A COMPACT DUAL-ELEMENT PIFA ARRAY FOR MIMO TERMINALS

This paper presents a study on a diversity antenna array using two modified planar invertedF antennas (PIFA) on a mobile handset operating at 5.2 GHz for the multiple input multiple output (MIMO) application. The S-parameters and the radiation patterns of the antennas are evaluated using Finite Integration Technique (FIT) simulation and experimental measurements. Diversity performance is also assessed by calculating the envelope correlation coefficients and the mean effective gain (MEG) of the antennas. Introduction: Multiple input multiple output (MIMO) systems have attracted a considerable interest as a practical approach to achieve significant increases in wireless channel capacity. Theoretical and experimental investigations have revealed significant improvements in capacity throughput for rich scattering environments when multiple transmitter and receiver antennas are deployed [1][2]. However, several practical aspects pose problems for achieving the predicted high data rates [3][4]. One of the most important aspects is that the antenna volume that multiple elements occupy need to be reduced, while providing the ability to achieve low envelope correlation and receive different signals arriving via different multipath routes. A planar inverted F antenna (PIFA) is commonly used in a mobile handset due to its compact size and good performance [5]. In this paper, we will investigate dual-element modified PIFA array mounted on the PCB of a mobile handset operating at 5.2 GHz. It not only retains the unique features of a conventional PIFA, but also offers good diversity performance. The proposed design is simulated using the Computer Simulation Technology (CST) Microwave Studio package, which utilizes the Finite Integral Technique (FIT) for electromagnetic computation [6]. The antenna is also experimentally verified in the Antenna Measurement Laboratory at Queen Mary, University of London (QMUL). Antenna performance is evaluated in terms of the far-field radiation pattern, envelope correlation coefficient and means effective gain (MEG). Antenna design: An experimental model of the PIFA array on a PCB (40 mm × 100 mm) is shown in Figure 1(a). It consists of two modified PIFAs which are mounted on the PCB of a mobile handset with a gap as shown in Figure 1(b). The two PIFAs are placed 20 mm apart on the PCB. The relative permittivity of the PCB's dielectric substrate is 4.7. Each element with a small ground plane (7 mm × 12 mm) constitutes a stand alone structure, as shown in Figure 1(c). The ground plane, as small as the antenna, is located between the PIFA and the PCB. As such, the PCB is no longer a ground plane for the PIFA and the influence of the PCB to the antenna is dramatically reduced. The height of the PIFA is reduced to 4 mm, which is almost half of (6 ~ 10 mm) a conventional PIFA [4], in order to retain the compactness of the whole antenna structure. But a small height of the PIFA results in a narrow bandwidth hence a parasitic element with height (HPE) of 3 mm was used to increase the bandwidth [7]. By optimising the design, the final dimensions of the modified PIFA model operating at the 5.2 GHz band is obtained as shown in Figure 1. Each PIFA was fed by a coaxial cable, where the coaxial cable was drill through the substrate of the PCB without touching the perfect electric conductor (PEC) layer on the PCB as shown in Figure 1(b). Antenna Diversity Analysis: Antenna diversity performance is evaluated from the correlation coefficient and relative signal strength levels between the received signals. The correlation coefficient of the received signals can be characterised by the complex correlation coefficient ρc and the envelope correlation coefficient ρe, which are related by [8] assuming that the received signals have a Rayleigh distributed enveloped and randomly distributed phase. Diversity gain can be achieved when the received signals from two antennas satisfy the following conditions [9]: where Pi is the average received power from each antenna element. By using the radiation pattern, the complex correlation is evaluated as follows [8]: where * * , , , , ( , ) ( , ) ( , ) ( , ) mn m n m n A E E E E θ θ φ φ θ φ θ φ θ φ θ φ = Γ + , in which E denotes the electric far field of the antenna and Γ is the cross-polarisation discrimination of the incident field. Γ of 6 dB which is the average in an urban fading environment [9] is assumed. The average received power i P , from each antenna element can be obtained from the radiation patterns by using the MEG. The MEG of an antenna is defined as the ratio of the mean received power to the mean incident power of the antenna [10]: 2 e c ρ ρ ≈ (1) 0.5 e ρ < and 1 2 P P ≅ (2)