Recent Developments and Challenges on Beam Steering Characteristics of Reconﬁgurable Transmitarray Antennas

: This paper highlights recent developments and challenges on beam steering characteristics of reconﬁgurable transmitarray antennas. It introduces the operating principle of beam form-ing/beam steering high gain transmitarray antennas to enable the user to opt for economical and high performance solutions. A transmitarray antenna typically consists of a source antenna and a phase transformation structure. The incident waves generated from the source antenna is tilted using the phase transformation structure in a desired direction to steer the beam. Moreover, the phase transformation structure alters the incident wavefront to a plane wavefront using phase change characteristics. In order to steer a beam to a speciﬁc desired angle, it can be divided into two methods. There is a method of applying a transmitarray with a variable transmission phase change or a method of changing the shape of the wavefront of the source antenna. This type of beam forming/beam steering high gain antenna has been mainly studied from the point of view of high efﬁciency, low proﬁle, and low cost. Several solutions of transmitarray unit cells have been presented in the literature, using PIN diodes, varactors, MEMS switches, and microﬂuids enable electronics to realize reconﬁgurable characteristics of transmitarray antennas. This paper analyzes the characteristics of various beam steering high gain reconﬁgurable transmitarrays (RTA) and highlights the future opportunities and challenges of the structure design for transmitarray antennas. This paper also highlights the challenges and gaps in terahertz and optical frequencies related to future work due to the structure complexity and lack of components’ availability. Moreover, the challenges and limitations related to multi-bit structures and dual-band requirements are presented.


Introduction
The high gain beam steering antennas are widely used in 5G wireless mobile communications, radio frequency (RF) wireless power transmission, and satellite communications [1][2][3]. In order to obtain high gain and beam steering characteristics, traditionally, antennas of the same type as phased array antennas, reflectarray antennas, and parabolic antennas have been designed [4][5][6]. However, these types of antennas have some disadvantages despite their excellent characteristics since the parabolic antenna and the dielectric lens antenna have a curved shape (i.e., non-planar structures), are expensive, and difficult to manufacture. In addition, a planar phased array antenna with high gain characteristics in the millimeter-wave band may have a complex feeding structure with losses that might influence the antenna's overall radio performance.
They are feasible for phase integration, their small size and low biasing complexity at lower to higher frequencies. Due to their advantage of being able to vary the transmission phase change freely, most of the beam forming/beam steering reconfigurable transmitarrays are designed by the former method. However, there is a disadvantage of the overall radio performance of the transmitarray antenna that may deteriorate due to its complexity of implementation or the additional loss of active elements. The latter method structurally changes the wavefront shape of the source antenna electrically or mechanically and has the advantage of structurally fixing the TA. However, there is a disadvantage of a large motor for arrays as mentioned in [44][45][46][47][48].
Significant research has been conducted in recent years [49][50][51][52][53]. A transmitarray antenna has been implemented with a source antenna (horn, microstrip patch antenna) and a planar array. The working principle of a transmitarray antenna uses a high transmittance and a transmission phase change characteristic of more than 360 degrees to control the wavefront of the source antenna. This type of transmitarray antenna is a high gain antenna and is beam forming, beam steering, and multi-beam [54][55][56]. It is designed as a beam control system such as PIN diodes, varactors, MEMS systems, and microfluids. These reconfigurable devices can be used in many applications. In order to perform beam forming/beam steering in the desired direction, it can be roughly divided into two methods. The total wavefront of the transmitarray antenna can be calculated as the sum of the wavefront of the source antenna and the wavefront of the plane wave passing through the transmitarray. This point is used to implement beam steering by a transmitarray with variable transmission phase change or when the wavefront shape of the source antenna is applied.
This paper is written in the following structure. Section 2 describes the beam forming/beam steering principle and characteristic analysis of a transmission antenna using a TA, and its relation to the aperture efficiency, which is an important performance indicator of the transmitarray antenna. In Section 3, the operating principle and results of various types of reconfigurable transmitarrays are presented, focusing on operation and devices that provide dual-band operation, dual-polarization capability, frequency agility, and other unique features. Section 4 discusses the future opportunities and challenges in the structure and characteristics of various beam steering high gains reconfigurable transmitarrays (RTAs). Future challenges and the scope of beam steering transmitarrays are discussed in Section 5. Finally, the conclusion is presented in Section 6.

Beam Steering/Beam Forming Principle of Transmitarray
The transmitarray antenna is an antenna that is capable of modifying the radiation pattern of a directional antenna such as a patch antenna or a horn antenna. A transmitarray has the capability of adjusting the wavefront of the source antenna and the incident wave. By using a structure that can adjust the size and phase of the incident wave, the transmission antenna can be used in various fields such as beam steering, beam focusing, and polarization control. Figure 1 shows a typical radiation model of a transmission antenna composed of a source antenna and a transmitarray. A transmitarray is formed by a feed, which is labeled as a source antenna where the source antenna is placed at a distance F from transmitarray. Focal distance (F) is calculated as a tradeoff between spillover losses and an increase in gain, aperture efficiency, and other parameters. The source antenna illuminated by incident wave to the first part of the array is called the receiver. The receiver is directly connected to group of phase shifters to control the beam wave front in the required direction. The other end of the phase shifters are coupled with a transmitter layer, which produces a phase shift in the incident wave that works in transmitter mode. The phase shifter of each unit cell provides γn path difference to the source point to obtain the desired scan angle θ. As shown in Figure 1, the direction of the incident wave is determined by radiation controlled by the transmission phase of the transmitarray. Transmitarrays are generally composed of several resonant unit cells with a spatial periodicity that forms a planar arrangement. the desired scan angle θ. As shown in Figure 1, the direction of the incident wave is determined by radiation controlled by the transmission phase of the transmitarray. Transmitarrays are generally composed of several resonant unit cells with a spatial periodicity that forms a planar arrangement. The principle of beam forming/beam steering transmitarrays is reported in [57][58][59][60][61][62]. There are incident EM waves generated from a feed source that passes through a transmitarray made up of Nth (n = 0, 1, …) elements of periodicity of "a" and produces phase change φn shown in Equation (1). φn = k0n Δs+ φ0 (1) where k ∘ = is the propagation constant of the free space-waves. Δs is the path difference between the nth and (n + 1th) elements after they pass through the transmitarray. Δs is defined here for the deflection angle θ of the transmitted wave with element dimensions a that is the path difference with the deflection angle Δs = a.sinθ. Now we can write the phase change as φn = φn+1 − φn.
Equations (2) and (3) demonstrate that if we define the deflection angle θ, we can calculate the phase change Δφ from Equation (2). Thus, both equations show inter-dependent relations for each other [63]. Similarly, the design and implementation of beam switching in arrays using a butler matrix have also been reported [64][65][66].
Transmitarray unit cells are usually designed based on microstrip patches, meta-materials [67,68], and frequency selective surfaces (FSS) [69,70]. The microstrip patch metasurface of the unit cell and its permeation characteristics are shown in [71]. The unit cell comprises of four dielectrics and five circular metal patches, and the equivalent circuit model can be expressed by series inductance and parallel capacitance in the passband. The transmission characteristics of the metasurface can be controlled by the radius (a) of the metal circular patch and the transmittance and transmission phase of the circular microstrip patch array transmitarray at 5.8 GHz. In this case, the size (W) of the square unit The principle of beam forming/beam steering transmitarrays is reported in [57][58][59][60][61][62]. There are incident EM waves generated from a feed source that passes through a transmitarray made up of nth (n = 0, 1, . . . ) elements of periodicity of "a" and produces phase change ϕ n shown in Equation (1).
where k • = 2π λ 0 is the propagation constant of the free space-waves. ∆s is the path difference between the nth and (n + 1th) elements after they pass through the transmitarray. ∆s is defined here for the deflection angle θ of the transmitted wave with element dimensions a that is the path difference with the deflection angle ∆s = a.sinθ. Now we can write the phase change as ϕ n = ϕ n+1 − ϕ n .
Equations (2) and (3) demonstrate that if we define the deflection angle θ, we can calculate the phase change ∆ϕ from Equation (2). Thus, both equations show inter-dependent relations for each other [63]. Similarly, the design and implementation of beam switching in arrays using a butler matrix have also been reported [64][65][66].
Transmitarray unit cells are usually designed based on microstrip patches, metamaterials [67,68], and frequency selective surfaces (FSS) [69,70]. The microstrip patch metasurface of the unit cell and its permeation characteristics are shown in [71]. The unit cell comprises of four dielectrics and five circular metal patches, and the equivalent circuit model can be expressed by series inductance and parallel capacitance in the passband. The transmission characteristics of the metasurface can be controlled by the radius (a) of the metal circular patch and the transmittance and transmission phase of the circular microstrip patch array transmitarray at 5.8 GHz. In this case, the size (W) of the square unit cell is 20 mm (0.38 λ 0 ), the dielectric constant (εr) and thickness (h) of the dielectric substrate are 2.2 and 3.2 mm, respectively. From 1 mm to 9.1 mm, a transmittance of 0.8 or more and a change in transmission phase of about 400 degrees can be obtained. A high gain transmission antenna using a metasurface composed of 10 × 10 circular patch array unit cells is presented. The total size of the transmitarray is 200 mm (about 4 λ 0 ), and the distance between the source antenna and the metasurface is 50 mm (about 1 λ 0 ). The total wavefront of the transmitarray antenna can be conceptually expressed as the sum of the wavefront of the source antenna and the transmission phase of the transmitarray, as illustrated in Figure 2. When the focal source antenna and transmitarray antenna are fixed for phase implementing high gain and shape of the wavefront, a straight line is demonstrated in Figure 2a. The focal source antenna is fixed, whereas phase change is implemented by an active transmitarray in Figure 2b. A high gain variable phase change is achieved in this scenario by changing the wavefront of the active transmitarray. It is illustrated in Figure 3 that the wavefront of the transmitarray antenna is fixed through the change in the wavefront shape of the source antenna using the phase transformation surface that helps to implement the change of phase and achieve the high gain steering transmitarray. Aperture efficiency is an important performance indicator of a transmitarray antenna. Aperture efficiency (η a ) is defined in terms of the taper efficiency (η t ), spillover efficiency (η s ), polarization efficiency (η pol ), transmission efficiency (η tran ), phase efficiency (η ph ), and random surface as in Equation (4). It can be calculated as the product of the error efficiency (η r ) [72].
Here, the efficiencies that mainly affect the aperture efficiency of the transmitarray antenna are the taper efficiency and the spillover efficiency. These two efficiencies are determined by the size and spacing of the source antenna and the metasurface. Taper and spillover efficiencies are related to the uniformity and amount of power reaching the metasurface from the source antenna, respectively, and are calculated by Equations (5) and (6).
where S and P rad are the area of the metasurface and the radiation power of the source antenna, respectively. It is vital to determine the source antenna, the distance between the source antennas and the plane array (F), and the size (D) of the transmitarray to optimize the aperture efficiency of the transmitted antenna using the TA. When the distance between the source antenna and the plane array is short, the spillover efficiency is high but the taper efficiency is low. Conversely, if F becomes longer, taper efficiency is high but spillover efficiency is lower. In addition, as the size of the transmitarray increases, the spillover efficiency is high but the taper efficiency decreases. Therefore, to maximize the aperture efficiency of the transmit antenna, it is necessary to optimize F/D. Maximum numerical aperture efficiency and the optimum gain of the source antenna following various F/D are shown in [42]. In this case the gain of the source antenna has become relatively low, the maximum value of the aperture efficiency decreases, but the distance F with maximum efficiency is reduced and a low-profile transmitarray antenna can be designed. In addition, a source antenna with a high gain may be applied to secure the maximum aperture efficiency, for example, when a patch antenna having a gain of 7.5 dBi is applied to a transmission antenna as a source antenna, the maximum aperture efficiency is about 62%, and F/D can be designed to 0.2.  When the distance between the source antenna and the plane array is short, the spillover efficiency is high but the taper efficiency is low. Conversely, if F becomes longer, taper efficiency is high but spillover efficiency is lower. In addition, as the size of the transmitarray increases, the spillover efficiency is high but the taper efficiency decreases. Therefore, to maximize the aperture efficiency of the transmit antenna, it is necessary to optimize F/D. Maximum numerical aperture efficiency and the optimum gain of the   When the distance between the source antenna and the plane array is short, the spillover efficiency is high but the taper efficiency is low. Conversely, if F becomes longer, taper efficiency is high but spillover efficiency is lower. In addition, as the size of the transmitarray increases, the spillover efficiency is high but the taper efficiency decreases. Therefore, to maximize the aperture efficiency of the transmit antenna, it is necessary to optimize F/D. Maximum numerical aperture efficiency and the optimum gain of the

Beam Steering/Beam Forming Using Reconfigurable Components
Several studies are available in the literature [73][74][75][76][77][78][79][80] to demonstrate the operations and functions of transmitarray beam steering. These are realised by utilizing different reconfigurable devices, materials, element designs, and operational parameters. However, every structure needs a requisite to obtain the beam steering/beam forming of the transmitarray antenna. Each transmitarray unit cell must acquire a transmission phase that can be Electronics 2022, 11, 587 7 of 24 tuned (varied) up to 360 degrees. Moreover, the transmission coefficient should remain constant throughout the operational bandwidth. This section focuses on demonstrating the electronically reconfigurable beam of transmitarray, transmission phase distribution on the transmitarray surface, and the phase shift of each unit cell. Reconfigurable beam steering can be achieved with different techniques. The most prominent one is electronic control accomplished, which is achieved by loading one or more active devices into the resonant elements designed on the unit cell such as (PIN diodes, varactors, and MEMS switches). Other techniques are on the utilization of tunable materials (liquid crystal, microfluidic systems). There is a detail Summary of beam steering reconfigurable transmitarrays with PIN diodes is presents in Table 1.

PIN Diodes
The author of [73] proposed a 2-BIT electronically reconfigurable beam steering transmitarray unit cell. A reconfigurable transmitarray unit cell is composed of six metal layers. The transmitter patch has an O shape slot, and two PIN diodes are mounted to the microstrip patch element. The receiver patch has etched on another O slot patch connected to a transmitter patch with a metallic via that transfer electromagnetic energy to the transmitter. This unit cell achieves 10.1-12.1% 3 dB transmission bandwidth. This paper is further extended in [74,75] to validate the array parameters of this technique. There are two transmitarrays of 14 × 14 and 20 × 20, reported as switchable circular and linear polarization. These are 1-bit and 2-bit phase resolution variable (tunable) unit cells. There are two and four PIN diodes mounted that allow transmission phase control for each design, respectively. The 1-bit prototype reconfigurable unit cell's measured gain is 20.8 dBi with a 3 dB transmission bandwidth of 14.6% at 29 GHz. The 2-bit design has a measured gain of 19.8 dBi with a 3 dB bandwidth of 16.2% at 29 GHz.
Moreover, a novel reconfigurable Yagi-Vivaldi transmitarray structure was reported in [76,77] for beam steering of the transmitarray at Ku-band. This reconfigurable transmitarray (RTA) primarily focused on a wide bandwidth and beam scanning at Ku-band. The RTA unit cell contains a tightly coupled microstrip. A Vivaldi antenna based on the transmitter and a Yagi antenna structure etched on it behaves as a transmitter. A pair of anti-parallel diodes are mounted on a slot-line to the microstrip phase shifter to obtain 180 • phase shift with the current reversal mechanism shown in Figure 4. It achieves 14% 1 dB bandwidth of a reconfigurable transmitarray with peak gain 22.3 dBi at 13.6 GHz. The RTA realized 25.6% aperture efficiency with ±60 degrees scan angle for both E-plane and H-plane. Two novel linearly polarized reconfigurable design transmitarrays have been investigated with PIN diode at Ku-band in [78,79]. An asymmetric dipole element PIN diode-based reconfigurable transmitarray is presented in [79]. This novel design has been investigated as a 1-bit dual-band linearly polarized reconfigurable transmitarray at Ku-band, as shown in Figure 5. This element has an active dipole for the receiver and an asymmetric passive dipole for the transmitter 1-bit transmission phase shift, obtained by integrating two diodes on an active patch in the reverse direction with the current reversal mechanism. To reduce the impact of diode resistance and improve insertion loss, two parasitic bypass dipoles alongside the main patches can be added. The dual-polarization parameter is realized by orthogonal interconnecting for the transmitter and receiver. A 10 × 10 elements array is designed, manufactured, and tested. It achieves measured aperture efficiency of 22.6% with a peak gain of 18.3 dB at 12.2 GHz. The 2D beam steering feature correspondence realized in a test environment with a scanning angle covers ±50 degrees. The maximum measured E-Plane and H-plane losses are 2.9 dB and 3.5 dB, respectively, as shown in Figure 5c. asitic bypass dipoles alongside the main patches can be added. The dual-polarization parameter is realized by orthogonal interconnecting for the transmitter and receiver. A 10 × 10 elements array is designed, manufactured, and tested. It achieves measured aperture efficiency of 22.6% with a peak gain of 18.3 dB at 12.2 GHz. The 2D beam steering feature correspondence realized in a test environment with a scanning angle covers ±50 degrees. The maximum measured E-Plane and H-plane losses are 2.9 dB and 3.5 dB, respectively, as shown in Figure 5c. Another novel C-band design of a transmitarray is discussed in [80], and some other applications of this array are explored in [80][81][82]. A C-band square ring patch with proximity coupled U-shape resonator transmitarray is proposed. A prototype of a 1-bit beam steering 10 × 10 transmitarray with performance is presented. A prototype of a square ring patch with a proximity coupled U-shape resonator with good/bad effects is also demonstrated. The dual-polarized beam scanning effect is experimentally verified in the test asitic bypass dipoles alongside the main patches can be added. The dual-polarization parameter is realized by orthogonal interconnecting for the transmitter and receiver. A 10 × 10 elements array is designed, manufactured, and tested. It achieves measured aperture efficiency of 22.6% with a peak gain of 18.3 dB at 12.2 GHz. The 2D beam steering feature correspondence realized in a test environment with a scanning angle covers ±50 degrees. The maximum measured E-Plane and H-plane losses are 2.9 dB and 3.5 dB, respectively, as shown in Figure 5c. Another novel C-band design of a transmitarray is discussed in [80], and some other applications of this array are explored in [80][81][82]. A C-band square ring patch with proximity coupled U-shape resonator transmitarray is proposed. A prototype of a 1-bit beam steering 10 × 10 transmitarray with performance is presented. A prototype of a square ring patch with a proximity coupled U-shape resonator with good/bad effects is also demonstrated. The dual-polarized beam scanning effect is experimentally verified in the test Another novel C-band design of a transmitarray is discussed in [80], and some other applications of this array are explored in [80][81][82]. A C-band square ring patch with proximity coupled U-shape resonator transmitarray is proposed. A prototype of a 1-bit beam steering 10 × 10 transmitarray with performance is presented. A prototype of a square ring patch with a proximity coupled U-shape resonator with good/bad effects is also demonstrated. The dual-polarized beam scanning effect is experimentally verified in the test setup. The maximum measured gain of the proposed transmitarray is 12 dBi, corresponding to maximum scan loss that reaches 2 dB with a scan angle of ±30 degrees.
Furthermore, a novel architecture of a reconfigurable coupling slot transmitarray is mentioned in [83]: a prototype of a 1-bit electronically reconfigurable transmitarray for Ku-band. Two orthogonal H-shape slots were integrated on patches that act as transmitter and receiver, respectively. Electromagnetic energy was transmitted by coupling a transmit line in between slot patches. Diodes were mounted on the coupling line to realize the electronic phase control in an anti-parallel direction to produce 180 • phase difference. The oblique incidence performance of the elements was maintained by sub-wavelength element spacing λ 0 /3. A 16 × 16 element 12.5 GHz reconfigurable transmitarray was designed and tested. The maximum gain of 17.0 dBi corresponds to an aperture efficiency of 14.0% presented in experimental results. The H-shape coupling slot transmitarray realizes a beam scanning angle within ±50 degrees for E-and H-planes. The 3 dB gain bandwidth retains 9.6% in the measured results. This work is further improved in [84]: a 16 × 16 elements reconfigurable transmitarray is proposed in this paper. The transmitter is designed as a C-shape feed probe placed beneath a rectangular patch to obtain the feathers of broad bandwidth and lower insertion loss. The receiver of the transmitarray is a U-shaped slot etched on a simple rectangular patch. To induce a reverse current path, two PIN diodes are symmetrically placed on the feed probe of the unit cell shown in Figure 6. The minimum measured insertion loss is realized at 0.47 dB for each phase state with 16% of 3 dB broad bandwidth achieved by stepped impedance matching performance. There are 256 elements in the feed shape probe transmitarray with 512 PIN diodes used in its manufacturing and testing. The maximum peak gain is 22.1 dBi at 12.5 GHz with an aperture efficiency of 21.2%. The 3 dB insertion bandwidth is realized at 12.3% with electronic beam scanning performance of ±60 degrees for two-dimensional H-and E-planes, respectively. This novel reconfigurable transmitarray is a good solution for many applications in wireless communication systems.
Another advanced technique structure of a transmitarray is reported in [85] with a polarization rotation unit cell. Two split circles are connected with two narrow strips with pin diodes mounted in between them. Polarization control structures sandwich the PIN diode loaded with a transmitter and receiver. The air gap is presented between the transmitter and receiver to mount diodes on patches easily, and bias circuits are etched on the polarization control structure. The phase difference 180 • is achieved for two outgoing polarized waves for 0 and 180 degree phase states. The polarization control structure reduces the blocking effect of dc-bias lines and improves the aperture efficiency of the structure. A 16 × 16 1-bit transmitarray is designed, fabricated, and tested in a test setup. Two-dimensional electronic beams scanning of ±40 degrees is realized by the polarization rotation element. The maximum measure 3 dB and 1 dB insertion loss of 45% and 17% are obtained, respectively, at 5.5 GHz. The aperture efficiency of the polarization rotation transmitarray is 18.4%.
patch. To induce a reverse current path, two PIN diodes are symmetrically placed on the feed probe of the unit cell shown in Figure 6. The minimum measured insertion loss is realized at 0.47 dB for each phase state with 16% of 3 dB broad bandwidth achieved by stepped impedance matching performance. There are 256 elements in the feed shape probe transmitarray with 512 PIN diodes used in its manufacturing and testing. The maximum peak gain is 22.1 dBi at 12.5 GHz with an aperture efficiency of 21.2%. The 3 dB insertion bandwidth is realized at 12.3% with electronic beam scanning performance of ±60 degrees for two-dimensional H-and E-planes, respectively. This novel reconfigurable transmitarray is a good solution for many applications in wireless communication systems.
(a) (b) Another advanced technique structure of a transmitarray is reported in [85] with a polarization rotation unit cell. Two split circles are connected with two narrow strips with pin diodes mounted in between them. Polarization control structures sandwich the PIN diode loaded with a transmitter and receiver. The air gap is presented between the transmitter and receiver to mount diodes on patches easily, and bias circuits are etched on the polarization control structure. The phase difference 180° is achieved for two outgoing polarized waves for 0 and 180 degree phase states. The polarization control structure reduces the blocking effect of dc-bias lines and improves the aperture efficiency of the structure. In [86], the authors have presented a 10 × 10 elements 1-bit reconfigurable transmitarray (RTA) with linear polarization at Ku-band. This transmitarray has the ability of beam steering for a wide range of frequencies. The reported unit cell consists of multiple-layer circular rings and substrates. Every element of the RTA has the capability to electronically control phase shift path difference for 0 and 180 states. The 3 dB gain bandwidth of the structure lies at 33% (12.5 GHz-17.5 GHz) at 15 GHz frequency. The effective aperture efficiency of the annular rings RTA is achieved up to 33.64% at 15 GHz. From 14 GHz to 17 GHz, two-dimensional beam steering with ±60 degrees may be realized.

Varactor Diodes
The literature review also presents the reconfigurable transmitarray (RTA) made up of varactors to accomplish beam forming/beam steering [87][88][89][90][91][92]. Varactor diodes are the best solution for continuous tuning by using electronic switches. Varactor diodes are mature technology; they are economical, have low insertion losses, can be modeled easily, and have voltage control switches. There are a wide range of diodes that can be used related to applications shown in Figure 7. It is important to choose the varactor diode that offers the best performance for many applications. A varactor diode capacitance ratio is defined as C max /C in , meaning a large capacitance ratio results in a wide range of tuning capacitance. There is a detail Summary of beam steering reconfigurable transmitarrays with varactor diodes shown in Table 2.
A varactor diode reconfigurable transmitarray (VD-RTA) is presented in [93,94] for beam steering performance. Each element of the transmitarray is composed of four substrate Rogger RO4350B double sided layers, shown in Figure 7. One pair can be tuned for 180 • phase shift control, and a whole pair of unit cells duplicated to earn 360 • phase shift. The electronic phase control of the element is obtained by loading each layer with varactor diodes. A theoretical 3 dB bandwidth of the unit cell is 1 GHz at 24.6, as mentioned in this paper, but it reveals that the insertion loss of this paper achieves −5 dB. Experimental results show that 3 dB insertion losses are quite high up to −12 dB for this transmitarray. Furthermore, this work is extended in [94] and an array of 6 × 6 elements of the abovementioned transmitarray is fabricated. This RTA has the experimental performance of beam scanning capability ±50 degrees at 24.6 GHz in both the H-plane and E-plane.
The electronic phase control of the element is obtained by loading each layer with varactor diodes. A theoretical 3 dB bandwidth of the unit cell is 1 GHz at 24.6, as mentioned in this paper, but it reveals that the insertion loss of this paper achieves −5 dB. Experimental results show that 3 dB insertion losses are quite high up to −12 dB for this transmitarray. Furthermore, this work is extended in [94] and an array of 6 × 6 elements of the abovementioned transmitarray is fabricated. This RTA has the experimental performance of beam scanning capability ±50 degrees at 24.6 GHz in both the H-plane and E-plane. Another novel varactor diode and pin diode base phase shifter transmitarray design is reported in [95]. A wideband varactor diode and pin diode base phase shifter reconfigurable transmitter gives continuous phase shift control at 5.8 GHz. This multilayer element structure transmitter (Tx) comprises an H-slot rectangular patch integrated with two diodes. A varactor diode is mounted on the receiver layer to build a 180° analog phase shifter combined with pin diodes to acquire 360° electronic phase shift. Theoretical transmission insertion loss includes 0.85 dB to 1.24 dB at 5.8 GHz frequency. A 3 dB bandwidth is obtained up to 17% of the required frequency. A 16 × 16 elements prototype of the transmitarray is designed, fabricated, and measured result tested. A maximum peak gain of 23.7 dBi at 5.8 GHz is achieved using the varactor diode-based phase shifter transmitarray. The 2D transmitarray beam scanning performances are maintained for ±60 degrees elevation and azimuth planes.
In addition, a frequency selective surface (FSS) varactor diode reconfigurable transmitarray with 2D beam scanning ability is reported in [96]. Each layer of the FSS loaded with a varactor diode enables beam scanning of the main beam of the original radiation pattern in both the E-plane and H-plane. A five-layer 5 × 5 FSS transmitarray integrated with varactor diodes coupled with a horn antenna feed network has been verified with theoretical and experimental results. Two simulated and fabricated models are developed with a peak gain of 20.2 dBi and 19.9 dBi, respectively, at 5.8 GHz frequency. Experimental results of 2D beam steering are realized approximately ±30 degrees in both the azimuth and elevation planes. The proposed RTA shows the potential for use in applications with better economical fabrication cost compared to the most direct alternative beam steering approaches.
A detailed case study has been presented in [97] of the practical constraints of twodimensional beam steering using a varactor diode-based meta-material at micro and mmwave frequencies. The practical impacts of the varactor diode utilized with a meta-material-based RTA are deeply observed, as shown in Figure 8. Various results of the internal resistance, capacitance, and inductance have been compared in the reported paper. Another novel varactor diode and pin diode base phase shifter transmitarray design is reported in [95]. A wideband varactor diode and pin diode base phase shifter reconfigurable transmitter gives continuous phase shift control at 5.8 GHz. This multilayer element structure transmitter (Tx) comprises an H-slot rectangular patch integrated with two diodes. A varactor diode is mounted on the receiver layer to build a 180 • analog phase shifter combined with pin diodes to acquire 360 • electronic phase shift. Theoretical transmission insertion loss includes 0.85 dB to 1.24 dB at 5.8 GHz frequency. A 3 dB bandwidth is obtained up to 17% of the required frequency. A 16 × 16 elements prototype of the transmitarray is designed, fabricated, and measured result tested. A maximum peak gain of 23.7 dBi at 5.8 GHz is achieved using the varactor diode-based phase shifter transmitarray. The 2D transmitarray beam scanning performances are maintained for ±60 degrees elevation and azimuth planes.
In addition, a frequency selective surface (FSS) varactor diode reconfigurable transmitarray with 2D beam scanning ability is reported in [96]. Each layer of the FSS loaded with a varactor diode enables beam scanning of the main beam of the original radiation pattern in both the E-plane and H-plane. A five-layer 5 × 5 FSS transmitarray integrated with varactor diodes coupled with a horn antenna feed network has been verified with theoretical and experimental results. Two simulated and fabricated models are developed with a peak gain of 20.2 dBi and 19.9 dBi, respectively, at 5.8 GHz frequency. Experimental results of 2D beam steering are realized approximately ±30 degrees in both the azimuth and elevation planes. The proposed RTA shows the potential for use in applications with better economical fabrication cost compared to the most direct alternative beam steering approaches.
A detailed case study has been presented in [97] of the practical constraints of twodimensional beam steering using a varactor diode-based meta-material at micro and mmwave frequencies. The practical impacts of the varactor diode utilized with a meta-materialbased RTA are deeply observed, as shown in Figure 8. Various results of the internal resistance, capacitance, and inductance have been compared in the reported paper.
Four prototypes are developed on different frequencies, including 5, 14, 28, and 60 GHz, to understand the effect of internal resistance Rs and internal inductance Ls of varactor diodes. It is concluded from the above study that internal resistance Rs have various impacts on RTAs. On the other hand, internal inductance Ls affects the proposed TA's bandwidth and operational frequency band response. As internal inductance Ls increases, higher frequencies shift towards lower frequencies, increasing the bandwidth of the reported elements. These result in a reduction in the overall phase shift accomplished by multilayers of unit cells that may have the impact of reducing the scanning range of TAs to perform beam steering with a transmitarray.  Four prototypes are developed on different frequencies, including 5, 14, 28, and 60 GHz, to understand the effect of internal resistance Rs and internal inductance Ls of varactor diodes. It is concluded from the above study that internal resistance Rs have various impacts on RTAs. On the other hand, internal inductance Ls affects the proposed TA's bandwidth and operational frequency band response. As internal inductance Ls increases, higher frequencies shift towards lower frequencies, increasing the bandwidth of the reported elements. These result in a reduction in the overall phase shift accomplished by multilayers of unit cells that may have the impact of reducing the scanning range of TAs to perform beam steering with a transmitarray.

MEMS Switches
In the past decade, dynamic progress and development have been seen for reconfig-

MEMS Switches
In the past decade, dynamic progress and development have been seen for reconfigurable devices in antenna and array applications. In radar and communications system applications, reconfigurable components' demand has increased day by day due to the multi-functionality and versatile adaptability of array systems. As a result, emerging technologies (such as MEMS) have been developed, and innovative solutions have recently been introduced. MEMS are an excellent solution to realize beam forming/beam steering characteristics in reconfigurable transmitarrays (RTAs). A detail Summary of beam steering reconfigurable transmitarrays with MEMS in Table 3.
These are miniature elements of structure created with semiconductor technology. They have remarkable advantages over other reconfigurable devices, such as being small in size, low power usage, and high-level integration. Several MEMS switches reconfigurable transmitarrays have been reported in [98][99][100][101][102][103][104] the literature for beam steering.  [34]. A four-metal layer unit cell with two identical substrates was used in the proposed unit cell. The O-slot patch acts as a transmitter (Tx), and U-slot patch behaves as a receiver (Rx) etched on RO4003 substrate connected with a metal probe. The electronic phase shift control is obtained by MEMS switches with series bias via two bias lines printed on the substrates. The proposed element retains low insertion losses of 0.8 dB and wide 3 dB bandwidth of 16%. A bandpass filter reconfigurable frequency selective surface (FSS) was reported in [100] using MEMS switches, as shown in Figure 9. A four-legged bridge was mounted with RF-MEMS switches and DC-bias lines to control the ON/OFF mechanism of the switches. There are 909 elements and 3636 RF-MEMS switch bridges integrated to control the phase shift for the beam steering of the array. MEMS switches FSS transmitarray elements gain 3.2 GHz, 3 dB bandwidth at 30 GHz, and 99.9% accuracy is achieved during the experimental test verified for the proposed array.   [34]. A four-metal layer unit cell with two identical substrates was used in the proposed unit cell. The O-slot patch acts as a transmitter (Tx), and U-slot patch behaves as a receiver (Rx) etched on RO4003 substrate connected with a metal probe. The electronic phase shift control is obtained by MEMS switches with series bias via two bias lines printed on the substrates. The proposed element retains low insertion losses of 0.8 dB and wide 3 dB bandwidth of 16%. A bandpass filter reconfigurable frequency selective surface (FSS) was reported in [100] using MEMS switches, as shown in Figure 9. A four-legged bridge was mounted with RF-MEMS switches and DC-bias lines to control the ON/OFF mechanism of the switches. There are 909 elements and 3636 RF-MEMS switch bridges integrated to control the phase shift for the beam steering of the array. MEMS switches FSS transmitarray elements gain 3.2 GHz, 3 dB bandwidth at 30 GHz, and 99.9% accuracy is achieved during the experimental test verified for the proposed array. A 2-bit programmable MEMS, antenna filter antenna (AFA), and millimeter-wave frequency reconfigurable lens-array are presented in [104]. A AFA lens-Array integrated with 1021 switches was fabricated on two quartz wafer substrates. It consisted of three metal layers, bound with an adhesive and integrated with two orthogonally oriented slots. This array explores various characteristics of design, fabrication, and test performance. A 2-bit programmable MEMS switches mechanism provides four possible states of phase shifts, including 0°, 90°, 180°, and 270°. Moreover, the 2-bit phase resolution reduces the directivity of the transmitarray due to phase quantization loss. The proposed prototype A 2-bit programmable MEMS, antenna filter antenna (AFA), and millimeter-wave frequency reconfigurable lens-array are presented in [104]. A AFA lens-Array integrated with 1021 switches was fabricated on two quartz wafer substrates. It consisted of three metal layers, bound with an adhesive and integrated with two orthogonally oriented slots. This array explores various characteristics of design, fabrication, and test performance. A 2-bit programmable MEMS switches mechanism provides four possible states of phase shifts, including 0 • , 90 • , 180 • , and 270 • . Moreover, the 2-bit phase resolution reduces the directivity of the transmitarray due to phase quantization loss. The proposed prototype shows an insertion loss that includes 1.4 dB to 1.6 dB for different phase states. AFA 2D beam scanning realized ±60 degrees for the azimuth and elevation planes at 32 GHz.

Microfluids
In the modern era, adaptability and versatility are fascinating characteristics of advanced communication systems. Microfluids have exotic parameters, flexible flow properties, and acquiescent elastic limits, and are mechanically flexible compared to copper or other metallic materials. These feathers are also helpful for innovative fabrication technologies such as 3D antennas, spraying antennas, and fluid conductive antennas. Compared to traditional high speed switching methods, beam steering reconfigurable antennas function with the reconfigurability of microfluid liquids related to electronic pumping and electro-mechanically controlled capillaries. Several articles for reconfigurable microfluid transmitarray beam steering are demonstrated in the preceding literature [105][106][107][108]. Additionally, a circularly polarized microfluid-based transmitarray antenna unit cell is reported in [108]. The reconfigurable unit cell was manufactured with two split rings slots produced so that microfluid can pass through this channel, as shown in Figure 10. Microfluid passes through these split slot channels to obtain control on the phase shift beam steering. Microfluid circulates through the symmetric circular rings of elements and rotates through the circular slots causing liquid displacement to be controlled and the phase shift to double the rotation angle. The circulation of microfluid passing through the circular rings delivers continuous 0-360 degrees of phase shift. A microfluid transmitarray prototype is manufactured at glass substrate, and a slot channel is etched by Polydimethyl-siloxane (PDMS) at 8.8 GHz frequency. The expected insertion loss includes 2-3 dB, reaching up to 1 dB liquid rotation. The proposed element insertion losses become remarkably high −10 dB with testation of waveguide characteristic setup. On the contrary, it takes time to reconfigure itself in the active mode, which reduces its effectiveness for applications. Each unit cell acquires many slot tubes to circulate microfluid inside the elements to gain reconfigurable characteristics. This feature will lead to an increase in the unit cell sizes to utilize the slot tubes.
shows an insertion loss that includes 1.4 dB to 1.6 dB for different phase states. AFA 2D beam scanning realized ±60 degrees for the azimuth and elevation planes at 32 GHz.

Microfluids
In the modern era, adaptability and versatility are fascinating characteristics of advanced communication systems. Microfluids have exotic parameters, flexible flow properties, and acquiescent elastic limits, and are mechanically flexible compared to copper or other metallic materials. These feathers are also helpful for innovative fabrication technologies such as 3D antennas, spraying antennas, and fluid conductive antennas. Compared to traditional high speed switching methods, beam steering reconfigurable antennas function with the reconfigurability of microfluid liquids related to electronic pumping and electro-mechanically controlled capillaries. Several articles for reconfigurable microfluid transmitarray beam steering are demonstrated in the preceding literature [105][106][107][108]. Additionally, a circularly polarized microfluid-based transmitarray antenna unit cell is reported in [108]. The reconfigurable unit cell was manufactured with two split rings slots produced so that microfluid can pass through this channel, as shown in Figure 10. Microfluid passes through these split slot channels to obtain control on the phase shift beam steering. Microfluid circulates through the symmetric circular rings of elements and rotates through the circular slots causing liquid displacement to be controlled and the phase shift to double the rotation angle. The circulation of microfluid passing through the circular rings delivers continuous 0-360 degrees of phase shift. A microfluid transmitarray prototype is manufactured at glass substrate, and a slot channel is etched by Polydimethylsiloxane (PDMS) at 8.8 GHz frequency. The expected insertion loss includes 2-3 dB, reaching up to 1 dB liquid rotation. The proposed element insertion losses become remarkably high −10 dB with testation of waveguide characteristic setup. On the contrary, it takes time to reconfigure itself in the active mode, which reduces its effectiveness for applications. Each unit cell acquires many slot tubes to circulate microfluid inside the elements to gain reconfigurable characteristics. This feature will lead to an increase in the unit cell sizes to utilize the slot tubes. Moreover, a plasma reconfigurable transmitarray/reflectarray antenna is introduced in [109]. The radiating incident waves with a source feed antenna are placed in different directions. A 13 × 13 elements array composed of 169 elements with reconfigurable plasma as a transmitarray/reflectarray antenna is demonstrated. The reflectarray antenna mode is exhibited due to the plasma ground plane acting as a perfect conductor and collision frequency of atoms 1 THz. Contrarily, when the plasma ground plane is not ionized, it behaves as a dielectric material to perform the transmitarray antenna mode. The phase shift of the array is controlled by argon gas density injected into symmetrical unit cells. Each element consists of a two cylindrical tubes setup on the top and bottom and a ground plate filled with argon (Arg) gas. DC-bias voltage is applied to the end of the cylinder. The Moreover, a plasma reconfigurable transmitarray/reflectarray antenna is introduced in [109]. The radiating incident waves with a source feed antenna are placed in different directions. A 13 × 13 elements array composed of 169 elements with reconfigurable plasma as a transmitarray/reflectarray antenna is demonstrated. The reflectarray antenna mode is exhibited due to the plasma ground plane acting as a perfect conductor and collision frequency of atoms 1 THz. Contrarily, when the plasma ground plane is not ionized, it behaves as a dielectric material to perform the transmitarray antenna mode. The phase shift of the array is controlled by argon gas density injected into symmetrical unit cells. Each element consists of a two cylindrical tubes setup on the top and bottom and a ground plate filled with argon (Ar g ) gas. DC-bias voltage is applied to the end of the cylinder. The frequency of plasma argon gas (Ar g ) is varied, leading to variation in the reflection coefficient from 0-305 degrees corresponding to the maximum reflection coefficient that reaches up to −8 dB. The continuous transmission phase shift control executes from 0-345 degrees. Moreover, the remaining phase control compensates due to plasma frequency variation. Plasma transmitarray/reflectarray is accomplished with a 2D beam scanning ability of ±30 degrees for the H-plane and E-Plane at 19.75 GHz and 19.39 GHz, respectively.

Beam Steering/Beam Forming Using Variable Source Antenna
The beam steering is acquired by changing the position of the primary feed source or using an electronically variable source antenna. To enhance the performance of beam steering, the concept of multi-focal transmitarray is introduced. Generally, two identical feed sources are placed where the phase error becomes minimal. The focal point is 33 degrees for a beam direction provided in bifocal transmitarray. There are some studies reported in the literature that demonstrate beam steering using variable source antennas. The early investigation of variable source antenna beam steering is demonstrated in [110,111]. Reference [47] presents a beam steering transmission antenna using an electrically variable source antenna. A novel variable source beam scanning transmitarray presents as shown in Figure 11. There is a 3D dielectric printing transmitarray illustrated on paper with a taper section to enhance the bandwidth of the transmitarray. A 50 × 50 elements transmitarray with variable source beam scanning capabilities and wide bandwidth transmitarray is validated with measured and experimental results. The measured radiation performance shows 1 dB bandwidth over 23% achieved at 30 GHz. The radiation performance of ±27 degrees is obtained in the E-plane at 30 GHz with variable source mechanisms. frequency of plasma argon gas (Arg) is varied, leading to variation in the reflection coefficient from 0-305 degrees corresponding to the maximum reflection coefficient that reaches up to −8 dB. The continuous transmission phase shift control executes from 0-345 degrees. Moreover, the remaining phase control compensates due to plasma frequency variation. Plasma transmitarray/reflectarray is accomplished with a 2D beam scanning ability of ±30 degrees for the H-plane and E-Plane at 19.75 GHz and 19.39 GHz, respectively.

Beam Steering/Beam Forming Using Variable Source Antenna
The beam steering is acquired by changing the position of the primary feed source or using an electronically variable source antenna. To enhance the performance of beam steering, the concept of multi-focal transmitarray is introduced. Generally, two identical feed sources are placed where the phase error becomes minimal. The focal point is 33 degrees for a beam direction provided in bifocal transmitarray. There are some studies reported in the literature that demonstrate beam steering using variable source antennas. The early investigation of variable source antenna beam steering is demonstrated in [110,111]. Reference [47] presents a beam steering transmission antenna using an electrically variable source antenna. A novel variable source beam scanning transmitarray presents as shown in Figure 11. There is a 3D dielectric printing transmitarray illustrated on paper with a taper section to enhance the bandwidth of the transmitarray. A 50 × 50 elements transmitarray with variable source beam scanning capabilities and wide bandwidth transmitarray is validated with measured and experimental results. The measured radiation performance shows 1 dB bandwidth over 23% achieved at 30 GHz. The radiation performance of ±27 degrees is obtained in the E-plane at 30 GHz with variable source mechanisms. Figure 11. (a) Proposed mechanism of beam scanning transmitarray; (b) actual implementation of variable source transmitarray (reprinted with permission of [47]).
There is a method to steer the beam by the structurally fixed transmitarray and the switching of the source antenna. Since the wavefront of the source antenna is variable, as described in Figure 3a the beam steering transmission antenna is realized by the combination with the transmission phase of the transmitarray. Five source antennas are selectively operated by applying two SP3T switching elements. Although the beam steering characteristics can be implemented relatively simply, there are also disadvantages of a limited beam steering angle and discretization. To improve such a drawback, a study was conducted to realize a continuous beam steering angle by operating two source antennas and adjusting the position and of each phase [48]. The combination of a two-dimensional metasurface and a planar source antenna capable of one dimensional phase control improves the gain of the existing one dimensional phased array antenna.
Moreover, an automotive radar application of a dual-layer 77 GHz printed circuit board transmitarray is presented [49]. There are four SIW antennas used as the primary feed source. Coplanar unit cells are linked with vias and printed on the opposite side of There is a method to steer the beam by the structurally fixed transmitarray and the switching of the source antenna. Since the wavefront of the source antenna is variable, as described in Figure 3a the beam steering transmission antenna is realized by the combination with the transmission phase of the transmitarray. Five source antennas are selectively operated by applying two SP3T switching elements. Although the beam steering characteristics can be implemented relatively simply, there are also disadvantages of a limited beam steering angle and discretization. To improve such a drawback, a study was conducted to realize a continuous beam steering angle by operating two source antennas and adjusting the position and of each phase [48]. The combination of a two-dimensional metasurface and a planar source antenna capable of one dimensional phase control improves the gain of the existing one dimensional phased array antenna.
Moreover, an automotive radar application of a dual-layer 77 GHz printed circuit board transmitarray is presented [49]. There are four SIW antennas used as the primary feed source. Coplanar unit cells are linked with vias and printed on the opposite side of the PCB. The coplanar unit cell was designed with concentric rings to gain a 1-bit phase shift of the transmitarray. Four substrate integrated waveguides were used as source antennas to obtain four beams to obtain a beam scanning range over ±15 degrees throughout the desired bandwidth. The transmitarray generates 18.5 dB of gain at 76.5 GHz, with good agreement in theoretical and experimental results. Alternatively, in [112], an optically transparent transmitarray is presented with a focal source (horn antenna) replaced with a 2 × 2 unit cells aperture couple stack microstrip patch antenna of a seven element array as shown in Figure 12. Beam steering is obtained by switching the position of seven 2 × 2 arrays to the next one. The choice of an aperture couple stack microstrip patch array over a horn antenna creates the transparent transmitarray's low profile and light weight. The proposed meshed double ring unit cells achieve a reduced transmission loss and slight dependence on the angle of incidence at the Ka-band. The unit cells contribute to a phase shift of 300 • with 1 dB reduced insertion loss at 28.5 GHz. Theoretical and measured results exhibit beam-wide scanning angles attaining ±30 degrees in the azimuth plane at 28.5 GHz. The maximum gain of the transparent transmitarray is 19.5 dBi, corresponding to the minimum SLL < −12.4 dB, ranging from 27.5 GHz to 29.5 GHz. This work is a remarkable candidate for reducing the visual impact for 5G communication systems and smart beam steering technology. the PCB. The coplanar unit cell was designed with concentric rings to gain a 1-bit phase shift of the transmitarray. Four substrate integrated waveguides were used as source antennas to obtain four beams to obtain a beam scanning range over ±15 degrees throughout the desired bandwidth. The transmitarray generates 18.5 dB of gain at 76.5 GHz, with good agreement in theoretical and experimental results. Alternatively, in [112], an optically transparent transmitarray is presented with a focal source (horn antenna) replaced with a 2 × 2 unit cells aperture couple stack microstrip patch antenna of a seven element array as shown in Figure 12. Beam steering is obtained by switching the position of seven 2 × 2 arrays to the next one. The choice of an aperture couple stack microstrip patch array over a horn antenna creates the transparent transmitarray's low profile and light weight. The proposed meshed double ring unit cells achieve a reduced transmission loss and slight dependence on the angle of incidence at the Ka-band. The unit cells contribute to a phase shift of 300° with 1 dB reduced insertion loss at 28.5 GHz. Theoretical and measured results exhibit beam-wide scanning angles attaining ±30 degrees in the azimuth plane at 28.5 GHz. The maximum gain of the transparent transmitarray is 19.5 dBi, corresponding to the minimum SLL < −12.4 dB, ranging from 27.5 GHz to 29.5 GHz. This work is a remarkable candidate for reducing the visual impact for 5G communication systems and smart beam steering technology. A planar multi-focal source transmitarray antenna beam scanning performance is explored in [113]. The proposed planar multi-focal transmitarray antenna pros are described in this article over the traditional transmitarray. A quad-focal phase shift distribution was studied by applying the particle swarm optimization technique for ±60 degrees beam scanning range for Ku-band transmitarray. The findings of this study show that a multifocal transmitarray antenna can be an excellent low-cost option for high gain beam forming applications. In normal conditions, multi-focal transmitarray constraints give an extra degree of freedom that cannot be achieved without variations in the lattice geometry or transmitarray curvature. However, the phase error can be addressed by optimizing the array aperture phase distribution mechanism. At 14.5 GHz, a 260 mm aperture diameter with F/D equal to 1 is built to exhibit quad-focal transmitarray. The azimuth plane ±60 degrees beam scanning performance is examined. This phenomenon shows exceptional beam scanning accomplishment for a quad-focal transmitarray. A planar multi-focal source transmitarray antenna beam scanning performance is explored in [113]. The proposed planar multi-focal transmitarray antenna pros are described in this article over the traditional transmitarray. A quad-focal phase shift distribution was studied by applying the particle swarm optimization technique for ±60 degrees beam scanning range for Ku-band transmitarray. The findings of this study show that a multi-focal transmitarray antenna can be an excellent low-cost option for high gain beam forming applications. In normal conditions, multi-focal transmitarray constraints give an extra degree of freedom that cannot be achieved without variations in the lattice geometry or transmitarray curvature. However, the phase error can be addressed by optimizing the array aperture phase distribution mechanism. At 14.5 GHz, a 260 mm aperture diameter with F/D equal to 1 is built to exhibit quad-focal transmitarray. The azimuth plane ±60 degrees beam scanning performance is examined. This phenomenon shows exceptional beam scanning accomplishment for a quad-focal transmitarray.

Future Challenges and Scope
Transmitarray technology has become mature in the last decade. Many research articles are available in the literature to demonstrate the fixed and reconfigurable beam transmitarray discussed in the previous sections. There are still gaps available to explore new characteristics in this area, in particular, for reconfigurable technology. Here, this section discusses different future aspects and challenges related to future technologies. The discussion is focused on the future of terahertz and optical frequencies for this technology, dual-band reconfigurable transmitarrays, and multi-bits transmitarrays.

Terahertz and Optical Frequencies
Terahertz and optical frequency bands ranging from 100 GHz to 3 THz have potential for next generation wireless communication with a vast scope of applications due to the lack of use and unexplored spectrum. For instance, a circularly polarized graphene-based transmitarray for 6 THz is explored in [38]. Another sub-THz anisotropic fixed-configuration three-layer transmitarray is investigated in [114]. A fixed beam wideband high efficient transmitarray for D-band is studied in [115]. The implementation of reconfigurable THz and optical frequencies has many challenges ahead. The working principles of THz and above frequencies are similar to millimeter and microwave frequencies to design a transmitarray. Reflectarrays and transmitarrays provide the concept of low insertion losses and simple electronic beam control solutions. The most prominent challenge is the fabrication of higher frequency switchable and other devices. Mostly, conductors, dielectric materials, and polymers are lossy and economical to manufacture for higher frequencies.
Beam control terahertz and optical frequency transmitarrays become a potential candidate for many applications of the terahertz spectrum, such as wireless cognition, sensing, imaging, etc. An effort ha been put into terahertz MEMS switches for a reconfigurable reflectarray at 120 GHz frequency [116]. However, MEMS switches are electrically large elements that can easily be loaded with the array. In particular, reconfigurable materials-based technologies should be considered. Some research articles have investigated liquid crystals, particularly graphene-based cells topology. Therefore, beam scanning reconfigurable transmitarrays realization at terahertz and optical frequencies represents a fascinating frontier in which technological challenges are likely to play a far more significant role than in lower frequency applications.

Multi-Bit Structures
To reduce 1-bit reconfigurable transmitarray phase quantization insertion loss, several designs were investigated for a 2-bit or 3-bit fixed beam (passive) transmitarray [117][118][119]. A multi-bit phase quantization transmitarray significantly improves execution for collimating characteristics over a 1-bit structure. The most important design principle for a transmitarray is insertion loss and phase quantization of the structure elements, in particular for beam steering (using reconfigurable electronic techniques). High phase quantization improves the directivity, aperture efficiency, and side lobe levels of the transmitarray. In an ideal case, a complete 360 • phase shift and high phase quantization are required to achieve high directivity and low side lobe levels. Multi-bit phase quantization for electronically reconfigurable transmitarray still has challenges. A large number of solid-state devices, bias network complications, bandwidth, and power ingestion are compared to a 1-bit transmitarray. Using a 2-bit reconfigurable transmitarray can improve 30-40% aperture efficiency, as indicated in [120]. Only one paper on a 2-bit electronically reconfigurable MEMS system is available [103]. Briefly, this structure consists of wafer technology with five MEMS with bias lines, and six independent bias lines to control the complex biasing network. Due to its complexity, size, and other structures, no attempt for multi-phase quantization has been performed yet for RF-MEMS switches. In [103], it is suggested that structure significantly affects high switching resistance and insertion losses of around 4.2-9.4 dB. Moreover, novel 2-bit electronically reconfigurable transmitarray experimental results have been demonstrated in [27] for Ka-band. This unit cell is fabricated with dimensions λ/2 × λ/2 × λ/8 at 29.5 GHz with six metal layers and three substrate layers. Four switches using PIN diodes are integrated on unit cells to achieve a 2-bit phase resolution of a reconfigurable transmitarray. An increase in the number of diodes from 2 to 4 produces an effect on insertion losses; the maximum measured insertion losses reach up to 2.03 dB, corresponding to the minimum insertion loss to any stage 1.39 dB at 29.5 GHz. This is a remarkable effort compared to the MEMS-based unit cell. This work is further extended in [75] to convert observed unit cells into a 14 × 14 reconfigurable array. The proposed unit cell array obtains 19.8 dB gain with 15.9% of aperture efficiency at 29.5 GHz for 2-bit resolution. A multi-bit electronically reconfigurable transmitarray has a significant gap for future research, particularly for a higher frequency. There is no reconfigurable phase quantization research updated with liquid crystal or microfluid at high frequency. Extending this technique to other configurations may be a practical way forward in the future for realizing high gain reconfigurable apertures.

Dual-Band Frequency
Dual-band transmitarray design is a remarkable challenge to obtain complete ±360 • transmission phase shift and insertion loss close to 0 dB in both frequency bands. We need to attain both parameters to gain optimal features of the dual-band frequencies. Dual-polarization is another significant parameter for satellite communication applications. Independent phase shift for a dual-band transmitarray becomes a more difficult task due to dual-polarization. For satellite communication applications, lower and higher band frequencies in the Ku-band (12.5/14.5) are close to each other for satellite communication applications. Resonance peaks are very close to each other, so this proximity impacts both transmission magnitude and the phase of dual-band frequencies. The fact that few investigations have been conducted on dual-bands is a challenge. In addition, numerous fixed beam dual-band structures have been studied in the literature [121][122][123]. It is noted that much less work has been carried out to investigate reconfigurable dual-band unit cells. The proposed unit cell is based on MMIC Technology for K/Ka-band active transmitarray, as reported in [121]. This paper is the first reconfigurable dual-band SiGe-BICMOS MMIC-based transmitarray design, fabricated and tested. For instance, applications for satellite communication two-way data-links acquire from 19.7 GHz to 21 GHz downlinks and 29.5 to 30.8 for uplinks. Electronically reconfigurable dual-band is a promising gap for future research for many applications such as satellite communication (Sat-Com) and 6G/5G cellular systems with all of its challenging problems. This electronically reconfigurable dual-band transmitarray also has the potential to facilitate revolutionary applications, which will be enabled by new thinking and developments in solid-state devices, applications, and systems. A detail Summary of beam steering methods with characteristics to conclude out whole work, see Table 4.

Conclusions
Combining a planar array with a variable transmission phase change with a fixed source antenna or a metasurface with a fixed transmission phase change and a source antenna with a variable wavefront shape yields the beam steering characteristics of the transmitarray antenna. This article intends to provide a quick overview to readers on recent developments and challenges on beam steering characteristics for transmitarray antennas. It explains the operation of beam forming/beam steering transmitarrays and various types of beam steering structures investigated along with their radiation characteristics. In addition, to secure the maximum aperture efficiency of the transmission antenna, it is necessary to calculate the optimal F/D according to the gain of the source antenna and design a metasurface with high transmission efficiency. Multiple electronic reconfigurable devices and components are demonstrated for the beam steering/beam forming characteristics of the reconfigurable transmitarray. A comprehensive overview of recently reported work in this regard is presented. This article also outlined the future demands and challenges that will make beam steering transmitarrays even more attractive as future prospects for various applications.