## 4 Antennas for Beam Steering and Beamforming
### Passive Antennas for Beam Steering
The study explored beam steering but did not address real-world challenges, such as hardware implementation for real-time beam control. While simulations showed promising results, real-world environmental factors like interference, material imperfections, and atmospheric absorption in the D-band range limit performance. In Ref. [95], a microstrip antenna array was designed and implemented to support beam steering, emphasizing compactness and low-profile characteristics at 2.4 GHz. It likely incorporated phase shifters and other techniques to achieve beam steering, enhancing the signal directionality for communication systems. The authors also included various configurations (e.g., linear and planar) to improve the gain and performance. They highlight an increased gain of up to 7 dBi and beam steering as significant benefits of using the microstrip antenna array. They also suggest that communication system applications, such as 4G and 5G networks or radar, require high data rates and precise signal directionality. However, the paper does not address challenges related to dielectric and conductor losses, especially for higher-frequency applications. The beam-steering technique, while helpful, can increase the design complexity, especially if implementing phase shifters or control systems. There was a limited discussion of real-world challenges like environmental interference, manufacturing tolerances, and system integration into commercial devices. Reference [96] focuses on the K-band frequency range (\(\sim\)18-27 GHz) with microstrip technology, which is suitable for compact and planar systems. The authors employed phased array principles to achieve directional beam steering, improving antenna efficiency in high-frequency communication. They used series feed techniques to distribute power uniformly across array elements. They also demonstrated a gain of 7.6 dBi and a beam range of 30 degrees. However, the closely spaced elements in the array can lead to interference, reducing the overall efficiency. High-frequency designs, particularly in the K-band, are more susceptible to environmental factors like atmospheric attenuation. Reference [97] focuses on scalable and integrated patch antennas for 28 GHz, a key frequency for 5G applications. It demonstrated an integrated beamforming solution for enhanced connectivity in automotive systems. It used high-efficiency power amplifiers in the antenna system to ensure effective signal transmission in 5G networks, offering maximum gains of 7.5 dBi and 21.8 dBi before and after amplifier integration. The proposed antenna system offers 5G signal coverage and capacity through beamforming and amplifier integration. It is tailored for high-speed and reliable communications in automotive environments, addressing challenges like Doppler effects. However, integrating power amplifiers with antennas can lead to thermal issues, especially in compact systems. Moreover, high-frequency signals, such as those at 28 GHz, are prone to attenuation due to weather conditions and obstacles. In Ref. [98], a multibeam antenna at 38 GHz, a critical band for 5G communication, was designed. The array enhances spatial coverage using a folded Butler matrix for compact beam steering, which reduces the circuit area while achieving precise beam steering. It also focused on integrating multiple components into a package to save space, which is crucial for 5G mobile and IoT devices. An excellent beam-steering performance with low sidelobe levels was demonstrated, improving the signal quality and reducing the size by 53.5% compared to conventional Butler matrices and phased arrays, making it suitable for space-constrained applications. Four beams were measured at \(\pm\)36\({}^{\circ}\) and \(\pm\)12\({}^{\circ}\) and offered maximum gains of 19.8 and 21 dBi, respectively. However, the multi-folded design can introduce additional losses, which affect the overall efficiency. In Ref. [99], antenna arrays were designed to operate at 28.8 GHz, focusing on miniaturization.