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.

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.