## 5 Future Challenges and Scope
### Dual-Band Frequency  
Dual-band transmitarray design is a remarkable challenge to obtain complete \(\pm\)360\({}^{\circ}\) transmission phase shift and insertion loss close to 0 dB in both frequency bands. We need to attain both parameters to gain optimal features of the dual-band frequencies. Dual-polarization is another significant parameter for satellite communication applications. Independent phase shift for a dual-band transmitarray becomes a more difficult task due to dual-polarization. For satellite communication applications, lower and higher band frequencies in the Ku-band (12.5/14.5) are close to each other for satellite communication applications. Resonance peaks are very close to each other, so this proximity impacts both transmission magnitude and the phase of dual-band frequencies. The fact that few investigations have been conducted on dual-bands is a challenge. In addition, numerous fixed beam dual-band structures have been studied in the literature [121; 122; 123]. It is noted that much less work has been carried out to investigate reconfigurable dual-band unit cells. The proposed unit cell is based on MMIC Technology for K/Ka-band active transmitarray, as reported in [121]. This paper is the first reconfigurable dual-band SiGe-BICMOS MMIC-based transmitarray design, fabricated and tested. For instance, applications for satellite communication two-way data-links acquire from 19.7 GHz to 21 GHz downlinks and 29.5 to 30.8 for uplinks. Electronically reconfigurable dual-band is a promising gap for future research for many applications such as satellite communication (Sat-Com) and 6G/5G cellular systems with all of its challenging problems. This electronically reconfigurable dual-band transmitarray also has the potential to facilitate revolutionary applications, which will be enabled by new thinking and developments in solid-state devices, applications, and systems. A detail Summary of beam steering methods with characteristics to conclude out whole work, see Table 4.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Beam** & **Control** & **Advantages** & **Disadvantages** \\ \cline{2-4}
**Steering** & **Method** & **Elements/Devices** & **Broadband transmission and differential** & **Complexity of design due to** \\ \hline
[74; 75; 76; 77; 78] & & PIN diodes & **phase shift characteristics** & **half via for biasing** \\
[82; 83] & & PIN diodes & **Coupling Slots TA, large-angle beam scanning capability** & **Results are not symmetric** \\
[84] & & PIN diodes & **Excellent beam steering performance** & **through out the obtain frequency** \\  & & & **Excellent beam steering performance** & **The degradation of antenna efficiency** \\
[90; 91] & Active & **Transmitarray** & **V**aractor diodes** & **Excellent beam steering performance, but complicated bias circuit** \\
[93] & & **V**aractor diodes** & **High gain, simple circuitry, narrow beam scanning** \\
[94] & & **V**aractor diodes** & **Parametric study of the unit cell, more stack unit cell high internal resistance** & **more stack unit cell high internal resistance** \\
[100] & & **MEMS** & **2 bit phase quantization, directivity loss** & **High insertion loss, sensitivity loss** \\
[105] & & **MEMS** & **High insertion loss, poor power efficiency** & **High insertion loss, poor efficiency** \\
[109] & & **Microfluids** & **Narrow beam steering angle** & **Narrow beam steering angle** \\ \hline
[112] & **Variable** & **Seven 2 \(\times\) 2 aperture couple source antenna** & **Reduce visual impact important for 5G** & **Discrete beam steering angle** \\
[113] & **Transmitarray** & **A** antenna multi-focal source & **Excellent beam steering performance, quad-focal phase shift distribution** & **Multiple focal source** \\ \hline \hline \end{tabular}
\end{table}
Table 4: Summary of beam steering methods with characteristics.