## 4 Antennas for Beam Steering and Beamforming
### Beam-Steering/Beamforming Techniques and Their Possible Features for CubeSat Communications  
By adjusting the phase and amplitude of signals at each antenna element in an array, beamforming enables enhanced signal strength, reduced interference, and better spectral efficiency. It is critical in modern communication systems like 5G networks, satellite communications, and radar systems. Beamforming can be categorized into analog beamforming (ABF), digital beamforming (DBF), and hybrid beamforming (HBF), each with distinct methodologies, advantages, and limitations. Figure 6: Beam steering with different feeds. In ABF, the signal phase and amplitude are controlled directly at the radio-frequency (RF) stage using analog devices such as phase shifters and attenuators. This technique generates a single beam by combining signals from all antennas before converting them to a baseband. It has several advantages, such as low complexity (simpler hardware implementation compared to digital systems, making it more cost-effective), power efficiency (consumes less power as it requires a single RF chain), and suitability for narrowband systems (well-suited for systems with limited bandwidth requirements). However, it also has some limitations like limited flexibility (generating only a single beam per array, restricting its capability to support multi-user scenarios), lack of interference mitigation (limited ability to perform complex spatial filtering, leading to a reduced performance in high-interference environments), and susceptibility to non-idealities (analog devices introduce phase errors and amplitude mismatches, impacting beamforming precision). The techniques utilized variable phase shifters for signal steering and employing RF combiners to merge signals from different antenna elements. DBF operates at the baseband or intermediate frequency (IF) stage. Signals from each antenna element are individually digitized and processed using digital signal processing (DSP) algorithms, allowing for precise control over beam characteristics. Its advantages are high flexibility (it can generate multiple beams simultaneously, supporting multi-user scenarios), interference management (enables advanced signal processing techniques for interference suppression and null steering), and wideband support (capable of handling wideband signals, making it suitable for modern broadband communication systems). However, each antenna element requires a dedicated RF chain, ADC/DAC (analogue to digital/digital to analogue), and DSP unit. It consumes significantly more power due to the extensive hardware and processing requirements and the complex hardware and software integration. Its applied techniques include the application of advanced DSP algorithms, such as adaptive beamforming, minimum variance distortionless response (MVDR), and eigen-beamforming. Digital filters are usually used for beam steering and shaping. HBF combines the principles of analog and digital beamforming to balance efficiency with hardware complexity. It employs analog beamforming at the RF stage and digital beamforming at the baseband stage. Its advantages are scalability (reduction in the number of required RF chains compared to digital beamforming while maintaining high performance), cost, and power efficiency (balances cost and power consumption by using fewer RF chains), and support for multi-user scenarios (can generate multiple beams with fewer resources than a fully digital system). Nevertheless, its performance is typically inferior to pure digital beamforming due to reduced flexibility in analog signal steering. It requires careful integration of analog and digital components, increasing the system's design complexity, and involve trade-offs in beam placement and resource allocation. The utilized technique is the decomposition of the beam-forming task into analog precoding and digital precoding.