Dual-Band Quad-Polarized Transmitarray for 5G Mm-Wave Application

A dual-band quad-polarized transmitarray (TA) is designed in a common aperture operating at 25.9/39.8 GHz. Using narrow strip patches as cells, the 25.9 GHz cells are arranged in the ±45° direction, and the 39.8 GHz cells are arranged in the 0/90° direction in the formed square gap of the 25.9 GHz cells to realize a compact design with four polarization directions. In each band of 25.9/39.8 GHz, the proposed TA is dual-polarization designed. Measurements show that the expected beams can be achieved, and their 3 dB gain bandwidth covers almost all of the 5G bands n258 (24.25–27.5 GHz) and n260 (37–440 GHz). This TA has potential application value in 5G construction.


I. INTRODUCTION
The function of transmitarrays (TAs) or reflectarrays (RAs) is to converge a divergent spherical wave into a plane wave by placing unit cells with different phase shifts at different positions on the plane. TAs and RAs [1]- [14] are both effective solutions to achieve high gain aperture antennas, but the TAs have no feed shielding effect, which is more convenient for installation and use. For the dual-band TA, the main challenge is to make the cells independently change the transmission phase in two bands with low transmission loss, but it has a wider application space. Most of the existing dual-band TAs [15]- [27] are single polarized, and only a few dualband TAs [28]- [31] adopt a dual polarized design, but they are single polarized in each band. Therefore, it is significant to design a dual band TA, which has dual-polarization designed in each band.
The 5G bands n258 (24.25-27.5 GHz) and n260 (37-440 GHz) are used for 5G mm-wave communication, so it is meaningful to design a dual-band quad-polarized TA operating at 25.9 GHz and 39.8 GHz. In this paper, a dual-band quad-polarized TA is proposed with ±45 • polarization at 25.9 GHz and 0/90 • polarization at 39.8 GHz. The transmission cell used here is a narrow strip patch [31].
The associate editor coordinating the review of this manuscript and approving it for publication was Tutku Karacolak .
First, dual polarization is realized by interleaving the linear polarization cells, then similar higher band cells are placed between the gaps of the lower band cells, and finally, the interaction between the cells is adjusted. To make the best use of space, the lower band cells are tilted 45 • , and the higher band cells are placed horizontally or vertically, which makes the design more compact. Because of the high frequency, the phase changes rapidly with the size, so the narrow side of the strip patch remains unchanged, and the size changes are reflected on the longer side. This kind of operation can reduce the influence of machining error and maximize the design of higher frequency cells under the same machining error.
The dual-band quad-polarized TA designed in this paper has a total of 1681 linear polarization units, including four polarization directions. The final experimental results show that each polarization direction has a good gain and cross polarization level performance.

A. INITIAL CELL
To conveniently describe the polarization direction of the electric field, a schematic diagram of the dual-band quad-polarized TA is shown here, in which each polarization direction can be represented by the angle with the x-axis, as shown in Fig. 1. The initial cell of the TA is shown in Fig. 2. It is made up of two rectangular strips that are perpendicular to each other and printed on each side of two identical 0.508 mm-thick dielectric substrates (Rogers RO4350B, ε r1 = 3.66, tan δ 1 = 0.0037). The two rectangular strips are connected by a metalized via, and the two substrates are connected by a 0.1 mm-thick adhesive film (Rogers4450F, ε r1 = 3.52, tan δ 1 = 0.0042), with the floor in the middle. This kind of transmit cell has the function of polarization rotation, and this kind of polarization rotation cell has a wider passband than that of a nonrotation cell [31].

B. DUAL-BAND QUAD-POLARIZED CELL
The −45 • -pol cell is placed at the four corners of the 45 •pol cell, and a uniform dual-polarization array configuration can be formed [32]. Then, the 0 • and 90 • polarized patch cells are placed in the square space in the dual polarization array, and a dual-band quad-polarized array configuration can be formed. The formation process is shown in Fig. 3. The four different polarization linear polarization arrays share one aperture. The ±45 • -pol cells are designed at 25.9 GHz, and 0 • /90 • -pol cells are designed at 39.8 GHz. The relationship between the polarization directions of ±45 • , 0 • and 90 • and the coordinate axis is shown in Fig. 1.
The design model of a dual-band quad-polarized cell in the simulation software is shown in Fig. 4. Floquet ports and  periodic boundary conditions are used to simulate the cell. The parameter identification of the cell size is the same as in Fig. 2, and the specific design size is shown in Table 1. The coefficient k in Table 1 is used to define different sizes of cells. Furthermore, the coefficient k is mainly used to adjust the size of variables a and c. Setting different k values can make the cell realize different phase responses. We set the value interval of k as 0.02 to ensure higher phase accuracy, so 16 different phases are used in first band and 8 different phases are used in second band. And due to the limited machining accuracy, it is meaningless to continue to reduce the value interval of k. Generally, the greater the distance between the two differently polarized cells, the better the cross-polarization transmission performance (since the transmission cell here VOLUME 9, 2021  has a 90 • polarization rotation, homopolar transmission is unnecessary). The L is determined by scanning parameters. Fig. 5 shows the relationship between the transmission S-parameter and L of 25.9 GHz cells, and as L increases, the copolarized transmission S21 decreases. When L is 5 mm, the copolarized transmission S21 of 25.9 GHz cell is low, and when L continues to increase, the copolarized transmission S21 decreases slightly, but its cross-polarized transmission S21 decreases. Therefore, 5 mm is chosen as the distance between the cells for consideration of transmission performance and polarization isolation. The 39.8 GHz cells are farther apart in electrical length, so suppression of copolarized transmission should not be considered too much.
The current simulation of the dual-band quad-polarized cell is shown in Fig. 6. When the 25.9 GHz or 39.8 GHz cell is excited separately, there is almost no current in the uninspired cell with different frequencies or polarizations. Therefore, the   symmetrically designed to achieve 0 • and 180 • phase shifts, so the cell only needs to meet the phase-shifting range of 0 • to 180 • when tuning the size. In addition, 360 • phase coverage can be achieved through mirror symmetry. Considering the problem of processing accuracy, the value spacing of k in Table 1 is set to 0.02, that is, the minimum change in size is 2.5 mil length. Designed at 25.9 GHz is 4-bit cells with k ranging from 0.93 to 1.07. Since 39.8 GHz cells are smaller in size, 3-bit cells are designed at 39.8 GHz, with k ranging from 0.58 to 0.64.
The amplitude-frequency response of the cells is shown in Fig. 7(a) and (b). The −3 dB transmission bandwidth of a single cell is approximately 3.6 GHz but shared by all 25.9 GHz cells is only 1.4 GHz and by all 39.8 GHz cells is only 1.3 GHz. In a very wide bandwidth, more than half of the cells have an interpolation loss between 0 dB and 2 dB, which can greatly reduce the average transmission loss, so the total gain bandwidth is wider when designing arrays.    at 39.8 GHz, and the total phase shift range of all cells covers almost 0-180 • . Since this kind of cell has a 1-bit response at a fixed size, it can achieve a 0 • or 180 • response by mirroring itself symmetrically, so the original cells plus the symmetrically operated cells can cover almost 1-360 • . Their respective simulation phase shifts are shown in Table 2, where the cells with coefficients k between 0.93 and 1.07 are defined as #1∼#8 (25.9 GHz cells) and the symmetrically operated cells are #9∼#16. The cells with k between 0.58 and 0.64 are defined as * 1 ∼ * 4 (39.8 GHz cells), and the symmetrically operated cells are * 5 ∼ * 8.

III. TRANSMITARRAY A. PHASE SURFACE EXTRACTION
When the antenna is fed by the horn, it is necessary to know the phase value received by each cell at each position of the array to determine the phase shift to be compensated. Therefore, it is necessary to extract the radiated electric field phase of the horn on the receiving surface of the array. To irradiate the radiation energy of the horn on the receiving surface of the array as much as possible, the F/D is 0.69. The horn selected here is a linear polarization standard gain horn lb-28-15 working in the Ka-band. Because the position of each polarization cell is different, the phase plane of each linear polarization array needs to be extracted separately. A total of four phase planes need to be extracted, as shown in Fig. 8, and the phase extraction value of each grid point is quantized according to the phase in Table 2.

B. SIMULATIONS AND MEASUREMENTS OF THE TA
The dual-band quad-polarized TA prototype is simulated and tested. The feed used here is a linear polarization standard gain horn lb-28-15. Because the test band is wide, we divided the frequency into several segments for calibration and testing. The array has 20 × 20 cells at 25.9 GHz/−45 • -pol, 21 × 21 cells at 25.9 GHz/45 • -pol, 20 × 21 cells at 39.8 GHz/0 • -pol and 21 × 20 cells at 39.8 GHz/90 • -pol. A total of 1681 linearly polarized cells were arranged alternately. The design model of the dual-band quad-polarized TA is shown in Fig. 9. The size of the substrate is 120 mm × 120 mm, and the area of radiation cells is 105 mm × 105 mm. Four linearly polarized arrays are excited by changing the excitation frequency and rotating direction of the horn.
To reduce the influence of the fixing device, an acrylic plate and nylon column are used to fix the TA. The physical model of the dual-band quad-polarized TA is shown in Fig. 10. The feed horn is fixed on the acrylic plate by bolts, and there are fixing holes to allow the horn to rotate on the acrylic plate. Here, only four central beams are simulated and tested, and the gain and pattern test results are shown in Fig. 11 and Fig. 12. Because the array is large and difficult to simulate, only one polarization is simulated at each band.
The test results show that the peak gain is 26.1 dBi/ 45 • -pol and 25.8/−45 • -pol in the lower band (23-229 GHz), which is approximately 1 dB lower than the simulation result (27.1/26.8 dBi), and its 3 dB gain bandwidth is approximately 24.2-27.6 GHz. The peak gain of 27.9 dBi in the higher band (37-442 GHz) is 1.2 dB lower than that of the simulation result (29.1 dBi), and its 3 dB gain bandwidth is approximately 37.7-41.5 GHz. The pattern in Fig. 12 is normalized with the simulated gain as a reference. The tested and simulated patterns have the same ascending and descending trend from 0 • to 90 • , and all sidelobes and cross-polarization levels tested are below −17 dB on average. The higher the angle is, the higher the cross-polarization level is. This is caused by the polarization rotation in the transmission cells. The polarization direction of the horn is the cross-polarization direction of the array. Therefore, when the horn radiates at a large angle, the gain overflows the array, which causes the cross-polarization level to become higher.
Taking 105 mm × 105 mm as the aperture size of the array, according to the test results, the actual aperture efficiency of the dual-band quad-polarized TA is 39.4%/45 • -pol, 36.8%/−45 • -pol at 25.9 GHz, and 25.3% at 39.8 GHz because of the large spacing of the higher band cells. Table 3 shows the parameter comparison between some existing dual-band transmission arrays and the proposed TA. Compared with the dual-band or dual-polarized transmission array, the array designed in this paper is not inferior in aperture efficiency, polarization purity, array size, gain bandwidth, and other parameters. Moreover, dual-polarization transmission is realized in each band, which is a highlight of this design.

IV. CONCLUSION
Using the common radiation aperture method, a dual-band quad-polarized TA operating at 25.9/39.8 GHz is presented. The 3 dB gain bandwidth can almost cover all of the 5G bands n258 (24.25-27.5 GHz) and n260 (37-40 GHz). It has been proven that the proposed TA can achieve good beam convergence and polarization independence at each polarization and can be used for 5G millimeter-wave applications.