electronics
Article A 28 GHz Broadband Helical Inspired End-Fire Antenna and Its MIMO Configuration for 5G Pattern Diversity Applications
Hijab Zahra 1,*, Wahaj Abbas Awan 2 , Wael Abd Ellatif Ali 3 , Niamat Hussain 4,* , Syed Muzahir Abbas 1,5 and Subhas Mukhopadhyay 1
1 School of Engineering, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia; [email protected] (S.M.A.); [email protected] (S.M.) 2 Department of Integrated IT Engineering, Seoul National University of Science and Technology, Seoul 01811, Korea; [email protected] 3 Department of Electronics & Communications Engineering, College of Engineering and Technology, Arab Academy for Science, Technology and Maritime Transport (AASTMT), Alexandria 1029, Egypt; [email protected] 4 Department of Computer and Communication Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Korea 5 BENELEC, Botany, Sydney, NSW 2019, Australia * Correspondence: [email protected] (H.Z.); [email protected] (N.H.)
Abstract: In this paper, an end-fire antenna for 28 GHz broadband communications is proposed with its multiple-input-multiple-output (MIMO) configuration for pattern diversity applications in 5G communication systems and the Internet of Things (IoT). The antenna comprises a simple
geometrical structure inspired by a conventional planar helical antenna without utilizing any vias. The presented antenna is printed on both sides of a very thin high-frequency substrate (Rogers
Citation: Zahra, H.; Awan, W.A.; Ali, RO4003, εr = 3.38) with a thickness of 0.203 mm. Moreover, its MIMO configuration is characterized W.A.E.; Hussain, N.; Abbas, S.M.; by reasonable gain, high isolation, good envelope correlation coefficient, broad bandwidth, and high Mukhopadhyay, S. A 28 GHz diversity gain. To verify the performance of the proposed antenna, it was fabricated and verified Broadband Helical Inspired End-Fire by experimental measurements. Notably, the antenna offers a wide −10 dB measured impedance Antenna and Its MIMO Configuration ranging from 26.25 GHz to 30.14 GHz, covering the frequency band allocated for 5G communication for 5G Pattern Diversity Applications. systems with a measured peak gain of 5.83 dB. Furthermore, a performance comparison with the Electronics 2021, 10, 405. https:// state-of-the-art mm-wave end-fire antennas in terms of operational bandwidth, electrical size, and doi.org/10.3390/electronics10040405 various MIMO performance parameters shows the worth of the proposed work.
Academic Editors: Reza K. Amineh Keywords: 28 GHz; end-fire antenna; millimeter-wave communication; IoT; Internet of things; and Paolo Baccarelli 5G antenna Received: 9 December 2020 Accepted: 3 February 2021 Published: 7 February 2021
1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in The fifth-generation (5G) of mobile communications is expected to revolutionize the published maps and institutional affil- manner in which communications take place globally. This is based on a user-centric iations. approach which will guarantee improved mobile broadband, and aims for a peak data rate of 20 Gbps [1]. Moreover, 5G technology is expected to upgrade the Internet of Things (IoT) capabilities of the current mobile network, which will be a transformation for future communication technologies [2]. Consequently, to achieve these highly promising features
Copyright: © 2021 by the authors. of 5G technology, antennas operating at the designated bands capable of supporting 5G Licensee MDPI, Basel, Switzerland. functionalities are required [3]. In addition, 5G antennas must support other features, such This article is an open access article as accurate beam coverage, and have the ability to meet the requirements of ever-changing distributed under the terms and scenarios [4]. In addition to reducing antenna losses to provide coverage with considerable conditions of the Creative Commons depth and width, precise control of the antenna pattern to direct the beam towards the Attribution (CC BY) license (https:// intended direction, and high spectral efficiency to allow the sharing of resources in the creativecommons.org/licenses/by/ time and frequency domain between several users, are highly desired [5]. 4.0/).
Electronics 2021, 10, 405. https://doi.org/10.3390/electronics10040405 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 405 2 of 15
The technology of the 5G frequency spectrum depends on three frequency bands (sub-1 GHz, sub-6 GHz, and millimeter waves), the most important frequency band of which is 28 GHz (n257) [6]. The 28 GHz band is considered to be the most tested band in the world, mainly due to the availability of large portions of the unutilized spectrum around it, and due to the global availability of this band [7]. Furthermore, this frequency is preferable for the reduced complexity of the devices because of the better propagation compared to other millimeter frequencies [8]. A considerable amount of research has been devoted to the design of antennas operating at 28 GHz using different optimization techniques [9–14]. One of the commonly used types is the microstrip antenna, due to its advantage of design simplicity and its compactness, as demonstrated in [9,10]. Metasurface-based antennas have the advantages of wide bandwidth and high gain at the expense of bigger dimensions and profiles [11,12]. Antennas with defected ground structures (DGS) offer the advantages of wider operational bandwidth and compact size, but the disadvantage of complex geometrical structures [13–16]. Moreover, arrays of antennas provide high gain and wide bandwidth, however, the structures are complex with bigger dimensions [17,18]. Several designs have been reported for 5G mm-wave applications [19–21], in which the single element gain is in the range of 2dB to 6 dB. In addition to other antenna types, end-fire antennas are also well-known due to their numerous advantages, including compact size, wideband, and high gain for radar and sensing applications. Although a large number of studies have used microwave frequency bands to obtain compact antennas with end-fire radiation patterns, few studies have been conducted using the mm-wave band spectrum. A dual-polarized end-fire antenna with an operational bandwidth of 3.03% was presented in [22], however, the antenna has a complex geometrical structure due to the presence of multiple vias and mesh grid structure. In [23], a broadband Yagi-Uda antenna is presented with a high gain of around 12 dB, but the antenna has a large dimension i.e., 5.6λC × 1.6λC. In [24,25], an SIW-fed antenna and planar helical antennas were proposed for V-band applications, respectively. Both presented works offer wide operational bandwidth of 11.67% and 25.5%, and high gain of 12 dB and 6.3 dB, respectively. However, these designs have a larger electrical size in conjunction with the presence of several vias that cause the structural complexity for mass production. Another interesting work was presented in [26], where a Yagi-Uda antenna with director and reflectors was designed to achieve a high gain of 7.82 dB and an impedance bandwidth of 12.5%. The antenna has a large overall size of 4.05λC × 1.89λC. Planar helical antennas have also been reported in [27,28], however, they also have complex designs with connecting via-holes. After an in-depth study of the state-of-the-art antennas, it can be concluded that the design of a compact size antenna with a low level of complexity and characterized by good mechanical robustness, which can be implemented on the same printed circuit board as other electronic devices, is needed for upcoming 5G communication systems. In this paper, an end-fire antenna is proposed for 28 GHz applications. The antenna is inspired by a planar helical shape for enhanced compactness to make it suitable for surface-mounted applications in 5G smart devices and IoT applications. The suggested antenna is characterized by a via-free simple geometrical structure, which significantly reduces the level of complexity combined with the advantages of wider bandwidth and higher gain. To fulfill the requirements of the multiple-input-multiple-output (MIMO) system, a single element was further utilized to design a two-element MIMO antenna characterized by a low envelope correlation coefficient (ECC), channel capacity loss (CCL), high mean effective gain (MEG), pattern diversity capability, and high efficiency. The remainder of the paper is organized as follows: antenna design is discussed in Section2, simulation and measurement results are discussed in Section3 and, finally, a conclusion is presented in Section4. Electronics 2021, 10,Electronics xElectronics FOR PEER2021 2021 REVIEW, 10, 10, 405, x FOR PEER REVIEW 3 of 15 33 of of 15 15
2. Antenna Geometry2.2. Antenna Antenna and Design Geometry Geometry Methodology and and Design Design Methodology Methodology This section presentsThisThis section sectionthe details presents presents for the the the proposed details details for for design the the proposed proposed geometry, design design and geometry,the geometry, design and and the the design design evolution and impactevolutionevolution of design and and impact impact parameters. of of design design parameters. parameters.
2.1. Antenna Geometry2.1.2.1. Antenna Antenna Geometry Geometry The geometricalThe Theconfiguration geometrical geometrical of configuration configurationthe proposed of ofend-fire the the proposed propos antennaed end-fire end-fire is shown antenna antenna in Figure is shownis shown in Figurein Figure1 . 1. The antenna has a compact size, with a length (AX) and width (AY), which correspond 1. The antenna Thehas a antenna compact has size, a compact with a length size, with (AX) a and length width (AX (A) andY), which width correspond (AY), which correspond to to 1.36λ ×λC × 0.9λ λC, whereλ λC is the free-space wavelength at the central frequency of 28 GHz. to 1.36λC × 0.9λC1.36, whereC λ0.9C is theC, where free-spaceC is wavelength the free-space at the wavelength central frequency at the central of 28 frequencyGHz. of 28 GHz. The presented antennaTheThe presented presented is imprinted antennaantenna on both is imprinted imprinted sides of Rogers on on both both RO4003 sides sides of substrate Rogers of Rogers RO4003 with RO4003 a thick-substrate substrate with with a thick- a thicknessness (H) of (H) only of only0.203 0.203 mm, mm,dielectric dielectric constant constant (𝜀 ) of(ε 3.38,) of 3.38,and dissipation and dissipation factor factor(tan δ) ness (H) of only 0.203 mm, dielectric constant 𝜀 of 3.38, and dissipationr factor (tan δ) of 0.0027 [29]. The(tanof antenna0.0027δ) of 0.0027 [29]. was The [fed29]. antennausing The antenna a groundedwas fed was using fedcoplanar using a grounded waveguide a grounded coplanar (GCPW) coplanar waveguide tech- waveguide (GCPW) (GCPW) tech- nique. The radiatortechnique.nique. consists The The radiatorof a radiator planar consists helical consists of antennaa ofplanar a planar with helical helicalan antennainspired antenna with V-shaped withan inspired an struc- inspired V-shaped V-shaped struc- θ ture, where bothstructure,ture, arms where are where expanded both both arms by arms are an expanded areangle expanded of θ .by The an by backside angle an angle of ofθ of. theThe. Thesubstrate backside backside con-of the of thesubstrate substrate con- consistssists of a of partial a partial ground ground plane plane with with a length a length L3. A L hole. A holewith witha radius a radius of 1 mm of 1 was mm etched was sists of a partial ground plane with a length L3. A hole with a radius3 of 1 mm was etched from the substrateetchedfrom to insertthe from substrate the substratescrew to ofinsert the to insert theend-launch screw the screw of connector, the of end-launch the end-launch which connector, is used connector, to whichfeed which is used is used to feed to the proposed antenna,feedthe proposed the as proposed depicted antenna, antenna, in Figure as depicted as 1d. depicted in Figure in Figure 1d.1 d.
(a) (a) (b ) (b) ( c ) (c) ( d ) (d) Figure 1. Geometry of proposed antenna (a) top view, (b) bottom view, (c) side view, and (d) 3-D Figure 1. FigureGeometry 1. Geometry of proposed of proposed antenna antenna (a) top view,(a) top ( bview,) bottom (b) bottom view, (view,c) side (c) view, side view, and (andd) 3-D (d) model3-D with end- model with end-launchmodel connector. with end-launch connector. launch connector. 2.2. Antenna Designing2.2. Antenna and Radiation Designing Mechanism and Radiation Mechanism 2.2. Antenna Designing and Radiation Mechanism The design methodologyThe design and methodology various steps and involved various in steps the designinvolved of inthe the proposed design of the proposed The design methodology and various steps involved in the design of the proposed antenna are depictedantenna in Figureare depicted 2. The inante Figurenna design2. The comprisesantenna design of the comprises following ofthree the following three antenna are depicted in Figure2. The antenna design comprises of the following three steps. steps. steps.
Prototype-2 Proposed design Prototype-1 Prototype-1 Prototype-2 Proposed design
Figure 2. Design evolutionFigureFigure 2. 2. Designof Design the proposed evolution evolution planar of of the the helix proposed proposed antenna. planar planar helix helix antenna. antenna.
Step-1: Design ofStep-1:Step-1: a T-shapedDesign Design monopole of of a a T-shaped T-shaped antenna monopole monopole operating antenna antenna at 28 operatingGHz. operating at at 28 28 GHz. GHz. Step-2: ModifyingStep-2:Step-2: the conventionalModifying Modifying the the T-shaped conventional conventional antenna T-shaped T-shaped to improve antenna antenna impedance to to improve improve band- impedance impedance band- band- width and end-fire radiation. width and end-firewidth radiation. and end-fire radiation.
Electronics 2021, 10, x FOR PEER REVIEW 4 of 15
Step-3: Design of helical inspired broadband antenna to cover 28 GHz band spectrum with an end-fire radiation pattern. First, a T-shaped GCPW fed monopole antenna was designed to resonate at the de- sired frequency (fc) of 28 GHz. The length (P1) of the monopole antenna was determined by using the following equation [30]: 𝑐 𝑃 = (1) 4 𝑓 𝜀
where c is the free-space speed of light which is approximately equal to 3 × 108 m s−1, and Electronics 2021, 10, 405 ɛeff is the effective dielectric constant of the substrate, which can be estimated 4by of 15using the following relation [30]: 𝜀 +1 𝜀 +1 𝑑 . (2) Step-3: Design of helical𝜀 inspired≈ broadband+ antenna 1+12 to cover 28 GHz band spectrum with an end-fire radiation pattern. 2 2 ℎ
whereFirst, ɛr is a the T-shaped dielectric GCPW constant fed monopole of the antennasubstrate, was d designedis the width to resonate of the atmonopole, the desired and h is thefrequency substrate (fc) thickness. of 28 GHz. The The bandwidth length (P1) of of the the monopole T-shaped antenna antenna was was determined 650 MHz, by ranging using the following equation [30]: from 27.78 GHz to 28.43 GHz, as shown in Figure 3. The antenna exhibits a bi-directional radiation pattern due to equal distribution cof current on both sides of the head of the T- P1 = √ (1) shaped antenna, as depicted in Figure 4,4 fwithc εe fa f maximum gain value of 3.96 dB, as illus- trated in Figure 5. The bandwidth of the T-shaped antenna was not enough to cover the c × 8 −1 globallywhere is allocated the free-space band speed spectrum of light of which 28 GHz is approximately with a range equalof 26.5–29.5 to 3 10 GHz.m s Moreover,, and the ε is the effective dielectric constant of the substrate, which can be estimated by using the reportedeff gain and radiation patterns were not suitable for 5G mm-wave communication following relation [30]: systems. − Thus, to further improveε the+ 1 bandwidthε + 1 of the antennad 0.5 and radiation characteristics, ε ≈ r + r 1 + 12 (2) the symmetric T-shapede fantenna f 2 designed2 in the firsth step was modified as an asymmetric T-shaped antenna, as depicted in Figure 2. The asymmetric structure changed the current where ε is the dielectric constant of the substrate, d is the width of the monopole, and distributionr on the surface and resulted in increased current flowing through the large h is the substrate thickness. The bandwidth of the T-shaped antenna was 650 MHz, armranging compared from 27.78 to the GHz short to 28.43arm of GHz, the asT-shaped shown inhead, Figure as3 depicted. The antenna in Figure exhibits 4. This a redis- tributionbi-directional of current radiation and pattern unbalanced due toequal geometrical distribution configuration of current onresulted both sides in more of the electro- magnetichead of the waves T-shaped bending antenna, in a asspecific depicted directio in Figuren and,4, with therefore, a maximum an end-fire gain value radiation of pat- tern,3.96 dBas, depicted as illustrated in inFigure Figure 5.5. Moreover, The bandwidth it was of the also T-shaped observed antenna that wasthe notimpedance enough band- widthto cover of thethe globally modified allocated T-shaped band antenna spectrum increased of 28 GHz from with 650 a range MHz of 26.5–29.5to 4300 MHz, GHz. with a rangeMoreover, of 26.9–31.1 the reported GHz, gain and and the radiationgain improved patterns from were 3.96 not GHz suitable to 5.11 for 5G dB, mm-wave as illustrated in Figurecommunication 3 and Figure systems. 5, respectively.
0
-5
-10
-15 | (dB) 11 |S -20 Prototype-1
-25 Prototype-2 Proposed -30 24 25 26 27 28 29 30 31 32 Frequency(GHz)
FigureFigure 3. |S11|| of of the variousvarious antennas antennas used used in thein th designe design of the of proposedthe proposed antenna. antenna. Thus, to further improve the bandwidth of the antenna and radiation characteristics, the symmetric T-shaped antenna designed in the first step was modified as an asymmetric T-shaped antenna, as depicted in Figure2. The asymmetric structure changed the current distribution on the surface and resulted in increased current flowing through the large arm compared to the short arm of the T-shaped head, as depicted in Figure4. This redistribution of current and unbalanced geometrical configuration resulted in more electromagnetic waves bending in a specific direction and, therefore, an end-fire radiation pattern, as depicted in Figure5. Moreover, it was also observed that the impedance bandwidth of the modified T-shaped antenna increased from 650 MHz to 4300 MHz, with a range Electronics 2021, 10, 405 5 of 15
Electronics 2021, 10, x FOR PEER REVIEWof 26.9–31.1 GHz, and the gain improved from 3.96 GHz to 5.11 dB, as illustrated in 5 of 15 Electronics 2021, 10, x FOR PEER REVIEW 5 of 15 Figures3 and5, respectively.
Prototype-IPrototype-I Prototype-II Prototype-II Proposed Proposed design design
Figure 4. Surface charge distribution (at 28 GHz) of various steps in the antenna design. FigureFigure 4. Surface 4. Surface charge charge distribution distribution (at 28 (at GHz) 28 GHz) of various of various steps steps in the in antenna the antenna design. design.
GainGain (dB) (dB) GainGain (dB) (dB) GainGain (dB) (dB) 6 6 6 6 6 6
-10 -10 -10 -10 -10 -10
Prototype-IPrototype-I Prototype-IIPrototype-II Prototype-III Prototype-III
Figure 5. Radiation pattern of various steps in the design of proposed antenna. FigureFigure 5. Radiation 5. Radiation pattern pattern of of various various steps steps in thethedesign design of of proposed proposed antenna. antenna.
In theInIn the finalthe final final step, step, step, a single a a single single turn turn turn helix-inspir helix-inspired helix-inspired antennaed antenna antenna was was was formed, formed, formed, as depicted as depicted inin Fig- in Fig- ureFigure 2.ure Helix 2.2 Helix. Helix antennas antennas antennas are well are wellwell known known known for for theirfor their their en end-fired-fire end-fire radiation radiation radiation pattern pattern if if they they if satisfythey satisfy satisfy the the the conditions for the axial mode of operation and their broadband operation. However, conditionsconditions for thefor axialthe axial mode mode of operation of operation and andtheir their broadband broadband operation. operation. However, However, the the the conventional helix antenna requires a larger volume, therefore, planar helix antennas conventionalconventional helix helix antenna antenna requires requires a larger a larger volume, volume, therefore, therefore, planar planar helix helix antennas antennas are are arebecoming becoming popular popular [31]. [31]. The The equi equivalentvalent modelsmodels of of an an uncoiled uncoiled single single turn turn helix helix and and a a con- becomingconventional popular helix [31]. are depictedThe equi invalent Figure models6. The planar of an uncoiled configuration single of theturn helix helix antenna and a con- ventional helix are depicted in Figure 6. The planar configuration of the helix antenna uses ventionaluses the helix following are depicted parameters, in Figure as discussed 6. The planar in [32 ]:configuration the diameter of of the the helix helix antenna (D), the uses the following parameters, as discussed in [32]: the diameter of the helix (D), the circum- thecircumference following parameters, of the helix√ as (C =discussedπD), the spacing in [32]: between the diameter helix turns of the (x), helix the total (D), length the circum- of ference of the helix (C2 = πD),2 the spacing between helix−1 turnsx (x), the total length of the ferencethe single of the turn helix (LT (C= = CπD),+ xthe), andspacing the pitch between angle helix (θ = sin turnsL (x),). In the most total planar length helix of the T antennas,single turn the (L topT = and √ bottom𝐶 +𝑥 arms), and are the connected pitch angle by means (θ = ofsin a−1 via). to In form most a singleplanar turn, helix anten- single turn (LT = √𝐶 +𝑥 ), and the pitch angle (θ = sin−1 ). In most planar helix anten- however,nas, the thetop insertion and bottom of a via arms results are inconnected additional by unwanted means of inductance