Download Enhanced Isolation in Compact UWB MIMO Antenna with Hilbert Fractal Defects and more Essays (university) Digital Signal Processing in PDF only on Docsity!
Journal Pre-proofs
Regular paper
Design and Analysis of a Compact UWB MIMO Antenna incorporating Frac- tal Inspired Isolation Improvement and Band Rejection Structures
Jeet Banerjee, Abhik Gorai, Rowdra Ghatak
PII: S1434-8411(20)30469- DOI: https://doi.org/10.1016/j.aeue.2020. Reference: AEUE 153274
To appear in: International Journal of Electronics and Commu- nications
Received Date: 27 February 2020 Revised Date: 22 April 2020 Accepted Date: 20 May 2020
Please cite this article as: J. Banerjee, A. Gorai, R. Ghatak, Design and Analysis of a Compact UWB MIMO Antenna incorporating Fractal Inspired Isolation Improvement and Band Rejection Structures, International Journal of Electronics and Communications (2020), doi: https://doi.org/10.1016/j.aeue.2020.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier GmbH.
Design and Analysis of a Compact UWB MIMO Antenna incorporating Fractal
Inspired Isolation Improvement and Band Rejection Structures
Jeet Banerjee^1 , Abhik Gorai^2 and Rowdra Ghatak^3 (^1) Department of ECE, School of Engineering and Technology, Adamas University, Kolkata, West Bengal, India. (^2) School of Electronics Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar 751024, India. (^3) Microwave and Antenna Research Laboratory, Electronics and Communication Engineering Department, National Institute of Technology Durgapur, 713209, India.
Abstract— This paper numerically proposes and experimentally investigates a compact coplanar waveguide fed two element ultra-wideband (UWB), multiple-input–multiple-output (MIMO) antenna array accompanying peak isolation and triple band notch mechanism. The proposed antenna consists of two identical printed monopole elements along with the incorporation of multiple 2nd^ order Hilbert defects in the ground structure and proposes to incorporate three different mechanisms to reduce the electromagnetic interference of the UWB antenna with existing narrowband systems. The measured impedance bandwidth for the intended antenna ranges from 2.7 GHz to 11.22 GHz. The incorporation of notch mechanisms provides interference suppression at the C-band (3.7-4.2 GHz), WLAN (5.15- 5.825 GHz), and X-band communication (7.9-8.4 GHz). The overall size of the intended UWB MIMO antenna is 30.75 mm × 37.80 mm. Moreover, the diversity parameters of the proposed UWB MIMO antenna are numerically calculated and empirically investigated, these calculations and investigations depicts high diversity performance. It is noteworthy to observe that the spacing between the elements is staggeringly reduced to 2 mm. The concurrence among the diverse numerical and experimental outcomes supports the candidature of the suggested layout for portable UWB as well as UWB MIMO systems.
KeyWords—Ultra wide-band (UWB), fractal, diversity antenna, Hilbert fractal, printed planar monopole (PPM).
1.Introduction
W ith the improvement and intensification of wireless communication, the necessity for elevated reliability (wireless link accessible to multipath fading) and high data rate is increasing [1]. To attain this objective without sacrificing additional spectrum or transmission power, the base stations and portable devices are equipped with diversity techniques [2]. However, placement of multiple antennas in a compact space will lead to severe mutual coupling among different antennas. Since the Federal Communication Commission (FCC) released 3.1-10.6 GHz for unlicensed usage and merchandising operation [3], UWB technology has been broadly studied, and numerous UWB antennas have been reported. However, the functionality of the UWB antennas is firmly interfered by several narrow bands preoccupying the UWB Spectrum. The notable interfering bands are Wireless local area network (WLAN) band, C-Band, Wi-MAX and satellite communication (X-Band). Short range communications encompasses low data carrying capacity as well as low reliability due to multipath fading effects. To prevail over these constraints, MIMO technology is unified with UWB technology and was first reported by Kaiser et al. [4].
The fractal structures, possessing space filling attributes, are extensively exploited for diminishing the gross proportions of the antenna [5].Investigations on fractal inspired antennas was popularized with the study of geometrical impact on antenna attributes and UWB trait due to the self-similar as well as space filling features of fractal shapes like Minkowski curve, Koch curve [5] [6].
diversity system with the use of fractal parasitic strips as compared to non-fractal counterparts (rectangular (non-fractal) parasitic strips).
- Use of novel band notching mechanism by exploiting connected fractal resonators across CPW feed line. The recommended linked fractal notch resonators are immensely compact and can play a decisive role in miniaturization of microwave resonators. The proposed linked notch resonators can be implemented in various system designs including displacement based sensors.
- Use of sleeve monopoles for achieving desirable impedance matching above 9.8 GHz, leading to bandwidth enhancement and complete coverage of the intended UWB spectrum.
The intended antenna is realized on an FR4 substrate (dielectric constant ε r= 4.4, and a loss tangent tan δ= 0.02) of thickness 1.6 mm. The evolutionary steps and the matching Scattering parameters are presented in Fig. 2 and are detailed in the following subsections.
2.Design principle of the antenna
The advocated diversity arrangement is illustrated in Fig. 1. Numerical verifications of the intended monopole antenna are executed, by employing commercialized EM simulation software CST Microwave StudioTM. The diversity arrangement of the monopoles is attained within a closely packed layout of 30. mm × 37.80 mm.
2.1Preliminary UWB Monopole Antenna Design
Fig. 2a depicts the fundamental UWB-Monopole antenna design having a wide impedance bandwidth (3.08 GHz-11.80 GHz) due to beveling and use of CPW-feed. Because of beveling of the monopoles, an unwrinkled transition from one resonant mode to another resonant mode can be attained which contributes to high impedance matching over ultra-wide band spectrum [20]. The monopole structure consists of a beveled elliptical patch and fed by a coplanar waveguide (CPW) line. The fundamental structure of the monopole is attained by combining two ellipse one along its major axis (with radius of the major axis (R 1 ) equal to 10.75 mm and the radius of the minor axis equal to 6.5 mm) to the other along its minor axis (with radius of major axis (R 2 ) equal to 8.5 mm and radius of minor axis equal to 5 mm) respectively.
2.2Realization of UWB MIMO antenna with reduced ground plane
Fig. 2b shows that, the two identical monopoles (already designed in Fig. 2a) are joined together and the height of the common ground plane is reduced to achieve proper impedance matching.
(a)
(b) Fig. 2. Variations of resonance characteristics and comparison of isolation enhancement between various sequential stages of antenna design (a) Variations of S 11 characteristics, (b) Comparison of SFig. 3. A comparison between the design complexity andlevel of isolation provided by 2 12 /S 21. nd (^) iterations of three fractal curves (Koch, Minkowski and Hilbert fractal respectively).
The next concern is regarding the enhancement of port to port isolation, which is established by engraving three 2nd^ order Hilbert defects in the common ground structure and the insertion of semi-fractal Koch parasitic strips behind the antenna, as discussed in the subsequent sections. To utilize the compact shared ground plane of the antenna, a space filling curve inspired by 2nd^ order Hilbert fractal geometry is incorporated for isolation enhancement. Fig. 3 depicts a comparison between, the levels of isolation provided by three famous fractal curves (Koch, Minkowski and Hilbert respectively) reported in [5], [6] and [25]. It can be clearly observed from Fig. 3, that the design complexity provided by utilizing three 2nd^ order Hilbert fractal curves is the least amongst all the three reported fractal DGS. The isolation level (|S 12 /S 21 |) provided by the Hilbert defects, is also found to be the highest. The miniaturized band rejection structures inspired by fractal geometries are the Hilbert slots on the radiators, and the connected fractal resonators (placed behind the antenna). It can be noted that the fractal inspired shapes were tactfully chosen as a compact space filling curve, but not necessarily analyzed as fractals.
2.3 Effect of fractal defects and parasitic semi-fractal Koch elements in isolation enhancement as well as in attaining dual band rejection characteristics
Fig. 2a (STEP 3) and Fig. 2b (STEP 3) shows the insertion of three 2nd^ order Hilbert fractal inspired defects in the common ground plane, engraving a pair of 2nd^ order Hilbert fractal slots over the radiating monopoles (PPM1 and PPM2), etching out a pair of thin quarter wavelength rectangular slots in the specific ground planes and finally introducing a pair of semi- fractal Koch parasitic elements behind the antenna. The introduction of undesired traits in the radiation patterns of closely dispersed radiators is caused majorly due to the introduction of surface waves. These surface waves are triggered as a consequence of mutual coupling between the radiators. So, there is a demand for additional structures, to curtail down the surface waves causing high coupling. The suggested methodology explores and implements the stop band attributes of Hilbert fractal DGS for preventing the flow of surface waves between the closely dispersed monopoles [21]. The ground plane of the proposed UWB MIMO antenna has been engineered with multiple 2 nd^ order Hilbert slots for isolation improvement. Initially, as shown as an inset in Fig. 4a, a single 2nd^ order fractal slot is engraved in the common rectangular ground plane which shows improvement in isolation (|S 12 /S 21 |≤ 15 dB) as compared to no engraved slots. A gradual rise in isolation (|S 12 /S 21 |) can be observed from Fig. 4a, after engraving the second and the third Hilbert fractal slots. It is important to understand that in this endeavor fractal slots are exploited to provide high isolation without enhancing the overall size of the antenna unlike, the physically spaced MIMO antennas. Figure 4a and b clearly depicts, that in presence of the three 2 nd^ order Hilbert fractal slots proportionate to the
(a)
(b) (c) Fig. 4. (a) S 12 /S 21 plot showing enhancement of port to port isolation with increasing count of 2nd^ order Hilbert fractal defects, (b) Current distribution over the common rectangular ground plane of the antenna, with no fractal slots (c) with three 2nd^ order Hilbert fractal slots.
(a)
(b)
(c) Fig. 5. Resolved diagram projecting reduction of the mutual coupling amongst a pair of neighboring antenna elements. (a) Without parasitic elements, (b) With parasitic elements and (c) Effect of various parasitic elements in isolation improvement (inset: current distribution on the antenna showing isolation without parasitic elements, with non-fractal parasitic elements and with semi-fractal Koch parasitic elements).
pair of rectangular (non-fractal) parasitic strips is employed behind the antenna. A small cutback in the mutual coupling between the monopoles is detected especially near 6.25 GHz and 10.5 GHz, without considerably influencing the impedance bandwidth of the antenna. Finally, the pair of non-fractal rectangular strips are replaced by a pair of semi-fractal Koch parasitic elements to investigate the influence of fractal parasitic elements in improving the isolation of the diversity system. It is evident from Fig. 5c, that using a pair of fractal inspired parasitic elements provides higher as well as wideband isolation as compared to its non- fractal counterparts. In addition, the parametric optimization of the semi-fractal parasitic elements also reveal that the proposed methodology is responsive to the corresponding position between the pair of semi-fractal parasitic elements and corresponding position among the radiating elements and the fractal parasitic elements [23]. During this observation port 1 is exclusively excited and the second port is terminated by matched load. Also, as a consequence of symmetrical layout, the result of port 1 is exclusively presented (excluding the response for the 2nd^ port).
Next, a pair of 2nd^ order Hilbert fractal slots is engraved on the planar monopoles (PPM1 and PPM2). The etching of these slots leads to the creation of a sharp band notch at C-band (3.7 GHz-4.2 GHz) frequencies, thereby suppressing the electromagnetic interference occurring in this band. Fig. 1 depicts the incorporation of 2nd^ order Hilbert slots on the radiators leading to sufficient miniaturization of the notch structure without influencing the radiation characteristics of the intended antenna. The gross length ST of the 2nd^ order Hilbert slot can be deduced from (2) and is established to be nearly proportionate to half of the guided wavelength at the equivalent notch frequency as indicated by (3) [24], [25].
ST 15 L 1 12 T (2)
notch eff (3)
T f S c 2
Here, L 1 stands for length of each side of the Hilbert slot, T stands for thickness of the slot, ST is the sum of all the line segments, εeff is the effective dielectric constant of the substrate and fnotch stands for the center frequency of C-band (3.95 GHz) systems. The resonant frequency of the notch band depends on the length and thickness of the slot. Fig. 6a firmly presents the intensification of surface currents over the 2nd^ order Hilbert slot at the desired notch frequency of 3.95 GHz, leading to a sharp band rejection at the intended (Wi- Max) notch frequency. Fig. 6a also depicts the parametric variation of the length (H 1 ) and thickness (HT) of the Hilbert fractal slots for the process of parameter optimization. It was noticed that enhancing the stretch of the Hilbert slots results in shifting of the mid frequency of the rejection band to lower frequencies of the UWB spectrum. Contemporary to this, reducing the length of the Hilbert slots lead to shifting of the rejection band to elevated bands (above 4 GHz). The thicknesses of the slots are also found to be largely influencing the mid frequency of the rejection band at 3.95 GHz. An increase in thickness of the slots lead to shifting of the center of the notch band to 4.35 GHz, whereas, a reduction in thickness of the fractal slots lead to shifting of the mid of the rejection band to 3.85 GHz. The final length and thickness of the slots were decided after a series of parametric optimizations. A pair of quarter wavelength (at 5.5 GHz) rectangular slots are engraved in the ground plane of the suggested antenna as illustrated by Fig. 1, for realizing band rejection attributes in the WLAN band (5.15-5.85 GHz). The length of the slots is numerically investigated by the equation (4).
R eff (4)
L f
c S
The length of the notch at its resonant frequency ( fR = 5.5 GHz) is presented employing (4). The numerically computed overall reach of the rectangular slots employing (4) is 8.29 mm, although the verified reach of the slot in the ground structure is 7.475 mm. The computed and verified lengths of the slots are in close agreement. The engraving of slots in the ground structure unsettles the current dissemination on the ground
structure and thus modifies the equivalent parameters over the defected region [21]. Fig. 6b illustrates the surface current distributions at 5.5 GHz, to justify the role of slots engraved in the ground structure. It is noteworthy to observe the concentration of current over the rectangular slots at the notch frequency (5. GHz). According to the parametric variations carried out in Fig. 6b, an elevation in the length of the rectangular slots resulting in shifting of the notch frequencies to the lower edges (4.8 GHz) of the UWB spectrum and vice a versa. This alteration also leads to shifting of the rejection band to 6.25 GHz (to the higher edges of the UWB spectrum). In the next phase, the parametric variation concerning the wideness of the rectangular slots is carried out. The study reveals that increasing the width of the slots leads to broadening of the rejection bandwidth leading to rejection of undesired frequency bands beyond WLAN (5.15-5.85 GHz) range.
2.4Introduction of sleeves for bandwidth enhancement as well as incorporation of third band rejection mechanism by exploring connected fractal resonators
Fig. 1 presents the incorporation of sleeves as an extension of the CPW-feed line for providing adequate impedance matching in the upper boundaries of the UWB spectrum (9.8 GHz and above). It is evident to observe from the return loss characteristics provided in Fig. 2a that the modifications carried out in the subsection 2.3 leads to severe impedance mismatch (S 11 >-10 dB) between 9. GHz and 11.6 GHz. To resolve this mismatch as well as to attain adequate impedance matching over the entire UWB spectrum, a pair of sleeves having identical dimensions of SL 1 × SL 3 is added to the proposed antenna as depicted in Fig. 1. The return loss characteristics illustrated in Fig. 2a, clearly infer that the addition of sleeves leads to higher impedance matching around 9. GHz. To aid the reasoning of bandwidth enhancement, the input impedance and input reactance curves with the presence and absence of the sleeves are plotted in Fig. 7. For the first case when sleeves are introduced on the ground plane, both resistance and reactance have marginal variation ranging between 9.6 GHz to 10. GHz; resistance varies from 46.86 Ω to 41.13 Ω, while reactance varies from 3.23 Ω to 14.74 Ω.As for the second case without the introduction of sleeves, resistance varies from 39.64 Ω to 30.92 Ω, while reactance varies from -3.42 Ω to 14.64 Ω. Thus, the contrast of both resistance and reactance is much less with frequency for the first case. This aids in attaining a much wider impedance bandwidth [26] at the upper edges of the UWB spectrum. Fig. 7 presents the aggregation of currents over the sleeves at 9.5 GHz and above, leading to generation of a sharp resonance at 9.5 GHz and ultimately resulting in the extension of the impedance bandwidth. An indispensable criterion for lowering the comprehensive cost, size, and mass of communication devices is the curtailing of microwave peripherals, including microwave resonators. It is customary that S-
Fig. 7.A study concerning the influence of sleeves on the input impedance, input reactance and the distribution of current (at 9.5 GHz) on the intended diversity antenna.(a) (b)
(c) Fig. 8.Bottom view of the CPW loaded with and without the connected 2nd^ order Hilbert fractal resonators: (a) Contra directional magnetic fluxes present in the slots of the CPW; (b) connected pair of 2nd^ order Hilbert fractal resonators displaced by HC 1 /2 from the symmetry plane of the CPW. (c) Parametric study of length (HC 1 ) of connection between 2ndorder connected Hilbert resonators and the position (HH 1 ) of the connected Hilbert fractal resonators.
3. Results and Discussion
3.1Fabricated Structure and its empirical analysis
Joint assessment of the numerical and empirical outcomes are executed to investigate the working of the suggested UWB MIMO antenna. The suggested antenna layout is realized in-house [see Fig. 9], and the S- parameters are confirmed utlizing Rhode and Schwarz ZVA 40 vector network analyzer. One port of the antenna is connected to the network analyzer, and 50-Ω load impedance is utilized to terminated the alternative port. It can be visibly perceived from Fig. 9 that, the empericalband of operationstretchesbetween2.7 GHz to 11.22 GHz (S 11 ≤ -10 dB), sharply excluding three pre-assigned narrow bands at 3.95 GHz (C-band), 5.5 GHz (WLAN) and 8.15 GHz (X-band). Fig. 9, also depicts the analogyamong thenumerical and emperical S 11 and S 21 attributes.The span of │S 21 │ is identified under - dB for majourity of the UWB band showing high isolation. But, few disparities among the numerical and empericaloutcomes are detected as a result of uncertainties of dielectric constant, loss tangent of the substrate, losses due to SMA connectors, fabrication tolerances and soldering losses.Even in presence of these disparities, the simulation and emperical outcomes depict high resemblance.
Fig. 10 depicts the normalized simulated as well as measured 2-D radiation patterns of diversity antenna. The 2-D radiation pattern for each radiator are studiedalong a pair of planes i.e. X- Z plane as well as X-Y plane at three frequencies 3.1 GHz, 5 GHz and 7 GHz. During measurements port 1 is exclusively excited, while port 2 is terminated using a 50Ω broadband load. It can be observed from Fig. 10a, b and c that at the lower end of the UWB spectrum (3. GHz), the patterns in the H-plane (X-Y plane) are less directional but, towards the higher frequency band it is nearly omnidirectional with multiple lobes because of higher order resonant modes. Monopole like (Dumbbell shaped) patterns is observed in the E-plane (X-Z plane). The radiation patterns are approaching omnidirectional in the H-Plane (X-Y plane),indicatingthe realization of large and steadyextent for UWB system application with the suggested arrangement. As an outcome of symmetrical arrangement of the monopoles, it can be concluded that, when port 2 is excited the radiation patterns are mirror image of the patterns obtained when port 1 is excited. The high ressemblance among the numerical and empirical results of 2-D plots,as well as the omnidirectional pattern of the H-plane justifies the attainment of substantial and steady coverage
(a)
(b)
(c)
Fig. 10.Radiation patterns of the proposed UWB MIMO antenna with port 1 excited and port 2 terminated with matched termination at (a) 3.1 GHz, (b) 5 GHz, (c) 7 GHz (Antenna is oriented in y-z plane)
of the UWB system functioning with the intended layout.Fig. 11a depicts the measured peak gain and radiation efficiency of the suggested diversity antenna with ports 1 excited and port 2 terminated with 50-Ω impedance load. The peak empirical gain is predominantly uniform across the intended band with an average value of 2.11 dBi. The radiation efficiencies of the diversity antennas is high in the lower bands and mid bands of UWB spectrum, but decrease to an average of 45.83 % at higher frequencies as shown by Fig. 11a. The reduction of radiation efficiencies is credited to the substantial enhancement of the dielectric loss of the (FR4) substrate with increasing frequency of operation. The measured efficiency in the UWB is above 66%, excluding the notch bands where the efficiency substantially falls to an average of 26%. The intense fall in efficiency permits the antenna to operate in the highly interfered environments. The multiplexing efficiency of the MIMO antenna is described as the dissimilarity betwixt the powers needed by MIMO (antenna under test) to the model standard MIMO antenna to attain a definite capacity [32]. For uniform 3D-angular power spectrum and peak signal-to-noise ratio, multiplexing efficiency (ηMUX) can be obtained using (5).
MUX ( 1 C^2 1 2 )
Where, ρC is the complex correlation coefficient among the pair of elements, and ECC≈ | ρC |^2 and ƞn is total efficiency of the nth^ antenna element. Fig. 11a presents high similarity among the total efficiency as well as the multiplexing efficiency.
4. Diversity Performance
Uncorrelated radiation patterns are a vital necessity for antenna elements to be constructively involved in pattern diversity. The envelope correlation coefficient (ECC) is utilized to estimate correlation among radiation patterns. The computations of ECC ( ρe ) can be executed utilizing the far field radiation pattern [33] or S-parameter technique [34]. It is comparatively easy to compute the ECC criterion employing the S-parameter approach, as exploring the 3D far field radiation pattern measurements for ECC computations becomes humdrum and knotty. As suggested in [35], the ECC ( ρe )can be computed from S-parameter under uniform propagation environment by utilizing equation (6).
2 12
2 22
2 21
2 11
2 11 12 21 22 S S S S
S S S S e
Under standard operating environment, the valuation of ECC, beneath 0.5 symbolize allowable verge of signal tampering [5] [36]. The effective arrangement of the diversity antenna is verified by Fig. 11b,depictingthe attainment of a nominal value of 0.035 for the ECC parameter all over the UWB
(a)
(b)
(c) (d) Fig. 11. (a) Measured realized gain, radiation efficiency and multiplexing efficiency (b) Measured ECC and DG, (c) total active reflection coefficient (TARC) and (d) mean effective gain (MEG).
An innovative technique for lowering electromagnetic pairing among nearly dispersed radiators as well as to suppress potential interference from existing narrowband systems has been presented in this contribution. The key of this design lies in proposing a novel isolation improvement mechanism by exploring multiple fractal defects and fractal parasitic elements along with the use of sleeves for bandwidth enhancement and linked fractal resonators for rejecting desired frequency bands. The employment of fractal based miniaturization techniques provides simple design methodology without involving complex decoupling structures. The numerical and empirical results have shown that the suggested antenna can attain a mutual coupling less than 20 dB throughout the UWB spectrum and can possess three sharp rejection bands centered at C-band, WLAN and X-band satellite communication. Results depict good impedance matching, omnidirectional radiation patterns and gain flatness in the intended ultra-wide bandwidth. The realized gains are above 2.11 dBi and radiation efficiency is low primarily at 66% as a consequence of the loss prone attributes of the economical FR4 substrate. This limitation can be reduced by using low loss substrate. The attained ECC value is observed to be satisfactorily beneath the practical margin of 0.5. The compact size, simple planar structure, ease of fabrication as well as excellent diversity and notch characteristics makes the proposed contribution a notable contender for handy UWB and portable diversity applications.
References
[1] M. A. Jensen and J.W. Wallace. A review of antennas and propagation for MIMO wireless communication. IEEE Transactions on Antennas and Propagation 2004; 52(11): 2810–2824. [2] R. G. Vaughan and J. B. Anderson. Antenna diversity in mobile communication. IEEE Transactions on Vehicular Technology 1987; 36(4): 149-172, 1987. [3] F Federal Communications Commission, Washington, DC, USA. Federal communications commission revision of Part 15 of the Commission’s rules regarding ultra-wideband transmission system from 3.1 to 10.6 GHz. ET-Docket 2002: 98–153. [4] T. Kaiser, F. Zheng and E. Dimitrov. An overview of ultra-wide-band systems with MIMO. Proceedings of the IEEE 2009; 97(2): 285–312. [5] Tripathi S, Mohan A, and Yadav S. A compact Koch fractal UWB MIMO antenna with WLAN band rejection. IEEE Antenna and Wireless Propagation Letters 2015; 14: 1565–1568. [6] Y. K. Choukiker, S. K. Sharma and S. K. Behera. Hybrid Fractal Shape Planar Monopole Antenna Covering Multiband Wireless Communications With MIMO Implementation for Handheld Mobile Devices. IEEE Transactions on Antennas and Propagation 2014; 62(3): 1483-1488. [7] J.-Y. Deng, L.-X. Guo, and X.-L liu. An ultrawideband MIMO antenna with high isolation. IEEE Antenna and Wireless Propagation Letters 2016; 15: 182-
[8] Q. Li, A. P. Feresidis, M. Mavridou, and P. S. Hall. Miniaturized double-layer EBG structures for broadband mutual coupling reduction between UWB monopoles. IEEE Transactions on Antennas and Propagation 2015; 63(3):1168–1171. [9] G. S. Lin, C. H. Sung, J. L. Chen, L. S. Chen and M. P. Houng. Isolation Improvement in UWB MIMO Antenna System Using Carbon Black Film. IEEE Antennas and Wireless Propagation Letters 2017; 16: 222-225. [10] L. Liu, S. W. Cheung and T. I. Yuk. Compact MIMO Antenna for Portable Devices in UWB Applications. IEEE Transactions on Antennas and Propagation 2013; 61(8): 4257-4264. [11] C. X. Mao and Q. X. Chu. Compact Coradiator UWB-MIMO Antenna with Dual Polarization. IEEE Transactions on Antennas and Propagation 2014; 62(9): 4474-4480. [12] M. Bilal, R. Saleem, H. H. Abbasi, M. F. Shafique and A. K. Brown. An FSS-Based Nonplanar Quad-Element UWB-MIMO Antenna System_._ IEEE Antennas and Wireless Propagation Letters 2017; 16: 987-990. [13] C. M. Luo, J. S. Hong and L. L. Zhong. Isolation Enhancement of a Very Compact UWB-MIMO Slot Antenna With Two Defected Ground Structures. IEEE Antennas and Wireless Propagation Letters 2015; 14: 1766-1769. [14] X. Zhao, S. P. Yeo and L. C. Ong. Planar UWB MIMO Antenna With Pattern Diversity and Isolation Improvement for Mobile Platform Based on the Theory of Characteristic Modes. IEEE Transactions on Antennas and Propagation2018; 66(1), 420-425. [15] J. F. Li, Q. X. Chu, Z. H. Li and X. X. Xia. Compact Dual Band-Notched UWB MIMO Antenna With High Isolation. IEEE Transactions on Antennas and Propagation 2013; 61(9): 4759-4766. [16] L. Liu, S. W. Cheung and T. I. Yuk. Compact MIMO Antenna for Portable UWB Applications With Band-Notched Characteristic. IEEE Transactions on Antennas and Propagation 2015; 63(5): 1917-1924. [17] R. Chandel, A. K. Gautam and K. Rambabu. Tapered Fed Compact UWB MIMO-Diversity Antenna With Dual Band-Notched Characteristics. IEEE Transactions on Antennas and Propagation 2018; 66(4): 1677-1684. [18] Y. Y. Liu and Z. H. Tu. Compact Differential Band-Notched Stepped-Slot UWB-MIMO Antenna With Common-Mode Suppression. IEEE Antennas and Wireless Propagation Letters 2017; 16: 593-596. [19] L. Kang, H. Li, X. Wang and X. Shi. Compact Offset Microstrip-Fed MIMO Antenna for Band-Notched UWB Applications. IEEE Antennas and Wireless Propagation Letters 2015; 14: 1754-1757. [20] S. R. Emadian and J. Ahmadi-Shokouh. Very Small Dual Band-Notched Rectangular Slot Antenna With Enhanced Impedance Bandwidth. IEEE Transactions on Antennas and Propagation 2015; 63(10), 4529-4534. [21] Jeet Banerjee, Anirban Karmakar, Rowdra Ghatak and Dipak Ranjan Poddar. Compact CPW-fed UWB MIMO antenna with a novel modified Minkowski fractal defected ground structure (DGS) for high isolation and triple band-notch characteristic. Journal of Electromagnetic Waves and Applications 2017; 31(15): 1550-1565. [22] A. C. K. Mak, C. R. Rowell and R. D. Murch. Isolation Enhancement Between Two Closely Packed Antennas. IEEE Transactions on Antennas and Propagation 2008; 56(11): 3411-3419. [23] Z. Li, Z. Du, M. Takahashi, K. Saito and K. Ito. Reducing Mutual Coupling of MIMO Antennas With Parasitic Elements for Mobile Terminals. IEEE Transactions on Antennas and Propagation 2012; 60(2): 473-481. [24] M. Barra, C. Collado, J. Mateu and J. M. O'Callaghan. Miniaturization of superconducting filters using Hilbert fractal curves. IEEE Transactions on Applied Superconductivity 2005; 15(3): 3841-3846. [25] J. Banerjee et al. A compact printed UWB monopole antenna with triple band notch characteristics. 2018 3rd International Conference on Microwave and Photonics (ICMAP) 2018: 1-2.
[26] H. -. Chen, H. -. Chen and W. -. Chen. Planar CPW-fed sleeve monopole antenna for ultra-wideband operation. IEEE Proceedings-Microwaves, Antennas and Propagation 2005; 152(6): 491-494. [27] A. K. Horestani, Z. Shaterian, J. Naqui, F. Martín and C. Fumeaux. Reconfigurable and Tunable S-Shaped Split-Ring Resonators and Application Band- Notched UWB Antennas. IEEE Transactions on Antennas and Propagation 2016; 64(9): 3766-3776. [28] R. Ghatak, M. Pal and P. Sarkar. Compact dual band bandpass filter using Bamileke fractal architecture shaped resonator. IEEE MTT-S International Microwave and RF Conference 2013; 1-4. [29] A. K. Horestani, C. Fumeaux, S. F. Al-Sarawi and D. Abbott. Displacement Sensor Based on Diamond-Shaped Tapered Split Ring Resonator. IEEE Sensors Journal 2013; 13(4), 1153-1160. [30] F. Martin, F. Falcone, J. Bonache, R. Marques and M. Sorolla, "Miniaturized coplanar waveguide stop band filters based on multiple tuned split ring resonators. IEEE Microwave and Wireless Components Letters 2003; 13(12): 511-513. [31] A. K. Horestani, M. Durán-Sindreu, J. Naqui, C. Fumeaux and F. Martín. Coplanar Waveguides Loaded with S-Shaped Split-Ring Resonators: Modeling and Application to Compact Microwave Filters. IEEE Antennas and Wireless Propagation Letters 2014; 13, 1349-1352. [32] Gautam A.K., Saini Anjali, AgarwalNiraj and Rizvi Z. Navaid. Design of a compact protrudent-shaped ultra-wideband multiple-input-multiple- output/diversity antenna with band-rejection capability. International Journal of RF and Microwave Computer-Aided Engineering 2019: 1-11. [33] A. Kulkarni and S. K. Sharma. A multiband antenna with MIMO implementation for USB dongle size wireless devices. Microwave and Optical Technology Letter 2012;54(8): 1990-1994. [34] A. Kulkarni and S. K. Sharma. Frequency reconfigurable microstrip loop antenna covering LTE bands with MIMO implementation and wideband microstrip slot antenna all for portable wireless DTV media player. IEEE Transactions on Antennas and Propagation 2012; 61(2), 964-968. [35] S. Blanch, J. Romeu, and I. Corbella. Exact representation of antenna system diversity performance from input parameter description.Electronics Letters 2003; 39(9), 705-707. [36] AbhikGorai, Anwesha Dasguptaand Rowdra Ghatak. A compact quasi-self-complementary dual band notched UWB MIMO antenna with enhanced isolation using Hilbert fractal slot .AEU-Int J Electron Commun 2018 ; 94: 36-41. [37] Chandrashekhar Rao Jetti, Venkateshwara Rao Nandanavanam. Trident shape strip loaded dual band-notched UWB MIMO antenna for portable device application. AEU-Int J Electron Commun 2018 ; 83: 11-21.
