With massive deployment, multiple-inputmultiple-output (MIMO) systems continue to take mobile communications to new heights, but the ever-increasing demands mean that there is a need to look beyond MIMO and pursue the next disruptive wireless technologies. Reconfigurable intelligent surface (RIS) is widely considered a key candidate technology block to provide the next generational leap. The first part of this article provides an updated overview of the conventional reflection-based RIS technology, which complements the existing literature to include active and semiactive RIS, and the synergies with cell-free massive MIMO (CF mMIMO). Then, we Manuscript

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Reconfigurable surfaces facilitating energy-efficient, intelligent surface wave propagation have recently emerged as a technology that finds applications in many-core systems and 6G wireless communications. In this paper, we consider the porositybased reconfigurable surface where there are cavities that can be filled on-demand with fluid metal such as Galinstan, in order to create adaptable channels for efficient radio propagation. We aim to investigate the propagation phenomenon of signal fluctuation resulting from the diffraction of discrete porosity and study how different porosity patterns affect this phenomenon. Our results cover the frequency range between 21.7GHz and 31.6GHz when the WR-34 waveguides are used as the transducers.

Surface wave inherently has less propagation loss as it adheres to the surface and minimizes unwanted dissipation in space. Recently, they find applications in network-on-chip (NoC) communications and intelligent surface aided mobile networked communications. This paper puts forward a reconfigurable surface wave platform (RSWP) that utilizes liquid metal to produce highly energy-efficient and adaptive pathways for surface wave transmission. Our simulation results illustrate that the proposed RSWP using Galinstan can obtain a 25dB gain in the electric field for a propagation distance of 35λ at 30GHz where λ denotes the wavelength. Moreover, less than 1dB loss is observed even at a distance of 50λ, and a pathway with right-angled turns can also be created with only a 3.5dB loss at the turn.

As an attempt to develop a reconfigurable surface architecture that can use liquid metal such as Galinstan to shape surface channels on demand, this paper considers a punctured surface where cavities are evenly distributed and can be filled with liquid metal potentially via digitally controlled pumps. In this paper, we look at the benefits of such architecture in terms of surface-wave signal enhancement and isolation, and examine how various system parameters impact the performance using full wave 3-dimensional electromagnetic simulations. It is shown that extraordinary signal shaping can be obtained.