Review of Recent Phased Arrays for Millimeter-Wave Wireless Communication

sensors

Review Review of Recent Phased Arrays for Millimeter-Wave Wireless Communication

Aqeel Hussain Naqvi and Sungjoon Lim * School of Electrical and Electronics Engineering, College of Engineering, Chung-Ang University, 221, Heukseok-Dong, Dongjak-Gu, Seoul 156-756, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-820-5827; Fax: +82-2-812-7431

Received: 27 July 2018; Accepted: 18 September 2018; Published: 21 September 2018

Abstract: Owing to the rapid growth in wireless data trafﬁc, millimeter-wave (mm-wave) communications have shown tremendous promise and are considered an attractive technique in ﬁfth-generation (5G) wireless communication systems. However, to design robust communication systems, it is important to understand the channel dynamics with respect to space and time at these frequencies. Millimeter-wave signals are highly susceptible to blocking, and they have communication limitations owing to their poor signal attenuation compared with microwave signals. Therefore, by employing highly directional antennas, co-channel interference to or from other systems can be alleviated using line-of-sight (LOS) propagation. Because of the ability to shape, switch, or scan the propagating beam, phased arrays play an important role in advanced wireless communication systems. Beam-switching, beam-scanning, and multibeam arrays can be realized at mm-wave frequencies using analog or digital system architectures. This review article presents state-of-the-art phased arrays for mm-wave mobile terminals (MSs) and base stations (BSs), with an emphasis on beamforming arrays. We also discuss challenges and strategies used to address unfavorable path loss and blockage issues related to mm-wave applications, which sets future directions.

Keywords: beamforming; beam-scanning; millimeter-wave (mm-wave); 5G; line-of-sight (LOS); phased arrays

1. Introduction Of the many fundamental inventions whose histories have been well documented, the origin of the antenna array is not generally known. There has been much focus on Guglielmo Marconi, who was a Nobel Prize-winning scientist, and his famous transatlantic wireless communication in December 1901 [1]. An array of 20 antenna elements was designed for the experiment. Unfortunately, strong winds destroyed the designed array system; therefore, a two-element antenna array was used to successfully transmit a repeated Morse code signal letter “S” from Poldhu, UK to St. John’s in Canada. However, another Nobel Prize-winning scientist, Luis Alvarez, was awarded the recognition globally for the discovery of the electronically scanning phased array. His innovation was initially triggered by the role of the U.S. in World War II, and was the reason for the development of Eagle, which was the ﬁrst reported radar-based bombing system. Until now, the use of phased arrays has been very common in the defense domain, and the design of warships and military jets focus on phased array radars.

1.1. Phased Array A phased array is deﬁned as a multiple-antenna system that electronically alters or directs the transmission or reception of an electromagnetic (EM) beam [1–3]. These systems can be realized by introducing a time variation or phase delay in every antenna’s signal path to compensate for the path differences in free space. Figure1 shows an illustration of a phased-array receiver with N-channels.

Sensors 2018, 18, 3194; doi:10.3390/s18103194 www.mdpi.com/journal/sensors Sensors 2018, 18, x FOR PEER REVIEW 2 of 33 Sensors 2018, 18, x FOR PEER REVIEW 2 of 33 introducingSensors 2018, 18 a, 3194 time variation or phase delay in every antenna’s signal path to compensate for the2 path of 31 introducingdifferences a intime free variation space. Figure or phase 1 shows delay anin everyillustration antenna’s of a signalphased path‐array to compensatereceiver with for N the‐channels. path differencesA uniform in antennafree space. spacing Figure d 1 was shows kept an between illustration consecutive of a phased antenna‐array receiverelements, with and N signals‐channels. from A Avariousuniform uniform paths antenna antenna were spacing combined spacing d wasd usingwas kept kepta variable between between‐delay consecutive consecutive block on antenna each antenna signal’s elements, elements, path [2].and and signals signals from from variousvarious paths paths were were combined combined using using a variable a variable-delay‐delay block block on on each each signal’s signal’s path path [2]. [2 ].

Figure 1. Block diagram of basic phased‐array receiver [2]. FigureFigure 1. Block 1. Block diagram diagram of basic of basic phased phased-array‐array receiver receiver [2]. [ 2]. Numerous designs and structures for low‐cost mm‐wave electronic scanning antennas have beenNumerous Numerousassessed. designs They designs contain and and structures structuresactive or passivefor low-costlow‐‐arraycost mm-wavemm structures,‐wave electronic electronic printed scanningplanar scanning arrays, antennas antennas reflect have havearrays, been beenorassessed. lensassessed. arrays. They They Each contain contain design active active may or consist passive-array or passive of different‐array structures, structures, radiating printed elementsprinted planar planar with arrays, various arrays, reﬂect properties,reflect arrays, arrays, or such lens or asarrays.lens narrowband arrays. Each Each design or designwideband, may may consist linear consist of or differentof circularly different radiating polarized, radiating elements digitalelements or with analogwith various various phase properties, properties,shifters, as such wellsuch as as variousnarrowbandnarrowband kinds or of wideband, array feeding linear linear structures. or or circularly circularly Two polarized, polarized, fundamental digital digital phased or or analog analog‐array phase phasestructures shifters, shifters, are as illustrated as well well as as variousinvarious Figure kinds kinds 2 [1]. of of array array feeding feeding structures. structures. Two Two fundamental fundamental phased phased-array‐array structures areare illustrated in in FigureFigure2 2[1 [1].].

(a) (b) (a) (b) Figure 2. BasicBasic architecture architecture of of ( a)) passive phased phased-array‐array architecture, and ( b) active phased-arrayphased‐array Figurearchitecture 2. Basic [[1]. 1architecture]. of (a) passive phased‐array architecture, and (b) active phased‐array architecture [1]. From a radio frequency (RF) perspective, passive phased arrays differ from typical radiating arraysFrom owingowing a radio toto the thefrequency addition addition (RF) of of phase phaseperspective, shifters, shifters, which passive which can phasedcan introduce introduce arrays a phase adiffer phase delay from delay in thetypical in array, the radiating array, and hence and arrayshencesteer owing the steer beam tothe the ofbeam theaddition antenna.of the of antenna. phase Passive shifters, Passive phased which phased arrays can doarrays introduce not containdo not a phasecontain active delay components active in componentsthe array, and include and and henceincludea transmitter steer a thetransmitter andbeam receiver. of and the receiver.antenna. Despite Despite this,Passive active this,phased phased-array active arrays phased do structures not‐array contain structures are active distributed, are components distributed, and contain and and includecontainactive a circuit transmitteractive components circuit and components receiver. to produce Despite to produce and this, amplify andactive amplify the phased power. the‐array Activepower. structures phased-array Active arephased distributed, designs‐array designs possess and containpossessmany active benefits many circuit benefits compared components compared to passive-array to to producepassive designs,‐ arrayand amplify designs, including the including reducedpower. reducedActive power phased losses power and‐ lossesarray noise designsand figures, noise possessfigures,flexible many designs,flexible benefits designs, and compared multi-function and multi to passive‐function characteristics‐array characteristics designs, [1]. including [1]. reduced power losses and noise figures, flexible designs, and multi‐function characteristics [1]. 1.2. Millimeter Millimeter-Wave‐Wave 5G Wireless Communication 1.2. Millimeter‐Wave 5G Wireless Communication Over the past few decades, the continuous development of new generations of communications technologyOver the haspast significantly significantly few decades, impacted the continuous thethe daily development lives and routines of new of generations people and ofresulted communications in a constant constant technologyincrease in has data significantly traffic traffic and impacted device connections the daily lives [4–7]. [4–7 and]. Different Different routines wireless of people services and resulted have been in a introduced introducedconstant increaseto the market in data trafficbecause and of devicethe rapid connections development [4–7]. of Different wireless wirelesscommunications services haveand mobile been introduced networking to techniques,the market because whichwhich has ofhas causedthe caused rapid smart developmentsmart devices devices to of become wirelessto become better communications known,better andknown, has and promptedand mobile has networking aprompted tremendous a techniques, increase in which the data has traffic caused in wirelesssmart devices networks to [6become,7]. Owing better to theknown, increasing and has number prompted of users a in

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Sensors 2018, 18, x FOR PEER REVIEW 3 of 33 wireless communications, it is predicted that the number of mobile connections will surpass 100 billion tremendous increase in the data traffic in wireless networks [6,7]. Owing to the increasing number of by the year 2020; hence, the Internet of Things (IoT) is becoming a more common concept [5]. The use users in wireless communications, it is predicted that the number of mobile connections will surpass of100 currently billion unusedby the year spectra 2020; is hence, therefore the Internet being highly of Things encouraged (IoT) is becoming because a of more the increasingcommon concept need for higher[5]. The data use rates of incurrently wireless unused communications spectra is [4therefore–11]. Millimeter-wave being highly encouraged (mm-wave) because communications of the systemsincreasing have need attracted for higher much data interest rates asin awireless next-generation communications technology [4–11]. and Millimeter are referred‐wave to (mm as‐ the fifth-generationwave) communications (5G) wireless systems communication have attracted systems, much interest which as are a next expected‐generation to be technology implemented and by are the earlyreferred 2020s to [ 5as]. the The fifth frequency‐generation spectrum (5G) wireless specified communication by the International systems, Telecommunicationwhich are expected to Union be (ITU)implemented for 5G communication by the early 2020s includes [5]. theThe 3.4–3.6 frequency GHz, spectrum 5–6 GHz, specified 24.25–27.5 by GHz,the International 37–40.5 GHz, andTelecommunication 66–76 GHz bands, Union while (ITU) the for Federal 5G communication Communications includes Commission the 3.4–3.6 (FCC)GHz, 5–6 has GHz, specified 24.25– the 27.5–28.3527.5 GHz, GHz 37–40.5 frequency GHz, bandand 66–76 for 5G GHz [5,6 ,bands,11]. while the Federal Communications Commission (FCC)The has benefit specified of using the 27.5–28.35 mm-waves GHz for frequency wireless band communication for 5G [5,6,11]. has been well known for a long time [4The–6,9 benefit–13]. of Compared using mm‐ withwaves 4G, for 5Gwireless wireless communication communication has been systems well known employ for a significantly long time different[4–6,9–13]. system Compared performance with 4G, scales 5G thatwireless require communication data rates of systems the order employ of several significantly gigabits-per-second different (Gbps),system as performance well as a veryscales high that require data flow data density, rates of the millisecond order of several level delay,gigabits crowded‐per‐second connections, (Gbps), andas well enhanced as a very spectral high data energy flow density, and cost millisecond factor level [5,11 ,delay,13,14]. crowded It is connections, generally agreed and enhanced that the signal-to-interference-plus-noisespectral energy and cost factor [5,11,13,14]. ratio (SINR) It is reduces generally considerably agreed that owingthe signal to extreme‐to‐interference free-space‐plus loss‐ noise ratio (SINR) reduces considerably owing to extreme free‐space loss and blockage experienced and blockage experienced by EM waves at high frequencies, particularly in the mm-wave bands [9]. by EM waves at high frequencies, particularly in the mm‐wave bands [9]. Figure 3 illustrates a typical Figure3 illustrates a typical heterogeneous 5G mobile network scenario. The use of highly directional heterogeneous 5G mobile network scenario. The use of highly directional antennas and their line‐of‐ antennas and their line-of-sight (LOS) propagation can effectively alleviate the signal interference in sight (LOS) propagation can effectively alleviate the signal interference in between the common between the common channels to or from other systems [14]. To achieve this, high-gain directional channels to or from other systems [14]. To achieve this, high‐gain directional antennas can be used at antennasboth the can transmitting be used at bothand receiving the transmitting ends, resulting and receiving in a significantly ends, resulting enhanced in a significantly SINR, a reduced enhanced SINR,Doppler a reduced effect, Dopplerand improved effect, data and security, improved and data can security, be used andin long can‐range be used mm in‐wave long-range point‐to mm-wave‐point point-to-point(P2P) communications (P2P) communications with an LOS link with [6,8,10,11,13,15–19]. an LOS link [6, 8The,10, 11path,13 loss,15– 19can]. be The reduced path lossby using can be reduceddirectional by using high directional‐gain antennas high-gain [8,9,20]. antennas However, [8,9 ,directional20]. However, antennas directional with antennasnarrow beams with narrow are beamsunsuitable are unsuitable for multiuser for multiusermobile communications mobile communications as they provide as they only provide limited only spatial limited coverage spatial coverage[6,9,20]. [ Moreover,6,9,20]. Moreover, directional directional beams need beams to be need steered to be either steered electronically either electronically or mechanically or mechanically to obtain toa obtain better asubstitute better substitute link for non link‐LOS for non-LOS communications communications [4,7,9,13–16,20–27]. [4,7,9,13 –Therefore,16,20–27]. major Therefore, obstacles major obstaclesfor the forimplementation the implementation of commercial of commercial mm‐wave mm-wave systems systemson a large on scale a large are scale their are high their cost high and cost andcompromised compromised performance performance [9,10,25]. [9,10, 25].

FigureFigure 3. 3. 5G5G heterogeneousheterogeneous mobile mobile network network scenario. scenario. 2. Millimeter-Wave Antenna Array for 5G Communication 2. Millimeter‐Wave Antenna Array for 5G Communication EvenEven though though mm-wave mm‐wave technology technology is generallyis generally acknowledged acknowledged as being as being promising promising for 5G for wireless 5G communicationwireless communication systems, there systems, exists there a gap exists between a gap thebetween current the mm-wave current mm designs‐wave designs and the and proposed the commercialproposed mm-wave commercial cellular mm‐ networkswave cellular [4–6,9 networks,11,13,22]. A[4–6,9,11,13,22]. few basic modifications A few basic are needed modifications in the practical are implementationneeded in the practical of mm-wave implementation cellular networks. of mm‐ Towave improve cellular the networks. performance To improve of wireless the communicationperformance systems,of wireless advanced communication antenna systems, array advanced architectures antenna are array used, architectures with names are suchused, with as phased names such arrays,

Sensors 2018, 18, 3194 4 of 31 Sensors 2018, 18, x FOR PEER REVIEW 4 of 33 beamformingas phased arrays, arrays, and beamforming multi-input multi-output arrays, and (MIMO) multitransceivers‐input multi [‐1output,7,9,10 ,12(MIMO),14–17 ,19transceivers,22,24,25,28 ]. However,[1,7,9,10,12,14–17,19,22,24,25,28]. user mobility, the random However, movement user mobility, of mobile the devices, random as movement well as indoor of mobile and devices, outdoor propagationas well as indoor in urban and environments outdoor propagation are essential in urban factors environments that affect are the essential architecture factors that of base affect station the (BS)architecture and mobile of station base (MS)station antennas (BS) and in mm-wavemobile station cellular (MS) communication antennas in systemsmm‐wave [13 ,cellular20,25,29 ]. Thus,communication to obtain a systems good link [13,20,25,29]. quality, beam-steering Thus, to obtain should a good belink applied quality, on beam both‐steering the BSs should and MSs,be as illustratedapplied on inboth Figure the BSs4. and MSs, as illustrated in Figure 4.

FigureFigure 4. 4.Beamforming Beamforming antennaantenna array scenario at at BS BS and and MS. MS.

Generally,Generally, most most of of the the recent recent mm-wave mm‐wave 5G5G researchresearch efforts are are based based on: on: (1) (1) SiGe SiGe compound compound semiconductorsemiconductor circuits circuits and and integrated integrated chips chips (ICs) (ICs) [[30–34]30–34] and (2) (2) large large-scale‐scale mm mm-wave‐wave phased phased arrays arrays andand beam-switching beam‐switching antennas antennas to to increase increase thethe antennaantenna and amplifier ampliﬁer gains gains within within the the mm mm-wave‐wave 5G 5G wirelesswireless communication communication links links [ 22[22,23].,23]. TheseThese studies have stimulated stimulated interest interest in in applications applications that that focusfocus on on compact compact and and low-power low‐power consumption, consumption, such as the the mm mm-wave‐wave 5G 5G backhaul, backhaul, access access terminals, terminals, andand BSs BSs [12 [12,14,16].,14,16]. With With thethe help of of prominent prominent mm mm-wave‐wave 5G 5G antenna antenna studies, studies, these these applications applications are arefurther further specified speciﬁed as as planar planar phased phased arrays arrays [35], [35], grid grid arrays arrays [36], [36], substrate substrate-integrated‐integrated waveguides waveguides (SIWs)(SIWs) [37 [37],], and and planar planar lens lens architectures architectures [ 38[38,39].,39].

2.1.2.1. Millimeter-Wave Millimeter‐Wave Antenna Antenna Arrays Arrays for for 5G 5G Mobile Mobile TerminalsTerminals TheThe application application of antennaof antenna technologies technologies for mm-wave for mm‐ 5Gwave cellular 5G cellular handsets handsets is still not is widespreadstill not withwidespread respect to with antennas respect architectures. to antennas architectures. Considering Considering the significance the significance and role ofand cellular role of phones cellular in thephones mobile in network the mobile industry, network mm-wave industry, mm antennas‐wave antennas for 5G cellular for 5G cellular handsets handsets are considered are considered as the trend-shiftingas the trend‐ technologyshifting technology for 5G mobile for 5G networks.mobile networks. The use The of ause mm-wave of a mm‐ transceiverwave transceiver within within a mobile a phonemobile presents phone significantpresents significant challenges challenges that should that should be considered be considered [7,16]. [7,16]. The importanceThe importance of these of challengesthese challenges is found is found because because of the of unique the unique scenarios scenarios related related to to the the use use of of mobilemobile phones. phones. Mobile Mobile phones are more compact in size relative to tablet PCs and wireless docking stations, and owing to phones are more compact in size relative to tablet PCs and wireless docking stations, and owing to the expanded use of smart phones, their usage scenarios are highly varied [7]. Secure and reliable the expanded use of smart phones, their usage scenarios are highly varied [7]. Secure and reliable communication link at Gbps speeds need to be guaranteed by the 5G transceivers within a mobile communication link at Gbps speeds need to be guaranteed by the 5G transceivers within a mobile phone. Among the key factors considered in the establishment of mm‐wave 5G networks, the antenna phone. Among the key factors considered in the establishment of mm-wave 5G networks, the antenna design requires the most extensive modifications. The antennas, which are technically described as designomnidirectional requires the antennas most extensive that transmit modifications. and receive The RF antennas,signals equally which in areall horizontal technically directions described in as omnidirectionalboth geometrical antennas planes, thatare the transmit focus of and resonant receive‐type RF antennas signals equally implemented in all in horizontal user devices. directions The inomnidirectional both geometrical antennas planes, inside are the mobile focus phones of resonant-type have gains that antennas generally implemented fall within the in userrange devices. of ‐8 Theto omnidirectional 0 dBi [28]. To compensate antennas insidefor the mobilehigh signal phones attenuation have gains at mm that‐wave generally frequencies fall within with theincreased range of -8 togain, 0 dBi the [ 28idea]. To of compensateantenna arrays for for the mobile high signal phones attenuation was proposed. at mm-wave frequencies with increased gain, theIn idea the wireless of antenna communication arrays for mobile industry, phones the design was proposed. of antennas for mobile phones is considered byIn RF the and wireless antenna communication engineers to industry,be an engineering the design art. of antennas This is because for mobile of phonesthe difference is considered in the by RFperformance and antenna of engineers antennas to when be an in engineering an ideal situation art. This in free is because space and of when the difference installed ininside the performance a handset.

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Sensors 2018, 18, x FOR PEER REVIEW 5 of 33 of antennas when in an ideal situation in free space and when installed inside a handset. It is not possibleIt is not for possible an antenna for an thatantenna is installed that is installed in a mobile in a mobile phone phone to behave to behave identical identical to one to one in free in free space withoutspace without considering considering real-time real scenarios.‐time scenarios. The induced The induced electric electric and magnetand magnet field-coupling field‐coupling due due to the antennato the antenna surface currents surface currents eventually eventually affect the affect characteristic the characteristic impedance impedance matching matching and designed and designed antenna efficiencyantenna inefficiency free space in free [16 ,space25,40 ].[16,25,40]. Moreover, Moreover, the radiation the radiation performance performance of the of antenna the antenna varies varies owing to theowing presence to the presence of other of electronic other electronic components components and the and metallic the metallic casing casing [41]. [41]. Most Most of the of spacethe space inside mobileinside phones mobile is phones taken upis taken by the up large by the liquid-crystal large liquid‐ displaycrystal display (LCD) (LCD) and battery. and battery. The metallic The metallic brackets locatedbrackets behind located the behind LCD effectively the LCD effectively limit the limit design the ofdesign current of current mobile mobile phone phone antennas antennas to the to devicethe edgesdevice [40 edges]. Moreover, [40]. Moreover, the use the of use certain of certain metallic metallic frames frames that that are are employed employed for for different different types types of of sensors, cameras, speakers, and microphone modules further reduce the available antenna space sensors, cameras, speakers, and microphone modules further reduce the available antenna space inside inside cellular phones [16]. Therefore, antenna arrays with single‐ and multilayer printed circuit cellular phones [16]. Therefore, antenna arrays with single- and multilayer printed circuit board (PCB) board (PCB) technologies, polarization diversity, and a wide scanning range will be presented in this technologies, polarization diversity, and a wide scanning range will be presented in this paper. paper. 2.1.1. Multilayer Phased-Array Antennas for 5G Mobile Terminals 2.1.1. Multilayer Phased‐Array Antennas for 5G Mobile Terminals In [10], Hong et al. presented the basic design concept of mm-wave 5G antennas for cellular In [10], Hong et al. presented the basic design concept of mm‐wave 5G antennas for cellular × phonesphones at at 28 28 GHz. GHz. In In thatthat study, the the design design of of 1 × 1 16 mesh16 mesh-grid‐grid antenna antenna-element‐element phased phased arrays arraysat the at thetop top and and bottom bottom positions positions of ofcellular cellular phones phones has hasbeen been realized realized using using two sets two of sets PCBs, of PCBs, as shown as shown in inFigure Figure 5d.5d. The The mesh mesh grid grid antenna antenna elements elements are arranged are arranged in slanted in slanted angles anglesof approximately of approximately 50° at ◦ 50 eachat eachcorner corner of the ofPCB. the The PCB. proposed The proposed mesh‐grid mesh-grid mm‐wave mm-waveantenna array antenna configuration array configuration exhibits a exhibitsfan‐beam a fan-beam radiation radiationcharacteristic, characteristic, as shown in asFigure shown 5e. The in Figure proposed5e. array The proposedantenna was array measured antenna wasin measuredthe anechoic in chamber the anechoic at Samsung chamber Electronics at Samsung headquarters Electronics located headquarters in Suwon, South located Korea. in Suwon,The Southmeasurement Korea. The results measurement show a 130° results and 12° show 3‐dB a 130 beamwidth◦ and 12 ◦in3-dB the beamwidthelevation and in azimuth the elevation planes, and azimuthrespectively. planes, The respectively. proposed array The proposed antenna exhibits array antenna a 10‐dB exhibitsimpedance a 10-dB bandwidth impedance of 1 GHz bandwidth at a 27.9‐ of 1 GHzGHz at center a 27.9-GHz frequency. center The frequency. antenna array The antenna inside the array cellular inside device the cellularhas a measured device haspeak a gain measured of more peak gainthan of 10.5 more dBi. than The 10.5 radiation dBi. The pattern radiation mismatch pattern between mismatch the free between‐space theand free-space integrated andscenarios integrated is scenariosbecause is of because diffraction of diffraction and refraction and between refraction the between chassis theand chassis antenna and elements antenna of elements cellular phones. of cellular phones.Figure Figure5f illustrates5f illustrates the radiation the radiation patterns of patterns a mm‐wave of a mm-wave antenna array antenna in specific array beam in specific directions beam for both free‐space and integrated scenarios. The measurement shows an angular scanning range of directions for both free-space and integrated scenarios. The measurement shows an angular scanning ±70°. The proposed antenna array configuration on the top and bottom of the cellular device achieved range of ±70◦. The proposed antenna array configuration on the top and bottom of the cellular device an almost spherical radiation coverage, as shown in Figure 5e. achieved an almost spherical radiation coverage, as shown in Figure5e.

(a) (b)

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(e)

(f)

FigureFigure 5. 5. ((aa)) Comparative illustration illustration of of the the standard standard cellular cellular antenna antenna and and mm mm-wave‐wave 5G antenna. 5G antenna. (b) (bProposed) Proposed antenna. antenna. (c) ( cPrototype) Prototype photograph photograph of of the the standalone standalone mm mm-wave‐wave antenna antenna array array with with coaxial coaxial connectors.connectors. ( d()d Photograph) Photograph of mm-waveof mm‐wave 5G cellular5G cellular antenna antenna array array integrated integrated inside inside a Samsung a Samsung handset andhandset zoomed-in and viewszoomed of‐in 5G views mm-wave of 5G antenna mm‐ array.wave (eantenna) mm-wave array. antenna (e) mm array‐wave conﬁguration antenna forarray 5G cellularconfiguration mobile terminals.for 5G cellular (f) Measured mobile terminals. and normalized (f) Measured radiation and patterns normalized for different radiation beam patterns directions for (Figuredifferent5f redrawn beam directions from [10 (Figure]). 5f redrawn from [10]).

InIn [ 42[42],], aa planarplanar quad-modequad‐mode widebandwideband phased array for a 5G mobile terminal terminal device device was was reported.reported. A A phased-array phased‐array design design consists consists of eight of eight antenna antenna elements elements with eight with MMPX eight (Micro-MiniatureMMPX (Micro‐ Connector)Miniature Connector) connectors. connectors. The proposed The proposed phased array phased is a array multilayer is a multilayer design comprising design comprising two Taconic two RF-30Taconic substrate RF‐30 substrate layers with layers thickness with thickness 0.762 mm, 0.762 having mm, a dielectrichaving a dielectric constant ofconstant 3 and loss of 3 tangent and loss of 0.0014,tangent vias, of 0.0014, and microvias. vias, and Bothmicrovias. layers Both were layers glued were with glued FR4 glue with having FR4 glue a thickness having a of thickness 0.2 mm, of and 0.2 a dielectricmm, and constant a dielectric of 4.3constant and tangent of 4.3 and loss tangent of 0.025. loss The of design0.025. The overview design of overview the proposed of the antennaproposed is shownantenna in is Figure shown6a. in A Figure coaxial-to-differential 6a. A coaxial‐to‐ striplinedifferential transition stripline feeding transition is proposed. feeding is Theproposed. antenna The is fedantenna by a differential is fed by a striplinedifferential feed. stripline The top feed. and The bottom top and dipole bottom antennas dipole were antennas connected were toconnected differential to striplinedifferential through stripline vias, through as shown vias, in as Figure shown6a. in The Figure proposed 6a. The array proposed antenna array generates antenna four generates distinctive four modesdistinctive when modes excited, when as excited, illustrated as illustrated in Figure 6inb, Figure with an 6b, endfire with an radiation endfire radiation pattern inpattern all modes. in all Themodes. measured The measured reflection reflection coefficients coefficients of the proposed of the proposed eight-element eight phased-array‐element phased antenna‐array are antenna presented are inpresented Figure6d. in The Figure main 6d. beam-scanning The main beam range‐scanning is from range−90 is◦ fromto +90 −90°◦. Figure to +90°.6g Figure illustrates 6g illustrates the scanning the resultsscanning of theresults proposed of the proposed beam-steering beam‐ phased-arraysteering phased antenna‐array antenna at 25, 27, at 25, 29, 27, and 29, 31 and GHz. 31 GHz. The finalThe prototypefinal prototype of the of phased-array the phased‐array antenna antenna with connectorswith connectors is shown is shown in Figure in Figure6i. 6i.

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(a) (b)

(c)

(d) (e)

(f) (g)

Figure 6. Cont.

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(h) (i)

FigureFigure 6. 6.(a )(a Three-dimensional) Three‐dimensional (3D) (3D) illustration illustration of of proposed proposed antenna antenna element. element. ((b)) Electric fields ﬁelds of ofproposed antenna’s antenna’s modes: modes: top top view view (left (left) and) and bottom bottom view view (right (right). ().c) (Geometryc) Geometry of proposed of proposed eight‐ eight-elementelement phased phased-array‐array antenna. antenna. (d) S‐ (parametersd) S-parameters of proposed of proposed phased phased-array‐array antenna. antenna. (e) 3D radiation (e) 3D radiationpatterns patterns of proposed of proposed phased phased array in array all modes. in all modes. (f) 3D view (f) 3D of viewantenna of antenna and MMPX and MMPXfeeding feedingstructure. structure.(g) Simulated (g) Simulated and measured and measured 3D radiation 3D radiation patterns patterns at different at different scanning scanning angles angles at 25, at27, 25, 29, 27, and 29, 31 andGHz. 31 GHz. (h) Overview (h) Overview of the of layers the layers in the in antenna the antenna structure. structure. (i) Prototype (i) Prototype of the phased of the phased-array‐array antenna. antenna.(Redrawn (Redrawn from [42]) from [42])

2.1.2.2.1.2. Multi-Polarized Multi‐Polarized Multilayer Multilayer Phased-Array Phased‐Array Antennas Antennas for for 5G 5G Mobile Mobile Terminals Terminals ToTo improve improve the the diversity diversity gain gain in in 5G 5G mobile mobile terminals, terminals, the the use use of of multi-polarized multi‐polarized antenna antenna arrays arrays isis a desirablea desirable solution solution to to overcomeovercome the polarization polarization mismatch mismatch problem. problem. A simple A simple demonstration demonstration dual‐ dual-polarizedpolarized mm mm-wave‐wave antenna antenna with with good good isolation isolation is is reported reported in in [43]. [43]. To To achieve achieve dual dual polarizations polarizations at at28 GHz, the topologytopology ofof aa side-by-sideside‐by‐side arrangement arrangement of of horizontal horizontal and and vertical vertical Yagi-Uda Yagi‐Uda antennas antennas is is usedused in in [44 [44].]. In In [45 [45],], an an aperture aperture antenna antenna array array with with a dual-polarized a dual‐polarized unidirectional unidirectional pattern pattern through through a backa back cavity cavity on multilayer on multilayer PCB PCB technology technology is proposed. is proposed. AA multi-polarized multi‐polarized mm-wave mm‐wave antenna antenna array array configuration configuration for for 5G 5G mobile mobile terminals terminals was was demonstrateddemonstrated in in [46 [46].]. The The proposed proposed workwork isis thethe continuationcontinuation of the previously reported reported work work of ofHong Hong et et al. al. The The reported reported research research focuses focuses on losses on losses incurred incurred by the polarization by the polarization mismatch mismatch to enhance tothe enhance mm‐wave the mm-wave transmission transmission and reception and efficiency. reception At efficiency. mm‐wave At frequencies, mm-wave frequencies,it is complicated it is to complicatedtransmit high to transmit‐power energy high-power because energy of well because‐understood of well-understood propagation propagationand absorption and losses. absorption The 5G losses.wireless The communication 5G wireless communication link budget linkestimation budget estimationhas become has more become inflexible more inflexibleowing to owingreal‐life toconstraints real-life constraints such as suchlimited as limitedbattery batterylife. Because life. Because mobile mobileantennas antennas integrated integrated inside inside the mobile the mobileterminals terminals face different face different angular angular motions, motions, polarization polarization mismatches mismatches between between transmit transmit (Tx) and (Tx) receive and receive(Rx) antennas (Rx) antennas have have become become an important an important loss loss factor factor for for mm mm-wave‐wave cellular cellular communication. communication. To Toovercome overcome the the polarization polarization mismatch mismatch loss loss factor, factor, two two different different antenna antenna-element‐element designs designs based based on the onantenna the antenna array arrayschematic schematic are demonstrated are demonstrated and investigated. and investigated. A coplanar A waveguide coplanar waveguide-fed‐fed horizontally horizontallypolarized planar polarized Yagi planar‐Uda Yagi-Udaantenna‐element antenna-element configuration configuration together togetherwith a multi with‐ aplate multi-plate antenna‐ antenna-elementelement topology, topology, which which excites excites a vertically a vertically polarized polarized electric electric field, field,is proposed, is proposed, as shown as shown in Figure in Figure7a. The7a. The16‐element 16-element phased phased-array‐array antenna antenna depicted depicted in Figure in Figure 7c 7isc isdesigned designed by by deploying deploying the the two twolinearly linearly polarized polarized antenna antenna elements elements alternatively alternatively along along the edgeedge ofof the the mobile mobile terminals terminals with with an an ◦ angularangular scanning scanning range range of of±80 ±80°.. By By maintaining maintaining the the distance distance at at less less than than 3 mm,3 mm, an an isolation isolation of of more more thanthan 40 40 dB dB was was achieved achieved between between the the horizontally horizontally polarized polarized and and vertically vertically polarized polarized multi-antenna multi‐antenna elements.elements. The The two two sets sets of of 16-element 16‐element phased-array phased‐array antennas antennas on on the the opposite opposite corners corners at at the the top top and and bottombottom sides sides of of the the cellular cellular device device provide provide maximum maximum spherical spherical coverage coverage and and polarization polarization diversity. diversity. AnAn antenna antenna array array with with a maximum a maximum height height of of 0.8 0.8 mm mm was was fabricated fabricated using using 10-layer 10‐layer FR-4 FR‐ lamination.4 lamination. TheThe dielectric dielectric constant constant and and tangent tangent loss loss at 28at 28 GHz GHz are are determined determined to beto be 4.2 4.2 and and 0.09, 0.09, respectively. respectively.

Sensors 2018, 18, 3194 9 of 31 Sensors 2018, 18, x FOR PEER REVIEW 9 of 33

(a)

(b)

FigureFigure 7. 7.( a(a)) Two Two discrete discrete 28-GHz 28‐GHz mm-wave mm‐wave antenna antenna elements, elements, i.e., i.e., the the horizontally horizontally polarized polarized planar planar Yagi-UdaYagi‐Uda antenna antenna (left), (left), and and the the vertically vertically polarized polarized multi-plate multi‐plate antenna antenna (right). (right). ( b(b)) Final Final topology topology of of verticallyvertically polarized polarized multi-plate multi‐plate antenna. antenna. ( c()c) 16-element 16‐element phased-array phased‐array conﬁguration configuration at at the the edge edge of of the the mobilemobile terminal. terminal. (d ()d Photograph) Photograph of of the the prototype prototype testing testing in anin an anechoic anechoic chamber chamber and and close-up close‐ viewup view of 5Gof mm-wave5G mm‐wave phased-array phased‐array antenna antenna (Redrawn (Redrawn from from [46]). [46]).

AA multi-polarizedmulti‐polarized antenna antenna array array that that integrates integrates the the horizontally horizontally and andvertically vertically polarized polarized quasi‐ quasi-YagiYagi antennas antennas together together into a intosingle a area single was area implemented was implemented and demonstrated and demonstrated in [47] for in 5G [47 mobile] for 5Gterminals, mobile terminals,achieving achievingpolarization polarization diversity diversityand size reductions. and size reductions. To achieve To polarization achieve polarization diversity diversityalong with along the with beam the‐ beam-scanningscanning capability, capability, dual dual-polarized‐polarized quasi quasi-Yagi-Uda‐Yagi‐Uda antennas antennas for for bothboth thethe cornercorner edges edges and and a laterala lateral array array design design were were addressed, addressed, as illustrated as illustrated in Figure in Figure8a. The 8a. proposed The proposed array antennaarray antenna was implemented was implemented on a three-layer on a three structure‐layer structure on a Rogers4003 on a Rogers4003 substrate withsubstrate a total with thickness a total ofthickness 1.93 mm, of which 1.93 mm, has a which dielectric has constanta dielectric of 3.38 constant and a of tangent 3.38 and loss a of tangent 0.0027. loss The implementedof 0.0027. The implemented horizontally polarized and vertically polarized antenna array designs for the lateral edge as well as the multi‐polarized antenna for the corner edges are shown in Figure 8b. The 10‐dB

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horizontallySensors 2018, 18 polarized, x FOR PEER and REVIEW vertically polarized antenna array designs for the lateral edge as well10 as of the 33 multi-polarized antenna for the corner edges are shown in Figure8b. The 10-dB impedance bandwidth isimpedance 25% with bandwidth a port isolation is 25% of with more a than port 20 isolation dB. The of peak more realized than 20 gain dB. The of the peak array realized exceeds gain 11.8 of dBi, the witharray an exceeds overall 11.8 efﬁciency dBi, with of morean overall than 80%.efficiency of more than 80%.

(a)

(b)

Figure 8.8. (a) Geometry of the mobile terminalterminal with array position inin laterallateral andand cornercorner edges.edges. ( b) Prototype photograph of the multi-polarizedmulti‐polarized quasi-Yagi-Udaquasi‐Yagi‐Uda antenna arrays for lateral and corner edges. (Redrawn from [[47])47])

2.1.3. Single-LayerSingle‐Layer Phased-ArrayPhased‐Array Antennas for 5G Mobile Terminals A novel design of a Vivaldi phased antenna array is presented in [48] [48] for 5G mobile terminals. A low-costlow‐cost N9000N9000 PTFEPTFE substratesubstrate waswas usedused forfor thethe proposedproposed phased-arrayphased‐array antenna.antenna. The proposedproposed array antenna comprises eight antipodal Vivaldi antennaantenna elements.elements. AA 50-50‐Ω Ω microstrip feed line feeds the taperedtapered arms arms on on the the upper upper side side of theof the antenna antenna and and is used is used at the at upper the upper end of end mobile of mobile PCB, as PCB, shown as inshown Figure in9 Figureb. The simulated9b. The simulated S-parameters S‐parameters of the eight of the Vivaldi eight elements Vivaldi withelements discrete with ports discrete are shown ports are in Figureshown9 inc. TheFigure antenna 9c. The has antenna an operating has an bandwidthoperating bandwidth of more than of 1more GHz than at a 1 28-GHz GHz at center a 28‐GHz frequency. center Thefrequency. surface-current The surface distribution‐current of distribution the proposed of 5G the array proposed antenna 5G at anarray operating antenna frequency at an ofoperating 28 GHz isfrequency shown in of Figure 28 GHz9b. is The shown proposed in Figure phased-array 9b. The proposed antenna has phased enough‐array beam-scanning antenna has rangeenough in beam the ϕ‐ ◦ ◦ directionscanning rangingrange in from the φ 0 directionto 70 , as ranging shown from in Figure 0° to9 70°,d,e. as shown in Figure 9d,e.

Sensors 2018, 18, 3194 11 of 31 Sensors 2018, 18, x FOR PEER REVIEW 11 of 33

(a)

(b) (c)

(e)

(d)

FigureFigure 9.9. ((a)) GeometryGeometry ofof thethe VivaldiVivaldi phased-arrayphased‐array antenna.antenna. ((bb)) Surface-currentSurface‐current distributiondistribution ofof proposedproposed phased-array phased‐array antenna antenna at 28at GHz.28 GHz. (c) Simulated (c) Simulated S-parameters S‐parameters of the proposedof the proposed Vivaldi antennaVivaldi arrayantenna elements. array elements. (d) Radiation (d) patternsRadiation of patterns the antenna of the at different antenna scanning at different angles. scanning (e) Realized angles. gain (e) patternsRealized of gain the patterns antenna of at differentthe antenna scanning at different angles. scanning (Redrawn angles. from (Redrawn [48]). from [48]).

AA phased-arrayphased‐array antenna antenna with with switchable switchable three-dimensional three‐dimensional (3D) scanning (3D) scanning for 5G mobile for 5G terminals mobile wasterminals proposed was in proposed [49]. Three in similar[49]. Three subarrays similar of subarrays patch antennas of patch arranged antennas along arranged the edge along of the the mobile edge terminalof the mobile were proposed.terminal were Each proposed. subarray consists Each subarray of eight consists microstrip of patcheight antennamicrostrip elements patch antenna (MPAs) ◦ andelements has a beam-scanning(MPAs) and has capability a beam‐ ofscanning±90 in capability the θ plane. of The±90° antenna in the wasθ plane. designed The onantenna the Nelco was N9000designed substrate on the havingNelco N9000 a thickness substrate of 0.787 having mm a with thickness a dielectric of 0.787 constant mm with of 2.2a dielectric and tangent constant loss ofof 0.0009.2.2 and Figure tangent 10 lossc illustrates of 0.0009. an Figure architecture 10c illustrates to implement an architecture feeding using to implement low-loss phase feeding shifters using with low a‐ loss phase shifters with a 4.5‐dB insertion loss for beam‐steering, and a microwave SP3T switch to switch between the subarrays. The proposed design has a 1‐GHz 10‐dB impedance bandwidth in the

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4.5-dBfrequency insertion range loss from for 21 beam-steering, to 22 GHz, as and shown a microwave in Figure SP3T10b. The switch proposed to switch phased between‐array the design subarrays. has Thea good proposed beam‐scanning design has range a 1-GHz of −90° 10-dB to +90° impedance with a gain bandwidth of more inthan the 12.5 frequency dBi, as rangeshown from in Figure 21 to 2210d,e. GHz, as shown in Figure 10b. The proposed phased-array design has a good beam-scanning range of −90◦ to +90◦ with a gain of more than 12.5 dBi, as shown in Figure 10d,e.

(a) (b)

(c)

(d) (e)

Figure 10. (a) Proposed view of 5G phased-arrayphased‐array antenna with fullfull ground plane. (b) S-parametersS‐parameters of proposed phased-arrayphased‐array antenna with eighteight elements.elements. (c)) Proposed phased-arrayphased‐array architecture. (d) 3D radiation patterns of each subarray at different scanscan angles.angles. (e) 2D realized gain patterns at different scan angles. (Redrawn from [[49])49])

The 3D beam coverage was achieved and proposed by Zhang et al. in [50]. Three planar phased subarray configurations, as shown in Figure 11b, were used to change/switch the beam pattern to

Sensors 2018, 18, 3194 13 of 31

Sensors 2018, 18, x FOR PEER REVIEW 13 of 33 The 3D beam coverage was achieved and proposed by Zhang et al. in [50]. Three planar phased subarraytheir distinct configurations, regions using as shown chassis in surface Figure‐ wave11b, were excitation. used toThe change/switch 3D spherical coverage the beam is pattern achieved to by their distinctmerging regions the usingbeam chassispatterns surface-wave of subarrays. excitation. The proposed The 3D antenna spherical has coverage a 2‐GHz is achieved10‐dB impedance by merging thebandwidth beam patterns at 28 GHz, of subarrays. as shown The in Figure proposed 11b. antennaA Nelco N9000 has a 2-GHz PCB substrate 10-dB impedance was used to bandwidth design the at 28antenna GHz, as with shown a dielectric in Figure constant 11b. A of Nelco 2.2 and N9000 a loss PCB tangent substrate of 0.0009. was usedThe substrate to design dimensions the antenna are with 65 a dielectricmm × 130 constant mm, with of 2.2 a thickness and a loss of tangent 0.764 mm. of 0.0009. A subarray The substratecomprises dimensions eight slot elements, are 65 mm each× with130 mm,a withdimension a thickness of 4.85 of 0.764mm × mm.0.5 mm. A subarrayThe consecutive comprises slot‐element eight slot distance elements, from each center with‐to‐ acenter dimension is 5.35 of mm. The whole phased‐array switches the main beam between subarrays in the φ direction, and 4.85 mm × 0.5 mm. The consecutive slot-element distance from center-to-center is 5.35 mm. The whole scans the beam in the θ direction with variable phase shifts, as shown in Figure 11c,d. All the elements phased-array switches the main beam between subarrays in the ϕ direction, and scans the beam in the between subarrays A, B and B, C have a mutual coupling lower than −12.3 dB and −9.4 dB, θ direction with variable phase shifts, as shown in Figure 11c,d. All the elements between subarrays respectively. The angular scanning range of subarrays A, B, and C point in the φ directions of ±73°, A, B and B, C have a mutual coupling lower than −12.3 dB and −9.4 dB, respectively. The angular ±128°, and ±20°, respectively. The efficiency of subarrays A, B, and C are 74.3%, 68%, and 72.2%, ± ◦ ± ◦ ± ◦ scanningrespectively. range of subarrays A, B, and C point in the ϕ directions of 73 , 128 , and 20 , respectively. The efficiency of subarrays A, B, and C are 74.3%, 68%, and 72.2%, respectively.

(a) (b)

(e) (f)

FigureFigure 11. 11.(a )(a Printed) Printed switchable switchable array array antennaantenna conﬁgurationconfiguration of of subarrays. subarrays. (b ()b S)‐ S-parameterparameter plots plots of of proposedproposed array array antenna antenna for for elements elements 4, 12, 4, and12, and 20 in 20 subarrays in subarrays A, B, A, and B, C,and respectively, C, respectively, using using discrete ports.discrete (c) Gainports. patterns (c) Gain withpatterns beam-steering with beam‐ insteering the ϕ indirection the φ direction of each subarrayof each subarray with all with elements. all (d)elements. Radiation (d gain) Radiation patterns gain with patterns 70◦ beam-scanning: with 70° beam at‐scanning:ϕ = 73◦ for at φ subarray= 73° for A, subarray at ϕ = 128 A,◦ atfor φ subarray= 128° B,for at ϕ subarray= 20◦ for B, subarray at φ = 20° C. (fore) Illustration subarray C. of (e proposed) Illustration phased-array of proposed antenna phased with‐array coaxial antenna feed with cables. (f)coaxial Prototype feed of cables. proposed (f) phased-arrayPrototype of antennaproposed with phased coaxial‐array feed antenna cables. with (Redrawn coaxial from feed [50 cables.]) (Redrawn from [50])

Sensors 2018, 18, 3194 14 of 31

Sensors 2018, 18, x FOR PEER REVIEW 14 of 33 Zhang et al. also reported a study of user effects on the proposed switchable array in user mode, i.e., talkingZhang mode et andal. also data reported mode, a atstudy 28 GHz.of user Figure effects on12 athe illustrates proposed differentswitchable setups array in in user user mode, mode, wherei.e., ‘’top” talking and mode ‘’bottom” and data indicate mode, the at 28 switchable GHz. Figure array 12a position illustrates at different the top setups of the in chassis user mode, near to the indexwhere ﬁnger ‘’top” and and at ‘’bottom” the bottom indicate of the the chassis switchable close array to the position hand at palm, the top respectively. of the chassis The near electrical to the propertiesindex of finger skin andand differentat the bottom tissues of were the chassis taken fromclose [ 50to ].the Parametric hand palm, results respectively. for the switchable The electrical array on theproperties top and bottom of skin ofand the different chassis tissues in talking were mode taken and from data [50]. usage Parametric mode results are shown for the in Figureswitchable 12b,c, respectively.array on Basedthe top on and the bottom parametric of the chassis analysis in talking and results, mode and it is data proposed usage mode that theare shown switchable in Figure array 12b,c, respectively. Based on the parametric analysis and results, it is proposed that the switchable has a better performance in terms of beam-switching, body loss, and realized gain on the chassis top array has a better performance in terms of beam‐switching, body loss, and realized gain on the chassis position when compared with the bottom of the chassis. It is also proposed to design an additional top position when compared with the bottom of the chassis. It is also proposed to design an additional arrayarray on the on bottom the bottom side side of the of the chassis, chassis, which which results results in in a a decreased decreased shadowing effect effect in in talking talking mode. mode.

(a)

(b) (c)

(d)

FigureFigure 12. (a 12.) User (a) User effect effect setup setup for: for: talk talk mode mode (left (left) and) and data data mode mode ((rightright). ( (bb)) Gain Gain plots plots of of each each subarraysubarray for top for and top bottomand bottom sides sides of chassis of chassis in in talking talking mode. mode. ( c(c)) Gain Gain plots of each each subarray subarray for for top top and bottomand bottom sides sides of chassis of chassis in data in data usage usage mode. mode. (d ()d Measurement) Measurement setupsetup for free free space space (left (left) and) and for for talkingtalking mode mode with with real personreal person (right (right). (Redrawn). (Redrawn from from [50 [50])])

Sensors 2018, 18, 3194 15 of 31 Sensors 2018, 18, x FOR PEER REVIEW 15 of 33

YuYu et et al. al. presented presented a study study and and design design of ofa novel a novel phased phased-array‐array antenna antenna operating operating at 28 GHz at 28 with GHz withbeam beam-steering‐steering applications applications for a for5G amobile 5G mobile terminal terminal with metallic with metallic casing in casing [41]. The in [ 41proposed]. The proposed beam‐ beam-steeringsteering array array comprises comprises two twosubarrays, subarrays, each eachhaving having eight eight identical identical elements elements on both on bothsides sides of a of a mobilemobile devicedevice with a metallic casing, casing, as as shown shown in in Figure Figure 13a. 13a. A A slot slot element element with with a cavity a cavity-backed‐backed structurestructure is is proposed, proposed, whichwhich is is easy easy to to fabricate fabricate on on the the metallic metallic casing casing of the of mobile the mobile terminal. terminal. An Animportant important factor factor is isto todetermine determine the the optimum optimum position position of the of proposed the proposed phased phased subarrays subarrays inside insidethe themobile mobile terminal terminal before before finalizing ﬁnalizing the the actual actual design design in inpractice. practice. The The length length of the of the slot slot is kept is kept at the at the half‐guided wavelength λg/2, i.e., 5.8 mm at an operating frequency of 28 GHz, and the corresponding half-guided wavelength λg/2, i.e., 5.8 mm at an operating frequency of 28 GHz, and the corresponding slotslot width width is is 1.5 1.5 mm mm withwith a cavity height height of of 4 4mm. mm. The The high high-gain‐gain directional directional radiation radiation pattern pattern of the of theslot slot element element is achieved. is achieved. Figure Figure 13c illustrates 13c illustrates the design the design of one of of the one proposed of the proposed eight‐element eight-element phased arrays. It is proposed to use small stepped pins soldered on the microstrip feed line feeding each phased arrays. It is proposed to use small stepped pins soldered on the microstrip feed line feeding element of the subarray. The microstrip feeding line is printed on a 0.254‐mm‐thick Rogers 5880 each element of the subarray. The microstrip feeding line is printed on a 0.254-mm-thick Rogers substrate having a dielectric constant of 2.2 and a loss tangent/of 0.0009. Each element has been 5880 substrate having a dielectric constant of 2.2 and a loss tangent/of 0.0009. Each element has been provided by the phase variation using 6‐bit phase shifters within a 28‐GHz front‐end RF integrated provided by the phase variation using 6-bit phase shifters within a 28-GHz front-end RF integrated circuit (RFIC) chip to accomplish beam‐steering, as illustrated in Figure 13e. Figure 13d shows the circuitblock (RFIC) diagram chip of the to accomplish eight‐element beam-steering, beam‐steering asphased illustrated array. in Figure 13e. Figure 13d shows the block diagram of the eight-element beam-steering phased array.

(a) (b)

(e)

Figure 13. Cont.

Sensors 2018, 18, 3194 16 of 31 SensorsSensors 2018 2018, ,18 18, ,x x FOR FOR PEER PEER REVIEW REVIEW 1616 of of 33 33

(f) (f) ((gg))

FigureFigureFigure 13. 13.13.( a ()(aa) Different) DifferentDifferent views viewsviews of ofof mobile mobilemobile terminal terminalterminal design. design. ((b (bb)) ) PhotographPhotograph Photograph ofof of mobilemobile mobile terminalterminal terminal withwith with proposedproposedproposed phased-array phasedphased‐‐arrayarray antennas. antennas.antennas. ( (cc)) Design Design of of proposed proposed eighteight eight-element‐‐elementelement antennaantenna antenna array.array. array. ((dd) () d BlockBlock) Block diagramdiagramdiagram of ofof eight-element eighteight‐‐elementelement beam-steering beambeam‐‐steeringsteering antennaantenna array. array. ((e (ee)) ) BlockBlock Block diagramdiagram diagram descriptiondescription description ofof of 2828‐ 28-GHz‐GHzGHz front‐end RFIC chip. (f) Simulated and measured input reflection coefficients and isolation between front-endfront‐end RFIC RFIC chip. chip. (f ()f Simulated) Simulated and and measured measured inputinput reflection reﬂection coefficients coefﬁcients and and isolation isolation between between array elements. (g) 2D radiation plots at different scanning angles (Redrawn from [41]). arrayarray elements. elements. (g ()g 2D) 2D radiation radiation plots plots at at different different scanningscanning angles (Redrawn (Redrawn from from [41]). [41]).

FigureFigure 13f 13f shows shows the the simulated simulated and and experimented experimented S S1111 and and S S2121 plots. plots. The The proposed proposed beam beam‐‐steeringsteering Figure 13f shows the simulated and experimented S11 and S21 plots. The proposed beam-steering phased-arrayphasedphased‐‐arrayarray has has has achieved achieved achieved an an an angular angular angular scanningscanning scanning range range of of 0° 0° 0◦ to toto 60° 60° 60 ◦at atat 28 28 28 GHz, GHz, GHz, as as asshown shown shown in in Figure inFigure Figure 13g. 13g. 13 g. The gain variation in the angular scanning range of ±60° is symmetrical with 15.6 dBi as peak gain. TheThe gain gain variation variation in in the the angular angular scanning scanning range range ofof ±±60°60◦ isis symmetrical symmetrical with with 15.6 15.6 dBi dBi as aspeak peak gain. gain. Two‐dimensional (2D) radiation plots for various scanning angles of the proposed phased array are Two-dimensionalTwo‐dimensional (2D) (2D) radiation radiation plots plots for for various various scanningscanning angles angles of of the the proposed proposed phased phased array array are are shownshown inin FigureFigure 13g.13g. shown in Figure 13g. YuYu etet al.al. alsoalso presentedpresented thethe effecteffect ofof thethe user’suser’s bodybody onon thethe gaingain andand impedanceimpedance matchingmatching ofof thethe Yu et al. also presented the effect of the user’s body on the gain and impedance matching of the proposedproposed phasedphased array,array, asas illustratedillustrated inin FigureFigure 14a,b.14a,b. TheThe analysisanalysis ofof thethe user’suser’s bodybody effectseffects onon thethe proposed phased array, as illustrated in Figure 14a,b. The analysis of the user’s body effects on the proposedproposed 1616‐‐elementelement phasedphased arraysarrays showsshows thatthat itit cancan achieveachieve aa reasonablereasonable gaingain ofof atat leastleast 6.96.9 dBi,dBi, proposed 16-element phased arrays shows that it can achieve a reasonable gain of at least 6.9 dBi, eveneven withwith thethe user’suser’s bodybody effects.effects. even with the user’s body effects.

((aa)) ((bb))

FigureFigureFigure 14. 14. 14. (( (aa))) EffectsEffects of of of user’s user’s user’s body body body on on the onthe proposed theproposed proposed eight eight‐ eight-element‐elementelement phased phased phased array array with with array different different with beam different beam‐‐ beam-scanningscanningscanning anglesangles angles positionedpositioned positioned onon thethe on leftleft the sideside left ofof side mobilemobile of mobile terminal.terminal. terminal. ((bb)) UserUser ( bbodybody) User effectseffects body onon effects thethe eighteight on‐ the‐ eight-elementelementelement phasedphased phased arrayarray array locatedlocated located alongalong along thethe the rightright right edgeedge edge ofof mobile ofmobile mobile terminalterminal terminal atat different atdifferent different beambeam beam-scanning‐‐scanningscanning angles.angles.angles. (Redrawn (Redrawn (Redrawn from from from [ 41[41]) [41])])

Table1 categorizes the performance of recent phased arrays for the 5G mobile terminal devices discussed in this section. A performance comparison has been made based on the beam-scanning capability, peak gain, and array elements of various phased arrays.

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Table 1. Performance variation of various phased arrays for 5G mobile terminals.

Ref # No. of Subarrays Elements Per Subarray fo (GHz) BW (GHz) Beam-Scanning (deg) Peak Gain (dBi) Substrate Array Position [10] 2 1 × 16 27.9 =1 ±70◦ 10.5 Multilayer-PCB Top and Bottom [46] 4 1 × 16 28 - ±80◦ - Multilayer-FR4 Four Corners [49] 3 1 × 8 21.5 =1 ±90◦ 12.5 Nelco N9000 Top [48] 1 1 × 8 28 >1 ±70◦ 12 N900 PTFE Top [50] 3 1 × 8 28 =2 ±73◦, ±128◦, ±20◦ 12.5 Nelco N9000 Top [41] 2 1 × 8 28 >2.5 ±60◦ 15.6 Metallic casing Left and Right [42] 1 1 × 8 28 >8 ±90◦ 12 Taconic RF-30 Top [47] 3 1, 1 × 4, 1 36 >2 - 11 Multilayer-PCB Top (Mid + Corners) Sensors 2018, 18, x FOR PEER REVIEW 19 of 33 Sensors 2018, 18, 3194 18 of 31

2.2. Millimeter‐Wave Phased Arrays for 5G Access Terminals 2.2. Millimeter-Wave Phased Arrays for 5G Access Terminals In mm‐wave cellular networks, backhaul systems and BSs will generally be deployed in crowdy environmentIn mm-wave on the cellular poles, networks, beacons, backhauland building systems tops. and The BSs combined will generally data to be or deployed from the in multiple crowdy environmentusers will be ontransmitted the poles, beacons,with less and delay building from the tops. mm The‐wave combined BS to datathe central to or from hub the through multiple various users willmm‐ bewave transmitted wireless with or optical less delay channels. from the The mm-wave communication BS to the range central for hub the throughoutdoor various mm‐wave mm-wave access wirelesslink would or optical be larger channels. when Thecompared communication to indoor range mm‐ forwave the communication outdoor mm-wave links, access but linkthe wouldpower beconsumption larger when of comparedthe user end to terminals indoor mm-wave cannot be communication increased. Because links, of the but high the powersignal attenuation consumption at ofmm the‐wave user frequencies, end terminals the cannot effective be increased.communication Because distance of the of high mm signal‐wave attenuationsystems is limited at mm-wave when frequencies,compared to the microwave effective communicationsignals. To reduce distance the increased of mm-wave path systems loss in isoutdoor limited environments, when compared and to microwaveowing to the signals. user mobility, To reduce a the suitable increased beamforming path loss in technique outdoor environments, is required for and implementation owing to the user in mobility,cellular communication a suitable beamforming access links technique in the mm is required‐wave frequency for implementation band. This in can cellular be realized communication by using accessthe mm links‐wave in theBS mm-waveequipped frequencywith high band.‐gain phased This can arrays, be realized where by there using is the a mm-waverelaxation BS of equipped size and withpower high-gain consumption phased requirements. arrays, where therePhased is aarrays relaxation at millimeter of size and‐wave power frequencies consumption present requirements. a high‐ Phaseddata‐rate arrays communication at millimeter-wave solution frequencies using high present bandwidth a high-data-rate and directional communication links between solution the BS using and highMS terminals. bandwidth and directional links between the BS and MS terminals.

2.2.1. Millimeter Millimeter-Wave‐Wave PCBPCB PhasedPhased ArraysArrays In [9], [9], Zhang Zhang et et al. al. presented presented a comparative a comparative analysis analysis of different of different antenna antenna array array architectures architectures and andbeamforming beamforming techniques techniques for foroutdoor outdoor mm mm-wave‐wave communication communication systems. systems. Four Four different different types types of array architectures, i.e., an 8 × 88 rectangular rectangular element element array, array, 64 circular element array, 61 hexagonalhexagonal element array,array, and and 16 16 cross-shaped cross‐shaped element element array array architectures architectures were were analyzed, analyzed, as as depicted depicted in Figurein Figure 15. The15. The 2D 2D radiation radiation plots plots that that were were retrieved retrieved from from 3D 3D radiation radiation patterns patterns are are illustratedillustrated in Figure 15.15. The analysis of different array architectures shows that the array architecture with circular elements has a larger coverage area with high gain and directivity compared to those of the other antenna array architectures.

(a) (b)

Figure 15. 2D radiation plots of ( a) 8 × 88 rectangular rectangular element element array, array, ( (bb)) 16 16 cross cross-shaped‐shaped element element array, array, ((c)) 64 circular elementelement array,array, andand ((dd)) 61 61 hexagonal hexagonal element element array array (Redrawn (Redrawn from from [ 9[9]).]).

SensorsSensors 20182018, ,1818, ,x 3194 FOR PEER REVIEW 2019 of of 33 31

In [13], a testbed with a bandwidth of 800 MHz at an operating frequency of 28 GHz, which was built toIn [test13], the a testbed practicality with aof bandwidth mm‐wave of cellular 800 MHz communications at an operating at frequency Samsung ofElectronics, 28 GHz, which (Suwon) was Korea,built to was test presented. the practicality The BS of antenna mm-wave array cellular consisted communications of (6 × 8) 48 antenna at Samsung elements Electronics, that are grouped (Suwon) asKorea, subarrays was presented. in groups of The three BS antenna antenna array elements consisted to reduce of (6 the× 8) complexity 48 antenna of elements the system that components. are grouped Eachas subarrays subarray in provides groups of a horizontal three antenna beam elements‐scanning to reduceof 110° thewith complexity the aid of a of connected the system phase components. shifter, ◦ mixer,Each subarray and an RF provides path. The a horizontal MS antenna beam-scanning array was composed of 110 with of two the antenna aid of a connectedsubarrays, phase where shifter, each subarraymixer, and provides an RF path. horizontal The MS beam antenna‐scanning array of was either composed 90° or 180°, of two with antenna four elements subarrays, designed where each on ◦ ◦ onesubarray of the providessides of the horizontal RF board. beam-scanning The design overview of either of 90 the orBS 180 and, MS with antenna four elements arrays are designed shown onin Figureone of 16a,b, the sides respectively. of the RF board. The design overview of the BS and MS antenna arrays are shown in Figure 16a,b, respectively.

(a) (b)

FigureFigure 16. (a)) TransceiverTransceiver array array antenna antenna for for BS. BS. (b) Transceiver(b) Transceiver array array antenna antenna for MS. for (c )MS. Block (c diagram) Block diagramof testbed of for testbed mm-wave for cellularmm‐wave communication cellular communication with beamforming with antennabeamforming array. (antennad) Testbed array. structure (d) Testbedbuilt at Samsungstructure Electronics,built at Samsung Korea. Electronics, (Redrawn Korea. from [13 (Redrawn]) from [13])

InIn [39], [39], Jiang Jiang et et al. al. proposed proposed a a novel novel array array architecture architecture for for beamforming beamforming and and multibeam multibeam massive massive MIMOMIMO systemssystems using using a metamaterial-baseda metamaterial‐based thin thin planar planar lens antenna. lens antenna. The proposed The proposed antenna comprisesantenna comprisesan EM lens an combined EM lens combined with an antennawith an antenna array. The array. EM The lens EM together lens together with an with antenna an antenna array array helps helpsto increase to increase the throughput the throughput and data and rate data of rate massive of massive MIMO MIMO (m-MIMO). (m‐MIMO). Both the Both lens the and lens antenna and antennaelements elements were fabricated were fabricated using PCB using technology. PCB technology. Figure 17 Figurea,b respectively 17a,b respectively presents presents the side the and side top andviews top of views the proposed of the proposed lens antenna. lens antenna. An array An of array a seven-element of a seven‐element SIW-fed SIW stacked‐fed patchstacked antenna patch antennaoperating operating at 28 GHz at is28 placedGHz is at placed the focal at regionthe focal behind region the behind EM lens. the EachEM lens. subarray Each comprisessubarray comprisesfour SIW-fed four square SIW‐fed patches square as patches radiating as patches.radiating Each patches. array Each element array is element connected is connected to the input to portthe inputusing port SIW-to-grounded-coplanar-waveguide using SIW‐to‐grounded‐coplanar (GCPW)‐waveguide transition (GCPW) feeding. transition The design feeding. overview The design of the overviewseven-element of the SIW-fed seven‐element stacked patchSIW‐fed antenna stacked array patch is given antenna in Figure array 17isb. given The proposedin Figure multilayer 17b. The proposedantenna array multilayer is fabricated antenna on array a Rogers is fabricated RO4003C on substrate a Rogers having RO4003C a dielectric substrate constant having of 3.55 a dielectric and loss constanttangent ofof 0.0027.3.55 and The loss thickness tangent of of the 0.0027. top and The bottom thickness layers of the is 0.508 top and mm bottom and 0.813 layers mm, is respectively. 0.508 mm and 0.813 mm, respectively. Both layers are joined together with a bonding layer of Rogers RO4450B,

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Bothwhich layers has a are dielectric joined together constant with of 3.54, a bonding loss tangent layer of of Rogers 0.004, RO4450B,and height which of 0.202 has mm. a dielectric A photograph constant of 3.54,the fabricated loss tangent prototype of 0.004, of and the height proposed of 0.202 seven mm.‐element A photograph array loaded of the with fabricated an EM lens prototype is shown of the in proposedFigure 17c. seven-element The measured array 10‐dB loaded impedance with matching an EM lens ranges is shown from in 25.8–28.9 Figure 17 GHz,c. The as measuredshown in Figure 10-dB impedance17d. The angular matching beam ranges‐scanning from range 25.8–28.9 of the GHz, proposed as shown phased in Figure array 17 isd. from The − angular27° to +27°. beam-scanning Figure 17e ◦ ◦ rangeshows of the the experimented proposed phased radiation array patterns is from − of27 theto proposed +27 . Figure phased 17e shows‐array thelens experimented integrated antenna radiation at patterns28 GHz. ofThe the measured proposed peak phased-array gain of the lens lens integrated‐fed phased antenna‐array atantenna 28 GHz. is 24.2 The measureddBi, with an peak efficiency gain of theof 24.5% lens-fed at a phased-array frequency of antenna27.5 GHz. is 24.2 dBi, with an efﬁciency of 24.5% at a frequency of 27.5 GHz.

(a)

(b)

Figure 17. Cont.

Sensors 2018, 18, 3194 21 of 31 SensorsSensors 2018 2018, ,18 18, ,x x FOR FOR PEER PEER REVIEW REVIEW 2222 of of 33 33

((ee))

FigureFigureFigure 17. ( a17. 17.) Side ( (aa)) Side Side view view view of seven-elementof of seven seven‐‐elementelement antenna antenna antenna array array combined combined with with with EM EM EM lens. lens. lens. ( (bb)) (Top Topb) Top view view view of of the the of the seven-elementsevenseven‐‐elementelement SIW SIW SIW fed fedfed by by aby stacked aa stacked stacked patch patchpatch antenna antennaantenna array. array. ( (cc)) ) PhotographPhotograph Photograph ofof of thethe the lenslens lens integratedintegrated integrated arrayarray array antennaantennaantenna prototype. prototype. prototype. (d) ( Experimented(dd)) Experimented Experimented and and and simulated simulated simulated 10-dB10 10‐‐dBdB impedance impedance bandwidth. bandwidth. bandwidth. ( (ee)) Measured (Measurede) Measured scan scan scan beamsbeams of of the the proposed proposed lens lens‐‐fedfed phased phased‐‐arrayarray antenna antenna (Redrawn (Redrawn from from [39]). [39]). beams of the proposed lens-fed phased-array antenna (Redrawn from [39]).

In [15InIn], mm-wave [15],[15], mmmm‐‐wavewave massive massivemassive MIMO MIMOMIMO with withwith digital digitaldigital beamforming beamformingbeamforming (DBF) (DBF)(DBF) architecture architecturearchitecture operating operatingoperating at a atat 28-GHz aa 2828‐‐GHzGHz frequencyfrequency withwith anan operatingoperating bandwidthbandwidth ofof 500500 MHzMHz hashas beenbeen proposed.proposed. TheThe proposedproposed frequency with an operating bandwidth of 500 MHz has been proposed. The proposed transceiver has a transceivertransceiver hashas aa (16(16 ×× 4)4) 6464‐‐elementelement antennaantenna arrayarray configuration.configuration. AA printedprinted YagiYagi‐‐UdaUda antennaantenna (16 × 4) 64-element antenna array configuration. A printed Yagi-Uda antenna element combined with a elementelement combinedcombined withwith aa microstripmicrostrip balunbalun structurestructure waswas chosenchosen asas thethe radiatingradiating elementelement inin thethe microstriptransceiver.transceiver. balun Owing structureOwing toto was thethe compact chosencompact as size,size, the ease radiatingease ofof fabrication,fabrication, element in highhigh the gain, transceiver.gain, andand lowlow Owing‐‐costcost factors,factors, to the compact thethe size, easeproposedproposed of fabrication, YagiYagi‐‐UdaUda high antennaantenna gain, elementelement and low-cost exhibitsexhibits factors, thethe bestbest the optionoption proposed toto bebe consideredconsidered Yagi-Uda antennaforfor mmmm‐‐wavewave element MIMOMIMO exhibits the besttransceiverstransceivers option to [51].[51]. be considered FigureFigure 18a18a depictsdepicts for mm-wave thethe designdesign MIMO overviewoverview transceivers ofof thethe printedprinted [51]. YagiYagi Figure‐‐UdaUda 18 antennaantennaa depicts element.element. the design overviewTheThe ofexperimentalexperimental the printed reflection Yagi-Udareflection coefficient antennacoefficient element. ofof thethe designeddesigned The experimental arrayarray isis lowerlower reflection thanthan coefficient − −1414 dBdB atat of aa the2828‐‐GHz designedGHz arrayoperatingoperating is lower thanfrequencyfrequency−14 dBwithwith at aa a bandwidthbandwidth 28-GHz operating ofof moremore thanthan frequency 33 GHz.GHz. with TheThe awidewide bandwidth azimuthalazimuthal of angularangular more than beambeam 3‐‐ GHz. The widescanningscanning azimuthal achievedachieved angular byby thethe beam-scanning proposedproposed DBFDBF‐‐based achievedbased phasedphased by the‐‐arrayarray proposed systemsystem is DBF-basedis ±67°,±67°, asas shownshown phased-array inin FigureFigure system 18b.18b. is ±67◦,TheThe as shown experimentalexperimental in Figure TxTx and18andb. RxRx The peakpeak experimental gainsgains areare 2929 Tx dBidBi and andand Rx 2727 peak dBi,dBi, respectively.respectively. gains are 29 dBi and 27 dBi, respectively.

((aa)) ((bb))

((cc))

Figure 18. (a) Design overview of proposed Yagi-Uda antenna element with its 3D far-field radiation pattern. (b) Radiation patterns of 16-element DBF-based phased array in azimuth plane. (c) Photograph of the proposed DBF-based massive MIMO transceiver system with 64-antenna elements. (Redrawn from [15]). Sensors 2018, 18, x FOR PEER REVIEW 23 of 33

Figure 18. (a) Design overview of proposed Yagi‐Uda antenna element with its 3D far‐field radiation Sensors 2018pattern., 18, 3194 (b) Radiation patterns of 16‐element DBF‐based phased array in azimuth plane. (c) 22 of 31 Photograph of the proposed DBF‐based massive MIMO transceiver system with 64‐antenna elements. (Redrawn from [15]). 2.2.2. Millimeter-Wave RFIC Phased Arrays 2.2.2.Along Millimeter with the‐Wave current RFIC Phased IC-based Arrays packaging technology, there also exists an integrated analog-basedAlong phased-arraywith the current solution IC‐based proposed packaging for technology, 5G mm-wave there also communication exists an integrated [52–69 analog]. In‐ [53], a 32-elementbased phased RFIC-based‐array solution phased proposed array for operating 5G mm‐wave at 28communication GHz, which [52–69]. supports In [53], dual a 32 polarization‐element andRFIC accurate‐based beam-scanning phased array operating capability at 28 with GHz, the which advantage supports ofdual high polarization output power, and accurate was presented. beam‐ Thescanning proposed capability RFIC reported with the advantage in [53] is of compact high output and power, efﬁcient was with presented. respect The to proposed size, and RFIC scaled, whichreported supports in [53] dual is polarization compact and at efficient the Tx and with Rx respect with in-packaged to size, and arrayscaled, technology. which supports The proposed dual polarization at the Tx and Rx with in‐packaged array technology. The proposed RFIC with a 32‐ RFIC with a 32-transceiver (TRx) element phased array is implemented using a 0.13-µm-size transceiver (TRx) element phased array is implemented using a 0.13‐μm‐size SiGe BiCMOS (Bipolar SiGe BiCMOS (Bipolar Complementary Metal Oxide Semiconductor) process, as presented in Complementary Metal Oxide Semiconductor) process, as presented in Figure 19a. A packaged Figuremodule 19a. Aof packagedthe proposed module four ofICs the with proposed 128 elements four ICs (32 with each 128 in four elements ICs) and (32 each64 dual in‐ fourpolarized ICs) and 64 dual-polarizedantenna elements antenna providing elements two 64 providing‐element concurrent two 64-element beams was concurrent proposed beams in [65] was with proposed a measured in [65] ◦ ◦ withbeam a measured‐scanning beam-scanning range of ±50° with range a 1.4° of resolution,±50 with as a 1.4shownresolution, in Figure 19b. as shown in Figure 19b.

Sensors 2018, 18, x FOR PEER REVIEW 24 of 33 (a)

(b)

FigureFigure 19. 19.( a(a)) RFIC architecture architecture and and schematic schematic block block diagram diagram of 16‐element of 16-element phased array. phased (b) 64 array.‐ (b) 64-elementelement TX TX phased phased-array‐array angular angular beam beam-scanning‐scanning of ±50° of (Redrawn±50◦ (Redrawn from [53,65]) from [53,65])

In [54], an IC‐based transceiver with a flip‐chip package phased array operating at 28–32 GHz was proposed for 5G mm‐wave communication. The proposed IC chip comprised four separate TRx channels along with 6‐bit phase shifters and a 4‐bit 14‐dB gain control. Figure 20a shows the block diagram of 2 × 2 TRx 5G mm‐wave phased arrays. The proposed antenna design shown in Figure 16b has a 10‐dB reflection coefficient of 0.5 GHz. The peak array gain is 18 dBi and 12 dBi for the Tx and Rx ends, respectively. The measured EIRP (Effective Isotropic Radiated Power) at 29 GHz is 24.5 dBm. The photograph of the flip‐chip test board with 2 × 2 antenna elements is shown in Figure 20b. Based on the proposed beamformer packaged module, a 32‐element (4 × 8) TRX array with PCB integrated microstrip antennas on a four‐layer PCB stack was proposed in [60]. The angular beam‐ scanning of ±50° in the azimuth and ±25° in the elevation plane was achieved, as shown in Figure 20c.

(a)

Sensors 2018, 18, x FOR PEER REVIEW 24 of 33

(b) Sensors 2018, 18, 3194 23 of 31 Figure 19. (a) RFIC architecture and schematic block diagram of 16‐element phased array. (b) 64‐ element TX phased‐array angular beam‐scanning of ±50° (Redrawn from [53,65]) In [54], an IC-based transceiver with a flip-chip package phased array operating at 28–32 GHz wasIn proposed [54], an forIC‐based 5G mm-wave transceiver communication. with a flip‐chip The package proposed phased IC chiparray comprised operating at four 28–32 separate GHz wasTRx proposed channels alongfor 5G with mm‐ 6-bitwave phase communication. shifters and The a 4-bit proposed 14-dB IC gain chip control. comprised Figure four 20 separatea shows TRx the channelsblock diagram along ofwith 2 × 6‐2bit TRx phase 5G mm-waveshifters and phased a 4‐bit arrays. 14‐dB gain The proposedcontrol. Figure antenna 20a designshows shownthe block in diagramFigure 16 ofb has2 × 2 a TRx 10-dB 5G reflection mm‐wave coefficient phased arrays. of 0.5 The GHz. proposed The peak antenna array gain design is 18 shown dBi and in Figure 12 dBi 16b for hasthe Txa 10 and‐dB Rx reflection ends, respectively. coefficient of The 0.5 measured GHz. The EIRP peak (Effective array gain Isotropic is 18 dBi Radiated and 12 dBi Power) for the at 29Tx GHz and Rxis 24.5 ends, dBm. respectively. The photograph The measured of the flip-chip EIRP (Effective test board Isotropic with 2 Radiated× 2 antenna Power) elements at 29 GHz is shown is 24.5 in dBm.Figure The 20b. photograph Based on the of the proposed flip‐chip beamformer test board with packaged 2 × 2 antenna module, elements a 32-element is shown (4 × in8) Figure TRX array 20b. withBased PCB on integratedthe proposed microstrip beamformer antennas packaged on a four-layer module, PCB a 32 stack‐element was proposed(4 × 8) TRX in [60array]. The with angular PCB ◦ ◦ integratedbeam-scanning microstrip of ±50 antennasin the azimuth on a four and‐layer±25 PCBin stack the elevation was proposed plane wasin [60]. achieved, The angular as shown beam in‐ scanningFigure 20 c.of ±50° in the azimuth and ±25° in the elevation plane was achieved, as shown in Figure 20c.

Sensors 2018, 18, x FOR PEER REVIEW 25 of 33 (a)

(b) (c)

FigureFigure 20. 20. (a()a Block) Block diagram diagram of of 2 2× ×2 transmit/receive2 transmit/receive 5G 5G phased phased array. array. (b ()b Photograph) Photograph of of the the flip ﬂip-chip‐chip testtest board board with with 2 × 2 2× transmit/receive2 transmit/receive 5G phased 5G phased array. array. (c) Measured (c) Measured E and E H and‐plane H-plane patterns patterns of 4 × 8 of transmit/receive4 × 8 transmit/receive 5G phased 5G array phased in array Rx mode in Rx (Redrawn mode (Redrawn from [54,60]). from [ 54,60]).

InIn [67], [67], a a multilayer multilayer 64 64-element‐element dual dual-polarized‐polarized antenna antenna-in-package‐in‐package assembly assembly with with four four-SiGe‐SiGe BiCMOSBiCMOS multi-chipmulti‐chip phasedphased array array operating operating at 28 GHzat 28 was GHz proposed was forproposed 5G mm-wave for 5G communication. mm‐wave communication.The proposed module The proposed has four module mounted has SiGe four BiCMOS mounted transceiver SiGe BiCMOS ICs supporting transceiver dual ICs supporting polarization dualin Tx polarization and Rx modes. in Tx The and proposed Rx modes. antenna The proposed array has antenna a 3-GHz array bandwidth has a 3 at‐GHz an operating bandwidth frequency at an operatingof 28 GHz. frequency The measured of 28 GHz. Tx gain The ismeasured around 35Tx dBigain for is thearound 64-element 35 dBi for antenna the 64 array.‐element The antenna angular ◦ array.beam-scanning The angular of beam±40 ‐scanningin the azimuth of ±40° and in the elevation azimuth plane and waselevation achieved, plane as was shown achieved, in Figure as shown 21c. in Figure 21c.

(a)

(b) (c)

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(b) (c)

Figure 20. (a) Block diagram of 2 × 2 transmit/receive 5G phased array. (b) Photograph of the flip-chip test board with 2 × 2 transmit/receive 5G phased array. (c) Measured E and H-plane patterns of 4 × 8 transmit/receive 5G phased array in Rx mode (Redrawn from [54,60]).

In [67], a multilayer 64-element dual-polarized antenna-in-package assembly with four-SiGe BiCMOS multi-chip phased array operating at 28 GHz was proposed for 5G mm-wave communication. The proposed module has four mounted SiGe BiCMOS transceiver ICs supporting dual polarization in Tx and Rx modes. The proposed antenna array has a 3-GHz bandwidth at an operating frequency of 28 GHz. The measured Tx gain is around 35 dBi for the 64-element antenna array.Sensors The2018 ,angular18, 3194 beam-scanning of ±40° in the azimuth and elevation plane was achieved, as shown24 of 31 in Figure 21c.

(a)

(b) (c)

Figure 21. (a) Overview of antenna array assembly with transceiver ICs. (b) Photograph of the assembled antenna array package with four transceiver ICs. (c) 64-element TX phased-array angular ◦ beam-scanning of ±40 (Redrawn from [67]).

Table2 shows the performance of recently developed phased arrays for 5G access terminals demonstrated in this section. A performance comparison was made based on the beam-scanning capability and array elements of various phased arrays. Sensors 2018, 18, 3194 25 of 31

Table 2. Performance variation of various phased arrays for 5G access terminals.

Array/IC Elements in TX/RX Polarizations Beam-Scanning Gain (Tx/Rx) TX/RX Package Speciﬁcation / Integration Ref # f (GHz) Technology Elements Package Per IC o (deg) (dBi) Interface Level [9] 32 × 32 1024 - 28 - - Single-layer PCB - (HUST) Array Antenna architecture (Samsung) MIMO [13] 6 × 8 48 - 28 ±110◦ 21 Single-layer PCB - Antenna transceiver [39] 1 × 7 7 - 28 ±27◦ 24.2 Multilayer PCB - Antenna only (Bell Labs) MIMO transceiver with [15] 16 × 4 64 - 28 ±67◦ 29/27 Single-layer PCB - DBF architecture (Samsung) Array [55] 4 × 8 32 - 27.925 ±30◦ 18 Single-layer PCB - Antenna architecture 0.13 µm [53,65] 16 × 2 128 2/2 28 ±50◦ 32/34 3 GHz IF (IBM) Antenna in package SiGe BiCMOS 0.18 µm [54,60] 2 × 2 4 1/1 29 ±50◦, ±25◦ 18/12 RF (UCSD) Antenna on PCB SiGe BiCMOS [61] 2 × 4 8 1/1 28 ±20◦ 14 28 nm CMOS Analog IQ BB (LG) Antenna in package [63] 4 × 6 24 2/2 28 ±45◦ 44/34 28 nm CMOS 6.5 GHz IF (Qualcomm) Antenna in package Multilayer PCB and (Qualcomm) Antenna on PCB with [66] 2 × 2 4 - 28 ±45◦ - - Flip-chip module packaged ICs Multilayer PCB + 3 GHz IF [67] 4 × 4 64 2/2 28 ±40◦ 35 (IBM) Antenna in package 4 SiGe BiCMOS interface Sensors 2018, 18, 3194 26 of 31

3. Discussion Although mm-wave technology is being considered overall for future 5G wireless communication systems, there remains the need for improvement and changes in current mm-wave architectures and the expected commercial designs to be used in mm-wave cellular communication networks [4,6,9]. A few basic improvements are required with respect to the deployment and implementation of mm-wave cellular networks. Sophisticated antenna array designs are proposed to improve the performance of wireless communication systems in terms of high gain and coverage area. Because there is a trade-off between the gain and bandwidth, phased arrays structures offer an increased angular beam-scanning range along with high gain [70]. Various types of phased-array structures have been developed in the last few years. For the BS array antenna, there remains a relaxation of the space factor for designing complicated phased-array designs with increased spatial beam-scanning. However, in the case of MS antennas, it is more demanding to implement phased arrays in cellular handsets because of the limited space, user mobility, and user effects. Therefore, in this article, we discussed in detail the design constraints and implementation challenges of mm-wave phased arrays for 5G mobile terminals (MSs) and access terminals (BSs). To obtain the directional fan-beam radiation patterns in both the vertical and horizontal directions, two separate phased arrays positioned on the top and bottom of the chassis of a cellular handset were demonstrated in [10]. In [49], 3D beam-scanning was achieved by employing a folded 3D structure using three subarrays at the top position, but there is a limitation in terms of the implementation of the 3D array structure in the cellular handset. To address this limitation, 3D coverage was achieved in [50] using the surface waves of three identical slotted subarrays by switching the main beam direction to separate regions. Each subarray behaves similar to a phased array to tilt the beam to a separate region. To address the polarization mismatch [7,46] and incurred losses owing to the mobility of the user and various kinds of motions experienced by the mobile terminals at different angles, multi-polarized, i.e., horizontally polarized and vertically polarized antennas are combined in a subarray to achieve the diversity gain, while enhancing the efficiency of the transmission and reception at mm-wave frequencies [47]. To achieve polarization diversity along with the enhanced bandwidth, printed horizontally polarized and vertically polarized quasi-Yagi-Uda antennas were implemented [51]. Enhanced beam-scanning is achieved by combining horizontally polarized and vertically polarized antennas in an array using the side-by-side placement of these subarrays in a cellular handset [44,47]. Because of the constraints of space and position in cellular handsets, much research has been performed on the structure of mm-wave phased arrays with different handset positions [28,37,41,50]. Most of the phased arrays for 5G mobile terminals are single-layer substrate planar phased arrays to better fit into the chassis of cellular phones [42,48–50]. Further research efforts have been attempted on multilayer mesh-grid phased arrays [7] and SIW phased arrays [37]. Although most research has been performed on low-cost substrate boards [71,72], such as PCB-based phased arrays, there is little compliance with the metallic frame casing of modern cellular devices [56,57]. However, the study in [41] introduced a set of two eight-element cavity-backed slotted subarrays on both sides of the metallic casing of a cellular device. The study in [40] illustrates the degree to which the human body is vulnerable to electromagnetic fields (EMF) owing to the designed phased-array antennas in terms of the specific absorption rate (SAR). The authors in [40] successfully reduced the SAR to 0.88 W/kg by using slotted subarrays on the top and back of the metallic casing of a cellular handset. To address the increased path loss at mm-wave frequencies and the lower power consumption compared to that of sub-6-GHz communication links, LOS communication using phased-array techniques with an array antenna gain is required. Various antenna array architecture designs for outdoor mm-wave communication with different beamforming techniques, i.e., analog, digital, and hybrid techniques, were analyzed [9]. The antenna array design with circular elements is more robust to vibrations and is therefore the best candidate for outdoor mm-wave communication in terms of beam gain, gain fluctuation, and beam misalignment of phased-array antennas. Sensors 2018, 18, 3194 27 of 31

Analog beamforming compensates for the high path loss by generating high beamforming gains, whereas digital beamforming presents a better performance with an enhanced structural complexity. Therefore, a combination of analog and digital beamforming array architectures on both the transmitter and receiver ends has become the focus of research in recent years days based on the trade-off between the ﬂexibility and performance in outdoor mm-wave communications [55,73]. Cost is an important factor that needs to be signiﬁcantly lowered owing to the widespread use of phased arrays in 5G wireless communication. Therefore, ICs that are based on silicon technology (SiGe or BiCMOS) [34,35,54,58,59] along with low-cost PCB technology [35,36,60] should be considered. Scalable, RFIC-based phased arrays that are capable of beam-scanning in both the horizontal and vertical planes currently attract much attention from researchers [60].

4. Conclusions In this review, article, we demonstrated recently proposed phased arrays for 5G mobile terminals and access terminals. These state-of-the-art phased arrays are compact in size, low-cost, and have enough beam-scanning and coverage range. The effects of the user’s body and human exposure to EM waves by these state-of-the-art phased arrays with 3D coverage designed on single-layer and multilayer substrates were illustrated. In addition, multi-polarized antenna arrays and the space constraints of mobile terminals were presented and discussed. Various beamforming techniques using state-of-the-art mm-wave phased-array architectures to mitigate the co-channel interference and to overcome the path loss were proposed.

Author Contributions: S.L. conceived the idea for the study and A.H.N. wrote the manuscript. S.L. revised the manuscript and ensured conciseness. Acknowledgments: This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (2018R1A4A1023826). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Parker, D.; Zimmermann, D.C. Phased arrays—Part 1: Theory and architectures. IEEE Trans. Microw. Theory Tech. 2002, 50, 678–687. [CrossRef] 2. Niknejad, A.M.; Hashemi, H. Integrated Beamforming Arrays. In mm-Wave Silicon Technology; Series on Integrated Circuits and Systems; Springer Science and Business Media: Berlin, Germany, 2008; ISBN 978-0-387-76561-7. 3. Carpentieri, E.; Elia, U.F.D.; Stefano, E.D.; Guida, L.D.; Vitiello, R. Millimeter-Wave Phased-Array Antennas. In Proceedings of the IEEE Radar Conference, Rome, Italy, 26–30 May 2008; pp. 1–5. 4. Dahlman, E.; Mildh, G.; Parkvall, S.; Peisa, J.; Sachs, J.; Selen, Y.; Skold, J. 5G wireless access: Requirements and realization. IEEE Commun. Mag. 2014, 52, 42–47. [CrossRef] 5. Marcus, M.J. 5G and “IMT for 2020 and beyond” [Spectrum Policy and Regulatory Issues]. IEEE Wirel. Commun. 2015, 22, 2–3. [CrossRef] 6. Rappaport, T.S.; Sun, S.; Mayzus, R.; Zhao, H.; Azar, Y.; Wang, K.; Wong, G.N.; Schulz, J.K.; Samimi, M.; Gutierrez, F. Millimeter wave mobile communications for 5G cellular: It will work! IEEE Access 2013, 1, 335–349. [CrossRef] 7. Hong, W.; Baek, K.H.; Ko, S. Millimeter-Wave 5G Antennas for Smartphones: Overview and Experimental Demonstration. IEEE Trans. Antennas Propag. 2017, 65, 6250–6261. [CrossRef] 8. Pi, Z.; Khan, F. An introduction to millimeter-wave mobile broadband systems. IEEE Commun. Mag. 2011, 49, 101–107. [CrossRef] 9. Zhang, J.; Ge, X.; Li, Q.; Guizani, M.; Zhang, Y. 5G Millimeter-Wave Antenna Array: Design and Challenges. IEEE Wirel. Commun. 2017, 24, 106–112. [CrossRef] 10. Hong, W.; Baek, K.H.; Lee, Y.; Kim, Y.; Ko, S.T. Study and prototyping of practically large-scale mmWave antenna systems for 5G cellular devices. IEEE Commun. Mag. 2014, 52, 63–69. [CrossRef] Sensors 2018, 18, 3194 28 of 31

11. Dehos, C.; González, J.; Domenico, A.; Kténas, D.; Dussopt, L. Millimeter-wave access and backhauling: The solution to the exponential data traffic increase in 5G mobile communications systems? IEEE Commun. Mag. 2014, 52, 88–95. [CrossRef] 12. Dussopt, L.; Bouayadi, O.E.; Alberto, J.; Luna, Z.; Dehos, C.; Lamy, Y. Millimeter-Wave Antennas for Radio Access and Backhaul in 5G Heterogeneous Mobile Networks. In Proceedings of the 9th European Conference on Antennas and Propagation, Lisbon, Portugal, 12–17 April 2015; pp. 1–4. 13. Kim, Y.; Lee, H.Y.; Hwang, P.; Patro, R.K.; Lee, J.; Roh, W.; Cheun, K. Feasibility of Mobile Cellular Communications at Millimeter Wave Frequency. IEEE J. Sel. Top. Signal. Process. 2016, 10, 589–599. [CrossRef] 14. Hur, S.; Kim, T.; Love, D.J.; Krogmeier, J.V.; Thomas, T.A.; Ghosh, A. Millimeter wave beamforming for wireless backhaul and access in small cell networks. IEEE Trans. Commun. 2013, 61, 4391–4403. [CrossRef] 15. Yang, B.; Yu, Z.; Lan, J.; Zhang, R.; Zhou, J.; Hong, W. Digital Beamforming-Based Massive MIMO Transceiver for 5G Millimeter-Wave Communications. IEEE Trans. Microw. Theory Tech. 2018, 1–16. [CrossRef] 16. Helander, J.; Sjoberg, D.; Gustafsson, M.; Zhao, K.; Ying, Z. Characterization of millimeter wave phased array antennas in mobile terminal for 5G mobile system. In Proceedings of the IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Vancouver, BC, Canada, 7–8 October 2015; pp. 7–8. 17. Rajagopal, S.; Surra, S.A.; Pi, Z.; Khan, F. Antenna array design for multi-Gbps millimeter wave mobile broadband communication. In Proceedings of the Global Telecommunications Conference (GLOBECOM 2011), Houston, TX, USA, 5–9 December 2011; pp. 1–6. 18. Wang, J.J.H.; Corporation, W.E. Wideband Wide-Scan Millimeter-Wave Phased Arrays for Enhanced Security/Privacy and Performance in 5G Mobile Wireless. In Proceedings of the IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, San Diego, CA, USA, 9–14 July 2017; pp. 1471–1472. 19. Bas, C.U.; Wang, R.; Psychoudakis, D.; Henige, T.; Monroe, R.; Park, J.; Zhang, J.; Molisch, A.F. A real-time millimeter-wave phased array MIMO channel sounder. In Proceedings of the IEEE 86th Vehicular Technology Conference: VTC2017-Fall, Toronto, ON, Canada, 24–27 September 2017; pp. 1–6. 20. Niu, Y.; Li, Y.; Jin, D.; Su, L.; Vasilakos, A.V. A survey of millimeter wave communications (mmWave) for 5G: Opportunities and challenges. Wirel. Netw. 2015, 21, 2657–2676. [CrossRef] 21. Brady, J.; Hogan, J.; Sayeed, A. Multi-beam MIMO prototype for real-time multiuser communication at 28 GHz. In Proceedings of the Globecom Workshops (GC Wkshps), Washington, DC, USA, 4–8 December 2016; pp. 1–6. 22. Han, S.; Chih-Lin, I.; Xu, Z.; Rowell, C. Large-scale antenna systems with hybrid analog and digital beamforming for millimeter wave 5G. IEEE Commun. Mag. 2015, 53, 186–194. [CrossRef] 23. Alexandropoulos, G.C. Position Aided Beam Alignment for Millimeter Wave Backhaul Systems with Large Phased Arrays. In Proceedings of the IEEE 7th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing (CAMSAP), Curacao, The Netherlands, 10–13 December 2017; pp. 1–5. 24. Hong, W.; Jiang, Z.H.; Yu, C.; Zhou, J.; Chen, P.; Yu, Z.; Zhang, H.; Yang, B.; Pang, X.; Jiang, M.; et al. Multibeam Antenna Technologies for 5G Wireless Communications. IEEE Trans. Antennas Propag. 2017, 65, 6231–6249. [CrossRef] 25. Björnson, E.; Van der Perre, L.; Buzzi, S.; Larsson, E.G. Massive MIMO in Sub-6 GHz and mmWave: Physical, Practical, and Use-Case Differences. arXiv, 2018; arXiv:1803.11023. 26. Cai, M.; Laneman, J.N.; Hochwald, B. Carrier Aggregation for Phased-Array Analog Beamforming with Beam Squint. In Proceedings of the 2017 IEEE GLOBECOM, Singapore, 4–8 December 2017. 27. Abbaspour-Tamijani, A.; Sarabandi, K. An Affordable Millimeter-Wave Beam-Steerable Antenna Using Interleaved Planar Subarrays. IEEE Trans. Antennas Propag. 2003, 51, 2193–2202. [CrossRef] 28. Helander, J.; Zhao, K.; Ying, Z.; Sjoberg, D. Performance Analysis of Millimeter Wave Phased Array Antennas in Cellular Handsets. IEEE Antennas Wirel. Propag. Lett. 2015, 1225.[CrossRef] 29. Rappaport, T.S.; Gutierrez, F.; Ben-Dor, E.; Murdock, J.N.; Qiao, Y.; Tamir, J.I. Broadband millimeter-wave propagation measurements and models using adaptive-beam antennas for outdoor Urban cellular communications. IEEE Trans. Antennas Propag. 2013, 61, 1850–1859. [CrossRef] 30. Adabi, E.; Floyd, B. mm-Wave Silicon Technology: 60 GHz and Beyond; Springer Science and Business Media: New York, NY, USA, 2008; ISBN 978-0-387-76558-7. Sensors 2018, 18, 3194 29 of 31

31. Lai, C.H.; Ma, T.G. Applications to Heterogeneous Integrated Phased Arrays. In Synthesized Transmission Lines: Design, Circuit Implementation, and Phased Array Applications; John Wiley & Sons: Singapore, 2016; pp. 96–125. 32. Emami, S.; Wiser, R.F.; Ali, E.; Forbes, M.G.; Gordon, M.Q.; Guan, X.; Lo, S.; McElwee, P.T.; Parker, J.; Tani, J.R.; et al. A 60GHz CMOS phased-array transceiver pair for multi-Gb/s wireless communications. In Proceedings of the 2011 IEEE International Solid-State Circuits Conference, San Francisco, CA, USA, 20–24 February 2011; Volume 40, pp. 164–165. [CrossRef] 33. Slezak, C.; Dhananjay, A.; Rangan, S. 60 GHz Blockage Study Using Phased Arrays. In Proceedings of the 2017 Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, USA, 29 October– 1 November 2017; pp. 1655–1659. 34. Natarajan, A.; Reynolds, S.K.; Tsai, M.D.; Nicolson, S.T.; Zhan, J.H.C.; Kam, D.G.; Liu, D.; Huang, Y.L.O.; Valdes-Garcia, A.; Floyd, B.A. A fully-integrated 16-element phased-array receiver in SiGe BiCMOS for 60-GHz communications. IEEE J. Solid-State Circuits 2011, 46, 1059–1075. [CrossRef] 35. Rebeiz, G.M.; Kim, S.Y.; Inac, O.; Shin, W.; Gurbuz, O.; Ou, Y.C.; Golcuk, F.; Kanar, T.; Ku, B.H. Millimeter-wave large-scale phased-arrays for 5G systems. In Proceedings of the Microwave, MTT-S International Symposium, San Francisco, CA, USA, 17–22 May 2015; pp. 28–30. 36. Chen, Z.; Zhang, Y.P. FR4 PCB grid array antenna for millimeter-wave 5G mobile communications. In Proceedings of the Microwave Workshop Series on RF and Wireless Technologies for Biomedical and Healthcare Applications (IMWS-BIO), Singapore, 9–11 December 2013; pp. 13–15. 37. Yang, Q.L.; Ban, Y.L.; Kang, K.; Sim, C.Y.D.; Wu, G. SIW Multibeam Array for 5G Mobile Devices. IEEE Access 2016, 4, 2788–2796. [CrossRef] 38. Ala-Laurinaho, J.; Aurinsalo, J.; Karttunen, A.; Kaunisto, M.; Lamminen, A.; Nurmiharju, J.; Raisanen, A.V.; Saily, J.; Wainio, P. 2-D Beam-Steerable Integrated Lens Antenna System for 5G E-Band Access and Backhaul. IEEE Trans. Microw. Theory Tech. 2016, 64, 2244–2255. [CrossRef] 39. Jiang, M.; Chen, Z.N.; Zhang, Y.; Hong, W.; Xuan, X. Metamaterial-Based Thin Planar Lens Antenna for Spatial Beamforming and Multibeam Massive MIMO. IEEE Trans. Antennas Propag. 2017, 65, 464–472. [CrossRef] 40. Bang, J.; Choi, J. A SAR Reduced MM-Wave Beam-Steerable Array Antenna with Dual-Mode Operation for Fully Metal—Covered 5G Cellular Handsets. IEEE Antennas Wirel. Propag. Lett. 2018, 1225.[CrossRef] 41. Yu, B.; Yang, K.; Sim, C.; Yang, G. A Novel 28 GHz Beam Steering Array for 5G Mobile Device With Metallic Casing Application. IEEE Trans. Antennas Propag. 2018, 66, 462–466. [CrossRef] 42. Syrytsin, I.; Zhang, S.; Pedersen, G.F.; Ieee, S.M.; Morris, A. Compact Quad-Mode Planar Phased Array with Wideband for 5G Mobile Terminals. IEEE Trans. Antennas Propag. 2018.[CrossRef] 43. Liu, S.T.; Hsu, Y.W.; Lin, Y.C. A dual polarized cavity-backed aperture antenna for 5G mmW MIMO applications. In Proceedings of the Microwaves, Communications, Antennas and Electronic Systems (COMCAS), Tel Aviv, Israel, 2–4 November 2015; p. 4. 44. Hong, W. Compact 28 GHz Antenna Array with Full Polarization Flexibility Under Yaw, Pitch, Roll Motions. In Proceedings of the 9th European Conference on Antennas and Propagation (EuCAP), Lisbon, Portugal, 13–17 April 2015; pp. 1–3. 45. Chuang, N.C.; Lin, H.S.; Lin, Y.C. Compact cavity-backed dual-polarized aperture antennas for millimeter wave MIMO applications. In Proceedings of the Microwaves for Intelligent Mobility (ICMIM), Nagoya, Japan, 19–21 March 2017; pp. 131–134. 46. Hong, W.; Ko, S.T.; Lee, Y.; Baek, K.H. Multi-polarized antenna array configuration for mmWave 5G mobile terminals. In Proceedings of the 2015 International Workshop on Antenna Technology (iWAT), Seoul, Korea, 4–6 March 2015; pp. 60–61. 47. Hsu, Y.W.; Huang, T.C.; Lin, H.S.; Lin, Y.C. Dual-Polarized Quasi Yagi-Uda Antennas with Endfire Radiation for Millimeter-Wave MIMO Terminals. IEEE Trans. Antennas Propag. 2017, 65, 6282–6289. [CrossRef] 48. Ojaroudiparchin, N.; Shen, M.; Pedersen, G.F. Design of Vivaldi antenna array with end-fire beam steering function for 5G mobile terminals. In Proceedings of the 2015 23rd Telecommunications Forum Telfor (TELFOR), Belgrade, Serbia, 24–26 November 2015; pp. 587–590. [CrossRef] 49. Ojaroudiparchin, N.; Shen, M.; Zhang, S.; Pedersen, G.F. A Switchable 3-D-Coverage-Phased Array Antenna Package for 5G Mobile Terminals. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 1747–1750. [CrossRef] Sensors 2018, 18, 3194 30 of 31

50. Zhang, S.; Chen, X.; Syrytsin, I.; Pedersen, G.F. A Planar Switchable 3-D-Coverage Phased Array Antenna and Its User Effects for 28-GHz Mobile Terminal Applications. IEEE Trans. Antennas Propag. 2017, 65, 6413–6421. [CrossRef] 51. Alhalabi, R.A.; Rebeiz, G.M. High-gain yagi-uda antennas for millimeter-wave switched-beam systems. IEEE Trans. Antennas Propag. 2009, 57, 3672–3676. [CrossRef] 52. Garg, R.; Natarajan, A.S. A 28-GHz Low-Power Phased-Array Receiver Front-End with 360◦ RTPS Phase Shift Range. IEEE Trans. Microw. Theory Tech. 2017, 65, 4703–4714. [CrossRef] 53. Sadhu, B.; Tousi, Y.; Hallin, J.; Sahl, S.; Reynolds, S.; Renstrom, O.; Sjogren, K.; Haapalahti, O.; Mazor, N.; Bokinge, B.; et al. A 28GHz 32-element phased-array transceiver IC with concurrent dual polarized beams and 1.4-degree beam-steering resolution for 5G communication. In Proceedings of the 2017 IEEE International Solid-State Circuits Conference (ISSCC), San Francisco, CA, USA, 5–9 February 2017; pp. 128–129. 54. Kibaroglu, K.; Sayginer, M.; Rebeiz, G.M. A quad-core 28-32 GHz transmit/receive 5G phased-array IC with ﬂip-chip packaging in SiGe BiCMOS. In Proceedings of the 2017 IEEE MTT-S International Microwave Symposium (IMS), Honololu, HI, USA, 4–9 June 2017; pp. 1892–1894. [CrossRef] 55. Roh, W.; Seol, J.Y.; Park, J.; Lee, B.; Lee, J.; Kim, Y.; Cho, J.; Cheun, K.; Aryanfar, F. Millimeter-Wave Beamforming as an Enabling Technology for 5G Cellular Communications: Theoretical Feasibility and Prototype Results. IEEE Commun. Mag. 2014, 52, 106–113. [CrossRef] 56. Chang, C.K.; Liao, W.J.; Tsai, C.C. Metal Body-Integrated Open-End Slot-Antenna Designs for Handset LTE Uses. IEEE Trans. Antennas Propag. 2016, 64, 5436–5440. [CrossRef] 57. Bang, J.; Hong, Y.; Choi, J. MM-Wave Phased Array Antenna for Whole-Metal-Covered 5G Mobile Phone Applications. In Proceedings of the International Symposium on Antennas and Propagation (ISAP), Phuket, Thailand, 30 October–2 November 2017; pp. 5–6. 58. Koh, K.J.; May, J.W.; Rebeiz, G.M. A millimeter-wave (4045 GHz) 16-element phased-array transmitter in 0.18-µm SiGe BiCMOS technology. IEEE J. Solid-State Circuits 2009, 44, 1498–1509. [CrossRef] 59. Hashemi, H.; Guan, X.; Komijani, A.; Hajimiri, A. A 24-GHz SiGe phased-array receiver-LO phase-shifting approach. IEEE Trans. Microw. Theory Tech. 2005, 53, 614–625. [CrossRef] 60. Kibaroglu, K.; Sayginer, M.; Rebeiz, G.M. A Low-Cost Scalable 32-Element 28-GHz Phased Array Transceiver for 5G Communication Links Based on a Beamformer Flip-Chip Unit Cell. IEEE J. Solid-State Circuits 2018, 53, 1260–1274. [CrossRef] 61. Kim, H. A 28-GHz CMOS Direct Conversion Transceiver with Packaged 2 × 4 Antenna Array for 5G Cellular System. IEEE J. Solid-State Circuits 2018, 53, 1245–1259. [CrossRef] 62. Kim, H. A 28GHz CMOS direct conversion transceiver with packaged antenna arrays for 5G cellular system. In Proceedings of the 2017 IEEE Radio Frequency Integrated Circuits Symposium (RFIC 2017), Honolulu, HI, USA, 4–6 June 2017; pp. 69–72. 63. Dunworth, J.D. A 28GHz Bulk-CMOS dual-polarization phased-array transceiver with 24 channels for 5G user and base station equipment. In Proceedings of the IEEE International Solid-State Circuits Conference-(ISSCC), San Francisco, CA, USA, 11–15 February 2018; pp. 70–72. 64. Du, J. Dual-polarized patch array antenna package for 5G communication systems. In Proceedings of the 11th European Conference on Antennas and Propagation (EUCAP), Paris, France, 19–24 March 2017; pp. 3493–3496. 65. Sadhu, B. A 28-GHz 32-Element TRX Phased-Array IC With Concurrent Dual-Polarized Operation and Orthogonal Phase and Gain Control for 5G Communications. IEEE J. Solid-State Circuits 2017, 52, 3373–3391. [CrossRef] 66. Jeong, N.S.; Ou, Y.; Tassoudji, A.; Dunworth, J.; Koymen, O.; Raghavan, V. A recent development of antenna-in-package for 5G millimeter-wave applications. In Proceedings of the 19th annual IEEE Wireless and Microwave Technology Conference (WAMICON 2018), Sand Key, FL, USA, 4–9 April 2018; pp. 1–3. 67. Gu, X. A multilayer organic package with 64 dual-polarized antennas for 28GHz 5G communication. In Proceedings of the IEEE International Microwave Symposium (IMS2017), Honololu, HI, USA, 4–9 June 2017; pp. 1899–1901. 68. Gu, X.; Sadhu, B.; Liu, D.; Baks, C.; Valdes-Garcia, A. Antenna-in-package design and module integration for millimeter-wave communication and 5G. In Proceedings of the The 2018 International Symposium on VLSI Design, Automation and Test, Hsinchu, Taiwan, 16–19, April 2018; pp. 1–2. Sensors 2018, 18, 3194 31 of 31

69. Valdes-Garcia, A. Circuit and antenna-in-package innovations for scaled mmWave 5G phased array modules. In Proceedings of the 2018 IEEE Custom Integrated Circuits Conference (CICC), San Diego, CA, USA, 8–11 April 2018; pp. 1–8. 70. Bondyopadhyay, P.K. The ﬁrst application of array antenna. In Proceedings of the IEEE International Conference on Phased Array Systems and Technology, Dana Point, CA, USA, 21–25 May 2000; pp. 29–32. 71. Niamat, H.; Kedze, K.; Park, I. Performance of a Planar Leaky-Wave Slit Antenna for Different Values of Substrate Thickness. J. Electromagn. Eng. Sci. 2017, 17, 202–207. [CrossRef] 72. Mok, K.Y.; Rhee, Y.C.; Yoon, J.H. Design of a Pot-Shaped Monopole Antenna with Dual Band Notched Characteristics for UWB Application. J. Electromagn. Eng. Sci. 2017, 17, 44–49. [CrossRef] 73. Kwon, D.S.; Lee, S.J.; Kim, J.W.; Ahn, B.K.; Yu, J.W.; Lee, W.S. An Eight-Element Compact Low-Proﬁle Planar MIMO Antenna Using LC Resonance with High Isolation. J. Electromagn. Eng. Sci. 2016, 16, 94–97. [CrossRef]

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