Design and Realization of 1.8-2.4 GHz MIMO 2 x 2 Antenna for Handset Application

Audhia Reza Telecommunication Engineering Undergraduate Program

School of Electrical Engineering and Informatics Institut Teknologi Bandung

Bandung, Indonesia audhia.reza@gmail.com

Joko Suryana Radio Telecommunication and Microwave Lab. School of Electrical Engineering and Informatics

Institut Teknologi Bandung Bandung, Indonesia

joko.suryana@stei.itb.ac.id

Abstract— In this paper, a dual antennas has designed and implemented for 2 x 2 MIMO handset application . The antenna element of the proposed dual port antennas of 2 x 2 MIMO handset is based on a Planar Inverted F-Antenna (PIFA) using meandering shorting strip which optimized to have frequency band operation at between 1.8 to 2.4 GHz for |S11| ≤ -10 dB. Each of the PIFA antenna used in this research has 20.5 mm x 16 mm x 9.5 mm size which is compact and small enough for the placement in handset which has a limited space. Several MIMO configuration applied for the two PIFAs to meet the required return loss (S11 and S22), mutual coupling (S12 and S21), and correlation coefficient while maintaing its size to be small enough for the placement in handset. Then, the proposed 2 x 2 MIMO antenna has been implemented on a low cost FR4 substrate . Moreover, effects of the usage of the antenna near human head and hand to the antenna parameters are also investigated.

Keywords-component; MIMO, PIFA, wideband, meandering shorting strip, diversity

I. INTRODUCTION Multipath fading is one of the main problems occur in

cellular communications. With the existence of fading, the signal could be distorted or lose SNR. This can lead to reduction in system performance such as increase in BER (Bit Error Rate) and reduction in data rate [1]. This problem is considered important especially in the next generation technology, IMT-Advanced with LTE (Long Term Evolution) as an emerging standard for example. Multipath fading, especially slow and flat fading, can be mitigated by diversity of the antenna used. This diversity technique could increase the SNR and improve system performance. Right now, MIMO is most popular technique that could provide space diversity, therefore MIMO is considered as one of main technology applied in 4G system to meet the IMT-Advanced standard.

In this paper, a MIMO 2 x 2 antenna have designed as a component of the MIMO system. The antenna used as the MIMO antenna is Planar Inverted F-Antenna (PIFA) type. PIFA has suitable characteristics for utilization as handset internal antenna because PIFA resonance at λ/4 thus reducing the overall space used. This would be a significant advantage because common microstripe antenna resonance at λ/2 so the

size reduction can be maximized until 50 %. Additionally, PIFA is low profile and has a small backward radiation which is good to reduce SAR (Specific Absorption Rate) that can reduce antenna performance in handset application. However, PIFA has a weakness of narrow bandwidth. This could be a major disadvantage in the rapid-growth of cellular communication technology which lead to the needs for a lot of frequency bands for different technology. At Indonesia, GSM uses 0.9 and 1.8 GHz frequency, UMTS uses 2,1 GHz frequency, LTE will use 0.7, 1.8, 2.1, 2.3, or 2.6 GHz frequency, and Wireless LAN-bluetooth uses 2.4 GHz frequency.

Several methods has been reported at various reports to modify the PIFA and create multiband or wideband characteristic [2]-[6]. In [2], the PIFA is added with an L-slot and U-slot at its planar element to achieve dual band operation. In [3], two parasitic patches applied to each sides of PIFA to achive triple band operation. In [4], a double layer patches utilized with T-slot to achieve multiband operation and the ground structure is modified to enhance the bandwidth. In [5], a planar inverted FF antenna is investigated to achieve ultra wideband characteristic. In [6], an H-slit is applied to the ground plane to achieve a wideband characteristic.

In this paper, design from [7] is used as a reference to achieve the required parameters. The method to achieve wideband characteristic is quite simple by adding a meandering shorting strip in place of a normal shorting strip in a regular PIFA. The fabrication is easier than most novel PIFA design reported and low profile. The meandering shorting strip become parasitic component to the PIFA thus creating an additional resonant at the lower frequency besides the fundamental PIFA frequency. The lower and upper frequency bands are quite close each other and unite to create a wider bandwidth. This additional resonant arises from the input impedance improvement at the lower band by the meandering shorting strip [8].

In section 2 the reference antenna is redesigned and modified to shift the resonant frequency to the desired frequency so it could consist 1.8-2.4 GHz band for wireless communication applications, i.e. DCS-1800, UMTS-2100, LTE 2.3 GHz, and WLAN-bluetooth. The design optimized to achieve the best characteristic on the antenna parameters. Next

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the MIMO configuration is made from the single antenna design. In section 3 the fabricated MIMO antenna measured and the results are compared with the simulation results. Finally the conclusion of the whole research is provided in section 4.

II. ANTENNA DESIGN

A. Single Antenna Design The fundamental resonant frequency of PIFA can be

calculated as :

With c, the speed of light, is 3 x 108 m/s, L1 and L2

represent length and width of the planar element of PIFA, εr

represents dielectric permitivitty of substrate (in this case air, εr

= 1) and W represents the shorting strip width. This frequency is the upper resonant frequency of PIFA. With regards of setting the operating band to 1.8-2.4 GHz, the upper frequency is set to be 2.2 GHz in preliminary design. The second resonant frequency made by the meandering shorting strip should be slightly lower than the 2.2 GHz frequency. From eq.1, obtained λ/4 = 34.5. L1 is set 20 mm, L2 is set 16.5 mm, and W is set 2 mm.

The design is made at a time-domain Antenna CAD simulator.The preliminary design result shows that two frequency bands is produced at 1.8 and 2.2 GHz, but S11 at 1.8 GHz is still too high > -10 dB and could not meet the requirement. Therefore optimization would be done at these category:

• Ground plane dimension • Feeding distance from the shorting • Height of the planar element from ground plane

Ground plane width reduction would reduce the S11 at lower band but increase S11 at the higher band as shown in figure 1, so adjustment should be made to compensate S11 reduction between lower and upper band.

Figure 1. Ground plane width effect on the antenna

The optimum width of ground plane is found to be 54 mm. At the other hand, ground plane length reduction would shift the outer boundary of lower and upper band outwards thus increasing the overall bandwidth, however excessive reduction

could increase S11 at the band between lower and upper band as shown in figure 2 and could make the wideband characteristic turn to multiband characteristic with stopband presence between the bands. The optimum length of the ground plane is found to be 23 mm.

Figure 2. Ground plane length effect on the antenna

Feeding distance would adjust the matching impedance of PIFA and it is found that in this case the best feed-shorting distance is the minimum 2 mm. Further reduction in the distance could not be done due to the presence of dielectric of connector used to feed the antenna. Lastly, the element planar height increase would reduce S11 at lower band but increase S11

at upper band as shown in figure 3 and conversely in height reduction.

Figure 3. Planar element height effect on the antenna

The final design of single antenna that gives the optimum result shown in figure 4.

Figure 4. Final design of single PIFA

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TABLE I. FINAL DESIGN SPECIFICATION OF PIFA

B. MIMO 2 x 2 Configuration Before applying the MIMO configuration, the single

antenna designed placed at the top of a PCB that would act as a handset component board. The S11 is simulated and obtained a result that S11 at the lower band increases again. Optimization in ground plane width is applied once again and obtained the optimum size of ground plane width is 49 mm now.

The MIMO configuration is applied by placing the second identical single antenna at the PCB. The placement and positioning of the two antennas is varied but the placement should not use more than allowed space of 65 x 100 mm2 PCB board. From several researches it is found that the minimum spacing required between two antenna in MIMO system is 0,5 typically 0,5 λ [9]. The spacing is counted with the feed as reference. Keeping this in mind, the optimum configuration possible in the space provided is a configuration of 1st antenna placed vertically at the top left corner and 2nd antenna placed horizontally at the bottom right corner. The space between the two antennas is 97.72 mm or equal as 0.715 λ. The configuration is shown at figure 5.

Figure 5. Proposed dual antennas for 2 x 2 MIMO handset configuration

The simulated return loss gives result that S11 and S22 < -10 dB for the exact band of 1.77-2.43 GHz as in figure 7. Mutual coupling also gives a good result, that is S12 dan S21 < -15 dB for the band as in figure 8. Lastly, correlation coefficient < 0.1 for the band. The simulation also calculate the diversity gain, antenna gain, and radiation efficiency provided by the 2 x 2 MIMO antenna. The result is diversity gain of 10 dB at the operating band of 1.77-2.43 GHz, gain at the range of 2.2-2.7 dBi at the operating band, and radiation efficiency of 98-100 % as illustrated in figure 10-17.

For investigating the human head and hand effects, we also has simulated the antenna characteristic in the usage of the 1.8-2.4 GHz dual antennas on human ear with the addition of head and hand model. The results show that the return loss incerases at 2.35-2.4 GHz band until > -10 dB while the mutual coupling and correlation coefficient is still good and meet the requirement. However, this should not concern too much because practical application at 2.4 GHz band, i.e. WLAN and bluetooth does not demand usage of handset at the ear.

III. REALIZATION AND MEASUREMENT Two verify the simulation results, the designed MIMO

antenna is implemented using FR4 Epoxy as the PCB board and copper as the antenna and its ground plane. The fabricated antenna is shown at figure 6.

Figure 6. Fabricated MIMO antenna

A. Return loss Figure 7 shows the comparison between return loss for

measurement data and simulation. S11 is < -10 dB at the 1.77-2.43 GHz band for simulation. The result show that S11 curve is deeper the band is wider at the measurement than simulation, that is 1.72-2.41 GHz. The result for S22 is also better for the measurement with the band expands from 1.74-2.49 GHz. It can be concluded that the specification required for return loss is met by the antena.

Figure 7. Return loss of the MIMO antenna

B. Mutual coupling Figure 8 shows that the mutual coupling between

measurement and simulation is slightly different at 2.3 GHz, but overall both gives the result that S12 dan S21 < -15 dB for the 1.8-2.4 GHz.

Figure 8. Mutual coupling of the MIMO antenna

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C. Correlation coefficient Coefficient correlation for measurement is approximated

from the scattering parameter using equation:

Figure 9. Correlation coefficient of the MIMO antenna

The correlation coefficient calculated from measurement data measured is lower than correlation coefficient from simulation. This is good because coefficient correlation should be < 0.1 for MIMO antenna. The differences could occur because the measured scattering parameters as shown in figure 7 and 8 are generally lower than S parameter from simulation.

D. Radiation pattern Radiation pattern is observated at center frequency of the

four applications desired, that is 1.8 GHz (DCS-1800), 2.1 GHz (UMTS), 2.3 GHz (LTE), and 2.4 GHz (WLAN-bluetooth) for each antenna.

1) 1st antenna

Figure 10. Freq = 1.8 GHz; E-plane (left), H-plane (right)

Figure 11. Freq = 2.1 GHz; E-plane (left), H-plane (right)

Figure 12. Freq = 2.3 GHz; E-plane (left), H-plane (right)

Figure 13. Freq = 2.4 GHz; E-plane (left), H-plane (right)

2) 2nd antennna

Figure 14. Freq = 1.8 GHz; E-plane (left), H-plane (right)

Figure 15. Freq = 2.1 GHz; E-plane (left), H-plane (right)

Figure 16. Freq = 2.3 GHz; E-plane (left), H-plane (right)

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Figure 17. Freq = 2.4 GHz; E-plane (left), H-plane (right)

E. Head and hand effect The measured return loss and mutual coupling for the

MIMO antenna is slightly different than the simulated data, but far better in comparison. S11 for measurement is < -10 dB for 1.72-2.34 GHz, meanwhile simulation gives S11 < -10 dB for 1.74-2.34 GHz. S22 for measurement is < -10 dB for 1.75-2.37 GHz, meanwhile simulation gives S22 < -10 dB for 1.89-2.35 GHz. Mutual coupling for measurement and simulation is < -15 dB for all frequency, included the frequencies outside the 1.8-2.4 GHz desired band.

Figure 18. Return loss of MIMO antenna with head effect

Figure 19. Mutual coupling of MIMO antenna with head effect

IV. CONCLUSIONS A dual antenna for 2 x 2 MIMO based on PIFA antenna for

handset application has been presented in this paper. The MIMO PIFA antenna has a wideband characteristic which accomplished by implementation of meandering shorting strip in place of normal shorting strip. The antenna dimension is 20.5 x 16 mm2 with the shorting width of 2 mm and planar height 9.5 mm from the ground plane. The antenna work at 1.8-2.4 GHz band (or exactly 1.77-2.43 GHz) for S11 < -10 dB thus can be used for various modern wireless communications such as DCS-1800, UMTS, LTE 2,3 GHz, and WLAN-bluetooth. Return loss of the antenna could be improved by ground plane width adjustment, planar height adjustment, and feed-shorting distance adjustment. The PIFA antennas are separated by 0.715 λ, meet the practical requirement of 0.5 λ. The handset space utilization of MIMO antenna is 65 x 100 mm2. The mutual coupling is < -15 dB and correlation coefficient < 0.1 for the operating band. Finally, the presence of human head and hand has been simulated and analysed its effect on performance of the antenna. In this case, the return loss is worse at frequency > 2.35, but this should not be primary concern as the WLAN application on this frequency does not demand the usage of handset near the human head.

V. REFERENCES [1] T. S. Rappaport, Wireless Communications Principles and Practice 2nd

Edition. USA: Prentice Hall, 2002. [2] P. Salonen, M. Keskilammi, and M. Kivikoski, "Dual-band and

wideband PIFA with U- and meanderline-shaped slot," in Proc. IEEE Antennas and Propagation Society Int. Symp. Dig., 2001, pp. 116-119.

[3] M. K. Karkkainen, "Meandered multiband PIFA with coplanar parasitic patches," IEEE Microw. Wireless Compon. Lett., vol. 15, October 2005.

[4] K. J. Lee, T.K Lee, and J. W. Lee, "Bandwidth enhanced planar inverted-F antenna with modified ground structure," in Microwave Conference, 2007. APMC 2007. Asia-Pacific, 2007, pp. 1-4.

[5] C. H. See et al., "Ultra-wideband planar inverted FF antenna," Electronics Letters, vol. 46, no. 8, pp. 549-550, 2010.

[6] F. Yang and Y. Rahmat-Samii, "Wideband dual parallel slot patch antenna (DPSPA) for wireless communications," in IEEE Antennas Propagat. Soc. Int. Symp. Dig., 2000, pp. 1650-1653.

[7] P. W. Chan, H. Wong, and E. K. N. Yung, "Wideband planar inverted-F antenna with meandering shorting strip," Electronics Letters , vol. 44, no. 6, pp. 395-396, 2008.

[8] Z. Zhang, Antenna Design for Mobile Devices. Singapore: John Wiley & Sons, Pte. Ltd., 2011.

[9] W. Lee, Mobile Communications Engineering. USA: McGraw-Hill, 1997.

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