## 4 Antennas for Beam Steering and Beamforming
### Beam-Steering/Beamforming Techniques and Their Possible Features for CubeSat Communications
One notable advancement is the utilization of millimeter-wave (mm-wave) frequencies, which offer significantly larger bandwidths than traditional frequency bands. Operating in the 30 to 100 GHz range, mmWave communications can support the high data rates essential for high-resolution Earth observation and real-time data relay applications. However, mmWave signals are susceptible to greater free-space path loss and atmospheric attenuation, necessitating sophisticated beamforming and beam-steering techniques to maintain reliable links [76; 77]. Beyond 5G, research has explored the terahertz (THz) frequency spectrum (0.3 to 10 THz) to further augment communication capabilities. THz frequencies promise ultra-high data rates due to the vast available bandwidth, making them attractive for future CubeSat missions requiring substantial data throughput. Nevertheless, THz communications face challenges such as severe atmospheric absorption and a limited transmission range, which can be mitigated through advanced signal-processing techniques like adaptive modulation and coding and the development of high-gain directional antennas. The integration of mmWave and THz technologies into CubeSat communications necessitates advancements in signal processing to address the challenges associated with high-frequency signal propagation. Techniques such as adaptive beamforming, which dynamically adjusts the directionality of the antenna array, can compensate for signal degradation and enhance link reliability. Implementing multiple-input, multiple-output (MIMO) systems can also exploit spatial diversity to improve data rates and link robustness. These approaches require sophisticated algorithms and real-time processing capabilities, underscoring the importance of high-performance onboard processing units in future CubeSat designs [78]. As mentioned before, beam steering and its techniques are slightly different. Beam steering is a pivotal technology in modern communication and radar systems, enabling the dynamic redirection of signal beams without needing mechanical movement. This capability is essential for applications such as 5G networks, satellite communications, and advanced radar systems, for which rapid and precise beam direction is crucial. Recent advancements have introduced innovative methods to enhance beam-steering performance, presenting unique challenges and outcomes. Two of the most common techniques that have been utilized to improve the performance of beamforming/ beam steering are applying metasurface and reconfigurable structures. Metasurfaces are ultra-thin, two-dimensional structures composed of sub-wavelength-sized elements that can manipulate electromagnetic waves in a controlled manner. By engineering these elements, metasurfaces can modulate the amplitude, phase, and polarization of incident waves, enabling advanced wavefront-shaping capabilities. In beamforming and beam-steering applications, metasurfaces offer a low-profile alternative to traditional bulky components, allowing for precise control over the direction and shape of electromagnetic beams. This precision is achieved by designing specific phase gradients across the metasurface, facilitating the steering of beams without mechanical movement. The advantages of metasurfaces include their compactness, lightweight nature, and the potential for integration into various platforms, making them suitable for applications like wireless communications and optical systems. However, challenges such as fabrication complexity, sensitivity to fabrication errors, and potential limitations in bandwidth and efficiency need to be addressed. Ongoing research aims to overcome these limitations, with developments in tunable and reconfigurable metasurfaces showing promise for dynamic beamforming and beam-steering applications [79]. Metasurface structures have been used to improve beam steering. For instance, Ref. [80] employed a reflective metasurface technique, incorporating metal slots around the radiating patch to enhance the gain.