## 3 Emerging and State-of-the-Art Technologies
### Beamforming Networks at mmWaves and THz
These devices enabled precise control over the phase and amplitude of the excitati  
Figure 7: **(a)** 3D beamforming; **(b)** 3D beamforming utilizing Butler matrix and Vivaldi array. antenna element, while the SPDT switches enabled dual-port excitation of the beamformer to produce additional beams. However, their use in mmWave systems is constrained by their high insertion loss and limited tuning ranges, which degrade performance at higher frequencies [110]. To address these limitations, more advanced beamforming architectures have been developed, including fully digital [111; 112; 113; 114] and hybrid analog-digital systems [115; 116; 117]. In a fully digital beamforming network, each antenna element receives a signal that is digitally processed and then converted to an analog signal using a digital-to-analog converter (DAC), as shown in Figure 5b. This approach provides unparalleled flexibility and precision in shaping the beam. However, at mmWave frequencies, fully digital systems face significant challenges. In general, DACs consume significant power, particularly at high sampling rates (\(f_{s}\)). The sampling must obey the Nyquist minimum rate \(f_{s}>2 BW\), where \(BW\) represents the operational channel bandwidth of the received signal. However, complex sampling techniques can be employed to reduce the required sampling rate by half, allowing \(f_{s}>BW\). In practical implementations, commercial ADCs and DACs, especially those used in software-defined radio (SDR) applications, often operate at slightly higher sampling frequencies (approximately \(f_{s}=2.2 BW\) for real signals or \(f_{s}=1.1 BW\) for complex (IQ) signals) to ensure optimal performance [118]. Notably, current 5G systems support channel bandwidths of up to 200 or 400 MHz, whereas 6G is expected to accommodate bandwidths ranging from 0.5 GHz to 2 GHz, posing new challenges in power efficiency and signal processing capabilities. The DACs required to generate signals with these requirements must operate at extremely high sampling rates, which can result in substantial power consumption and reduced system efficiency. Hybrid beamforming networks provide a practical solution for mmWave applications [113] and are essential for THz frequency bands. By integrating analog and digital processing, these systems efficiently handle the high data rates and massive connectivity demands of 6G while reducing power consumption and hardware complexity [119]. The signal undergoes initial digital processing before being converted into an intermediate analog form. Subsequent operations, including up-conversion, amplification, and filtering, take place in the analog domain before transmission through the antenna elements, as illustrated in Figure 5c. By minimizing the number of DACs and leveraging analog signal processing, hybrid architectures achieve an optimal balance among complexity, performance, and energy efficiency, making them well suited for high-frequency applications. However, implementing hybrid beamforming in 6G introduces several challenges. Propagation loss compensation at THz frequencies requires high-gain arrays to maintain reliable communication. The beam squint effect is also an important issue [120], where the wide bandwidths of THz communications cause beams to shift in different directions at varying frequencies, leading to array gain loss. Furthermore, hardware constraints pose a significant hurdle, as hybrid beamforming in ultra-massive MIMO (or multi-antenna) systems demands managing large numbers of antennas and RF chains, increasing both hardware complexity and power consumption. Lastly, beam management is crucial in dynamic environments [121], where user mobility and rapid changes in channel conditions require efficient beam alignment and tracking mechanisms to ensure seamless connectivity.