Broadband Dual-Polarized Single-Layer Reflectarray Antenna With Independently Controllable 1-Bit Dual Beams

An independently controllable dual-beam reflectarray antenna is presented using a broadband dual-polarized single-layer 1 bit unit cell. The unit cell independently provides two-state phase compensation for two orthogonally linearly polarized waves. The 180° reflective phase difference between the two states is achieved by tuning the magnetic resonance of State 0 and the electrical resonance of State 1. With its two resonances close to each other, the unit cell has a reflective phase difference of 180° ± 20° between two states over a broad bandwidth of 27.2–51.1 GHz. The cross-polarization levels of below −30 dB ensure the high isolation between two polarizations. Using the proposed unit cell, a fixed dual-beam reflectarray antenna is designed and excited by a dual-polarized horn to show the ability of independently controlling two orthogonally linearly polarized waves. At 33 GHz, the beams direct to 20° and −15° for the feeding of horizontally and vertically polarized ports, respectively. The 1.5 dB gain bandwidth is greater than 20% for both polarizations. The proposed dual-polarized reflectarray antenna proves that, for two orthogonally linearly polarized incident waves, the independently controllable fixed dual beams can be realized using the 1 bit phase compensation and single-layer structure.

fifth-generation (5G) mobile communication systems [1], [2]. The MU-MIMO system needs dynamic multiple beams with low interference and high gain to guarantee reliable millimeter-wave (mmWave) communications. Thus, for such a system, the antenna with independently controlled multiple beams, high isolation, and high gain is highly preferred.
The metasurfaces have been widely studied in recent years and have presented many functions, such as invisibility [3], spatial reuse [4], polarization reuse [5], [6], and multiple-function electromagnetic (EM) waves' control [7]. The reflective metasurface with the ability of polarization reuse has the potential to independently control multiple beams with different polarizations and beam directions, which can achieve polarization diversity and spatial diversity for MU-MIMO systems. Some independently controllable dual-polarized dual-beam reflectarray antennas are presented using polarization reused reflective metasurface [8]- [14]. The key issue to realize an independently controllable dual-beam dual-polarized metasurface is the design of dual-polarized unit cells, which can independently provide the phase control for different polarized waves.
So far, several types of unit cells have been presented to realize independent phase control for different polarizations. For two orthogonally linear polarizations, the multilayer patches [8] and crossed dipoles [11], [12] structure are used to design independently controllable dual-polarized unit cells. Changing orthogonal geometrical dimensions can independently control the phase compensation for horizontally polarized waves and vertically polarized waves, respectively. The single-layer square patch loaded with two delay lines can also be used to independently control reflective phases, which has reconfigurable potential by changing the length of the two delay lines [9]. Furthermore, for dual circular polarizations, the Berry phase compensation and dynamic phase compensation can be utilized to achieve the independently controllable dual-circularly polarized unit cells with wide working bandwidth [13], [14], but the method is difficult to realize of linearly polarized unit cells.
The challenges for independently controllable dual-linearly polarized unit cell designs are simplifying the structure and broadening the bandwidth. For the unit cell design, the single-layer structure often has a relatively narrow working bandwidth, and the broad bandwidth usually needs a multilayer 0018-926X © 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
structure. However, both simplifying structure and broadening bandwidth are usually required for the mmWave antennas. Moreover, for the further design of the dynamic beam control, a simple structure and 1 b phase compensation usually are preferred [15]- [18]. In this work, a broadband dual-polarized 1 b unit cell is proposed to achieve an independently controllable dual-beam reflectarray antenna. The unit cell with fixed two states is designed on the single-layer substrate, which features a low cross-polarization level and broad bandwidth of 180 • ± 20 • phase difference between the two states. It can independently control reflective phases of the horizontally polarized waves and the vertically polarized waves, so the two independent beams for their respective polarization can be generated using a single-layer reflective metasurface. To verify the independent control of orthogonal linear polarizations, a fixed dual-beam reflective metasurface is designed, and a dual-polarized horn antenna is used to provide dual-polarized incident waves. The directions of reflective beams are designed at 20 • and −15 • elevation angles when, respectively, excited by two ports of the dual-polarized horn. The proposed single-layer reflective metasurface also has the potential to realize independently dual-polarized dual-beam scanning using four electronic switches instead of four stubs on the 1 bit unit cells, which may benefit the antenna design of MU-MIMO systems.
The rest of this article is organized as follows. A dual-polarized 1 bit unit cell is proposed, and the principles of operating and independent control are presented in Section II. Section III describes the 1 b dual-polarized dual-beam reflectarray antenna design with proposed unit cells. Simulation and measurement results are shown and compared with those in references in Section IV. Finally, conclusions are drawn in Section V.

II. DUAL-POLARIZED 1 BIT UNIT CELL
The EM waves in which the E-field paralleling to the x-axis are assumed as horizontally polarized waves and the EM waves in which the E-field paralleling to the y-axis are assumed as vertically polarized waves.

A. Structure of the Unit Cell
The structure of the proposed independently controllable dual-polarized 1 bit unit cell is shown in Fig. 1, and the prototype is a dependently controllable dual-polarized unit cell in [19]. The essential components of the unit cell, including a 2 × 2 square patch array, are printed on a piece of the grounded dielectric substrate. The square patch array is positioned in the center of the unit cell. There are two dark gray stubs, denoted by S x , along with the longitudinal slot and between the square patch array, which is symmetric with the center of the unit cell. Two stubs S x are used to connect the square patches along the x-direction, which is utilized to control the phase compensation for horizontally polarized waves. For horizontally polarized waves, the unit cell with and without the two dark gray stubs presents State 1 and State 0, respectively. The two light gray stubs, denoted by S y , are along with the transverse slot between the square patch array. These light gray stubs are similar to the two dark gray stubs and are utilized to connect the square patches along the y-direction, which can control the phase compensation for vertically polarized waves.
The unit cell structure is symmetric about the xoz-and yoz-planes, as shown in Fig. 1(a). The dark gray stubs and light gray stubs are used to independently control horizontally and vertically polarized waves, respectively. The unit cell is a symmetric structure, which brings a similar working principle for the two polarizations. Thus, the horizontally polarized incident waves will be discussed as an example.

B. Principle of 1 bit Phase Compensation
For the proposed unit cell, the principle of 1 bit phase compensation can be analyzed by the magnetic resonance and the electric resonance of the states. For State 0, the unit cell acts as a magnetic wall, which provides a 0 • reflective phase. For State 1, the unit cell works as an electric wall, which provides a 180 • reflective phase.
Without loss of generality, the incident waves are assumed to be horizontally polarized. As shown in Fig. 2, for State 0 of S x , currents on the patch, currents on the ground, and displacement currents between the patch and ground form an electric loop, the loop parallels to the horizontal direction and brings a magnetic resonance; therefore, the unit cell provides a 0 • phase compensation for horizontal waves. For State 1 of S x , the whole patch works like an electric dipole, and the dipole parallels to the horizontal direction and brings an electric resonance; therefore, the unit cell provides a 180 • phase compensation for horizontal waves.
Because the trend of the reflective phase for the two states against the frequency is similar, if the magnetic resonance S y = "11." (e) Side view, current on the patch, current on the ground, and displacement current, S x S y = "00." (f) Side view, current on the patch, current on the ground, and displacement current, S x S y = "01." (g) Side view, current on the patch and the ground, S x S y = "10." (h) Side view, current on the patch and the ground, S x S y = "11." (i) Bottom view, current on the ground, S x S y = "00." (j) Bottom view, current on the ground, S x S y = "01." (k) Bottom view, current on the ground, S x S y = "10." (l) Bottom view, current on the ground, S x S y = "11. "   TABLE I PARAMETERS OF THE PROPOSED UNIT CELL frequency for State 0 is close to the electric resonance frequency for State 1, the unit cell can realize wide bandwidth for 1 bit phase compensation with an approximate 180 • reflective phase difference. Moreover, for horizontally polarized incident waves, less current is on the stubs for S y ; on the square patch array, the sum of the currents is almost along the x-direction, as shown in Fig. 2. Therefore, high polarization isolation of the unit cell can be realized. Although the discussion of the unit cell is for horizontally polarized incident waves, a similar analysis also can be used for vertically polarized incident waves.
From the abovementioned analysis, to realize the 180 • reflective phase difference between two states, the frequency of the magnetic resonance and the electric resonance should be close enough, which means that the equivalent electric loop's length for State 0 should be twice the equivalent electric current length for State 1. The length of the patch l, the wide of the cross slot w, and the thickness of the substrate h determine the frequency of the magnetic resonance and the electric resonance. The thickness of the substrate usually is limited by the printed circuit board (PCB) process, so the length of the patch and the wide of the cross slot should be carefully designed and optimized. The relationship between the parameters and the resonance frequencies is presented in Fig. 3. The Taconic TLY with ε r = 2.2 and h = 1.016 mm is used as the substrate, and the design and analysis are in the Q-band for the application of mmWave band.
To reveal the relationship between the parameters and help the unit cell design, the two resonance frequencies along with the changing of two parameters are shown in Fig. 3(a) and (b). In Fig. 3(a), the magnetic resonance shifts to a higher frequency as either w increases or l decreases. However, in Fig. 3(b), the electric resonance shifts to a higher frequency when either w or l decreases. The opposite frequency trend of w will lead to an intersection between frequencies for the magnetic resonance and the electric resonance. The frequency differences of the magnetic resonance and the electric resonance under different values of w and l are presented in Fig. 3(c), and it is obvious that there is a red area in the figure, which indicates that the frequencies of magnetic resonance and electric resonance are very close. As shown in Fig. 3(a) and (b), by choosing parameters w and l within this red area, the proposed unit cell can achieve an approximate 180 • reflective phase difference for two states at an arbitrary frequency from 30 to 42 GHz. For example, w = 2.46 mm and l = 0.24 mm are chosen for the 1 b unit cell design, which makes an approximate 180 • reflective phase difference at 36.5 GHz. Other parameters of the unit cell are shown in Table I.

C. Independent Control of the Dual-Polarized Unit Cell
Based on the abovementioned analysis, the two states of S x and S y can provide broadband 1 bit phase compensation for the horizontally polarized waves and the vertically polarized waves, respectively. The independent control of S x and S y guarantees independent 1 bit phase control for two linearly polarized waves. Furthermore, as shown in Fig. 2, the current distributions are almost the same for the same states of S x and different states of S y under horizontally polarized incident waves, which means that the states of S y hardly influence the reflective phases for horizontally polarized waves. For vertically polarized waves, it easily deduces that the states of S x hardly influence the reflective phases, and the phases can be independently controlled by S y . Therefore, by controlling of S x and S y , the independent 1 bit phase control for two orthogonally linearly polarized incident waves can be realized using the proposed unit cell.
The desired reflective phases for different states of S x and S y are listed in Table II. These states present four kinds of states for the proposed unit cell, as shown in Fig. 2, and the independent phase control for two polarizations can be achieved using these states. The simulated performances of the unit cells are presented in Figs. 4-6. Under horizontally polarized incident  waves, the states of S y has little effect for the reflective phase, as shown in Fig. 4, which corresponds to the current analysis of Fig. 2. The 1 bit reflective phase for horizontally polarized waves can independently control from 27.2 to 51.1 GHz, and 180 • ±20 • reflective phase difference can be achieved over this bandwidth. The reflection loss is less than 0.12 dB in the working bandwidth, as shown in Fig. 5. The low cross-polarization levels for different states ensure the high isolation for the dual-polarized unit cell, as shown in Fig. 6. Due to the symmetrical structure, a similar performance can be realized for vertically polarized waves, by controlling the states of S y . The good performance of the proposed unit cell with fixed states is a requirement for a reconfigurable design. Also, there are some challenges in the reconfigurable design using the proposed unit cell. For example, the performance of the reconfigurable unit cell is influenced by the working bandwidth of PINs and the complexity of biasing circuits. If a beam-steerable reflectarray is designed, more PINs will be required than that in the conventional design, which needs only one switch for each polarization because the proposed unit cell needs two stubs for each polarization.
The proposed independently controllable dual-polarized 1 bit unit cell with a simple single-layer structure, wide bandwidth, and low cross-polarization performance indicates that an independently controllable 1 bit dual-beam dual-polarized reflectarray antenna can be achieved, which will be discussed in Section III.

III. REFLECTARRAY ANTENNA DESIGN
Using the proposed unit cells, the independently controllable 1 b dual-polarized reflectarray antenna with fixed dual beams is then designed. A dual-linearly polarized horn antenna with two ports is used as the feeding antenna, which provides two orthogonally linearly polarized incident waves. The gain of the horn antenna is about 14.5 dBi with a 3 dB beamwidth of 25.1 • at 33.0 GHz. The diameter of the array is 131.2 mm, about 14.4λ 0 , where λ 0 is the free-space wavelength at the frequency of 33 GHz. The total number of the unit cells is 872, and the intercell spacing is 0.43λ 0 . The horn antenna is above the center of the reflective metasurface, and the aperture of the horn is parallel to the metasurface. The ratio of focal length to aperture diameter (F/D) is set as 1.0 in the reflectarray antenna design.
When the positions of the horn antenna and the metasurface are determined, the initial phase on the reflective surface causing by spherical incident waves of the horn antenna can be extracted with the help of CST Microwave Studio. Let θ and ϕ denote the elevation angle and the azimuth angle, respectively. The targeted beam directions for the horizontally and vertically polarized beams are θ H = 20 • , ϕ H = 0 • and θ V = −15 • , ϕ V = 0 • , respectively. The precise phase compensation ψ H,mn of the mth column and the nth row (m, n = 1, 2, . . .) unit cell for the horizontally polarized beam can be calculated by where ϕ H,mn is the initial phase of the {m, n}th unit cell for horizontal polarization, d is the spacing between the unit cells, and φ H is a reference phase to provide an additional degree of freedom for the reflectarray design, which can be chosen from 0 to 2π and has a significant effect on the sidelobe level (SLL) [20], [21]. The 1 b phase compensation of the {m, n}th unit cell for horizontal polarization can be given by where mod (ψ H,mn , 2π) means the modulo of the precise phase compensation dividing 2π. The phase of (π/2) and (3π/2) can be mapped to State 0 and State 1 of S x , respectively, as shown in Fig. 7(a). Using the same method, the state of S y for vertical polarization can also be determined, as shown in Fig. 7(b). Then, the state distributions of all unit cells on the reflective metasurface can be obtained, as shown in Fig. 7(c). The beam directions for the horizontal and vertical polarizations can be independently controlled by adjusting the dark gray stubs (S x ) and the light gray stubs (S y ) of the unit cells.

A. Measurement Results
The designed 1 bit dual-beam dual-polarized reflective metasurface is fabricated and measured. The photograph of the fabricated reflectarray antenna with its fixture is shown in Fig. 8. The feeding horn antenna and the reflective metasurface are assembled using plastic screws and a 3-D-printing fixture. The radiation patterns and the gains of the reflectarray prototype are verified in the far-field anechoic chamber.
The simulated and measured far-field performances of the designed reflectarray with the feeding horn are compared in Figs. 9 and 10. The normalized radiation patterns in the xoz-plane are shown in Fig. 9. The undistorted pencil-shaped beams are realized from 31.0 to 35.0 GHz with the measured SLL less than −14 dB for both polarizations. The measured SLLs are higher than the simulated ones because of the influences of the fixture and the cables. The fluctuation of gains is less than 1 dB, and the variation of the beam directions is less than 1 • during this bandwidth. The measured cross-polarization of both beams is below −20 dB, indicating high polarization isolation. The measured aperture efficiency is about 20% in the working bandwidth.
As shown in Fig. 10, the measured 1.5 dB gain bandwidth and 2 dB gain bandwidth are greater than 20.6% and 28.6%, respectively, for both polarizations. The measured gains at 33.0 GHz for the horizontal and vertical polarizations are 25.0 and 25.8 dBi, respectively. The measured results agree well with the simulated ones, which demonstrate that the independent polarization control can be achieved using the proposed broadband dual-polarized unit cell.

B. Comparisons and Discussion
Table III compares the proposed independently controllable 1 bit dual-beam dual-polarized reflectarray antenna with reported dual-linearly polarized reflectarray antennas. Because all antennas are fixed dual beams, the Layer num. in the table does not consider the control circuits for beam scanning. For the dual-linearly polarized unit cell design, the rectangle patches and dipoles are commonly used, and the different reflective phases of dual polarizations can be achieved by changing orthogonal geometrical dimensions. The multilayer structure is utilized to realize sufficient reflective phase range and broaden the bandwidth of the unit cell, and some unit cells control different polarization on the different layers. However, for single-layer unit cells, it is difficult to realize independent dual-polarization control with broadband performance. Due to the simple 1 bit phase compensation, the proposed unit cell has the advantages in wide bandwidth, single-layer structure,  and high polarization isolation. The proposed 1 bit unit cell achieves the bandwidth of 62.1%, wider than reported unit cells.
The independently controllable 1 bit dual-polarized dual-beam reflectarray antenna can be achieved using the proposed unit cells. Compared with the reported multilayer independently controllable dual-linearly polarized dual-beam reflectarray antennas and single-layer/multilayer dependently controllable dual-linearly polarized dual-beam reflectarray antennas, the proposed 1 bit reflectarray antenna has the advantage in independently controllable dual-polarized dual beams and a single-layer structure and has a comparable gain bandwidth. Furthermore, the compact 1 bit phase compensation method, the single-layer structure, and the broadband unit cell with fixed states bring a potential for proposed independently controllable 1 bit dual-beam dual-polarized reflectarray antenna to achieve dual-beam scanning capability.

V. CONCLUSION
An independently controllable 1 bit dual-polarized unit cell with broad phase bandwidth, high polarization isolation, and simple single-layer structure has been proposed. By manipulating the magnetic resonance frequency and the electrical resonance frequency of the two states, the 1 bit reflective phase of horizontally and vertically polarized EM waves have been independently controlled. Using the proposed unit cell, the independently controllable 1 bit dual-polarized dual-beam reflectarray antenna has been designed, simulated, and fabricated. The proposed dual-beam reflectarray antenna can independently control horizontally and vertically polarized beams to the desired directions. The achieved performance of the proposed fixed dual-beam reflectarray antenna demonstrates that the independent control for orthogonal linear polarizations can be achieved using a simple single-layer structure and 1 bit phase compensation. This may provide a new antenna design for MU-MIMO applications.