Spatial multiplexing in far-field (FF) multiple-input multiple-output (MIMO) systems typically relies on the multipath provided by scatterers. Recent studies have revealed that near-field (NF) MIMO systems can exploit spatial multiplexing even in line-of-sight (LoS) propagation. Through singular value decomposition (SVD), an orthogonal basis consisting of the channel’s right singular vectors can be obtained to formulate a beamforming matrix. This matrix maps multiple data streams into orthogonal sub-channels, enabling spatial multiplexing. However, the SVD-based beamforming matrix, composed of entries with varying magnitudes and phases, generally requires a costly full digital array architecture. To achieve spatial multiplexing with low-complexity beamforming, this paper explores the channel’s structural characteristics of NF-LoS MIMO systems. Specifically, we consider a general point-to-point MIMO setup for a pair of uniform linear arrays (ULAs). We first investigate the ability of the transmitter (Tx) array to resolve paths to different Rx antennas under the NF spherical wave model. Based on the derived resolution, we explicitly formulate a quasi-orthogonal (QO) basis consisting of NF beamfocusing vectors, ensuring that the normalized magnitude of any two QO vectors remains sufficiently small to meet practical application requirements. By appropriately arranging these QO vectors into a beamforming matrix, we transform the spatial channel into a beamspace channel, where the beams serve as QO sub-channels. Furthermore, we utilize these QO beams or sub-channels for beamspace modulation (BM), where the Tx dynamically selects beams to deliver information via beam index modulation and via spatial multiplexing. Importantly, the beamformer designed for selecting QO beams, consisting of constant-magnitude entries, can be implemented using only analog phase shifters. Compared with the existing SVD based BM, the proposed scheme simplifies implementation in hybrid array architectures and achieves high spectrum efficiency with reduced computation complexity. We also show the feasibility of the proposed scheme in practical scenarios, including imperfect channel state information (CSI), finite-resolution phase shifters, mixed LoS and none-line-of-sight (NLoS) channels, and uniform planar arrays (UPAs).

Reconfigurable intelligent surfaces (RISs) have shown the potential to improve signal-to-interference-plus-noise ratio (SINR) related coverage, especially at high-frequency communications. However, assessing electromagnetic field exposure (EMFE) and establishing EMFE regulations in RIS-assisted large-scale networks remain open issues. This paper proposes a stochastic geometry (SG) based framework to characterize SINR and EMFE in such networks for downlink and uplink scenarios. Particularly, we carefully consider the association rule with the presence of RISs, accurate antenna pattern at base stations (BSs), fading model, and power control mechanism at mobile devices in the system model. Under the proposed framework, we derive the marginal and joint distributions of SINR and EMFE in downlink and uplink, respectively. The first moment of EMFE is also provided. Additionally, we design the compliance distance (CD) between a BS/RIS and a user to comply with the EMFE regulations. To facilitate efficient identification, we further provide approximate closed-form expressions for CDs. From numerical results of the marginal distributions, we find that in the downlink scenario, deploying RISs may not always be beneficial, as the improved SINR comes at the cost of increased EMFE. However, in the uplink scenario, RIS deployment is promising to enhance coverage while still maintaining EMFE compliance. By simultaneously evaluating coverage and compliance metrics through joint distributions, we demonstrate the feasibility of RISs in improving uplink and downlink performance. Insights from this framework can contribute to establishing EMFE guidelines and achieving a balance between coverage and compliance when deploying RISs.

Deploying reconfigurable intelligent surfaces (RISs) offers the potential to improve coverage performance in mil-limeter wave (mmWave) communications. However, electric and magnetic field (EMF) exposure related to base station (BS) trans-mission and RIS reflection is still unclear. This paper provides an analytical framework to evaluate the EMF exposure in RIS-assisted mmWave cellular networks. The proposed framework provides insights to establish technical guidelines for ensuring EMF exposure within a safe limit. For example, by considering the compliance distance (CD) set for BSs, we explore the necessity of designing a similar CD for RISs.

Millimeter-wave (mmWave) is highly susceptible to obstacles and requires significant directivity of beams. To address these challenges, a promising solution is to deploy antenna arrays at base stations (BSs) and reconfigurable intelligent surfaces (RISs). Antenna arrays enable 3D beamforming and provide highly directional beams, while RISs create favorable transmission environments. In this paper, we propose an analytical framework to quantify the coverage performance of the RIS-assisted mmWave cellular network with 3D beamforming. Modeling obstacles and RISs with a line Boolean model and BSs with a Poisson point process (PPP), we provide the distance distribution between a UE and its associated RIS by evaluating the impact of the RIS orientation. Moreover, we adopt a 3D sectorized flat-top antenna model to characterize the interference introduced by dynamic directional beams (i.e., mainlobe/sidelobe). Furthermore, we derive the signal-to-interference ratio (SIR) coverage probability, where an equivalent PPP is proposed to enhance the computation efficiency. Numerical results validate the accuracy of our analysis and show that the proposed network achieves significant coverage improvement. We also investigate the impacts of the key parameters on the coverage probability, providing useful insights for deploying RISs and antenna arrays in mmWave cellular networks.

Installing more base stations (BSs) into the existing cellular infrastructure is an essential way to provide greater network capacity and higher data rates in the 5th-generation cellular networks (5G). However, a non-negligible amount of the population is concerned that such network densification will generate a notable increase in exposure to electric and magnetic fields (EMF) over the territory. In this paper, we analyze the downlink, uplink, and joint downlink&uplink exposure induced by the radiation from BSs and personal user equipment (UE), respectively, in terms of the received power density and exposure index. In our analysis, we consider the EMF restrictions set by the regulatory authorities such as the minimum distance between restricted areas (e.g., schools and hospitals) and BSs, and the maximum permitted exposure. Exploiting tools from stochastic geometry, mathematical expressions for the coverage probability and statistical EMF exposure are derived and validated. Tuning the system parameters such as the BS density and the minimum distance from a BS to restricted areas, we show a trade-off between reducing the population’s exposure to EMF and enhancing the network coverage performance. Then, we formulate optimization problems to maximize the performance of the EMF-aware cellular network while ensuring that the EMF exposure complies with the standard regulation limits with high probability. For instance, the exposure from BSs is two orders of magnitude less than the maximum permissible level when the density of BSs is less than$20 \text {BSs/km}^{2}$.

A new generation of aerial vehicles is hopeful to be the next frontier for the transportation of people and goods, becoming even as important as ground users in the communication systems. To enhance the coverage of aerial users, appropriate adjustments should be made to the existing cellular networks that mainly provide services for ground users by the down-tilted antennas of the terrestrial base stations (BSs). It is promising to up-tilt the antennas of a subset of BSs for serving aerial users through the mainlobe. With this motivation, in this work, we use tools from stochastic geometry to analyze the coverage performance of the adjusted cellular network (consisting of the up-tilted BSs and the down-tilted BSs). Correspondingly, we present exact and approximate expressions of the signal-to-interference ratio (SIR)-based coverage probabilities for users in the sky and on the ground, respectively. Numerical results verify the analysis accuracy and clarify the advantages of up-tilting BS antennas on the communication connectivity of aerial users without the potential adverse impact on the quality of service (QoS) of ground users. Moreover, it is unveiled that there exists an optimal value of the up-tilted/down-tilted BS density ratio for maximizing the coverage probability of the aerial or ground users.