## 3 Emerging and State-of-the-Art Technologies
### Beamforming Networks at mmWaves and THz
The base station antenna gain is calculated using the maximum directivity of a planar array, assuming total efficiency \(\epsilon_{eff}=1\); thus, \(G_{t}=\epsilon_{eff}D_{max}\), where \(D_{max}=4\pi A_{eff}/\lambda^{2}\), with \(A_{eff}\) being the effective area [100]. For a realistic scenario, we incorporate penetration losses of 5.5, 6.2, and 7.8 dB at 3.5, 10, and 20 GHz, respectively, as reported in [101]. Similarly, we consider shadowing and fading losses of 6, 7, and 8 dB at 3.5, 10, and 20 GHz, respectively, based on findings from [102; 103; 104]. Figure 6 shows the range achieved by each antenna configuration under these conditions. Figure 5: Types of BFN architectures [20]: (**a**) analog beamforming, (**b**) digital beamforming, (**c**) hybrid beamforming, and (**d**) switched-beam hybrid beamforming. Figure 6: Maximum distance maintaining a link for three different antenna arrays’ configurations. This figure was inspired by [87]. An example of a 3D beamforming topology [105] is illustrated in Figure 7, and it is utilized to enhance system capacity and efficiency by enabling precise transmit and receive functions with narrow beams in both azimuth and elevation angles. This analog 3D beamformer is constructed using a stack of vertical beamformers (BMs), which are driven by either a single horizontal BM or a stack of horizontal BMs. To achieve ultra-wideband performance, the planar array is composed of planar-printed Vivaldi antenna elements, ensuring high directivity and efficient spatial coverage. Both adaptive phased-array beamforming and switched beamforming will play a crucial role in coverage extension in 6G non-terrestrial networks (NTN), enabling seamless connectivity across air, sea, and space. This involves the integration of LEO (low Earth orbit) satellites, GEO (geostationary) satellites, and HAPS to provide global coverage. Since LEO satellites traverse the sky at high speeds and for short durations (up to 30 min), real-time beam steering is essential to continuously track mobile ground terminals, ensuring stable and seamless connectivity. Scalable, reconfigurable, and large antenna arrays are required to serve multiple ground terminals simultaneously. A relevant example of this is a 1024-element SATCOM phased array, as discussed in [106]. The key challenges for phased-array beamformers will be high power consumption and increased computational complexity, especially in large-scale systems, as well as cost-effective implementation, particularly for LEO terminals that require real-time tracking. On the other hand, less complex switched beam beamformers can be effectively deployed in GEO satellites to cover vast geographic areas with predefined beams and in HAPS platforms to provide extensive ground coverage without requiring continuous steering, thereby minimizing power consumption. They are also well suited for backhaul communication between terrestrial networks and NTN. A 3D beamformer featuring a 16 \(\times\) 16 (256 element) array or larger, powered by a stack of multiple beamform modules similar to the design shown in Figure 7b, is expected to significantly improve coverage and performance for 6G networks. Although the main issue in extreme coverage extension is long-distance signal propagation, particularly in NTN where signals must travel thousands of kilometers while maintaining low latency and high reliability, 3D beamforming is essential for handovers between terrestrial 6G networks and NTN systems, significantly enhancing spectral efficiency. Additionally, it enables high-gain directional beams, optimizing signal strength and ensuring efficient long-distance communication in NTN environments. To integrate 3D beamformers into the aforementioned platforms, a miniaturized and compact design with enhanced performance accuracy is essential for optimal efficiency and seamless deployment. The introduction of tunable and reconfigurable components, such as phase shifters [107], attenuators [108], and SPDT switches [109], marked a significant step forward.