Research on High-Gain Millimeter-Wave and Terahertz Antennas for 5G and Beyond


Student thesis: Doctoral Thesis

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Award date10 Feb 2021


At the dawn of the fifth generation (5G) wireless communications era, the ever increasing demand for higher data transmission rate continues to drive carrier frequencies into the millimeter-wave (MMW) and terahertz (THz) bands for improved channel capacities. However, the increase of the operating frequency imposes significant challenges in MMW and THz antenna design; it incurs high atmospheric absorption loss, significant metal and dielectric material losses, and limited fabrication tolerance. In this thesis, we present several high-gain antenna solutions to address these challenges by developing a phase modulation technique for lens antennas design, adopting orbital angular momentum (OAM) for multiplexing, and exploring an amplitude modulation technique for leaky-wave antennas design.

First, the aperture phase modulated technique is developed to design THz lens antennas. Discrete dielectric lenses (DDL) are an appealing high-gain THz antenna candidate, owing to their advantageous characteristics such as low loss, simple feeding networks, arbitrary aperture phase control, and ease of fabrication. Moreover, we utilize the emerging 3-D printing technology and the superior geometric flexibility it provides to design and fabricate novel DDL antennas that can generate high-gain circularly polarized waves. In addition to fixed-beam generation, reconfigurable beams based on a dual-lens antenna system are also explored, which include a two-dimensional (2-D) beam-scanning Bessel launcher and a 3-D focus scanning metalens. The reconfigurable beams of these antennas are simply realized by in-plane rotation of two thin and light-weight DDLs.

Second, the inherent orthogonality among different eigenstates enables OAM beams to provide a theoretically unlimited number of channels, thereby increasing the transmission channel capacity. We design three OAM lens antennas based on the phase modulation technology. To overcome the intrinsic divergence characteristic of OAM beams at radio frequencies (RF), we design a novel 3-D printed DDL antenna that generates non-diffractive OAM beams covering an intended longitudinal region operating at 300 GHz. Two synthesis methods based on the geometric optics and alternating projection method are developed to synthesize the aperture phase distribution of the DDL. Furthermore, we propose a novel 3-D printed DDL antenna with OAM mode reconfigurability operating at 300 GHz. The antenna consists of a stationary lower DDL and an upper in-plane rotatable DDL, fed by a stationary pyramidal horn. The DDL pair can transform the quasi-Gaussian beams from the feed source into vortex waves, and its OAM mode number, l, can be dynamically reconfigured to 0, ±1, or ±2 via a simple mechanical in-plane rotation of the upper DDL panel at specific angles relative to the stationary lower one. Moreover, we design a holographic-inspired flat lens antenna capable of simultaneously transmitting two independent coaxially propagating OAM. Due to limitation on the fabrication capability and the lack of two excitation sources at 300 GHz, the antenna is designed to operate at 60 GHz. The feed sources do not carry OAM modes, but the flat lens can transform the spherical wave front from the two feed sources into transmitted vortex waves with the OAM mode numbers configured to l = −1 and +1 simultaneously. Other mode combinations are also feasible. Both the simulation and measurement results demonstrate that design of these lens antennas effectively realizes non-diffractive OAM beams, reconfigurable OAM modes and OAM multiplexing.

Finally, as a complementary modulation technique to previous phase modulation, we propose an amplitude modulation technique to design new leaky-wave antennas. The concept of amplitude-modulated (AM) leaky-wave antenna is inspired by the well-known amplitude modulation technique from classical communications theory. Leaky-wave antennas are also attractive as high-gain antennas at MMW and THz frequencies owing to their advantages of high directivity, low profile, frequency beam-scanning capabilities, and simple feeding structure. We theoretically demonstrate that spatial amplitude modulation corresponds to shifting the spatial spectrum of the modulating wave up to the carrier spatial frequency. To justify the proposed concept, two AM leaky-wave antennas based on corrugated transmission lines are designed: a sinusoidally AM leaky-wave antenna for high-gain applications and a beam-forming AM leaky-wave antenna, whose modulated wave generates flat-top radiation pattern. Both simulation and measurement results verify the concept and design of the AM leaky-wave antennas.