The 2nd International Conference on

Applied Electromagnetic Technology (AEMT) 2018

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Two Dual-Feed U-Slot Antennas in the Metal-framed

Smartphone for 2 × 2 LTE LB and 4 × 4 LTE M/HB

MIMO Operations

Imee Ristika Rahmi Barani

Institute of Communications

Engineering

National Sun Yat-sen University

Kaohsiung, Taiwan

wangym@ema.ee.nsysu.edu.tw

Song-En Lin, Kin-Lu Wong

Department of Electrical

Engineering

National Sun Yat-sen University

Kaohsiung, Taiwan

Wei-Yu Li

Information and Communications

Research Laboratories

Industrial Technology Research

Institute

Hsinchu, Taiwan

Abstract—Two dual-feed U-shaped open-slot (U-slot)

antennas are proposed for LTE MIMO operation. The two U-slot

antennas (U-slot1 and U-slot2) are of same structures and

dimensions, with each one providing two decoupled antennas to

respectively cover 824~960/1710~2690 MHz for the LTE

low/middle/high bands (L/M/HB) and 1710~2690 MHz for LTE

middle/high bands (M/HB). That is, U-slot1 and U-slot2 together

provide two LTE LB antennas and four LTE M/HB antennas,

making them suitable for 2 × 2 LTE LB MIMO and 4 × 4 LTE

M/HB MIMO operations. Additionally, low envelope correlation

coefficients (ECCs) for all four LTE M/HB antennas are obtained

and the ECCs between the two LTE LB antennas are much less

than 0.1. Furthermore, the slot openings of U-slot1 and U-slot2

are placed at the short-edge frames and 10.5 mm away from the

corners, making the user’s hand effects to be low. Details of the

proposed antennas are presented.

Keywords— smartphone antennas; multi-input multi-output

(MIMO) antennas; open-slot antennas; LTE MIMO antennas; 2 ×

2 MIMO; 4 × 4 MIMO

I. INTRODUCTION

In order to improve channel capacity and spectrum efficiency, multi-input multi-output (MIMO) system is applied to the present fourth-generation (4G) mobile communications [1]. This system requires at least two antennas embedded in the mobile terminal to operate 2 × 2 MIMO operation. Furthermore, it is also known that even higher channel capacity and spectrum capacity can be achieved by applying 4 × 4 Long Term Evolution (LTE) MIMO [2]. In this operation, four uncorrelated antennas are embedded in the mobile terminal to transmit multiple data simultaneously [3].

Recently, several designs of smartphone antennas for 4 × 4 LTE MIMO operations have been reported [2], [4]-[9]. However, the design that is suitable for metal-framed LTE smartphone is very limited. The application of metal-frame structure in the modern slim smartphone is preferred to increase the robustness thereof. One antenna design that is

suitable for metal-framed LTE smartphone is open-slot antenna (OSA) [10]-[18]. However, due to inadequate space in the metal-framed smartphone, the reported OSAs [10]-[18] are not applicable for the 4 × 4 LTE MIMO operation.

This paper presents two dual-feed U-shaped open-slot (U-slot) antennas for the LTE MIMO applications in the metal-framed smartphone. The antennas are formed based on the previously reported dual-feed U-slot antenna [10]. The two U-slot antennas, namely U-slot1 and U-slot2, are respectively placed at the short edges of smartphone. U-slot1 comprises Ant1 and Ant2 that respectively cover 824~960/1710~2690 MHz for the LTE low/middle/high bands (L/M/HB) and 1710~2690 MHz for LTE middle/high bands (M/HB) [19]. U-slot2, which includes Ant3 and Ant4, has the same structures and dimensions as those of U-slot1, hence same antenna performances are obtained. That is, U-slot1 and U-slot2 together provide two LTE LB antennas and four LTE M/HB antennas, making them suitable for 2 × 2 LTE LB MIMO and 4 × 4 LTE M/HB MIMO operations.

Additionally, relatively low user’s hand holding effects are obtained owing to the slot openings of U-slot antennas that are placed in the short edge frames. This arrangement can reduce the possibility of user’s hand to touch and to affect the antenna performances. Furthermore, good envelope correlation coefficients (ECCs) of below 0.1 in the LTE LB and below 0.2 in the LTE M/HB can also be achieved. This result will be promising for the 2 × 2 LTE LB MIMO and 4 × 4 LTE M/HB MIMO operations.

In this study, the configuration of the two dual-feed U-slot antennas is described in Section II. Measurement results of the fabricated prototype are discussed in Section III. The user’s hand effects on the antenna performances are analyzed in Section IV. The conclusion is presented in Section V.

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II. TWO DUAL-FEED U-SLOT ANTENNAS FOR LTE MIMO

OPERATIONS

The geometry of two dual-feed U-slot antennas is shown in Fig. 1. U-slot1 is placed at the top edge of the smartphone and comprises Ant1 and Ant2, whereas U-slot2 is placed at the bottom edge thereof and includes Ant3 and Ant4. Ant3 and Ant4 are identical to Ant1 and Ant2, respectively. All of the antennas are based on OSA structure with Ant1 and Ant3 covering LTE L/M/HB in 824~960/1710~2690 MHz and Ant2 and Ant4 covering LTE M/HB in 1710~2690 MHz.

The antennas are printed in the 150 mm × 78 mm system circuit board. The system circuit board is made of FR4 with 0.8 mm thickness. The metal frame is fabricated using copper-coated FR4 with 0.8 mm thickness. The dual-feed U-slot antennas have the same width of 3 mm. The length of longer open slot (slot 1) and shorter open slot (slot 2) are 44 mm and 24 mm, respectively. The slot openings are placed symmetrically at the short edges of the metal frame with 10.5-mm distance from the left and right side edges thereof. Such arrangement makes the possibility of the user’s hand touching and affecting the antenna performances to be reduced.

Fig. 2 shows the equivalent antenna structures with feed networks for U-slot1 (Ant1 and Ant2) along the top edge of the smartphone. Owing to the virtual shorting of the microstrip feedline, slot 1a of Ant1 and slot 2a of Ant2 (see Fig. 1 and Fig. 2) can also excite a 0.25-wavelength open-slot resonant

Fig. 1. Geometry of two dual-feed U-slot antennas.

Fig. 2. Equivalent antenna structures with feed networks for U-slot1 (Ant1 and Ant2) along the top edge of smartphone.

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Fig. 3. Simulated S parameters for U-slot1. U-slot2 (Ant3 and Ant4) at the bottom edge is also present.

modes [20]. Slot 1a has a length of 26 mm, whereas slot 2a has a length of 19.5 mm. Moreover, with the addition of matching networks (Feed 1 and Feed 2), the antennas can cover the desired bandwidth.

Fig. 3 shows the simulated S parameters for U-slot1 (Ant1 and Ant2) along the top edge with U-slot2 (Ant3 and Ant4) at the bottom edge is also present. The Ansys HFSS 17.2 is used to obtain the simulated results [21]. It is shown that Ant1 can cover LTE L/M/HB and Ant2 can cover LTE M/HB. Good isolation of better than 8 dB can be achieved between Ant1 and Ant2. The antenna performance and operating principle of U-slot2 are the same as those of U-slot1 and will not be repeated for brevity. Although the impedance matching of some frequencies is more than -6 dB, the measured antenna efficiencies are still generally more than 40%. This is acceptable for practical application [22] and will be discussed later in Section III.

III. EXPERIMENTAL RESULTS AND DISCUSSION

A. Antenna Performance

The two U-slot antennas are fabricated and tested as shown in Fig. 4. The measured and simulated S parameters of Ant1 and Ant2 in the fabricated prototype are presented in Fig. 5. Agreement between the measured and simulated results in Fig. 5(a) for S11 and S22 and in Fig. 5(b) for S21 is

Fig. 4. The photo of the fabricated two dual-feed U-slot antennas.

Fig. 5. Measured and simulated S parameters of Ant1 and Ant2 in the fabricated prototype. (a) S11 and S22. (b) S21.

seen. Ant1 can cover both LTE LB in 824~960 MHz and LTE M/HB in 1710~2690 MHz, whereas Ant2 can cover LTE M/HB in 1710~2690 MHz. The measured and simulated S parameters of Ant3 and Ant4 in the fabricated prototype are presented in Fig. 6. The results are similar to those in Fig. 5, in which Ant3 covering LTE L/M/HB and Ant4 covering LTE M/HB. This shows that by simply mapping the dual-feed U-

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Fig. 6. Measured and simulated S parameters of Ant3 and Ant4 in the fabricated prototype. (a) S33 and S44. (b) S43.

slot antenna to the other short-edge metal frame, good multiple antenna performances can be obtained. Good agreement can be achieved between the measured and simulated results.

Measured and simulated antenna efficiencies of the fabricated prototype are shown in Fig. 7. The results for Ant1 and Ant2 are shown in Fig. 7(a) and those for Ant3 and Ant4 are shown in Fig. 7(b). Fair agreement between the measured and simulated antenna efficiencies is seen. The measured antenna efficiencies for Ant1 in the LTE L/M/HB are about 40%. For Ant2 in the LTE M/HB, the measured antenna efficiency is better than about 45%. Slightly varied results are seen for U-slot2, however the obtained antenna efficiencies can be accepted for mobile communications [22].

Fig. 8(a) shows the calculated ECCs from measured radiation patterns of the fabricated prototype for Ant1 and Ant3 in the LTE LB. Those for Ant1, Ant2, Ant3, and Ant4 in LTE M/HB are shown in Fig. 8(b). Very low ECCs of less than 0.02 in the LTE LB are seen, indicating good correlation between antennas. The ECCs for Ant1, Ant2, Ant3, and Ant4 in the LTE M/HB are all below 0.2 with some even less than 0.03. The results are promising to obtain high channel capacity

for 2× 2 LTE LB MIMO and 4 × 4 LTE M/HB MIMO operations, which will be discussed in the following section.

0.5 1 1.5 2 2.5 30.8 0.9 1

0.5 1 1.5 2 2.5 30.8 0.9 1

Fig. 7. Measured and simulated antenna efficiencies of the fabricated prototype. (a) Ant1 and Ant2. (b) Ant3 and Ant4.

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Fig. 8. Calculated ECCs from measured radiation patterns of the fabricated prototype. (a) Ant1 and Ant3 in the LTE LB. (b) Ant1, Ant2, Ant3, and Ant4 in the LTE M/HB.

1.5 2 2.5 3

0.8 0.85 0.9 0.95 1

Fig. 9. Calculated channel capacity of the fabricated prototype with Rayleigh multipath environment and 20-dB signal-to-noise ratio (SNR) at the receiver side. (a) Ant1 and Ant3 for 2 × 2 LTE LB MIMO operation. (b) Ant1 and Ant2 for 2 × 2 LTE M/HB MIMO and Ant1, Ant2, Ant3, and Ant4 for 4 × 4 LTE M/HB MIMO operations.

B. MIMO Performance

Fig. 9(a) shows the calculated channel capacity of the fabricated prototype applied in 2 × 2 LTE LB MIMO. Those of Ant1 and Ant2 for 2 × 2 LTE M/HB MIMO and Ant1, Ant2, Ant3, and Ant4 for 4 × 4 LTE M/HB MIMO operations are shown in Fig. 9(b). For the 2 × 2 LTE LB MIMO operation, the obtained channel capacities are about 8.5~9.3 bps/Hz. On the other hand, the channel capacities for the 4 × 4 LTE M/HB MIMO operation are about 18.4~20.4 bps/Hz. The results for 2 × 2 LTE M/HB MIMO with Ant1 and Ant2 in the previously reported Ref. [10] are also shown in Fig. 9(b) for comparison. Note that the channel capacity for the 4 × 4 LTE M/HB MIMO case is about two times of the 2 × 2 LTE M/HB MIMO operation. The results show that the proposed two dual-feed U-slot antennas are promising to be implemented in 2 × 2 LTE LB MIMO and 4 × 4 LTE M/HB MIMO operations.

IV. USER’S HAND EFFECTS

ON THE ANTENNA PERFORMANCES

The user’s hands effects on the proposed fabricated prototype are also analyzed. The user’s hand is arranged in one-hand data-mode position according to CTIA (cellular telephone industries association) standard for mobile device having width greater than 73 mm and less than 92 mm [23].

Fig. 10. Measured S parameters of U-slot2 (Ant3 and Ant4) for the fabricated prototype in free space and with the user’s hand in one-hand data mode.

Frequency

(MHz)900 1900 900 1900

Ant1 44.2% 69.3% 23.8% 53.7%

Ant2 - 62.1% - 47.7%

Ant3 44.2% 69.3% 21.6% 48.7%

Ant4 - 62.1% - 40.5%

Free space With hand

Fig. 11. Simulated antenna efficiencies in free space and with the user’s hand in one-hand data mode.

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The measured S parameters of U-slot2 (Ant3 and Ant4) for the fabricated prototype in free space and with the user’s hand are shown in Fig. 10. From the result in the figure, some frequency shifts of the measured reflection coefficients are seen. The changes in the impedance matching are caused by the dielectric loading of the user’s hand that can shift the resonant frequency. However, the reflection coefficients can be generally less than -6 dB in the desired band. That is, the antennas can still cover the operating bands with the user’s hand presence. The results for U-slot1 (Ant1 and Ant2) are not shown in this study for brevity due to very small difference compared to the proposed free-space results.

The simulated antenna efficiencies with the user’s hand presence are shown in the Fig. 11. The simulation is performed on SEMCAD X from SPEAG [24]. The study is carried out at two frequencies of 900 and 1900 MHz. It is seen that the efficiencies of all antennas with the user’s hand presence are decreased significantly compared to the free-space case. The changes in the antenna efficiencies are due to the absorption of some of the transmitted power by the hand tissues. Moreover, due to the frequency shift caused by the dielectric loading of the hand, the antenna efficiency also reduced.

V. CONCLUSION

Two U-slot antennas placed at two short edges of the metal-framed LTE MIMO smartphone have been proposed. U-slot1 (Ant1 and Ant2) are placed at the top edge metal frame and can cover LTE L/M/HB in 824~960 MHz/1710~2690 MHz and LTE M/HB in 1710~2690 MHz, respectively. U-slot2 (Ant3 and Ant4) are easily obtained by disposing U-slot1 at the bottom short edges of the smartphone. Good performances for the 2 × 2 LTE LB MIMO and 4 × 4 LTE M/HB MIMO operations can also be achieved. The channel capacities of the proposed antennas for the 2 × 2 LTE LB MIMO and 4 × 4 LTE M/HB MIMO operations can respectively reach about 9 and 20 bps/Hz. The proposed antennas also show good result with the user’s hand presence. The obtained results show that the proposed two U-slot antennas will be promising for practical metal-framed LTE MIMO smartphone.

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