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.

Cellular communication standards have been established to ensure connectivity across most urban environments, complemented by deployment hardware and facilities tailored for city life. At the same time, numerous initiatives seek to broaden connectivity to rural and developing areas. However, with nearly half the global population still offline, there is an urgent need to drive research toward enhancing connectivity in areas and conditions that deviate from the norm. This article delves into innovative communication solutions not only for hard-to-reach and extreme environments but also introduces “hard-to-serve” areas as a crucial, yet underexplored, category within the broader spectrum of connectivity challenges.We explore the latest advancements in communication systems designed for environments subject to extreme temperatures, harsh weather, excessive dust, or even disasters such as fires. Our exploration spans the entire communication stack, covering communications on isolated islands, sparsely populated regions, mountainous terrains, and even underwater and underground settings. We highlight system architectures, hardware, materials, algorithms, and other pivotal technologies that promise to connect these challenging areas. Through case studies, we explore the application of 5G for innovative research, long range (LoRa) for audio messages and emails, LoRa wireless connections, free-space optics, communications in underwater and underground scenarios, delay-tolerant networks, satellite links, and the strategic use of shared spectrum and TV white space (TVWS) to improve mobile connectivity in secluded and remote regions. These studies also touch on prevalent challenges such as power outages, regulatory gaps, technological availability, and human resource constraints, where we introduce the concept of peri-urban hard-to-serve areas where populations might struggle with affordability or lack the skills for traditional connectivity solutions. This article provides an exhaustive summary of our research, showcasing how 6G and future networks will play a crucial role in delivering connectivity to areas that are hard-to-reach, hard-to-serve, or subject to extreme conditions (ECs).

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.

As 6G emerges, cellular systems are envisioned to integrate sensing with communication capabilities, leading to multi-faceted communication and sensing (JCAS). This paper presents a comprehensive cross-layer overview of the Hexa-X-II project's endeavors in JCAS, aligning 6G use cases with service requirements and pinpointing distinct scenarios that bridge communication and sensing. This work relates to these scenarios through the lens of the cross-layer physical and networking domains, covering models, deployments, resource allocation, storage challenges, computational constraints, interfaces, and innovative functions.

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.

6G goals, such as improved coverage, flexibility to different network scenarios, increased throughput, higher robustness and reliability could be advanced with the use of new access and flexible topologies. In this contribution, three enablers of new access and flexible topologies, which contribute to the aforementioned goals, are presented, namely network of networks, multi-connectivity and E2E context awareness management. Relevant studies of each enabler that impact the system architecture are described, along with the corresponding overviews of potential solutions. In addition, each enabler has been mapped to the 6G E2E system blueprint proposed by the Hexa-X-II project and its impact to the system, as well as the relevant stakeholders have been identified.

Single antenna sensors in the rapidly emerging Internet-of- Things (IoT) are attractive due to their simplicity and low cost. However, determining their own positions autonomously using only a single antenna is challenging. This paper presents a novel approach for autonomous downlink localization of single-antenna receivers using Reconfigurable Intelligent Surfaces (RIS) and a tracking process using the complex extended Kalman filter (EKF). Simulation results show that the considered RIS-aided wireless radio system can provide accurate localization and continuous fast tracking down to the centimeter level, especially when multiple RISs are deployed. Furthermore, various factors affecting the system performance are analyzed in detail.

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.