The communications sector is currently navigating a pivotal transition, shifting from traditional infrastructure toward a future defined by AI-native intelligence and 6G connectivity. The 6G market is poised for an explosive surge; early research and pre-commercial trials are expected to accelerate this year, with the global 6G market projected to leap within the upcoming years. In this upcoming 6G ecosystem, reconfigurable antennas are emerging from laboratory concepts to potential enabling agents for the new paradigm of communications. Unlike traditional systems with fixed antennas, reconfigurable antennas vary their physical geometry to exploit the spatial dimension of wireless channels, utilizing materials such as liquid metals, ionic solutions, or switchable radio-frequency pixels. This reconfigurability provides additional degrees of freedom that yield massive diversity gains and improved reliability.
In the context of 6G-LEADER, our team from UC3M focuses on reconfigurable antennas made of a metal that is liquid at room temperature, that is, liquid antennas. A defining characteristic of these antennas is their flexibility and their capacity to flow. It allows them not only to withstand severe mechanical deformations – such as stretching, bending, and twisting – but also to vary their radiation parameters while maintaining the ability to return to their original shape.
Current solutions are based on multiple-input multiple-output (MIMO) technologies adopted since the 4th generation of communications and their latest extensions and improvements, such as multi-user MIMO and massive-MIMO. Generally, these validated technologies work by exploiting diversity and spatial multiplexing. By increasing the number of elements of an antenna array, more directive transmissions can take place. In consequence, the interference between different transmissions or different users can be minimized, increasing data rates and/or multiplexing gains. Nonetheless, these solutions may face a scalability problem on the path toward the 6th generation of communications. The number of elements cannot grow indefinitely, as it would boost hardware costs and energy consumption.
Therefore, reconfigurable devices are foreseen to develop into a suitable option for certain use cases in which the different configurations of the device would compensate for the required specifications. Note that reconfigurability allows a single device to be compared to multi-element antenna arrays. By reducing the required hardware, costs and energy consumption can be reduced, while the performance is maintained. Additionally, reconfigurable devices are able to change their radiation parameters, such as polarization, resonating frequency, or radiation patterns, which can unlock new use cases allowed by the strengths of these novel technologies.
Particularly, eGaIn is a bio-compatible metal that is, it is not toxic, nor flammable, nor radioactive. It presents good conductivity and electrical features, but its most appealing characteristic is that it is liquid at room temperature. It can be displaced through microfluidic channels using mechanical pumps or syringes. However, more advanced implementations utilize continuous electrowetting (CEW), a mechanism that moves the eGaIn using electric impulses (as low as 1V) instead of mechanical parts, enabling reconfiguration speeds in the range of 10 to 100 mm/s. Our research group is focused on implementing this technique in a proof of concept that will be developed as part of 6G-LEADER.
However, to successfully incorporate liquid antenna technologies we need to overcome some challenges. Firstly, developing fluid hardware designs undoubtedly requires multidisciplinary research, as one cannot decouple communications and fluid dynamics. Furthermore, effective reconfiguration processes typically occur at a much slower rate (on the order of mm/s) than communication data rates (in the order of GHz). Consequently, it is important to consider the use cases in which the use of liquid antennas makes sense. Regarding signal processing, the overwhelming number of possible configurations makes finding the optimal configuration a demanding task. That is, acquiring channel data or applying state-of-the-art pre- or post-coding techniques. Fortunately, AI or ML learning-aided solutions are already being applied in this context to mitigate this problem and are also being studied within 6G-LEADER. Finally, the lack of commercial applications for these materials and technologies may slow the deployment of validated systems, as currently they are under validation in laboratories.
To sum up, the incorporation of novel mechanisms represents a challenge that must be overcome to match the expectations of a growing industry, both in capacity and in requirements. Nevertheless, a successful incorporation of liquid antenna technologies may help to pave the way toward new generation communications systems, motivating the research in the context of the 6G-LEADER project.
