1. Introduction

1.2. Related Work

1.2.1. Metamaterial-Based Antennas

Metamaterials in THz antennas offer unique properties like perfect absorption and transmission, enhancing sensing, imaging, and communication systems. These artificially engineered materials are structured periodically to exhibit exceptional characteristics [20]. THz metamaterial-based sensors utilize pseudo-surface plasma effects to amplify incident THz waves, improving signal quality. By incorporating semiconductors in multilayer structures, design flexibility is increased while maintaining negative permittivity for metamaterial integrity [21]. Active meta devices and topological photonic crystals are advancing THz applications, enabling control over free-space propagation and on-chip waveguides. The future holds promise for reconfigurable topological waveguides and topologically-protected meta devices in advanced THz technologies. Developments in metamaterial antenna design have focused on enhancing performance and flexibility. Intelligent methods like machine learning and topology optimization are being employed for meta surface design, enabling precise control over electromagnetic wave fronts. Experimentally validated dipole frameworks are being utilized to describe the properties of tunable metamaterial radiators, paving the way for improved meta surface array antenna designs [22]. Additive manufacturing, particularly 3D printing, is revolutionizing the fabrication of RF devices, antennas, and metamaterials, offering new material properties and geometries for enhanced applications. Composite right/left-handed transmission line (CRLH-TL) metamaterials are providing extra design freedom for creating high-performance, multifunctional antennas with re-configurability, catering to various applications like MIMO systems and wireless power harvesting [23]. These advancements signify a shift towards more efficient, adaptable, and compact antenna solutions in modern wireless communication systems.

Figure 1. Whittaker et al. 3D-printed perorated dielectric transmitarray (TA) with total dimensions of 180 × 180 × 32 mm.

Figure 1. Whittaker et al. 3D-printed perorated dielectric transmitarray (TA) with total dimensions of 180 × 180 × 32 mm.

Metamaterial-based THz antennas offer advantages like high interaction impedance, compactness, and multi-band operation for diverse applications such as medical uses [24]. These antennas enable high-speed data rates up to 1 Tbps, massive bandwidth of up to 100 GHz, and low latency connectivity crucial for 6G systems [25]. Challenges include the impact of atmospheric losses at high frequencies, the need for high-gain antenna arrays, and mitigating performance degradation when placed near human body models [26]. Additionally, the design complexity and material selection for achieving desired properties pose challenges in developing efficient THz antennas. Despite these challenges, metamaterial-based THz antennas hold immense potential for advancing communication, imaging, and material analysis applications in the THz band.

1.3. Printed and Flexible THz Antennas

Printed antenna technologies for THz frequencies offer innovative solutions for high-gain, lowcost antennas. Li and Luk proposed an open resonator antenna design at 0.1 THz, utilizing a planar cavity with a loaded phase-shifting periodic structure to achieve high gain and wide bandwidth. Wu et al. introduced a near-field focus lens antenna operating at THz frequencies, utilizing 3D printing for fabrication, showcasing 3D focus-scanning capabilities [27]. The integration of printed circuit boards and flexible printed circuits for efficient broadband antennas is discussed, highlighting the benefits and limitations of on-chip and off-chip designs [28]. These advancements in printed antenna technologies at THz frequencies demonstrate the potential for cost-effective, high-performance antennas suitable for various applications.

THz antenna design have shown promising developments in printed and flexible structures. Studies have explored hybrid designs incorporating materials like graphene and gold to enhance efficiency and flexibility [29]. Additionally, open resonator antennas with phase-shifting periodic structures have been proposed to achieve high gain and cost-effectiveness, utilizing threedimensional printing technology for manufacturing [30]. Furthermore, the use of innovative materials like Gallium-indium alloy (EGaIn) and Polyethylene naphthalate two formic acid glycol ester (PEN) has enabled the creation of flexible microstrip antenna arrays with superior mechanical properties and efficiency, catering to the demands of microwave energy transmission systems [31]. These collective efforts signify a significant stride towards enhancing the performance and applicability of THz antennas in modern communication systems.

Figure 2. Chen et al. Inkjet-printed UWB antenna.

Figure 2. Chen et al. Inkjet-printed UWB antenna.

There are several uses and advantages for printed and flexible antennas in THz systems. Highspeed wireless communications are a good fit for hybrid antenna designs that use materials like gold and graphene, which exhibit increased efficiency [32]. Furthermore, in one-dimensional THz applications, 3D-printed lens antennas can enhance beam-scanning and gain performance. Further, low transmission loss and single-mode properties of 3D printed flexible and stretchy terahertz waveguides allow for a wide range of applications in ultra-flexible and stretchable THz devices [33]. These developments show that extremely efficient THz antennas have broad potential for use, particularly in wearable technology and body-centric wireless communication.1.3 Reconfigurable and Smart Antennas

Terahertz (THz) applications benefit from reconfigurable antennas because they provide wireless communication systems with flexibility and adaptability. Because of their special qualities, graphene-based antennas have potential for reconfigurability. Beam reconfigurable directional antennas using graphene components, reconfigurable structured light for augmenting THz production, and multi-band patch antennas based on graphene have all been suggested [34,35]. Moreover, printed ridge gap waveguides with programmable capabilities for flexible terahertz systems have been implemented using graphene. Moreover, Substrate-Integrated Waveguide (SIW) filters that use graphene have been made tunable in the THz range [36]. The aforementioned developments underscore the possibility of reconfigurable graphene-based antennas to augment efficiency and usefulness inside THz wireless systems.

Smart and reconfigurable THz antenna designs have demonstrated creative methods for improving performance with graphene-based materials. Compact graphene-based MIMO antennas with good isolation and wideband capabilities are among these designs; they provide effective and reversible solutions for THz applications [37]. Furthermore, with the use of graphene-based components, terahertz dipole antennas with compound reconfigurability have been created, providing control over radiation pattern, frequency, and polarization state [38]. Additionally, a reconfigurable graphene antipodal Vivaldi antenna with increased gain has been shown [39]. This antenna employs frequency selective surfaces in addition to reconfigurability by modification of the graphene chemical potential. These developments demonstrate how graphene-based materials may be used to create smart, programmable THz antennas for a range of uses.

Figure 3. Jin et al. The structure of the graphene-based dipole antenna. (a) top view, (b) side view.

Figure 3. Jin et al. The structure of the graphene-based dipole antenna. (a) top view, (b) side view.

Figure 4. Jin et al. Structure of the CRTA. a) Top view. b) Side view.

Figure 4. Jin et al. Structure of the CRTA. a) Top view. b) Side view.

Through the adjustment of antenna array weights, adaptive beamforming techniques are essential in improving signal reception. Challenges such as unexplained mismatches and problems with the signal-to-noise ratio (SNR) are tackled in different ways. Improving resilience against mismatches may be achieved by using steering vector (SV) estimates in an interference-plus-noise covariance matrix (INCM) reconstruction method [40]. An alternative technique efficiently suppresses interferences under system defects by using Gauss–Legendre quadrature (GLQ) for INCM reconstruction and SV estimation. Furthermore, in non-ideal conditions, a unique robust beamforming approach achieves good performance and resilience against multiple mismatches by combining steering vector estimation with interference estimates [41]. These methods demonstrate developments in adaptive beamforming and address issues to maximize signal reception while reducing interference.

Improving smart THz antenna systems requires a significant contribution from machine learning. These technologies enable adaptive learning by quickly analyzing large volumes of data from different antenna configurations with the use of machine learning algorithms [42]. Furthermore, in order to optimize geometric parameters and suggest appropriate antennas, intelligent antenna synthesis techniques make use of ensemble models and support vector machines [43]. This results in excellent classification and parameter prediction accuracy. Real-time self-adaptive beamforming for 6G terahertz wireless communications in huge MIMO systems is made possible by deep reinforcement learning, which also predicts spatial phase profiles for desired radiation patterns. Furthermore, antenna designs may be optimized with the use of machine learning models like Extra Tree Regression, which drastically cut down on simulation time and resource needs [44].

1.4. Terahertz Antenna Arrays and MIMO Systems

A key component of Terahertz (THz) technology are antenna arrays. They are necessary to prevent air losses at high frequencies, achieve large data speeds, and provide dependable communication. In order to construct 6G wireless communication systems, THz antennas, antenna arrays, and Multiple Input Multiple Output (MIMO) systems are popular research subjects [19]. Researchers have been working on improving THz antennas employing a variety of substrates and materials, including polyimide, quartz, copper, graphene, and more, for Internet-of-things, imaging, and sensing applications [45]. THz antenna technology has also been used with reflector antennas, horn array antennas, and metamaterial substrates in radio astronomy, deep space exploration, and atmospheric remote sensing. For space communication, high-gain THz antenna arrays with notable gains and efficiency at particular frequencies have been developed.

Novel designs such as a patch antenna array with via and slot loading for D-band applications are examples of terahertz antenna arrays that offer high radiation gain and broad impedance matching. Furthermore, the creation of improved terahertz pulses with scalable directional gain and low-profile assembly has been made possible by the combination of InP/InGaAs unitraveling-carrier photodiodes (UTC-PDs) with planar bowtie antennas [46]. Additionally, a high-power, compact surface-emitting terahertz source with resonant-tunneling diodes (RTDs)-based active antenna has been presented [47]. It exhibits increased directivity via coherent oscillation and has promise for use in 6G communication and THz imaging. These innovations demonstrate how terahertz antenna technology is developing in order to achieve improved functionality and performance.

Figure 5. Koyama et al. The THz source chip prototype features an RTD chip measuring 3.2 × 3.2 mm², housing 36 active antennas, coupling lines, a common feed line, and wire bonding pads. Each antenna integrates a 170 × 170 μm² square patch antenna with two RTDs buried within a 7 μm-thick dielectric layer of benzocyclobutene (BCB) and silicon dioxide (SiO2). Metal vias establish electrical connections between the RTDs and the upper and lower electrodes of the patch antennas, enabling bias power feeding to the diodes and facilitating THz oscillation of the antennas.

Figure 5. Koyama et al. The THz source chip prototype features an RTD chip measuring 3.2 × 3.2 mm², housing 36 active antennas, coupling lines, a common feed line, and wire bonding pads. Each antenna integrates a 170 × 170 μm² square patch antenna with two RTDs buried within a 7 μm-thick dielectric layer of benzocyclobutene (BCB) and silicon dioxide (SiO2). Metal vias establish electrical connections between the RTDs and the upper and lower electrodes of the patch antennas, enabling bias power feeding to the diodes and facilitating THz oscillation of the antennas.

Multiple-Input Multiple-Output (MIMO) systems provide huge connectivity and good spectrum efficiency. Higher data rates and greater connection are made possible by THz-band NOMA communications, which use MIMO systems for antenna arrays [48]. Furthermore, the use of MIMO technology to Quantum Key Distribution (QKD) systems operating at THz frequencies is discussed. In these systems, the use of multiple antennas improves security and secret key rates are improved in situations when eavesdropping is restricted. Moreover, channel sparsity and channel estimation issues are addressed by combining ultra-massive MIMO with intelligent reflecting surfaces in THz communications, which improves system capacity and performance [49]. MIMO systems have a great deal of promise to enhance system performance in terms of spectrum efficiency, connection, security, and overall THz communication scenarios.

Figure 6. Zarini et al. Schematic of beamspace technology: (a) architectural components; (b) analog beam selection.

Figure 6. Zarini et al. Schematic of beamspace technology: (a) architectural components; (b) analog beam selection.

Terahertz (THz) MIMO systems face challenges and offer performance enhancements. Challenges include the impact of antenna element directivity and self-induced inter-symbol interference on THz MIMO system performance [50]. Innovative techniques such as tailored photonic crystal substrates and graphene-based patch antenna arrays have been proposed to increase performance, leading to notable reductions in total path loss and significant capacity improvements in THz MIMO systems [51]. Furthermore, difficulties like near-field region enlargement and channel sparsity must be controlled, however integrating ultramassive MIMO and intelligent reflecting surface systems can successfully overcome coverage restrictions and line-of-sight obstructions in THz communications [52]. THz MIMO systems can greatly increase capacity and open the door for upcoming ultra-high-speed wireless networks by utilizing these developments and resolving related difficulties.

2. Conclusions

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