Reflective Metasurface With Steered OAM Beams for THz Communications

The abundant spectrum resources and low beam divergence of the terahertz (THz) band, can be combined with the orthogonal propagation property of orbital angular momentum (OAM) beams to multi-fold the capacity of wireless communication systems. Here, a reflective metasurface (RMTS) is utilized to enhance the coverage of the high gain THz OAM beams by enabling the non-line-of-sight (NLoS) component by reshaping the planar wavefront of the incident wave into the helical wavefront, so that it is re-directed towards the direction of interest. This can contribute to alleviating the concern of the small aperture size, since improving the channel capacity can be achieved at the low spectrum blocks of the THz band (larger aperture size). For validation, three $90 \times 90$ mm RMTSs are simulated, fabricated, and tested in the frequency range 90–110 GHz, to re-direct single and dual OAM beams towards the desired location.


I. INTRODUCTION
For a long time, terahertz (THz) applications were confined to imaging and sensing as pertinent to the scarcity of high-efficiency THz devices [1]. However, the rapid development of THz technologies has attracted attention to the THz gap (between 100 GHz and 10 THz) to be used in several applications associated with security, astronomical observation, and localization [2], [3], [4]. Nevertheless, the growth of the world's hunger for higher data rates and low latency wireless communications will deplete the available frequency resources [5], which has motivated researchers to resort to the THz regime. To integrate delay-sensitive and data-demanding applications, driving up the channel frequency to the THz regime can be embraced to satisfy the demand for higher capacities [6]. Thus, employing THz spectrum blocks will enable operators to improve wireless communications and provide a plethora of non-communication applications.
In addition to utilizing higher frequencies, the spatial distribution of propagated waves can be employed as a The associate editor coordinating the review of this manuscript and approving it for publication was Walid Al-Hussaibi . new modulation procedure to enhance the system's throughput. For example, orbital angular momentum (OAM) beams can improve channel capacity and spectral efficiency for short-range THz communications [7]. The orthogonality among different OAM modes enables exchanging of data simultaneously through several streams at the same frequency, compared to sending a single beam in traditional antennas. This screw dislocation of phase can be expressed mathematically by e jlϕ , where l denotes the mode's number or the changes of the wavefront around the intensity null and ϕ represents the azimuthal angle [8], [9]. By that, no additional resources, such as power, time, and frequency, will be required to deliver services to users [10]. Employing OAM beams means that a new generation of transceivers will begin, since these beams depend on the angular momentum compared to the linear momentum of traditional devices, which is restricted to one-dimensional motion.
Generating THz OAM beams can bring multi-fold benefits related to improving the bandwidth, channel capacity, and spectral efficiency. That returns to the vast bandwidth of the THz band and the additional degree of freedom provided by the OAM beams. Not to forget that the beam divergence decreases at short wavelengths, so the OAM beam divergence reduces at higher frequencies [11]. Adding to that, the short wavelength of THz frequencies will minimize the size of structures, so hundreds of antennas can be packed together, which contributes to reducing the divergence problem of OAM waves as well. Besides, this integration can reinforce both security and integrated sensing and communications.
Despite the promising solution provided by THz OAM waves, the high directivity of waves can lead to poor network coverage in the absence of the line-of-sight (LoS) component. The blockage of THz links would make them highly susceptible to attacks, even in short-range communications [12]. That returns to poor diffraction and scattering abilities of THz waves as well as being exposed to atmospheric attenuation. Accordingly, coverage enhancement should be given high priority. On the other hand, THz waves suffer from a lack of strong electromagnetic (EM) response from naturally occurring materials [13]. Engineered metasurfaces are a potential solution to enable the exotic EM properties and develop efficient and compact THz components. One of the advantages of using metasurfaces is manipulating the anomalous reflection of the arbitrary incident EM wavefront by two-dimensional (2D) metamaterials [14], [15].
In the latest report of the international telecommunication union (ITU), the RIS and OAM technologies at THz frequencies have been proposed as promising solutions to realize a full utilization of the spatial dimensions and enhance the performance of antennas [16]. Intelligent reflecting surface (IRS) has drawn the attention of the research society since it can play a critical role in the transition towards highly efficient and coverage-enhanced networks [17]. The developed networks can deliver high-quality-of-service (QoS) to loads of subscribers by sensing the environment, recycling existing radio waves, and enabling non-line-of-sight (NLoS) communications [18], [19]. The integration between RIS and OAM at THz frequencies can solve the limited bandwidth issue of RIS structures, reduce the divergence of OAM, and improve spectral efficiency. Moreover, the fabrication complexity at the high THz spectrum blocks can be alleviated by employing low THz frequencies. Thus, high spectral efficiency can be maintained at the desired frequency bands.
The rest of the paper is organized as follows: Section II discusses the motivation and contributions of our work. Afterwards, the design procedures of the unit cell and RMTSs for single-and dual-steered OAM beams are examined in Section III. Following that, simulation and experimental results are presented and compared in Section IV. Finally, Section V summarizes the contents of this paper.

II. MOTIVATION AND CONTRIBUTIONS
Frequencies at low spectrum blocks of the THz band (below 300 GHz) are not exposed to high levels of molecular absorption, allowing for short-range communications up to a few hundred meters [20]. Accordingly, it would be doable to operate THz IRSs to bypass the obstacles between the base stations (BSs) and users to achieve NLoS communications via short-distance links. Loads of advantages can be gained by utilizing IRSs, associated with introducing low-profile and power-efficient antennas that make them environment-friendly structures with the capability to perform full-band transmission.
The system's throughput can be enhanced even more by increasing the spectral efficiency to obtain the maximum utilization of the available bandwidth. The inherent orthogonality among OAM modes can be operated to simultaneously split data between multiple data modes for the same frequency. The generation of helical waves has been addressed in the literature by various structures of different features. The majority of the state-of-the-art designs are restricted to producing a single OAM state [21], [22], or are not feasible to be fabricated in the immediate term [23]. Nevertheless, many successful designs introduced multi-OAM states at the expense of increasing the system's complexity, price, and beam divergence through concentric uniform circular arrays (UCAs) [24]. The capability of the THz-IRS structure to flexibly control the phase of incident EM waves promotes employing this low-cost and highly reliable solution to generate different OAM modes. This paper introduces a reflective metasurface (RMTS) not only to enable NLoS communications but also to highlight the capability of these structures to convert the planar wavefront of the impinged beam into a helical wavefront (see Fig. 1). The large number of scatterers with a controllable phase of RIS can be utilised to overcome the disadvantages of traditional methods to generate OAM beams [10]. Here, directed beams with unique OAM modes are produced by three RMTSs in the band between 90 and 110 GHz. At this range of frequencies, reduced modal power coupling among the modes and less distortion occurs [25]. The power-free THz-RMTS can provide good coverage, enhanced bandwidth, high spectral efficiency, and low-cost secured communications. To the best of the authors' knowledge, the proposed design is the first fabricated and tested OAM-based THz RMTS. VOLUME 11, 2023

III. RMTS DESIGN
This section reviews the design process and EM concepts pertinent to the RMTS to generate directed OAM beams. A comparison between two unit cells will be introduced first, followed by discussing and rendering the required phase distributions for single and dual OAM beams.

A. UNIT CELL
The phasing scatterers are prone to patterning, fabrication, or even analysis approximation errors. The quantization error can happen due to the low accuracy of the fabrication process, where choosing the wrong dimension (or rotation) of the selected elements will cause a difference between the desired and quantized phases. Besides, the unit cell should provide a complete phase cycle (360 • ) so that the closest quantized phase can be realized. However, to maintain periodicity, the dimensions of unit cells should be small relative to the wavelength. Unit cells of less than (λ/2) are usually employed to reduce the phase quantization error and achieve a smoother phase variation. The phase error can be expressed by: where ψ d co−pol (m, n) represents the needed reflection phase of the mn th unit cell and ψ r co−pol (m, n) is the realized co-polar reflection phase [26].
Nevertheless, the transition towards higher frequencies raises the difficulty of fabrication. Accordingly, the trade-off between the phase error and fabrication complexity should be thoroughly considered. Multi-layer or stacked unit cells can be utilized as an alternative solution since they improve the phase range and reduce the lattice size [27]. Furthermore, lattice and scatterer can be square, rectangular or hexagonal shaped. For this reason, the reflection coefficients of square and hexagonal-shaped patches were investigated here as functions of the width w, as shown in Figs. 2(a) and (b). It is clear in Figs. 2(c) and (d), the hexagonal lattice delivers better performance for the reflection phase and magnitude compared to the coefficients of the square element. Knowing that the hexagonal lattice offers good performance in respect of dispersion isotropy and polarization sensitivity [28].
Accordingly, the element and lattice of the proposed design are selected to be hexagonal, as displayed in Fig. 2(a). The parameters of the proposed scatterer are optimized to achieve an adequate reflection phase range and keep the reflection magnitude close to 0 dB [29]. Adopting hexagonal elements of λ/2.5 at 110 GHz (λ/3 at 90 GHz) with a single copper patch as a variable to compensate for the phase can bring loads of benefits, since improved control can be achieved by reducing the empty spaces among elements. Moreover, the fabrication accuracy increases when a single solid patch is distributed over the surface.
The configuration of the proposed unit cell is depicted in Fig. 2(a). It is a wideband stacked element with a Rogers RT5880 substrate of h 1 = h 2 = 0.127 mm thickness. The hexagons' diameter is around s = 1.1 mm, whereas the parameter w varies between 0.1 to 1 mm to guarantee a phase range of around 270 • at the desired bandwidth. The achieved phase range can be considered sufficient for the proposed RMTSs since they will be illuminated by plane waves. That can be observed in the obtained phase distributions, as we will see in the next section. Fig. 2(c) illustrates the reflection phase and magnitude responses as functions of w in the frequency range between 90 and 110 GHz. Results were obtained by CST Studio Suite with the periodic boundary condition and Floquet excitation.

B. ORBITAL ANGULAR MOMENTUM
When a plane wave impinges the metasurface, it induces surface currents on the RMTS. By manipulating the phase of each element, the received beam can be reshaped and reflected. This modulation usually mimics the shape of the desired beam. For example, the phase distribution could be made of concentric circles to generate the spherical wavefront of pencil beams. Again, that can be utilized to produce vortex beams carrying OAM where the geometries should follow a helical phase distribution of e j.l.ϕ (l refers to the mode number and ϕ represents the azimuthal angle). To obtain a vortex with a specific mode l for metasurfaces with a feeder as in reflectarray antennas (RAs), each mn th element should reflect a phase of [30]: where k 0 refers to the free space wave number, and the distance between the desired scatterer and the feeding horn is denoted by R mn .û b is the beam direction, ⃗ r mn stands for its vector, the point (x m , y n ) represents the element's position, and ψ 0 refers to a constant phase. Therefore, to change the beam direction,û b and ⃗ r mn should be configured based on the elevation and azimuth angles θ and ϕ, respectively.

1) SINGLE OAM BEAM DIRECTED TOWARDS A DESIRED ANGLE
It can be noticed that the main difference between RA and RMTS is the source of the incident wave. An RA usually would have a feeder, so the distance between the RA and feeder, called focal length (F), determines the phase distribution to compensate for the potential reception delay among the distributed elements. Nonetheless, it is assumed that the RMTS will be illuminated by a plane wave [31]. For simplicity, the phase distribution of the RMTS can be acquired by substituting the distance between the transmitter of the plane wavefront and metasurface (D) in (2), so the phase can be obtained by: where θ and ϕ can be used to change the course of the beam towards the required direction.

2) DUAL OFF-CENTRED OAM BEAMS DIRECTED TOWARDS DESIRED ANGLES
The conventional routines to produce multiple beams depend mainly on superposition or holography [32], [33]. The phase distribution can be obtained through the sum of phase sets of multiple beams, so it is called the superposition technique. Therefore, the desired waves are radiated from different discontinuities on the surface. Whereas in holographic structures, the interference pattern is described by the surface impedance, which should represent the phase variation of a reference wave and the phase projection of an objective wave. The main drawbacks of the previously mentioned methods are related to the high sidelobes and challenging requirements for manufacturing at high frequencies [34].
The needed phase distribution to generate dual-directed OAM beams by the proposed RMTS can be obtained by modifying the formula introduced for RAs in [23]. In principle, all the forms of generating OAM beams create delay paths for the incident wave to re-generate vortices. By making the phase delay lines follow a fork shape, the circular paths can be converted into several branches, as shown in Fig. 4(a). Therefore, the main contribution of the phase will be near the fork lines, whereas the ambient lines will work on replicating the signal. Accordingly, only that part is considered in this work. Besides, elements with a size of around λ/2.5 at 110 GHz were chosen for all the studied RMTSs of 90 ×90 mm to achieve smoother OAM phase variation. Thus, the phase distribution of both beams is accommodated in a single RMTS. Producing dual beams requires new centres O k (x m t , y n t ) at which waves would be radiated, so the phase delay distribution of each beam can be obtained based on its position in the metasurface as follows [23]: The total phase distribution, shown in Fig. 4(a), is computed by substituting (4) in (3) as follows: Accordingly, the prototype of the proposed RMTS with dual OAM beams steered towards θ = −25 • & ϕ = 0 • and θ = +25 • & ϕ = 0 • of the fourth mode is depicted in Fig. 4(b). The RMTS contains 82 × 82 stacked unit cells VOLUME 11, 2023  and an overall size of 90 × 90 mm. It should be noted that prototypes were fabricated with the low-cost printed circuit board (PCB) technique.

IV. SIMULATION AND EXPERIMENTAL RESULTS
In this section, the design principle of the presented RMTSs in the range 90-110 GHz is validated through simulation and measurement results as follows:

A. SIMULATION RESULTS
Numerical simulations were conducted to demonstrate the performance of the RMTSs in CST Microwave Studio Suite for single and dual directed OAM beams at 90 and 110 GHz. A plane wave tool is used to illuminate the metasurface. The phase and magnitude distributions of the electric field (Efield) of directed OAM beams can be detected by rotating the observation cross-section. So, it would be perpendicular to the propagation direction. The needed rotation can be obtained by using transformation matrices by transforming the cross-section with an angle of ±45 around the x-axis. The E-field properties of the RMTS with dual directed OAM beams can be obtained by following similar steps. Taking into account that the cross-section needs to be transformed two times to observe the E-field of each beam (with an angle of ±25 around the x-axis). The phase delay lines are shown in Figs. 6(a) and (b) for the phase and magnitude, obtained by a cross-section parallel to the RMTS. On the other hand, the E-field phase distribution of the fourth OAM mode is indicated in Fig. 6(c) with spiral lines. Besides that, the energy gap can be seen in the E-field magnitude distribution Fig. 6(d). Here, a spectral analysis of the Fourier transform is employed to assess the purity of the generated OAM beams. The sampling phase F (φ) is extracted along a circle, so the Fourier relationship can be written as [35]: where a l is the OAM spectrum.  the transmitter α is close to 10 • at the specified range of frequencies. Thus, the distance (di) between the emitter and the surface can be given by: where L denotes the width of the RMTS. Thus, to illuminate a 90 × 90 mm metasurface, di should be around 51.4 cm for the range between 90 and 110 GHz. Since the OAM beams are steered towards different angles, a platform with a protractor was needed to accurately attain the required rotations. Figs. 8(a), (c), and (e) illustrate the system configuration of the discussed RMTSs. The first configuration started by rotating the arm to be located at 45 • as shown in Fig. 8(a). The measured amplitude of the E-field is over 17.5 dB, as demonstrated in Fig. 8(b) (black dashed plot). Since only a half protractor is supported by the platform, to do the measurement for the second RMTS, we rotated it around its z-axis by 180 • to direct the beam towards the receiver as portrayed in Fig. 8(c). Accordingly, around 16.5 dB is achieved for the measured amplitude of the E-field pattern, displayed in Fig. 8(d). The last setup was performed twice to measure the dual directed OAM beams at 90 GHz. In the first time, the arm was located at −25 • to measure the reflected OAM beam. In the following measurement, the RMTS was rotated around its axis to direct the 25 • OAM beam to the receiver, as Fig. 8(e) exhibits. Fig. 8(f) confirms the generation of OAM through the observed singularity in the magnitude distribution of the measured E-field.
At the same time, Figs. 8(b), (d), and (f) compare the simulated results (continuous blue lines) and measured results (dashed black lines). The good alignment between these results validates the simulation results and the introduced concept to build the RMTSs. The next phase of the measurement is accomplished by changing the angle between the receiver (fixed on the protractor arm) and the RMTS to determine the range of angles that are still viable to detect OAM beams, as a way to check the RMTSs sensitivity or misalignment cases. The protractor arm was varied with a step of ±1 • for a range of ±10 • . By that, the receiver's location was varied for fixed transmitter and metasurface locations. Figs. 9(a), (b), and (c) confirms that the proposed RMTSs were capable of reflecting OAM beams for only ±5 • range of angles.
However, some scenarios may not require the continuous reshaping of the wireless transmission environment. An RMTS can be built to reconfigure the wireless propagation environment for specific scenarios to avoid blockage of the BSs links caused by obstacles. For this reason and to reduce the design's complexity, power consumption, and cost, multiple adjacent RMTSs can be employed without operating active elements. Thus, by directing different OAM beams towards the same receiver, OAM multiplexing can be achieved.

V. CONCLUSION
This paper proposed three reflective metasurfaces with single and dual directed OAM beams to tackle the poor network coverage of THz waves in the absence of LoS communications. The integration between the OAM and THz RMTS technologies can improve spectral efficiency through a low-cost and low-profile solution. The presented metasurfaces of 90 × 90 mm were simulated, fabricated, and tested to verify the capability to control and steer the wavefront of the EM waves at high frequencies. The discussed environment-friendly device can be a potential solution to be utilised for future wireless sixth-generation (6G) communications.