## 3 Emerging and State-of-the-Art Technologies
### Techniques for Reconfigurable RF Front-End Systems and Antennas  
Following the emerging trends of low-cost, frequency-agile, and reconfigurable communication systems for 6G, the development of novel tunable devices is essential. Reconfigurable RF front-end will play a pivotal role in the miniaturization and simplification of future base stations. With 6G expected to unify various licensed spectra, including satellite communication, terrestrial networks, radar systems, and more, there will be a significant demand for wideband systems, particularly in base stations. The development  
Figure 9: Groove gap waveguide 3D Butler matrix [94]. of such wideband systems poses considerable challenges, as it further complicates the design and operation of RF transceivers. To address this, an effective strategy is the design and implementation of tunable microwave devices and antennas. These devices would enable the dynamic adjustment of system response based on the utilized frequency band, reducing complexity and minimizing insertion losses particularly at mmWave frequencies. In microwave technology, tunability refers to the electronic control of different characteristic of a device. For example, in antennas, the ability to control the radiation pattern and operation frequency is a highly valuable asset. Traditionally, the most widespread technique for introducing electronic control in microwave systems involves the use of semiconductors, such as PIN diodes, varactor diodes, or transistors acting as switches [145; 146]. Typically, these switches connect or disconnect specific elements from the main structure of the device, or they introduce variable capacitance to modify its properties. For instance, a continuously tunable SIW filter was demonstrated using varactors integrated into a resonator patch [147], and a 1-bit reconfigurable metasurface was proposed in [148]. A tunable fiftenna for X band applications were proposed in [149] where a novel superstrate-loading technique was applied permitting the frequency control using only one varactor. The measured antenna's tunability reached 35%, providing more than 20 dB out-of-band radiation suppression. As illustrated in Figure 10a, a PIN diode is employed between the arms of a Jerusalem cross, enabling the metasurface to switch between transmission and reflection modes. However, a significant limitation of such tuning mechanisms lies in the discrete nature of their tuning states, which can restrict applications or require a large number of switches to achieve finer control [150; 151]. The increased number of switching elements also leads to higher power consumption, particularly for diodes, as they require a bias voltage for their operation. Thus, minimizing the number of diodes is an active area of research. Moreover, as a two-terminal device, the PIN diode shares a single physical point for both the control port (biasing) and the RF signal. This dual use complicates the design of the biasing network, which must integrate the DC and RF signals while ensuring proper isolation between them. Designing effective solutions to these challenges is crucial for advancing reconfigurable microwave technology. On the other hand, RF microelectromechanical systems (MEMS) offer superior isolation, lower insertion losses, and higher power-handling capabilities compared to traditional switching mechanisms [152]. Due to their compact size (typically smaller than 1 mm), MEMS devices are particularly attractive for miniature frequency-reconfigurable antennas [152; 153], where the effective length of the antenna is adjusted by adding or removing parts of the radiating element. Recently, significant research efforts have focused on developing MEMS for mmWave bands, leveraging their advantages in the design of novel variable phase shifters [154] and attenuators [155] for beamforming networks [156; 157].