2014 Loughborough Antennas and Propagation Conference (LAPC)
10 - 11 November 2014, UK
A Reconfigurable Dual-Band MIMO Antenna System for Mobile Terminals Rifaqat Hussain and Mohammad S. Sharawi Electrical Engineering Department King Fahd University of Petroleum and Minerals (KFUPM) Dhahran 31261, Saudi Arabia Email: rifaqat,
[email protected] Abstract—In this paper, a varactor diode based two element reconfigurable, dual-band, multiple input multiple output (MIMO) antenna is presented. The single antenna element is a modified printed inverted F-shape antenna (PIFA) with radiating lines and a folded patch. This proposed structure is grounded by a metallic wall for size optimization and compactness. The proposed design is very versatile as it can be used to cover wide frequency bands for dual-band operation. The well known frequency bands covered are GSM-750, GSM-870, LTE-900, LTE1800, with several other bands as well. The proposed design provides at least -12 dB isolation between its antenna elements. The dimensions of the proposed single element design were 12×30 mm2 with a ground plane area of 65×120 mm2 .
I.
I NTRODUCTION
In modern wireless communications, the exponential growth of wireless services resulted in an increasing demand of the data rate requirements and the reliability of data. The number of services that these devices may offer are increasing tremendously. These services include high-quality audio/ video calls, online video streaming, video conferencing and online gaming. All these demanding features require wide operating bandwidth or covering several frequency bands [1]. To surmount the high data rate requirement, due to continuous escalation in the wireless handheld devices services, reconfigurable multiple-input multiple-output (MIMO) antenna systems can be used. MIMO antenna systems are adopted to increase the wireless channel capacity. The key feature of a MIMO antenna system is its ability to multiply data throughput with enhanced data reliability using the available bandwidth and hence results in improved spectral efficiency [2]. Printed inverted-F antenna (PIFA) are widely used in mobile handsets because of their simple structure, easy integration, compact size and can be installed above other components [3]–[5]. However, in MIMO applications, closely spaced PIFA elements with common ground plane exhibit poor isolation. Isolation improvement between MIMO elements is required for each elements. Several designs were reported with improved the isolation between their MIMO elements [6], [7]. MIMO systems are being utilized in 4G wireless standards to multiply data throughput with increased reliability and are more suitable for in multipath wireless channels. Most reconfigurable antennas were designed to operate as single element without any analysis of MIMO performance [8]. In addition, most of the available designs for MIMO applications in mobile terminals and wireless handheld devices cover high
978-1-4799-3662-5/14/$31.00 ©2014 IEEE
110
Figure 1.
Proposed MIMO antenna (a) HFSS model (b) fabricated model.
frequency bands [9]–[11]. In [9], four elements MIMO was presented working at 2.4 GHz. A quad band MIMO antenna was proposed in [10]. This 2×1 MIMO antenna covered four frequency bands i.e. 2.4∼2.5 GHz, 3.4∼3.6 GHz, 5.15∼5.35 GHz, and 5.75∼5.875 GHz. A dual band PIFA antenna was prseneted in [11] for PCS and WiMAX application. In [12], dual band PIFA with meandered and folded patch was presented for mobile phone applications. The proposed design was compact, covering the frequency bands of 930∼950 MHz and 1706∼1865 MHz. A modified version of [12] was presented in [13]. The quad band MIMO antenna was compact with single element dimension of 12×30 mm2 with good isolation. The proposed design was suitable for mobile handsets in MIMO application. The proposed antenna design was covering frequency bands for the LTE bands 746∼787 MHz, 1850∼1990 MHz, 1920∼2170 MHz, and 3600∼3700 MHz for Wi-Fi applications. Both these design [12], [13] are suitable for fixed frequency operation without any flexibility to tune to other bands. Moreover, the given design exhibit a small bandwidth in lower bands of operation below 1 GHz. In this work, we propose a novel antenna structure based on modified PIFA. The proposed design is compact and suitable for smart phone and other small handheld wireless devices with high isolation and low correlation coefficient. Moreover, the proposed design is reconfigurable completely analyzed for MIMO performance metrics. The distinguishing feature of the proposed design is its dual band and its tunability over a wide frequency range including the well
Figure 4. (a) Fabricated Modified MIMO PIFA antenna, (b) Fabricated antenna bottom side, (c) Fabricated antenna top view.
design was fabricated on an FR4 substrate with ground plane area of 120×65 mm2 . The size of the circuit board is assumed to be the size of mobile handset size.
Figure 2. Detailed schematic of reconfigurable MIMO antenna (a) Top view (b) Bottom view (c) Side View (d) Front view.
Figure 3.
Varactor diode biasing circuitry.
known LTE and GSM bands. It exhibits a wide tunable range from 750∼1030 MHz in lower frequency band while in the second band the tunable frequency range is from 1540 ∼ 1940 MHz. This is achieved by applying DC voltage to its varactor diodes. II.
A NTENNA D ESIGN D ETAIL
The proposed varactor based reconfigurable modified PIFA antennas are shown in Fig. 1. The HFSS and fabricated models are shown in Figs. 1(a) and (b), respectively. The proposed
111
The reconfigurable MIMO elements were mounted on the top corners of the main board with a coaxial feed. The total height of the whole system was 5.8 mm. The two reconfigurable antennas are exactly similar in structure with dimensions of 30×12 mm2 . The detailed schematic of the two MIMO antenna is shown in Fig. 2. Fig. 2(a) shows the top view of the reconfigurable MIMO elements while Fig. 2(b) shows the bottom view of the antenna. The bottom layer of the antenna consists of radiating lines and the coaxial feed. The two antenna elements were fed from the bottom side of elevated board. Varactor diodes were embedded on the top side of the antenna to connect the two different radiating parts, thus providing variable capacitance at the junction. The change in capacitance by varying the applied voltage results in different resonating bands and thus providing reconfigurability. The top and bottom layers of the elevated board were shorted through metallic wall at one side of the board. Figs. 2(c) and 2(d) show the side and front views of the PIFA setup, respectively. Both MIMO elements were short circuited through a shorting wall to GND plane to adopt to the concept of PIFA for compact and optimized design for lower LTE and GSM bands (below 1 GHz). Each antenna element is embedded with varactor diodes to change the capacitance of the current path and hence resonate at different frequency bands. The biasing circuitry for a single varactor diode is shown in Fig. 3. The circuit is a series combination of an RF choke of 1 µH in series with 2.1 kΩ resistor, connecting in series with both terminals of varactor diode. The variable voltage was applied to change the capacitance of varactor diode in reverse bias fashion. The varactor diode used in this design was BB-145. The biasing circuitry was used to bias the varactor and at the meantime, isolate the DC and RF parts of the antenna.
Figure 5.
Simulated reflection coefficient at antenna 1.
Figure 6.
Measured reflection coefficient at antenna 1.
Figure 7.
Simulated Mutual coupling curves between the two antennas
Figure 8.
Measured Mutual coupling curves between the two antennas.
Table I.
S IMULATED AND MEASURED fc AND BW FOR TWO BANDS
The fabricated model of the proposed design is shown in Fig. 4. Fig. 4(a) shows the complete 2-element MIMO antenna system while Figs. 4(b) and (c) shows the top and bottom layers of the fabricated elevated PIFA structure, respectively.
Simulated
Measured
fc (MHz)
BW (MHz)
fc (MHz)
BW (MHz)
Ant-1
820
27
840
60
Ant-2
820
27
845
58
Ant-1
1700
210
1660
160
Ant-2
1700
210
1665
165
Band-1
III.
S IMULATION AND M EASUREMENTS R ESULTS
The antenna was fabricated and its s-parameters were measured. The measured results showed close agreement with the simulation ones. A slight shift in the resonant frequencies of the fabricated antenna was attributed to the difference between the material properties of the substrate defined in the simulation design and the one used for fabrication. In addition, lumped and active elements are modeled in HFSS using the specification given in the date sheet. The real values might have some differences which might be the cause of shift in the frequency.
112
Band-2
The proposed design is basically dual-band PIFA. Fig. 5 shows the simulated reflection coefficient curves of antenna1 by changing the capacitance of the varactor diode from 0.1 pF to 10 pF. There is a smooth change of resonance from 750 ∼1160 MHz in the lower band while the second band smoothly changes from 1540∼1900 MHz. Increasing
Figure 9.
Simulated Gain at 825 MHz (a) Antenna 1 (b) Antenna 2.
Figure 10.
the capacitance tends to decrease the resonating frequency of PIFA. Fig. 6 shows the measured reflection coefficient for antenna 1 at port. Fig. 7 shows the simulated mutual coupling between the two antenna elements. The isolation is quit good between the MIMO antnnas and is less than -12dB. The used varactor (BB-145 ) had a minimum capacitance of 2.5 pF at 6V. Over this region, the measured results are well matched with simulated ones. The details of simulated and measured center resonances frequencies (fc ) and -6dB bandwidth (BW) for input biasing voltage equal to 4 volts are shown in Table I. Fig. 8 shows the measured mutual coupling between two antenna elements at various biasing points. The minimum measured -6 dB bandwidth in the lower band was 27 MHz while in the upper band was 210 MHz for all resonant frequencies. Table II.
ACKNOWLEDGMENT This work was supported by KACST, Saudi Arabia through the National Technology Plan for Science and Technology (NSTIP), under project number 12-ELE3001-04. R EFERENCES [1]
[2]
[3]
S IMULATED P EAK GAIN , E FFICIENCY AND E NVELOP CORRELATION COEFFICIENT
Peak Gain (dBi)
Efficiency (%η)
Ant-1
0.436
63
Ant-2
0.495
63.12
Ant-1
4.330
86
Ant-2
4.334
86.11
Band-1
[4] ρe
[5]
0.053
Band-2
[6] 0.032
[7]
The simulated gain pattern of the two antennas at 825 MHz and 1700 MHz are shown in Figs. 9 and 10. The simulated peak gains and efficiencies (%η) for both antennas at both bands (with biasing voltage equal to 4 Volts) are given in Table II. The value of envelope correlation coefficient (ρe ) at both bands (825 MHz, 1700 MHz) are tabulated in Table II and show acceptable MIMO performance. IV.
[8]
[9]
[10]
[11]
C ONCLUSION
In this paper, a novel structure modified PIFA with varactor diode loading is presented. The proposed design is well suited for lower frequency end of LTE and GSM bands. The frequency bands covered are GSM-750, GSM-870, LTE-900, LTE-1800, with several other bands as well. The correlation coefficient of the proposed design was less than 0.3 with compact structure and thus suitable for MIMO applications in mobile phone handset and other small handheld wireless devices.
113
Simulated Gain at 1700 MHz (a) Antenna 1 (b) Antenna 2.
[12]
[13]
K. A. Obeidat, B. D. Raines, R. G. Rojas, and B. T. Strojny, “Design of frequency reconfigurable antennas using the theory of network characteristic modes,” IEEE Transactions on Antennas and Propagation, vol. 58, no. 10, pp. 3106–3113, 2010. H. Liu, S. Gao, and T. Loh, “Compact mimo antenna with frequency reconfigurability and adaptive radiation patterns,” IEEE Antennas and Wirelesss Propagation Letters, pp. 269–272, 2013. Y.-X. Guo, M. Y. W. Chia, and Z. N. Chen, “Miniature built-in multiband antennas for mobile handsets,” IEEE Transactions on Antennas and Propagation, vol. 52, no. 8, pp. 1936–1944, 2004. Y.-X. Guo and H. S. Tan, “New compact six-band internal antenna,” IEEE Antennas and Wireless Propagation Letters, vol. 3, no. 1, pp. 295–297, 2004. P. Ciais, R. Staraj, G. Kossiavas, and C. Luxey, “Compact internal multiband antenna for mobile phone and wlan standards,” Electronics Letters, vol. 40, no. 15, pp. 920–921, 2004. S. Zhang, B. K. Lau, Y. Tan, Z. Ying, and S. He, “Mutual coupling reduction of two pifas with a t-shape slot impedance transformer for mimo mobile terminals,” IEEE Transactions on Antennas and Propagation, vol. 60, no. 3, pp. 1521–1531, 2012. J. Lim, Z. Jin, C. Song, and T. Yun, “Simultaneous frequency and isolation reconfigurable mimo pifa using pin diodes,” IEEE Transactions on Antennas and Propagation, vol. 60, no. 12, pp. 5939–5946, 2012. ´ Quevedo-Teruel, and E. Rajo-Iglesias, “ReconfigM. N. M. Kehn, O. urable loaded planar inverted-f antenna using varactor diodes,” Antennas and Wireless Propagation Letters, IEEE, vol. 10, pp. 466–468, 2011. S. Ghosh, T.-N. Tran, and T. Le-Ngoc, “Miniaturized four-element diversity pifa,” IEEE Antenna and Wireless Propagation Letters, vol. 12, no. 12, pp. 396–400, 2013. R. A. Bhatti, J.-H. Choi, and S.-O. Park, “Quad-band mimo antenna array for portable wireless communications terminals,” IEEE Antennas and Wireless Propagation Letters, vol. 8, pp. 129–132, 2009. J. Byun, J.-H. Jo, and B. Lee, “Compact dual-band diversity antenna for mobile handset applications,” Microwave and Optical Technology Letters, vol. 50, no. 10, pp. 2600–2604, 2008. H.-T. Chen, K.-L. Wong, and T.-W. Chiou, “Pifa with a meandered and folded patch for the dual-band mobile phone application,” IEEE Transactions on Antennas and Propagation, vol. 51, no. 9, pp. 2468– 2471, 2003. M. K. Meshram, R. K. Animeh, A. T. Pimpale, and N. K. Nikolova, “A novel quad-band diversity antenna for lte and wi-fi applications with high isolation,” IEEE Transactions on Antennas and Propagation, vol. 60, no. 9, pp. 4360–4371, 2012.
