Design and Diversity Performance of Optimized Dual-Element PIFA antennas for MIMO Handsets

Qiong Wang (1) Dirk Plettemeier *(1), Hui Zhang(1), Klaus Wolf(1) and Eckhard Ohlmer(2)

(1) Dresden University of Technology, Chair for RF and Photonics, 01062 Dresden, Germany. Email: dirk.plettemeier@tu-dresden.de

(2) Dresden University of Technology, Vodafone Chair Mobile Communications Systems, 01062 Dresden, Germany. Email: eckhard.ohlmer@ifn.et.tu-dresden.de

ABSTRACT: An optimized multiple-input-multiple-output (MIMO) antenna system is proposed. It is based on the traditional planar inverted-F antenna (PIFA) design, accommodated on a printed circuit board (PCB) for mobile handsets and operating at 21.6GHz. The PIFA dimension is reduced by 30% with meander structure etched on the conventional PIFA. Diversity performances of a parallel dual-element Meander PIFA array are evaluated in different statistical propagation environments for both without chassis and with chassis configurations. It is found that an optimized Meander PIFA with chassis can still keep nearly the same good diversity performance as the case without chassis. It has also been shown that the dual-element Meander PIFA can work well in different statistical environments. INTRODUCTION Much research effort has been put on multiple-input–multiple-output (MIMO) antenna systems which are considered to be one of the key enablers for higher data rates and more robust wireless communications systems. The major challenge of MIMO antenna design for wireless handset applications is to design small antenna elements with a small inter antenna element spacing such that the antennas fit into the limited handset volume. By contrast a high isolation between antenna elements is required in order to achieve the anticipated diversity and multiplexing gains. Additionally the antenna design has to account for the fact that handsets have to cope with a variety of propagation environments with different statistical properties, which will affect the diversity performance. A number of typical parameters have been widely used to describe the diversity performance including mean effective gain (MEG), MEG ratio, envelope correlation coefficient (ECC) and effective diversity gain (EDG) [1]. Meandered PIFA has been generally employed for size reduction and multi-band implementation compared with conventional PIFA [2], [3], [4]. Various meander structures are used on PIFA antenna for different applications. [2] had the meander etched in the middle of the conventional PIFA to get a double resonance characteristic. In order to get a wideband characteristic, [5] used meandered shorting strip while [6] had a meandered PIFA with two coplanar parasitic patches and [7] modified the meandered PIFA using a chip-resistor load in place of the shorting port. In this study, a meandered PIFA structure is first proposed based on the traditional planar inverted-F antenna. A size reduction by 30% in the optimized meandered PIFA has been obtained compared with the conventional PIFA. Then the diversity performance of a parallel dual-element meandered PIFA array with chassis and without chassis configuration is evaluated in a variety of statistical propagation environments. ANTENNA DESIGN The optimized PIFA applies the meander structure etched in the patch structure of the traditional PIFA, as shown in Fig. 1(a). The optimized PIFA in this paper is named Meander PIFA. The Meander PIFA is shorted to the PCB by a metallic strip and fed by the standard 50Ω RG405 coaxial cable. The Meander PIFA is mounted on a FR-4 PCB substrate of handset. The PCB dimension is the widely-used handset dimension 100x40 mm. The distance between the patch and the PCB is 5mm. The dimensions of the meander on the patch are shown in Fig. 1(b). The optimized patch size is 14.6mm (L) x 7mm (W) which reduces significantly the length of the patch compared with the traditional patch PIFA, in which the patch size is around 20mm (L) x 7mm (W). As it can be seen, the length of the patch has been improved by as high as 30%, which is an advantage as far as the limited PCB volume is concerned. The optimized Meander PIFA operates at the required 2.6GHz with a 10-dB bandwidth of 270MHz. 1 The authors acknowledge the support of Tyco Electronics,‘s-Hertogenbosch, Netherlands.

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Fig. 2 shows the prototype of the Meander PIFA element on the FR-4 PCB for single element and parallel dual elements. Measurements of return loss and radiation pattern have been carried out in the anechoic chamber. The simulated and measured S-parameters are in very good coincidence in the 2-3GHz frequency band as shown in Fig. 3. The Meander PIFA has an excellent resonance performance at the required center frequency of 2.6GHz and a desirable bandwidth performance.

(a) (b)

Fig. 1 Configuration views of (a) Meander PIFA on the PCB and (b) meander dimension.

(a) (b)

Fig. 2 Prototype of the Meander PIFA (a) single element and (b) parallel dual elements.

(a) (b)

Fig. 1 Simulated and measured S parameters of (a) single element and (b) parallel dual elements.

DIVERSITY PERFORMANCE Diversity performance is the critical merit of the multiple element antenna system for handset applications. Classical diversity parameters include MEG, MEG ratio, ECC and EDG. The MEG [8] is a parameter for single antenna elements, while the antenna branch power ratio / is used to measure the difference in signal power levels of the single elements. In a good diversity system, the power levels of the signals received by the two antennas must not be too different, which means k equal one is the best value one should try to get close. The correlation

5mm

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measurement

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coefficient [9] of the received signals is characterized by the envelope correlation coefficient (ECC) ρ . The EDG is the difference between the combined cumulative distribution function (CDF) and ideal single antenna CDF at a certain CDF-level, normally 1%. For selection combining scheme with 1% CDF level, the EDG has been approximately expressed in [10] which illustrates an ideal EDG value of 10dB providing 100% radiation and zero correlation. For handset applications, the MIMO antenna should keep as far as possible from other circuits and battery on the PCB in order to avoid the adverse influence. We assume a parallel dual-element orientation on one end of the PCB in this study as shown in Fig.4 (a). Meanwhile, we put the antennas and the PCB in a typical handset chassis to simulate a more realistic situation. Fig. 4 (b) shows the configuration of the parallel dual-element MIMO antenna array in the chassis with front, back and side views. The introduction of the chassis shifts the resonance frequency from 2.6GHz to 2.4GHz. Hence, an optimization of the Meander PIFA dimension is indispensable in order to get a desirable resonance and bandwidth performance. The optimized PIFA length finally comes to be 13mm and the height of the PIFA is 4.5mm. Compared with the original 14.6mm length and 5mm height, the Meander PIFA volume is considerably reduced. Assuming the antennas and the PCB board vertical to the ground plane, the diversity performance in different statistical propagation environments are tabulated in table I for both without and with chassis configurations. A uniform distribution is reasonably assumed in azimuth direction for the arrival angles of the incident radio. Gaussian and Laplacian distributions are typically used in the elevation direction [11]. As can be seen, the ECC in different propagation environments is less than 0.01 for antennas without chassis. The MEG values for both elements vary within 2dB. The MEG ratio between antenna 1 and antenna 2 is nearly 0dB and MEGs keep almost the same values. The optimized Meander PIFA with chassis gives nearly the same good results as original Meander PIFA without chassis. Low correlation and low MEG ratio are beneficial to achieve good diversity performance. Table II compares the ECC and EDG in different propagation environments for both antennas with and without chassis. Maximum and minimum value ranges are given. The maximum ECC values for antennas with chassis are slightly higher compared to antennas without chassis, but can still be less than 0.45. The EDG values for both configurations are almost in the same which ranges from 7~9.5dB. A high diversity performance can be concluded based on this high diversity gain. High diversity gain of the Meander PIFA can be attributed to high efficiency of the antenna as well as low correlation. It seems obviously that parallel-elements Meander PIFA shows a good diversity performance. Taking the chassis influence into account, the optimized Meander PIFA can still keep nearly the same good diversity performance. Low correlation, good MEG values and high diversity gain have been achieved for this Meander PIFA. CONCLUSIONS A Meander PIFA antenna in 2.6GHz band has been proposed for MIMO handset applications based on the traditional planar inverted-F antenna on a PCB. Antenna dimension has been significantly reduced compared with the traditional PIFA. Diversity performances of a parallel dual-element Meander PIFA array have been evaluated in different statistical propagation environments for both without chassis and with chassis configurations. Accommodation in chassis brings a more compact Meander PIFA with a good resonance performance. It has also been found that an optimized Meander PIFA with chassis can still keep nearly the same good diversity performance as without chassis.

(a) (b)

Fig. 4 Parallel dual-element MIMO antenna configurations (a) without chassis and (b) with chassis.

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Tab. 1 ECC and MEG and MEG ratio of the parallel dual-element PIFA in statistical propagation models

(a) Without N95 chassis

Propagation

models

ECC

MEG (dB) MEG ratio

(dB) MEG1 MEG2

Gaussian indoor 0.001 -3.85 -3.84 -0.01

Gaussian outdoor 0.049 -4.74 -4.74 0

Laplacian indoor 0.002 -4.47 -4.46 -0.01

Laplacian outdoor 0.013 -5.52 -5.51 -0.01

(b)With N95 chassis

Propagation

models

ECC

MEG (dB) MEG ratio

(dB) MEG1 MEG2

Gaussian indoor 0.003 -3.92 -3.93 0.01

Gaussian outdoor 0.082 -4.39 -4.43 0.04

Laplacian indoor 0.0001 -4.58 -4.60 0.02

Laplacian outdoor 0.057 -4.88 -4.93 0.05

Tab. 2 ECC and EDG comparisons for antenna configuration without chassis and with chassis

Propagation

models

ECC EDG (dB)

Without

chassis

With

chassis

Without

chassis

With

chassis

Gaussian indoor 0.067 0.081 9.1~9.5 9.0~9.4

Gaussian outdoor 0.331 0.368 7.7~9.5 7.4~9.4

Laplacian indoor 0.103 0.124 9.0~9.5 8.8~9.4

Laplacian outdoor 0.385 0.422 7.4~9.5 7.1~9.4

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