## 8 Conclusions
Unit cell miniaturization may be further explored to improve phase quantization errors. * Layer separation [61; 102]: This approach may lead to an increased number of layers. * Concentric cell [67]: The designed cell has a modified Jerusalem cross resonator and a modified complementary split ring resonator to tune transmission properties at two frequencies independently. The overall cell is concentric but at the cost of complex geometry for the overall cell. Nevertheless, each technique has its own advantages and disadvantages, necessitating careful consideration in the metasurface design. From analyzing various unit cell topologies, several research gaps have been identified. All-metallic phase-shifting cells, suitable for space applications, allow operation under high power without requiring expensive laminates. However, these structures suffer from narrow bandwidth. Exploring waveguide-based dual-band all-metallic phase-shifting cells could offer higher bandwidth along with improved structural rigidity. Composite metal-dielectric phase-shifting unit cells can provide an improved bandwidth response, but most of these composite cells require multiple layers of metal and dielectric material to achieve maximum phase range at dual frequency bands, increasing the overall thickness of the structure. Dual-band subwavelength cells with fewer metal-dielectric layers that can offer a better phase range while reducing the number of alternating layers can be further explored. The incident field for the majority of feed antennas is concentrated at the center, so high transmission magnitude cells can be placed in the center, while low transmission magnitude cells with desired phase shifts can be positioned towards the periphery. This approach of amplitude tapering, coupled with height reduction, can be explored to achieve better performance. Moreover, most dual-band cells have been optimized for normal angles of incidence. The stability of a unit cell's response under oblique angles of incidence at the two frequency bands needs further investigation. The various planar cell topologies discussed in this review can be easily integrated into existing RF front-end systems for SATCOM and COTM platforms. The choice of material for implementation depends significantly on the nature of the application. As an example, all-metallic structures are well-suited for space applications due to their ability to withstand extreme temperatures and harsh conditions. In contrast, composite metal-dielectric materials are more appropriate for applications requiring wider bandwidth and commercial COTM operations. Another hurdle associated with all-metal metasurfaces is that, since these panels are manufactured using laser cutting, high-frequency structures may require a level of precision that may not be feasible with the existing technologies. In such cases, printed structures could be preferred for high-frequency operations. However, low-loss laminates may be expensive, which could be a limitation for the large-scale production of printed metasurfaces. Although several variations of passive metasurfaces have been reported, reconfigurability, as seen in active reflectarrays, has not been extensively explored. Further research into reconfigurable metasurfaces is needed to dynamically control the beam direction. Moreover, the miniaturization of cells, while maintaining independent dual frequency response and achieving a wider bandwidth in dual frequency bands, remains a significant challenge for designing shared aperture antennas based on passive metasurfaces that can provide compact and cost-effective solutions with lower power consumption for the future communication systems catering to the ever increasing demands for global connectivity. All the technologies discussed in the article are essentially state-of-the-art and have undergone constant evolution in the short time since their inception.