Ultra-Wideband Diversity MIMO Antenna System for Future Mobile Handsets

sensors

Article Ultra-Wideband Diversity MIMO Antenna System for Future Mobile Handsets

Naser Ojaroudi Parchin 1,* , Haleh Jahanbakhsh Basherlou 2 , Yasir I. A. Al-Yasir 1 , Ahmed M. Abdulkhaleq 1,3 and Raed A. Abd-Alhameed 1,4

1 Faculty of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, UK; [email protected] (Y.I.A.A.-Y.); [email protected] (A.M.A.); [email protected] (R.A.A.-A.) 2 Bradford College, West Yorkshire, Bradford BD7 1AY, UK; [email protected] 3 SARAS Technology Limited, Leeds LS12 4NQ, UK 4 Department of Communication and Informatics Engineering, Basra University College of Science and Technology, Basra 61004, Iraq * Correspondence: [email protected]; Tel.: +44-734-143-6156

Received: 12 March 2020; Accepted: 14 April 2020; Published: 22 April 2020

Abstract: A new ultra-wideband (UWB) multiple-input/multiple-output (MIMO) antenna system is proposed for future smartphones. The structure of the design comprises four identical pairs of compact microstrip-fed slot antennas with polarization diversity function that are placed symmetrically at different edge corners of the smartphone mainboard. Each antenna pair consists of an open-ended circular-ring slot radiator fed by two independently semi-arc-shaped microstrip-feeding lines exhibiting the polarization diversity characteristic. Therefore, in total, the proposed smartphone antenna design contains four horizontally-polarized and four vertically-polarized elements. The characteristics of the single-element dual-polarized UWB antenna and the proposed UWB-MIMO smartphone antenna are examined while using both experimental and simulated results. An impedance bandwidth of 2.5–10.2 GHz with 121% fractional bandwidth (FBW) is achieved for each element. However, for S 6 dB, this value is more than 130% (2.2–11 GHz). The proposed UWB-MIMO smartphone 11 ≤ − antenna system offers good isolation, dual-polarized function, full radiation coverage, and sufficient efficiency. Besides, the calculated diversity performances of the design in terms of the envelope correlation coefficient (ECC) and total active reflection coefficient (TARC) are very low over the entire operating band.

Keywords: double-fed slot antenna; MIMO system; mobile terminals; polarization diversity; UWB technology

1. Introduction Ultra-Wideband (UWB) technology provides some superiority, such as high-speed data transmission, low cost, and easy manufacture [1]. However, such a popular topic of technology suﬀers from multipath fading in practical applications [2]. Precisely, the multiple-input multiple-output (MIMO) technique was raised to resolve this issue [3,4]. The combination of UWB and MIMO technologies, the use of space multipath, parallel transmission of multiple signals, can obtain obvious multiplexing gain and diversity gain, and achieve a stable signal transmission in distance [5,6]. UWB technology provides wide bandwidth with low power usage. Unlike narrowband technologies, like Bluetooth and Wi-Fi, this technology might be more suitable for short-distance communications [7]. However, it is natively more precise, uses less power, and can transmit data over a wider frequency (up to several GHz). A precise angle combined with a precise distance, which leads the ability for a

Sensors 2020, 20, 2371; doi:10.3390/s20082371 www.mdpi.com/journal/sensors Sensors 2020, 20, 2371 2 of 19 mobile handset to pinpoint an object to a reasonably precise location in space, as well as to recognize its surroundings. While the UWB technology is not new, its first implementation into a modern smartphone has been used by the world’s smartphone makers, such as Apple [8]. It should be noted that other technologies, like Bluetooth and Wi-Fi, are still useful since they have a longer range. Through MIMO technology with multiple antennas, the UWB can measure the angle of arrivals for the signals, as well as secure access to a myriad of the system [9]. Standard MIMO systems tend to employ two or four elements in a single physical package. However, a high number of antenna radiators are employed for the massive MIMO system [10,11]. When compared with previous generations, a large number of antenna elements, operating concurrently, is expected to be applied for 5G communications. 2 2 MIMO systems are successfully deployed for 4G cellular networks, × while, for the massive MIMO operation of 5G communications, it is expected to employ a large number of elements, since the greater number makes the system more resistant to interferences [12,13]. Therefore, the multi-antenna structure with minimum mutual coupling employed for generating polarization and pattern diversity for reliable communication in order to load the MIMO operation into a smartphone for 5G communication. The 5G network also needs fundamental technologies to enable small cells, beamforming, full duplexing, MIMO, and millimeter-wave (MM-Wave) [14,15]. Simple structured and compact antenna elements with sufficient impedance bandwidth and isolation are desirable to be integrated into 5G smartphone platforms, in accordance with the requirement of mobile networks [16–18]. Among various MIMO antennas, microstrip antenna elements are more applicable due to their promising features, such as compact-size, Omani-directional radiation, broad impedance bandwidth, ease of integration, and manufacturing [19,20]. Several MIMO 5G smartphone antennas have been recently proposed [21–38]. However, these smartphone antennas either suffer from narrow impedance-bandwidth (either single-band or dual/multi-band) or use single-polarized uniplanar antenna resonators occupying large spaces of the mainboard, which leads to increasing the complexity of the mobile phone system. In addition, in most of the reported antennas, all of the candidate bands of sub 6 GHz 5G, including LTE Band 7 (2.550–2.650 GHz), N77 (3.3–4.2 GHz), N78 (3.3–3.8 GHz), N79 (4.4–5.0 GHz), and LTE band 46 (5150–5925 MHz) are not supported. Additionally, in the designs of many reported MIMO smartphone antennas, it is common to avoid placing elements in vertical and choose instead to place them parallel, which prevents achieving the polarization diversity. However, in this paper, the antenna elements are both perpendicular and parallel to each other to exhibit the diversity function. In [21,22], uniplanar and double-layer antenna elements with only 200 MHz bandwidths are proposed for 5G smartphone applications. Tightly arranged orthogonal-mode pairs with design complexity is proposed in [23] for 5G smartphones. In [24,25], CPW-fed antenna arrays with large-clearance are introduced for sub 6 GHz MIMO handset. In [26–28], narrow-band antenna arrays with 100 MHz are represented to be integrated at the corners of the mainboard. The proposed antennas in [29,30] use slot resonators in the ground plane, providing polarization diversity function at 3.6 GHz. A multi-element multi-layer antenna with the operation band of 4.4–4.7 GHz is proposed in [31]. The proposed antennas in [32–34] have uniplanar configurations which make them difficult to fabricate and integrate with the circuit system. In addition, the proposed smartphone antenna array in [35] with wide impedance bandwidth employing only two radiators, which is not sufficient for 5G MIMO operation. Moreover, broadband MIMO arrays with 2 GHz bandwidths are reported in [36,37]. The employed radiators are not planar and careful consideration is required in the fabrication process. In [38], an eight-element PIFA array arranged on an artificial magnetic plane with less possible screen-space is proposed. In this manuscript, we present a new design of broadband MIMO antenna systems providing polarization diversity and ultra-wide impedance bandwidth. To the best of our knowledge, this is the first integration of multi UWB antennas with polarization and pattern diversity onto a MIMO smartphone system. The smartphone antenna is designed to work at 2.5–10.2 GHz, covering different wireless systems, including LTE-4G, sub 6 GHz 5G, WLAN, WIMAX, X-band, etc. The structure of the smartphone MIMO array design comprises four pairs of orthogonally dual-polarized slot Sensors 2020, 20, 2371 3 of 19

Sensors 2020, 19 FOR PEER REVIEW 3 radiators located at four edge corners of the smartphone PCB. By exciting the radiator from the patterndifferent and feeding polarization ports, two diversity orthogonally function. polarized A low‐cost waves FR‐ are4 dielectric generated, is used which as leadsthe PCB the substrate. radiation patternA prototype and polarization sample of the diversity proposed function. design A low-costwas fabricated FR-4 dielectric and then is usedtested. as theGood PCB impedance substrate. Abandwidth prototype and sample isolation of the are proposed achieved. design The characteristics was fabricated of and the thensingle tested. radiator Good and impedanceits MIMO packagebandwidth are and elaborated isolation below. are achieved. The characteristics of the single radiator and its MIMO package are elaborated below. 2. Dual‐Polarized UWB Slot Antenna 2. Dual-Polarized UWB Slot Antenna Figure 1 provides the geometry of the designed UWB diversity antenna design. As shown, the designFigure is composed1 provides of thean open geometry‐ended of circular the designed‐ring slot UWB radiator diversity with outer antenna and design. inner radiuses As shown, of r3 andthe design r2 differently is composed fed ofby an a open-endedpair of symmetrical circular-ring semi slot‐arc radiator‐shaped with microstrip outer and‐line inner in radiuses orthogonal of r3 positions.and r2 diff erentlyAs shown fed in by Figure a pair of1a, symmetrical 50‐Ohm SMA semi-arc-shaped (SubMiniature microstrip-line version A) connectors in orthogonal are employed positions. Asfor shownboth antenna in Figure feeding1a, 50-Ohm ports. Table SMA 1 (SubMiniature lists the dimensions version of A) the connectors proposed UWB are employed diversity for antenna both element.antenna feedingThe antenna ports. is Table designed1 lists on the an dimensions FR4 dielectric of the whose proposed permittivity UWB diversity and loss antenna tangent element. are 4.4 Theand 0.025, antenna respectively. is designed The on antenna an FR4 dielectric has an overall whose dimension permittivity of W andS × L lossS × h tangentS = 34 × 34 are × 4.41.6 andmm3 0.025,. It is operatingrespectively. at the The frequency antenna has range an overall of 2.5 dimensionto 10.2 GHz. of The W impedanceL h = 34 matching34 1.6 band mm of3. Itthe is proposed operating S × S × S × × slotat the antenna frequency is governed range of 2.5by toseveral 10.2 GHz. parameters, The impedance such as the matching dimensions band ofof thethe proposed open‐ended slot circular antenna‐ ringis governed slot and by the several semi‐arc parameters,‐shaped radiation such as the stub, dimensions as well as of the the small open-ended strip pairs circular-ring protruded slot to increase and the thesemi-arc-shaped matching band radiation and mutual stub, coupling, as well as especially the small at strip high pairsfrequencies. protruded All of to these increase parameters the matching were optimizedband and mutual to enable coupling, an UWB especially operating at frequency high frequencies. band with All sufficient of these parameters mutual coupling were optimized inferior to to ‐ 10enable dB. an UWB operating frequency band with sufficient mutual coupling inferior to 10 dB. −

(a) (b) (c)

Figure 1.1. OverallOverall view view of theof single-elementthe single‐element Ultra-Wideband Ultra‐Wideband (UWB) antenna(UWB) design,antenna (a )design, three-dimensional (a) three‐ (3D)dimensional view, (b )(3D) top, view, and (c ()b bottom) top, and layers. (c) bottom layers.

Table 1. Parameter values of the proposed UWB diversity designs.

ParameterParameter WS WSL S LS hS hS Wf Wf Lf Lf d d Value (mm) Value (mm) 34 34 34 34 1.6 1.6 5.5 5.5 3 3 7.5 7.5 Parameter hS WLW1 L1 r ValueParameter (mm) 1.6 hS 1 W 11 L 1.5 W1 1.5 L1 11.5 r ValueParameter (mm) r1 1.6 r2 1 r3 11 Wsub 1.5 Lsub 1.5 Hsub 11.5 ValueParameter (mm) 8.5 r1 8 r2 14 r3 75 Wsub 150 Lsub 1.6 Hsub Value (mm) 8.5 8 14 75 150 1.6 Various conﬁgurations of the antenna are shown in Figure2. Simulated S parameter results of the antenna design with a conventional circular-ring slot radiator (Figure2a), with an open-ended circular-ring slot radiator (Figure2b), and the presented design (Figure2c) are shown in Figure3a–c, respectively. As illustrated, by converting the circular-ring slot radiator into the open-ended circular-ring slot, not only the frequency bandwidth of the antenna but also the isolation characteristic of the design have been signiﬁcantly improved. Finally, by adding pairs of small strips under the antenna feed-lines, the antenna provides good matching with improved-bandwidth and well-isolated characteristics at the desired frequency band (2.5–10 GHz).

(a) (b) (c)

Sensors 2020, 19 FOR PEER REVIEW 3 pattern and polarization diversity function. A low‐cost FR‐4 dielectric is used as the PCB substrate. A prototype sample of the proposed design was fabricated and then tested. Good impedance bandwidth and isolation are achieved. The characteristics of the single radiator and its MIMO package are elaborated below.

2. Dual‐Polarized UWB Slot Antenna Figure 1 provides the geometry of the designed UWB diversity antenna design. As shown, the design is composed of an open‐ended circular‐ring slot radiator with outer and inner radiuses of r3 and r2 differently fed by a pair of symmetrical semi‐arc‐shaped microstrip‐line in orthogonal positions. As shown in Figure 1a, 50‐Ohm SMA (SubMiniature version A) connectors are employed for both antenna feeding ports. Table 1 lists the dimensions of the proposed UWB diversity antenna element. The antenna is designed on an FR4 dielectric whose permittivity and loss tangent are 4.4 and 0.025, respectively. The antenna has an overall dimension of WS × LS × hS = 34 × 34 × 1.6 mm3. It is operating at the frequency range of 2.5 to 10.2 GHz. The impedance matching band of the proposed slot antenna is governed by several parameters, such as the dimensions of the open‐ended circular‐ ring slot and the semi‐arc‐shaped radiation stub, as well as the small strip pairs protruded to increase the matching band and mutual coupling, especially at high frequencies. All of these parameters were optimized to enable an UWB operating frequency band with sufficient mutual coupling inferior to ‐ 10 dB.

(a) (b) (c)

Figure 1. Overall view of the single‐element Ultra‐Wideband (UWB) antenna design, (a) three‐ dimensional (3D) view, (b) top, and (c) bottom layers.

Table 1. Parameter values of the proposed UWB diversity designs.

Parameter WS LS hS Wf Lf d Value (mm) 34 34 1.6 5.5 3 7.5 Parameter hS W L W1 L1 r Value (mm) 1.6 1 11 1.5 1.5 11.5 Sensors 2020, 20Parameter, 2371 r1 r2 r3 Wsub Lsub Hsub4 of 19 Value (mm) 8.5 8 14 75 150 1.6 Sensors 2020, 19 FOR PEER REVIEW 4

Figure 2. Various structures of the UWB slot antenna, (a) antenna with a circular‐ring slot radiator, (b) the antenna with open‐ended circular‐ring slot, and (c) the proposed UWB antenna design.

Various configurations of the antenna are shown in Figure 2. Simulated S parameter results of the antenna design with a conventional circular‐ring slot radiator [Figure 2a], with an open‐ended circular‐ring slot radiator [Figure 2b], and the presented design [Figure 2c] are shown in Figures 3a, b, and c, respectively. As illustrated, by converting the circular‐ring slot radiator into the open‐ended circular‐ring slot, not only the frequency bandwidth of the antenna but also the isolation characteristic (a) (b) (c) of the design have been significantly improved. Finally, by adding pairs of small strips under the antennaFigure feed 2. Various‐lines, the structures antenna of the provides UWB slot good antenna, matching (a) antenna with with improved a circular-ring‐bandwidth slot radiator, and well‐ isolated(b) the characteristics antenna with open-endedat the desired circular-ring frequency slot, band and (2.5–10 (c) the proposed GHz). UWB antenna design.

(a)

(b)

(c)

Figure 3. S parameters of the (a) antenna with a circular-ring slot radiator, (b) the antenna with open-ended circular-ring slot, and (c) the proposed UWB antenna design.

Sensors 2020, 20, 2371 5 of 19

Sensors 2020, 19 FOR PEER REVIEW 5 The current densities of the antenna (port 1) at different resonance frequencies (including 3.8, 4.2, 7.1, and 9.1 GHz) are presented in Figure4 and discussed in the following. It is obvious that the maximum distributions of the surface current have been mainly concentrated around the ring‐slot maximum distributions of the surface current have been mainly concentrated around the ring-slot cut cut of the ground plane, since it is the main resonator of the design, which provides different of the ground plane, since it is the main resonator of the design, which provides different resonances resonances of the antenna. Additionally, the semi‐arc‐shaped microstrip feeding lines current is very of the antenna. Additionally, the semi-arc-shaped microstrip feeding lines current is very active at activediff erentat different frequencies. frequencies. However, However, the current the is highlycurrent concentrated is highly concentrated around the edge around of the the open-ended edge of the opencircular-ring‐ended circular at lower‐ring frequencies, at lower as frequencies, can be observed as can in Figure be observed4a,b. This in can Figures be explained 4a and from b. This Figure can3. be explainedIt is observed from Figure that by 3. converting It is observed the circular-ring that by converting slot radiator the circular to the open-ended‐ring slot radiator circular-ring to the slot, open‐ endedthe isolationcircular characteristic‐ring slot, the of the isolation antenna Scharacteristic/S has been of improved the antenna significantly S11/S22 (from has been12 to lessimproved than 11 22 − signi25ficantly dB), according (from − 12 to to the less obtained than − results25 dB), from according Figure3 a,b.to the Moreover, obtained the results semi-arc-shaped from Figures radiation 3a and b. − Moreover,stub and the the semi protruded‐arc‐shaped small rectangularradiation stub strips and in the the ground protruded plane small appear rectangular very active strips with high in the groundcurrent plane densities appear at thevery upper active frequencies with high (7.1 current and 9.1 densities GHz), as at shown the upper in Figure frequencies4c,d. This is(7.1 mainly and 9.1 GHz),because as shown of their in positive Figures impact 4c and on d. further This enhancementis mainly because of the antennaof their performance,positive impact especially on further at enhancementhigher frequencies, of the antenna as shown performance, in Figure3c. especially at higher frequencies, as shown in Figure 3c.

(a) (b) (c) (d)

FigureFigure 4. Current 4. Current densities densities from from Port Port 1 1 at at resonances: resonances: ((aa)) 3.8,3.8, ( b(b)) 4.2, 4.2, ( c()c 7.1,) 7.1, and and (d ()d 9.1) 9.1 GHz. GHz.

TheThe reflection reflection and and transmission transmission coefficients coefficients (S(S1111 andand SS2121)) ofof the the multi-resonance multi‐resonance/UWB/UWB-MIMO‐MIMO antennaantenna with with various parameters parameters are investigated.are investigated. Figures Figures5–8 show 5–8 the resultsshow ofthe varying results fundamental of varying parameters r, L, r , and W . In the simulation of the designed antenna, when one parameter changes, fundamental parameters1 r,1 L, r1, and W1. In the simulation of the designed antenna, when one the rest of the parameters are kept the same as the parameters that are listed in Table1. Since the parameter changes, the rest of the parameters are kept the same as the parameters that are listed in structure is symmetric, it is sufficient to only show the S11 and S21. Figure5 shows the e ffects of r Table 1. Since the structure is symmetric, it is sufficient to only show the S11 and S21. Figure 5 shows (outer radius of the main resonator) on the S-parameters. It can be observed that as L increases, the effects of r (outer radius of the main resonator) on the S‐parameters. It can be observed that as L the lower frequency bandwidth shifts down slowly to a lower band and also the upper operation increases, the lower frequency bandwidth shifts down slowly to a lower band and also the upper frequency moves fast toward a lower band, which means that the length of the slot radiator changes its operationcapacitance frequency characteristic moves [fast39,40 toward]. As seen, a lower by changing band, which the size means of r fromthat the 12 to length 16 mm, of thethe lowerslot radiator and changesupper its frequencies capacitance of the characteristic antennas move [39,40]. from As 3.5 seen, to 1.5 by GHz changing and 11 to the 8 GHz, size of respectively. r from 12 to In addition,16 mm, the lower and upper frequencies of the antennas move from 3.5 to 1.5 GHz and 11 to 8 GHz, respectively. by changing the values of r, the mutual coupling (S21) of the design is changed, especially at middle In addition,frequencies, by aschanging illustrated the in values Figure of5b. r, the mutual coupling (S21) of the design is changed, especially at middle frequencies, as illustrated in Figure 5b.

Sensors 2020, 20, 2371 6 of 19 Sensors 2020, 19 FOR PEER REVIEW 6 Sensors 2020, 19 FOR PEER REVIEW 6

((aa)) ((bb))

Figure 5. (a)S11 and (b)S21 characteristics for diﬀerent values of r. FigureFigure 5. 5. ( a(a) )S S1111 and and ( (bb)) S S2121 characteristicscharacteristics forfor different values of r.r.

FiguresFiguresFigures 66 6and and and7 represent 77 representrepresent the thethe impact impactimpact of L of(length L (length of arc-shaped of arc‐shaped radiation radiationradiation patch) patch)patch) and r1 and(innerand rr1 1(inner radius(inner ofradiusradius the slot of of the resonator)the slot slot resonator) resonator) on the S-parameters onon thethe SS‐‐parametersparameters of the designed of the designed UWB dual-polarized UWBUWB dualdual‐‐polarizedpolarized antenna. antenna. antenna. It is found It It is is 1 that,foundfound unlike that, that, unlike unlike the outer the the outer radiusouter radius radius of the ofof main thethe mainmain resonator resonator (r), (r), L and L and r1 rrmainly1 mainlymainly a affectffaffectect the the the lower lower operation operationoperation bandband of of the the antenna.antenna. antenna. As As shown shown in in Figure Figure 6 6a,a, asas LL decreasesdecreases fromfrom 121212 tototo 6 66 mm, mm,mm, the thethe lower lowerlower frequency frequency bandwidthbandwidth changes changes fromfrom 22 tototo 444 GHz.GHz.GHz. It It should should be noted that changing changing thethe values values of of L L does does not not significantlysignificantlysignificantly impact impact the the transmissiontransmission coecoefficientscoefficientsfficients (or mutual coupling) that that are are characteristic characteristiccharacteristic of of the thethe antennaantenna (shown(shown (shown in in in Figure Figure Figure6 b). 6b).6b). Figure FigureFigure7 illustrates 77 illustratesillustrates the the simulated simulated S-parameter SS‐‐parameterparameter results resultsresults of the ofof the antennathe antenna antenna for for various values (9 to 7 mm) of the inner radius (r1) of the slot resonator. It is clear that, when the variousfor various values values(9 to (9 7 to mm) 7 mm)of the of inner the inner radius radius (r1) of (r the) of slot the resonator. slot resonator. It is clear It is that,clear whenthat, when the inner the radiusinnerinner radius ofradius the slotof of the the varies slotslot from variesvaries 9 to fromfrom 7 mm, 99 toto the 77 mm, lower the operation lower operation bandwidth bandwidthbandwidth of the design ofof thethe tunes designdesign from tunes tunes 2.1 to 3.1fromfrom GHz. 2.1 2.1 to Into 3.1 3.1 addition, GHz. GHz. In In this addition, addition, parameter thisthis parameterparameter value changing value alsochanging affects also the affectsaffects isolation thethe ofisolation isolation the mutual of of the the coupling mutual mutual characteristiccouplingcoupling characteristic characteristic of the design, of of the the as design,design, illustrated asas illustratedillustrated in Figure7 inb. Figure It is evident 7b. ItIt isis that, evidentevident by the that,that, reduction byby the the reduction reduction of the inner of of the inner radius of the slot, the isolation of S2121 has been significantly improved. radiusthe inner of theradius slot, of the the isolation slot, the of isolation S21 has of been S significantlyhas been significantly improved. improved.

(a) (b) (a) (b)

Sensors 2020, 19 FOR PEER REVIEW 7 Figure 6. (a)S11 and (b)S21 characteristics for diﬀerent values of (a) L.

(a) (b) (a) (b) Figure 6. (a) S11 and (b) S21 characteristics for different values of (a) L. Figure 6. (a) S11 and (b) S21 characteristics for different values of (a) L.

(a) (b)

Figure 7. (a)S11 and (b)S21 characteristics for diﬀerent values of r1.

(a) (b)

Figure 7. (a) S11 and (b) S21 characteristics for different values of r1.

(a) (b)

Figure 8. (a) S11 and (b) S21 characteristics for different values of W1.

The effects of another important design parameter, the employed rotated strip on the slot resonator (W1), on antenna performance are investigated through simulation, as presented in Figure 8. It can be found from Figure 8a that the isolation of the resonant bands at lower and upper frequencies changes for different values of W1. Meanwhile, the change of the mutual coupling with the variation of is W1 is not significant. Figure 9a provides the simulated efficiencies (radiation and total) of the diversity antenna design. As shown from the figure that more than 75% radiation efficiency is obtained over the ultra‐wide impedance bandwidth of the antenna with the maximum value of 90% at the lower frequencies. This is caused by the enhanced isolation characteristic of the antenna. Moreover, it is evident that the UWB antenna exhibits good total efficiency results of 50%~75%. Figure 9b depicts the maximum gain and directivity results versus the investigated frequency range. It is shown that, although the gain varies with the operation frequency, its level is still more than 3.1 dBi within the frequency band of interest. It is seen that the difference between directivity and maximum gain results is small, which is mainly because of the high‐efficiency characteristic of the antenna. The overall variation of the gain and directivity is within 3 dBi in the entire frequency band, which is very good for wideband and multiband systems.

Sensors 2020, 19 FOR PEER REVIEW 7

(a) (b)

(a) (b) Sensors 2020, 20, 2371 7 of 19 Figure 7. (a) S11 and (b) S21 characteristics for different values of r1.

(a) (b)

Figure 8. (a)S11 and (b)S21 characteristics for diﬀerent values of W1.

The effects of another important design parameter, the employed rotated strip on the slot resonator (W1), on antenna(a) performance are investigated through simulation,(b) as presented in Figure8. It can be found from Figure8a that the isolation of the resonant bands at lower and upper frequencies Figure 8. (a) S11 and (b) S21 characteristics for different values of W1. changes for different values of W1. Meanwhile, the change of the mutual coupling with the variation of isThe W1 iseffects not significant. of another Figure important9a provides design the parameter, simulated the e ffiemployedciencies (radiation rotated strip and total)on the of slot the resonatordiversity antenna (W1), on design. antenna As performance shown from are the investigated figure that more through than 75%simulation, radiation as e presentedfficiency is in obtained Figure 8.over It thecan ultra-widebe found impedancefrom Figure bandwidth 8a that the of the isolation antenna of with the the resonant maximum bands value at oflower 90% atand the upper lower frequenciesfrequencies. changes This is causedfor different by the values enhanced of W1 isolation. Meanwhile, characteristic the change of of the the antenna. mutual Moreover,coupling with it is theevident variation that theof is UWB W1 is antenna not significant. exhibits Figure good total 9a provides efficiency the results simulated of 50%~75%. efficiencies Figure (radiation9b depicts and total)the maximum of the diversity gain and antenna directivity design. results As versus shown the from investigated the figure frequency that more range. than It 75% is shown radiation that, efficiencyalthough theis obtained gain varies over with the theultra operation‐wide impedance frequency, bandwidth its level is of still the more antenna than with 3.1 dBithe maximum within the valuefrequency of 90% band at ofthe interest. lower frequencies. It is seen that This the is di causedfference by between the enhanced directivity isolation and maximum characteristic gain resultsof the antenna.is small, whichMoreover, is mainly it is evident because that of the the high-e UWBffi antennaciency characteristic exhibits good of total the antenna. efficiency The results overall of 50%~75%.variation of Figure the gain 9b and depicts directivity the maximum is within 3 dBigain in and the entiredirectivity frequency results band, versus which the is veryinvestigated good for frequencywidebandSensors 2020 , range. and19 FOR multiband PEERIt is shownREVIEW systems. that, although the gain varies with the operation frequency, its level is8 still more than 3.1 dBi within the frequency band of interest. It is seen that the difference between directivity and maximum gain results is small, which is mainly because of the high‐efficiency characteristic of the antenna. The overall variation of the gain and directivity is within 3 dBi in the entire frequency band, which is very good for wideband and multiband systems.

(a) (b)

Figure 9. (a) Radiation/total eﬃciencies, maximum gain and (b) diversity results of the diversity antenna.

In the UWB antenna(a) system, the time-domain characteristics are(b) equally as important as the frequencyFigure 9. domain. (a) Radiation/total In the time efficiencies, domain maximum method, angain important and (b) diversity factor indicatingresults of the the diversity characteristic antenna. of the antenna is the fidelity factor. The values of the system fidelity factor vary between 0 and 100%. A systemIn the fidelity UWB factor antenna value system, of 100% the shows time‐ thatdomain the receivedcharacteristics signal perfectlyare equally fits as the important input signal as [41the]. Thefrequency fidelity domain. is employed In the as time a factor domain of similarity method, between an important the input factor and indicating received signal the characteristic and is obtained, of asthe follows: antenna is the fidelity factor. The values of the system fidelity factor vary between 0 and 100%. A R + system fidelity factor value of 100% shows that the∞ receiveds(t)r(t signalτ) perfectly fits the input signal [41]. − The fidelity is employed as a Ffactor= Max ofτ similarityq −∞ between the input and received signal and(1) is R + 2 R + 2 obtained, as follows: ∞ s(t) dt. ∞ r(t) dt −∞ −∞

𝑠 𝑡 𝑟𝑡 𝜏 𝐹𝑀𝑎𝑥 (1) 𝑠𝑡 𝑑𝑡. 𝑟𝑡 𝑑𝑡

where s(t) and r(t) are the input and received signals, respectively. Two identical configurations of the dual‐polarized UWB antenna, including side‐by‐side and face‐to‐face orientations with a 100 mm shift of their center points, are studied. The antenna is excited by using a modulated Gaussian pulse.

(a) (b)

Figure 10. Input/output signal pulses for (a) side by side and (b) face to face scenarios.

A relatively good similarity has been achieved for the RX and TX pulses, as shown in Figure 10. In addition, using (1) the fidelity factor of the reference antenna pair are calculated and good results have been obtained (equal to 0.85 and 0.75, respectively). A prototype sample of the proposed UWB dual‐polarized antenna is fabricated and tested. Figure 11 shows the placements of the fabricated

Sensors 2020, 19 FOR PEER REVIEW 8

(a) (b)

Sensors 2020 , 20, 2371 (a) (b) 8 of 19

Figure 9. (a) Radiation/total efficiencies, maximum gain and (b) diversity results of the diversity antenna. where s(t)Inand ther(t) UWBare antenna the input system, and received the time‐ signals,domain characteristics respectively. Twoare equally identical as conﬁgurationsimportant as the of the dual-polarizedfrequency domain. UWB antenna, In the time including domain side-by-side method, an important and face-to-face factor indicating orientations the withcharacteristic a 100 mm of shift of theirthe center antenna points, is the fidelity are studied. factor. The values antenna of the is excited system fidelity by using factor a modulated vary between Gaussian 0 and 100%. pulse. A Asystem relatively fidelity good factor similarity value of 100% has beenshows achieved that the received for the signalRX and perfectlyTX pulses, fits the as input shown signal in Figure [41]. 10. In addition,The fidelity using is (1)employed the ﬁdelity as a factorfactor ofof thesimilarity reference between antenna the pairinput are and calculated received signal and good and resultsis haveobtained, been obtained as follows: (equal to 0.85 and 0.75, respectively). A prototype sample of the proposed UWB dual-polarized antenna is fabricated and tested. Figure 11 shows the placements of the fabricated 𝑠𝑡𝑟𝑡 𝜏 prototype in measurement setups of S-parameters and radiation patterns, respectively. The measured 𝐹𝑀𝑎𝑥 (1) S-parameters (S11&S21) of the fabricated sample is examined while using the vector network analyzer 𝑠𝑡 𝑑𝑡. 𝑟𝑡 𝑑𝑡 and illustrated in Figure 12a. It is found that the fabricated prototype works properly and it provides acceptable agreement with the simulations. As shown, a good frequency bandwidth (S11 10 dB) of where s(t) and r(t) are the input and received signals, respectively. Two identical configurations≤ − of 2.5–10 GHz is achieved for the fabricated dual-polarized UWB antenna. However, for S 6 dB, the dual‐polarized UWB antenna, including side‐by‐side and face‐to‐face orientations with a 10011 mm≤ − this valueSensors could 2020, 19 beFOR from PEER 2.2–11REVIEW GHz. In addition, it is evident that the mutual-coupling characteristic9 shift of their center points, are studied. The antenna is excited by using a modulated Gaussian pulse. of the design is less than 10 dB over the entire UWB operation frequency band. prototype in measurement− setups of S‐parameters and radiation patterns, respectively. The measured S‐parameters (S11&S21) of the fabricated sample is examined while using the vector network analyzer and illustrated in Figure 12a. It is found that the fabricated prototype works properly and it provides acceptable agreement with the simulations. As shown, a good frequency bandwidth (S11 ≤ − 10 dB) of 2.5–10 GHz is achieved for the fabricated dual‐polarized UWB antenna. However, for S11 ≤ − 6 dB, thisSensors value 2020 could, 19 FOR be PEERfrom REVIEW 2.2–11 GHz. In addition, it is evident that the mutual‐coupling characteristic9 of the design is less than −10 dB over the entire UWB operation frequency band. prototype in measurement setups of S‐parameters and radiation patterns, respectively. The measured S‐parameters (S11&S21) of the fabricated sample is examined while using the vector network analyzer and illustrated in Figure 12a. It is found that the fabricated prototype works properly and it provides acceptable agreement with the simulations. As shown, a good frequency bandwidth (S11 ≤ − 10 dB) of 2.5–10 GHz is achieved for the fabricated dual‐polarized UWB antenna. However, for S11 ≤ − 6 dB, this value could be from( a2.2–11) GHz. In addition, it is evident that the mutual(b‐)coupling characteristic of the design is less than −10 dB over the entire UWB operation frequency band. Figure 10. Input/output signal pulses for (a) side by side and (b) face to face scenarios.

(a) (b)

Figure 10. Input/output signal pulses for (a) side by side and (b) face to face scenarios.

A relatively good similarity has been achieved for the RX and TX pulses, as shown in Figure 10.

In addition, using (1) the fidelity factor of the reference antenna pair are calculated and good results have been obtained (equal(a )to 0.85 and 0.75, respectively). A prototype sample(b) of the proposed UWB dual‐ polarized antenna is fabricated and tested. Figure 11 shows the placements of the fabricated

(a) (b)

Figure 11. Photographs( aof) the antenna in measurements setup of (a) S‐parameters(b) and (b) radiation patterns. Figure 11. Photographs of the antenna in measurements setup of (a) S-parameters and (b) radiation patterns.

(a) (b)

Figure 11. Photographs of the antenna in measurements setup of (a) S‐parameters and (b) radiation patterns.

(a) (b)

Figure 12. Measured and simulated (a) S-parameters and (b) ECC results of the proposed antenna.

(a) (a) (b) (b)

Figure 12. Measured and simulated (a) S‐parameters and (b) ECC results of the proposed antenna.

In order to validate the capability of a dual‐polarized MIMO antenna design, envelope correlation coefficient (ECC),(a) total active reflection coefficient (TARC),(b) and its diversity gain (DG) propertiesFigure are 12. three Measured important and simulated parameters (a) S‐parameters to be investigated and (b) ECC results [42,43]. of the The proposed acceptable antenna. limits of standards are ECC < 0.5, TARC< −10, and DG near 10 dB. These characteristics of MIMO antenna can be calculatedIn order from to thevalidate S‐parameter the capability results whileof a dualusing‐polarized the below MIMO formula: antenna design, envelope correlation coefficient (ECC), total active reflection coefficient (TARC), and its diversity gain (DG) properties are three important parameters to be investigated [42,43]. The acceptable limits of standards are ECC < 0.5, TARC< −10, and DG near 10 dB. These characteristics of MIMO antenna can be calculated from the S‐parameter results while using the below formula:

Sensors 2020, 20, 2371 9 of 19

In order to validate the capability of a dual-polarized MIMO antenna design, envelope correlation coefficient (ECC), total active reflection coefficient (TARC), and its diversity gain (DG) properties are three important parameters to be investigated [42,43]. The acceptable limits of standards are ECC < 0.5, TARC < 10, and DG near 10 dB. These characteristics of MIMO antenna can be calculated from the − S-parameter results while using the below formula:

2 Smm∗ Smn + Snm∗ Snn ECC = (2) 2 2 2 2 1 Smm Smn 1 Snm Snn ∗ − | | − | | − | | − | | s 2 2 (Smm + Smn) + (Snm + Snn) TARC = (3) − 2 Sensors 2020, 19 FOR PEER REVIEW q 9 DG = 10 1 (ECC)2 (4) − 𝐷𝐺 101𝐸𝐶𝐶 (4) Figure 12b illustrates the calculated ECC characteristic of the antenna. It can be observed that the calculated ECC results are very low entire the UWB operation band (less than 0.01). Figure 13a,b Figure 12b illustrates the calculated ECC characteristic of the antenna. It can be observed that illustratethe the calculated TARC andECC DGresults characteristics are very low entire of the the antenna, UWB operation respectively. band (less It than is also 0.01). found Figures in Figure13a 13a that the TARC value of the dual-polarized UWB-MIMO design is less than 20 dB in the frequency and b illustrate the TARC and DG characteristics of the antenna, respectively.− It is also found in range ofFigures 2.5–10 13a GHz. that the The TARC diversity value gainof the functiondual‐polarized of the UWB designed‐MIMO design antenna is less over than its − operation20 dB in the band is more thanfrequency 9.95 dB range over of the2.5–10 entire GHz. operating The diversity frequency gain function band, of the as designed can be seen antenna in Figureover its operation13b. band is more than 9.95 dB over the entire operating frequency band, as can be seen in Figure 13b.

(a) (b)

FigureFigure 13. 13.Calculated Calculated (a)) TARC TARC and and (b) ( bDG) DG results results from from the measured the measured and simulated and simulated results. results.

In addition, two-dimensional (2D) radiation patterns of the fabricated antenna have been measured. Figure 14 shows the measured/simulated radiation patterns of the antenna at selected frequencies, including 3, 6, and 9 GHz. These three frequencies are chosen form the lower, middle, and upper frequencies, respectively. In this design, the xz plane is H-plane (ϕ = 0◦) and yz-plane is E-plane (ϕ = 90◦) for the proposed antenna. From Figure 14, we can see that the antenna can gives dumbbell-like radiation characteristics in E-plane and nearly Omni-directional patterns in H-plane [44,45]. It was found that the radiation patterns of the UWB antenna deteriorate more or less with increasing frequency. However, the radiation properties are almost stable. The peak gains of the UWB dual-polarized antenna over its operation band are measured and compared with simulations, as illustrated in Figure 15a. Almost stable constant gain values are achieved over the antenna operation band, as can be observed. However, the antenna gain is increased from 3.2 to nearly 5.8 dB, which is caused by the deteriorated radiation at the higher band. As a well-known fact, a UWB antenna should cover a wide band. Therefore, the analysis of group delay is very important [46]. When a signal goes through a filter, the signal will distort in both amplitude and phase. This distortion depends on the characteristics of the designed filter, which can determine the communication quality. By representing the transmitting and receiving antennas as a filter, the group delay at the operation band is very important for designing UWB antennas. The group delay is defined as the measurement

Figure 14. Measured (dash line ) and simulated (solid line) results of the antenna patterns at (a) 3 GHz, (b) 6 GHz, and (c) 9 GHz.

Sensors 2020, 19 FOR PEER REVIEW 9

𝐷𝐺 101𝐸𝐶𝐶 (4)

Figure 12b illustrates the calculated ECC characteristic of the antenna. It can be observed that the calculated ECC results are very low entire the UWB operation band (less than 0.01). Figures 13a and b illustrate the TARC and DG characteristics of the antenna, respectively. It is also found in Sensors 2020, 20, 2371 10 of 19 Figures 13a that the TARC value of the dual‐polarized UWB‐MIMO design is less than −20 dB in the frequency range of 2.5–10 GHz. The diversity gain function of the designed antenna over its operation band is more than 9.95 dB over the entire operating frequency band, as can be seen in Figure 13b. of the signal transition time through a device. For the UWB antennas, the altered inductance properties result in a group delay of UWB antennas. The group delay characteristic can be deﬁned through the derivative of phase, which is expressed as:

∂ϕ(ω) τ(ω) = (5) − ∂(ω) where ϕ(ω) and ω are the phase and angular frequencies, respectively. Figure 15b illustrates the measured/simulated group delay of the designed UWB antenna. The result shows that the group delay variation is less than 1 ns in the entire operation band. This indicates a linear phase response and good pulse handling capability for ( thea) proposed antenna. Hence, the antenna is(b) useful in the UWB impulse radio and microwave imaging. Figure 13. Calculated (a) TARC and (b) DG results from the measured and simulated results.

Sensors 2020, 19 FOR PEER REVIEW 10

In addition, two‐dimensional (2D) radiation patterns of the fabricated antenna have been measured. Figure 14 shows the measured/simulated radiation patterns of the antenna at selected frequencies, including 3, 6, and 9 GHz. These three frequencies are chosen form the lower, middle, and upper frequencies, respectively. In this design, the xz plane is H‐plane (φ = 0º) and yz‐plane is E‐ plane (φ = 90º) for the proposed antenna. From FigURE 14, we can see that the antenna can gives dumbbell‐like radiation characteristics in E‐plane and nearly Omni‐directional patterns in H‐plane [44,45]. It was found that the radiation patterns of the UWB antenna deteriorate more or less with increasing frequency. However, the radiation properties are almost stable. The peak gains of the UWB dual‐polarized antenna over its operation band are measured and compared with simulations, as illustrated in Figure 15a. Almost stable constant gain values are achieved over the antenna operation band, as can be observed. However, the antenna gain is increased from 3.2 to nearly 5.8 dB, which is caused by the deteriorated radiation at the higher band. As a well‐known fact, a UWB antenna should cover a wide band. Therefore, the analysis of group delay is very important [46]. When a signal goes through a filter, the signal will distort in both amplitude and phase. This distortion depends on the characteristics of the designed filter, which can determine the communication quality. By representing the transmitting and receiving antennas as a filter, the group delay at the operation band is very important for designing UWB antennas. The group delay is defined as the measurement of the signal transition time through a device. For the UWB antennas, the altered inductance properties result in a group delay of UWB antennas. The group delay characteristic can be defined through the derivative of phase, which is expressed as:

𝜏𝜔 (5)

Where 𝜑𝜔 and 𝜔 are the phase and angular frequencies, respectively. Figure 15 b illustrates the measured/simulated group delay of the designed UWB antenna. The result shows that the group delay variation is less than 1 ns in the entire operation band. This indicates a linear phase response Figure 14.FigureMeasured 14. Measured (dash (dash line )line and ) and simulated simulated (solid (solid line) line) results results of of the the antennaantenna patterns patterns at at (a ()a 3) 3 GHz, and GHz,good (b pulse) 6 GHz, handling and (c) 9capability GHz. for the proposed antenna. Hence, the antenna is useful in the (b) 6UWB GHz, impulse and (c )radio 9 GHz. and microwave imaging.

(a) (b)

FigureFigure 15. Measured 15. Measured and and simulated simulated (a ()a antenna) antenna gain gain and ( (bb) )group group delay delay over over its ultra its ultra wide wide bandwidth. bandwidth.

Table 2 provides a comparative summary of the fundamental properties of the presented design with the reported dual‐polarized antennas available in the literature [47–50]. When compared with reported designs, the presented antenna has a simple structure with a compact, as summarized in Table 2. In addition, a wider impedance bandwidth providing more than 120% FBW is achieved for the proposed antenna. As shown, in contrast with the reported designs in the literature, the proposed dual‐port diversity antenna exhibits lower ECC results. Stable and almost constant gain values are achieved for the antenna over the antenna UWB operation band, which is very good for wideband and multiband systems. Furthermore, as explained earlier, high radiation efficiencies are achieved

Sensors 2020, 20, 2371 11 of 19

Table2 provides a comparative summary of the fundamental properties of the presented design with the reported dual-polarized antennas available in the literature [47–50]. When compared with reported designs, the presented antenna has a simple structure with a compact, as summarized in Table2. In addition, a wider impedance bandwidth providing more than 120% FBW is achieved for the proposed antenna. As shown, in contrast with the reported designs in the literature, the proposed dual-portSensors 2020, diversity19 FOR PEER antenna REVIEW exhibits lower ECC results. Stable and almost constant gain values are11 achieved for the antenna over the antenna UWB operation band, which is very good for wideband and for the proposed design. Therefore, the antenna is suitable for different wireless applications, such as multiband systems. Furthermore, as explained earlier, high radiation eﬃciencies are achieved for the radar, microwave imaging, and cellular communications, due to these attractive features. proposed design. Therefore, the antenna is suitable for diﬀerent wireless applications, such as radar, microwave imaging, and cellular communications, due to these attractive features. Table 2. Comparison between the Design and the Recently Reported Dual‐Polarized UWB Antennas.

TableReference 2. Comparison between the DesignFBW (%) and the Recently ReportedSize (mm Dual-Polarized2) Gain UWB (dB) Antennas. ECC [47] 115% (2.9–11 GHz) 105 × 105 2–7 ‐ Reference FBW (%) Size (mm2) Gain (dB) ECC [48] 112% (3–11.5 GHz) 90 × 90 4–9 ‐ [47] 115% (2.9–11 GHz) 105 105 2–7 - [49] 120% (3–12 GHz) × 76.25 × 52.25 4–6 < 0.05 [48] 112% (3–11.5 GHz) 90 90 4–9 - × [[50]49] 120% (3–12107% GHz) (3–10 GHz) 76.25 52.2572 × 72 4–6 3–6 ‐ <0.05 × [[51]50] 107% (3–10120% GHz) (3–12 GHz) 72 7266.25 × 66.25 3–65 - < 0.1 × [[52]51] 120% (3–12112% GHz) (3.1–11.8) 66.25 66.2561 × 68 5 1–7 <0.1 < 0.02 × [52] 112% (3.1–11.8 GHz) 61 68 1–7 <0.02 [53] 114% (3–11 GHz) × 57 × 57 5 < 0.02 [53] 114% (3–11 GHz) 57 57 5 <0.02 [54] 114% (3–11 GHz) × 56 × 56 3–5 < 0.02 [54] 114% (3–11 GHz) 56 56 3–5 <0.02 × [[55]55] 120% (3–12120% GHz) (3–12 GHz) 40.5 40.540.5 × 40.5 5 5 <0.1 < 0.1 × [[56]56] 114% (3–11114% GHz) (3–11 GHz) 35 3535 × 35 4.6 4.6 <0.3 < 0.3 × Proposed Antenna Antenna 121% (2.5–10.2121% (2.5–10.2 GHz) GHz) 34 3434 × 34 4–6 4–6 <0.01 < 0.01 ×

3. The Proposed UWB Diversity Antenna System Figure 1616 shows shows the the schematic schematic of the of MIMO the MIMO mobile-phone mobile‐ antennaphone antenna system. Assystem. shown, As it comprisesshown, it fourcomprises identical four pairs identical of compact pairs microstrip-fedof compact microstrip slot antennas‐fed slot with antennas polarization with diversitypolarization function diversity that arefunction placed that symmetrically are placed symmetrically at diﬀerent edge at different corners ofedge the corners mainboard of the with mainboard a standard with dimension a standard of 75dimension150 1.6of 75 mm × 3150. However, × 1.6 mm it3 is. However, also possible it is to also arrange possible the to design arrange in di theﬀerent design sizes in ofdifferent the handset sizes × × mainboard,of the handset due mainboard, to the compact due to sizes the ofcompact the employed sizes of diversitythe employed slot elements. diversity slot elements.

(a) (b) (c)

Figure 16. 16. (a()a Side,) Side, (b) ( top,b) top, and and (c) bottom (c) bottom views views of the ofUWB the‐multipleUWB-multiple-input‐input/multiple/multiple-output‐output (UWB‐ (UWB-MIMO)MIMO) antenna antenna system system design design for smartphone for smartphone applications. applications.

Figure 17 depicts thethe SS parametersparameters (including (including S Snnnn andand SSmnmn).). As As shown, shown, the the antenna antenna exhibits good S parameters, similar to the single-elementsingle‐element diversity antenna, covering the frequency spectrum of 2.5–10 GHz. In addition, the mutual coupling of the design is below 10 over the operation frequency 2.5–10 GHz. In addition, the mutual coupling of the design is below −−10 over the operation frequency of the smartphone antenna, which meets the basic requirements for MIMO operation. As expected, the maximum mutual coupling of the MIMO design is between the closely spaced diversity elements (S21 (port 1 and port 2, for example) with a fixed size of the antenna element (WS × LS). In addition, the mutual coupling characteristics of other port pairs is less than − 16 dB. This function makes the design flexible to be arranged in different sizes of PCB. Therefore, it is possible to get satisfactory results for smaller motherboard dimensions.

Sensors 2020, 20, 2371 12 of 19 the maximum mutual coupling of the MIMO design is between the closely spaced diversity elements (S (port 1 and port 2, for example)) with a fixed size of the antenna element (W L ). In addition, 21 S × S the mutual coupling characteristics of other port pairs is less than 16 dB. This function makes the − design flexible to be arranged in different sizes of PCB. Therefore, it is possible to get satisfactory Sensors 2020, 19 FOR PEER REVIEW 12 results for smaller motherboard dimensions.

(a) (b) Sensors 2020, 19 FOR PEER REVIEW 12 FigureFigure 17. 17. ((aa)S) Snnnn and ( b))S Smn characteristicscharacteristics of of the the simulated simulated MIMO MIMO design. design.

UsingUsing (1), (1), the the fidelity fidelity factor factor of of the MIMO antenna design is also investigated.investigated. In the proposed MIMO smartphonesmartphone antenna antenna array, array, the the fidelity fidelity factor factor is the is highest the highest between between the closely-spaced the closely diversity‐spaced antennadiversity pairs antenna with pairs the side-by-sidewith the side orientation,‐by‐side orientation, averaging averaging 0.90. It should 0.90. It beshould noted be that noted the that fidelity the factorfidelity can factor be decreased can be decreased as the distance as the distance increases increases and vice and versa vice [57 versa]. Accordingly, [57]. Accordingly, for other for possible other arrangementspossible arrangements of the antenna of the pairs antenna with pairs longer with distances, longer distances, the fidelity the factor fidelity drops factor to 0.70 drops in most to 0.70 cases. in However,most cases. the However, value is stillthe value reasonably is still good reasonably and acceptably good and low. acceptably low. Figure 18 illustrates the radiation efficiency (R.E.) and total efficiency (T.E.) properties of the proposed design. According to the results, it can be concluded that the MIMO design provides sufficient efficiencies over its entire operation band [58,59]. Three-dimensional (3D) radiation patterns of each element at 6 GHz (middle frequency)(a) have been plotted in Figure 19. It should be noted(b) that the shapes of the radiation patternsFigure for the 17. antenna (a) Snn and elements (b) Smn are characteristics a bit different of the from simulated single-element, MIMO design. which makes it more suitable for the smartphone application. The radiation patterns of the smartphone antenna not only can coverUsing different (1), sides the fidelity of the smartphone factor of the board, MIMO but antenna also generate design different is also polarizations, investigated. which In the is proposed a unique functionMIMO smartphone for MIMO design, antenna as mentioned array, the above fidelity [60 ].factor This is mainlythe highest due to between the corner the placement closely‐spaced of the antennadiversity pairs antenna as well pairs as the with big the ground side‐by plane‐side of orientation, the MIMO design.averaging As 0.90. can be It observedshould be from noted Figure that 19the, differentfidelity factor sides (includingcan be decreased sides 1, as 2, the 3, anddistance 4, as increases shown in and Figure vice 16 versaa) of the[57]. PCB Accordingly, board are for covered other bypossible different arrangements elements with of the different antenna polarization. pairs with Therefore,longer distances, in total, the the fidelity designed factor UWB drops smartphone to 0.70 in antennamost cases. contains However, four horizontally-polarized the value is still reasonably and four good vertically-polarized and acceptably low. radiation elements.

Figure 18. Efficiencies of the MIMO design.

FigureFigure 18. 18. EEfficienciesﬃciencies of of the the MIMO MIMO design. design.

Figure 19. Radiation patterns of the diversity antenna element with directivity value at 5.5 GHz.

Figure 18 illustrates the radiation efficiency (R.E.) and total efficiency (T.E.) properties of the proposed design. According to the results, it can be concluded that the MIMO design provides

Figure 19. Radiation patterns of the diversity antenna element with directivity value at 5.5 GHz.

Figure 18 illustrates the radiation efficiency (R.E.) and total efficiency (T.E.) properties of the proposed design. According to the results, it can be concluded that the MIMO design provides

Sensors 2020, 19 FOR PEER REVIEW 12

(a) (b)

Figure 17. (a) Snn and (b) Smn characteristics of the simulated MIMO design.

Using (1), the fidelity factor of the MIMO antenna design is also investigated. In the proposed MIMO smartphone antenna array, the fidelity factor is the highest between the closely‐spaced diversity antenna pairs with the side‐by‐side orientation, averaging 0.90. It should be noted that the fidelity factor can be decreased as the distance increases and vice versa [57]. Accordingly, for other possible arrangements of the antenna pairs with longer distances, the fidelity factor drops to 0.70 in most cases. However, the value is still reasonably good and acceptably low.

Sensors 2020, 20, 2371 13 of 19 Figure 18. Efficiencies of the MIMO design.

Sensors 2020, 19 FOR PEER REVIEW 13

sufficient efficiencies over its entire operation band [58–59]. Three‐dimensional (3D) radiation patterns of each element at 6 GHz (middle frequency) have been plotted in Figure 19. It should be noted that the shapes of the radiation patterns for the antenna elements are a bit different from single‐ element, which makes it more suitable for the smartphone application. The radiation patterns of the smartphone antenna not only can cover different sides of the smartphone board, but also generate different polarizations, which is a unique function for MIMO design, as mentioned above [60]. This is mainly due to the corner placement of the antenna pairs as well as the big ground plane of the MIMO design. As can be observed from Figure 19, different sides (including sides 1, 2, 3, and 4, as shown in Figure 16a) of the PCB board are covered by different elements with different polarization. FigureFigure 19. Radiation19. Radiation patterns patterns of of the the diversity diversity antenna element element with with directivity directivity value value at 5.5 at GHz. 5.5 GHz. Therefore, in total, the designed UWB smartphone antenna contains four horizontally‐polarized and Thefour proposed Figurevertically 18 ‐illustrates UWBpolarized dual-polarized radiationthe radiation elements. antenna efficiency system (R.E.) wasand fabricatedtotal efficiency and (T.E.) its S-parameters, properties of radiation the Sensors 2020, 19 FOR PEER REVIEW 13 patterns,proposed and gaindesign. characteristics According to are the measured. results, it can Figure be concluded 20a,b show that the the top MIMO and bottomdesign provides layers of the prototype.sufficient The efficiencies MIMO smartphone over its entire antenna operation is implemented band [58–59]. on a Three cheap‐dimensional FR4 substrate (3D) with radiation an overall size ofpatterns 75 150 of each1.6 element mm3. Figureat 6 GHz 20 (middlec illustrates frequency) the measurement have been plotted setup in of Figure the fabricated 19. It should antenna. be × × As cannoted be observed, that the shapes 50-Ohm of the loads radiation are employed patterns for for the the antenna elements, elements not underare a bit measurement different from insingle order‐ to measureelement, the characteristics which makes it of more the MIMO suitable system for the andsmartphone also to eliminateapplication. the The mutual radiation eﬀects patterns from of the the other smartphone antenna not only can cover different sides of the smartphone board, but also generate elements. Figure 21a,b depict the simulated and measured Snn and Smn results of the MIMO design, different polarizations, which is a unique function for MIMO design, as mentioned above [60]. This respectively. It should be noted that, due to the similar behavior of the elements, it is not necessary to is mainly due to the corner placement of the antenna pairs as well as the big ground plane of the showMIMO all of the design. achieved As can S-parameters. be observed from Similar Figure Snn 19,results different with sides ultra-wide (including bandwidth sides 1, 2, are 3, and obtained 4, as for the antennashown in elements, Figure 16a) as of shown the PCB in Figureboard are 21 covereda. In addition, by different all S elementsmn results with of different the design polarization. are less than 10 dB.Therefore, Good agreement in total, the is designed observed UWB for smartphone the presented antenna MIMO contains smartphone four horizontally antenna‐polarized when compared and − (a) (b) (c) with thefour simulations. vertically‐polarized radiation elements. Figure 20. Fabricated prototype, (a) top view, (b) bottom view, and (c) feeding mechanism.

The proposed UWB dual‐polarized antenna system was fabricated and its S‐parameters, radiation patterns, and gain characteristics are measured. Figures 20a and b show the top and bottom layers of the prototype. The MIMO smartphone antenna is implemented on a cheap FR4 substrate with an overall size of 75 × 150 × 1.6 mm3. Figure 20c illustrates the measurement setup of the fabricated antenna. As can be observed, 50‐Ohm loads are employed for the elements, not under measurement in order to measure the characteristics of the MIMO system and also to eliminate the mutual effects from the other elements. Figures 21a and b depict the simulated and measured Snn and Smn results of the MIMO design, respectively. It should be noted that, due to the similar behavior of the elements, it is not necessary to show all of the achieved S‐parameters. Similar Snn results with ultra‐wide bandwidth are obtained for the antenna elements, as shown in Figure 21a. In addition, all (a) (b) (c) Smn results of the design are less than −10 dB. Good agreement is observed for the presented MIMO smartphoneFigureFigure 20. antennaFabricated 20. Fabricated when prototype, compared prototype, ( awith ()a top) top the view, view, simulations. ( b(b)) bottombottom view, view, and and (c ()c feeding) feeding mechanism. mechanism.

ultra ‐wide bandwidth( aare) obtained for the antenna elements, as shown in(b Figure) 21a. In addition, all Smn results of the design are less than −10 dB. Good agreement is observed for the presented MIMO Figure 21. Measured and simulated (a) Snn and (b) Smn characteristics. smartphone antennaFigure 21.whenMeasured compared and with simulated the simulations. (a)Snn and (b)Smn characteristics.

The 2D‐polar radiation patterns of two adjacent elements (including Ant. 1 and Ant. 2) are measured and compared with the simulations due to similar performances of the radiation elements

(a) (b)

Figure 21. Measured and simulated (a) Snn and (b) Smn characteristics.

The 2D‐polar radiation patterns of two adjacent elements (including Ant. 1 and Ant. 2) are measured and compared with the simulations due to similar performances of the radiation elements

Sensors 2020, 20, 2371 14 of 19

The 2D-polar radiation patterns of two adjacent elements (including Ant. 1 and Ant. 2) are measuredSensors 2020 and, 19 comparedFOR PEER REVIEW with the simulations due to similar performances of the radiation elements14 of the smartphone antenna. Figure 22 plots the measured and simulated radiation patterns of the antennas of the smartphone antenna. Figure 22 plots the measured and simulated radiation patterns of the at 3, 6, and 9 GHz. It is found that the fabricated smartphone antenna can provide quasi-omnidirectional antennas at 3, 6, and 9 GHz. It is found that the fabricated smartphone antenna can provide quasi‐ radiation patterns at different frequencies of its operation bandwidth [61]. The experimental and omnidirectional radiation patterns at different frequencies of its operation bandwidth [61]. The simulation results agree well with each other, as can be observed. Next, we evaluate the TARC and experimental and simulation results agree well with each other, as can be observed. Next, we evaluate ECCthe characteristics TARC and ECC of characteristics the proposed of UWB the MIMOproposed smartphone UWB MIMO antenna. smartphone Since antenna. these two Since parameters these considertwo parameters the mutual consider effects, the they mutual can provideeffects, they more can meaningful provide more measures meaningful of the measures designed of MIMO the antennadesigned performance MIMO antenna than theperformance reflection than coeffi thecient. reflection coefficient.

FigureFigure 22. 22.Measured Measured (dash (dash line line ) ) and and simulatedsimulated (solid line) line) radiation radiation pattern pattern results results for for Ant. Ant. 1 and 1 and Ant.Ant. 2 at 2 at (a )(a 3) GHz,3 GHz, (b ()b 6) 6 GHz, GHz, and and ( c(c)) 9 9 GHz. GHz.

Figure 23a,b depict the ECC and TARC parameters, as calculated from the measured/simulated results. It can be observed from Figure 23a that the designed MIMO antenna provides very low ECC (less than 0.01) through its entire operation band. Furthermore, it is clear from Figure 23b that the UWB MIMO antenna design exhibits low TARC characteristics over the ultra-wide bandwidth (2.5–10 GHz). As seen, the design has less than 20 dB TARC values at different frequencies. The fundamental − properties of the proposed MIMO diversity antenna system are compared with the recently reported MIMO antenna designs and are listed in Table3[ 21–38]. As clearly seen, in contrast to the recently proposed designs, the designed MIMO antenna offers ultra-wide frequency bandwidth of 2.6–10 GHz (more than 120% FBW),(a a) unique characteristic that none of the other(b) MIMO antenna designs cited have. InFigure addition, 23. Calculated it can be seen(a) ECC that and the ( presentedb) TARC results MIMO of antennathe UWB exhibits smartphone higher antenna performances from in terms ofmeasured/simulated efficiency, gain level,results. ECC, and TARC characteristics. Furthermore, the proposed MIMO

Sensors 2020, 19 FOR PEER REVIEW 16

Sensors 2020, 20, 2371 15 of 19 smartphone antenna exhibits radiation pattern diversity and also supports diﬀerent polarizations (includingFigure vertical 22. Measured/horizontal) (dash line at di ) ﬀanderent simulated four sides (solid of line) the radiation mainboard, pattern unlike results most for Ant. of the 1 and reported MIMOAnt. designs. 2 at (a) 3 GHz, (b) 6 GHz, and (c) 9 GHz.

(a) (b)

FigureFigure 23. 23. CalculatedCalculated ((aa)) ECCECC and ( b)) TARC results results of of the the UWB UWB smartphone smartphone antenna antenna from from measuredmeasured/simulated/simulated results. results.

TableFigures 3. Comparison 23a and between b depict the Proposedthe ECC Smartphone and TARC Antenna parameters, and the Reported as calculated MIMO Antennas. from the measured/simulated results. It can be observed from Figure 23a that the designed MIMO antenna Ref. Design Type B.W. (GHz) Eﬃciency (%) Size (mm2) ECC provides very low ECC (less than 0.01) through its entire operation band. Furthermore, it is clear [21] Inverted-F 3.4–3.6 - 120 70 - from Figure 23b that the UWB MIMO antenna design exhibits low TARC characteristics× over the [22] Monopole 3.4–3.6 35–50 150 75 <0.40 ultra‐wide bandwidth (2.5–10 GHz). As seen, the design has less than − 20 dB× TARC values at [23] Tightly Arranged Pairs 3.4–3.6 50–70 150 73 <0.07 × different[24] frequencies. Integrated The Waveguide fundamental properties 3.4–3.6 of the proposed 50–80 MIMO diversity 150 75 antenna system<0.2 × are [compared25] with Modiﬁed the recently PIFA reported MIMO 3.25–3.85 antenna designs 40–75 and are listed 150 in Table75 3 [21–38].<0.01 As × clearly[26] seen, in contrast Loop to the recently proposed 2.55–2.6 designs, the 48–63designed MIMO 136 antenna68 offers< 0.15ultra‐ × [27] SCS patch-slot 3.55–3.65 52–76 150 75 - × [28] Monopole-Slot 2.55–2.68 48–63 136 68 <0.15 × [29] Slot 3.4–3.8 50–75 150 75 <0.01 × [30] circular-slot loop 3.3–3.9 60–80 150 75 <0.01 × [31] Monopole 4.55–4.75 50–70 136 68 - × [32] Inverted-F 3.4–3.6 55–60 100 50 - × [33] Self-Isolated Monopole 3.4–3.6 60–70 150 75 <0.015 × [34] Inverted-L Monopole 3.3–5 40–60 136 68 <0.2 × [35] Identical Monopole 2–6 30–60 124 64 - × [36] U-Slot 3.3–6 40–75 150 75 <0.12 × [37] Loop 3.3–5 40–70 150 75 <0.1 × [38] Inverted-F 3.3–4.5 20–75 150 75 <0.6 × This Work Diversity Ring-Slot 2.6–10.2 (122%) 60–80 150 75 <0.007 ×

Another important function is the channel capacity loss (CCL) of the MIMO system [43], which mainly depends on the S-parameters of the MIMO system with the accepted limit of 0.4 bps/Hz and ≤ can be calculated while using the below formulas: CCL = log det ψR (6) − 2    ρ11 ρ18   ···  R =  . .. .  ψ  . . .  (7)    ρ ρ  81 ··· 88 where, 2 2 ρii = 1 ( Sii + Sij ) − | | ρ = (S S + S S ) for i,j = 1, ... ,8 ij − ii∗ ij ∗ji ij Sensors 2020, 20, 2371 16 of 19

Figure 24a illustrates the calculated channel capacity loss from the measured and simulated results of the proposed UWB-MIMO smartphone antenna. It is shown that the MIMO antenna design offers a very low CCL band: less than 0.3 bps/Hz over the operation band of 2.7~10 GHz is obtained. Sensors 2020, 19 FOR PEER REVIEW 18 Furthermore, the calculated channel capacity (CC) is investigated in Figure 24b in order to further study the MIMO operation of the presented UWB antenna system. The CC is defined as the maximum 𝑆𝑁𝑅 possible transmission rate, such𝐶𝐶 that the 𝐸 probability 𝑙𝑜𝑔 𝑑𝑒𝑡 𝐼of error𝐻 is arbitrarily𝐻 small. It is defined as: (8) 𝑛 SNR T where the channel matrix Hscale can= be calculated using:+ CC E log2 det I HscaleHscale (8) nT where the channel matrix Hscale can𝐻 be calculated 𝜌, using: 𝐻.. 𝜌, (9)

Hscale = √ρscale,RX Hi.i.d √ρscale,TX (9) As shown, the calculated CC within the entire operating frequency is around 40 bps/Hz, whereas,As shown, for the the ideal calculated case, it CCis about within 46 the bps/Hz entire operating[62]. According frequency to the is aroundobtained 40 ECC, bps/Hz, TARC, whereas, and forCCL the results, ideal case, it can it is be about calculated 46 bps/ Hzthat [62 the]. Accordingproposed UWB to the obtainedsmartphone ECC, antenna TARC, andhas CCLprovided results, worthy it can beMIMO calculated performance. that the proposed UWB smartphone antenna has provided worthy MIMO performance.

(a) (b)

Figure 24. 24. CalculatedCalculated (a) (channela) channel capacity capacity loss (CCL) loss (CCL)and (b) andchannel (b) capacity channel (CC) capacity of the (CC) smartphone of the smartphoneantenna. antenna.

4. Conclusions A new design structure of eight-porteight‐port UWB-MIMOUWB‐MIMO mobile-phonemobile‐phone antenna is proposed for future mobile handsets. Its Its configuration configuration consists consists of of four four pairs pairs of of diversity diversity slot slot antenna antenna elements elements located located at atfour four edge edge corners corners of of the the PCB PCB mainboard mainboard with with an an FR FR-4‐4 dielectric. dielectric. The The circular circular-ring‐ring slot radiator is converted to an open-endedopen‐ended slot by adding a rectangular strip in order to enhance the port isolation and increase increase the the impedance impedance-matching‐matching of of the the closely closely-spaced‐spaced ports. ports. The antenna The antenna elements elements exhibit exhibit ultra‐ ultra-wide impedance bandwidth, covering 2.6–10 GHz (S 10 dB), and provide radiation pattern wide impedance bandwidth, covering 2.6–10 GHz (S11 ≤ −1110≤ dB), − and provide radiation pattern and andpolarization polarization diversity diversity function. function. Acceptable Acceptable fundamental fundamental properties properties in in terms terms of of input input-impedance,‐impedance, antenna gain, efficiency, efficiency, radiation patterns, and user user-e‐effectffect are achieved for the proposed design. Low ECC, TARC, DG,DG, andand CCLCCL characteristicscharacteristics havehave beenbeen obtained.obtained. Moreover, the performanceperformance of thethe designed UWB-MIMOUWB‐MIMO smartphone antenna for different different user-handuser‐hand/user/user-head‐head scenarios has been studied and susufficientfficient eefficiencyfficiency performancesperformances are obtained.obtained. The antenna has a simple and planar structure under the premise of coveringcovering didifferentfferent frequencies,frequencies, which makes it suitablesuitable for MIMOMIMO operation in future smartphonesmartphone applications.applications.

Author Contributions: Writing—original draft preparation, N.O.P., H.J.B., Y.I.A.A.-Y., A.M.A. and R.A.A.-A.; Author Contributions: writing—original draft preparation, N.O.P., H.J.B., Y.I.A.A.‐Y., A.M.A. and R.A.A.‐A.; writing—review and editing, N.O.P. and R.A.A.-A.; investigation, N.O.P., H.J.B., Y.I.A.A.-Y. and A.M.A.; resources, N.O.P.writing—review and R.A.A.-A.; and editing, for other N.O.P. cases, alland authors R.A.A. have‐A.; investigation, participated. AllN.O.P., authors H.J.B., have Y.I.A.A. read and‐Y. agreedand A.M.A.; to the publishedresources, versionN.O.P. and of the R.A.A. manuscript.‐A.; for other cases, all authors have participated. Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement H2020‐MSCA‐ITN‐2016 SECRET‐722424.

Acknowledgments: Authors wish to express their thanks to the support provided by the innovation programme under grant agreement H2020‐MSCA‐ITN‐2016 SECRET‐722424.

Conflicts of Interest: The authors declare no conflict of interest.

Sensors 2020, 20, 2371 17 of 19

Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET-722424. Acknowledgments: Authors wish to express their thanks to the support provided by the innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET-722424. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Song, Y.; Guo, N.; Qiu, R.C. Towards a real-time UWB MIMO test bed for sensing and communications. Proc. IEEE Southeastcon. 2011, 59–63. 2. Federal Communication Commission. First Report and order Revision of part 15 of the Commission’s Rules Regarding Ultrawideband Transmission System; Federal Communication Commission: Washington, DC, USA, 2002. 3. Chandel, R.; Gautam, A.K.; Rambabu, K. Design and packaging of an eye-shaped multiple-input– multiple-output antenna with high isolation for wireless UWB applications. IEEE Trans. Compon. Packag. Manuf. Technol. 2018, 8, 635–642. [CrossRef] 4. Mazloum, J.; Ghorashi, S.A.; Ojaroudi, M.; Ojaroudi, N. Compact triple-band S-shaped monopole diversity antenna for MIMO applications. Appl. Comput. Electromagn. Soc. J. 2015, 30, 975–980. 5. Rajagopalan, A.; Gupta, G.; Konanur, A.S.; Hughes, B.; Lazzi, G. Increasing channel capacity of an ultrawideband MIMO system using vector antennas. IEEE Trans. Antennas Propag. 2007, 55, 2880–2887. [CrossRef] 6. Ojaroudi, N.; Ghadimi, N. Dual-band CPW-fed slot antenna for LTE and WiBro applications. Microw. Opt. Technol. Lett. 2014, 56, 1013–1015. [CrossRef] 7. Zhang, R.; Song, L.; Jaiprakash, A.; Talty, T.; Alanazi, A.; Alghaﬁs, A.; Tonguz, O. Using ultra-wideband technology in vehicles for infrastructure-free localization. arXiv 2019, arXiv:1902.07764. 8. Mearian, L. Ultra Wideband (UWB) Explained (and Why It’s in the iPhone 11). Available online: https: //www.computerworld.com/article/3490037 (accessed on 31 December 2019). 9. Swedberg, C. Mobile phone companies to demonstrate UWB payments. RFID J. 2020. 10. Rao, Q.; Wilson, K. Design, modeling, and evaluation of a multiband MIMO/diversity antenna system for small wireless mobile terminals. IEEE Trans. Compon. Packag. Manuf. Technol. 2011, 59, 410–419. [CrossRef] 11. Jensen, M.; Wallace, J. A review of antennas and propagation for MIMO wireless communications. IEEE Trans. Antennas Propag. 2004, 52, 2810–2824. [CrossRef] 12. Larsson, E. Massive MIMO for next generation wireless Systems. IEEE Commun. Mag. 2014, 52, 186–195. [CrossRef] 13. Parchin, N.O.; Basherlou, H.J.; Al-Yasir, Y.I.; Abdulkhaleq, A.M.; Abd-Alhameed, R.A.; Excell, P.S. Eight-port MIMO antenna system for 2.6 GHz LTE cellular communications. Progress Electromagn. Res. C 2020, 99, 49–59. [CrossRef] 14. Al-Yasir, Y.I.A.; Abdullah, A.S.; Ojaroudi Parchin, N.; Abd-Alhameed, R.A.; Noras, J.M. A new polarization-reconﬁgurable antenna for 5G applications. Electronics 2018, 7, 293. [CrossRef] 15. Ojaroudiparchin, N.; Shen, M.; Pedersen, G.F. Small-size tapered slot antenna (TSA) design for use in 5G phased array applications. Appl. Comput. Electromagn. Soc. J. 2018, 32, 193–202. 16. Zhang, Z. Antenna Design for Mobile Devices; Wiley-IEEE Press: Hoboken, NJ, USA, 2011. 17. Ojaroudi, N. Circular microstrip antenna with dual band-stop performance for ultra-wideband systems. Microw. Opt. Technol. Lett. 2014, 56, 2095–2098. [CrossRef] 18. Ojaroudi, N. Design of microstrip antenna for 2.4/5.8 GHz RFID applications. In Proceedings of the GeMic 2014, German Microwave Conference, RWTH Aachen University, Aachen, Germany, 10–12 March 2014. 19. Valizade, A.; Ghobadi, C.; Nourinia, J.; Ojaroudi, N.; Ojaroudi, M. Band-notch slot antenna with enhanced bandwidth by using Ω-shaped strips protruded inside rectangular slots for UWB applications. Appl. Comput. Electromagn. Soc. (ACES) J. 2012, 27, 816–822. 20. Ojaroudi, Y.; Ojaroudi, N.; Ghadimi, N. Circularly polarized microstrip slot antenna with a pair of spur-shaped slits for WLAN applications. Microw. Opt. Technol. Lett. 2015, 57, 756–759. [CrossRef] 21. Zhao, X.; Yeo, S.P.; Ong, L.C. Decoupling of inverted-F antennas with high-order modes of ground plane for 5G mobile MIMO platform. IEEE Trans. Antennas Propag. 2018, 66, 4485–4495. [CrossRef] Sensors 2020, 20, 2371 18 of 19

22. Wong, K.-L.; Lu, J.-Y.; Chen, L.-Y.; Li, W.Y.; Ban, Y.L. 8-antenna and 16-antenna arrays using the quad-antenna linear array as a building block for the 3.5-GHz LTE MIMO operation in the smartphone. Microw. Opt. Technol. Lett. 2016, 58, 174–181. [CrossRef] 23. Sun, L.; Feng, H.; Li, Y.; Zhang, Z. Compact 5G MIMO mobile phone antennas with tightly arranged orthogonal-mode pairs. IEEE Trans. Antennas Propag. 2018, 66, 6364–6369. [CrossRef] 24. Abdullah, M.; Ban, Y.L.; Kang, K.; Li, M.Y.; Amin, M. Eight-element antenna array at 3.5GHz for MIMO wireless application. Progress Electromagn. Res. C 2017, 78, 209–217. [CrossRef] 25. Parchin, N.O.; Al-Yasir, Y.I.; Basherlou, H.J.; Abd-Alhameed, R.A.; Noras, J.M. Orthogonally dual-polarised MIMO antenna array with pattern diversity for use in 5G smartphones. IET Microw. Antennas Propag. 2020. 26. Li, M.-Y. Eight-port orthogonally dual-polarised MIMO antennas using loop structures for 5G smartphone. IET Microw. Antennas Propag. 2017, 11, 1810–1816. [CrossRef] 27. Parchin, N.O.; Al-Yasir, Y.I.; Noras, J.M.; Abd-Alhameed, R.A. Dual-polarized MIMO antenna array design using miniaturized self-complementary structures for 5G smartphone applications. In Proceedings of the 13th European Conference on Antennas and Propagation (EuCAP), Krakow, Poland, 31 March–5 April 2019. 28. Li, M.-Y.; Ban, Y.L.; Xu, Z.Q.; Wu, G.; Kang, K.; Yu, Z.F. Eight-port orthogonally dual-polarized antenna array for 5G smartphone applications. IEEE Trans. Antennas Propag. 2016, 64, 3820–3830. [CrossRef] 29. Parchin, N.O.; Al-Yasir, Y.I.A.; Ali, A.H.; Elfergani, I.; Noras, J.M.; Rodriguez, J.; Abd-Alhameed, R.A. Eight-element dual-polarized MIMO slot antenna system for 5G smartphone applications. IEEE Access 2019, 9, 15612–15622. [CrossRef] 30. Parchin, N.O.; Al-Yasir, Y.I.; Basherlou, H.J.; Abdulkhaleq, A.M.; Sajedin, M.; Abd-Alhameed, R.A.; Noras, J.M. Modiﬁed PIFA array design with improved bandwidth and isolation for 5G mobile handsets. In Proceedings of the 2019 IEEE 2nd 5G World Forum (5GWF), Dresedn, Germany, 30 September–2 October 2019. 31. Xu, S.; Zhang, M.; Wen, H.; Wang, J. Deep-subwavelength decoupling for MIMO antennas in mobile handsets with singular medium. Sci. Rep. 2017, 7, 12162. [CrossRef] 32. Al-Hadi, A.A.; Ilvonen, J.; Valkonen, R.; Viikan, V. Eight-element antenna array for diversity and MIMO mobile terminal in LTE 3500MHz band. Microwave Opt. Technol. Lett. 2014, 56, 1323–1327. [CrossRef] 33. Zhao, A.; Zhouyou, R. Size reduction of self-isolated MIMO antenna system for 5G mobile phone applications. IEEE Antennas Wirel. Propag. Lett. 2019, 18, 152–156. [CrossRef] 34. Jiang, W.; Liu, B.; Cui, Y.; Hu, W. High-isolation Eight-Element MIMO array for 5G smartphone applications. IEEE Access 2019, 7, 34104–34112. [CrossRef] 35. Zhekov, S.S.; Tatomirescu, A.; Franek, O.; Pedersen, G.F. Study of the interaction user head-ultrawideband MIMO antenna array for mobile terminals. Proc. Int. Conf. Electromagn. Adv. Appl. 2016, 930–933. 36. Zhang, X.; Li, Y.; Wang, W.; Shen, W. Ultra-wideband 8-port MIMO antenna array for 5G metal-frame smartphones. IEEE Access 2019, 7, 72273–72282. [CrossRef] 37. Zhao, A.; Ren, Z. Wideband MIMO antenna systems based on coupled-loop antenna for 5G N77/N78/N79 applications in mobile terminals. IEEE Access 2019, 7, 93761–93771. [CrossRef] 38. Liu, D.Q.; Luo, H.J.; Zhang, M.; Wen, H.L.; Wang, B.; Wang, J. An extremely low-proﬁle wideband MIMO antenna for 5G smart-phones. IEEE Trans. Antennas Propag. 2019, 67, 5772–5780. [CrossRef] 39. Kamalvand, A.; Ghobadi, C.; Nourinia, J.; Ojaroudi, M.; Ojaroudi, N. Omni-directional/multi-resonance CPW-fed small slot antenna for UWB applications. ACES J. 2013, 28, 829–835. 40. Ojaroudi, N.; Ojaroudi, M.; Amiri, S. Enhanced bandwidth of small square monopole antenna by using inverted U-shaped slot and conductor-backed plane. Appl. Comput. Electromagn. Soc. (ACES) J. 2012, 27, 685–690. [CrossRef] 41. Ojaroudi, N.; Ojaroudi, M.; Ghadimi, N. UWB omnidirectional square monopole antenna for use in circular cylindrical microwave imaging systems. IEEE Antennas Wireless Propag. Lett. 2012, 11, 1350–1353. [CrossRef] 42. Dastranj, A. Optimization of a printed UWB antenna: Application of the invasive weed optimization algorithm in antenna design. IEEE Antennas Propag. Mag. 2017, 59, 48–57. [CrossRef] 43. Sharawi, M.S. Printed MIMO Antenna Engineering; Artech House: Norwood, MA, USA, 2014. 44. Parchin, N.O.; Jahanbakhsh Basherlou, H.; Al-Yasir, Y.I.; Abdulkhaleq, A.M.; Patwary,M.; Abd-Alhameed, R.A. A new CPW-fed diversity antenna for MIMO 5G smartphones. Electronics 2020, 9, 261. [CrossRef] 45. Parchin, N.O.; Abd-Alhameed, R.A.; Shen, M. Gain improvement of a UWB antenna using a single-layer FSS. In Proceedings of the PhotonIcs & Electromagnetics Research Symposium (PIERS), Xiamen, China, 17–20 December 2019. Sensors 2020, 20, 2371 19 of 19

46. Siahkal-Mahalle, B.H.; Ojaroudi, M.; Ojaroudi, N. Enhanced bandwidth small square monopole antenna with band-notched functions for UWB wireless communications. Appl. Comput. Electromagn. Soc. (ACES) J. 2012, 27, 759–765. 47. Zhu, F.; Gao, S. Development and implementation of a dual-polarized slot-loaded patch antenna for ultra-wideband imaging systems. In Proceedings of the URSI General Assembly and Scientiﬁc Symposium (URSI GASS), Beijing, China, 16–23 August 2014. 48. Zhu, F.; Gao, S.; Ho, A.T.; Abd-Alhameed, R.A.; See, C.H.; Brown, T.W.; Xu, J. Ultra-wideband dual-polarized patch antenna with four capacitively coupled feeds. IEEE Trans. Antennas Propag. 2014, 62, 2440–2449. 49. Ram Krishnaa, R.V.S.; Kumar, R. Design and investigations of a microstrip fed open V-shape slot antenna for wideband dual slant polarization. Eng. Sci. Technol. Int. J. (JESTECH) 2015, 18, 513–523. [CrossRef] 50. Yahya, R.; Nakamura, A.; Itami, M.; Denidni, T.A. A novel UWB FSS-based polarization diversity antenna. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 2525–2528. [CrossRef] 51. Augustin, G.; Chacko, B.P.; Denidni, T.A. Dual Port Ultra Wideband Antennas for Cognitive Radio and Diversity Applications in Advancement in Microstrip Antennas with Recent Applications; InTech: Rijeka, Croatia, 2013. 52. Kishna, R.V.R.; Kumar, R.; Kushwaha, N. An UWB dual polarized microstrip fed L-shape slot antenna. Int. J. Microw. Wireless Technol. 2016, 8, 363–368. [CrossRef] 53. Ram Krishnaa, R.V.S.; Kumar, R. A dual-polarized square-ring slot antenna for UWB, imaging, and radar applications. IEEE Antennas Wireless Propag. Lett. 2016, 15, 195–198. [CrossRef] 54. Xiong, L.; Gao, P. Compact dual-polarized slot UWB antenna with CPW-fed structure. Proc. IEEE Antennas Propag. Soc. Int. Symp. (APSURSI’12) 2012, 1–2. 55. Irene, G.; Rajesh, A. A dual-polarized UWB–MIMO antenna with IEEE 802.11ac band-notched characteristics using split-ring resonator. J. Comput. Electron. 2018, 17, 1090–1098. [CrossRef] 56. Zhu, J.; Li, S.; Feng, B.; Deng, L.; Yin, S. Compact dual polarized UWB quasi-self-complementary MIMO/diversity antenna with band-rejection capability. IEEE Antennas Wireless Propag. Lett. 2016, 15, 905–908. [CrossRef] 57. Tiang, S.S.; Hathal, M.S.; Anwar, N.S.; Ain, M.F.; Abdullah, M.Z. Development of a compact wide-slot antenna for early stage breast cancer detection featuring circular array full-view geometry. Int. J. Antennas Propag. 2014, 1–11. [CrossRef] 58. Ojaroudiparchin, N.; Shen, M.; Fr, G. Design of Vivaldi antenna array with end-ﬁre beam steering function for 5G mobile terminals. In Proceedings of the 23rd Telecommunications Forum Telfor (TELFOR), Belgrade, Serbia, 24–26 November 2015; pp. 587–590. 59. Parchin, N.O.; Jahanbakhsh Basherlou, H.; Al-Yasir, Y.I.; Ullah, A.; Abd-Alhameed, R.A.; Noras, J.M. ‘Multi-band MIMO antenna design with user-impact investigation for 4G and 5G mobile terminals. Sensors 2019, 19, 456. [CrossRef] 60. Ojaroudiparchin, N.; Shen, M.; Zhang, S.; Pedersen, G.F. A switchable 3D-coverage phased array antenna package for 5G mobile terminals. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 1747–1750. [CrossRef] 61. Ojaroudi, N.; Ojaroudi, Y.; Ojaroudi, S. Compact ultra-wideband monopole antenna with enhanced bandwidth and dual band-stop properties. Int. J. RF Microw. Comput. Aided Eng. 2014, 25, 346–357. [CrossRef] 62. Chang, L.; Yu, Y.; Wei, K.; Wang, H. Polarization-orthogonal co-frequency dual antenna pair suitable for 5G MIMO smartphone with metallic bezels. IEEE Trans. Antennas Propag. 2019, 67, 5212–5220. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).