# A Low-Profile Dielectric Resonator Antenna Array for OAM Waves Generation at 5G NR Bands
## 1 Introduction  
At present, wireless technologies with high-speed internet connectivity and higher data rates are in high demand. However, current wireless technologies belonging to the third generation (3G) and fourth generation (4G) cannot meet the demands of fifth-generation (5G) wireless requirements. Moreover, they cannot be used for long-distance communication and low-power, wide-area technology. Thus, 5G technology has become a most challenging and interesting topic in wireless research and will open up new opportunities to overcome existing development limitations. 5G New Radio (5G NR) is a new air interface defined by the 3 rd Generation Partnership Project (3 GPP) in Release 15 and consecutive releases [1]. It is specifically developed for future 5G operations. Recently, the 3 GPP has released a new specification and has permitted the following new spectrums for future 5G NR working below the Sub-6 GHz frequency bands, namely Band n77, Band n78, and Band n79 [2]. Band n77 ranges from 3.3 to 4.2 GHz and is currently being used in the United States. Most Asian and European countries presently use Band n78, which ranges from 3.3 to 3.8 GHz. Meanwhile, Band n79 ranges from 4.4 to 5 GHz. The main advantages of the 5G are higher data rates, lower latency, massive capacity, and higher quality of service [3, 4]. The electromagnetic (EM) momentum includes linear and angular momentum. Angular momentum consists of spin angular momentum (SAM) and orbital angular momentum (OAM), which define its polarization state and phase structure distribution, respectively [5]. SAM has been used for decades [6; 7; 8; 9; 10; 11; 12], while OAM was discovered by [PERSON] et al. in 1992 [13]. The authors combined the concept of OAM with the idea of an optical vortex. In an optical vortex, the planes of the constant phase of the electric and magnetic vector fields form a corkscrew or helicoid running in the direction of propagation. Since then, OAM has been widely developed in the optical and quantum domains [10; 11; 12]. OAM is carried by EM waves, which have the helical phase factor of exp (\(il\varphi\)), where \(l\) is the OAM mode number and \(\varphi\) is the transverse azimuthal angle [13]. For SAM, the rotation of the \(E\) and \(H\) fields is around the main beam axis. However, for OAM, the rotation of the \(E\) and \(H\) fields is around the propagation direction [14]. The existence of the phase distribution exp (\(il\phi\)) indicates that on a plane perpendicular to the propagation axis, the phase of an OAM wave changes continuously by \(2\pi l\) upon one complete cycle around the beam axis. In addition, an OAM wave exhibits a helical wavefront, with \(l\) representing the number of wavefront twists within one wavelength. OAM has several distinct advantages compared to SAM; for example, it can provide an infinite number of states or modes since the OAM mode number \(l\) is a limitless integer. Circularly polarized (CP) waves carry SAM with only two orthogonal states: either right-hand circular polarization (RHCP) or left-hand circular polarization (LHCP) [15]. In addition, the beams with different OAM modes are orthogonal to each other, and they can be multiplexed or demultiplexed together [16]. Due to the orthogonality, the spectrum efficiency and channel capacity can be increased. Hence, more users can use the same frequency channel without extending the bandwidth [17; 18; 19]. For OAM applications, various antennas have been designed and optimized. Various methods can be used to generate OAM modes, such as spiral phase plate (SPP) [20; 21; 22], parabolic reflector antenna [23; 24], metasurface [25; 26], and uniform circular array (UCA) antenna [27; 28; 29; 30; 31]. The fundamental principle of SPP is based on changing the output phase of the beams in proportion to the azimuthal angle around the center according to the OAM mode number. The SPP is widely used because of its simple structure. However, this method can only generate one OAM state once the structure is decided.