Abstract
Electromagnetic (EM) waves with orbital angular momentum (OAM) provide a new degree of freedom for channel multiplexing to improve the capacity of wireless communication. For OAM-based systems, it is important to design specific configurations to generate vortex radios. In this paper, a reconfigurable reflectarray antenna is proposed with independent control of dual polarizations. A reflective cell is proposed by properly assigning the variable capacitances of four varactors, which are placed between metal square rings of each unit. The varactors of each unit are divided into two groups and the capacitance value of each group controls the reflection phase for a single linear polarization. By using the equivalent circuit model, the reflective units and array can be designed efficiently. Smooth phase variation and good reflection efficiency are achieved. Then, the reflectarray is set into sectors and a simple phase-shifting surface model is used to generate vortex beam. Each sector is realized with reflective units satisfying desired reflection phases for different modes. This kind of OAM-generating method can reduce the required variation range of reflection phase and provide more choices for a specific OAM mode combination with dual polarization, which is helpful to reduce mutual coupling between the two linear polarizations. Finally, full-wave simulations show that the 0, ±1, ±2 modes of vortex beam are successfully generated at 3.5 GHz with arbitrary combination in dual-polarization, which is also supported by OAM modes purity and reflection efficiency analysis. Therefore, in our design, the reconfigurable OAM and spin angular momentum (SAM), related with polarization, can be utilized simultaneously and independently for high-capacity wireless communication.
Highlights
Vortex waves carrying orbital angular momentum (OAM) have infinite orthogonal states in theory, while electromagnetic wave with spin angular momentum (SAM) has only two orthogonal states
Studies focus on OAM-carrying beams in optical region,[2,3] but recently, many studies indicate that OAM can be applied to radio frequencies
When we obtain reflection phase curve with frequency ranging from 2.5 GHz and 4.5 GHz, we introduce it to Advanced Design System (ADS) to make a fitting between the full-wave model in HFSS and the equivalent circuit model
Summary
Vortex waves carrying orbital angular momentum (OAM) have infinite orthogonal states in theory, while electromagnetic wave with spin angular momentum (SAM) has only two orthogonal states. Tennant and Allen demonstrated that multiple OAM modes could be produced by using time-switched arrays.[6] Gradually, studies on OAM-carrying beams for radio communication have become a hotspot. It may be argued that OAM is only a subset of multiple-in-multiple-out (MIMO) communication systems,[7] OAM has the potential to compliment the performance of the MIMO system, such as low receiver computational complexity.[8] Within an OAM radio system, it is essential to explore functional antennas to generate vortex beams. Different from the work in Ref. 11, where a dual polarized metasurface able to generate one fixed OAM state was designed, our proposed reflectarray can generate 25 combinations of OAM modes reconfigurably by changing capacitance values of varactors, keeping dual polarization original characteristics.
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