## 3 Emerging and State-of-the-Art Technologies
### Angular Coverage
SIWs are fabricated using a dielectric substrate with metalized top and bottom layers and vertical via along the sides to form a pseudo-waveguide structure. This configuration reduces losses compared to traditional printed lines while supporting higher power handling, all while maintaining the benefits of printed circuits, such as compactness and easy integration [126]. Despite these advantages, SIWs still suffer from dielectric losses [127]. To mitigate this issue, air-filled SIWs have garnered significant interest. In these designs, the dielectric propagation medium is replaced by air, resulting in substantially lower losses. However, the fabrication process for air-filled SIWs is more complex, requiring precise techniques to maintain structural integrity and performance [128; 129]. An even more advanced alternative is the use of full metallic waveguides with internal ridges or grooves [130; 131; 132]. These waveguides offer minimal propagation losses and compact designs suitable for mmWave frequencies. The feasibility of such structures has been enabled by breakthroughs in micromanching technologies, which allow the precise milling of metallic blocks into intricate and sophisticated geometries. An example of a groove gap waveguide 3D beamformer fabricated using additive manufacturing is shown in Figure 9. By leveraging these innovations, passive beamforming networks for mmWave applications can be realized with ridge, groove, inverted microstrip, or microstrip ridge waveguides, which introduce negligible RF signal losses. This significant reduction in losses enhances the overall efficiency of the communication system, aligning with the stringent power efficiency requirements of 6G technologies. The development of mmWave beamformers, whether based on switched-matrix or hybrid topologies, has already garnered significant interest from researchers worldwide. However, progress in this area has been relatively slow compared to the rapid pace of 5G and anticipated advances of 6G. Table 2 lists some notable work on beamforming network implementations, highlighting various architectures such as Butler matrices, Nolen matrices, and fully digital phased arrays. For each case, the beamforming type, frequency range, physical dimensions, fabrication technology, and number of supported beams are summarized to provide a comparative overview of analog, digital, and hybrid approaches across different platforms. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Reference** & **BF Network Type** & **BF Type** & **Frequency Range (GHz)** & **Size (in \(\lambda\))** & **Technology** & **Beams** \\ \hline \([95]\) & 3 \(\times\) 3 Nolen Matrix & Analog & 2.52–4.2 & 0.82\(\lambda\)\(\times\) 0.75\(\lambda\) & Microstrip on FR4 & 3 \\ \hline \([96]\) & Modified Blass Matrix & Analog & TDD & 24–40 & 2.6\(\lambda\)\(\times\) 2\(\lambda\) & Microstrip & 8 \\ \hline \([97]\) & Nolen & Analog & 29.6–30.4 & 22\(\lambda\)\(\times\) 9\(\lambda\) & RGW & 4 or 12 \\ \hline \([105]\) & 4 \(\times\) 4 and 8 \(\times\) 8 Butler Matrix & Analog & 6–16 & 0.74\(\lambda^{2}\) (4 \(\times\) 4), 2.6\(\lambda^{2}\) (8 \(\times\) 8) & Microstrip & 8 \\ \hline \([109]\) & 4 \(\times\) 4 Butler Matrix & Analog & 26-30 & 0.16\(\lambda\)\(\times\) 0.10\(\lambda\) & 28 nm CMOS & 7 \\ \hline \([111]\) & 2D Planar Hhased Array & Digital & 28 & 0.5\(\lambda\)\(\times\) 4\(\lambda\) & PCB/RFIC & 20 \\ \hline \([112]\) & SDR-Based & Digital & 26–30 & 5.6\(\lambda\)\(\times\) 14.2\(\lambda\) & SIW + TSA + SDR & 8 \\ \([122]\) & Dual Series Nolen Matrix & Analog & 2.8–3.2 & 1.3\(\lambda\)\(\times\) 1.75\(\lambda\) & Microstrip & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of beamforming network characteristics.