A novel compact broadband and radiation efficient antenna design for medical IoT healthcare system

Zaheer Ahmed Dayo; Muhammad Aamir; Shoaib Ahmed Dayo; Imran A. Khoso; Permanand Soothar; Fahad Sahito; Tao Zheng; Zhihua Hu; Yurong Guan; Zaheer Ahmed Dayo; Muhammad Aamir; Shoaib Ahmed Dayo; Imran A. Khoso; Permanand Soothar; Fahad Sahito; Tao Zheng; Zhihua Hu; Yurong Guan

doi:10.3934/mbe.2022180

Mathematical Biosciences and Engineering

2022, Volume 19, Issue 4: 3909-3927. doi: 10.3934/mbe.2022180

Previous Article Next Article

Research article

A novel compact broadband and radiation efficient antenna design for medical IoT healthcare system

1.
College of Computer Science, Huanggang Normal University, Huangzhou 438000, China
2.
Department of Industrial Engineering, Universita Degli Studi Di Salerno (University of Salerno)-Via Giovanni Paolo II, Fisciano (SA) 132-84084, Italy
3.
College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, China
4.
School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
5.
College of Electronic and Communication Engineering, Beijing University of Posts and Telecommunication, Beijing 100879, China
6.
Deputy General Manager, Nanjing Hurys Intelligent Technology Company Limited, Nanjing, China

Academic Editor:Sandeep Pirbhula

Received: 28 December 2021 Revised: 25 January 2022 Accepted: 07 February 2022 Published: 11 February 2022

This paper investigates and develops a novel compact broadband and radiation efficient antenna design for the medical internet of things (M-IoT) healthcare system. The proposed antenna comprises of an umbrella-shaped metallic ground plane (UsMGP) and an improved radiator. A hybrid approach is employed to obtain the optimal results of antenna. The proposed solution is primarily based on the utilization of etching slots and a loaded stub on the ground plane and rectangular patch. The antenna consists of a simple rectangular patch, a 50 Ƹ microstrip feed line, and a portion of the ground plane printed on a relatively inexpensive flame retardant material (FR4) thick substrate with an overall compact dimension of 22 × 28 × 1.5 mm³. The proposed antenna offers compact, broadband and radiation efficient features. The antenna is carefully designed by employing the approximate calculation formulae extracted from the transmission line model. Besides, the parameters study of important variables involved in the antenna design and its influence on impedance matching performance are analyzed. The antenna shows high performance, including impedance bandwidth of 7.76 GHz with a range of 3.65Ƀ11.41 GHz results in 103% wider relative bandwidth at 10 dB return loss, 82% optimal radiation efficiency in the operating band, reasonable gain performance, stable monopole-shaped radiation patterns and strong current distribution across the antenna lattice. The suggested antenna is manufactured, and simulation experiments evaluate its performance. The findings indicate that the antenna is well suited for medical IoT healthcare systems applications.
Citation: Zaheer Ahmed Dayo, Muhammad Aamir, Shoaib Ahmed Dayo, Imran A. Khoso, Permanand Soothar, Fahad Sahito, Tao Zheng, Zhihua Hu, Yurong Guan. A novel compact broadband and radiation efficient antenna design for medical IoT healthcare system[J]. Mathematical Biosciences and Engineering, 2022, 19(4): 3909-3927. doi: 10.3934/mbe.2022180

Related Papers:

Abstract

This paper investigates and develops a novel compact broadband and radiation efficient antenna design for the medical internet of things (M-IoT) healthcare system. The proposed antenna comprises of an umbrella-shaped metallic ground plane (UsMGP) and an improved radiator. A hybrid approach is employed to obtain the optimal results of antenna. The proposed solution is primarily based on the utilization of etching slots and a loaded stub on the ground plane and rectangular patch. The antenna consists of a simple rectangular patch, a 50 Ƹ microstrip feed line, and a portion of the ground plane printed on a relatively inexpensive flame retardant material (FR4) thick substrate with an overall compact dimension of 22 × 28 × 1.5 mm³. The proposed antenna offers compact, broadband and radiation efficient features. The antenna is carefully designed by employing the approximate calculation formulae extracted from the transmission line model. Besides, the parameters study of important variables involved in the antenna design and its influence on impedance matching performance are analyzed. The antenna shows high performance, including impedance bandwidth of 7.76 GHz with a range of 3.65Ƀ11.41 GHz results in 103% wider relative bandwidth at 10 dB return loss, 82% optimal radiation efficiency in the operating band, reasonable gain performance, stable monopole-shaped radiation patterns and strong current distribution across the antenna lattice. The suggested antenna is manufactured, and simulation experiments evaluate its performance. The findings indicate that the antenna is well suited for medical IoT healthcare systems applications.

References

[1]	M. S. Islam, M. T. Islam, M. A. Ullah, G. K. Beng, N. Amin, N. Misran, A modified meander line microstrip patch antenna with enhanced bandwidth for 2.4 GHz ISM-band internet of things (IoT) applications, IEEE Access, 7 (2019), 127850–127861. https://doi.org/10.1109/ACCESS.2019.2940049 doi: 10.1109/ACCESS.2019.2940049
[2]	M. Bansal, B. Gandhi, IoT based development boards for smart healthcare applications, in 2018 4^th International Conference on Computer Communication Automation (ICCCA), (2018), 1–7. https://doi.org/10.1109/CCAA.2018.8777572
[3]	K. R. Jha, B. Bukhari, C. Singh, G. Mishra, S. K. Sharma, Compact planar multistandard MIMO antenna for IoT applications, IEEE Trans. Antennas Propag., 66 (2018), 3327–3336. https://doi.org/10.1109/TAP.2018.2829533 doi: 10.1109/TAP.2018.2829533
[4]	B. J. Falkner, H. Zhou, A. Mehta, T. Arampatzis, D. Mirshekar-syahkal, H. Nakano, A circularly polarized low-cost flat panel antenna array with a high impedance surface meta-substrate for satellite on-the-move medical IoT applications, IEEE Trans. Antennas Propag. 69 (2021), 6076–6081. https://doi.org/10.1109/TAP.2021.3070011 doi: 10.1109/TAP.2021.3070011
[5]	B. Pradhan, S. Bhattacharyya, K. Pal, IoT-based applications in healthcare devices, J. Healthcare Eng., 2021 (2021). https://doi.org/10.1155/2021/6632599 doi: 10.1155/2021/6632599
[6]	T. Han, L. Zhang, S. Pirbhulal, W. Wu, V. H. C. de Albuquerque, A novel cluster head selection technique for edge-computing based IoMT systems, Comput. Networks, 158 (2019), 114–122. https://doi.org/10.1016/j.comnet.2019.04.021 doi: 10.1016/j.comnet.2019.04.021
[7]	S. Pirbhulal, H. Zhang, Md. E. E. Elahi, H. Ghavyat, S. C. Mukhopadhyay, Y. T. Zhang, et al., A novel secure IoT-based smart home automation system using a wireless sensor network, Sensors (Switzerland), 17 (2017), 1–19. https://doi.org/10.3390/s17010069 doi: 10.3390/s17010069
[8]	M. Pazhoohesh, M. S. Javadi, M. Gheisari, S. Aziz, R. Villa, Dealing with missing data in the smart buildings using innovative imputation techniques, in IECON 2021–47th Annual Conference of the IEEE Industrial Electronics Society, (2021), 1–7, https://doi.org/10.1109/iecon48115.2021.9612650
[9]	A. H. Sodhro, S. Pirbhulal, M. Qaraqe, S, Lohano, G. H. Sodhro, N. R. Junejo, et al., Power control algorithms for media transmission in remote healthcare systems, IEEE Access, 6 (2018), 42384–42393. https://doi.org/10.1109/ACCESS.2018.2859205 doi: 10.1109/ACCESS.2018.2859205
[10]	S. Pirbhulal, H. Zhang, S. C. Mukhopadhyay, C. Li, Y. Wang, G. Li, et al., An efficient biometric-based algorithm using heart rate variability for securing body sensor networks, Sensors (Switzerland), 15 (2015), 15067–15089. https://doi.org/10.3390/s150715067 doi: 10.3390/s150715067
[11]	Y. Guan, M. Aamir, Z. Rahman, A. Ali, W. A. Abro, Z. A. Dayo, et al., A framework for efficient brain tumor classification using MRI images, Math. Biosci. Eng., 18 (2021), 5790–5815. https://doi.org/10.3934/MBE.2021292 doi: 10.3934/MBE.2021292
[12]	A. H. Sodhro, S. Pirbhulal, A. K. Sangaiah, S. Lohano, G. H. Sodhro, Z. Luo, 5G-based transmission power control mechanism in fog computing for internet of things devices, Sustainability, 10 (2018), 1–17. https://doi.org/10.3390/su10041258 doi: 10.3390/su10041258
[13]	S. B. Baker, W. Xiang, I. Atkinson, Internet of things (IoT) for smart healthcare: technologies, challenges and opportunities, IEEE Access, 5 (2017), 26521–26544. https://doi.org/10.1109/ACCESS.2017.2775180 doi: 10.1109/ACCESS.2017.2775180
[14]	S. Pirbhulal, H. Zhang, W. Wu, S. C. Mukhopadhyay, Y. T. Zhang, Heartbeats based biometric random binary sequences generation to secure wireless body sensor networks, IEEE Trans. Biomed. Eng., 65 (2018), 2751–2759. https://doi.org/10.1109/TBME.2018.2815155 doi: 10.1109/TBME.2018.2815155
[15]	Z. A. Dayo, Q. Cao, Y. Wang, S. Pirbhulal, A. H. Sodhro, A compact high-gain coplanar waveguide-fed antenna for military RADAR applications, Int. J. Antennas Propag., 2020 (2020), 1–10. https://doi.org/10.1155/2020/8024101 doi: 10.1155/2020/8024101
[16]	T. Gayatri, N. Anveshkumar, V. K. Sharma, A compact planar UWB antenna for spectrum sensing in cognitive radio, in International Conference on Emerging Trends in Information Technology and Engineering, (2020), 1–5. https://doi.org/10.1109/ic-ETITE47903.2020.384
[17]	P. Soothar, H. Wang, C. Xu, Z. A. Dayo, B. Muneer, K. Kanwar, A compact broadband and high gain tapered slot antenna with stripline feeding network for H, X, Ku and K band applications, Int. J. Adv. Comput. Sci. Appl., 11 (2020), 239–244. https://doi.org/10.14569/IJACSA.2020.0110731 doi: 10.14569/IJACSA.2020.0110731
[18]	S. Das, H. Islam, T. Bose, N. Gupta, Ultra wide band CPW-fed circularly polarized microstrip antenna for wearable applications, Wirel. Pers. Commun., 108 (2019), 87–106. https://doi.org/10.1007/s11277-019-06389-9 doi: 10.1007/s11277-019-06389-9
[19]	G. Bozdag, M. Secmen, Compact wideband tapered-fed printed bow-tie antenna with rectangular edge extension, Microw. Opt. Technol. Lett., 61 (2019), 1394–1399. https://doi.org/10.1002/mop.31733 doi: 10.1002/mop.31733
[20]	L. Guo, M. Min, W. Che, W. Yang, A novel miniaturized planar ultra-wideband antenna, IEEE Access, 7 (2019), 2769–2773. https://doi.org/10.1109/ACCESS.2018.2886799 doi: 10.1109/ACCESS.2018.2886799
[21]	M. Li, Y. Zhang, M. C. Tang, Design of a compact, wideband, bidirectional antenna using index-gradient patches, IEEE Antennas Wirel. Propag. Lett., 17 (2018), 1218–1222. https://doi.org/10.1109/LAWP.2018.2839900 doi: 10.1109/LAWP.2018.2839900
[22]	Y. H. Lee, E. H. Lim, F. L. Bong, B. K. Chung, Compact folded C-shaped antenna for metal-mountable UHF RFID applications, IEEE Trans. Antennas Propag., 67 (2019), 765–773. https://doi.org/10.1109/TAP.2018.2879853 doi: 10.1109/TAP.2018.2879853
[23]	N. Jaglan, S. D. Gupta, B. K. Kanaujia, S. Srivastava, Band notched UWB circular monopole antenna with inductance enhanced modified mushroom EBG structures, Wirel. Networks, 24 (2018), 383–393. https://doi.org/10.1007/s11276-016-1343-7 doi: 10.1007/s11276-016-1343-7
[24]	P. K. Jain, B. R. Sharma, K. G. Jangid, S. Shekhawat, V. K. Saxena, D. Bhatnagar, Elliptical shaped wide slot monopole patch antenna with crossed shaped parasitic element for WLAN, Wi-MAX, and UWB application, Microw. Opt. Technol. Lett., 62 (2020), 899–905. https://doi.org/10.1002/mop.32100 doi: 10.1002/mop.32100
[25]	S. Baudha, M. V. Yadav, A novel design of a planar antenna with modified patch and defective ground plane for ultra-wideband applications, Microw. Opt. Technol. Lett., 61 (2019), 1320–1327. https://doi.org/10.1002/mop.31716 doi: 10.1002/mop.31716
[26]	Y. Zhao, C. Wang, Y. Deng, C. Xie, A novel compact ultra-wideband antenna with quad notched bands based on s-sCRLHs resonator, Wirel. Pers. Commun., 97 (2017), 4667–4679. https://doi.org/10.1007/s11277-017-4744-8 doi: 10.1007/s11277-017-4744-8
[27]	N. Gupta, J. Saxena, K. S. Bhatia, R. Kumar, A compact CPW-fed planar stacked circle patch antenna for wideband applications, Wirel. Pers. Commun., 116 (2021), 3247–3260. https://doi.org/10.1007/s11277-020-07847-5 doi: 10.1007/s11277-020-07847-5
[28]	A. Joshi and R. Singhal, Probe-Fed Hexagonal Ultra Wideband Antenna Using Flangeless SMA Connector, Wireless Pers. Commun., 110 (2020), 973–982. https://doi.org/10.1007/s11277-019-06768-2 doi: 10.1007/s11277-019-06768-2
[29]	K. P. Ray, S. S. Thakur, Modified trident UWB printed monopole antenna, Wirel. Pers. Commun., 109 (2019), 1689–1697. https://doi.org/10.1007/s11277-019-06646-x doi: 10.1007/s11277-019-06646-x
[30]	S. Y. A. Fatah, E. K. Hamad, W. Swelam, A. Allam, M. F. A. Sree, H. A. Mohamed, Design and implementation of UWB slot-loaded printed antenna for microwave and millimeter wave applications, IEEE Access, 9 (2021), 29555–29564. https://doi.org/10.1109/ACCESS.2021.3057941 doi: 10.1109/ACCESS.2021.3057941
[31]	Q. Y. Guo, H. Wong, Wideband and high-gain fabry–pérot cavity antenna with switched beams for milimeter-wave applications, 67 (2019), 4339–4347. https://doi.org/10.1109/TAP.2019.2905781
[32]	R. V. Gatti, R. Rossi, M. Dionigi, Single-layer line-fed broadband microstrip patch antenna on thin substrates, Electronics (Switzerland), 10 (2021), 1–14. https://doi.org/10.3390/electronics10010037 doi: 10.3390/electronics10010037
[33]	M. V. Yadav, S. Baudha, A miniaturized printed antenna with extended circular patch and partial ground plane for UWB applications, Wirel. Pers. Commun., 116 (2021), 311–323. https://doi.org/10.1007/s11277-020-07716-1 doi: 10.1007/s11277-020-07716-1
[34]	K. P. Ray, Design aspects of printed monopole antennas for ultra-wide band applications, Int. J. Antennas Propag., 2008 (2008), 1–8. https://doi.org/10.1155/2008/713858 doi: 10.1155/2008/713858
[35]	S. Baudha, V. D.Kumar, Miniaturized dual broadband printed slot antenna with parasitic slot and patch, Microw. Opt. Technol. Lett., 56 (2014), 2260–2265. https://doi.org/10.1002/mop.28567 doi: 10.1002/mop.28567
[36]	H. Fallahi, Z. Atlasbaf, Bandwidth enhancement of a cpw-fed monopole antenna with small fractal elements, AEU Int. J. Electron. Commun., 69 (2015), 590–595. https://doi.org/10.1016/j.aeue.2014.11.011 doi: 10.1016/j.aeue.2014.11.011
[37]	Y. Z. Cai, H. C. Yang, L. Y. Cai, Wideband monopole antenna with three band-notched characteristics, IEEE Antennas Wirel. Propag. Lett., 13 (2014), 607–610. https://doi.org/10.1109/LAWP.2014.2313178 doi: 10.1109/LAWP.2014.2313178
[38]	C. Kumar, D. Guha, Higher mode discrimination in a rectangular patch: new insight leading to improved design with consistently low cross-polar radiations, IEEE Trans. Antennas Propag., 69 (2021), 708–714. https://doi.org/10.1109/TAP.2020.3016506 doi: 10.1109/TAP.2020.3016506
[39]	S. Kundu, S. K. Jana, A compact umbrella shaped UWB antenna for ground-coupling GPR applications, Microw. Opt. Technol. Lett., 60 (2018), 146–151. https://doi.org/10.1002/mop.30928 doi: 10.1002/mop.30928
[40]	H. T. Hu, F. C. Chen, Q. X. Chu, A wideband u-shaped slot antenna and its application in MIMO terminals, IEEE Antennas Wirel. Propag. Lett., 15 (2016), 508–511. https://doi.org/10.1109/LAWP.2015.2455237 doi: 10.1109/LAWP.2015.2455237
[41]	S. K. Palaniswamy, K. Malathi, A. K. Shrivastav, Palm tree structured wide band monopole antenna, Int. J. Microw. Wirel. Technol., 8 (2016), 1077–1084. https://doi.org/10.1017/S1759078715000434 doi: 10.1017/S1759078715000434
[42]	M. Sharma, Y. K. Awasthi, H. Singh, Design of compact planar triple band-notch monopole antenna for ultra-wideband applications, Wirel. Pers. Commun., 97 (2017), 3531–3545. https://doi.org/10.1007/s11277-017-4684-3 doi: 10.1007/s11277-017-4684-3
[43]	P. Kumar, A. Chaturvedi, Design and development of single & dual resonant frequency antennas for moisture content measurement, Wirel. Pers. Commun., 114 (2020), 565–582. https://doi.org/10.1007/s11277-020-07382-3 doi: 10.1007/s11277-020-07382-3
[44]	P. V. Naidu, A. Malhotra, A small ACS-fed tri-band antenna employing C and L shaped radiating branches for LTE/WLAN/WiMAX/ITU wireless communication applications, Analog Integr. Circuits Signal Process., 85 (2015), 489–496. https://doi.org/10.1007/s10470-015-0637-5 doi: 10.1007/s10470-015-0637-5
[45]	Z. A. Dayo, Q. Cao, P. Soothar, M. M. Lodro, Y. Li, A compact coplanar waveguide feed bow-tie slot antenna for WIMAX, C and X band applications, in 2019 IEEE International Conference on Computational Electromagnetics (ICCEM) Shanghai China, (2019), 1–3, https://doi.org/10.1109/COMPEM.2019.8779099
[46]	Z. A. Dayo, Q. Cao, Y. Wang, P. Soothar, A compact high gain multiband bow-tie slot antenna, in 2019 International Applied Computational Electromagnetics Society Symposium-China (ACES), 1 (2019), 1–2. https://doi.org/10.23919/ACES48530.2019.9060736
[47]	M. A. Ullah, T. Alam, M. T. Islam, A UHF CPW-fed patch antenna for nanosatellite store and forward mission, Microsyst. Technol., 26 (2020), 2399–2405. https://doi.org/10.1007/s00542-020-04780-2 doi: 10.1007/s00542-020-04780-2
[48]	S. Naser, N. Dib, Design and analysis of super-formula-based UWB monopole antenna and its MIMO configuration, Wirel. Pers. Commun., 94 (2017), 3389–3401. https://doi.org/10.1007/s11277-016-3782-y doi: 10.1007/s11277-016-3782-y
[49]	N. Prasad, G. Mithilesh, Development of a reconfigurable and miniaturized CPW antenna for selective and wideband communication, Wirel. Pers. Commun., 95 (2017), 2599–2608. https://doi.org/10.1007/s11277-017-3942-8 doi: 10.1007/s11277-017-3942-8
[50]	L. Zhang, Y. Sun, Y. He, S. W. Wong, C. Mao, L. Gei, et al., A quad-polarization reconfigurable antenna with suppressed cross polarization based on characterstics mode theory, IEEE Trans. Antennas Propag., 69 (2021), 636–647. https://doi.org/10.1109/TAP.2020.3016384 doi: 10.1109/TAP.2020.3016384
[51]	Z. A. Dayo, Q. Cao, Y. Wang, P. Soothar, I. A. Khoso, G. Shah, et al., A compact high gain multiband bowtie slot antenna with miniaturized triangular shaped metallic ground plane, Appl. Comput. Electromagn. Soc. J., 36 (2021), 935–945. https://doi.org/10.47037/2021.ACES.J.360717 doi: 10.47037/2021.ACES.J.360717
[52]	A. Kurniawan, S. Mukhlishin, Wideband antenna design and fabrication for modern wireless communications systems, in Procedia Technology (Iceei), 11 (2013), 348–353. https://doi.org/10.1016/j.protcy.2013.12.201
[53]	Z. A. Dayo, Q. Cao, Y. Wang, P. Soothar, B. Muneer, B. S. Chowdhry, A compact broadband high gain antenna using slotted inverted omega shape ground plane and tuning stub loaded radiator, Wirel. Pers. Commun., 113 (2020), 499–518. https://doi.org/10.1007/s11277-020-07227-z doi: 10.1007/s11277-020-07227-z
[54]	K. E. Kedze, H. Wang, Y. Kim, I. Park, Design of a reduced-size crossed-dipole antenna, IEEE Trans. Antennas Propag., 69 (2021), 689–697. https://doi.org/10.1109/TAP.2020.3016392 doi: 10.1109/TAP.2020.3016392
[55]	W. Mazhar, D. M. Klymyshyn, G. Wells, A. A. Qureshi, M. Jacobs, S. Achenbach, Low-profile artificial grid dielectric resonator antenna arrays for mm-wave applications, IEEE Trans. Antennas Propag., 67 (2019), 4406–4417. https://doi.org/10.1109/TAP.2019.2907610 doi: 10.1109/TAP.2019.2907610
[56]	P. Soothar, H. Wang, B. Muneer, Z. A. Dayo, B. S. Chowdhry, A broadband high gain rapered slot antenna for underwater communication in microwave band, Wirel. Pers. Commun., 116 (2021), 1025–1042. https://doi.org/10.1007/s11277-019-06633-2 doi: 10.1007/s11277-019-06633-2
[57]	L. Z. Song, P. Y. Qin, S. Maci, Y. J. Guo, Ultrawideband conformal transmitarray employing connected slot-bowtie elements, IEEE Trans. Antennas Propag., 69 (2021), 3273–3283. https://doi.org/10.1109/TAP.2020.3037785 doi: 10.1109/TAP.2020.3037785
[58]	J. Wu, C. Wang, Y. X. Guo, A compact reflector antenna fed by a composite S/Ka-band feed for 5G wireless communications, IEEE Trans. Antennas Propag., 68 (2020), 7813–7821. https://doi.org/10.1109/TAP.2020.3000858 doi: 10.1109/TAP.2020.3000858
[59]	S. Soltani, P. Lotfi, R. D. Murch, Design and optimization of multiport pixel antennas, IEEE Trans. Antennas Propag., 66 (2018), 2049–2054. https://doi.org/10.1109/TAP.2018.2800759 doi: 10.1109/TAP.2018.2800759
[60]	Z. A. Dayo, Q. Cao, Y. Wang, S. U. Rahman, P. Soothar, A compact broadband antenna for civil and military wireless communication applications, Int. J. Adv. Comput. Sci. Appl., 10 (2019), 39–44. https://doi.org/10.14569/ijacsa.2019.0100906 doi: 10.14569/ijacsa.2019.0100906
[61]	J. Y. Li, R. Xu, X. Zhang, S. G. Zhou, G. W. Yang, A wideband high-gain cavity-backed low-profile dipole antenna, IEEE Trans. Antennas Propag., 64 (2016), 5465–5469. https://doi.org/10.1109/TAP.2016.2620607 doi: 10.1109/TAP.2016.2620607
[62]	T. H. Jung, S. C. Jung, H. K. Ryu, H. S. Oh, J. M. Woo, Ultrawideband planar dipole antenna with a modified taegeuk structure, IEEE Antennas Wirel. Propag. Lett., 14 (2015), 194–197. https://doi.org/10.1109/LAWP.2014.2359936 doi: 10.1109/LAWP.2014.2359936

Reader Comments

Your name:*

Email:*
© 2022 the Author(s), licensee AIMS Press. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0)