Reconfigurable Intelligent Surfaces (RISs) are potential enablers of future wireless communications and sensing applications and use-cases. The RIS is envisioned as a dynamically controllable surface that is capable of transforming impinging electromagnetic waves in terms of angles and polarization. Many models have been proposed to predict the wave-transformation capabilities of potential RISs, where power conservation is ensured by enforcing that the scattered power equals the power impinging upon the aperture of the RIS, without considering whether the scattered field adds coherently of destructively with the source field. In effect, this means that power is not conserved, as elaborated in this paper. With the goal of investigating the implications of global and local power conservation in RISs, this work considers a single-layer metasurface based RIS. A complete end-to-end communications channel is given through polarizability modeling and conditions for power conservation and channel reciprocity are derived. The implications of the power conservation conditions upon the end-to-end communications channel are analyzed.

The novel frequency shifting reconfigurable intel-ligent surface (FS-RIS) is built by a new generation of meta-materials and circuits, which is able to perform an operation that changes the frequency spectrum of the signal that was originally transmitted. The use of this FS- RIS for communication is still in its infancy. In this work, a millimeter wave (mmWave) communication link aided by FS-RIS is proposed. The FS-RIS is able to down-convert the carrier frequency of the mm Wave signal and spread the control information to the cell by using a more reliable and stable link, due to the fact that lower bands have a lower attenuation. A novel multi-frequency beam-management procedure (BMP) relying on FS-RIS is given. The proposed method is faster than existing methods since it makes use of wider beams without suffering from range penalization thanks to the better propagation in sub-6 GHz bands, unlike the hierarchical BMP. Overall, this paper provides a new way to exploit FS-RIS in wireless communications, which has the potential to change the way we look at spectrum and interference management.

The reflective intelligent surface (RIS) that is com-monly considered has linear properties and does not change the frequency of the impinging signal. This is different for the emerging RIS designs, capable of doing non-linear operations on the reflected signal. We show how nonlinear RIS operation can significantly reduce the overhead for channel estimation and localization. We treat communication and sensing for cyclic-prefixed orthogonal frequency-division multiplexing (OFDM)-based system. Introducing controllable artificial Doppler-shifts through non-linear RISs enables identification of the propagation path by the frequency shift. In this way, both the direct path and multiple RIS-assisted paths can be estimated by transmission of a single OFDM-symbol. Thus, with only a minimal penalty in terms of channel estimation accuracy, this scheme achieves performance that for linear RISs would require coding across multiple OFDM-symbols. This paper considers the linear minimum mean squared error (LMMSE) channel estimator and presents four different methods to choose the pilot sequence and the artificial Doppler-shifts, in order to minimize the mean squared error (MSE) of the channel estimation.

Frequency shifting-reconfigurable intelligent surfaces (FS-RISs) can be built by a new generation of meta-materials and circuits, which can perform an operation that changes the original frequency spectrum of the transmitted signal. Its exploitation for wireless communications is still in its infancy. In this work, a millimetre wave (mmWave) cellular network aided by FS-RIS is proposed to satisfy the new challenging communication services. The FS-RIS is able to down-convert the carrier frequency of the mmWave signal and spread the control information to the cell by using a more reliable and stable link, relying on better propagation within the lower bands, and a novel multi-frequency beam-management procedure is given. The proposed method can rapidly discover all users in the entire cell by using different sizes of beams. It avoids range penalization due to the better propagation in sub-6 GHz bands, unlike the hierarchical BMP. Additionally, the proposed technique only requires one base station (BS), and hence the delay overhead caused by the information exchange among several BSs is fully avoided, unlike the case of coordinated multi-point (CoMP). Overall, this paper provides a new way to exploit FS-RIS in wireless systems, which can potentially change the way we look at spectrum and interference management for the efficient exploitation of communication applications.

Due to a production error in[1], there is an error with the definition of the real and imaginary part operators. They should be defined as Re{} and Im{}. However, the {} are missing in all the equations. We apologize for this error.

This paper evaluates the performance of multi-user massive multiple-input multiple-output (MIMO) systems in which the base station is equipped with a dynamic metasurface antenna (DMA). Due to the physical implementation of DMAs, conventional models widely-used in MIMO are no longer valid, and electromagnetic phenomena such as mutual coupling, insertion losses and reflections inside the waveguides need to be considered. Hence, starting from a recently proposed electromagnetic model for DMAs, we formulate a zero-forcing optimization problem, yielding an unconstrained objective function with known gradient. The performance is compared with that of full-digital and hybrid massive MIMO, focusing on the impact of insertion losses and mutual coupling.

Dynamic metasurface antennas (DMAs) arise as a promising technology in the field of massive multiple-input multiple-output (mMIMO) systems, offering the possibility of integrating a large number of antennas in a limited —and potentially large— aperture while keeping the required number of radio-frequency (RF) chains under control. Although envisioned as practical realizations of mMIMO systems, DMAs represent a new paradigm in the design of signal processing techniques (such as beamforming) due to the constraints inherent to their physical implementation, for which no complete models are available yet. In this work, we propose a complete and electromagnetic-compliant narrowband communication model for a generic DMA based system. Specifically, the model accounts for: i) the wave propagation and reflections throughout the waveguides that feed the antenna elements, ii) the mutual coupling both through the air and the waveguides, and iii) the insertion losses. Also, we integrate the electromagnetic model in the conventional digital communication model, providing a complete and useful framework to design and characterize the performance of these systems. Finally, the accuracy of the model is verified through full-wave simulations.

This paper proposes a communication model for multiuser multiple-input multiple-output (MIMO) systems based on large intelligent surfaces (LIS), where the LIS is modeled as a collection of tightly packed antenna elements. The LIS system is first represented in a circuital way, obtaining expressions for the radiated and received powers, as well as for the coupling between the distinct elements. Then, this circuital model is used to characterize the channel in a line-of-sight propagation scenario, rendering the basis for the analysis and design of MIMO systems. Due to the particular properties of LIS, the model accounts for superdirectivity and mutual coupling effects along with near field propagation, necessary in those situations where the array dimension becomes very large. Finally, with the proposed model, the matched filter transmitter and the weighted minimum mean square error precoding are derived under both realistic constraints: limited radiated power and maximum ohmic losses.