the free space wave number corresponding to the resonant frequency and is the speed of light. Calculated frequency using (8) is 3.55 GHz which is very near to the simulated frequency i.e. 3.6 GHz. Fig. shows the field pattern when and mode are excited. These patterns depict the dominant mode excitation for and in b and a dimensions respectively. IV. SOME PARAMETRIC STUDY One of the challenging task when working with MIMO antennas is the isolation between two antenna ports especially when using single radiator and symmetric feeding mechanism. Many techniques have been used to improve isolation but at the cost of more complexity in the design

3 Z feed lines along the DR. Values of each feed line position is listed in Table II. Y V. RESULTS AND DISCUSSIONS Fig. 4 shows the impedance plot at 3.6 GHz. It is clear from figure that the real part is nearly 50 Ω and imaginary part is at almost zero. (a) Z X (b) Fig.. Electric field distribution at 3.6 GHz (a) mode (b) mode. Fig. 4. Impedance plot the design at 3.6 GHz Fig. 5 shows the S - Parameters where S and S are the same because of the design symmetry. The impedance bandwidth is 4.% ( GHz), which covers the entire WiMAX band [15]. Radiation efficiency is 90% through both the ports at 3.6 GHz and gain is 5.87 dbi respectively. Fig. 3. S 1 for various feed line positions along DR. fabrication. In this work high isolation is achieved by moving the feed line position along dielectric resonator. Fig. 3 shows different levels of isolation when both symmetric feed line are moved along the periphery of DR from position P1 to P4 (shown in Fig. 1(b)). It is clear from figure that isolation is maximum for position P4. It is because coupling level changes with the movement of the Fig. 5. S-Parameters of the proposed antenna Fig. 6 shows E-plane and H-plane through port1 and port at 3.6 GHz respectively. Fig. 5(a) to 5(d) reveals that cross-polarization is almost at 8 db in E and H-planes respectively. Furthermore radiation patterns are in the broad side direction which varifiy that dominant modes have been excited along x- and y-direction respectively.

4 Co-Polarization Cross-Polarization ECC (a) (b) Frequency (GHz) Fig. 7. Envelop correlation coefficient (ECC) using (11) (c) Fig. 6. Co and X-polarization at 3.6 GHz (a) E-plane port1 (b) H-plane port1 (c) E-plane port (d) H-plane port For MIMO performance of the proposed design, envelop correlation coefficient (ECC) and diversity gain (DG) are analyzed. ECC provides correlation between two signals at the receiving end. In this paper ECC is calculated using both S-Parameters and 3-D radiation patterns using (11) and (1) respectively [11][16]. When determining ECC using S-parameters, it takes into account the surface current on the ground plane only, whereas (1) also accounts for coupling caused by 3-D radiation patterns. Fig. 7 shows that ECC is almost zero at the band of interest. ECC is calculated from (1) is found to be in (1) gives the 3D pattern when port is excited. (d) Diversity Gain (db) Frequency (GHz) Fig. 8. Diversity gain (DG) using (13). ( ) ( ) (11) All of the above results clearly indicate that the proposed design is suitable for WiMAX applications. (, ). (, ) (, ) (, ) (1) The last and the very important parameter for MIMO performance analysis is diversity gain (DG). DG can be calculated using the relation (13) [17]. Fig. 8 shows that the value of DG is almost 10 db which guarantees good MIMO performance. where ( ) 10 (13) VI. CONCLUSION This paper presents a rectangular MIMO DRA designed for WiMAX application. Two orthogonal radiating modes are excited at an overlapped frequency range around 3.6 GHz. Two symmetric feed lines are extended on top of the dielectric resonator to increase the electrical length for good impedance matching. Impedance bandwidth of 300 MHz with high isolation of 6 db has been obtained with only 8 mm height of the design. Results of ECC, DG and co and cross polarization clearly indicates that the proposed design is a suitable candidate to be used for MIMO application at WiMAX band.

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