This letter introduces a reconfigurable intelligent surface (RIS)-assisted modulation scheme tailored for non-coherent receivers. Employing a RIS of dual-polarized elements, we manipulate the polarization state of the reflected signals to enable a differential polarization shift keying modulation scheme while simultaneously beamforming the reflected signal towards the receiver. Subsequently, an additional differential phase shift keying (DPSK) modulation layer is superimposed under two distinct deployments, where either the source or the RIS performs the DPSK modulation. Furthermore, the analytical performance is investigated, and a comparison with benchmark schemes is evaluated.

Polarization is a fundamental property of electromagnetic radio signals but often neglected in localization studies. In this paper, we study the potential benefits of integrating the polarization dimension into localization applications. We develop a three-dimensional (3D) geometric channel model between a base station (BS) and user equipment (UE), both equipped with dual-polarized (DP) antennas, which offers fundamental insights into the angles of departure (AoD) from the BS to the UE as well as the 3D orientation of the UE. From the model, we identify the degrees of freedom (DoF) provided by the polarization dimension for localization solutions by evaluating the rank of the equivalent Fisher information matrix. Subsequently, we leverage these DoF to introduce three distinct localization applications: (i) 3D orientation estimation, (ii) 2D AoD estimation, and (iii) mixed 2D position and 1D orientation estimation for vehicular scenarios. Furthermore, for the three localization applications we identify their regions of operation in terms of the ranges of the angles of interest, to avoid any ambiguity occurrence through the estimation process, thereby guaranteeing unique solutions. Finally, we derive the Cramér-Rao lower bounds and numerically establish the efficiency of the proposed estimators.

We propose a joint polarization and spatial modulation (JPSM) scheme using reconfigurable intelligent surface (RIS). In this scheme, a RIS equipped with dual-polarized (DP) reflecting elements is used to assist the communication between a transmitter of a single polarized antenna and a receiver equipped with a uniform linear array of DP antennas, while additionally encoding the reflected waves on the RIS to perform JPSM scheme. The information data is encoded in terms of the receiver DP antenna index as well as the polarization state of the received signal. Furthermore, we develop exhaustive and heuristic RIS phase shift design solutions to enable the RIS-JPSM scheme. Moreover, both an optimum maximum likelihood and a low complexity greedy detectors are formulated. The proposed scheme enhances the data rate by operating higher-order polarization modulation in comparison to the conventional RIS-based spatial modulation scheme.

Reconfigurable intelligent surfaces (RIS) have emerged as a promising technology for 6G networks. In this study, we explore a novel use case for RIS: passive localization and tracking of a RIS-equipped object using monostatic sensing, where the fixed transmitter and receiver share the same single antenna, using OFDM signals. We develop a low-complexity algorithm that achieves centimeter-level accuracy using only 6 MHz bandwidth, and by applying temporal coding to random RIS phase profiles, separating signals from undesired multipath sources. In addition, we evaluate the impact of model uncertainty on the performance of the algorithm.

Reconfigurable intelligent surface (RIS) is considered as a promising technology for future wireless communication, localization and sensing systems. An RIS is a thin planar array that consists of multiple sub-wavelength-sized reflecting elements, each of which can induce independent and controllable interactions with the incident signal. An RIS can actively control the propagation channel, creating energy-efficient smart radio environments. Meanwhile, polarization is a fundamental property of electromagnetic (EM) waves; it refers to the orientation of the electric field component relative to the direction of propagation. The EM polarization provides additional degrees of freedom (DoF) to the wireless channel beyond the well-exploited time, frequency, and spatial dimensions.

In this thesis, we explore the RIS technology and EM polarization as two potential key enablers for future wireless communication and localization systems. First, we exploit the RIS technology to address a fundamental bottleneck for point-to-point multiple-input multiple-output (MIMO) communications in line-of-sight environments. Specifically, we deploy multiple RISs as controllable scatterers to create an artificial scattering environment, thereby providing additional spatial DoF and restoring the spatial multiplexing gain of MIMO systems.

Second, we present two studies that focus on the RIS operation regarding its efficient position of deployments and the channel state information acquisition. In the former study, we analyze the positional impact of the RIS on the achievable rate for single and multiple antenna systems. We consider not only the impact of propagation losses but also the impact of channel richness with multipath components to identify the efficient region of RIS deployments. In the latter study, we develop two distinct channel estimation schemes based on the superimposed pilot signals to reduce the significant pilot overhead required in RIS-aided wideband multi-user communications. Then, we incorporate iterative channel estimation and data detection techniques. Furthermore, we develop an RIS reflection design using an alternating optimization to maximize channel power.

Third, we unlock the RIS capability to control the polarization states of reflected signals by considering dual-polarized RIS. We utilize the RIS as an information modulator by performing three distinct polarization-based modulation schemes: binary polarization modulation, joint differential polarization and phase modulation, and joint polarization and spatial modulation. 

Finally, we study the potential benefits of integrating the polarization dimension into localization applications. We identify the DoF provided by polarization for localization. Subsequently, we leverage these DoF to introduce three distinct localization applications which enable three dimensional (3D) orientation estimation, 2D angle of departure estimation, and mixed 2D position and 1D orientation estimation for vehicular scenarios.

We propose a novel binary polarization shift keying modulation scheme for a line-of-sight environment by exploiting the polarization control ability of the reconfigurable intelligent surface (RIS). The RIS encodes the information data in terms of the polarization states of either the reflected wave from the RIS or the composite wireless channel between an RF source and receiver. In the first case, polarization mismatch correction becomes essential at the receiver. In the second case, the RIS pre-codes the reflected wave to compensate for the polarization mismatch which allows non-coherent demodulation at the receiver.

We propose a novel reconfigurable intelligent surface (RIS)-aided differential polarization shift keying modulation scheme for a line-of-sight environment. In this scheme, the RIS exploits the state of polarization (SoP) of the reflected waves over two successive reflection frames to encode the data bit. In particular, the RIS either preserves the SoP of the reflected wave similar to the previous reflection frame or switches it to another orthogonal SoP as a function of the information data bits. The proposed scheme allows non-coherent data detection without the need for polarization mismatch estimation and compensation processes at the receiver.    

Reconfigurable intelligent surface (RIS) is a promising technology for wireless communications applications. A RIS is a thin planar array that consists of multiple reflecting elements, each of which is connected to a tunable chip and can induce a controllable reflection coefficient to the incident signal. The RIS is a nearly passive unit as it only leverages the reflection on its elements and it does not consume any power for transmission. Furthermore, RISs can actively control the propagation channel, by accurately tuning the reflection coefficients of their elements to fit a specific need. The advantage of the RIS arises in creating energy-efficient smart radio environments wherein the wireless channel becomes an optimization variable. In this thesis, we study some of the potential applications and deployments for the RIS in wireless communications. First, the multiple-input multiple-output channel matrix in line-of-sight (LoS) environments turns out to be rank deficient such that spatial multiplexing becomes unattainable. Thus, we exploit the RISs to create additional degrees of freedom by synthesizing a sort of multi-path propagation. Then, we optimize the transmit covariance matrix and the reflection coefficients of the RISs using an alternating optimization algorithm to maximize the achievable rate. Alternatively, we propose different schemes to enhance the composite channel power which would result in an improvement to the achievable rate. Second, we characterize the efficient regions of RIS deployments with single and multiple antenna systems in Rician fading channels. We show that in RIS-aided single antenna systems, near RIS deployments relative to the transmitter or receiver are always better than far deployments. Moreover, we show that in RIS-aided multiple antenna systems, the efficient regions of operations are highly dependent on the propagation environment itself. In LoS environments, both the near and far deployments can result in substantial achievable rate gains. However, as the channel becomes richer with multipath, near deployments gradually become more efficient than the far deployments. Third, we propose the RIS to act as an access point for information transfer by exploiting the polarization control ability of the RIS. In particular, the RIS alternates the polarization state of the reflected waves to perform conventional as well as differential polarization shift keying (PolSK) modulation schemes. In RIS-aided conventional PolSK, two different schemes are proposed. In the first scheme, the receiver corrects for the polarization mismatch loss that occurs in the wireless channel. In the second scheme, the RIS additionally pre-codes the reflected wave to compensate for the polarization mismatch. In RIS-aided differential PolSK, the detection process is independent of the polarization mismatch. Thus, there is no need for a polarization mismatch compensation process by either the receiver or the RIS.

In line-of-sight (LoS) environments, point-to-point (P2P) multiple-input multiple-output (MIMO) channel matrix turns out to be rank deficient such that spatial multiplexing becomes unattainable. In this paper, we propose the deployment of distributed intelligent reflecting surfaces (IRSs) to act as artificial scatterers and synthesize a sort of multi-path propagation such that additional degrees of freedom are created. We show that given the far-field deployment of the IRS, it simply resembles a full-duplex relay with a single effective reflection coefficient. However, to maximize the channel capacity both the effective reflection coefficients of all IRSs and the transmit covariance matrix should be jointly optimized, which is a nonconvex optimization problem. Thus, we develop an alternating optimization algorithm to iteratively find a sub-optimal solution. Moreover, we propose different schemes to enhance the composite channel power which would result in an improvement to the achievable rate. Our simulation results show that the deployment of distributed IRSs with P2P MIMO systems in LoS environments increases the rank of the channel matrix, and improves the achievable rate by making spatial multiplexing possible.

We study the positional impact of an intelligent reflecting surface (IRS) on the achievable rate for single and multiple antenna systems. We show that in IRS-aided single antenna systems, it is always best to place the IRS as close as possible to the transmitter or receiver since the large-scale fading for IRS-reflected links is the main factor that characterizes the performance gain. However, for IRS-aided multiple antenna systems, the propagation environment has an important role in characterizing the efficient regions of IRS placement. In the case of a line-of-sight environment, the channel matrix turns out to be rank-deficient. Thus, both far and near IRS placements result in significant achievable rate improvements where the former provides a substantial additional degree-of-freedom, while the latter results in a power gain. Furthermore, as the wireless channel becomes richer with multipath, the rank of the channel matrix increases. Thus, the efficient far placement regions gradually shrink until they disappear in the case of a Rayleigh fading channel where IRS near placements become more efficient than far placements as they result in higher power gains.