## 3 Emerging and State-of-the-Art Technologies
### Angular Coverage  
An important issue in mmWave hybrid or switched beam matrix beamformers is related to continuous angular coverage. Specifically, at mmWaves, increased gain is needed to mitigate propagation losses, thus corresponding to narrow beamwidth. When this is successfully achieved, the consecutive switched beams may overlap at very low levels, below \(-3\) dB. This problem can be mitigated by introducing a group of intermediate orthogonal beams, as suggested by [PERSON] et al. [122], which is also shown in Figure 8. This is achieved by introducing constant-value phase shifters between the Nolen or Butler matrix outputs and the antenna ports, selectable through digitally controlled RF switches. At even higher frequencies, a third or fourth group of additional orthogonal beams may be needed if the radiation pattern of the element is highly directive. As displayed in Figure 8, a \(4\times 8\) Nolen matrix can be expanded by introducing an intermediate group of three additional beams, leading to a total of seven beams and improving the overlapping point to \(-3\) dB. As future 6G networks adopt larger antenna arrays and operate at higher frequencies, such as the THz band, beamforming will become increasingly complex. To address this challenge, AI, particularly ML, is being leveraged to optimize beamforming strategies, enhancing efficiency, adaptability, and overall network performance [123; 124]. The optimization process involves several key aspects, including enhanced beam selection, where AI identifies the optimal transmission beam, reducing latency and improving signal quality. Additionally, adaptive beam management allows AI-driven algorithms to enable real-time beam tracking and selection, ensuring seamless communication in dynamic environments. Furthermore, Improved channel estimation leverages AI to predict channel conditions with greater accuracy, enabling more efficient and adaptive beamforming adjustments. However, several challenges must be addressed. One major issue is algorithm selection, as identifying the most suitable AI model for specific beamforming scenarios remains difficult. Another critical concern is security, as AI-based beamforming systems must be safeguarded against adversarial attacks to ensure network integrity and reliability. The development of 6G technology presents substantial challenges in PCB manufacturing, particularly in ensuring signal integrity and minimizing losses. Advancements in digital and RF systems so far rely heavily on PCB technologies, where transmission lines are typically incorporated into planar or multilayer structures to support seamless integration and high-performance operation. However, at frequencies above 10 GHz, RF propagation through dielectric materials introduces significant losses. In the context of 6G communication systems, where power efficiency is paramount, minimizing transmission and insertion losses in devices is critical. Consequently, alternative transmission line technologies have been explored to address these challenges. One promising solution is substrate integrated waveguides (SIWs), a hybrid technology that combines the advantages of metallic rectangular waveguides and printed transmission lines [125]. An inevitable requirement for the selection of mmWave transmission lines is to avoid dielectric losses, which are unacceptably high even at microwave frequencies. Additionally, the metallic  
Figure 8: (**a**) Schematic diagram of the \(4\times 8\) network implementing the switched-line-phase-shifter (SLPS) technique and (**b**) their corresponding results where three more beams were introduced [122]. surfaces must be made perfectly smooth (metal spreading) to minimize current paths, since at mmWaves, the penetration depth is very short. Thus, waveguides based on air or with isolated metallic parts are needed. The trend is to devise waveguides with a planar structure but with air or gap-isolated conductors.