Reconfigurable Reflectarrays and Array Lenses for Dynamic Antenna Beam Control: a Review 3

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. XX, NO. YY, MONTH 2013 1 Reconfigurable Reflectarrays and Array Lenses for Dynamic Antenna Beam Control: A Review Sean Victor Hum, Senior Member, IEEE, and Julien Perruisseau-Carrier, Senior Member, IEEE

Abstract—Advances in reflectarrays and array lenses with gain antenna alternatives. Recently, researchers have become electronic beam-forming capabilities are enabling a host of new interested in electronically tunable versions of reflectarrays possibilities for these high-performance, low-cost antenna archi- and array lenses to realize reconfigurable beam-forming. By tectures. This paper reviews enabling technologies and topologies of reconfigurable reflectarray and array lens designs, and surveys making the scatterers in the aperture electronically tunable a range of experimental implementations and achievements that through the introduction of discrete elements such as varactor have been made in this area in recent years. The paper describes diodes, PIN diode switches, ferro-electric devices, and MEMS the fundamental design approaches employed in realizing recon- switches within the scatterer, the surface as a whole can be figurable designs, and explores advanced capabilities of these electronically shaped to adaptively synthesize a large range of nascent architectures, such as multi-band operation, polarization manipulation, frequency agility, and amplification. Finally, the antenna patterns. At high frequencies, tunable electromagnetic paper concludes by discussing future challenges and possibilities materials such as ferro-electric films, liquid crystals, and even for these antennas. new materials such as graphene can be used to as part of Index Terms—Reconfigurable antennas, reflectarrays, reflector the construction of the reflectarray elements to achieve the antennas, array lenses, transmitarrays, lens antennas, antenna ar- same effect. This has enabled reflectarrays and array lenses rays, microstrip arrays, varactors, semiconductor diodes, micro- to become powerful beam-forming platforms in recent years electro-mechanical systems (MEMS), beam steering. that combine the best features of aperture antennas and phased arrays. They offer the simplicity and high-gain associated with I.INTRODUCTION their reflector / lens counterparts, while at the same time HE NEED for low-cost, reconfigurable antenna beam- providing fast, adaptive beam-forming capabilities of phased T forming is widespread in many existing and next- arrays using a fraction of their hardware and associated cost. generation wireless and sensing systems. High-gain pencil- They are also highly efficient, since there is no need for beam or multi-beam synthesis is paramount to many systems transmission line feed networks as in the case of phased arrays. including satellite communications, point-to-point terrestrial This paper reviews the development of reconfigurable reflec- links, deep-space communication links, and radars. Traditional tarray (RRA) and reconfigurable array lens (RAL) technology. aperture antennas such as reflectors and lenses provide a rela- While extensive and impressive advances in reflectarray and tively low-cost and straightforward solution for achieving high array lens technology have been made over the past 50 years, antenna gain. Their downside is that adaptive beam-steering this paper focuses on key experimental achievements that is only possible through the use of mechanical scanning, have been made in the area of reconfigurable variations of and adaptive beam-shaping is also similarly elusive unless these architectures, which have been primarily made in the more sophisticated feeding systems are considered. On the past decade or so. Hence, its purpose is not to provide a other hand, phased antenna arrays provide electronic flexibility review of the architectures specifically, but focus more on the in exciting the elements, allowing for reconfiguration and mechanisms and innovation by which the architectures can be scanning of the beam pattern in real time. The disadvantage realized in reconfigurable form. of phased arrays, however, is their large hardware footprint, This paper is organized as follows. It begins by discussing as each array element (or sub-array as the case may be) needs the basic operation of reflectarrays and array lenses and re- to be connected to a dedicated transceiver module leading to viewing advances in the underlying architectures in Section II. very high implementation cost. Phased arrays also diminish Then, the paper introduces the underlying technologies for en- arXiv:1308.4593v1 [physics.optics] 21 Aug 2013 in efficiency at millimeter-wave frequencies due to the use of abling reconfigurability in Section III. Section IV presents ba- transmission-line feeding networks which become increasingly sic concepts for introducing reconfigurability to reflectarrays, lossy at high frequencies. focusing on single-band, single-polarization beam-scannable Reflectarrays and array lenses are interesting hybrids be- reflectarrays. This discussion progresses to more advanced tween aperture antennas and antenna arrays. They have been concepts presented in Section V, which presents implemen- studied extensively in the past 20 years due to their attractive tations providing dual-band operation, dual-polarization capa- qualities, namely their low-profile nature, ease of manufactur- bility, frequency agility, and other unique capabilities. Recon- ing, low weight, good efficiency, and overall promise as high- figurable array lenses, and their close relation and similarity in operating principles to the reflectarray, are discussed in The authors are with the Edward S. Rogers Sr. Department of Elec- Section VI, The paper includes with a discussion of a number trical and Computer Engineering, University of Toronto, Canada, and of future challenges to the field in Section VII, to inspire the Adaptive Micro Nano Wave Systems Group, LEMA/Nanolab, Ecole´ Poltechnique Fed´ erale´ de Lausanne (EPFL), Switzerland, respectively. (email: readers about research that lies ahead. Finally, conclusions are [email protected], julien.perruisseau-carrier@epfl.ch) drawn in Section VIII. 2 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. XX, NO. YY, MONTH 2013

II.REFLECTARRAY AND ARRAY LENS BACKGROUNDAND elements must provide a large range of phases to accommodate HISTORY the geometry of the reflectarray, and the phases must be as Reflectarrays and array lenses originally evolved as indepen- linear with frequency as possible, if good bandwidths are to be dent architectures for approximating the behavior of reflector achieved. Additionally, the magnitude of the scattered wave is and lens antennas, respectively. Here, the basic history and ideally the same as that of the incident wave. Steady research operating principle of each architecture is described briefly. progress on reflectarrays has allowed design and analysis techniques for the structures to mature significantly in recent years [3]. Fast, fully vectorial analysis techniques enable one A. Reflectarray Principles and Development to predict attributes such as cross-polarization performance, The reflectarray concept was first developed by Berry in the the effect of varying angles of incidence on the elements, and 1960s, and utilized short-circuited waveguide sections to com- so on. pensate for the phase shifts needed to collimate waves from a Most reflectarray designs in the literature present a variety feed antenna into a pencil beam [1]. Interest in reflectarrays of designs for fixed reflectarrays whereby the ∆φmn terms are did not really begin in earnest, however, until planar antennas static. Linearly-polarized designs can be realized by varying (namely, microstrip patch antennas) were popularized in the the shape and size of patch elements [4], slots [5], loops [6], 1990s, which is when most advances in reflectarrays began to and other element shapes [7]. Elements can also be coupled to be made [2]. Hence, the discussions in this paper are most transmission line stubs of varying lengths to vary the scattered concerned with a planar reflectarray, which is illustrated in phase [8]. In circularly-polarized (CP) designs, both of these Figure 1(a). approaches can be utilized by acting on the scattered phase of each polarization independently. There also exists a third option for CP designs, whereby the element can be physically rotated to directly manipulate the phase shift [9][10]. Many reflectarray elements capitalize on resonances in the scatterer to achieve the large phase shift between the incident and scattered waves. Hence, the effect tends to be narrowband and much of the recent research on fixed-beam reflectarrays has been devoted to realizing broadband, or multi-band, de- signs. While a complete list would be too long to present here, approaches to achieve wideband element designs tend to focus on either coupling multiple resonances together [11]–[13] or (a) Reflectarray (b) Array lens coupling antenna elements to true time delay (TTD) lines [14]. Fig. 1. Spatially-fed array architectures Multi-band designs are also similarly achieved by stacking multiple resonators together [15], or overlaying resonators on A basic reflectarray collimates waves from a nearby feeding the same metal layer [16]. Most recently, the use of sub- antenna into a pencil beam by applying a phase correction to wavelength elements has been identified as an effective means the scattered field at each element on the reflectarray surface. for improving reflectarray bandwidth [17]. This essentially For the case of a reflectarray with a feed whose phase center makes the reflectarray look more like an artificial impedance is located at the origin O as shown, the phase of the scattered surface [18] whose localized reflection coefficient can be field from the entire reflectarray must be constant in a plane controlled over a larger bandwidth [19][20]. As we will see normal to the direction rˆ0 of the desired beam so that, in Section IV, the impedance surface concept is not dissimilar 0 ~ from modern wideband implementations of reflectarrays. k0(rmn − Rmn · rˆ0) − ∆φmn = 2πN, (1) where k is the wavenumber in free space, ~r0 is the position 0 mn B. Array Lens Principles and Development vector of the mnth element, R~ mn is a position vector of the mnth element relative to (0, 0, f), f is the focal length, ~r0 is Array lenses, also known as constrained lenses and trans- the desired direction of the pencil beam and N = 0, 1, 2,... mitarrays, were first realized by controlling the delay of A phase shift ∆φmn is introduced between the incident and an electromagnetic wave as it passed through a discrete scattered field by the mnth reflectarray element. structure [21]. They attracted significantly more interest once However, it is important to point out that reflectarrays can do planar antenna technologies were available, and waves could more than synthesize pencil beams. They are popular options be coupled to delay lines connecting the input and output array for contoured-beam synthesis as well as multi-feed systems, elements composing the array lens [22]. Microstrip elements for which more advanced design methods must be pursued. were very popular for exploring early array lenses [23], though Additionally, fast vectorial analysis techniques allow for the parallel efforts, while not strictly array lenses, were extensively prediction of cross-polarization, the effect of varying the angle investigated in the context of spatial power combiners [24]. A of incidence, and so on [3]. schematic of an array lens is shown in Figure 1(b). Most of the design effort in reflectarrays has been in real- Similar to reflectarrays, the goal of an array lens is to izing suitable fixed elements that synthesize the desired phase typically to collimate waves from a feed into a pencil beam on shift as some part of the element’s geometry is varied. These the output side of the lens. Hence, the beam-forming equation RECONFIGURABLE REFLECTARRAYS AND ARRAY LENSES FOR DYNAMIC ANTENNA BEAM CONTROL: A REVIEW 3

(1) is the same, except that the desired pencil beam appears by the increased demand for adaptability or multi-functionality on the opposite side of the surface as the reflectarray shown in radar and communication systems. As a result emerging in Figure 1(a). A key difference in the design of array lens technologies have been consolidated (e.g. MEMS) and exotic elements is that in addition to exhibiting a large phase range solutions recently introduced, such as photo-conductive [32], and low insertion loss, the element should produce low (ideally macro-mechanical [33], fluidic [34], and graphene-based [35] zero) reflection from the input side of the element. Unlike reconfiguration techniques. Table I provides an overview of reflectarrays, where the pencil beam can be potentially directed the main properties and suitability of the technologies. Other in the specular direction to minimize reflection losses, power criteria such as power handling and required control voltage is permanently lost to specular reflections in array lenses. also have to be considered in practice. It is important to Originally, array lenses were conceived as the inter-coupling emphasize that different entries in the table are not always of antenna elements on the input side of the lens to corre- independent and should be regarded as general qualitative sponding elements on the output side of the lens, as shown assessment; in practice the definition of a specific application in the inset of Figure 1(b). The simplest phasing mechanism and requirements for a specific RRA or RAL design would of array lenses is a length of transmission line chosen for the allow a more accurate selection of the optimal technology. required phase shift [23][25]. However, in principle any two- TABLE I port network can be used to provide the phase shift provided SELECTED TECHNOLOGIES FOR THE IMPLEMENTATION OF RRA AND it can be encapsulated within the array lens. RALS AND QUALITATIVE ASSESSMENT OF A FEW RELATED PROPERTIES The phasing network does not necessarily need to be a (‘+’ , ‘0’, AND ‘-’ SYMBOLSREFERTOGOOD, NEUTRAL, ANDPOOR, RESPECTIVELY). guided-wave transmission line circuit. The input and out- put antenna elements can be coupled via other microwave structures, such as slots, which can be patterned to provide a specific frequency response [26], including potentially the phase shift. Additionally, phase-shifting of circularly-polarized radiation from the feed can be accomplished using element rotation [27]. Furthermore, similar to reflectarrays, array lenses can be composed of resonant scatterers that couple together to impose the required phase shift on the incident wave [28][29]. Type Technology Maturity - reliability Integration (incl. biasing) D/A control Complexity (cost) Loss (microwave / THz) Bias power consumption Linearity p-i-n diodes + - D + -/- - 0 Lumped Essentially, the array lens becomes a nonuniform frequency Varactor diodes + - A + -/- + - elements selective surface (FSS) when realized in this way, except that RF-MEMS 0 + D1 + +/0 + + the local insertion phases of the elements become the primary Hybrid Ferro-electric 0 + A 0 0/- + 0 design objective, rather than the overall magnitude response thin film Liquid crystal 0 0 A 0 -/+ 0 0 (filtering effect) of a fully periodic FSS [30]. Tunable Graphene - + A 0 -/+ + - Examining (1), it can be readily seen that the phase shift materials Photo-conductive 0 - A? 0 -/- - - Fluidic 0 - A 0 0/+ + 0 ∆φmn could be adaptively controlled in order to provide dynamic beam-forming or beam-synthesis capabilities from 1While analog MEMS is possible, digital MEMS devices have been proven reflectarrays and array lenses alike. This potential capability to be more reliable / repeatable. in reflectarrays was identified early on in their develop- ment [31] as a significant advantage. In the next section, The solutions in Table I are classified according to whether tunable technologies that enable this reconfigurable phase shift the control is made using variable lumped element to be are presented, and the subsequent sections will provide specific embedded in the array unit cell, or via the distributed control details on how a wide variety of adaptive beam-forming of some material property. Most designs so far use lumped platforms can be realized from these technologies. elements, and in particular semiconductors elements such as p-i-n and varactor diodes [36][37]. This is mainly due to the III.ENABLING RECONFIGURATION TECHNOLOGIES maturity and availability of off-the-shelf components, but also There are various enabling technologies for the dynamic to the fact that this technology does not require advanced control of electromagnetic waves in RRAs and RALs, which fabrication facilities or expertise. To overcome the well-known differ significantly in terms of maturity, availability, perfor- limitations of such technologies, RF-MEMS technology was mance, or other characteristics such as integration and biasing employed [38]–[40], the most prominent properties of which complexity, or the suitability to a given frequency range. being very low loss up to mm-wave frequencies, virtually Therefore it is crucial to select the best technology for a given zero power consumption, high linearity, and possibility of implementation and set of requirements. Though a detailed monolithic integration. One limitation of MEMS technology review on reconfiguration technologies is beyond the scope for RRAs and RALs is that analog control generally does not of this paper, it is important here to overview the main provide sufficient reliability or temperature stability, and thus solutions available to the antenna designer and highlight their two-state digital elements are used, similar to the use of p-i-n key properties regarding RRA and RAL implementations. diodes in semiconductor technology. This implies increased There has been significant progress in the development and unit cell and biasing network complexity. Ferroelectric thin- application of reconfiguration technology platforms for anten- films have also being used to implement RRAs [41]. This nas and other microwave devices in recent years, mainly driven technology has the advantage of providing analog control in 4 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. XX, NO. YY, MONTH 2013 a monolithic fabrication process and using very low power. IV. BASIC RECONFIGURABLE REFLECTARRAY However, losses quite higher than those achievable with APPROACHES MEMS. There are three general approaches employed in the design The DC biasing network is a particularly acute issue in of basic reconfigurable reflectarrays, which are summarized in RRA and RALs, since in general each cell of the array must Figures 2(a)-(c). Here, we define a basic reflectarray design as be controlled independently, potentially resulting in thousands one operating at a single frequency on a single polarization; of control lines. Technologies offering a maximum of 1 bit more advanced designs will be considered in Section V. The control per lumped element such as p-i-n diode and most RF- majority of reflectarray designs in this category manipulate MEMS technologies will result in a larger number of biasing the phase of the scattered field from the elements by changing commands, resulting in a tradeoff between performance and characteristics of a resonator composing the elements. One of complexity when selecting the elementary phase resolution. many possible approaches is shown in Figure 2(a), whereby This issue is related to the well-known phase quantization a tunable capacitor is integrated with the resonator. Hence, effects in antenna arrays [42]: phase errors made at each if an electronically tunable phase shift is desired, a tuning element due to the finite number of available phase states mechanism can be incorporated into the resonators to make result in reduced gain and rising side lobe levels. For this this possible. It is also possible to evoke a phase shift from the reason, in large arrays it might be interesting to consider phase element by transitioning received space-waves by the element resolution of reflective elements as low as 1-bit [43]–[45]. In to guided-waves, phase-shifting the wave using a guided-wave any case, the biasing network has to be carefully designed circuit such as a transmission line stub, and then re-radiating not to affect the device and scattering performance. In this the resulting wave. This approach is shown in Figure 2(b). regard, it is important to note that advanced MEMS processes Hence, to make the phase shift dynamic electronic phase- readily include highly resistive layers allowing realizing very shifting circuits can potentially be employed in the guided- high impedance bias line transparent to the EM waves, which wave portion of the element, resulting in an antenna / phase- is extremely convenient for the biasing network design. shifter / antenna signal flow. Finally, for CP waves, electronic means for element rotation can be considered to produce the Though MEMS is becoming a mature technology and necessary phase shifts, as shown in Figure 2(c). Each of these can provide excellent properties up to V or W band, new three techniques are elaborated upon in more detail in the technologies are still needed to address the growing interest following sections. in mm-wave and THz frequencies for communication and sensing. This issue is especially relevant for RRAs and RALs, whose space-feeding is essential for reducing loss in feeding of the array element as frequency increases. In this context recently liquid crystal (LC) technology has been considered for sub-millimeter-wave frequencies [46]. It has been proposed to address upper terahertz or even infrared frequencies using RIEL AND LAURIN: DESIGN OF AN ELECTRONICALLY BEAM SCANNING REFLECTARRAY 1261

on the nonradiating side of the reflectarray has been proposed in graphene [35][47]. Interestingly, these emerging technologies [12]. The proposed element receives a linearly polarized wave, amplifies it with a field-effect transistor, adjusts its phase using a allow simple biasing via a single electrode per cell since the determined length of transmission line and retransmits the wave in the(a) orthogonal polarization. Tunable Using this element, the authors(b) of Guided- (c) Element rotation [12] realized two fixed-beam X-band amplifying reflectarrays, material properties are controlled in an analog fashion. where one of them was used as a spatial power combiner. resonator wave 1822 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 4, APRIL 2012 The authors have recently proposed a novel tunable reflec- tarray element that consists of a resonant microstrip patch aper- ture-coupled to a transmission line loaded with two varactor The geometry of a single element of the reflectarray is shown Another important aspect when comparing lumped element diodes [13]. The measured results of this element demonstrated in Fig. 1(b). This element consists of one ring slot resonator a full phase tuning range of more than 360 and improved loss and eight inductive radial stubs .Theinner performances compared to previously reported elements having and the outer radii of the ring slot are and , respectively. continuous phase tuning. Using a similar element, a complete and tunable material technologies for the design of RRA The angle between any two adjacent stubs is 45 . The length and beam scanning reflectarray has been developed. Its scanning performances as well as a loss analysis are presented in this the width of the radial stubs are and , respectively. Eight or RAL cells concerns modelling and design. In particular, paper. The breadboard used for experimental demonstration in- p-i-n diodes are respectively connected in cluded only 30 elements in order to minimize complexity and parallel to the inductive radial stubs .The the design of a lumped elements based cell can be carried cost, at the detriment of aperture efficiency. This number of ele- switching of these diodes is equivalent to the mechanical ro- ments is however sufficient to observe the beam scanning func- tation of the reflectarray element. At any moment in time, two tionality. of the diodes with angular positions that differ by 180 are re- out representing it by a multi-port scattering matrix where verse-biased to ensure their high-impedance states; meanwhile the other six diodes are forward-biased to obtain low-impedance II. DESIGN AND CHARACTERIZATION OF THE TUNABLE states for these diodes. the effect of the lumped elements is included via circuit- ELEMENT The axis is defined as the axis parallel to the two stubs The aperture-coupled tunable element is shown in Fig. 1. The with the diodes in the high-impedance state. Assume that at based post-processing. This not only allows a single full-wave patch, with dimensions of 19.5 mm 15.2 mm, was printed on Fig. 1. Reflectarray element (a) side view, not to scale (b) bottom view (dc bias a certain moment in time, diodes and are in the high- a 25- thick polyimide membrane with a relative dielectric circuits not shown). impedance state and the axis is parallel to the stubs constant of 3.4. The microstrip lines and the ground plane with Fig. 1. (a) Spiraphase-type reflectarray based on ring slot resonators with and . In this case, the linearly polarized incident wave with (d) Varactor-tuned resonator [52] (e) Tunedswitchable stub radial [53] stubs. (b) Geometry(f) ofSpiraphase a single element. [54] simulation of the cell for obtaining all the different states of the slot were printed on both sides of a 0.5-mm thick Duroid polarization plane parallel to the axis cannot excite a con- 5880 substrate (with relative dielectric constant of 2.2). The slot Ideally, the diode and the line segment implementing siderable electromagnetic fieldinthestubs and .Inaddi- the cell [48], but also allows for other interesting analyses dimensions are 15.4 mm 0.76 mm and the lines have a 50- should be lossless. To achieve a 360Forvariation more than on the three phase decades, spiraphase-type elements [4] tion, the other six stubs are short-circuited by the diodes in the impedance.Fig. 2.The patch Reconfigurable is separated from the ground plane reflectarray and of , a 180 phase approaches variation musthave be obtained been and considered for corresponding. Also, promising a spatial phase shiftersexamples for circu- low-impedance state. Thus, for the incident waveofthispo- slot by a 3-mm thick Rohacell 71 foam with a relative dielec- 180 phase difference between larlyand polarizedmust be phasedmaintained, array applications. Nevertheless, when larization, the reflectarray is equivalent to a frequency selective such as the average or maximum voltage induced on each tric constant of 1.11. The three substrates were bonded together which is done by making a quarterthe concept wavelength was longer initially than presented, the particular properties of surface (FSS) based on ring slot resonators. Therefore, when the using two 25- thick adhesive films. . The proposed element is designed for a linearly polarized the classic spiraphase element based on half-wave dipoles were perimeter of the ring slot approximately equals the wavelength In the configuration of Fig. 1, the total impedance of the incident E-field perpendicular to the slot axis. criticized for its poor transformation of the switch impedances , the FSS is transparent to this wave. When a metal screen is element [49] or some computation related to the sensitivity of transmission line seen by the slot is the series combination of A design frequency of 5.4 GHzas well was as forused its for reduced the sake frequency band [17]. Later, spiraphase- and . By varying the reverse bias voltage applied si- of demonstration. An Aeroflex-Metelics MGV-100-20 hyper- placed at a distance from the FSS, this incident wave is type arrays based on thin conductor radiators [18], [19], mi- reflected by the array with a reflection coefficient approxi- multaneously on the two diodes, the impedance terminating abrupt varactor diode was used, withcrostrip a total radiators capacitance of different configurations [20], ring slots with the cell response to faults in the lumped control devices [50]. the lines varies, thus creating a phase variation in the reflection varying from 1.7 to 0.27 pF when varies from 0 to 20 V. mately equal to one. different angular positions of metal shorts [21], and ring slot On the other hand, the linearly polarized incident wave with coefA.ficient Tunable Resonator. Using a ApproachWith this diode, the measured variedresonators over a 186 withphase switchable range radial stubs [22] were proposed. In simplified model with only one Floquet mode, it can be shown with a maximum loss of 1.1 dB [13]. To measure the reflec- polarization plane orthogonal to the axis excites a signifi- However it is worth noting here that accurate results require a recent paper [23], it was experimentally proven that the ele- that when the aperture-coupled antenna is perfectly matched, tion performances of the element, it was placed at the end of a cant electromagnetic fieldinstubs and . Then, the capac- ment described in [22] provides the optimal transformation of the microstrip lines are lossless and radiation leakage from the square waveguide of dimensions , and the wave- itance of the diodes and in the high-impedance state and rigorous correction of parasitics related to the introduction of slot canWhile be neglected, thefixed phase variation reflectarrays of the reflection co- guide was modify excited by a wave. thethe This switch resonator setup impedances is equivalent for to minimizing dimensions dissipative loss. to the inductance of the outer metal of the ring form a series reso- efficient is the same having a plane wave incident on an inInfinite this periodic paper, array we with prove the suitability of an optimized nant circuit. Thus, at the series resonant frequency, the incident the lumped port in the full-wave simulator [48][51]. Obvi- aschange the phase variation oftheir the reflection resonant coefficient affecting frequency,incidence angle andspiraphase-typein hence the H-plane, whereelement phaseis based on shift, a ring slot resonator recon- with wave of this polarization is reflected by the array with a reflec- the plane wave incident on the patch. This fact has been vali- the wavelength in free space. At 5.4switchable GHz, the corresponding radial stubs in-for circularly polarized reconfigurable tion coefficient approximately equal to 1. dated as follows. Firstly, a full-wave simulation of an infinite cidence angle is 50.3 . In the simulationsmillimeter-wave of the element, reflectarray the applications. Therefore, this reflectarray element provides different reflec- ously, this separate computation of cell response and control periodicfigurable array of the cell loaded elements with diodes was done toachieve obtain same setup this was used so using that both theSpecially simulations electronic developed and measure- methods tuning. of numerical simulation Elec- based tion coefficients and for the two linearly polarized waves as a function of . Then, was calculated by consid- ments of its reflection coefficienton correspond infinite periodic to a plane and wavefinite array approaches were used to an- with polarization planes parallel and orthogonal to the axis , ering the series combination of two 50- transmission lines, of with . In fact, the passivealyze structure the recon of thefigurable element, reflectarray. Initially, the infinite ap- respectively. Now, if one assumes that a normally-incident cir- elements is virtually impossible for technologies relying on lengthstronicand , respectively, means each of for them terminated changing by the placed on one the end of the resonant waveguide,proach was simulated was frequency applied with Ansoftto optimize the refl ofectarray patches element based cularly polarized plane wave (CPW) travels toward the array in bias-dependent impedance whose characteristics were ob- HFSS using a 3-port network includingon p-i-n one diodeport for switches the incident at 36.5 GHz and to estimate the possible the negative -direction, the electric field vector of the incident tained from measurements. The phase and magnitude of and reflected waves and two other ports with 50- termi- the distributed control of some material property, which thus have been known for a long time,scanning for sectorexample, of the reconfigurable through reflectarray. The finite the ap- wave can be expressed as follows: were then compared to those of . The results showed that the nations on the microstrip line, oneproach on each was side then of used the slot to predict in the radiation characteristics for phase variations of and follow each other very closely. the planes of and shown in Fig. 1(b). The results of these require full-wave solutions for each material state and provide use of frequency-agile patches employingthe reconfigurable varactor reflectarray. diodes [55], (1) II. PRINCIPLE OF OPERATION where is the complex magnitude of the incident wave, and fewer possibilities for advanced optimization methods. and hence the first electronically tunableAreflectarray reflectarray with spiraphase-type elements element is shown in are the unit vectors in the -and -directions, respectively, Fig. 1(a). This reflectarray contains multiple ring slot res- and is the free-space wavenumber. onators with switchable radial stubs arranged at the nodes According to [17], the electric field vector of the reflected of a rectangular grid with periodicity and along the - wave can be expressed as a sum of two CPWs that propagate in and -directions, respectively. The reflectarray elements are the positive -direction printed on a substrate with relative dielectric permittivity of and dielectric thickness of . These elements are situated at a distance over a metal screen. A feed horn is used to illuminate (2) the reflectarray and each element is configured to introduce a required phase shift to redirect the reflected wave. where is the angle between the axes and . RECONFIGURABLE REFLECTARRAYS AND ARRAY LENSES FOR DYNAMIC ANTENNA BEAM CONTROL: A REVIEW 5 was based on this frequency-agile patch design [56]. However, the scattering behavior of the element. Figure 3(a) shows the it is important to properly couple the choice of the tuning equivalent circuit for the mushroom-style AMC which realizes element to the size of the patch in order to achieve the large a parallel LC circuit because of the intrinsic inductance of the phase ranges achievable with comparable fixed elements, and patch/via combination and the fringing capacitance between this early design only achieved about 180◦ of phase range. patches. A generalized AMC composed of, for example, More phase range was achieved from this varactor-loaded floating patches can be thought of has being capacitive if patch concept by contemplating different loading schemes the elements are sub-wavelength. This leads to the equivalent for the patch [52][57] and coupling the varactor to patches circuit shown in Figure 3(b) which illustrates the equivalent of appropriate size [58]. It is also possible to use micro- capacitance of the cell placed an electrical distance βh in front electrical-mechanical systems (MEMS) varactors for the same of a short-circuit, representing the ground plane on the rear purpose [39]. Figure 2(d) shows an example of integrating of the surface. The substrate, being illuminated by a TEM varactor diodes into the structure of a patch antenna to achieve wave, acts as a transmission line, which is a typical concept phase agility. from frequency selective surfaces [66]. Finally, Figure 3(c) Essentially, these techniques can be thought of as changing shows a possible equivalent circuit of a reflectarray element, the effective electrical length of the resonator. Hence, a wide which differs from that shown in (b) because the elements variety of techniques have been contemplated to implement in a traditional reflectarray are typically comparable to a reflectarray elements based on this concept. Switches in the wavelength. Therefore, owing to the distributed nature of the form of PIN diodes and micro-electrical-mechanical systems scatterer, more sophisticated circuits are needed to represent (MEMS) have been integrated with patches to control the the reactance block X shown in the figure [57], or even other current path and corresponding resonator length [40][59][60]. circuit models entirely [36][67]. Such methods depend on modelling techniques that allow for the analysis of the effect of tunable lumped element devices on the large scale electrical scattering characteristics of the device [48][61]. In addition to using lumped element devices to effect changes in resonator lengths, more exotic techniques have also been contemplated, such as photo-induced plasmas for changing the length of slots coupled to reflectarray ele- (a) AMC (b) AIS (c) Reflectarray ments [32]. Fig. 3. Equivalent circuits of reflectarray unit cells The resonant frequency of a simple patch element also can be manipulated in a distributed fashion by varying the Impedance surfaces can be easily adapted to have a tunable dielectric constant of the substrate, which is the operating reflection phase. For example, the capacitance between the principle of reflectarray elements using dielectrics with tunable patches (which appears in Figures 3(a) and (b)) can be made properties such as liquid crystals [46][62][63]. Ferro-electric adjustable by placing a tunable capacitor such as a varactor films have also been employed for in semi-distributed ele- diode across the gaps. Tunable impedance surfaces have been ments [64][65]. demonstrated for use as plane-wave re-direction surfaces [68] Reflectarrays share many traits in common with artificial though the bias network can theoretically be reconfigured impedance surfaces (AISs). Since reflectarray elements allow for such surfaces to work as tunable reflectarrays. While the phase of the scattered field to be manipulated arbitrarily, the downside of this approach is that many more tunable setting the phase shift to be uniform across the surface changes components are needed due to the sub-wavelength size of its electrical characteristics from that of a plain conductor. For the unit cell, the reduced unit cell size also provides for example, if the phase shift it set to 0◦, then the reflectarray improved bandwidth characteristics [17] leading to potentially surface resembles the well-known artificial magnetic conduc- broadband reflectarray performance. Bandwidth-related issues tor [18], even though structurally the reflectarray element may for tunable reflectarrays are discussed in more detail in Sec- be quite different from a mushroom structure. In fact, the tion VII. main differences between a reflectarray and an AIS are: i) the dimensions of reflectarray elements are usually spaced around half a wavelength whereas in AISs the spacings tend to be B. Guided-Wave Approach smaller; ii) the dispersion characteristics of reflectarray cells Rather than controlling the resonance of a scatterer as are not usually engineered to suppress surface waves; and discussed so far, it is also possible to control the phase iii) the local phase of the reflectarray unit cells is varied in shift by a guided-wave approach, as symbolically depicted accordance with the beam to be synthesized, while AISs are in Figure 2(b). In this case the incoming space-wave is first fully periodic. coupled by an antenna to a guided-wave. The guided-wave is Equivalent circuit modelling of reflectarray unit cells also then phase shifted, and is finally re-radiated, resulting in an closely parallels those developed for AISs. Each cell of a antenna–phase-shifter–antenna topology. This technique was reflectarray element can be see as a scatterer placed within first applied to fixed-beam antennas, and then extended to a periodic (Floquet) waveguide [3]. At a specific angle of RRAs by using dynamically controllable phase shifters as incidence, an equivalent circuit can be synthesized for the discussed in the remainder of the section. The guided-wave cell and the input reflection coefficient Γ used to describe approach presents both advantages and disadvantages when 6 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. XX, NO. YY, MONTH 2013 compared to the tunable resonator technique. First, unit cells of the former type are generally easier to optimize. Indeed, while the modelling complexity of both approaches is quite similar (if the approach described in Section IV-A is used when addressing tunable resonator cells), the fact that the antenna and phase shifter can be optimized separately in the guided-wave approach results in simpler design procedure. For instance, in a digital design it is quite straightforward to achieve equi-spaced phase states in the guided-wave phase shifter, whereas doing so with a tunable resonator can require complex optimization which might still lead to sub-optimal phase distributions [48]. Another advantage of this technique is that wideband behavior is more easily obtained since simple guided-wave phase shifters can be designed to provide true- Fig. 4. Illustration of the guided-wave approach to RRA phase control: time delay capability. Further comments on bandwidth of patches elements are aperture coupled to a 1-bit delay line embedding a reflectarrays are provided in Section VII-A. p-i-n diode. Two antenna elements share the same phase shifter for reducing Though not strictly required, most reconfigurable guided- complexity [69]. wave cells are implemented in multi-layer configurations [37], which will generally increase fabrication complexity and ther- mal issues. However, in some applications it might be desirable was initially applied to the reflectarray and the associated op- to have the tuning element shielded from the antenna aperture. eration principle and derivations are well-known [9]. Here we Several RRAs or unit cells have been developed based summarize a slightly more general formulation for RRAs [74] on the guided-wave approach, a few notable examples of (the case of the lens array is available elsewhere [27]). which are briefly described here. A design using antennas Let us consider a general unit cell such that the unit cell is aperture-coupled to delay lines embedding two varactor diodes rotated an angle ψ as depicted in Figure 2(c). Assume a right- allowed achieving a continuous tuning over a 360◦ range with hand-polarized feed hence an incident right-hand CP wave maximum loss of 2.4 dB at 5.4 GHz [53]. Other authors travelling towards the cell, proposed, as previously done in usual phased array antennas, E~ inc = A(ˆa + jaˆ )ejk0z. (2) to arrange reflectarray cells into sub-arrays to reduce the x y number of control elements [69]. Gathering of the elements by It can easily be shown that the reflected field can be written pairs was implemented in a full array demonstrator of 122 sub- in the general form arrays, demonstrating the possibility of cost and complexity ref jk0z jk0z saving without significant reduction in the performance of E~ = ΓcoA(ˆax − jaˆy)e + ΓxpA(ˆax + jaˆy)e (3) the antenna. Note that a similar ‘gathering’ approach could also be used in the tunable-resonator approach, such as done with previously in a Fabry-Perot antenna [70]. 1  Γ = Γ (ψ = 0)ej2ψ = (s0 − s0 ) + js0 e+j2ψ, A large ‘guided-wave’ RRA having more than 25,000 re- co co 2 11 22 12 flecting elements was fabricated for millimeter-wave imaging (4) system operating in the 60-GHz band [37]. To manage the and complexity of this system, the unit cell for this RRA consists 1 0 0 Γxp = (s11 + s22), (5) of microstrip patch directly connected to a 1-bit reflective 2 transmission line embedding a p-i-n diode. MEMS technology where where Γco and Γxp are the co-polar and cross-polar CP has also been also considered here, and a fully-operational reflection coefficients, and the primed scattering parameters monolithic MEMS RRA at 26 GHz was designed and fabri- correspond to the fundamental Floquet harmonics of x0- and cated [71], while cells using surface mount MEMS elements y0-polarized waves in the primed coordinate of Figure 2(c). were also implemented [72]. In both cases thermal losses were Note that even for a cell whose pattern is symmetrical around 0 0 several dB despite the use of MEMS technology, which is y such as in Figure 2(c), s12 is not in fact zero since a below the performance that can be achieved using MEMS periodic arrangement of such cells along x and y is itself not technology and the tunable resonator approach [40]. Intuitively symmetrical around y0. this results from the fact that in the guided-wave approach all The principle of operation and requirements for the cells are incoming power is flowing through the tuning circuitry (i.e. the now easily deduced from (2)–(5). First, in order to suppress the phase shifter), while the tunable resonator approach is a more reflected cross-polarized field one must ensure that |Γxp| ≈ 0, distributed control mechanism where part of the scatterer is which according to (5) requires the phase of the linear- subjected to low induced currents and lower losses result. polarized reflection coefficients along x0 and y0 axis to differ by about 180◦ (assuming similar losses along both axes). In C. Rotation Technique for Circularly-Polarized Waves practice this is achieved by making the element resonate along A clever alternative to the above methods, though restricted y at the design frequency, while being weakly excited by a x- to CP, is that of the ‘rotation technique’ [73]. This principle oriented incident electric field. Once this condition is met, (3) RECONFIGURABLE REFLECTARRAYS AND ARRAY LENSES FOR DYNAMIC ANTENNA BEAM CONTROL: A REVIEW 7 and (4) show that the phase of the desired reflected circular- instance a shared aperture for widely-spaced transmit / receive polarized wave is simply twice the angular orientation ψ of frequencies, and dual-polarization as needed in many radar and the element on the surface. This is the essence of the rotation satcom applications. Additionally, flexible frequency or polar- technique: the reflected phase of the CP wave co-polarized ization can be provided for cognitive radio applications [76]. wave can be simply controlled by rotating the elementary In this context it is fundamental to remark that the RRA resonator along the reflector. (and to a certain extend the RAL as well) concept is inherently As in the case of the previous methods, the rotation tech- favorable to multi-reconfiguration when compared to standard nique has been implemented in fixed configurations [9][27], phased arrays. This is because the implementation of more but has also been proposed for dynamic phase control. In advanced control of the aperture surface comes with reduced this latter case the independent rotation of each element added complexity when compared to that needed in phased must obviously be implemented by electrical means. This was array. For instance, polarization or frequency flexibility in a proposed as early as the 1970’s by integrating diodes in a phased array would generally also require the implementation rotation-invariant geometry [73], so that selectively actuating of reconfigurable matching networks, adding significant com- some of the lumped elements implements the ‘electromagnetic plexity, loss, and power consumption. Such an issue does not rotation’ of the element. An example is illustrated in Fig- exist in RRAs since there is no need to match the elementary ure 5(a), where the rotation of a dipole is implemented [38] by cell, which by definition reflects all non-dissipated incoming switching the desired pairs of branches (slots [10] and metal energy. As a result various advanced RRA capabilities have split rings [74] have also been used). Figure 5(b) shows a been proposed recently. So far these studies essentially focused full array implementation of the concept. However, to the best on demonstrating the capability at unit cell level, and are of our knowledge, no operational full reflectarray with actual briefly commented on the remainder of this section. dynamic beam-scanning has been implemented so far, with the above examples demonstrating so-called ‘frozen’ MEMS array A. Dual-polarization Cells implementation for complexity reasons. The use of a micro- motor for implementing the rotation has also been proposed Reflectarray cells utilizing two polarizations with indepen- and implemented in a unit cell [75], but not a full array dent control of the phase of each LP component, which configuration. would allow independently scanning two LP beams have been experimentally demonstrated [77],[78]. The principle of such cells is illustrated in Figure 6(a) and (b), where a microstrip ring resonator is loaded by two varactor diodes pairs ‘A’ and ‘B’ [77]. In the case of the y-polarized incident field compo- nent, the varactors ‘B’ have no effect on the reflection phase because they are located in zeros of the current distribution, by symmetry, whereas the elements ‘A’ allow the control of the reflection phase for this polarization. In the case of the x component, the control elements ‘A’ are now in zeros of the current distribution and the reflection phase is controlled by ‘B’. The element of Figure 6(a) is of the ‘tunable resonator approach’ type described in Section IV-A. However as in Fig. 5. Reflectarray using the element rotation technique for beam-scanning the case of the single-LP cell, the dual-LP element can for CP [38]. (a) Example of elementary cell, (b) Array implementation using also be implemented using the ‘guided-wave approach’ of frozen MEMS states. Section IV-B [79] [80]. In that case perfect symmetry is difficult to achieve but cross-polarization can still be made very low. In fact, the element in [80] is more robust than V. ADVANCED CONCEPTSIN RECONFIGURABLE the initial demonstration of [77] in terms of response under REFLECTARRAYS oblique incidence. The research on reconfigurable RRAs logically first focused More recently the implementation of a reflectarray allowing on the control of a single linearly-polarized (LP) beam. the independent control of two CP beams of opposite polar- These activities confirmed that the reflectarray approach is ization but the same frequency [81]. Since such a capability an advantageous solution in electronically-controlled antenna cannot be achieved via a single-layer reflectarray, here a multi- arrays, and motivates considering more advanced capabilities layer structure must be adopted, as shown in Figure 7. The top in terms of operating frequency and polarization. Specifically, layer must be transparent to one polarization, while reflecting the idea here is to maintain dynamic local phase control for the other with the desired phase. The bottom layer can then beam-scanning/shaping, while simultaneously achieving one be simply implemented as any single-CP reflectarray. This or several additional capabilities in terms of dual-polarization, interesting concept has not been demonstrated experimentally polarization flexibility, multi-frequency, or frequency-tunable yet in a true reconfigurable mode at the time of publication, operation. Such advanced operation modes would even fur- but its implementation will come with similar possibilities and ther the interest in reconfigurable reflectarrays, providing for issues as other reflectarrays, with the additional constraint of 8 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. XX, NO. YY, MONTH 2013

90◦ phase shift step needed for conversion from LP to CP. A final important note is that high variation in the losses of the cell for different phases will strongly impact on the quality of the polarization control, hence effort must be focused on achieving similar loss in the different cell states [80].

C. Dual-band Cells Multi-band reflectarrays have been proposed in the past in fixed configurations. In general such reflectarrays are designed by implementing an ensemble of reflecting cells for each (a) Varactor- (b) Cell reflection phase along x axis when controlled unit cell varying the voltages of both pairs of diodes desired frequency, that are then arranged in a single or over for the independently ‘A’ and ‘B’, demonstrating the independent multiple layers depending on the application requirement, beam-scanning of two polarization control in particular on the relative spacing between the desired single-LP frequencies. However, multi-band operation in beam-scanning Fig. 6. Polarization reconfiguration reflectarrays has only been recently considered. A first proof- of-concept of such an operation mode was recently provided considering CP for both frequencies [74], as depicted in having as many as three layers all requiring embedded control Figure 8. This was based on the element rotation technique elements. described in Section IV-C, using MEMS to electronically implement the required rotation at each cell while preserving low loss at the operating frequencies 24 GHz and 35 GHz. Measured radiation patterns of frozen full array prototypes are also shown in the figure.

Fig. 7. Principle of the dual-CP reflectarray concept of [81]

Several applications do require dual-polarization but only with a beam common to both LP components. For instance this is the case of a line-of-sight communication between a reflectarray and a moving terminal, where the two LP components can be used as two different communication channels (so-called frequency-reuse). Such elements logically also provide an alternative to the elementary rotation principle for single-CP reflectarrays discussed in Section IV-C.

B. Polarization-flexible cells The possibility to dynamically control the polarization of the beam synthesized by a reflectarray is another very interesting Fig. 8. Principle of a reflectarray for independent beam-scanning of two CP beams at different frequencies in the millimeter wave range [74]. (a) scanning prospect for cognitive radio applications, among others. In is achieved by independently rotating slip-ring elements corresponding to each fact, cells allowing the independent control of two linear frequency using MEMS switches (b) measured switched-beams on frozen polarizations such as presented in the previous section allows prototypes at the two operation frequencies. achieving such a capability as well. Consider the unit cell of Figure 6(a) and a single LP incident jk0z field oriented such that E~i = E0(ˆex +e ˆy)e . The reflected D. Frequency-agile Reflectarray Elements −jk0z field is E~r = (ρxE0eˆx + ρyE0eˆy)e , where ρx and ρy It is well known the performance of reflectarrays in terms of are the reflection coefficients of the cell along the x and y bandwidth is limited, and techniques for wideband operation axes, respectively. Since, as explained in Section V.A, the cell are more difficult to implement in beam-scanning cells than in allows to independently control Γx and Γy, it is possible to fixed array. In this context, achieving the bandwidths required independently control both the polarization and the phase of for some applications might be very challenging. An example E~r. This principle can be used when there is at least a 2- is satellite broadcasting, with downlink / uplink bands of 10.7– bit resolution for each component, since this corresponds to a 12.75 GHz / 14.0–14.5 GHz at Ku band. RECONFIGURABLE REFLECTARRAYS AND ARRAY LENSES FOR DYNAMIC ANTENNA BEAM CONTROL: A REVIEW 9

Though the bandwidth constraint cannot be overcome by lenses, except that the output is collimated to be collected by frequency tuning if a very large instantaneous bandwidth the feed horn on the output side of the lens. is required, frequency reconfiguration is a viable option for In a reflectarray, the active device is engineered into the selectively receiving / transmitting, or for frequency-hopping unit cell such that the reflection coefficient from the unit cell systems and cognitive radio. For such a design to be useful, is greater than unity. There are two ways to achieve this, as obviously the tuning frequency range must be much wider illustrated in Figure 10. In a co-polarized reflectarray, the input than the bandwidth achievable with a single-frequency design, and output polarizations are the same, necessitating the use of depending on the requirements and implementation. In this a reflection-mode amplifier (RMA). Achieving stability in such context a reflectarray cell able to dynamically control the designs is extremely challenging, since the stability condition reflection phase at a variable frequency was recently presented is in [82]. As shown in the measured results of Figure 9, it |ΓA(ω)GA(ω)| ≤ 1 (6) achieves a continuous tuning range of more than 270◦ of phase range for any desired frequency within a range larger than where ΓA is the input reflection coefficient of the antenna 1:1.5. The principle of operation of the cell is also symboli- composing the reflectarray unit cell, and GA is the gain of the cally explained in Figure 9. The reconfigurable cell combines reflection-mode amplifier. This condition must be met over two switches and a varactor to tune the cell frequency response the entire operating frequency range of the amplifier, which in a coarse and fine manner, respectively. As a result, the can be very challenging. A more common approach is to cell can adjust the reflection phase at a variable operating utilize a cross-polarized reflectarray design where the input frequency over large and continuous phase-frequency ranges. and output polarizations are orthogonal, which employs a two- The length of the cell sections are designed so the spacing port dual-polarization antenna as the reflectarray element. This between the resonances of the four switch configuration is affords some isolation between the input and isolation, and the uniform and identical to the maximum frequency shift induced stability condition can be approximated as [84] by the varactor. |GA(ω)S12(ω)| ≤ 1 (7)

On Off On Off where GA is the gain of a two-port amplifier connecting the input and output ports of the antenna, and S12 is the coupling arg(SYY) On On Off Off between the two ports. This condition is much easier to meet [deg] and has been exploited in the design of fixed-pattern active 180º reflectarrays [85][86]. PIN diode 90º

Varactor analog phase control 0º for beam-scanning PIN diode frequency tuning range -90º

-180º (a) Co-polarized (b) Cross-polarized 1.75 2.0 2.25 2.5 2.75 3.0 3.25 3.5 Freq. [GHz] coarse tuning (switching) Fig. 10. Active reflectarray unit cell types fine tuning (reactive loading) A natural extension to these designs is to incorporate recon- Fig. 9. Measured reflection phase of frequency-reconfigurable reflectarray cell in a RWG simulator and operation principle [82]. figurability into the beam pattern. The use of active devices compensates some of the loss that might be incurred by the tuning mechanism while at the same time increasing the gain of the system and providing power combining capabilities for E. Active Reflectarrays transmitters. Though this area is still growing, there have been several cross-polarized designs employing two-port phase- Recently, there has been significant interest in integrating shifters in cascade with amplifiers connecting the two polariza- active devices in the form of amplifiers with antenna ar- tion ports of patch antennas. Experimental results employing rays for a variety of reasons, such as increasing the overall phase shifters based on IQ modulators [87] and reflection- gain of the antenna, compensating for losses, and in the mode phase shifters [84] have been recently presented. Active case of transmitters, power-combining for high EIRPs. At array lens designs have also been proposed [88], though high frequencies, especially in the millimeter-wave frequency presently beam-forming is achieved by employing multiple range, transistor sizes become very small necessitating the feeds. use of power-combining networks to achieve high output powers from power amplifiers. Losses in transmission line- based power combining networks become pronounced in this VI.ARELATED ARCHITECTURE:THE ARRAY LENS frequency range, which spurred considerable interest in spatial As discussed in Section II-B, the array lens topology is a power combiners (SPCs) in the 1990s and 2000s [83][24]. variant of a spatially-fed antenna array whereby one side of the Essentially, spatial power combiners operate similar to array array is illuminated by the feed and the radiation is produced 10 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. XX, NO. YY, MONTH 2013 on the opposite side. Reconfigurable versions of this topology ments are desired, because it simplifies the biasing control have several advantages over their reflectarray equivalents. of the array lens immensely. In this case, resonators gen- First, array lens designs are free from feed blockage effects, erally need to be separated by a significant electrical dis- which may be a consideration in small apertures. Also, in tance (e.g. one quarter-wavelength) in order for the resonators addition to far-field beam-forming, array lenses also have to produce the required phase range while maintaining an the capacity to form focal points in the vicinity of the lens acceptable reflection coefficient seen looking into the unit aperture which can be useful in applications requiring adaptive cell [93]–[96]. However, the increased electrical distance not focusing, such as microwave hyperthermia. only increases the physical thickness of the lens, it also As is the case with electronically tunable reflectarrays, introduces an inter-layer coupling mechanism which has been reconfigurable array lens designs can be grouped into similar shown to potentially lead to spurious radiation in undesired categories according to the mechanism by which phase shifting directions [95]. is achieved by the unit cell. Each of these approaches is The other option is to use an arrangement of dissimilar res- described in more detail in the following sections. onators in order to alleviate the need to separate the resonators 1) Tunable Scatterer Approach: As described earlier, a key by a large distance. For example, tunable patch resonators can difference between the unit cells in reflectarrays and array be coupled to a capacitively-tuned slot resonator to effectively lenses is that in an array lens, the phase of the wave must realized a triple-pole response using a very thin structure [90]. be manipulated with a minimum of reflection and insertion Theoretically, achieving thin tunable array lenses is possible loss, whereas in reflectarrays a strong reflection is generally by closely coupling together capacitive and inductive surfaces guaranteed because of the use of a ground plane. Intrinsic to to form a tunable FSS [89], but would require tunable capaci- this process is also the fact that the wave interacts with the tances on the capacitive surface (straightforward to implement) scatterer twice during its transit from the feed to the aperture and tunable inductors (more challenging to implement) on the plane, meaning that a single-pole resonator is all that is inductive surface to achieve best overall performance. The required to produce nearly 360◦ of phase shift in a reflectarray. main disadvantage of an approach using dissimilar layers the This contrasts significantly with the situation in an array lens. layers need to be tuned separately each other in order to Resonators the incoming wave interacts with can be seen as form the right pole trajectories to maximize the phase range introducing a single pole response into the transfer function while minimizing the insertion loss through the unit cell. This modelling the input/output characteristic of a unit cell, as is can complicate the biasing and control of such surfaces, and well known in the field of frequency selective surfaces. Hence, combined with the need for thin array lenses, has motivated multi-pole designs have been widely employed to tailor the research on the next approach. response of FSSs using resonators of different types [26], or 2) Guided-Wave Approach: In this approach, the array by coupling layers of inductive and capacitive elements [89], to elements composing the input of the array lens are connected achieve a desired magnitude response for filtering applications. to the array elements composing the output of the array However, the adaptation of resonators to tunable surfaces has lens via a two-port guided-wave network. In reconfigurable several important implications on the design of RAL unit cells. designs, this network must be electronically tunable, and, as Considering the pole/zero behavior in the complex plane we have seen in Section V-E, can potentially incorporate gain is highly useful in understanding the design of array lens as well. elements based on tunable resonators [90]. Assuming a res- Only a handful of reconfigurable array lenses of this type onator pole can be arbitrarily manipulated, the insertion loss have been experimentally demonstrated. Borrowing the ter- and phase are dictated by the distance from, and the angle minology of SPCs, a “tray” approach can be taken whereby made with, the pole to the operating frequency point in the phase-shifting circuits are integrated with the input and output complex frequency plane, respectively. At a fixed operating faces of the lens in a three-dimensional manner [97], with the frequency, in order for the insertion magnitude of the unit cell primary drawback being a thick structure that is more difficult to remain constant, the pole must be manipulated such that it to manufacture. Other approaches tend towards “tile” forms of moves in a circular arc in the left-hand plane around the center integration, employing designs that use varactor diode-tuned frequency. Achieving such an ideal trajectory is impossible in bridged-T phase shifters [95][98] or MEMS switches to adjust most designs. Furthermore, this discussion illustrates that a the delay through a bandpass structure [99]. single pole is capable of contributing, at most, up to 180◦ While research on tunable array lenses is still in its infancy, of phase shift in the transfer function. Early tunable array the potentially thin nature and bandwidth of the guided-wave lens designs employing only a single-pole response hence approach makes it an attractive topology. Figure 11 shows a achieved very low phase ranges [91]. Hence a minimum of recent example of an experimental prototype exhibiting a 10% two, and preferably three or more, resonators are required to fractional bandwidth at 5 GHz. meet the phase requirements of beam-forming. As a result, 3) Element Rotation Approach: Reconfigurable array lenses even fixed array designs tend to require multi-layer structures for handling CP waves are still at an early developmental stage. of resonators [29], unless one settles for 1-bit (0◦ / 180◦) Fixed CP array lenses have been explored that incur the phase phase-shifting which can simplify the cell somewhat [92]. shift by manipulating the LP components of a CP wave [27] In a similar way, in RALs, designs achieving the required or by employing the element rotation technique [100]–[102]. levels of phase agility have been accomplished using different However, to our knowledge, array lenses employing this types of resonators. In many cases, identical resonator ele- approach have yet to be realized in reconfigurable form. RECONFIGURABLE REFLECTARRAYS AND ARRAY LENSES FOR DYNAMIC ANTENNA BEAM CONTROL: A REVIEW 11

The desired TTD behavior can be easily achieved using the guided-wave approach. Fixed designs employing aperture- coupled microstrip delay lines [105] have been shown to substantially improve the bandwidth of reflectarrays to the 10% range. As discussed earlier, this approach can be adapted to provide beam-steering [106]. However, since the amount of time delay that can be created within the space constraints of the cell is limited, and the bandwidth of such element is Fig. 11. Experimental array lens [95] also conditional to the wideband matching of the resonating element with the phase shifter. For array lenses, the bandwidth similarly depends on their VII.ONGOINGAND FUTURE CHALLENGES implementation. Classical array lenses were based on transmis- sion lines connecting the input and output elements. However, A. Bandwidth extension and transformation optics approaches array lenses based on resonant FSS structures have smaller A well-known limitation of reflectarrays and array lenses is bandwidths, though this situation has been alleviated through their limited operating bandwidth, and this is currently a very the use of miniaturized element FSSs (MEFSSs) [89]. active area of research. Bandwidth limitations fundamentally Similar to reflectarrays, wideband RALs must employ either originate from the fact that in order to achieve ideal band- wideband phase shifters coupled to wideband elements, or re- width characteristics, the array elements must produce a true configurable TTD structures. As wideband element and phase time delay (TTD) response. Most reflectarray and array lens shifter designs are widespread in the literature, ultimately elements can only approximate such a response over a narrow the bandwidth limitations stem from issues arising in the bandwidth. Addressing this limitation for RRAs and RALs is compact integration of the elements with the phase shifters particularly challenging, for reasons that are outlined below. in the former approach [95]. Regarding the latter approach, In the case of reflectarrays, bandwidth constraints are usu- again, switched structures, which trade off bit resolution for ally alleviated by employing one of two approaches. The bandwidth, attempt to implement TTD structures for enhanced first is to increase the phase bandwidth of the elements by bandwidth [99]. attempting to approximate the TTD response over a limited Ways to further improve the bandwidth of spatially-fed band. This can be achieved by employing multi-resonant ele- apertures is an area of active research, with particularly ments, such as stacked patches [12] and concentric loops [13] promising solutions arising from the field of transformation to name a few popular techniques. These approaches have optics (TO) [107]. In the synthesis of wideband apertures using been applied with success in fixed reflectarrays, improving a TO approach, the desired field transformation (e.g. from bandwidth from around 3% in the case of single-resonant el- a spherical feed to a plane-wave pencil-beam) is defined ements to around 12% in the case of multi-resonant elements. spatially in one spatial domain, and the metric-invariant prop- The challenge with adapting this to reconfigurable designs is erty of Maxwell’s equations can be employed to achieve that these element designs employed coupled resonators to the same wave propagation in another spatial domain filled improve the bandwidth of the element. By varying the size with a region of inhomogeneous dielectrics. In the case of and shape of the individual resonators, one not only changes reflectarrays, this region is a cover that is placed over a flat the resonant frequency of each constituent resonator, but also reflector (ground plane) composing the reflector, while in array the inter-resonator coupling. In electronically tunable variants, lenses, the region defines the lens itself. Conceptually, the the resonator frequency can be readily controlled through case in Figure 12 illustrates the transformation between the integration with tunable components, as we have seen in warped space (virtual space) and the regular space (physical Section IV-A, but the inter-resonator electromagnetic coupling space) succinctly. Essentially, the material cover implements is not affected significantly by the tuning because the geometry a spatially-distributed TTD system to ensure all rays from of the resonator remains fixed. Hence, more sophisticated the feed are delayed appropriately by the aperture. Moreover, tuning techniques, that also employing tunable devices to since the transformation does not yield large zones of materials vary the inter-resonator coupling, are required to improve the exhibiting unusual electromagnetic properties (e.g. a refractive bandwidth of multi-resonant element designs [103]. Using this index less than 1), the dispersion associated with such materi- technique, the element phase bandwidth can be effectively als is avoided yielding a potentially very broadband transfor- multiplied by the number of resonators employed in the unit mation device. This approach has been used to successfully cell. design flat reflectors [108] and lenses [109][110] in principle. Resonators composing the element can also be designed to Researchers have contemplated beam-steerable be co-planar, which tends to reduce the coupling effect and versions [111] provided the electromagnetic properties allow the individual resonances of each element to play a of the dielectric region can be manipulated. Effective material larger role in controlling the bandwidth. Unit cells composed implementations based on metallic conductors have recently of three parallel dipole resonators situated over a tunable liquid been investigated as a means for realizing the dielectric crystal substrate have been proposed [104] and experimentally region, while providing reflectors with very large potential verified [46] as a means for extending the bandwidth of LC beam-scanning ranges [112]. A similar implementation cells to 8%. can be pursued for array lenses. Such conductors can be 12 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. XX, NO. YY, MONTH 2013

C. Very large apertures realized using compound apertures focal point focal point Realizing very large, high-gain apertures composed of re- configurable unit cells, while theoretically possible, poses many practical constraints in terms of the number of devices required (and associated cost), bias network complexities, and

T in some cases, device power requirements. However, many TO reflector high-gain cost-effective solutions can be realized by combin- PEC ing adaptive spatially-fed arrays with fixed apertures. The most common of these is a parabolic or flat main reflector (which (a) Virtual space (b) Physical space itself could be realized as a fixed reflectarray) illuminated Fig. 12. Transformation between virtual (a) and physical spaces (b) in the by a sub-reflector composed of a reflectarray. Such dual- TO approach reflector combinations can be used to emulate their traditional counterparts, such as the Cassegrain antennas [114] and offset reflectors [115]. In particular, employing a reconfigurable sub- loaded with tunable components to yield ultra-wideband reflectarray (or array lens) is an effective way to manage implementations of spatially-fed apertures, which may benefit the cost of the antenna system, since the primary large-area many applications in the future. A disadvantage of this aperture is not reconfigurable. The tradeoff is that the overall approach is that the dielectric region tends to be quite thick in scanning range of the antenna system is reduced, often to just order to facilitate the field transformation, creating interesting a few degrees depending on the system geometry. However, challenges for future research. many applications do not require large scan ranges, such as atmospheric limb-sounding, certain satellite applications, B. Mitigating Nonlinear Behavior etc. Liquid-crystal reflectarrays have successfully been inte- A major challenge facing designers of electronically tunable grated as sub-reflectors into dual-reflector systems for such reflectarrays and array lenses is the linearity of the underlying purposes [116], and extensions of this technique to other tuning technology. Table I summarizes the technologies and configurations may be a practical way forward in realizing shows their corresponding linearity. RRAs and RALs are very high gain reconfigurable apertures in the future. being proposed for satellite and RADAR applications, which employ transmitters with very high output powers. Hence, the illumination of the aperture may induce nonlinear behaviour D. Towards Terahertz and Optical Frequencies in the underlying technologies, leading to harmonic and inter- The application of the reflectarray and lens-array concepts to modulation distortion (IMD). In the area of communication higher frequencies has recently attracted significant attention. systems, particularly satellite communications, there are strict For instance, at optical frequencies fixed configurations have limitations on the allowable adjacent channel interference and been proposed using metals in the plasmonic regime [117]– harmonic levels produced by the transmitter. Even passive [120] and lower-loss dielectric scatterers [121]. Though higher inter-modulation is a source of compliance failure in these operation frequencies obviously entail significant novel prac- transmitters, indicating that ultra-linear tuning technologies tical considerations, the operating principle is essentially the must be employed should reconfigurable apertures find prac- same as in prior art at lower frequencies. tical use in these applications. Dynamic beam control is now also considered at higher Semiconductor technologies, while mature and widespread, frequencies. Potential applications at terahertz frequencies are are the most prone to this problem. Varactor diodes inte- numerous both for sensing and communication, where the grated in the aperture, for example, can easily have their reflectarray and lens-array concept should provide a low-loss capacitance modulated at the frequency of the illuminating and relatively simple solution for electronic beam control. signal, causing phase modulation that manifests itself in terms At optical frequencies, applications mostly relate to sensing of distortion of the scattered signal. For example, the IMD but flexible free-space interconnects or even visible light performance of a single-pole reflectarray element has been communication could be of interest in the future. evaluated in a waveguide simulator [36], illustrating that even Early attempts to beam-scanning at particularly high fre- under modest illumination power levels, the creation of odd- quencies include the on-going effort to produce a MEMS- order distortion is significant for varactor diode-tuned elements based reflectarray at 120 GHz [122]. However, such frequen- which ultimately may relegate such apertures to receive-only cies can be considered as an upper bound for the applicability applications. Researchers are keenly aware of this and efforts of standard RF-MEMS technology, among others, because the to document the linearity of new designs is undertaken in most MEMS elements become too electrical large to efficiently be recent studies [92]. integrated within the array unit cell. Therefore, it is particularly Nevertheless, addressing this challenge remains a funda- relevant to consider enabling technologies based on reconfig- mental concern for designers. Ultimately, MEMS technology urable materials. For instance, there have been experimental may be the solution to this problem [39][113]. Other exotic demonstrations of reflectarray unit cells using LC crystals, materials possessing large relaxation times, such as liquid based on the principle that the LC anisotropic permittivity crystals, may also be contenders as the underlying technologies tensor is modified by an applied bias field [46]. Though this mature. demonstration was done at sub-millimeter-wave frequencies, RECONFIGURABLE REFLECTARRAYS AND ARRAY LENSES FOR DYNAMIC ANTENNA BEAM CONTROL: A REVIEW 13 the use of LC has proven very efficient at optical frequencies [7] M. Bozzi, S. Germani, and L. 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