A Compact Quad-Element UWB-MIMO Antenna System with Parasitic Decoupling Mechanism

applied sciences

Article A Compact Quad-Element UWB-MIMO Antenna System with Parasitic Decoupling Mechanism

Fatima Amin 1, Rashid Saleem 1,* , Tayyab Shabbir 1 , Sabih ur Rehman 2 , Muhammad Bilal 3 and M. Farhan Shaﬁque 4

1 Department of Telecommunication Engineering, University of Engineering and Technology, Taxila 47050, Pakistan; [email protected] (F.A.); [email protected] (T.S.) 2 School of Computing and Mathematics, Charles Sturt University, Port Macquarie NSW 2444, Australia; [email protected] 3 Department of Telecommunication Engineering, Balochistan University of Information Technology, Engineering and Management Sciences, Quetta 87300, Pakistan; [email protected] 4 Center for Advance Studies in Telecommunications, COMSATS University Islamabad, Park Road, Tarlai, Kalan, Islamabad 45550, Pakistan; farhan.shaﬁ[email protected] * Correspondence: [email protected]

Received: 30 April 2019; Accepted: 5 June 2019; Published: 11 June 2019

Abstract: This research work proposes a compact four-port multiple-input multiple-output (MIMO) antenna that operates in the whole license free ultra-wideband (UWB) spectrum of 3.1–10.6 GHz. Spatial diversity has been introduced by arranging these antennas in close proximity without developing a strong mutual coupling. Antenna elements are evolved from a conventional rectangular patch antenna whereas a customized decoupling structure is introduced on the back side of the substrate to achieve the desired isolation level. The parasitic decoupling structure consists of different features which are resonant at different frequencies offering a whole UWB coverage. In addition to the decoupling structure a dumbbell shaped stub has also been introduced to the partial ground plane to suppress the mutual coupling. The overall measured isolation among elements is more than 20 dB. Different MIMO performance parameters have also been investigated from the measured results. Whole MIMO system measures 0.41 λo 0.44 λo at 3.1 GHz. The MIMO system is intended × for high data rate and short-range communication devices used in wireless personal area networks.

Keywords: channel capacity loss (CCL); decoupling; envelope correlation coefficient (ECC); isolation; multiple-input multiple-output (MIMO); total active reflection coefficient (TARC); ultra-wideband (UWB)

1. Introduction In recent years, data rate and channel capacity issues have been expansively investigated, since high throughput is imperative for modern wireless devices [1]. The unlicensed ultra-wideband (3.1 GHz–10.6 GHz) spectrum is considered promising for short range wireless systems that require high data rate. However, power restriction on license free UWB spectrum reduces its benefits for its applications in modern wireless systems [2]. Since multiple-input multiple-output (MIMO) technology is known to offer better data-rate for same channel thus the concept of incorporating UWB with MIMO antenna system offers a potential solution to enhance data rate [3]. The resulting UWB-MIMO antennas provide greater throughput and better channel capacity. Despite of various advantages achieved with MIMO antennas in modern communication systems, some challenges are also associated with them. Placement of multiple antennas in a close vicinity with a shared ground plane introduces strong mutual coupling among antennas resulting in reduced diversity

Appl. Sci. 2019, 9, 2371; doi:10.3390/app9112371 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2371 2 of 13 gain. Thus, one of the main challenges is to maintain isolation among antennas by reducing their mutual coupling. Recent literature reports a number of techniques to achieve isolation among multiple radiating elements [4]. These techniques include stub loading [5,6], defected ground structure (DGS) [7], neutralization lines [8], frequency selective surfaces (FSS) [9], metamaterial isolator [10], patterned grounds [11], meander line resonator [12], decoupling structure and parasitic elements [13], etc. Another challenge in UWB-MIMO antennas is to incorporate multiple antennas in limited space as per the miniaturization demands of radio frequency (RF) front-ends of modern antenna systems by keeping mutual coupling at a desired minimum level. The literature reports extensive research work on UWB-MIMO antennas [14–29]. A four-element UWB-MIMO antenna for portable wireless applications is reported in [14]. The stepped slot antenna is used as a single radiating element. Isolation level of 22 dB is achieved with this arrangement. However, no specific decoupling mechanism is used for isolation. Another quad-element UWB-MIMO with band notching is presented in [15]. The staircase shaped decoupling and defected ground structure provides the isolation level of 22 dB. This antenna is designed for portable wireless systems. The MIMO antenna design in [16] proposes two symmetric half-slot radiating elements. A Y-shaped slot introduced in the ground plane provides isolation at lower frequencies of UWB by increasing the current path between the two ports. This system has compact dimensions but isolation level of 15 dB is achieved only at lower frequencies of UWB band. Another two-element UWB-MIMO notched band antenna reported in [17] has a T-shaped stub in the ground plane that helps in achieving 20 dB isolation level. A four element UWB-MIMO is presented in [18]. The orthogonal arrangement of four monopoles provides an isolation level of 15 dB as well as the polarization diversity. However, the isolation level of 15 dB with a larger size in comparison is achieved. Moreover, the MIMO performance parameters like total active reflection coefficient (TARC) and channel capacity loss (CCL) are left unevaluated with antenna efficiency of 70%. Another four-port UWB-MIMO system is presented in [19]. The system has four circular monopole antennas with defected ground plane and electromagnetic bandgap (EBG) structures. This system has much larger antenna dimensions and offers the isolation level of 17.5 dB. Split Ring Resonator (SRR) based quad-element MIMO antenna is proposed in [20]. This antenna configuration is designed for LTE, WiMAX and Bluetooth. However, the isolation of only 14 dB is achieved with a much larger size. In another study a two-element UWB-MIMO antenna is presented [21]. The two quarter elliptical ring resonators achieve the isolation of 15 dB with a comparatively large antenna size. This system has low peak gain and efficiency in comparison. Moreover, important parameters like TARC and CCL are left unevaluated. In [22] a two-element U-shaped MIMO configuration for handheld devices is presented. The antenna achieves the required isolation with a decoupling slot introduced in the ground plane. However, the proposed design operates in the lower UWB band only with much larger size. The efficiency of antenna is 60% and certain MIMO performance parameters are also left uncalculated. Another two element UWB-MIMO antenna with band-notching is presented in [23]. The T-shaped stub in the ground plane provides necessary isolation between antennas. This design has an isolation of 15 dB with the much larger size in comparison. The TARC and CCL values are not evaluated as well. A four-element MIMO antenna is presented in [24]. The proposed system operates in LTE band with sufficient MIMO performance. This system has much larger size with the minimum isolation level of 10.5 dB. Moreover, the efficiency and gain are also low. Another four-element annular slot-based MIMO antenna is designed for UWB applications in [25]. The antenna system is meant for cognitive radio platform and operates up to 7.65 GHz. This presented system has low isolation level of 13 dB with larger size and low performance in terms of efficiency and gain comparatively. Likewise, in another study, reported in [26], a four-element MIMO system for LTE band 1 and 4 is discussed. This system achieves isolation of 10 dB in limited frequency band of 1.77 GHz–2.51 GHz. Another semi-elliptical two-element UWB-MIMO antenna is reported in [27]. This design has larger size along with the isolation level of 18 dB. The MIMO performance is evaluated in terms of ECC and CCL. Four monopoles based MIMO antenna is discussed in [28]. The design possesses the band rejection ability because of band-stop design insertion on the ground plane. The structure acts as an LC Appl. Sci. 2019, 9, x 3 of 13

of 17 dB with larger size. The design in [29] has four quasi-self-complementary (QSC) antenna elements. This system possesses polarization diversity but with much larger size comparatively. The isolation level of 20 dB is achieved but certain MIMO performance parameters are left unevaluated. In this work, a highly compact semi-elliptical shaped quad-element UWB-MIMO antenna is presented. The antenna possesses two distinct mechanisms for coupling reduction. The ground plane is modified by introducing defects as well as stubs have been attached to the ground plane to reduce Appl.coupling Sci. 2019 between, 9, 2371 side-by-side elements. A parasitic decoupling structure is designed and placed3 of 13on the back side of substrate which offers multiple resonances in order to ensure wideband decoupling band-stopfor across filterelements, and rejectsthese structures the wireless provide localarea isolat networkion better (WLAN) than 20 band. dB in However,the entire frequency this system band. has isolationDiversity of performance 17 dB with larger of the size. prop Theosed design MIMO in [antenna29] has four is established quasi-self-complementary by evaluating the (QSC) performance antenna elements.parameters This like system envelope possesses correlati polarizationon coefficient diversity (ECC), buttotal with active much reflection larger sizecoefficient comparatively. (TARC), Thechannel isolation capacity level loss of 20 (CCL) dB is achievedand diversity but certain gain (DG). MIMO performance parameters are left unevaluated. InThe this rest work, of the a highlypaper is compact organized semi-elliptical as follows: Section shaped 2 quad-elementdescribes the proposed UWB-MIMO design antenna in detail is presented.followed by The Section antenna 3, possesses in which two the distinct isolation mechanisms mechanisms for coupling for mutual reduction. coupling The groundreduction plane are isdiscussed. modified byThe introducing simulated defectsand measured as well asresults stubs of have the been proposed attached MIMO to the antenna ground are plane discussed to reduce in couplingSection 4. between A comprehensive side-by-side comparison elements. A of parasitic the proposed decoupling design structure is presented is designed in Section and placed5 and onthe thepaper back is sideconcluded of substrate in Section which 6. offers multiple resonances in order to ensure wideband decoupling for across elements, these structures provide isolation better than 20 dB in the entire frequency band. Diversity2. Antenna performance Configuration of the proposed MIMO antenna is established by evaluating the performance parameters like envelope correlation coefficient (ECC), total active reflection coefficient (TARC), channel 2.1. Single UWB Antenna Configuration capacity loss (CCL) and diversity gain (DG). TheA low-cost rest of the 20 papermm × is20 organized mm FR-4 assubstrate follows: of Section thickness2 describes 1 mm, relative the proposed permittivity design of in 4.4 detail and followeddielectric by loss Section tangent3, in of which 0.02 is the used isolation to design mechanisms the single for antenna. mutual The coupling antenna reduction geometry are has discussed. evolved Thefrom simulated a conventional and measured rectangular results patch. of The the bottom proposed corners MIMO of the antenna radiator are are discussed chamfered, in while Section semi-4. Acircles comprehensive of radius 2.5 comparison mm are subtracted of the proposed from both design sides of is the presented patch. Rectangular in Section5 slitsand are the introduced paper is concludedat the top edge in Section of each6. radiating patch. The resulting semi-elliptical shape provides good impedance matching in the band of interest. A microstrip feed line with chamfered top edge is used to excite 2.antenna Antenna elements Configuration with matched impedance. All modification steps and their effect on the reflection coefficient is shown in Figure 1a. 2.1. Single UWB Antenna Configuration 2.2. UWB-MIMOA low-cost 20 Antenna mm Configuration20 mm FR-4 substrate of thickness 1 mm, relative permittivity of 4.4 × and dielectric loss tangent of 0.02 is used to design the single antenna. The antenna geometry has The proposed MIMO antenna design has four identical radiating elements. Four antennas evolved from a conventional rectangular patch. The bottom corners of the radiator are chamfered, arranged in MIMO configuration are shown in Figure 1b. Rear view of the MIMO antenna is shown while semi-circles of radius 2.5 mm are subtracted from both sides of the patch. Rectangular slits are in Figure 1c. Partial ground plane with tapered top edges with certain defects have been introduced introduced at the top edge of each radiating patch. The resulting semi-elliptical shape provides good on the rear-side to achieve wide impedance bandwidth. The ground plane is defected with slits and impedance matching in the band of interest. A microstrip feed line with chamfered top edge is used stubs on the both sides. Antenna 1, 2 and 3, 4 are placed side-by-side whereas antenna 1, 4 and 2, 3 to excite antenna elements with matched impedance. All modification steps and their effect on the are in across configuration with respect to each other. reflection coefficient is shown in Figure1a.

(a)

Figure 1. Cont. Appl.Appl. Sci. Sci. 20192019, 9, ,x9 , 2371 4 of4 13 of 13

(b) (c)

FigureFigure 1. 1.UWB-MIMOUWB-MIMO antenna antenna design:( design:(a) singlea) single antenna antenna element element evolution evolution (b) (bfront) front view view of offinal ﬁnal

design;design; (c) (rearc) rear view view of final of ﬁnal design; design; L = L40,W= 40,W = 43,= fl 43,= 6, f lfw= =6, 1.8, fw l1= =1.8, 3, w l11 = 1.5,l3, w2 1= =7, 1.5,lw2 =2 4,= l7,3 =w 1.3,2 = 4, w3l =3 =6, 1.3l4 =, 0.6,w3 =w46, = l10,4 = l0.6,5 = 4, w w4 5= = 10,12, ll56 = 13,4,w w56 == 2.6,12, ll67 = 5.1,13, wl8 6= =1.7,2.6, l9 = l7 5.3,= 5.1, l10 = l8 3.4,= 1.7, l11 = l 90.8,= 5.3, w7 =l10 4,= l123.4 , = 2,wl11 8= =0.8 2, ,l13 w =7 0.5,= 4, w l129 = 5.5,2,w 8l14= =2, 3, l 13d = 2,0.5, r1 w= 92.5,= 5.5, r2 = l 143, =r33, = d1.5,= 2, a r=1 9.1,= 2.5, b = r 29.8= 3,(all r3 dimensions= 1.5, a = 9.1, areb =in9.8 mm).(all dimensions are in mm).

2.3.2.2. Equivalent UWB-MIMO Circuit Antenna Modeling Configuration CircuitThe proposedmodeling of MIMO a UWB antenna antenna design is generally has four carried identical out in radiatinga time domain elements. simulation Four because antennas of arrangedextremely in wide MIMO bandwidth. configuration The are modeling shown in is Figure carried1b. out Rear by view matching of the MIMOthe input antenna impedance is shown or in admittanceFigure1c. of Partial the antenna ground with plane the with lumped tapered equivalent top edges model. with certainOne way defects of modeling have been a UWB introduced antenna on is theby rear-sideconsidering to achieve it a combination wide impedance of various bandwidth. overlapping The ground adjacent plane resonators. is defected If with a parallel slits and RLC stubs resonatoron the both is considered sides. Antenna as a unit 1, 2element, and 3, 4 a are number placed of side-by-side them is considered whereas to antenna be attached 1, 4 and in series 2, 3 are as in shownacross in configuration Figure 2a. Based with on respect the required to each other.resonances, the number of these RLC resonant elements can be added or subtracted. 2.3. Equivalent Circuit Modeling The values of these lumped components are optimized through a schematic solver where the values Circuitof each modelingelement is of tuned a UWB to antennamatch the is generallyenvelop of carried the impedance out in a time curve domain and resonance simulation peaks. because Theof proposed extremely UWB wide antenna bandwidth. element The comprises modeling of is three carried resonant out by peaks matching within the the input UWB impedance band, one or is admittancearound 3 GHz, of the second antenna is witharound the 7 lumped GHz and equivalent third is model.around One 11.5 way GHz. of modelingTherefore, a UWBthree antennaparallel is resonantby considering circuits itare a combination included in of the various equivalent overlapping circuit adjacent model. resonators. The equivalent If a parallel circuit RLC lumped resonator componentsis considered optimized as a unit at element, R1 = 80 Ω a, numberR2 = 95 Ω of, R them3 = 110 is consideredΩ, L1 = 1.4 nH, to be L2 attached = 0.4 nH, in L series3 = 0.21 as nH, shown C1 = in 1.92Figure pF, C2a.2 = Based1.25 pF on and the C required3 = 0.82 pF resonances, the number of these RLC resonant elements can be added or subtracted. The values of these lumped components are optimized through a schematic solver where the values of each element is tuned to match the envelop of the impedance curve and resonance peaks. The proposed UWB antenna element comprises of three resonant peaks within the UWB band, one is around 3 GHz, second is around 7 GHz and third is around 11.5 GHz. Therefore, three parallel resonant circuits are included in the equivalent circuit model. The equivalent circuit lumped components optimized at R1 = 80 Ω,R2 = 95 Ω,R3 = 110 Ω,L1 = 1.4 nH, L2 = 0.4 nH, L3 = 0.21 nH, C1 = 1.92 pF, C2 = 1.25 pF and C3 = 0.82 pF.

(a) (b)

Figure 2. Equivalent circuit: (a) real impedance of UWB antenna; (b) comparison of reflection coefficients. Appl. Sci. 2019, 9, x 4 of 13

(b) (c)

Figure 1. UWB-MIMO antenna design:(a) single antenna element evolution (b) front view of final

design; (c) rear view of final design; L = 40,W = 43, fl = 6, fw = 1.8, l1 = 3, w1 = 1.5,l2 = 7, w2 = 4, l3 = 1.3,

w3 = 6, l4 = 0.6, w4 = 10, l5 = 4, w5 = 12, l6 = 13, w6 = 2.6, l7 = 5.1, l8 = 1.7, l9 = 5.3, l10 = 3.4, l11 = 0.8, w7 = 4, l12 = 2,w8 = 2, l13 = 0.5, w9 = 5.5, l14 = 3, d = 2, r1 = 2.5, r2 = 3, r3 = 1.5, a = 9.1, b = 9.8 (all dimensions are in mm).

2.3. Equivalent Circuit Modeling Circuit modeling of a UWB antenna is generally carried out in a time domain simulation because of extremely wide bandwidth. The modeling is carried out by matching the input impedance or admittance of the antenna with the lumped equivalent model. One way of modeling a UWB antenna is by considering it a combination of various overlapping adjacent resonators. If a parallel RLC resonator is considered as a unit element, a number of them is considered to be attached in series as shown in Figure 2a. Based on the required resonances, the number of these RLC resonant elements can be added or subtracted. The values of these lumped components are optimized through a schematic solver where the values of each element is tuned to match the envelop of the impedance curve and resonance peaks. The proposed UWB antenna element comprises of three resonant peaks within the UWB band, one is around 3 GHz, second is around 7 GHz and third is around 11.5 GHz. Therefore, three parallel resonant circuits are included in the equivalent circuit model. The equivalent circuit lumped components optimized at R1 = 80 Ω, R2 = 95 Ω, R3 = 110 Ω, L1 = 1.4 nH, L2 = 0.4 nH, L3 = 0.21 nH, C1 = 1.92 pF, C2 = 1.25 pF and C3 = 0.82 pF Appl. Sci. 2019, 9, 2371 5 of 13

(a) (b) Appl. Sci. 2019, 9, x 5 of 13 FigureFigure 2. 2.Equivalent Equivalent circuit: circuit: (a ()a real) real impedance impedance of of UWB UWB antenna; antenna; (b ()b comparison) comparison of of reﬂection reflection coe coefficients.ﬃcients. 3. Isolation Mechanisms 3. Isolation Mechanisms Two distinct techniques are employed to achieve isolation for side-by-side and across antenna Two distinct techniques are employed to achieve isolation for side-by-side and across antenna arrangements. A horizontal parasitic decoupling strip is designed and placed in the middle of the arrangements. A horizontal parasitic decoupling strip is designed and placed in the middle of the rear-side for isolating antennas placed across and diagonally. Isolation among side-by-side antennas rear-side for isolating antennas placed across and diagonally. Isolation among side-by-side antennas is is achieved through a dumbbell shaped stub attached to the ground plane. The same structure is also achieved through a dumbbell shaped stub attached to the ground plane. The same structure is also produced on the top layer between antennas. The fabricated UWB-MIMO antenna design is shown produced on the top layer between antennas. The fabricated UWB-MIMO antenna design is shown in in Figure 3. Figure3.

(a) (b)

FigureFigure 3. Fabricated 3. Fabricated UWB-MIMO UWB-MIMO antenna: antenna: (a) front (a) frontview; view; (b) rear (b) view. rear view.

3.1.3.1. Decoupling Decoupling Structure Structure for for Side-by-Side Side-by-Side Radiators Radiators TheThe dumbbell dumbbell shaped shaped stub stub attached attached to tothe the ground ground plane plane provides provides isolation isolation between between side-by-side side-by-side radiatingradiating elements. elements. The The circles circles are are etched etched in inthe the center center of ofthe the dumbbell dumbbell structure structure and and semi-circles semi-circles are are subtractedsubtracted from from its its sides sides in inorder order to toeffectively eﬀectively introduce introduce the the desired desired capacitive capacitive and and inductive inductive loading loading of ofthe the ground ground plane. plane. The The decoupling decoupling structure structure is isshown shown in inFigure Figure 1a.1a. Replicas Replicas of of these these stubs stubs are are placedplaced on on the the top top side side between between antennas antennas as aswell. well. These These strips, strips, on on the the top top side, side, act act as astransmission transmission line line resonatorsresonators and and effectively eﬀectively reduce reduce the the mutual mutual coup couplingling between between the the side-by-side side-by-side radiating radiating elements. elements. DecouplingDecoupling performance performance better better than than 20 20 dB dB is isachieved achieved between between side-by-side side-by-side antennas. antennas.

3.2.3.2. Decoupling Decoupling Structure Structure for for Across Across Radiators Radiators TheThe mutual mutual coupling coupling reduction reduction between between antennas antennas placed placed across across is isachieved achieved by by a customized a customized parasiticparasitic decoupling decoupling structure. structure. The The structure structure consists consists of of several several resonant resonant features features combined combined to to form form a a horizontalhorizontal strip placedplaced inin thethe middle middle of of the the substrate’s substrate’s rear-side. rear-side. The The decoupling decoupling strip strip consists consists of threeof three distinct sections which are joined together horizontally. Two circles and a hash sign slot are etched in the central section whereas squares and rectangular slots are etched in the outer section. These distinct structures combine to form a wide-band multi-resonant structure. It provides isolation better than 20 dB between antennas placed across. This customized structure is shown in Figure 1b.

3.3. Analysis of Parasitic Decoupling Structure The resonant frequency analysis of the parasitic decoupling structure is shown in Figure 4. It is performed by using wave ports in a full wave electromagnetic solver (Ansys HFSS®). The analysis setup and overall transmission loss over the UWB band is shown in Figure 4. Top and bottom side of an air box containing the resonant structure is excited with wave ports whereas the front and backside of the box is assigned perfect magnetic conductors. The other two sides (left and right) are assigned with the perfect electric conductor boundary conditions. It is apparent that different resonant components of the decoupling structure contribute in achieving the isolation between antennas placed across or diagonally. An overall isolation of 20 dB is provided by this parasitic structure. Appl. Sci. 2019, 9, 2371 6 of 13 distinct sections which are joined together horizontally. Two circles and a hash sign slot are etched in the central section whereas squares and rectangular slots are etched in the outer section. These distinct structures combine to form a wide-band multi-resonant structure. It provides isolation better than 20 dB between antennas placed across. This customized structure is shown in Figure1b.

(a) (b)

FigureFigure 4. 4.Analysis. Analysis. (a ()a Analysis) Analysis setup; setup; (b ()b transmission) transmission loss. loss.

4.4. Results Results and and Discussions Discussions

4.1. Induced Surface Currents 4.1. Induced Surface Currents TheThe surface surface currents currents at 4 at GHz, 4 GHz, 6 GHz 6 GHz and 10 and GHz 10 are GHz shown are inshown Figure in5 .Figure Only one 5. Only antenna one element antenna iselement made to is radiate made andto radiate others and are others terminated are terminat at matcheded at impedance matched impedance in order to in observe order to the observe eﬀect ofthe radiation.effect of Thereradiation. are strongThere inducedare strong surface induced currents surface on currents the non-radiating on the non-radiating elements which elements are caused which byare coupled caused ﬁelds. by coupled Both side fields. and facingBoth side elements and faci developng elements these induced develop currents. these induced The introduction currents. The of decouplingintroduction structure of decoupling helps in structure suppressing helps these in suppressing coupling, in these Figure coupling,5a the current in Figure plot 5a at the 4 GHz current is shownplot at and 4 GHz it can is shown be observed and it thatcan be the observed coupled that currents the coupled are now currents visible onare the now outer visible section on the of outer the decouplingsection of structurethe decoupling which meansstructure that which these sectionsmeans that resonate these at sections lower frequency resonate which at lower is supported frequency bywhich Figure is4 bsupported as well. Likewise by Figure the 4b current as well. plot Likewise at 6 GHz the iscurrent shown plot in Figure at 6 GHz5b and is shown the introduction in Figure 5b ofand decoupling the introduction structure of suppresses decoupling these structure currents. suppres Theses middle these currents. section as The well middle as side section sections as well is responsibleas side sections for this is suppression. responsible Finallyfor this Figuresuppression5c displays. Finally the higherFigure frequency 5c displays current the higher distribution frequency at 10current GHz and distribution the middle at section 10 GHz of theand decoupling the middle structure section of in the particular decoupling is responsible structure for in thisparticular current is suppression.responsible Itfor is this also current evident suppression. from Figure 4Itb. is also evident from Figure 4b.

(a) (b)

Figure 4. Analysis. (a) Analysis setup; (b) transmission loss.

4. Results and Discussions

4.1. Induced Surface Currents The surface currents at 4 GHz, 6 GHz and 10 GHz are shown in Figure 5. Only one antenna element is made to radiate and others are terminated at matched impedance in order to observe the effect of radiation. There are strong induced surface currents on the non-radiating elements which are caused by coupled fields. Both side and facing elements develop these induced currents. The introduction of decoupling structure helps in suppressing these coupling, in Figure 5a the current plot at 4 GHz is shown and it can be observed that the coupled currents are now visible on the outer section of the decoupling structure which means that these sections resonate at lower frequency which is supported by Figure 4b as well. Likewise the current plot at 6 GHz is shown in Figure 5b and the introduction of decoupling structure suppresses these currents. The middle section as well as side sections is responsible for this suppression. Finally Figure 5c displays the higher frequency current distribution at 10 GHz and the middle section of the decoupling structure in particular is responsibleAppl. Sci. 2019 ,for9, 2371 this current suppression. It is also evident from Figure 4b. 7 of 13

(a) (b)

Appl. Sci. 2019, 9, x 7 of 13 (c) Figure 5. Surface current distribution of UWB-MIMO antenna: (a) 4 GHz; (b) 6 GHz; (c) 10 GHz. Figure 5. Surface current distribution of UWB-MIMO antenna: (a) 4 GHz; (b) 6 GHz; (c) 10 GHz. 4.2. Impedance Matching 4.2. Impedance Matching The impedance matching response with and without decoupling structures for antenna The impedance matching response with and without decoupling structures for antenna elements elements is shown in Figure 6. The modification in the patch design and partial ground plane helps is shown in Figure6. The modiﬁcation in the patch design and partial ground plane helps in achieving in achieving impedance matching in the desired frequency band. The decoupling structures on the impedance matching in the desired frequency band. The decoupling structures on the other hand other hand also improve the impedance matching especially in the lower and upper band. The also improve the impedance matching especially in the lower and upper band. The simulated and simulated and measured return loss results are both below 10 dB in the entire UWB frequency band. measured return loss results are both below 10 dB in the entire UWB frequency band.

(a)

(b)

FigureFigure 6. 6. SimulatedSimulated and and Measured Measured reflection reﬂection coefficient coeﬃcient ( (aa)) antenna antenna 1 1 and and 2; 2; ( (b)) antenna antenna 1 and and 3.

4.3. Isolation/Decoupling Performance The simulated and measured isolation/decoupling performance for all relative placements are shown in Figure 7. Isolation better than 20 dB is achieved in the entire band for all configurations. Two distinct isolation mechanisms, stub loading in the ground plane as well as parasitic decoupling element provide significant mutual coupling reduction among antennas. Since the diagonally placed elements 1, 3 and 2, 4 are spatially apart so they have minimum coupling even without the use of decoupling structure. However, the strong coupling between antenna 1, 2 and 3, 4 are greatly suppressed by use of dumbbell shaped decoupling structure. The high frequency coupling in across elements 1, 4 and 2, 3 are reduced remarkably by employing the parasitic decoupling structure.

(a) Appl. Sci. 2019, 9, x 7 of 13

Figure 5. Surface current distribution of UWB-MIMO antenna: (a) 4 GHz; (b) 6 GHz; (c) 10 GHz.

4.2. Impedance Matching The impedance matching response with and without decoupling structures for antenna elements is shown in Figure 6. The modification in the patch design and partial ground plane helps in achieving impedance matching in the desired frequency band. The decoupling structures on the other hand also improve the impedance matching especially in the lower and upper band. The simulated and measured return loss results are both below 10 dB in the entire UWB frequency band.

(a)

(b)

Appl. Sci.Figure2019, 96., 2371Simulated and Measured reflection coefficient (a) antenna 1 and 2; (b) antenna 1 and 3. 8 of 13

4.3. Isolation/Decoupling Performance 4.3. Isolation/Decoupling Performance The simulated and measured isolation/decoupling performance for all relative placements are shownThe in simulatedFigure 7. Isolation and measured better than isolation 20 dB/decoupling is achieved performance in the entire for band all relativefor all configurations. placements are Twoshown distinct in Figure isolation7. Isolation mechanisms, better stub than loading 20 dB is in achieved the ground in theplane entire as well band as for parasitic all conﬁgurations. decoupling elementTwo distinct provide isolation significant mechanisms, mutual coupling stub loading reduct inion the among ground antennas. plane as wellSince as the parasitic diagonally decoupling placed elementselement provide1, 3 and signiﬁcant2, 4 are spatially mutual apart coupling so th reductioney have minimum among antennas. coupling Since even the without diagonally the use placed of decouplingelements 1, structure. 3 and 2, 4However, are spatially the apartstrong so coup theyling have between minimum antenna coupling 1, 2 evenand 3, without 4 are greatly the use suppressedof decoupling by use structure. of dumbbell However, shaped the decoupling strong coupling structure. between The high antenna frequency 1, 2 and coupling 3, 4 are in greatlyacross elementssuppressed 1, 4 byand use 2, of3 are dumbbell reduced shaped remarkably decoupling by employing structure. the The parasitic high frequency decoupling coupling structure. in across elements 1, 4 and 2, 3 are reduced remarkably by employing the parasitic decoupling structure.

Appl. Sci. 2019, 9, x 8 of 13 (a)

(b)

(c)

Figure 7. Mutual coupling reduction among antenna elements: ( (aa)) side-by-side side-by-side antennas; antennas; ( (bb)) across antennas; ( c) diagonally-across antennas.

4.4. MIMO MIMO Performance Performance Parameters Parameters The diversity performance of of MIMO antennas is evaluated through certain parameters like channel capacity loss loss (CCL), envelope correlation coefficient coefficient (ECC), diversity gain gain (DG) and total active reflection reflection coefficient coefficient (T (TARC).ARC). For an acceptable MIMO pe performancerformance CCL and ECC should be lessless than than 0.5 0.5 whereas whereas TARC TARC should be below 0 dB [30]. [30]. ECC, ECC, DG DG and and TARC TARC are are calculated using using the the relationsrelations given in [3]. [3]. ECC ECC values values can can be be approximated approximated through through scattering scattering parameters parameters instead instead of of far far field patterns. It gives an accurate estimate of the envelope correlation without 3D measurement of the radiation pattern. The relation is assumed for rich isotropic scattering environment and is given in Equation (1): ∗ ∗ |SS SS| (1) 1 |S| |S| 1 |S| |S| For MIMO systems, multiple antennas affect each other when actively involved in transmission or reception therefore they change the overall operating bandwidth and efficiency. TARC is a new metric that has been used to incorporate the overall combined effect of all antennas in a MIMO system and is calculated by using the Equation (2):

Diversity gain is related to the calculated ECC value of the MIMO antenna system. The relation between ECC and DG is given by the Equation (3) 𝐷𝐺 101|𝐸𝐶𝐶| (3) Appl. Sci. 2019, 9, 2371 9 of 13

field patterns. It gives an accurate estimate of the envelope correlation without 3D measurement of the radiation pattern. The relation is assumed for rich isotropic scattering environment and is given in Equation (1): 2 S∗ S12 + S∗ S22 11 21 (1) (1 ( S S 2))(1 ( S 2 S 2)) − | 11|2−| 21| − | 22| − | 12| For MIMO systems, multiple antennas affect each other when actively involved in transmission or reception therefore they change the overall operating bandwidth and efficiency. TARC is a new metric that has been used to incorporate the overall combined effect of all antennas in a MIMO system and is calculated by using the Equation (2): v u 2 2 tu S11 + S12 + S13 + S14 + S21 + S22 + S23 + S24 + | | 2 | | 2 S31 + S32 + S33 + S34 + S41 + S42 + S43 + S44 | | | | (2) 4

Diversity gain is related to the calculated ECC value of the MIMO antenna system. The relation between ECC and DG is given by the Equation (3) q DG = 10 1 ECC 2 (3) − | | These parameters for the proposed MIMO antenna are shown in Figure8. All the parameters have values within the suitable range for all the possible placement of four antennas. CCL is less than 0.3 whereas ECC is less than 0.2, thus ensuring the diversity performance of proposed MIMO antenna. TARC gives the overall match of the MIMO system with the value 8 dB. Other parameters − like radiation efficiency and peak gain also highlight performance of the proposed antenna system. The radiation efficiency and peak gain of the proposed MIMO antenna is shown in Figures8e and9, respectively. The antenna has reasonable gain values in the entire frequency band with the highest value of 4 dB. The proposed antenna has radiation efficiency above 90% in the complete frequency band thus making it an efficient radiation system.

4.5. Radiation Characteristics The normalized E and H plane radiation patterns taken from simulations and well as measurements at 4 GHz, 6 GHz and 10 GHz are shown in Figure 10. Both E- plane and H- plane patterns are omni-directional in nature and radiate in all directions as intended. Slight disagreement between measured and simulated patterns is observable because of the measurement setup constraints and fabrication tolerances. Overall the radiation envelope of measured and simulated patterns is similar which ensures the eﬀective radiation behavior. Appl. Sci. 2019, 9, x 9 of 13

These parameters for the proposed MIMO antenna are shown in Figure 8. All the parameters have values within the suitable range for all the possible placement of four antennas. CCL is less than 0.3 whereas ECC is less than 0.2, thus ensuring the diversity performance of proposed MIMO antenna. TARC gives the overall match of the MIMO system with the value −8 dB. Other parameters like radiation efficiency and peak gain also highlight performance of the proposed antenna system. The radiation efficiency and peak gain of the proposed MIMO antenna is shown in Figures 8e and 9, respectively. The antenna has reasonable gain values in the entire frequency band with the highest value of 4 dB. The proposed antenna has radiation efficiency above 90% in the complete frequency bandAppl. Sci.thus2019 making, 9, 2371 it an efficient radiation system. 10 of 13

(a) (b)

(e) Appl. Sci. 2019, 9, x 10 of 13 FigureFigure 8. 8.MIMOMIMO performance performance parameters: parameters: (a ()a )CCL; CCL; (b (b) )ECC; ECC; (c (c) )DG; DG; (d (d) )TARC; TARC; ( (ee)) radiation radiation efficiency. eﬃciency.

FigureFigure 9. 9.Simulated Simulated and and measured measured peak peak gain. gain.

(a)

(b)

(c)

Figure 10. E and H plane radiation patterns: (a) 4 GHz; (b) 6 GHz; (c) 10 GHz. Appl. Sci. 2019, 9, x 10 of 13

Figure 9. Simulated and measured peak gain.

Appl.patterns Sci. 2019 is ,similar9, 2371 which ensures the effective radiation behavior. 11 of 13

(a)

(b)

(c)

FigureFigure 10.10. EE andand HH planeplane radiationradiation patterns:patterns: ((aa)) 44 GHz;GHz; ((bb)) 66 GHz;GHz; ((cc)) 1010 GHz.GHz.

5. Comparison with the Existing Literature The proposed MIMO system is designed for UWB applications. The existing literature reports several UWB-MIMO antennas. However, the proposed design is highly competitive to the reported designs in terms of isolation, size/profile and MIMO parameters. This MIMO antenna not only has a compact size but also exhibits efficient performance. A comprehensive comparison with the recent literature is shown in Table1. The proposed work is compared with planar two-port and four-port reported designs. It has the smallest MIMO antenna dimensions of 40 43 1 mm3 and highest × × isolation of 20 dB in comparison with reported designs in Table1. Moreover, proposed design has desirable values of ECC, CCL and TARC and the highest values of gain and efficiency in comparison, whereas most of the reported designs in the table are not evaluated in terms of CCL and TARC. Appl. Sci. 2019, 9, 2371 12 of 13

Table 1. Comparison with the existing literature. ) 3 ECC CCL Ports TARC ciency (%) (GHz) Ref. No Geometry Peak Gain ﬃ Size (mm E Isolation (dB) Frequency Band

[18] Planar 4 36 36 1.6 15 - <0.5 - 2.5–4 70 3.1–10.6 × × [19] Planar 4 60 60 1.6 17.5 - <0.3 <0.4 8 91.2 3–16.2 × × [20] Planar 4 45 45 1.6 14 - <0.25 - 4 91 2.2–3.8/2.3–5.7/2.4, 5.1–5.8 × × [21] Planar 2 26 60 1.6 15 - <0.01 - 3.5 80 3.1–10.6 × × [22] Planar 2 66 52 1.5 15 - <0.02 - 1.8–4 66 Lower UWB band × × [23] Planar 2 38.5 38.5 1.6 15 - <0.02 - 1.4–3.6 75 3.1–10.6 × × [24] Planar 2 70 150 1 >10.5 - <0.5/<0.1 -- 40/80 0.699–0.960/1.710–2.690 × × [25] Planar 4 120 60 1.5 13 10 <0.248 - 3.2 75 1.77–2.51 × × − [26] Planar 4 68 98 1.5 10 - <0.1 - 2.73 71 1.66–2.7 × × [27] Planar 2 50 28 1.6 18 - <0.12 <0.4 - - 3.1–10.6 × × [28] Planar 4 50 39.8 1.524 17 - - - 2.5–6 - 2.7–5.1/5.9–12 × × [29] Planar 4 50 50 1 20 - <0.3 - - - 3.1–10.6 × × This work Planar 4 40 43 1 20 8 <0.2 <0.3 4 92 3.1–10.6 × × −

6. Conclusions In this manuscript, a planar and compact quad-element MIMO antenna suitable for UWB applications is presented. The novel semi-elliptical radiating elements are realized on low-cost FR-4 substrate. Various design modiﬁcations in the patch itself and ground plane ensure the wideband impedance matching. Isolation level of at least 20 dB is achieved for the entire band owing to the two distinct isolation mechanisms of parasitic resonators and ground plane with reactive stub loading. Simulated and measured results of the proposed design are in good agreement. The diversity performance criteria indicate suitability of the proposed design for UWB-MIMO applications. The proposed antenna system is an excellent candidate for WPAN applications for consumer electronics.

Author Contributions: F.A. performed antenna designing and software simulation under the supervision of R.S. T.S., S.u.R., and M.B. managed the writing of the paper. M.F.S. provided fabrication, testing and measurement facilities. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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