We hereby present a novel interference mitigation strategy specifically designed to enhance the quality of service that a typical terrestrial user equipment (UE) would experience after the occurrence of a calamity. In particular, we devise a novel stochastic geometry framework where the functioning ground base stations are modeled as an inhomogeneous Poisson point process, and promote proper silencing as an effective solution to improve both coverage and reliability (which is usually overlooked in emergency scenarios); in particular, the latter is evaluated by means of the signal-to-interference-plus-noise ratio (SINR) meta distribution performance metric. The derived downlink performances assume Rayleigh fading conditions for all wireless links. The numerical results show insightful trends in terms of both average coverage probability (which is optimized by choosing the best area for applying the silencing strategy) and SINR meta distribution, depending on: distance of the UE from the disaster epicenter (henceforth intended as the center of the area where the BS can be damaged), disaster radius (also referring to the latter area), and quality of resilience of the terrestrial network. The aim of this paper is therefore to prove the effectiveness of proper silencing in emergency scenarios, at least from the coverage and reliability perspectives.

The sub-6 GHz spectrum and the millimeter wave frequency band, with its huge spectrum, have been exploited in the past years to meet the traffic demands of wireless communication networks. However, the limited and licensed spectrum of these bands cannot support the massive connectivity requirements, the exponential growth of data traffic, and the strict quality-of-service requirements for 6G and beyond wireless systems. Extremely high frequencies, such as optical and terahertz, which offer much wider transmission bandwidths with extreme data rate capabilities, are expected to play key roles in the 6G and beyond era. As a result, future-generation wireless networks will transition from single-band and heterogeneous networks to multi-band networks (MBNs), where various frequency bands coexist. Despite the great potential of MBNs, they face novel challenges from channel modeling, transceiver and antenna design, programmable simulation platforms, standardization activities, and resource allocation. This paper provides a tutorial overview from the communication design perspective of the various frequency bands, elaborating on the above issues. Then, we introduce and examine typical MBN architectures for future networks and provide a detailed overview of state-of-the-art resource allocation problems for existing MBNs that typically operate on two frequency bands. The considered resource allocation optimization problems and solution techniques are discussed comprehensively. We then identify key performance metrics and constraint sets that should be considered for resource allocation optimization in future MBNs and provide numerical results to depict how various system parameters and user behaviors can influence their performance. Finally, we present several potential research issues as future work for the design and performance optimization of MBNs.

Reconfigurable intelligent surface (RIS) technology and its integration into existing wireless networks have recently attracted much interest. While an important use case of said technology consists in mounting RISs onto unmanned aerial vehicles (UAVs) to support the terrestrial infrastructure in post-disaster scenarios, the current literature lacks an analytical framework that captures the networks’ topological aspects. Therefore, our study borrows stochastic geometry tools to estimate both the average and local coverage probability of a wireless network aided by an aerial RIS (ARIS); in particular, the surviving terrestrial base stations (TBSs) are modeled by means of an inhomogeneous Poisson point process, while the UAV is assumed to hover above the disaster epicenter. Our framework captures important aspects such as the TBSs’ altitude, the fact that they may be in either line-of-sight or non-line-of-sight condition with a given node, and the Nakagami-m fading conditions of wireless links. By leveraging said aspects we accurately evaluate three possible scenarios, where TBSs are either: (i) not aided, (ii) aided by an ARIS, or (iii) aided by an aerial base station (ABS). Our selected numerical results reflect various situations, depending on parameters such as the environment’s urbanization level, disaster radius, and the UAV’s altitude.

Digital divide refers to the gap between populations at different socio-economic levels with respect to their access to the Internet and communication technologies in general. In this chapter, we summarize the recent developments aimed at bridging such inequality from a technical and economic standpoint. After a preliminary discussion on alternative backhaul paradigms, we focus on space-, air-, and ground-based communication paradigms, as well as their integration. Subsequently, we discuss the affordability of these solutions, as it is primarily the economic aspect which hinders digital inclusion.

Even though achieving global connectivity represents one of the main goals of 5G and beyond wireless networks, exurban areas are still suffering frequent outages due to the lack of proper telecom infrastructures, which are often available only in urban areas. Indeed, cellular network design is usually capacity-driven, and thus base stations’ densities mostly follow population and especially revenue densities. Contextually, we focus on one of the most promising solutions to provide sufficient and reliable coverage in far-flung areas: aerial base stations, which consist of unmanned aerial vehicles carrying cellular base station equipment. In this paper, we extensively discuss the problem of bridging the so-called urban-rural digital divide (i.e., the connectivity gap between urban and rural areas) from various perspectives. First, we showcase various alternative solutions, and compare conventional terrestrial networks with aerial networks from a techno-economic point of view. Then, we highlight the topological aspects of rural environments and explain how they can affect the actual design of cellular networks. In addition, we investigate both the coverage probability and the reliability of the communication links via simulations, proving that the integration of aerial base stations can be quite promising in a 6G perspective. Finally, we propose two original extensions of our case study as open problems.

For acquisition of narrow-beam free-space optical (FSO) terminals, a Global Positioning System (GPS) is typically required for coarse localization of the terminal.  However, the GPS signal may be shadowed, or may not be present at all,  especially in rough or unnameable terrains.   In this study, we propose a lidar-assisted acquisition of an unmanned aerial vehicle (UAV) for FSO communications in a poor GPS environment. Such an acquisition system consists of a lidar subsystem and an FSO acquisition subsystem: The lidar subsystem is used for coarse acquisition  of the UAV, whereas, the FSO subsystem is utilized for fine acquisition to obtain the UAV’s accurate position. This study investigates the optimal allocation of energy between the lidar and FSO subsystems to minimize the acquisition time. Here, we minimize the average acquisition time, and maximize the cumulative distribution function of acquisition time for a fixed threshold. We learn that an optimal value of the energy allocation factor exists that provides the best performance of the proposed system.

In this paper, the outage probability of maritime free-space optical communication system is analyzed. Maritime turbulence channel is modeled by Lognormal distribution. Channel model includes the combined effect of turbulence, pointing error, angle-of-arrival (AOA) fluctuations, and attenuation. Pointing error and AOA fluctuations are assumed to be Beckmann and Rayleigh distributed, respectively. Turbulence power spectrum is considered to present the Kolmogorov characteristics and Rytov variance of a propagating Gaussian beam is obtained. The probability density function, cumulative distribution function and outage probability of maritime communication channel are obtained analytically. Outage performance of a maritime communication link is given depending on various parameters of the maritime environment, the Gaussian beam, and the communication system.

This paper investigates the performance of high altitude platforms (HAPS) based free-space optical (FSO) communication links including HAPS-to-ground station (downlink), ground-to-HAPS (uplink) and HAPS-to-HAPS (horizontal link) communications. The effects of  attenuation loss, atmospheric turbulence, pointing error and angle-of-arrival (AOA) are taken into account. Also, the application of adaptive optics correction, one of the most effective turbulence mitigation techniques, is analyzed using the Zernike polynomials representation. Closed-form expressions are obtained for probability density function (PDF), cumulative distribution function (CDF), Rytov variance, adaptive optics filter function and outage probability mainly in terms of Meijer’s G function when both no adaptive optics correction is used and adaptive optics correction is applied. Some selected results are presented depending on the various  parameters such as the HAPS altitude, the ratio of vertical and horizontal deviations, beam waist, Zenith angle, height of ground station, receiver aperture diameter, channel state threshold and wind speed. The performance improvement with adaptive optics correction is investigated by removing different Zernike modes. We show that the downlik performance outperforms the uplink, the performance of the horizontal link sharply increases above certain altitude and communication links benefit from the adaptive optics correction up to a certain level in terms of performance improvement.

Motivated by the need for ubiquitous and reliable communications in post-disaster emergency management systems (EMSs), we hereby present a novel and efficient stochastic geometry (SG) framework. This mathematical model is specifically designed to evaluate the quality of service (QoS) experienced by a typical ground user equipment (UE) residing either inside or outside a generic area affected by a calamity. In particular, we model the functioning terrestrial base stations (TBSs) as an inhomogeneous Poisson point process (IPPP), and assume that a given number of uniformly distributed unmanned aerial vehicles (UAVs) equipped with cellular transceivers is deployed in order to compensate for the damage suffered by some of the existing TBSs. The downlink (DL) coverage probability is then derived based on the maximum average received power association policy and the assumption of Nakagami-m fading conditions for all wireless links. The proposed numerical results show insightful trends in terms of coverage probability, depending on: distance of the UE from the disaster epicenter, disaster radius, quality of resilience (QoR) of the terrestrial network, and fleet of deployed ad-hoc aerial base stations (ABSs). The aim of this paper is therefore to prove the effectiveness of vertical heterogeneous networks (VHetNets) in emergency scenarios, which can both stimulate the involved authorities for their implementation and inspire researchers to further investigate related problems.

This study evaluates the effect of application of spatial diversity technique, namely multiple-input multiple-output (MIMO), on the outage performance of free-space optical (FSO) communication systems operating in atmospheric turbulent medium for both slant path (downlink and uplink) and horizontal links. FSO communication links cover ground-to-high altitude platform system (HAPS), HAPS-to-ground, and HAPS-to-HAPS configurations. System model includes the combined effect of attenuation (resulting from absorption and scattering), angle-of-arrival (AOA) fluctuations, atmospheric turbulence, and pointing error. Atmospheric turbulent channel is characterized by Gamma-Gamma distribution. The Hoyt and Rayleigh distributions are used to model pointing error and AOA fluctuations, respectively. The performance of MIMO FSO communication links is presented depending on the various environmental and system parameters. Results show that the MIMO application in both transmitter and/or receiver can be a prominent alternative concerning the performance improvement for FSO communication links.

We analyze the outage probability of integrated ground-air-space free-space optical (FSO) communication links for different atmospheric turbulence channel models. The performance of ground to high altitude platforms (HAPS), HAPS to geostationay Earth orbit (GEO) satellite, HAPS to HAPS and ground to GEO satellite is investigated for various link configurations (downlink, horizontal, and uplink) and parameters such as zenith angle, channel state, horizontal and vertical deviations, altitude, beam waist, receiver aperture diameter, wind speed, and visibility. The atmospheric attenuation (the effects of fog, cloud and volcanic activities), atmospheric turbulence, angle of arrival (AOA) fluctuations and pointing error are included in the considered model. Closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) are obtained for Lognormal, Gamma and exponentiated Weibull distributed channel models. The benefit of using HAPS is shown by comparison of outage probabilities for direct ground to GEO satellite and HAPS assisted ground to GEO satellite links.

The number of disasters has increased over the past decade where these calamities significantly affect the functionality of communication networks. In the context of 6G, airborne and spaceborne networks offer hope in disaster recovery to serve the underserved and to be resilient in calamities. Therefore, this paper surveys the state-of-the-art literature on post-disaster wireless communication networks and provides insights for the future establishment of such networks. In particular, we first give an overview of the works investigating the general procedures and strategies for counteracting any large-scale disasters. Then, we present the possible technological solutions for post-disaster communications, such as the recovery of the terrestrial infrastructure, installing aerial networks, and using spaceborne networks. Afterward, we shed light on the technological aspects of post-disaster networks, primarily the physical and networking issues. We present the literature on channel modeling, coverage and capacity, radio resource management, localization, and energy efficiency in the physical layer and discuss the integrated space-air-ground architectures, routing, delay-tolerant/software-defined networks, and edge computing in the networking layer. This paper also presents interesting simulation results which can provide practical guidelines about the deployment of ad hoc network architectures in emergency scenarios. Finally, we present several promising research directions, namely backhauling, placement optimization of aerial base stations, and the mobility-related aspects that come into play when deploying aerial networks, such as planning their trajectories and the consequent handovers.