Construction of a Cost-Effective Phased Array Through High-Efficiency Transmissive Programable Metasurface

Programmable metasurfaces have shown great potential in the areas of low-complexity phase array systems in comparison with the conventional phased array antennas. In this document, a 1-bit transmissive programable metasurface with high efficiency is proposed for the cost-effective beam-steering phased array. The designed transmissive metasurface is made up of reconfigurable cells with perfect 1-bit phase tuning and less transmission losses. Through dynamically programming the 1-bit code distributions of the metasurface, real-time scanning pencil beams in desired directions can be created.


INTRODUCTION
Recently, metasurfaces are widely known as a kind of two-dimensional periodic structures with subwavelength scale, owing to its independent talent of delicate modulation on electromagnetic waves [1]. Based on the advantages of low loss, easy integration and simple preparation, metasurfaces have shown extensive application prospects and become a research hotspot in modern physics, information science and the related interdisciplinary [2][3][4].
Within microwave domain, metasurfaces also arouse a wide range of investigation, such as radiation improvement [5][6][7], perfect absorbers [8,9], OAM-EM wave generation [10][11][12][13], scattering reduction [14][15][16], holographic imaging [17,18], and energy harvesting [19]. Metasurfaces particularly demonstrate the unique abilities in waveform shaping through the phase manipulation of the building units in microwave metasurfaces. Reflective metasurfaces were demonstrated to generate the high-intensity pencil beams or high-purity vortex waves through the full 2π phase tuning [20,21]. Polarization-insensitive transmissive metasurface with hexagonal unit configuration is proposed to stimulate vortex beam at any polarization [22]. The ultra-thin transmissive metasurfaces were designed for high-efficiency wavefront modulation in circular polarization, based on photon spin Hall effect or Pancharatnam-Berry phase theory [23,24]. Nevertheless, for all the foregoing designs, only the fixed directional or shaped beams can be created once the metasurfaces are constructed, which restrict the application for dynamic control in electromagnetic wave.
The programmable metasurfaces integrated with lumped components could be very practical for the fine-grained modulation over electromagnetic waves, which also show great advantages at improving the incomplexity and cost for dynamic beam forming or scanning, in comparison with conventional phased array systems that are composed of massive digital or analog phase shifters [25]. Digital or programmable metamaterials are firstly proposed in 2014 to expand the concept of metamaterials by using dynamic sequences of "0" and "1" bits [26]. Ulteriorly, the metasurface unit can obtain a clear alteration in resonant property through integrating lumped components, and the programmable reflective metasurfaces are thus actually constructed for the dynamic modulation of the electromagnetic waves [27]. The digital reflective metasurfaces were constructed by designing reconfigurable cells combined with the PIN diodes for binary phase states [28,29]. Later, two-bit reflective programable metasurfaces were designed by integrating two PIN diodes or a voltage-controlled varactor diode to modulate the resonant characteristics of the reconfigurable units [30,31]. Since the feed shielding effects and the phase distortions are usually very serious for the reflective metasurfaces, the programmable transmissive metasurfaces are then proposed to avoid these deficiencies [32]. Metasurface unit integrated with C-shaped patch and U-shaped slot serving as the receiver and transmitter was designed to construct the digital transmissive metasurface [33]. Metasurface unit with the combination of a couple of C-shaped patches and ring slots was adopted to achieve 1-bit phase resolution [34]. Later, the equivalent magnetic dipole unit was also presented by combining the rectangular patch and sideshorted patch [35]. However, all these transmissive metasurfaces are suffering from very high transmission losses due to the insufficient unit architecture design, which lead to the low aperture efficiency not exceeding 20.0%.
In this document, a high-efficiency transmissive 1-bit digital metasurface is designed for the construction of the cost-effective phased array. Through dynamically programming the 1-bit code distributions of the metasurface when biasing the integrated PIN diodes, real-time scanning pencil beams in the desired directions can be created and the numerical simulation results demonstrate the availability of the proposed metasurface.

UNIT DESIGN AND CONFIGURATION
The component of the designed transmissive programable metasurface is a reconfigurable programmable cell with binary phase modulation. The 1-bit unit is constructed with multilayer metallic structure along with two dielectric substrates and a bonding film, the structure of which is shown in Figure 1A.
The two dielectric substrates have the same parameters with the dielectric constant of 3.55 and the thickness of 1.524 mm, and the thickness of the bonding film is 0.1 mm. The metallic structure is composed of the transmitting patch, the ground plane, the bias layer and the receiving patch. The transmitting patch is designed in front with a rectangular patch loaded by an O-slot and two PIN diodes, while the same-sized receiving patch is set on the reverse side and integrated by an U-slot, and the two patches constitute an integral whole by a metallized central via-hole through the ground. In addition, the transmitting patch is connected with the ground through a pair of symmetric via-holes, while the receiving patch is connected with a pair of rectangular distributed capacitors in the bias layer in similar way for biasing purpose.
To reduce the influence of the bias lines on the designed unit, the bias lines are designed using extremely narrow linewidths and located near the ground plane. Detailed structure and dimensions of the proposed metasurface cell are also given in Figure 1.
The PIN diode M/A-COM Flip Chip MA4FCP305 is chosen to construct and modulate the unit, and the parameter modelling of the PIN diode is characterized as lumped components for the binary ON/OFF states [36]. When biasing the PIN diode with a forward current, the ON state along with a series of a resistance R ON 2.1 Ω would be employed; and for a reverse bias voltage, the OFF state along with a parallel of a capacitance C OFF 0.05 pF would be applied for the PIN diode. The 1-bit phase modulation of the unit can be accomplished through biasing the integrated PIN diodes. The simulated scattering parameters of the designed metasurface cell are plotted in magnitude and phase for both π/0 states, as shown in Figure 2. When PIN diode I is at the ON state and PIN diode II is at the OFF state, the metasurface cell would be operating at the π-state; the simulated S 11 is less than -10 dB from 7.23 to 7.82 GHz, and the simulated S 12 is above -2 dB from 7.17 to 7.89 GHz. When PIN diode I is at the OFF state and PIN diode II is at the ON state, the metasurface cell would be working at the 0-state; now the simulated S 11 is less than -10 dB from 7.21 to 7.79 GHz, and the simulated S 12 is above -2 dB from 7.13 to 7.86 GHz. The phase displacement of the binary cell states could maintain constant at nearby π with relatively small variations.

METASURFACE CONFIGURATION AND BEAM-SCANNING VERIFICATION
In the actual metasurface design, the transmissive programable metasurface is constructed with 400 units and an overall dimension of 400 × 400 mm 2 , and the overall structure of the designed metasurface is also shown in Figure 1A. The focal source of a standard waveguide horn is placed in front of the  To generate a scanning pencil beam in the desired direction, the phase compensation Δϕ mn for the metasurface cells should satisfy the formula, where k 0 is the wavenumber, r → ′ mn is the position vector of the mnth unit, R → mn is the position vector of the mnth unit relative to (0, 0, f ), and r 0 is the desired direction of the pencil beam. All these parameters are defined as shown in Figure 3.
To get the overall code distribution on the metasurface, the binary phase states for each unit could be obtained by furtherly quantizing the continuous phase shift Δϕ mn , To furtherly validate the effectiveness of the transmissive programmable metasurface, steering pencil beams with a wide scanning range of at least 45°and an angular spacing of 15°are then numerically generated. The optimal quantized code distributions for the four directional beams are generated using Equation (2) and implemented as shown in Figure 4, which can be simply controlled by the modulation of the ON/ OFF states of the PIN diodes. The simulated far-field radiation patterns for the scanning pencil beams in four directions are plotted in Figure 5. For the broadside radiation, a directional beam is obtained with a simulated gain over 25.3 dB, corresponding to an aperture efficiency over 27% by using the following definition: where G is the simulated gain and A is the metasurface aperture area. When furtherly considering the quantization loss of about 3 dB for a typical 1-bit phase resolution, the metasurface aperture efficiency could be expected to exceed 50% theoretically. As the scanning angle of the steering beam increases, the main-lobe maximum gain would decrease due to the diminution of the effective metasurface aperture area, which is in accordance with the traditional phased array antenna. For all the four directional scanning beams, the side-lobe levels are −10 dB lower than that of the main lobes.  Frontiers in Physics | www.frontiersin.org October 2020 | Volume 8 | Article 589334 5 CONCLUSION In summary, a high-efficiency transmissive programable metasurface with 1-bit phase modulation is presented for the construction of a cost-effective phased array. The expected performances with real-time dynamic scanning beams are investigated, and scanning pencil beams in four directions with low side-lobe levels are generated through dynamically programming the 1-bit code distributions on the metasurface, which verify the effectiveness of the presented transmissive programable metasurface design.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.