Journal of Renewable Natural Resources Bhutan

ISSN: 1608-4330

 A Compact CPW-Fed Circularly Polarized Square Slot Antenna for UWB Applications Saeid Alizadeh G. and Ch. Ghobadi Islamic Azad University, Urmia Branch, and Department of Electrical Engineering, Urmia, Iran E-mail: saeid,[email protected]

ABSTRACT A new design compact circular polarized square slot antenna (CPSSA) with two inverted-L metallic strips is presented. The broadband circular polarization (CP) is realized by using two inverted-L strips around two opposite corners and adjusts its dimension. The ultra-wide band operation is achieved by using the vertical tuning stub. The antenna is designed, fabricated, and measured. It is shown that the proposed CPSS antenna has 37.5% circular polarization characteristic with combination of previous CP techniques. The miniaturized CPSS antenna dimension is only 25×25× 0.8 mm , which is 57.6% smaller than the previous structures operates over the frequency band between 2.8 and 11.2 GHz for VSWR < 2. Throughout this letter, details of the improvement process for S11 and axial ratio (AR) properties are presented and discussed.

Keywords:

Ultra Wide Band, Circularly polarized, Axial Ratio, Square Slot antenna.

I. INTRODUCTION recently, a significant number of researchers are investigating studies for the development of Ultra Wideband (UWB) antennas. These researches are due to the merits of UWB antenna such as its enabling high data transmission rates, simple hardware configuration and low power consumption in communication applications. The UWB antennas of these systems generally need small sizes, wideband and non-dispersive properties. The commercial application of UWB systems approved by the Federal Communications Commission (FCC) to be used in a frequency band between 3.1-10.6GHz in 2002. Modern wireless communications are in need of broad impedance matching bandwidths, broad 3dB axial ratio bandwidths, compact, low profile, coplanar structures and antennas compatible with the printed circuit boards (PCBs). Printed slot antennas with different feeding structures can fulfill these necessitates. On the other hand, for deploying a transceiver system without polarization mismatches, circular polarization (CP) is of the promising solutions for system performance enhancement providing better mobility and weather penetration than linearly polarized (LP) antennas [1-2]. In recent years, various layouts and designs of broadband circularly polarized square slot antenna can provide broad impedance and axial ratio bandwidths. Also, the right-hand CP

Fig. 1. Geometry of the proposed CPSS antenna for UWB applications

Saeid Alizadeh G. and Ch. Ghobadi

Bhu.J.RNR. Vol 3;7, 369-376: 2015

and the left-hand CP can be achieved simultaneously with various techniques in these antennas [1-20]. Some of the techniques employed for excitation and expansion of the CP bandwidth of the slot antennas in the literature are being stated as: Embedding two inverted-L grounded strips around two opposite corners of the slot [11] embedding lightning shaped feed-line and inverted-L grounded strips [18], embedding three equal size inverted-L grounded strips around corners of the slot [18], embedding tuning stubs in feed line structure [19]. In this communication, a new design compact circular polarization square slot antenna (CPSSA) for UWB systems with combination of the techniques introduced in [5], [11], [16], [18], [19] is presented. As indicated in TABLE. I, the miniaturized antenna dimension is 57.6% smaller than the previous CPSS antenna structures. The measurements indicate it has an impedance bandwidth of 2.8-11.2GHz (4:1,120%), which is 4 times wider than previous structures. The achieved fractional circular polarization bandwidth (FCPBW) is up to 37.5% cover 5150–5350/5725–5825 MHz (specified by IEEE 802.11a) bands for wireless standard technologies [18].

Fig. 2. Four improved prototypes of the proposed CPSS antenna. II. CPSS ANTENNA CONFIGURATION AND DESIGN Fig. 1 depicts the physical structure and configuration of the proposed single layer CPW-fed CPSS antenna. The configuration of this antenna is a combination of the square slot CP antenna [5] and the techniques introduced in [11], [16], [18], [19-23]. As it is indicated in the Fig. 1, the proposed CPSS antenna consists of a T-shape main patch embedded to feed line (L ). The presented antenna uses two inverted-L shaped strips around two opposite corners of the grounded slot and a tuning stub in feeding structure. Two main features are considered in the designing procedure: 1) enhancing the impedance bandwidth and 2) widening the 3dB Axial Ratio (AR) band width. Through extensive simulations it was found that by embedding a T-shaped patch to the feeding mechanism (Ant III-Ant IV) and adjusting its parameters to the optimized values make the impedance bandwidth wider. The circular polarization (CP) excitation of the proposed circularly polarized square slot antenna (CPSSA) is mainly performed through investment of two inverted-L shape strips located around two opposite corners of the square slot with lx × ly and dx × dy dimensions [11], [16], [18], [19]. By embedding inverted-L shape metallic strips, it is expected that a wideband circular polarization will be generated. Using the feedline in conjunction with grounded inverted-L shaped and T-shaped strips leads to a large ARBW. However this large ARBW may not be guaranteed by the VSWR≤ 2. Therefore, a tuning stub is embedded in the CPSSA structure to widen the impedance bandwidth with key techniques introduced in [11], [16], [18], [19] (Called Ant IV). The proposed CPSS antenna structure show (Fig. 1), right and left handed circularly polarized (RHCP and LHCP) radiations will generate in the +z and −z directions, respectively. The size of the ground plane (GND) will affect the antenna performance, specially the AR characteristic. Therefore we should choose GND plane with a suitable size. The proposed CPSS antenna is printed on a commercially cheap FR4-epoxy substrate with ϵ = 4.4, tag(δ)=0.024 and compact dimension of 25×25×0.8(= ℎ) . The width W of the CPW feed-line is fixed at 3 to achieve 50-Ω characteristic impedance. The horizontal feed section (± direction) is separated from the left and right hand of the feed by a gap of g = 0.3mm. To terminate the feed-line a standard SMA is used. III. RESULTS AND DISCUSSION CPSS antenna characteristics and its parametric analysis are performed using Ansoft™ High Frequency Structure Simulator (HFSS) to find the optimized parameters. The impedance bandwidth and Axial Ratio (AR) of the CPSSA are measured

using the Agilent 8722ES vector network analyzer (VNA). First of all to clarify the improvement process, four prototypes of the CPSS antenna are designed and presented as follows (See Fig.2): Ant I includes only a feed line; Ant 370

Saeid Alizadeh G. and Ch. Ghobadi

Bhu.J.RNR. Vol 3;7, 369-376: 2015

II contains a T-shape patch and two inverted-L shape metallic strips; Ant III has a T- shape patch, two inverted-L strips; and Ant IV uses a T-shaped patch, two inverted-L strips in conjunction with a vertical tuning stub; The measured and simulated results along associated structural dimensions of four CPSS antenna were summarized in TABLE.II. The proposed antenna have (ℓ , w , w , ℓ , lx × ly and dx × dy) key parameters. These key parameters, based on the parametric analysis of the antenna by HFSS, are optimized to achieve the maximum impedance bandwidth and a wide circular polarization BW. Throughout the studies presented in this section, all other parameters that have not been mentioned are fixed to the values shown in Fig. 1. As shown in TABLE.II, embedding of inverted-L metal strips to ground plane has a great effect on the ARBW of Ant I. The 3dB ARBW achieved for the Ant II is about 6.7% (5.6 − 6.0 GHz). The simulation results in TABLE II indicates that, this structure (the inverted-L grounded strips) that leads to large ARBW may not have a satisfying (VSWR ≤ 2) impedance matching [21], [22]. The simulation results also show that if, ly, lx, dy and dx of the inverted-L strips are set equal to 7mm(= 0.375 L) and the tuning stub is not widened (Ant II-Ant III, w =0) [11], [16], [18], [19] 3-dB fractional ARBW will be reduced to 0%. Through extensive simulations and experiments, we have found that choosing ly, lx, dy and dx of the inverted-L strips equal to 5mm(= 0.275 L), based on optimized results presented in TABLE.II, not only increases the AR band width but also improve impedance bandwidth (Ant III-Ant IV). Therefore, embedding patch to feed line and using tuning stub [19], [18] choose from TABLE.I techniques to increase impedance bandwidth. As is depicted in TABLE II, insertion of the T-Shape patch to the feed line structure, improves the impedance matching bandwidth of Ant II (called Ant III). It means that increasing ℓ (0 to 1.5mm) and w (1 to 5mm) values will easily introduce the additional band (9.7-10.7 GHz). Our simulation on techniques used in TABLE.I results show that the modified feed structure (vertical tuning stub) [11], [18], [19] plays an important role in increasing the impedance bandwidth in comparison with other techniques. Therefore, a tuning stub (perpendicular to the feed line) is added in feeding structure to widen the impedance bandwidth (called Ant IV). Through extra simulations we have found that width (w ) of the tuning stub has a great effect on improvement of impedance matching in the 3-dB axial ratio band [19]. The vertical tuning stub is formed by extending the feed section to right by a length of ℓ and a width of . As depicted in Figs. 1 and 2 the vertical tuning stub is very close to the inverted-L Shape strip around the lower right corner of the square slot [19]. As revealed in TABLEs. I and II and Fig. 3, the Axial Ratio result of Ant IV is very similar to previous CPSS antennas, where their impedance bandwidths are quite different. Note that Ant IV has already attained a 3dB ARBW (4.5 − 6.3 GHz).

TABLE I SUMMARY OF MEASURED AND SIMULATED CHARACTERSISTICS OF CPBW AND IMPEDANCE BAND WIDTH ENHANCEMENT TECHNIQUES SOME CPSS ANTENNAS IN COMPARISION WITH PROPOSED ANTENNA; ε IS THE RELATIVE PERMITIVITY. Ref .

[1]

[2]

[3]

CPBW and impedance bandwidth enhancement Techniques Shorting annular and square ringslot antennas Introducing proper asymmetry in the ring slot structure and feeding the ring slot using a microstrip line at 45° from the introduced asymmetry Embedding 45° strips around square slot corners

[4]

Widened Tuning Stub

[5]

Protruding a T-shaped metallic strip from the ground plane toward the slot center and feeding the

( = 2) − , MHz 13111757 13471533 1500~30 00 1560~28 80 17502624 371

MH z

3 ,%

Size

169 7

16251769

140×140× 1.6 mm

4. 4

149 2

14601525

80×80× 1.6 mm

4. 4

0

0

0

0

70×70× 1.6 mm 72×72× 1.6 mm

4. 4 4. 4

596 9

18502070

70×70× 1.6 mm

4. 4

Saeid Alizadeh G. and Ch. Ghobadi

Bhu.J.RNR. Vol 3;7, 369-376: 2015

[6]

[7]

[8] [9] [10 ]

[11 ]

[12 ]

square slot antenna using a protruded signal strip at 90° to the T-shaped strip A widened L-type strip along the diagonal line of the square slot A cross patch inclined diagonally with respect to the square slot is placed at the center of the square slot Using a shorted square-ring slot Using corrugated structure around square slot and meander feed line Using an inverted-L tuning stub protruded from the signal line of the CPW-fed Using two inverted-L grounded strips around two opposite corners of the slot and a widened tuning stub protruded into the slot from the CPW-fed Exciting antenna by an L-shaped strip with a taper end, connected in series to a microstrip-line-fed located along the diagonal line of the circular-slot

[13 ]

Loading two spiral slots in the ground plane

[14 ]

Using L-shaped microstrip feed line The ring slot connects to the dual slots of CPW and is shorted for the circular polarization purpose Using a lightening-shaped feed line protruded from the signal line of the feeding CPW with a pair of inverted-L grounded strips

[15 ] [16 ]

[17 ] [18 ] [19 ] [20 ]

Utilizing the protruded arc-shaped metallic strip Embedding three inverted L-shape strip to the ground plane and using the vertical stub to the feed line Embedding two L-shape strip to the ground plane and using the tuning stub and slit in the feed line using two parallel monopoles and embedding a crane-shaped strip in the ground plane to achieve circular polarization

2200~32 00

240 0

2200~26 00

100×100× 1.6 mm

4. 4

17202400

195 0

18502050

70×70× 1.6 mm

4. 4

19452760 1350~15 50

235 0 195 5

60×64× 1.6 mm 70×70× 1.6 mm

4. 4 4. 4

1772– 2591

222 0

2200~25 00 1350~15 50 1880– 2560

60×60× 0.8 mm

4. 4

16003055

266 5

23003030,27. 4

60×60× 0.8 mm

4. 4

30304450

373 5

29104560

100×100× 1.6 mm

4. 4

15001700 20002400 19002800

160 0 220 0 245 0

15001700 20002400 19003000

70×70× 1.6 mm

4. 4

60×60× 1 mm

2. 55

2227 ~2578

247 5

24502500

40×40× 1.6 mm

4. 26

20233421

274 5

20753415,48. 8

60×60× 0.8 mm

4. 4

32004100 48005800

370 0

34004000

40×40× 0.8 mm

4. 4

20007071

357 5

20305120

60×60× 0.8 mm

4. 4

267413124

596 9

49936945

60×60× 0.8 mm

4. 4

14001650 18002200

154 0 198 5

14801600 18802090

70×70× 1.6 mm

4. 4

372

Saeid Alizadeh G. and Ch. Ghobadi

Bhu.J.RNR. Vol 3;7, 369-376: 2015

Using two inverted-L grounded strips around two opposite corners of the slot and two tuning stub in feed line and ground plane perpendicular to the AR axis

Thi s wo rk

267413124

551 6

45206513

25×25× 0.8 mm

4. 4

TABLE II: SUMMARY OF MEASURED AND SIMULATED CHARACTERSISTICS OF SOME CPSS ANTENNAS IN THE TABLE, THE IMPEDANCE BAND WIDTH IS THE FREQUENCY RANGE WHERE THE VSWR IS EQUAL TO OR LESS THAN 2; f IS THE CENTER FREQUENCY OF THE 3-DB AR BAND WIDTH. ( = 4.4, = 0.024, ℎ = 0.8 , = 25 , = 18.5 , =3 , = 0.3 )

Ant I Ant II

0 5

0 5

0 5

0 5

0 0

0 0

0 0

0 0

0 0

0 0

Ant III

5

5

5

5

0

0

5

1.5

0

0

Ant III

5

7

5

5

0

0

5

1.5

0

0

5

7

5

5

1.5

3

5

1.5

0

0

5

5

5

5

1.5

3

5

1.5

0

5

5

5

5

1.5

3

5

1.5

0

Ant IV (Sim.) Ant IV (Sim.) Ant IV(Me a.)

( = ) − , MHz 3937-6250 4250-6850 3350-4400 9700-1075 3348-4389 9710-1070

G Hz

,%

0 0

0 5630-6023, 6.7%

0

5711-5993, 4.8%

0

0

3125-11082

0

0

0

3125-11063

5.5

4453-6579, 38.5%

0

2802-11232

5.5

4502-6582, 37.5%

As also indicated in Fig. 3, the measured impedance bandwidth for Ant IV has an operating frequency range from 2.8 to 11.2 GHz and 3dB ARBW of 37.5% (4502 − 6582 MHz). For the optimum design of Ant IV, ℓ = 3 and =2 are adopted in this communication. Fig. 3 indicates the close correspondence between the measured and simulated curves for the Ant IV with optimized Values presented in TABLE.II. Our simulations focusing on which dimensions of , , , affects the 3-dB AR band and impedance matching of CPSS antenna show if , , , of the inverted-L strips are set equal to 5mm (= 0.275L) and tuning stubs are widened according TABLE.II dimensions in Ant IV we will have maximum impedance matching and axial ratio band width (37.5%). The tuning vertical stub, when properly widened, can not only enhance the coupling among the feed line and inverted-L strips (lower right corner), but also further perturb the current distribution in the square slot [18], [19]. In process of varying the ground size, there are two ways of changing the size. One way is to change the ground plane’s size (G) without changing the slot’s size (L). The other way is to change the sizes of the ground plane and the square slot at the same time. Through simulation results, we have found that the proposed antenna is actually very sensitive to ground plane size. The simulation results show, if G is changed from 25mm to 28mm, corresponding to a G/L ratio of 1.351, the measured 3dB AR of the Ant IV is 25% (5.8-7.5GHz), which is completely covered by its impedance band of 347511062MHz. Next, with G changed to 35mm, corresponding to a G/L ratio of 1.55, a CPSSA with 3dB ARBW more 373

Saeid Alizadeh G. and Ch. Ghobadi

Bhu.J.RNR. Vol 3;7, 369-376: 2015

than 17% is difficult to design with this structure. Unfortunately, the impedance bandwidth of G/L > 1.55 antenna narrower than its 3dB ARBW, and, even worse the impedance band of G/L < 1.35 antenna does not overlap with its 3dB AR band. Therefore, for the optimum design of Ant IV, G/L ratio fixed at 1.351. To improve the impedance matching for the antennas with the G/L ratio that is significantly deviated from 1.351, more structural parameters in the proposed CPSS antenna should be changed, or additional techniques according TABLE.I results should be used into the antenna design, which is complicated design structure. We simulate the surface current distribution on the Ant IV at 5.6GHz, the minimum point of axial ratio. The simulation results of surface current distribution for Ant IV are shown in Fig. 4. It is observed that the surface current distribution in 180° and 270° are equal in magnitude and opposite in phase of 0° and 90° . The simulated and measured maximum gain is shown in Fig. 3. It can be seen the both simulated and measured gain are between 1.9 − 3.2 within the impedance bandwidth from 2.8 − 11.2 GHz. The measured peak gain is 3.12 at 10.37 GHz while the simulated peak gain is 2.67 . The measured results of the normalized radiation patterns of the miniaturized CPSS antenna IV are presented in Fig. 5 with reasonable agreement between measurement and simulation. It is also interesting to notice that the proposed CPSS antenna has 120% impedance bandwidth, which is 4 times wider than the impedance bandwidth in [11] and [18] with similar CP technique and also 5 times wider than TABLE.I CPSS antennas. Moreover, the dimension of the proposed CPSS antenna is much smaller than those of antennas found in the TABLE.I (at least 57.6%). The photograph of the realized CPSS antenna is shown in Fig. 6.

Fig. 3. Measured and simulated reflection coefficient, axial ratio and gain of Ant IV.

°

9

374

°

Saeid Alizadeh G. and Ch. Ghobadi

Bhu.J.RNR. Vol 3;7, 369-376: 2015

18

°

27

°

Fig. 4. The surface current distribution on the Ant IV at 5.6GHz.

Fig. 5. Measured and simulated normalized radiation pattern of the Ant IV at 5.5GHz.

Fig. 6. The photograph of realized CPSS antennas IV. CONCLUSION A miniaturized broadband CPW-fed circularly polarized square slot antenna for UWB applications has been demonstrated. Some prototypes have been implemented with combination of previous CPSS antennas techniques. In addition to the advantage of simple geometry for the proposed antenna, measured results show that the obtained 3-dB ARBW can reach as large as about 37.5% with a fractional impedance bandwidth of greater than 129%. The proposed antenna uses two pair inverted-L strips in opposite corners for the excitation of two orthogonal resonant modes for CP radiation. Our simulation results show that the embedded tuning stubs in antenna structure plays an important role in increasing the impedance bandwidth and ARBW. The dimension of the proposed antenna is much smaller than those of antennas found in the literature. As a result, the proposed simple CPSS antenna to be employed in future mobile devices such as IEEE 802.11a technologies. REFERENCES [1] W.-S. Chen, C.-C. Huang, and K.-L. Wong, “Microstrip-line-fed printed shorted ring-slot antennas for circular polarization”, Microwave Opt Technol Lett 31, PP. 137–140, 2001. [2] Kin-Lu Wong; Chien-Chin Huang; Wen-Shan Chen; “Printed ring slot antenna for circular polarization”, IEEE Trans. Antennas Propag., vol. 50, pp.75-77, Aug. 2002. 375

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[3] Jyh-Ying Chiou; Jia-Yi Sze; Kin-Lu Wong; “A broad-band CPW-fed strip-loaded square slot antenna ”, IEEE Trans. Antennas Propag., vol. 51, pp. 719 – 721, Apr. 2003. [4] Horng-Dean Chen; “Broadband CPW-fed square slot antennas with a widened tuning stub”, IEEE Tran. Antennas Propag. Vol. 51, pp. 1982-1986, Aug. 2003. [5] J. Y. Sze, K. L. Wong, and C. C. Huang, “Coplanar waveguide-fed square slot antenna for broadband circularly polarised radiation”, IEEE Trans. Antennas Propag., vol. 51, pp. 2141–2144, Aug. 2003. [6] Y. B. Chen, X. F. Liu, Y.C. Jiao and F. S. Zhang, “CPW-fed broadband circularly polarized square slot antenna”, Electronics Letters., Vol 42, pp.1074-1075, Sep 2006. [7] Chou, C.C.; Lin, K.H.; Su, H.L.;; “Broadband circularly polarised crosspatch- loaded square slot antenna”, Electronics Letters., Vol 43, pp.485-486, Aug 2007. [8] Kow-Ming Chang, Ren-Jie Lin, I-Chung Deng and Qing-Xiang Ke “A novel design of a microstrip-fed shorted square-ring slot antenna for circular polarization”, Microwave Opt Techno Lett, Vol. 49, pp. 1684–1687, July 2007. [9] C.H. Chen, E.K.N. Yung and B.J. Hu, “Miniaturized CPW-fed circularly polarized corrugated slot antenna with meander line loaded”, Electronics Letters, Vol. 43, Dec. 2007. [10] Sze, J.-Y.; Wang, J.-C.; Chang, C.-C.; “Axial-ratio bandwidth enhancement of asymmetric-CPW-fed circularlypolarized square slot antenna”, Electronics Letters., Vol 44, pp.1048-1049, Aug 2008. [11] J. Y. Sze and C. C. Chang, “Circularly polarized square slot antenna with a pair of inverted-L grounded strips”, IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 149–151, 2008. [12] L. Y. Tseng and T. Y. Han, “Microstrip-fed circular slot antenna for circular polarization”, Microwave Opt. Technol. Lett., vol. 50, pp.1056–1058, Apr. 2008. [13] C. Chen and E. K. N. Yung, “Dual-Band Dual-Sense Circularly-Polarized CPW-Fed Slot Antenna With Two Spiral Slots Loaded”, IEEE Trans. Antennas Propag., vol. 57, no. 6, June 2009. [14] Zhou, C.-Z. ; Fu, G. ; Chen, Q. ; “Wideband circularly polarized square slot antenna with parasitic patch” Antennas, Propagation and EM Theory, 2008. ISAPE 2008. 8th International Symposium, pp.158, March 2009. [15] C.-L. Tsai, S.-M. Deng, and L.-W. Liu, “A compact shorted rectangular-ring slot antenna fed by a CPW for circularly polarized wave operations in the WLAN 2.4 GHZ band”, Microwave Opt Technol Lett, Vol.51, PP. 2229–2232, Sep.2009. [16] J.-Y Sze, Chung-I. G. Hsu, Z.-W. Chen, and C.-C. Chang, “Broadband CPW-Fed Circularly Polarized Square Slot Antenna With Lightening-Shaped Feed-line and Inverted-L Grounded Strips”, IEEE Tran. Antennas Propag., Vol. 58, pp. 973-977, March 2010. [17] M.-J. Chiang, T.-F. Hung, and S.-S Bor. “Dual-band circular slot antenna design for circularly and linearly polarized operations”, Micro Opt. Technol. Lett, Vol. 52, No. 12, pp.2717-2721, Dec. 2010. [18] Felegari, N.; Nourinia, J.; Ghobadi, C.; Pourahmadazar, J.; “Broadband CPW-Fed Circularly Polarized Square Slot Antenna With Three Inverted-L-Shape Grounded Strips ”, IEEE Antenna Propagation. Let, Vol. 10, pp. 274277, April 2011. [19] Pourahmadazar, J.; Ghobadi, C.; Nourinia, J.; Felegari, N.; Shirzad. H.; “Broadband CPW-Fed Circularly Polarized Square Slot Antenna With Inverted-L Strips for UWB Applications”, IEEE AWPL, Vol. 10, April 2011. [20] Chen, C.-H.; Yung, E. K. N.; “Dual-Band Circularly-Polarized CPW-Fed Slot Antenna With a Small Frequency Ratio and Wide Bandwidths”, IEEE Trans. Antennas Propag., vol. 59, pp. 1379-1384, Apr. 2011. [21] W. L. Stutzman and G. A. Thiele, “Antenna theory and design”, 2nd ed., Wiley, New York, NY 1998. [22] W. L. Stutzman, “Polarization in Electromagnetic Systems”, Norwood, MA, Artech House, 1993. [23] Karamzadeh, S.; Rafii, V.; Kartal, M.; Virdee, B.S., "Modified circularly polarised beam steering array antenna by utilised broadband coupler and 4 × 4 butler matrix," in Microwaves, Antennas & Propagation, IET , vol.9, no.9, pp.975-981, 6 18 2015

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