This article has been accepted for publication in a future issue of this journal, but has not been fully edited Content may change prior to final publication Citation information: DOI 10.1109/LAWP.2015.2505669, IEEE Antennas and Wireless Propagation Letters > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Reconfigurable Antenna for Future Spectrum Reallocations in 5G Communications Le Huy Trinh, Student Member, IEEE, Fabien Ferrero, Member, IEEE, Leonardo Lizzi, Member, IEEE, Robert Staraj, Member, IEEE, and Jean-Marc Ribero Abstract— This article presents a reconfigurable antenna for mobile terminals with extended band coverage obtained by using a Digitally Tunable Capacitor (DTC) The antenna structure is matched permanently over high frequency bands including DCS/PCS, UMTS, LTE 1800/2600, and 3.5GHz bands Concerning the sub-GHz bands, several reconfigurable states enable a full coverage of LTE 600/700 and GSM 850/900 standards, as well as expected spectrum reallocations for 5G communications With dimensions of 40 mm × 10 mm × mm for the antenna and 130 mm × 70 mm × 0.8 mm for the whole Printed Circuit Board (PCB), this structure is compatible with any mobile terminal Fig The geometry of the proposed antenna and chassis dimension Index Terms— handset antenna, multi-band antenna, reconfigurable antenna, Digitally Tunable Capacitor, 5G communications I INTRODUCTION T HE continuing exponential growth of data traffic on mobile devices is forcing network providers and authorities to find new frequency bands As suggested in [1], the main trends for bands extension are in the UHF TV spectrum and in the frequencies higher than GHz Thanks to the lower bandwidth requirement of digital television compared to the analog one, the portion of spectrum from 698 to 806 MHz has been free up in US Compared to higher frequencies, the 700 MHz waves have excellent propagation characteristics to easily penetrate buildings and to cover large geographic areas with few infrastructures This tendency is going to be extended lower in frequency because the FCC is planning to reallocate as much as 120 MHz of the 600 MHz broadcast TV spectrum for cellular market [2] For the higher bands, possible spectrum reallocation includes parts of 3600-4200MHz and 4400-4990MHz [3], which are good candidates for small coverage with high datarate capacity The arrival of these new frequency bands adds new challenges for antenna designers First, as the compatibility with 2G, 3G and 4G is required, the number of operating band is naturally increasing Secondly, the volume dedicated to the antenna cannot be expanded, resulting in the design of smaller antennas Manuscript submitted on April, 2015 L.H Trinh is with University of Information Technology, National University Ho Chi Minh City, Ho Chi Minh City, Vietnam F Ferrero, L Lizzi, R Staraj and J.-M Ribero are with the Univ Nice Sophia Antipolis, CNRS, LEAT, UMR 7248, 06903 Sophia Antipolis, France (e-mail: fabien.ferrero@unice.fr) Fig Detailed unwrapped antenna geometry and DTC model compared to the lower used wavelength [4] To cover the new frequency bands above 3GHz, a more complex structure of the antenna using parasitic elements is usually enough to achieve the desired coverage However, concerning low frequency bands, passive antenna technology reaches its limits Nowadays, a large number of reconfigurable antennas based on different tuning techniques have been considered Varactor diodes have been extensively used for DVB-H reception [5], [6] However, because of low radiofrequency (RF) power handling of this component, its application is generally limited to the receiving mode The discrete frequency tuning by means of PIN diodes is also known to be a popular technique [7] In this case, the inconvenient of varactor diodes is solved, but the high DC power consumption is a huge drawback of this semiconductor device in mobile applications Other components, which are commonly used for reconfigurable antennas are MEMS In [8] the advantages of MEMS as the small insertion loss, the low consumption and the higher power handling with respect to PIN diodes or varactor diodes are presented However, currently the spreading of MEMS-based solutions is mainly limited by the complexity in the 1536-1225 (c) 2015 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information This article has been accepted for publication in a future issue of this journal, but has not been fully edited Content may change prior to final publication Citation information: DOI 10.1109/LAWP.2015.2505669, IEEE Antennas and Wireless Propagation Letters > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Fig Simulated antenna total efficiency Fig Simulated antenna VSWR in LB for different capacitances Fig Simulated antenna VSWR in MB and HB bands for different capacitances design of these components In this paper, we propose a multi-band reconfigurable antenna topology to fulfill the coverage of frequency bands: a low band for LTE 600/700 and GSM 850/900 standards (LB), a middle band for DCS/PCS, UMTS, LTE 1800/2600 (MB) and high band for emerging 3.5 GHz standard (HB) As reconfiguring component we use a CMOS Digitally Tunable Capacitor (DTC), as it combines high power handling (34 dBm), low power consumption (150A), a low supply voltage (2.3 to 3.6V) and a good capacitance scale in a compact packaging It is controlled with I2C or SPI protocols and has 32 different states [9] In [10], it has been demonstrated that high-Q antennas can suffer from losses due to RF reconfigurable components To overcome this problem, we propose a structure with a tradeoff between low-Q and high-Q antennas The system has a low-Q factor in the 860960MHz thanks to two coupled resonances and a high-Q for the 630-860MHz band, which is obtained by a single resonance The rest of the paper is structured as follows In Sect II, the antenna design process is described with the help of numerical simulations In order to validate the proposed antenna design, experimental measurements of the antenna prototype are presented in Sect III Some concluding comments end the paper (Sect IV) II ANTENNA DESIGN The geometry of the proposed antenna (Figs and 2) Fig Simulated current distribution at different frequencies is based on the capacitive coupling principle between a fed monopole and parasitic elements The antenna has a compact design (40 mm ×10 mm × mm) and it is placed on top of a FR4-Epoxy substrate with a size of 130 mm × 70 mm × 0.8 mm, (r = 4.4, tan = 0.02) The structure is composed by three independent elements (Fig 2) The driven element in the center is a meandered monopole fed by a 50Ω microstrip line (orange) The overall length (84mm) of this monopole is equal to a quarter wavelength at 900 MHz Its third order and fifth order modes are adjusted to 1750 MHz and 3500 MHz using the two strips (pink) An additional resonance is created in the LB using the left shorted parasitic element (yellow) This shorted parasitic element is connected to the ground plane through a variable capacitor For the lower capacitance value, the coupling between the driven and the parasitic element is tuned to enlarge as much as possible the bandwidth as shown in Figure for 0.61pF Then, for higher capacitance CTot, a narrow band can be reconfigured to cover the 630-860 MHz frequency range On the right side, a ground strip (blue) is used to cover the 2.5 GHz band Finally, a new resonance is obtained at 2.7 GHz thanks to an optimization of the shape of the meander monopole as shown in Figure The combination of these two different solutions enables the correct matching of the antenna over the MB The antenna geometrical parameters are optimized using ANSYS HFFS EM solver The final parameters values (expressed in mm) are shown in Fig 1536-1225 (c) 2015 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information This article has been accepted for publication in a future issue of this journal, but has not been fully edited Content may change prior to final publication Citation information: DOI 10.1109/LAWP.2015.2505669, IEEE Antennas and Wireless Propagation Letters > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Fig The fabricated prototype and the control unit TABLE I MEASURED ANTENNA MATCHING FOR DIFFERENT CTOT DTC State 10 11 12 13 14 15 CDTC (pF) 0.90 1.29 1.42 1.56 1.70 1.83 1.96 2.10 2.24 2.37 2.59 2.65 2.78 2.92 3.05 3.19 CTot (pF) 0.61 0.77 0.81 0.86 0.90 0.93 0.96 1.00 1.03 1.05 1.10 1.11 1.13 1.15 1.17 1.19 BW (MHz) 790-980 760-840 742-805 734-792 725-778 721-762 716-751 710-740 704-730 697-721 690-713 683-705 677-698 674-692 670-688 666-684 DTC State 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 CDTC (pF) 3.35 3.49 3.63 3.80 3.94 4.08 4.26 4.39 4.54 4.68 4.85 4.99 5.13 5.27 5.46 5.60 CTot (pF) 1.21 1.23 1.25 1.27 1.28 1.30 1.31 1.33 1.34 1.35 1.37 1.38 1.39 1.40 1.41 1.42 Fig Simulated and measured antenna VSWR in the low frequency band BW (MHz) 663-680 660-677 657-673 655-670 652-667 650-664 647-661 645-659 643-656 641-654 639-652 638-650 636-648 634-646 632-644 631-643 The simulated VSWR curves in the lower frequency bands are presented in Fig When the capacitor value CTot is tuned between 0.61 pF to 1.8 pF, the reflection coefficient of the antenna is modified For the 0.61 pF capacitance, a bandwidth of 170 MHz between 820 MHz and 990 MHz is obtained For higher capacitance values, a narrow band can be tuned from 600 to 860MHz with an instantaneous 3:1 VSWR bandwidth always larger than 10 MHz The capacitance tuning has a small impact on the MB and HB as shown in Fig The antenna is almost constantly matched between 1.7 to 2.7 GHz and 3.5 to 3.75 GHz with a 3:1 VSWR, despite a small overshoot at 1.9GHz for Ctot=0.61pF Total efficiency was simulated using HFSS and the model of the DTC presented in Fig Values for Rs and Cs were extracted from component datasheet According to the results shown in Fig 5, simulated total efficiencies are roughly from -5 dB to -2 dB for the LB, from -3 dB to -0.4 dB for MB and from -3 dB to -1 dB for the HB To further confirm the antenna’s principle and to clearly show which parts of the antennas are responsible for the radiation in the different bands, the study of the current distribution has been made As shown in Fig 6, four frequencies have been considered At 630 MHz, the current is mainly located on the meander monopole and the parasitic element, and especially on the strip used to connect the capacitor to the ground plane At 920 MHz strong currents appear only on the meander monopole At 2100 MHz, the capacitive effect between the ground strip and the feeding line is clearly visible A strong Fig Measured antenna VSWR in the middle and high frequency bands Fig 10 Measured and simulated antenna total efficiency in the low frequency band current is also observed near the capacitor, which explains the influence on the VSWR in this band Finally, at 3500 MHz, the strongest current density is located on strip III PROTOTYPE AND CONCEPT VALIDATION Fig shows the prototype of the proposed antenna The unwrapped radiating structure shown in Fig has been printed on the FR4 substrate and successively folded thanks to the mechanical milling of the edges to be bent The tunable capacitor used to reconfigure the antenna is the Peregrine PE64905 DTC On the back of the PCB, an I2C system used to control the DTC is placed It consists of a Mbed NXP LPC 1768 microcontroller and three AA batteries which are placed in the holder In shunt configuration, the DTC can provide capacitance values from 0.9 pF to 5.6 pF Consequently, in order to achieve the needed 0.61 pF capacitance value (Fig 3), a series capacitor Cs = 1.9 pF 1536-1225 (c) 2015 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information This article has been accepted for publication in a future issue of this journal, but has not been fully edited Content may change prior to final publication Citation information: DOI 10.1109/LAWP.2015.2505669, IEEE Antennas and Wireless Propagation Letters > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Fig 11 Measured and simulated antenna total efficiency in the middle and high frequency bands is connected to the DTC The resulting measured capacitance values (CTot=CDTCCs/(CDTC+Cs) as well as the bandwidths of the antenna obtained in the different DTC configurations are summarized in Tab I Thus, the maximal Ctot is limited to 1.42pF As it can be noticed, the antenna enables continuous coverage with high frequency resolution control from 631 to 980MHz The VSWR values of the prototype, shown in Figs (LB) and (MB & HB), show a fair agreement between simulation and measurement In the LB, the proposed antenna can operate from 630 MHz to 960 MHz by changing the DTC value with VSWR lower than In the MB and HB, VSWR is slightly modified when DTC value is tuned, but it is always lower than for capacitance values (CTot) higher than 1.03 pF For lower capacitance, an overshoot on the VSWR at GHz is observed Consequently, the frequency bands from 1700 MHz to 2700 MHz and from 3500 MHz to 3800 MHz are also covered The total efficiency of the antenna has been measured by using a StarLab-Satimo chamber Since the lower frequency limit of the Starlab-Satimo chamber is 700 MHz, the total efficiency of antenna has been measured only for frequencies above this value Some slight differences between simulations and measurements are visible in the lower frequency band However, the agreement is quite fair considering the dB accuracy of the Starlab-Satimo station The total efficiency level is sufficient for mobile handset applications It varies from -4 dB at 730 MHz to -2 dB at 860 MHz, from -6 dB to 0.5 dB in the 1710-2700 MHz band and from -2.5 dB to -1.5 dB in the range 3500-3800MHz Finally, Fig 12 illustrates the simulated and measured radiation patterns of the proposed antenna at 920, 2100, and 3500 MHz, respectively The measured results fairly agree with the HFSS simulation The radiation behavior at the different frequencies is quasi-omnidirectional, thus being suitable for mobile terminal scenarios IV CONCLUSION A reconfigurable-multiband antenna is presented for today mobile standards and expected spectrum reallocations for 5G communications Thanks to the compact size, this antenna is a good candidate for mobile devices, and especially for MIMO systems in which more than one antenna must be integrated in a very small volume Fig 12 Simulated (dotted line) and measured (solid line) radiation pattern of the proposed antenna ACKNOWLEDGMENT The authors would like to thank the CREMANT for its support in measurements REFERENCES [1] S Sesia, I Toufik, and M Baker, LTE-The UMTS Long Term Evolu-tion: From Theory to Practice Wiley Publishing, 2009 [2] Federal Communications Commission, “Mobile Spectrum Holdings Report and Order - FCC14-63,” Federal Communications Commission, Washington, D.C 20554, Jun 2014 [3] Huawei (2013, February) Whitepaper On Spectrum [Online] Available: http://www.huawei.com/ilink/en/download/HW_204545 [4] R.F Harrington, “Efect of Antenna Size on Gain, Bandwidth, and Efficiency”, Journal of Research of the National Bureau of Standards-D Radio Propagation, vol 64D, no 1, JanuaryFebruary, pp 1-12, 1960 [5] M.A.C Namien, A Sharaiha, S.Collardey and K Mahdjoubi, “An Electrical small frequency reconfigurable antenna for DVBH”, Proc IEEE Int Workshop Antenna Technology (iWAT), 2012 [6] F Canneva, F Ferrero, J Ribero, R Staraj, “Reconfigurable minature antenna for DVB-H standard”, Proc IEEE Antennas and Propagation Society International Symposium (APSURSI), 2010 [7] B Mun, C Jung, M-J Park, and B Lee, “A Compact FrequencyReconfigurable Multiband LTE MIMO Antenna for Laptop Applications”, IEEE Antennas Wireless Propag Lett., vol 13, pp 1389-1392, 2014 [8] S.C Del Barrio, A Morris and Gert F Pedersen, “Antenna Miniaturization with MEMS Tunable Capacitors: Techniques and Trade-Offs”, Int Journ Antenna and Propagation, vol 2014, Article ID 709580, pages, 2014 doi:10.1155/2014/709580 [9] Peregrine Semiconductor Corp., “UltraCMOS Digitally Tunable Capacitor (DTC) 100-3000 MHz”, Document No 70-0335-06, 2012 [10] Del Barrio, S.C.; Pelosi, M.; Pedersen, G.F., "On the efficiency of frequency reconfigurable high-Q antennas for 4G standards," Electron Lett., vol.48, no.16, pp.982-983, 2012 1536-1225 (c) 2015 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information ... expected spectrum reallocations for 5G communications Thanks to the compact size, this antenna is a good candidate for mobile devices, and especially for MIMO systems in which more than one antenna. .. accepted for publication in a future issue of this journal, but has not been fully edited Content may change prior to final publication Citation information: DOI 10.1109/LAWP.2015.2505669, IEEE Antennas... small frequency reconfigurable antenna for DVBH”, Proc IEEE Int Workshop Antenna Technology (iWAT), 2012 [6] F Canneva, F Ferrero, J Ribero, R Staraj, Reconfigurable minature antenna for DVB-H standard”,