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\begin{document}

\title{Performance Evaluation of a Compact THz MIMO Antenna for 6G Wireless Communications}

\author{
\IEEEauthorblockN{Ahmed S. I. Amar}
\IEEEauthorblockA{
\textit{Egyptian Technical Research and Development Centre}\\
Cairo, Egypt\\
ahmed.s.i.amar.adc@alexu.edu.eg
}
\and
\IEEEauthorblockN{Mohamed Boumalkha}
\IEEEauthorblockA{
\textit{Department of Physics, Faculty of Sciences}\\
\textit{Moulay Ismail University}\\
Meknes, Morocco
}
\and
\IEEEauthorblockN{Ahmed Samir}
\IEEEauthorblockA{
\textit{Department of Electronics and Communication Engineering}\\
\textit{Uruk University}\\
Baghdad, Iraq\\
Ahmed.S.ahmed@uruk.edu.iq
}
\and
\IEEEauthorblockN{Zakaria Che Muda}
\IEEEauthorblockA{
\textit{Faculty of Engineering and Quantity Surveying}\\
\textit{INTI International University}\\
Nilai, Malaysia
}
\and
\IEEEauthorblockN{Ahmed Solyman}
\IEEEauthorblockA{
\textit{Department of Engineering}\\
\textit{Glasgow Caledonian University}\\
Glasgow, United Kingdom
}
\and
\IEEEauthorblockN{Tamer Shamseldin}
\IEEEauthorblockA{
\textit{Technical Research Centre}\\
Cairo, Egypt\\tamer@alumni.purdue.edu
}
}


\maketitle

\begin{abstract}
This study presents a compact 4-element MIMO antenna array designed for THz and 6G communications. The antenna features a straightforward design, utilizing modified T-shaped monopole elements fed by microstrip lines, positioned on opposite sides of a compact board with overall dimensions of $105 \times 105$ $\mu$m$^2$. The proposed MIMO antenna demonstrates broadband functionality, operating effectively from 3.7 THz to 4.3 THz. The array exhibits strong potential in terms of key performance metrics, including S-parameters, radiation patterns, and gain/efficiency levels. This research work directly supports SDG 9 (Industry, Innovation, and Infrastructure).
\end{abstract}

\begin{IEEEkeywords}
6G, monopole antenna, MIMO, THz networks.
\end{IEEEkeywords}



\section{Introduction}
The evolution toward 6G wireless networks is expected to transform global communication by supporting a wide range of emerging applications such as self-driving vehicles and large-scale Internet of Things (IoT) ecosystems \cite{b1}. To realize these scenarios, systems must deliver extremely high data throughput, ensure dependable low-latency links, and provide uninterrupted connectivity. Achieving these capabilities depends heavily on the exploitation of terahertz (THz) frequencies.

The THz spectrum, spanning approximately 0.1--10 THz, offers immense potential but also introduces distinctive challenges, including strong atmospheric absorption, shorter transmission ranges, and severe propagation losses. These issues threaten signal quality and system reliability. Consequently, the design of advanced, compact printed antennas capable of mitigating these limitations is a key enabler for the successful deployment of future 6G communication platforms \cite{b2}.

Printed antennas offer a promising solution for meeting the rigorous demands of 6G communication. These antennas are characterized by their lightweight, low-profile nature and ease of integration into various devices, making them particularly suitable for modern mobile applications. However, the antennas must exhibit broad bandwidth, high radiation efficiency, and low mutual coupling, especially in MIMO configurations \cite{b3, b4, b5}. 

A significant challenge in designing printed antennas for 6G applications is achieving high data throughput in the presence of multipath fading and interference, which are exacerbated at higher frequencies. Current literature emphasizes the need for innovative antenna configurations that maintain high isolation between elements while optimizing radiation patterns \cite{b6}. Moreover, the demand for broadband functionality in printed antennas is becoming increasingly critical in 6G systems, as it enables simultaneous operation across multiple frequency bands. This capability not only enhances spectral efficiency but also ensures compatibility with existing technologies, making it easier to transition from current 5G to 6G networks. Recent research indicates that dual-band printed antennas can significantly enhance system capacity by enabling concurrent data transmission across different frequency bands, thereby facilitating more efficient network management \cite{b7, b8, b9}.

To address these pressing challenges, the development of a novel four-element printed MIMO antenna array, specifically designed for 6G and THz applications, is crucial. The overall configuration of the suggested design is represented in Fig. 1. As shown, this array features compact modified T-shaped monopole elements that are printed on a minimal $105 \times 105 \times 8$ $\mu$m$^3$ substrate, effectively balancing compactness with performance. The design’s characteristics were examined using CST 2024 \cite{b10}. Table I provides the specific parameter values for the suggested MIMO antenna design.

% --- Figure 1 ---
\begin{figure}[htbp]
\centerline{\includegraphics[width=0.45\textwidth]{IMG_2248.jpeg}}
\caption{Schematic of the THz MIMO array.}
\label{fig1}
\end{figure}



\begin{table}[htbp]
\caption{Parameter Values for the Suggested MIMO Antenna Design}
\label{tab1}
\begin{center}
\begin{tabular}{|c|c||c|c|}
\hline
\textbf{Parameter} & \textbf{Value ($\mu$m)} & \textbf{Parameter} & \textbf{Value ($\mu$m)} \\
\hline
\hline
W  & 105 & W3 & 5  \\ \hline
L  & 35  & L3 & 20 \\ \hline
W1 & 15  & W4 & 20 \\ \hline
L1 & 5   & L4 & 24 \\ \hline
W2 & 20  & W5 & 20 \\ \hline
L2 & 34  & W6 & 8  \\ \hline
\end{tabular}
\end{center}
\end{table}




\section{Single-Element Monopole Resonator}

This section discusses the critical parameters of the single-element THz antenna. The T-shaped monopole structure was selected due to its compact geometry and ability to support wideband operation, making it highly suitable for integration into next-generation wireless devices \cite{b11, b12}. 

Figure \ref{fig2} illustrates both the physical configuration of the antenna and its simulated reflection coefficient ($S_{11}$) response. The results indicate that the antenna achieves an impedance bandwidth ranging from 3.7 THz to 4.3 THz at the –10 dB reference level, aligning well with the requirements of the 6G frequency spectrum.


\begin{figure}[htbp]
\centerline{\includegraphics[width=0.48\textwidth]{IMG_2249.jpeg}}
\caption{S11 result of the monopole antenna.}
\label{fig2}
\end{figure}

\begin{figure}[htbp]
\centerline{\includegraphics[width=0.4\textwidth]{IMG_2250.jpeg}}
\caption{Surface current distributions at 4 THz.}
\label{fig3}
\end{figure}


The simulated surface current distributions at 4 THz are illustrated in Fig. \ref{fig3}. As shown, the current is predominantly distributed along the entire modified T-shaped radiating element, with the longer section of the structure actively participating in radiation. This corresponds to a longer effective electrical length, which supports resonance at 4 THz. As observed, the current is mainly concentrated in the lower and middle segments of the T-shaped element.



The antenna's 3D radiation pattern at its resonance frequency is illustrated in Fig. \ref{fig4}. As shown, a well-defined and quasi-omni-directional radiation pattern is observed for the designed antenna, making it suitable for wide coverage communications.



\begin{figure}[htbp]
\centerline{\includegraphics[width=0.4\textwidth]{IMG_2251.jpeg}}
\caption{3D radiation pattern of the single-element antenna at its resonance frequency (4GHz).}
\label{fig4}
\end{figure}



\begin{figure}[htbp]
\centerline{\includegraphics[width=0.4\textwidth]{IMG_2252.jpeg}}
\caption{Efficiency results.}
\label{fig5}
\end{figure}


Figure \ref{fig5} presents the simulated efficiency characteristics of the antenna across its resonant frequencies. The results confirm that the element consistently maintains high performance throughout its operational bandwidth. Specifically, radiation efficiency exceeds 99\%, while the overall (total) efficiency remains above 90\% across the entire band. At the resonance points, both metrics rise above 98\%, underscoring the suitability and robustness of the proposed structure for 6G wireless applications. 

In addition, the gain performance illustrated in Fig. \ref{fig6} shows that the monopole antenna achieves values in the range of 5.5--6 dBi, which contributes to improved signal strength and enhances the overall effectiveness of the design \cite{b6}.



\begin{figure}[htbp]
\centerline{\includegraphics[width=0.45\textwidth]{IMG_2253.jpeg}}
\caption{Maximum gain result of the F-shaped antenna.}
\label{fig6}
\end{figure}



\section{The Suggested 4-Elements MIMO Design}


This section outlines the key characteristics of the THz MIMO antenna, with its overall schematic presented in Fig. \ref{fig1}. 


\begin{figure}[htbp]
     \centering
     \begin{subfigure}[b]{0.5\textwidth}
         \centering
         \includegraphics[width=\textwidth]{IMG_2254.jpeg}
         \caption{Snn results of the MIMO elements.}
         \label{fig:Snn}
     \end{subfigure}
\\ 
     \begin{subfigure}[b]{0.5\textwidth}
         \centering
         \includegraphics[width=\textwidth]{IMG_2255.jpeg}
         \caption{Sn1 results of the MIMO elements.}
         \label{fig:Sn1}
     \end{subfigure}
     \caption{(a) Snn and (b) Sn1 results of the MIMO elements.}
     \label{fig:fig7}
\end{figure}



The S-parameter results, including the reflection coefficients ($S_{nn}$) for the four elements and mutual coupling ($S_{n1}$) between the elements, are displayed in Fig. \ref{fig:fig7}. These results demonstrate the antenna's dual operational coverage, ranging from 3.7 to 4.3 THz at the -10 dB threshold, effectively covering critical frequencies for emerging 6G applications. 

The elements exhibit identical reflection coefficients, and the resonators show consistently low mutual coupling, remaining below -22 dB across their operational band. This identical bandwidth coverage, along with low coupling, ensures minimal interference between elements, making the design highly efficient for MIMO operation.






\begin{figure}[htbp]
\centerline{\includegraphics[width=0.45\textwidth]{IMG_2256.jpeg}}
\caption{(a) 3D and (b) 2D radiations of the antenna elements at 3.5 THz.}
\label{fig8}
\end{figure}


The 3D representations of the radiated field at the resonance frequency (4 THz) for the antenna elements within the main configuration of the suggested THz MIMO antenna are shown in Fig. \ref{fig8}. These visualizations confirm well-defined radiation patterns at the target frequency, ensuring efficient operation. The results demonstrate satisfactory radiation coverage and gain levels, delivering consistent performance across various sides of the mainboard, thus providing robust and ample coverage for the intended applications. These highlight the antenna's capability to meet the demands of advanced communication systems \cite{b7}.


Figure \ref{fig9} illustrates the simulated total efficiency of the antenna element across the operating frequency range. The results consistently indicate strong performance, with total efficiency remaining above 90\% throughout the entire band, reflecting the robustness of the proposed structure and the effectiveness of the integration. 


\begin{figure}[htbp]
\centerline{\includegraphics[width=0.45\textwidth]{IMG_2257.jpeg}}
\caption{Simulated total efficiency of the antenna element across the operating frequency range.}
\label{fig9}
\end{figure}


Furthermore, the maximum gain values presented in Fig. \ref{fig10} show that the elements achieve gains between 5.5 and 6.5 dBi over the operating spectrum. These outcomes collectively confirm that the antenna configuration offers both high efficiency and satisfactory gain, making it well-suited for THz communication.


\begin{figure}[htbp]
\centerline{\includegraphics[width=0.45\textwidth]{IMG_2258.jpeg}}
\caption{Maximum gain values of the MIMO antenna elements over the operating spectrum.}
\label{fig10}
\end{figure}




\begin{figure}[htbp]
\centerline{\includegraphics[width=0.4\textwidth]{IMG_2259.jpeg}}
\caption{Envelope Correlation Coefficient (ECC) results.}
\label{fig11}
\end{figure}

\begin{figure}[htbp]
\centerline{\includegraphics[width=0.45\textwidth]{IMG_2260.jpeg}}
\caption{Diversity Gain (DG) Results.}
\label{fig12}
\end{figure}

To assess the MIMO performance, key parameters such as the Envelope Correlation Coefficient (ECC) and Diversity Gain (DG) were evaluated. The results are illustrated in Figs. \ref{fig11} and \ref{fig12}, respectively. 

As shown in Fig. \ref{fig11}, the ECC consistently stays below 0.01 throughout the operational bandwidth. This low ECC indicates minimal signal correlation between the antenna elements, ensuring independent operation of each element. This independence enhances spatial diversity and maximizes the overall system capacity. 

Furthermore, Fig. \ref{fig12} illustrates the DG, which ranges from 9.995 to 10 dB across the bandwidth. A DG near 10 dB indicates robust diversity, effectively combating multipath fading and ensuring stable, reliable communication \cite{b4}.





\section{Conclusion}
This paper introduced a new design of a MIMO antenna featuring low-profile, microstrip-fed modified T-shaped monopole resonators with broadband functionality, specifically designed for 6G applications. The design, as detailed in the schematic, consists of four resonators positioned on different sides of the board, optimized to perform from 3.7 THz to 4.3 THz. Simulations and analyses of the MIMO array demonstrate promising results across key parameters, confirming its suitability for advanced wireless communication systems.

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\end{thebibliography}


\end{document}

