a super‑wideband and high isolation mimo antenna system

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A super‑wideband and high isolation MIMOantenna system using a windmill‑shapeddecoupling structure

Yu, Chenyin; Yang, Shuhui; Chen, Yinchao; Wang, Wensong; Zhang, Li; Li, Bin; Wang, Ling

2020

Yu, C., Yang, S., Chen, Y., Wang, W., Zhang, L., Li, B., & Wang, L. (2020). A super‑widebandand high isolation MIMO antenna system using a windmill‑shaped decoupling structure.IEEE Access, 8, 115767‑115777. doi:10.1109/ACCESS.2020.3004396

https://hdl.handle.net/10356/145634

https://doi.org/10.1109/ACCESS.2020.3004396

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Received May 31, 2020, accepted June 11, 2020, date of publication June 23, 2020, date of current version July 2, 2020.

Digital Object Identifier 10.1109/ACCESS.2020.3004396

A Super-Wideband and High Isolation MIMOAntenna System Using a Windmill-ShapedDecoupling StructureCHENYIN YU 1, SHUHUI YANG1, (Member, IEEE), YINCHAO CHEN2, (Senior Member, IEEE),WENSONG WANG3, (Member, IEEE), LI ZHANG1, BIN LI1, AND LING WANG11Department of Communication Engineering, Communication University of China, Beijing 100024, China2Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, USA3School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798

Corresponding author: Shuhui Yang ([email protected])

ABSTRACT In this paper, a novel super-wideband multiple-input multiple-output (SWB-MIMO) antennasystem with high isolation has been proposed, each antenna element consisting of a spade-shaped radiationpatch, a defected ground structure (DGS) as well as a windmill-shaped decoupling structure. The ellipticalslot etched on the radiation patch and the rectangular DGS are jointly utilized to make better the radiationperformance and impedance bandwidth. Both of four-element and eight-element SWB-MIMO antennasystems are simulated and analyzed, respectively. A prototype of the four-element SWB-MIMO antennasystem with total dimensions of 58 mm × 58 mm × 1 mm was fabricated to verify the concept of designusing an FR4material substrate.Measurement and simulation results indicate that the proposed SWB-MIMOantenna system can work in the frequency band ranging from 2.9 to 40 GHz with a mutual coupling of lowerthan –17 dB. Also, excellent performance has been achievedwith a low envelop correlation coefficient (ECC)of lower than 0.04, high multiplexing efficiency (ηmux) of more than–3.0 dB, stable gain of up to 13.5 dBi aswell as quasi-omnidirectional radiation properties. This design provides an important guideline for obtaininga super-wide MIMO antenna.

INDEX TERMS Multiple-input multiple-output (MIMO) antenna, high isolation, super-wideband (SWB),defected ground structure (DGS), windmill-shaped decoupling structure, meander technology.

I. INTRODUCTIONWith the rapid development of the high-frequency andmicrowave communication technology such as the fifth-generation (5G) wireless networks and the internet of things(IoT), the need for high channel capacity and reliability isdramatically increasing for smart city, smart home wear-able devices, and other applications. Ultra-wideband (UWB)system has attracted extensive attention because of its richspectrum resource, fast transmission rate, strong confiden-tiality, and low power consumption. The radiation powerof the UWB system is relatively low, which makes it dif-ficult to realize long-distance transmission. The alternativesolution is to utilize the UWB technology and the MIMOtechnology to improve the anti-multipath fading capability.

The associate editor coordinating the review of this manuscript andapproving it for publication was Raheel M. Hashmi.

The transmission distance of the UWB system can besignificantly enhanced, while the radiated power of theantenna would not be increased.

Wideband and high isolation characteristics are of particu-lar interest for MIMO antenna researchers, however, to meetboth of the requirements remains a challenging task. Severalapproaches to achieve UWB characteristics include thoseusing the gradual geometry structure [1], resonant struc-ture [2], and slot structures [3]-[5]. A compact stepped trans-mission line (TL)-loaded wideband antenna was introducedand printed on an RT/Duroid 5880 substrate in [1], whichcould cover 2.1-11.5 GHzwith a 50mm× 50mm× 1.52mmsize. A wideband antenna with double rejection zeros wasimplemented in [2] by using a slot and quarter-wavelengthresonator simultaneously. By using this method, the band-width of the antenna is raised up to 32% ranging from4.2 to 5.6 GHz, and a filter theory-based feeding network

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https://orcid.org/0000-0002-9779-0250

C. Yu et al.: SWB and High Isolation MIMO Antenna System Using a Windmill-Shaped Decoupling Structure

is verified to improve the bandwidth. A similar methodusing multiple step slots to achieve broadband was reportedin [3]–[5], where the multiple slots were designed to formmore resonance modes at adjacent frequencies. In particu-lar, the stepped slot antenna proposed in [4] can make theimpedance bandwidth to reach 3.2-13.2 GHz and have notchcharacteristics to suppress interference from WLAN sys-tems. Operating bandwidth of 97%, VSWR of less than 2.3,and the maximum antenna gain of 4.0 dBi could be obtainedby the designed 50 � microstrip fed UWB slot antenna,loaded with a tapered line impedance transformer using fourexciting resonance modes. High isolation characteristics canbe achieved by the current neutralization, introducing stub,as well as parasitic elements [6]-[8]. A compact UWB-MIMOslot antenna with a total size of 48 mm × 48 mm waspresented in [6], working in the bandwidth of 2.5-12 GHz.The band-notched property was realized by using a split-ringresonator (SRR) structure, and a T-shaped slot was appliedon the ground plate to increase the isolation between radi-ators which was below –15 dB during the entire workingband. The rectangular slot and T-shaped stub etched onthe ground plane were also applied in the dual-polarizedUWB-MIMO antenna [7], so that the coupling among theradiators was dropped below –20 dB, and the overall sizewas reduced. A neutralization-line structure was verified toobtain high isolation in [8]. Because the neutralization-linetook up very little space, there was no need to make com-plex modifications to the ground like traditional decou-pling structures. Although the isolation could be achievedbelow –19 dB, this design was only for the frequency bandaround 2.4 GHz.

Various featured studies are focusing on UWB-MIMOantennas and several techniques have been discussed toenhance the isolation [9]-[15]. A compact tapered-fedMIMOantenna was designed in [9] with a dual-band-notchedcharacteristic and a high isolation of lower than –22 dBachieved by an L-shaped pattern in the ground plane. In [10],an F-shaped stub was introduced in the ground plane fordecoupling between antenna elements for high isolation.In [11], a T-shaped slot was etched on the ground planeof a dual-band UWB-MIMO Vivaldi antenna, and the portisolations between antenna elements were improved. Theseresearches have achieved both compact size and high iso-lation, and their antennas are limited at two ports withthe impedance bandwidths of being less than 20 GHz.A four-element UWB-MIMO antenna was designed byusing the polarization diversity in [12]. In [13], an inte-grated four-element UWB-MIMO antenna based on fre-quency reconfiguration was fabricated for cognitive radio(CR) applications. Both presented in [12], [13] require arelatively complex manufacturing process. Differently, a sep-arated staircase-shaped structure to reduce coupling wasintroduced in [14], and the designed UWB-MIMO antennawith four elements could perform well in miniaturizationand high isolation. Koch fractal geometry was employed toachieve desired miniaturization and wideband phenomena

for the designed UWB-MIMO antenna fabricated on theFR4 substrate in [15].

Although UWB-MIMO antennas have attracted tremen-dous research interests in wireless communications, it is hardto find a reported MIMO antenna with a super impedancebandwidth being greater than 20 GHz. In this paper,a compact super-wideband MIMO (SWB-MIMO) antennasystem with super-wide impedance bandwidth and highisolation is presented. The four-element and eight-elementSWB-MIMO antenna systems are also analyzed, respec-tively, and the results show excellent characteristicsof SWB and high isolation. Then, a prototyped four-elementSWB-MIMO antenna system is fabricated to verify the con-cept of design. The spade-shaped radiation patch can createan SWB bandwidth. The windmill-shaped structure is imple-mented to act as the decoupling function, achieving isolationof less than –17 dB within the frequency band of interest.The SWB-MIMO antenna system has achieved a super-widebandwidth of 37.1 GHz, ranging from 2.9 to 40GHz. It coversmany wireless communication bands, including worldwideinteroperability for microwave access (WiMAX), wirelesslocal area network (WLAN), K/Ku-band, 5G band, etc.As smart cities gradually grow up, the IoT and 5G wirelesscommunication technologies are increasingly widely used.The proposed SWB-MIMO antenna system has a wide appli-cation prospect, including the smart city having smart med-ical treatment, house, logistics, energy, industry, agriculture,etc., as shown in Fig. 1. All simulations are conducted usingthe electromagnetic simulation software Ansys HFSS, andthe measurements are implemented in an anechoic chamberwith the vector network analyzer R&S R© ZVA 67.

FIGURE 1. Application of the SWB-MIMO antenna system in smart city.

II. SWB-MIMO ANTENNA DESIGN AND ANALYSISA. SWB-MIMO ANTENNA ELEMENT DESIGNThe geometry of the antenna element of the proposedSWB-MIMOantenna system is shown in Fig. 2. The radiationpatch is a spade-shaped structure placed on the top surface ofa printed circuit board (PCB). The ground plane on the bottom

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FIGURE 2. Geometry of proposed SWB-MIMO antenna element. (a) Topview, and (b) bottom view.

surface of the PCB is specially patterned for enhancing thebandwidth. The PCB substrate is 1 mm FR4 material witha loss tangent of 0.02 and permittivity of 4.4. The metalliclayers, including the radiation patch and the ground pattern,are made up of 0.035 mm thick copper film. As shownin Fig. 3, the design process of the proposed SWB-MIMOantenna element is investigated from three types of UWBantennas, termed Type I, Type II, and Type III, respectively.The Type III is the proposed SWB-MIMO antenna element.The Type I is the initial antenna, starting with a rectangularmicrostrip patch. To improve the radiation property, the edgeof the spade patch is cropped, which forms Type II. Theradiation patch has an elliptical slot with the major radius Rof 3.5 mm and ratio of 0.7 between the elliptical short andthe long axes etched in the center for improving impedancematching. As shown in Type III, the radiation patch with anelliptically-slotted semicircle area is fedwith amicrostrip linebacked with an edge-cropped groove ground.

FIGURE 3. Investigated three types of UWB antennas. (a) Type I,(b) Type II, and (c) Type III (Proposed SWB-MIMO antenna element).

Fig. 4 displays the magnitude of S11 for the three typesof UWB antennas represented in Fig. 3. For the sake ofcomparison, the current distributions for the three types ofUWBantennas at 10GHz are presented in Fig. 5. It is found inFig. 4(a) that the initial rectangular antenna (Type I) performspoorly, which cannotmeet the expectation. In order to achievethe impedance matching of the UWB antenna, the upper edgeof the spade-shaped patch is cropped, the lower edge of the

FIGURE 4. Magnitude of S11 in dB for three types of UWB antennas.

patch is modified to an arc and an elliptical slot is etched onthe patch to get broadband characteristics in Type II antenna.It is equivalent to increase the effective length of the radiator.Correspondingly the resonant frequency is reduced, whichis an efficient way to realize the patch miniaturization andto improve radiation performance [16]. Thus, the enhancedversion, Type II, as seen in Fig. 4, is significantly improvedfrom the bandwidth almost by 20 GHz. The current dis-tributions of Type II antenna are much wider at the loweredge of the radiation patch as displayed in Fig. 5(b). Also,the current intensity is improved in the areas near the uppercut angle part and the elliptical slot. By adding a defectedground structure (DGS) to the Type III, the 10-dB impedancebandwidth of S11 is further widened from 3.3 to 27 GHz as

FIGURE 5. Simulated current distributions of three types of UWBantennas at 10 GHz. (a) Type I, (b) Type II, and (c) Type III, theproposed SWB-MIMO antenna element.

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depicted in Fig. 4. As expected, Type III antenna’s currentdistributions (see Fig. 5(c)) show that the current intensityon the two beveled edges of the ground is higher than thatof Type II antenna. The current distributions of the proposedSWB-MIMO antenna element at 5, 10, and 20GHz are shownin Fig. 6. It could be found that the current distributionsare mainly concentrated on the lower edge of the arc of theradiation patch, the edge of the defected ground, and thefeeding line. Although the current distribution at the feedingline varies slightly with frequency, there is little difference inthe approximate area of the distribution. This phenomenonfurther illustrates that modifying the radiation patch and theDGS do play an important role in improving the performanceof UWB antennas.

FIGURE 6. Simulated current distributions of proposed SWB-MIMOantenna element at (a) 5 GHz, (b) 10 GHz, and (c) 20 GHz.

B. DECOUPLING STRUCTURE AND SWB-MIMOANTENNA SYSTEMAs shown in Fig. 7(a), the adjacent antenna elementsare firstly placed orthogonal to each other in order toimprove the isolation among them. The scattering parameters(S-parameters) are simulated and shown in Fig. 8. It canbe seen that such an arrangement greatly improves theantenna system’s impedance bandwidth ranging almost from2.9 through 38 GHz with only a small fraction of S11 beinglarger than –10 dB. However, it is found that the magnitudeof S31 is at the level –13 dB or larger, which obviously doesnot meet the isolation requirement for MIMO antennas.

Next, for the purpose of further decoupling among theMIMO antenna elements, as seen in Fig. 7(b), a symmetricstaircase-shaped patch is applied on the center of the ground

FIGURE 7. Evolution of proposed SWB-MIMO antenna system.SWB-MIMO antenna system (a) without decoupling structure, (b) with astaircase-shaped decoupling structure, and (c) with a windmill-shapeddecoupling structure.

FIGURE 8. Simulated S-parameters of SWB-MIMO antenna systemwithout decoupling structure.

plane performed as a stepped impedance transformer. Sincea multi-stepped impedance transformer exhibits widebandcharacteristics, it is a good candidate for achieving high isola-tion in UWB-MIMO antenna systems [14]. As seen in Fig. 9,this structure improves the coupling level between antennaelements in the high frequency band, and the isolation levelachieves less than –15 dB in the whole frequency band.However, the S-parameters are deteriorated in the low fre-quency band which cannot maintain a SWB characteristic.Specifically, the magnitudes of S21, S31 and S41 are reduced,but they are still at the level of –15 dB in the frequency bandranging from 2.9 to 38 GHz.

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FIGURE 9. Simulated S-parameters of SWB-MIMO antenna system with astaircase-shaped decoupling structure.

To address the issue of deteriorated S11, it is necessary toincrease the order of the stepped impedance transformer toprovide a wider operation band for improving the impedancematching. When the number of steps reaches infinity andthe length of the step shrinks infinity, the asymptote-shapedstructure will be formed as shown in Fig. 7(c). In otherwords, the staircase-shaped decoupling structure is furtherevolved into the windmill-shaped one. Fig. 10 depicts thatthe finalized SWB-MIMO antenna system almost extends thebandwidth to be from 2.9 to 40 GHz. Meanwhile, the magni-tudes of S21, S31, and S41 are of less than –17 dB in the wholefrequency band.

FIGURE 10. Simulated S-parameters of SWB-MIMO antenna system witha windmill-shaped decoupling structure.

By analyzing the distribution of the surface current offour proposed SWB-MIMO antenna elements with differ-ent decoupling structures at 10 GHz, the decoupling effectwith the best strength of the windmill-shaped structure isfurther verified, as displayed in Fig. 11. For simplicity,the four antenna elements constituting the antenna sys-tems are referred to as antennas 1, 2, 3 and 4 as shownin Fig. 11(c). It can be seen that if port 1 of antenna 1 isexcited, the high mutual coupling is produced among antennaelements, as shown in Fig. 11(a). Mutual coupling espe-cially between antennas 1 and 3 is significantly degraded by

FIGURE 11. Simulated current distribution of SWB-MIMO antenna system(a) without decoupling structure, (b) with the staircase-shapeddecoupling structure, and (c) with the windmill-shaped decouplingstructure.

adding a staircase-shaped structure on the bottom layer of theantenna system. However, there is still much mutual couplingamong antennas 1, 2 and 4 as seen in Fig. 11(b). The modifieddecoupling structure using the meander structure is placed inthe middle of the bottom layer of SWB-MIMO antenna sys-tem for further decoupling among the four antenna elements.As depicted in Fig. 11(c), there is almost no mutual couplingamong the radiators except for only some small couplingcurrents existing in antennas 1 and 2. Hence, a high isolationfor the proposed SWB-MIMO antenna could be achieved.

C. SIMULATION AND ANALYSIS OF EIGHT-ELEMENTSWB-MIMO ANTENNA SYSTEMAs shown in Fig. 12, the proposed eight-elementSWB-MIMO antenna system is studied. It is equivalent tohaving two four-element SWB-MIMO antenna systems, bothof which are arranged in parallel and diagonal topologies,respectively, along with the interval distance of 12 mm,termed Case 1 and Case 2. In order to further investigatetheir performance, simulation results of S-parameters for theboth cases are shown in Fig. 13-16. It can be observed thatthe distance among ports is an important factor affectingthe isolation level of the entire eight-element SWB-MIMOantenna system. The mutual coupling between the closestports on the two sub-antennas is the highest. Port 1 and port 2are selected to add excitation sources respectively in Case 1,and port 1 and port 3 are selected to add excitation sourcesrespectively in Case 2. Comparing the simulated resultsin Fig. 13 with those in Fig. 14, it can be found that the valuesof S11 and S22 associated with port 1 and port 2 in Case 1 are

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FIGURE 12. Proposed eight-element SWB-MIMO antenna system.(a) Case 1: Parallel placement of two proposed SWB-MIMO antennasystems, and (b) Case 2: diagonal placement of two proposedSWB-MIMO antenna systems.

FIGURE 13. Simulated S-parameters of proposed eight-elementSWB-MIMO antenna system with antenna 1 excited, shown in Case 1.

almost the same. The S-parameters with port 1 excited inCase 1 representing the isolations among port 1 and theports 5-8, are significantly lower than those with port 2excited in Case 1. This is due to the port 1 is further awayfrom those ports than port 2. The results demonstrate thatthe isolations of eight-element SWB-MIMO antenna systemperform well and are all lower than –19 dB, satisfying therequirements of MIMO antenna systems. Similarly, theseS-parameters of the designed eight-element SWB-MIMOantenna system with port 1 and port 3 excited in Case 2 are




lower than –24 dB and –17 dB in the whole impedancebandwidth, respectively. Obviously, compared with the onein Case 2, the eight-element SWB-MIMO antenna systemin Case 1 not only has a more compact structure, but alsohas a more stable performance with high isolation. It isworth mentioning that both of the cases have outstandingradiation characteristics in terms of high isolation and wideimpedance bandwidth. In particular, the S-parameters ofports 1-4 and ports 5-8 are exactly the same to those ofthe four-element SWB-MIMO antenna system, respectively.

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The eight-element SWB-MIMO antenna systems inCase 1 and Case 2 have already performed well in terms ofbandwidth and isolation, not to mention the situation of fur-ther distances. It also indicates that the proposed four-elementSWB-MIMO antenna system has more extended implemen-tation to obtain multiple ports, and it is a great potential forwide wireless communication system applications.

III. MEASUREMENT AND DISCUSSION OF SWB-MIMOANTENNA SYSTEMA. FABRICATION AND MEASUREMENTThe proposed four-element SWB-MIMO antenna system isprototyped andmeasured. Optimized geometrical dimensionsof the antenna element shown in Fig. 2 are as follows: L1 =29, L2 = 11, L3 = 8.5, L4 = 1, L5 = 3.25, L6 = 8.5,W1 = 22, W2 = 1.93, W3 = 20, W4 = 5, W5 = 5.5,W6 = 2, h1 = 20.5, h2 = 10.5, R = 8.5, all in mm. Thegeometrical parameters of the decoupling structure depictedin Fig. 6(c) are given as: L7 = 58, W7 = 58, x1 = 10,x2 = 22, x3 = 3.7, r = 2, all in mm. Fig. 17 showsthe photograph of the fabricated four-element SWB-MIMOantenna system. The measured and simulated S-parametersof the proposed SWB-MIMO antenna system essentiallyagree well with each other, as plotted in Fig. 18. It is foundthat the fabricated antenna system achieves the impedance

FIGURE 17. Fabrication photograph of proposed four-elementSWB-MIMO antenna system. (a) Top view, and (b) bottom view.

FIGURE 18. Measured and simulated S-parameters for proposedfour-element SWB-MIMO antenna system.

bandwidth of 2.9 to 40 GHz as well as corresponding decou-pling level of below –17 dB. As shown in Fig. 18, the mea-sured S-parameters of the proposed SWB-MIMO antennasystem are not totally identical with the simulated resultsat high frequencies. However, the measured S11 is below−10 dB and the measured isolation is lower than −17 dBin the entire bandwidth which performs better than the sim-ulated data. The trends of both the measured and simu-lated curves match well and both show the designed SWBcharacteristics. Fig. 19 shows the two-dimensional radiationpatterns of the proposed four-element SWB-MIMO antennasystem in the xoy-plane and xoz-plane, which are measuredby using NSI far field anechoic chamber at the frequenciesof 3.5, 5.8, 13, 18, 25 and 30 GHz. Obviously, the antennasystem’s radiation pattern is almost omnidirectional, andcan maintain stable radiation characteristics at differentfrequencies.

B. MIMO SYSTEM PARAMETERSServal important parameters are analyzed to assess the per-formance of the proposed SWB-MIMO antenna system. Theenvelope correlation coefficient (ECC) is employed to evalu-ate the degree of independence between antenna elements,which could be computed by using the S-parameters asfollows [17]

ECCij =

∣∣Sii ∗ Sij + Sji ∗ Sjj∣∣2(1− |Sii|2 −

∣∣Sji∣∣2) (1− ∣∣Sjj∣∣2 − ∣∣Sij∣∣2) (1)

where ECCij is the ECC between ith and jth antenna elements.The ECCs of the proposed SWB-MIMO antenna system aredisplayed in Fig. 20. It can be seen that the ECCs are below0.04 within the frequency band of interest, and meet thecommunication requirements, which are lower than 0.5 [14].

The diversity gain (DG) is also an important performanceindicator of the SWB-MIMO antenna system against channelfading. It can be written as [10]

DG = 10√1− (ECC)2 (2)

Usually, a larger DG value indicates a better MIMO antennaperformance. As shown in Fig. 21, the obtained DG is more

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FIGURE 19. Measured and simulated radiation patterns in the xoy-planeand xoz-plane at the frequencies of (a) 3.5 GHz, (b) 5.8 GHz, (c) 13 GHz,(d) 18 GHz, (e) 25 GHz and (f) 30 GHz.

than 9.5 dBi within the bandwidth. It is observed that theproposed four-element SWB-MIMO antenna system has alarger DG and smaller ECC at the same time. Fig. 20 showsthe peak gain and multiplexing efficiency of the designedSWB-MIMO antenna system. In [18], the multiplexing effi-ciency (ηmux) is written as

ηmux =

√(1− |ρc|2

)ηiηj (3)

FIGURE 20. Measured ECC and diversity gain of proposed SWB-MIMOantenna system.

where ρc is themagnitude of the complex correlation betweenthe ith and jth antenna elements, ηi is the total efficiency ofthe ith antenna element. ρc and ECC have the relationshipof |ρc |2 ≈ ECC [18]. As show in Fig. 21, it indicates thatthe ηmux is higher than –3 dB within the operating frequencyrange. The peak gain is more than 4.3 dBi through the entirebandwidth, and the maximum value is 13.5 dBi at 28.7 GHz.Obviously, both of the parameters highly meet the require-ments of MIMO wireless communication systems.

FIGURE 21. Measured peak gain and multiplexing efficiency of proposedSWB-MIMO antenna system.

The channel capacity loss (CCL) is another importantparameter to check the performance of an MIMO antennasystem [23]. The CCL defines the maximum reachable limit

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TABLE 1. Comparisons of proposed SWB-MIMO antenna system and published literatures.

of the information transmission rate. The limit is smaller,the signal is easier to transmit with less loss. For a four-portMIMO antenna system, it is mathematically expressed as:

Closs = − log2 det(XR)

(4)

where

XR=

X11 X12 X13 X14X21 X22 X23 X24X31 X32 X33 X34X41 X42 X43 X44

(5)

Xmm = 1−4∑

n=1

|Smn|2 (6)

Xmn = −(S∗mmSmn + S

∗nmSmn

)(7)

where S∗ is the complex conjugate of S. Fig. 22 shows thesimulated and measured CCLs for the proposed SWB-MIMOantenna system. Except thst in a small band near 4.8 GHz,where there is a high level of S11 around –10 dB, the CCLof the proposed SWB-MIMO antenna system is below 0.6bits/s/Hz in most of the operating bandwidth.

The total active reflection coefficient (TARC) is definedas the square root of the total reflected power divided bythe square root of the total incident power. It represents themutual couplings and random signal combinations amongthe ports and can be calculated from the S-parameters. For afour-port MIMO antenna system, TARC is expressed as [23]:

TARC =

√√√√ 4∑i=1

|bi|2/√√√√ 4∑

i=1

|ai|2 (8)

where ai and bi are the incident and reflected signals, respec-tively, and they are related by the relation of the S-parameters:

b = Sa (9)

FIGURE 22. Simulated and measured CCL of SWB-MIMO antenna system.

By combining equations (5) and (6), the TARC representedby the S-parameters is derived as:

TARC =

√√√√√ 4∑i=1

∣∣∣∣∣Si1 +4∑

n=2

Sinejθn−1

∣∣∣∣∣2/

2 (10)

TARC =

√√√√√√√√∣∣S11 + S12ejθ1 + S13ejθ2 + S14ejθ3 ∣∣2+∣∣S21 + S22ejθ1 + S23ejθ2 + S24ejθ3 ∣∣2+∣∣S31 + S32ejθ1 + S33ejθ2 + S34ejθ3 ∣∣2+∣∣S41 + S42ejθ1 + S43ejθ2 + S44ejθ3 ∣∣2

2(11)

where θi is the phase of the ith input signal. The simulatedand measured TARC for the proposed SWB-MIMO antennasystem is presented in Fig. 23. For the same reason as CCL,except at the frequencies around 4.8 GHz the TARC is below–10 dB for almost all the frequency band of 2.9-40 GHz.This further proves the performance of the proposedSWB characteristic.

All the simulated and measured results agree well in naturewith some small differences. The deviations may be causedby the narrow band soldering structures, test connectors,

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FIGURE 23. Simulated and measured TARC of SWB-MIMO antennasystem.

connecting cables for the instruments, fabrication tolerancesand test environment. It is worth pointing out that, the super-wide band characteristics of the proposed SWB-MIMOantenna system require a better test environment than thosefor the narrow-band or low frequencyMIMO antenna system.

C. PERFORMANCE COMPARISONComparison of the fabricated four-element SWB-MIMOantenna system with other published UWB-MIMO works iscarried out in Table 1. It summarizes serval MIMO antennaparameters, including size, substrate material, impedancebandwidth, gain, isolation, ECC, and number of ports. It canbe found that the designed SWB-MIMO antenna systemis easy to fabricate, and performs well in terms of gainof up to 13.5 dBi and isolation of lower than –17 dB.Although its size is not the smallest one, it does achievea super-wide bandwidth which is far wider than otherworks.

IV. CONCLUSIONIn this paper, a novel SWB-MIMO antenna system witha high isolation and super-wide impedance bandwidth hasbeen presented. The SWB-MIMO antenna element includesa spade-shaped radiation patch and a DGS, which couldimprove the impedance bandwidth. A prototype of thefour-element SWB-MIMO antenna system has been fabri-cated. The measured results and simulated data are agreedwell, and the operating frequency range is from 2.9 to40 GHz. A windmill-shaped decoupling structure is proposedto enhance the port isolation at the level of lower than –17 dB.The excellent performance with quasi-omnidirectional radia-tion pattern, high isolation, and low ECC is also obtained.Also, the eight-element SWB-MIMO antenna system is ana-lyzed and performs well in the high isolation and SWBbandwidth. The designed SWB-MIMO antenna system hasthe potential to be applied in various wireless communicationsystems associated with IoT and 5G techniques in smartcities.

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CHENYIN YU received the B.S. degree fromthe Communication University of China (CUC),in 2017, where she is currently pursuing the mas-ter’s degree with the Department of Communi-cation Engineering. Her current research interestsinclude multiple-input multiple-output antennasand metasurfaces.

SHUHUI YANG (Member, IEEE) received theB.Sc. degree from Zhejiang University, in 1994,the M.Sc. degree from the Beijing BroadcastingInstitute, in 1997, and the Ph.D. degree fromthe Institute of Microelectronics of the ChineseAcademy of Sciences, in 2003.

From 2003 to 2015, he was a Professor withBeijing Information Science and Technology Uni-versity. He joined the Communication Universityof China, as a Professor, in 2015, where he is

currently the Chairmanwith theDepartment of Communication Engineering.His current research interests include multiple-input multiple-output anten-nas, filters, and metasurfaces. He is a Senior Member of the Chinese Instituteof Electronics (CIE).

YINCHAO CHEN (Senior Member, IEEE)received the Ph.D. degree in electrical engineeringfrom the University of South Carolina.

He has worked with the University of Illinoisat Urbana-Champaign (UIUC), The Hong KongPolytechnic University (HKPU), and the Univer-sity of South Carolina (USC). He is one of thecoauthors or editors for three academic booksand ten book chapters in electrical engineering.He has published more than 200 academic papers

in international journals and conference proceedings. His current researchinterests include signal integrity for high-speed circuits, RF and integratedmicrowave circuits, and integrated microstrip antennas.

WENSONG WANG (Member, IEEE) receivedthe Ph.D. degree from the Nanjing Universityof Aeronautics and Astronautics, Nanjing, China,in 2016. From 2013 to 2015, he was a VisitingScholar with the University of South Carolina,Columbia, SC, USA. He joined Nanyang Tech-nological University, Singapore, as a ResearchFellow, in 2017. His current research interestsinclude RF/microwave components and systems,inter/intra-chip wireless interconnect, power wire-

less transfer, signal integrity, and rail non-destructive real-time monitoring.

LI ZHANG received the B.Eng., M.Eng., andPh.D. degrees from the CommunicationUniversityof China, in 2000, 2003, and 2017, respectively.

She joined the Communication University ofChina, in 2003, where she is currently anAssociateProfessor. Her current research interests includemicrowave circuits, electromagnetic compatibility(EMC), and optimization algorithms in electro-magnetic simulations.

BIN LI received the B.Sc. and Ph.D. degrees fromNorthern Jiaotong University, China, in 2001 and2007, respectively.

Since 2008, she has been a Lecturer with theCommunication University of China. Her currentresearch interests include optical fiber commu-nication, millimeter-wave devices, and terahertzcomponents.

LING WANG received the B.Sc. degree fromChangjiang University, in 1997, and the M.Sc.and Ph.D. degrees from the Beijing Institute ofTechnology, in 2000 and 2004, respectively.

Since 2005, she has been an Instructor withthe Communication University of China. She wasan Associate Professor, in 2011. Her researchinterests include microwave filters and signalintegrity (SI).

VOLUME 8, 2020 115777

a super‑wideband and high isolation mimo antenna system

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