## 7 Future Challenges and Opportunities  
Novel circuit antenna co-integration needs to be a solution for active beamforming antenna arrays for 5G telecommunication applications in the sub 6 GHz (FR1) and near-30 GHz (FR2) mmWave frequency bands. Active integrated antennas can be designed with nontraditional functionalities (e.g., tailored power combining, active load modulation for power amplifiers [111], active source modulation for low-noise amplifiers, and RF filtering [112]) that help to minimize power loss, particularly at mmWave frequencies. Antenna arrays with high directivity are implemented for fixed-beam communication at frequencies beyond 100 GHz. Most high-efficiency mmWave array antennas are waveguide (WG) structures that are typically designed with multilayer H-plane split blocks that are galvanically connected. This method results in a hollowed WG multilayer structure formed by stacking machined metal plates. Due to crucial connectivity power dissipation and physical constraints, conventional systems incorporating phase shifter ICs attached to each element may not be practical at 100G+ frequency. Other systems, such as those based on reconfigurable antenna elements \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** & **Geometry** & **Decoupling** & **Effective** & **Resolution(dB)** & **Channel** & **ECC** \\
**Antennas** & **Geometry** & **Techniques** & **(GHz)** & **(GHz)** & **Capacity Loss** & **ECC** \\ \hline [101] & Non Planar & Decoupling based on FSS & 3–11 & 20 & \textless{}0.20 & \textless{}0.20 \\ \hline [102] & Non Planar & \begin{tabular}{c} Parasitic structure \\ for decoupling \\ \end{tabular} & 3.1–10.6 & 20 & \textless{}0.70 & \textless{}0.1 \\ \hline [103] & Non Planar & \begin{tabular}{c} Configuration based on 3D \\ element distance \\ \end{tabular} & 3–11 & 20 & - & \textless{}0.5 \\ \hline [104] & Planar & \begin{tabular}{c} Decoupling structure based \\ on grounded slits \\ \end{tabular} & 2–11 & 15 & - & \textless{}0.2 \\ \hline [89] & Planar & Orthogonal polarization & 2.5–2.7 & 12 & - & \textless{}0.12 \\ \hline [105] & Planar & \begin{tabular}{c} Decoupling structure of \\ ENG-NZI metamaterial \\ \end{tabular} & 3.4–3.6, 4.8–5.0 & 12 & \textless{}0.08 & \textless{}0.15 \\ \hline [106] & Planar & DN (secoupling network) & 3.4–3.6 & 14 & - & \textless{}0.2 \\ \hline [107] & Planar & \begin{tabular}{c} Element positioning + \\ geometrical slotting \\ \end{tabular} & 3.2–5.92 & 14.5 & - & \textless{}0.1 \\ \hline [108] & Planar & \begin{tabular}{c} Port distance plus \\ positioning of elements \\ \end{tabular} & 3.2–6.1 & 18 & - & \textless{}0.21 \\ \hline [109] & Planar & \
\begin{tabular}{c} Parasitic structure \\ for decoupling \\ \end{tabular} & 3–11 & 20 & \textless{}0.35 & \textless{}0.0025 \\ \hline [110] & Planar &
\begin{tabular}{c} Decoupling structure of \\ ENG-NZI metamaterial \\ \end{tabular} & 3–11 & 28 & \textless{}0.30 & \textless{}0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of Different Techniques employed for Mutual Coupling Reduction/Decoupling. equipped by varactor or PIN diodes, suffer from comparable problems, as well as design complexity, high cost, and time-consuming assembly. Innovative array antenna designs enabling two-dimensional (fullspace) beam control and effective beam steering are ideal for integration and hybrid packaging alongside active IC modules. Antenna subarray modularization (ASM) is one of the important technologies in this context for making the system practical and robust [112]. ASM will be investigated in order to determine optimally sparse topologies to relax the physical restrictions of antennas while meeting beamforming objectives with a lower number of amplitude-phase controls per element and remaining design compatible with semiconductor technology. One specific goal is to reduce signal latency while operating several phase shifters along the same ASM module.