RECONFIGURABLE GRAPHENE-BASED MULTI-INPUT-MULTI-OUTPUT ANTENNA FOR SIXTH GENERATION AND BIOMEDICAL APPLICATIONS

: In this paper, a small MIMO antenna with a semi-hexagonal form is developed for use in terahertz applications. The suggested antenna consists of four radiating components printed on a Silicon Dioxide substrate that is 90 ×90 µ𝑚 2 , with a thickness of 10 µm. The radiating components have been positioned in an orthogonal orientation to produce excellent isolation and miniaturization of the MIMO system. The suggested MIMO antenna works for all the (0.1-10)THz bands with different values of chemical potential with wide impedance bandwidth (S11 ≤ -10dB) in the frequency range of 2.4 to more than 10 THz, With a co-reflection coefficient less than -20 dB over the whole operating band, with a return loss -50 dB. The MIMO antenna has a maximum gain of 8.4 dBi and a steady diversity performance across all the working bands. According to the high-performance characteristics, the suggested graphene MIMO antenna design can be used for many applications in the THz band, including 6G high-speed wireless communication systems, security scanning, biomedical applications, IoT (Internet of Things), and sensing


Introduction
Recent years have seen an increased demand for the unused frequency spectrum.For higher carrier frequency, future communication systems focus on the THz area, more channel capacity, and better data speeds [1].On the electromagnetic spectrum, terahertz radiation is emitted between infrared and microwave radiation in the range of 0.1-10 THz.hence has numerous features with these other forms of radiation.Similar to infrared and microwave radiation, terahertz radiation is non-ionizing and travels straight through materials.Identical to microwave radiation, Terahertz radiation may pass through various non-electrically conducting materials [2].
Between (2027 and 2030) years anticipated that the sixth-generation (6G) wireless communication system, a new wireless communication paradigm, will be deployed.This system will have the full support of artificial intelligence.The infrastructure for sixthgeneration (6G) wireless communications will demand frequency working at terahertz frequencies [3,4].Due to a lack of available equipment, materials, measurements [5], detectors, and sources that work within the terahertz band, the band of the electromagnetic spectrum that was concerned with the development of the 6 G with terahertz frequency was the one that got the most amount of study [6].

Original Research
Beyond the 5G system, some fundamental concerns that need to be solved include a more significant system capacity, higher data rate [7,8], an enhanced quality of service (QoS), reduced latency, and a better level of security compared to the 5G system [9].
The capacity to manage massive data volumes and very high data rates per device is essential for wireless networks that support 6G technology.It would also follow the tendencies of past generations, including adopting new technology and services.The new offerings are A.I., 3D mapping, intelligent wearables, implants, driverless cars, sensing, and computing reality gadgets [10].In certain circumstances, it is expected that the per-user data rate in 6G will be close to one terabit per second [1,11].It will deliver a simultaneous wireless connection one thousand times greater than the 5G system.Metals, particularly gold, are the material of choice for constructing antennas that operate in the THz frequency regime.The poor conductivity of metal at the THz frequencies band, compared to its conductivity at D.C. frequencies, allows for increased field penetration into metal [12].So, to reduce the losses in metallic antennas, it is proposed to investigate the use of nonmetallic materials.Graphene is the material of choice to get minimum losses in the THz region and is a brandnew substance made up of a single atom of graphite, a kind of carbon with a variety of exceptional electrical and mechanical properties [13].It is considered the strongest and thinnest known material sheet.Its conductivity can be changed to behave like a semiconductor or a metal, which will make it an excellent choice for high-frequency electronics [14].
THz applications, such as imaging, spectroscopy, and others, have seen significant advancements in the last years [12].Metals like gold are frequently used in the construction of antennas that operate at THz bands.Metallic antennas work at THz frequency scales and can penetrate more deeply and produce less radiation because the conductivity of metal is lower when measured at terahertz frequencies than it is at D.C. frequencies.[13].To cut losses in smaller antennas, research into the use of nonmetallic materials is advised.The preferred material for reducing losses in the THz region is graphene.[14].
The need for a higher transmission capacity, a wider bandwidth, and Modern wireless communication methods have become more prevalent, which has led to better use of the available frequency range.The technically advanced advancement is utilizing many antenna components at both ends of the network.These wireless systems are called MIMO, meaning multiple inputs and outputs [15].Without boosting transmit power or bandwidth, it offers a higher data rate [16].It is a workaround for the data rate limitations imposed on single-input, single-output (SISO) systems.Additionally, MIMO may be implemented in different networks to improve the system's dependability, the speed at which data is sent, and the channel's performance [3,17].In a MIMO antenna, the decreased distance between the antennas will increase the mutual coupling, resulting in more correlation coefficients and reduced efficiency and gain [18].This paper presents a wideband, effective, and reconfigurable MIMO antenna based on graphene for THz applications.

Conductivity of Graphene
Graphene is a substance that consists of only two dimensions and has a thickness of one atomic [18].The graphene surface conductivity may be broken down into two distinct categories: the first category, known as the intra-band, is Predominant in the below five THz, while the second category, known as the inter-band, predominates in the higher frequency ranges.Their effects on the surface conductivity of graphene can be seen on both sides of the equation [19].Graphene's surface impedance may be calculated based on its conductivity using the equation provided below, where   is the applied voltage [20].

The Designed Single Antenna
Fig. 1(a),(c) shows the proposed semi-hexagonal graphene microstrip patch antenna connected to a feed line with a width of Wf=3 µm, and its length Lf=8 µm that is designed by the microstrip equation in [21].Two triangular-shaped slots have been carved into the radiating element's lower edge.A middle quadrilateral slot with a partial ground plane obtained Lg is equal to the quarter of the wavelength, and Wg is optimized as in Fig. 1 On the silicon dioxide (Sio2) dielectric substrate's front surface 40 x 45 µ 2 with   =3.9 , The height is 10 µ, and two sub-layers above the silicon dioxide are the Alumina and silicon crystalline, each one has a thickness 1 µ.The antenna is composed of graphene elements with a relaxation time of 0.1 ps and a thickness of 0.001 nm.Table 1 shows the optimal dimensions of the designed antenna.The proposed antenna is developed and optimized by the full-wave electromagnetic computer modeling technique CST 2020.Fig. 2 depicts the primary antenna setup, which works at the multi-band in the THz band and minimum return loss close to -50 dB.Fig. 3 shows the radiation far-field, displaying a directivity of the main lob is 10.7dBi at f=9 THz.Fig. 4 depicts the gain IEEE maximum value 11.06 dB at 9.5 THz.The distance between each antenna is set at 28 µm according to (⁄2), as represented in Table 1, to lessen the co-reflection coefficient and boost the performance.

S-Parameters
The designed MIMO antenna works in multiband with different values of µc that present a reconfigurable MIMO antenna that covers all the frequency bands (0.The chemical potential will also shift whenever we adjust the D.C. voltage, but the return loss remains lower than -10 dB.The chemical potential changed the MIMO antenna bandwidth in Fig. 7,8,9, and 10.Because the antenna's chemical potential may be changed in response to the D.C. voltage that is delivered, it is possible to tune the antenna to a particular frequency. This indicates that the return loss  11 at specific frequencies is less than -10dB, but at other frequencies, it is not.The return loss  11 at µc (0.1,0.3,0.5,0.7,1)shown in Fig. 11 shows that this MIMO antenna works for the entire (0.1-10) THz frequency band, so a reconfigurable MIMO antenna result.

The far-field radiation pattern
The planned MIMO antenna far-field is depicted in Fig. 12 and 13, which show the realized gain that rises as the frequency of the band of interest increases.A higher data rate for long-distance transmission is needed for the development of the MIMO design, which ensures the graphene antenna's compliance in the THz frequency band.When attempting to determine whether or not the ports correlate with one another, the envelope correlation coefficient (ECC) must demonstrate that each channel is independent.It is less than 0.003 for all the THz bands, as shown in Fig. 15.The ECC value should be relatively small.And fall between 0 and 0.5.MIMO antennas must have very low ECC, which could be computed by using the S-parameters using the equation that shown below [22].The equation used to calculate the diversity gain of the proposed MIMO antenna is as follows [22](6): To get a diversity benefit close to 10 dB, the correlation across ports should ideally be lower than 0.5.The outcomes of modeling diversity gain among radiating element ports are illustrated in Fig. 16.The calculated value of the diversity gain of the graphene MIMO antenna is less than 9.99 over the whole spectrum, demonstrating that the suggested antenna has a suitable performance.

Conclusions
The design and numerical analysis of a reconfigurable four-element MIMO antenna intended for use in THz applications have been completed.In great depth, this study describes the development, design, and geometry of a valuable antenna for THz applications.The graphene MIMO antenna has a minimum return loss of around -50 dB and operates in a multiband of THz band with different values of µc.The suggested MIMO antenna demonstrates a peak gain 8.4 dB in 10 THz, and its ECC is closer to 0.003 dB for each two ports of the MIMO antenna.This study concludes that raising the voltage applied to graphene strips reduces graphene surface impedances, resulting in the resonance frequency shifting towards higher frequencies.Therefore, the suggested MIMO antenna is appropriate for various THz applications due to its high gain, excellent diversity performance and steady radiation pattern examples of these applications include the diagnosis of breast cancer, the measurement of sugar, and the detection of drugs.

Figure 1 .
The single design (a) front, (b) back, and(c) side view.

Fig. 14
Fig.14 depicts the simulation of gain vs frequency for the present graphene-based MIMO antenna, demonstrating that the gain IEEE is above zero dB for all (0.1-10)THz since the graphene MIMO antenna works in THz band, with the peak gain value being 8.1dBi at 9.2 THz.

Figure 16 .
Figure 16.The DG for the MIMO antenna

Table 1 .
The measurements of a single design variables Value (µm) variables Value (µm)