Graphene-Based Full-Duplex Antenna for Future Generation Communication in THz Frequency Range

A proximity-coupled graphene patch-based full-duplex antenna is proposed for terahertz (THz) applications. Te antenna provides a 10dB impedance bandwidth of 6.06% (1.76 − 1.87 THz). Te input ports of the proposed design are isolated from each other by − 25dB. Te aspect ratio of the graphene-based radiating patch and the physical parameters of the antenna is selected for obtaining the single-mode operation. Te dimensions (length and width of graphene) of the proposed antenna have been opted to operate in two higher-order orthogonal modes, and these modes remain intrinsically isolated. Te utilization of the graphene material provides fexibility in tuning the antenna response. Graphene-based patch exhibits good electrical conductivity, electrical conductance controllability, and plasmon properties. Te graphene-based antennas perform better than their metallic counterparts, especially in the THz frequency range. Te radiation properties of the graphene material are more prominent due to no-ohmic losses. Moreover, its chemical potential may be altered by applying a bias voltage to its surface conductivity, which modifes the surface impedance value of graphene. Terefore, with a small footprint, graphene acts as an excellent radiator at extremely high frequencies.


Introduction
With the advancement in high-speed data communication, terahertz (THz) components have been investigated for the present and future through innovative processes and technologies. Antennas in the THz frequency ranges (0.1 to 10 THz) have been extensively investigated and engineered for diferent applications. Expanding compact transceiver subsystems of data transmission at low power with ultrafast speed and superbroad bandwidth-like features are desirable for future generation communication [1][2][3][4][5][6]. Microwave and millimetre wave communication systems use metallic antennas/radiators in transceiver systems. However, they display poor conductivity at higher frequencies or the THz range [7][8][9]. Consequently, it is required to discover the replacement of metallic radiators in diferent devices operating in THz frequencies. Switching carrier frequencies to the terahertz band is a natural substitute for meeting future-generation needs, i.e., high-speed data communications [10][11][12]. However, compared to lower frequency antenna systems, the THz antennas have specifc characteristics, including higher transmission path loss and additional molecular losses due to the absorption of the radiation energy [13]. In recent times, to combat this, many strategies have been employed. Te highly directional antennas have been suggested to overcome path loss issues and to improve the channel capacity [14]. On the other hand, conventional radio frequencies (RF) and optical transceivers have numerous shortcomings, such as their bulky size, design complexity, and energy consumption [15,16]. Tese constraints have prompted researchers to investigate novel nanomaterials as the foundation for next-generation electronics beyond silicon. Graphene is one of the most promising substitutes [17][18][19][20][21][22][23][24][25][26].
Graphene exhibits good electrical conductivity, electrical conductance controllability, and plasmon properties. In case graphene is integrated into antenna systems, it shows improved radiation properties than typical counterparts in metal antennas at extremely high frequencies. Tis is due to its good conductivity and the fact that its chemical potential may be altered by applying a bias voltage to its surface conductivity, which varies the value of the surface impedance of graphene. Moreover, its conductivity can be tuned either by doping during manufacturing or by applying an external electrostatic direct current (DC) voltage [14,[27][28][29]. Recently, several graphene-based antenna designs have been reported for diferent applications. A graphene-based antenna was reported for the ultrashort range impulse communication and biosensing applications in [30,31], respectively. Te graphene material was loaded in the realization of plasmonic antennas [32]. A mathematical model was computed for a planar graphene antenna with a triangular-shaped radiator [33]. As mobile communications evolve into the ffth generation, to ofer a high-speed data rate and reduced latency, multi-input-multioutput (MIMO) antenna systems were investigated. Various MIMO antennas operating in microwave, millimetre wave (mmWave), and THz frequency ranges were reported in [26,[34][35][36].
A full-duplex antenna system is required to deal with practical wireless communication connectivity. In general, a full-duplex antenna system comprises two indispensable components, i.e., duplexers and antennas. Traditionally, they are designed independently and integrated with the help of a suitable transmission line. However, incorporating these components increases the system complications in limited space and leads to colossal power consumption due to a massive number of radio-frequency chains. In addition, such confguration increases the complexity and is less favourable in array extension. Hence, the concept of a selfduplex antenna was introduced in recent research [37][38][39][40][41][42][43]. Tese antennas allow a more straightforward solution to enhance the performance of a two-way communication system. Te main concern of THz antennas is their low gain and radiation efciency. Tis enforces exploring a suitable way to implement THz antennas with duplexing (fltering) functionality. To the authors' knowledge, no work has been reported for full-duplex design in the THz communication range. Here, a graphene-based full-duplex antenna for THz communications is introduced. A single graphene patch is excited with the help of a proximity-coupled two-port feeding mechanism. Te design maintains the simple confguration of the planar feeding technique. Te graphene has proven capability to exhibit good radiation characteristics, owing to the absence of Ohmic losses and surface waves. Tese attractive features of graphene make it a preferred choice over a metallic antenna, especially at the THz frequency range.
Te diference in the architecture of the conventional and the self-duplex antenna for dual-band operation is illustrated in Figure 1. Te duplex antenna is a radiofrequency component that supports the separation or combination of two frequency channels (bands) and allows them to operate on a single antenna module. Duplex antennas are provided with low-frequency and high-frequency ports. Te idea can be further extended as N-frequency with N-input ports. Here, N is the number of frequency channels.

Graphene-Based Full-Duplex Antenna Modelling
Te geometrical view of the proposed graphene-based selfduplex antenna is shown in Figure 2. Tis confguration contains a layered structure of two substrates with proximity-coupled feeding systems. Te silicon dioxide is used as a dielectric substrate with a permittivity of 3.8. Te substrate of height (h1) is placed just above the perfect electric conductor (PEC). Another substrate-2 of height (h2) is placed just above it. Te PEC-based 50 Ω microstrip feed lines are sandwiched between these two silicon-based dielectric substrates. A thin flm of the graphene material is polished on the top layer of the dielectric substrate-2. Te thickness of the graphene flm is optimally chosen as 0.001 mm. Tis flm acts as a radiator when it is excited by proximity-coupled feeding. To realize the full-duplex functionality, an orthogonal feeding system is used. Te aspect ratio of the proposed design is maintained in such a way that both microstrips feed excite the orthogonal higher-order modes. Moreover, the resonant frequency corresponding to the excited feed can be controlled by changing the corresponding dimension of the rectangular patch. Te impedance-matching characteristics of the patch  can be optimised by changing the length of the microstrip line. Also, the resonance of the patch can be tuned by changing the length of the microstrip line, as it controls the magnitude of the coupling energy with the graphene patch. Te graphene patch is modelled by adopting the mechanism explained in [25]. Te parameters of the graphene layer, including its thickness (t) and relaxation time (τ), are opted such that the antenna can be realized physically. Tese parameters are designated in such a way that a suitable positive value of the external electrostatic DC voltage is required for the tuning of chemical potential (μc) and the surface conductivity (σg) of graphene material. In the antenna structure, a metal gate layer is implanted at the top of the graphene patch for applying the DC gate voltage. Te length and width of the metallic gate layer are kept equal to the graphene patch so that its Fermi level can be maintained uniformly when the external voltage biasing is performed. Te surface conductivity of graphene varies with radian frequency (ω), scattering rate (Γ � 1/τ), temperature (T), applied magnetostatic bias feld (Bo), and μC. Te chemical potential is a function of the applied electrostatic feld (Eo). Kubo's formalism reports that σg is a combination of Hall's and diagonal conductivity [25]. Te Hall's conductivity of graphene becomes zero for Bo � 0, as given in [6]. Tus, σg is only due to Eo, which is generally called diagonal conductivity. Te diagonal conductivity of graphene is the composition of interband and intraband transitions [1][2][3][4]. Tus, σg is only due to the intraband contribution in the operating frequency band of the designed two-port antenna.

Working Principle and Result Analysis
Te dimensions of the proposed design are provided in Table 1. Te length and the width of the patch antenna are optimised in such a way that the antenna ofers the two distinct frequency bands for simultaneous transmitting and receiving channel operation in the frequency range of THz frequency applications.
Finally, the optimised design operates at a lower frequency band of around 1.8 THz and a higher frequency band of about 2.0 THz frequency. Furthermore, the tuning of the antenna response is possible by varying the chemical potential of the graphene material, a feature available in CST MWS. Figure 3 shows the electric feld distributions at the operating frequencies at Port-1 and Port-2. It can be clearly evidenced that the antenna operates in higher-order modes. As shown in Figure 3(a), the electric feld shows three half-wave variations along the length of the graphene patch. On the other hand, it ofers two half-wave variations when Port-2 is on. It is observed that the incident feld is radiated through the appropriate aperture without being transmitted to another port. Tereby, input port isolation is enhanced and maintained. Finally, the optimised design with parameters is shown in Table 1. Te lower resonant frequency occurs at 1.83 THz and is generated when Port-1 is excited, and the antenna radiates along the broader side of the rectangular graphene patch. Similarly, the resonance at upper resonant frequency is generated from Port-2 when it is excited along the narrower side of the rectangular graphene patch. Te length of the radiating patch is chosen to be much larger than half of the guide wavelength at the resonant frequencies, so it shows functionality at higher-order modes corresponding to Port-1 and Port-2.
Te frequency response of the proposed design in terms of S-parameters is shown in Figure 4. Te design shows that the coupling of the energy from Port-1 to Port-2 and vice versa is below −20 dB. Te same can be inferred from Figure 3 that the resonant frequency of each channel can be tuned without afecting the other channel by maintaining the same level of port isolation. Also, it operates in diferent high-order modes. Te 3D far-feld radiation pattern at the resonant frequencies is shown in Figure 5. Te proposed design shows unidirectional radiation patterns due to the large ground plane at the bottom. Te peak directivity of the antenna in the broadside direction is noted as 2.44 and 2.79 dBi at 1.83 and 2.06 THz, respectively. Moreover, the proposed THz design has a compact structure and simple design procedure, which would make it attractive to choose for upcoming communication systems. Te proposed design ofers the fexibility of using one device and one board layout to cover dual-frequency bands and substitutes the conventional design mechanism.
3.1. Future Applications of THz Technology. THz range of the electromagnetic spectrum has increasingly been used in numerous sectors for practical purposes, and research on the use of THz technology in communications, radar, imaging, sensors, and other areas has been reported in recent years.

Conclusion
A proximity-coupled graphene patch-based two-port duplexing antenna is realized for simultaneous transmitting and receiving channel operation in the frequency range of THz frequency applications. Te physical parameters of the antenna can be selected for desired operating frequency. Te operating frequency maintains a small frequency ratio, and the same can be manipulated further. Te antenna maintains isolation better than 20 dB between input ports which helps to realize duplexing functionality and radiation characteristics as a single unit. Te proposed design ofers the fexibility of using one device and one board layout to cover dual-frequency bands and substitutes the conventional design mechanism. Te proposed design possesses an overall compact footprint, which makes this design extremely useful for applications in forthcoming wireless communication systems.

Data Availability
No data were used to support the fndings of this study.