The exploitation of millimeter wave (mmWave) and sub-Terahertz (sub-THz) bands is expected to be one of the main pillars for the development of future cellular networks due to the high available bandwidth they provide. The existence of Line-of-Sight (LOS) link between a user equipment (UE) and an access point (AP) is a prerequisite for connection establishment in these networks, as the wireless links in these bands are very sensitive to blockage effects. This can be achieved by densifying APs within a network area. An arising challenge is the efficient exploitation of the available bandwidth of a given network. In this paper, the maximization of the number of served UEs in modern mmWave and sub-THz cellular networks is investigated and achieved by deploying a Maximum Flow Algorithm for UE-AP association (MFUA) to optimize bandwidth allocation, assuming that every AP will have a finite and predefined amount of bandwidth which they can share among UEs. MFUA determines the maximum flow between two given nodes of a graph corresponding to a specific network, where the capacity of its edges is known. An extensive simulation campaign was carried out revealing that the use of MFUA utilizes bandwidth more effectively compared to the reference method and improves the system performance, leading to the maximization of number of served UEs. The examined test cases include static and time-evolving scenarios.

This work presents accurate closed-form approximations for the probability density function (PDF) of the sum of independent and identically distributed (i.i.d.) fading envelopes and square fading envelopes, experienced by indoor and outdoor channels operating in the D-band. In order to provide a simple yet accurate analytical expression for these PDFs, we propose the use of the Pearson family of distributions, which are mainly characterized by their skewness and kurtosis. Our analysis has demonstrated that among the seven types of Pearson distributions, the type I Pearson distribution provides the most accurate fit to the exact PDF of envelopes and squared envelopes, for a diverse range of indoor and outdoor wireless propagation scenarios. The distribution parameters are estimated using moments-matching techniques, while the requisite moments are obtained through a measurement campaign conducted in indoor as well as outdoor environments. Specifically, with regard to the indoor scenarios, the following locations have been considered: A mall, an airport, and an entrance hall. With regard to the outdoor scenarios, the following locations have been selected: A university campus, a residential area, and a city center. Our results are further applied to analyze the average bit error rate performance and the ergodic capacity of equal-gain combining (EGC) and maximal-ratio combining (MRC) receivers, the sum rate of rate splitting multiple access (RSMA) systems with EGC diversity reception, as well as the coverage probability of wireless systems employing intelligent reflecting surfaces (IRS). Our analysis is supported by extensive numerical results and Monte Carlo simulations.

Due to the fact that large swathes on Earth still lack broadband communication coverage, especially in remote/rural areas and developing countries, there have been several attempts, starting from 3GPP Release 17, to lay out the architectural amendments needed for the integration of terrestrial networks with their non-terrestrial counterparts. Such attempts have led to recent projects regarding such integration that consider either 5G/5G-Advanced networks or more revolutionary approaches for the forthcoming 6G networks. In this manuscript, we give an overview of the architectural framework, technical innovations, and considered use cases of the Horizon Europe ETHER project.

As the demands of users increase continuously over time, especially in 5G and beyond 5G era, accurate performance evaluation is essential for design and optimization of cellular networks. To this end, stochastic geometry and specifically, uniform Poisson Point Processes (PPPs) have been employed in the past for the performance analysis of such networks, due to their mathematical tractability and adequate accuracy they provide. The distribution of the distance between a user node and its tagged access point (AP) is an important metric, which is derived from the statistical properties of PPPs. However, the presence of obstacles within the network's deployment area, has not attracted much attention from the research community. In this paper, we study the distribution of the distance between a user node and its tagged AP in areas with obstacles, where network nodes are distributed according to a PPP. Numerical results accompanied with simulations, are presented to verify the proposed analysis.

In this paper, we examine the potential for a reconfigurable intelligent surface (RIS) to be powered by energy harvested from information signals. This feature might be key to reap the benefits of RIS technology’s lower power consumption compared to active relays. We first identify the main RIS power-consuming components and then propose an energy harvesting and power consumption model. Furthermore, we formulate and solve the problem of the optimal RIS placement together with the amplitude and phase response adjustment of its elements in order to maximize the signal-to-noise ratio (SNR) while harvesting sufficient energy for its operation. Finally, numerical results validate the autonomous operation potential and reveal the range of power consumption values that enables it.

The authors propose a coordinated multi‐cell beamforming technique for signal‐to‐interference plus noise ratio (SINR) balancing under multiple base station power constraints. Instead of balancing SINR of all users in all cells to the same level, the authors’ propose a new approach to balance SINR of users in various cells to different maximum possible values. This has the ability to allow users in cells with relatively more transmit power or better channel condition to achieve a higher balanced SINR than that achieved by users in the worst‐case cells. This multi‐level SINR balancing problem is solved using SINR constraints based SINR balancing criterion and subgradient method. The simulation results support the optimality of the results through comparison with semi‐definite programming‐based optimisation.

Hierarchical Quadrature Amplitude Modulation is considered to enhance the aggregate Spectral Efficiency (SE) in broadcast systems operating under fading conditions. Closed-form and series representations for SE are derived assuming that the user terminals are uniformly distributed as to their distance from the Base Station and employ Maximum Likelihood detection. The modulation parameters are adjusted to optimize system performance. Numerical results and Monte Carlo simulations show that by adjusting the constellation diagram of the Hierarchical Modulation scheme, the SE performance may be significantly enhanced, especially under fading conditions.

The authors investigate a spectrum sharing peer‐to‐peer relay network where multiple source nodes with multiple antennas communicate with their desired destination nodes with multiple antennas through a multiple‐input and multiple‐output (MIMO) relay. The authors establish the duality between uplink and downlink peer‐to‐peer MIMO channels with any number of antennas at each node and demonstrate mean square error (MSE) of a downlink peer‐to‐peer network can be achieved in a virtual uplink network with the same total network transmission power constraint. By applying this result, the authors develop an iterative algorithm to optimise the source, relay and receiver processing matrices such that the weighted MSE of the retrieved signal at the receivers is minimised. The simulation results demonstrate satisfactory performance of the proposed algorithm.

We propose a two-way relay based spatial multiplexing technique for a cognitive radio relay network (CR). The relay coefficients and the transmission powers are optimized to maximize the worst-case user signal-to interference and noise ratio (SINR), while ensuring interference leakage from the relays to the primary users (PUs) in the network is below a threshold. We solve this problem through an iterative procedure that uses semidefinite and geometric programming along with bisection search method. We evaluate the performance of the proposed scheme in terms of the mean SINR for different number of relays and transmission power at the relays.

The authors present an alternative moments-based approach for the performance analysis of dual-hop relaying communications systems over generalised fading channels. A unified analytical approach for the computation of the nth moment of the harmonic mean of N≥2 arbitrarily distributed random variables (RVs) is proposed. On the basis of this statistical result, we analyse the outage and the average error rate performance of dual-hop cooperative wireless systems employing amplify-and-forward (AF) relays. Moreover, our newly derived results can be applied to evaluate the average sum rate of AF relaying systems. Extensive numerical and simulation results are presented to substantiate the proposed analysis.

In this paper, we consider a two-way amplify-and-forward relaying scheme, which consists of multiple transceivers and r relay nodes. Assuming that both the transceivers and the relays are equipped with single antennas, we deploy a signal-to-interference and noise-ratio (SINR) balancing technique, where the smallest of the transceivers SINRs is maximized under a total transmit power constraint. We solve this problem through an iterative procedure that uses semidefinite and geometric programming along with bisection search methods. We evaluate the performance of the proposed scheme in terms of the mean SINR for various relays and power at the relays.

A versatile fading distribution that generalises many commonly used models for multi-path and shadow fading is the so-called generalised-K (KG) distribution. By considering the product of N independent but not necessarily identically distributed (n.i.d.) squared KG random variables (RVs), we derive exact closed-form expressions for the moment generating function, probability density function, cumulative distribution function and the moments of the cascaded KG fading channel. Expressions for the ergodic capacity and the error performance of different digital modulation schemes are also obtained. Based on the above mentioned formulas, we analyse the performance of orthogonal space-time block codes over multiple-input multiple-output keyhole KG fading channels. Various performance evaluation results demonstrate the proposed mathematical analysis.

In this paper, new expressions for the probability density function (pdf) of the sum of non-identical independent squared η-μ random variables are derived. Based on our newly derived results, we provide a pdf-based approach for the performance analysis of 1-D and 2-D RAKE receivers in the context of an asynchronous direct sequence code division multiple access system operating over η-μ fading channels. For the considered system, useful performance metrics such as the outage probability, the channel capacity and the average bit error probability are investigated. Extensive numerical and computer simulation results are presented that illustrate the proposed mathematical analysis.

New results for the multichannel Nakagami-m fading model with an arbitrary correlation matrix are presented in this paper. By using an efficient tridiagonalization method based on Householder matrices, the inverse of the Gaussian correlation matrix is transformed to tridiagonal, managing to derive a closed-form union upper bound for the joint Nakagami-m probability density function and an exact analytical expression for the moment generating function of the sum of identically distributed gamma random variables. Our analysis considers an arbitrary correlation structure, which includes as special cases the exponential, constant, circular, and linear correlation ones. Based on the proposed mathematical analysis, we obtain a tight union upper bound for the outage probability of multibranch selection diversity receivers as well as exact analytical expressions for the outage and the average error probability of multibranch maximal-ratio diversity receivers. Our analysis is verified by comparing numerically evaluated with extensive computer simulation performance evaluation results, showing the usefulness of the proposed approach.

This paper provides exact-form expressions for the average symbol error probability (ASEP) of various digital modulation schemes with maximal ratio combining (MRC) diversity over L independent, not necessarily identically distributed (n.i.d.) eta-mu fading channels. The derived expressions are given in terms of the Lauricella and Appell hypergeometric functions and include several others available in the literature such as those for Nakagami-m and Hoyt. General asymptotic ASEP expressions are also derived for all the considered modulation schemes which provide useful insights regarding the factors affecting the performance of the considered system.

In this letter, closed-form outage probability expressions are presented for an -relays dual-hop plus a direct link network, in which the decode-and-forward relaying protocol is employed. Our analysis significantly extends previous results on Rayleigh fading, considering a Nakagami- fading environment with either equal or distinct second hops fading parameters to average power ratios. Various numerical examples illustrate the proposed analysis.

New results for the multichannel Nakagami-m fading model with an arbitrary correlation matrix are presented. By using an efficient tridiagonalization method based on Householder matrices, a union upper bound for the joint Nakagami-m probability density function and an infinite series representation for the moment generating function of the sum of gamma random variables are derived. Based on the proposed mathematical analysis, a tight union upper bound for the outage probability of multibranch selection diversity, as well as, exact analytical expressions for the outage and average error probability of multibranch maximal-ratio diversity receivers operating over identically distributed and arbitrarily correlated Nakagami-m fading channels are obtained. Our analysis is verified by comparisons of numerically evaluated results with extensive computer simulation ones.

In this paper, new results for the multivariate Rayleigh distribution with an arbitrary correlation matrix are presented. By using an efficient tridiagonalization method based on Householder matrices, closed-form union upper bounds for its joint probability density and cumulative density functions are derived. Based on the proposed mathematical analysis, a tight union upper bound and an analytical approximation for the outage probability of multibranch selection diversity receivers operating over identically distributed and arbitrarily correlated Rayleigh fading channels are obtained. Our analysis is verified by comparing numerically evaluated results with extensive computer simulation ones.

An infinite series representation for the moment generating function of the sum of squared arbitrarily correlated Rayleigh random variables is presented. Based on the derived formula, corresponding analytical expressions for the probability density and cumulative distribution functions are extracted. As an application for the aforementioned sum, exact analytical expressions for the outage and the average error probability, as well as, the channel average spectral efficiency of multibranch maximal-ratio diversity receivers operating over identically distributed and arbitrarily correlated Rayleigh fading channels are obtained. Our analysis is verified by comparing numerically evaluated results with extensive computer simulation ones.

In this paper, for an L-relays dual-hop plus a direct link wireless network, in which the decode-and-forward relaying protocol is employed, closed-form expressions for the end-end outage probability are presented. Our analysis considers a Nakagami-m fading environment with either equal or distinct second hops fading parameter to average power ratios. Various numerical examples illustrate the proposed analysis.

In this paper the impact of the position of user's hand holding a multiple-input-multiple-output (MIMO) terminal on system performance and more specifically on channel capacity is investigated. We consider a 4times4 MIMO system with a personal digital assistant (PDA) equipped with a compact array of 4 patch elements. Radiation field patterns of the antenna elements in the presence of the other elements as well as in the presence of a user's hand are simulated. Different positions of the user's hand are taken into account. The covariance matrix of the receiving antennas is evaluated by means of the obtained patterns. A correlation-based channel model is considered, where the antenna patterns are incorporated. Extensive simulations showed that the performance of the MIMO systems varies significantly with the position of the user's hand.

Future wireless communication systems are expected to provide seamless inter-working between existing and 3G radio networks providing the user with a wide variety of services, while maintaining a large area of coverage and minimum user QoS requirements. In this paper, a new mechanism that implements interoperability between HSDPA and WLAN systems is proposed. The proposed interoperability mechanism is activated via the optimization of a suitably defined cost function, which takes into account all the appropriate system level parameters that trigger the interoperability process. The performance evaluation of the proposed scheme is assessed by means of a software - based simulation platform. A number of simulations have been carried out in order to demonstrate the performance enhancements achieved by the proposed mechanism in terms of user throughput, handovers statistics, and system throughput.

In this paper the system level performance of an HSDPA (high speed downlink packet access) MIMO network, which combats the effects of antenna correlation by applying novel reconfigurable techniques is investigated. Reconfigurability is achieved at the link level by applying a linear precoding scheme at the transmitter space-time block encoder. An appropriate link to system interface has been developed, taking into account the linear precoding technique. The system level performance metrics of interest are the average system throughput and the number of satisfied users. Simulation results are presented in order to demonstrate the performance enhancements achieved by the application of reconfigurable techniques compared to the conventional ones.

An experimental point-to-point coverage prediction model based on field measurements taken in the region of Athens in the 900 MHz band is discussed. The developed model, incorporating a new method for effective height estimation in 3-dimensions, addresses the case of mobile radio propagation over urban and suburban areas with non-uniform terrain morphology in which negative effective height values are also present. The effectiveness of the proposed model is proved by comparing actual transmission loss values, for the specific area, to prediction values obtained by this model as well as by a number of known models.

Coverage prediction is essential for designing a mobile radio system operating in the 800/900 MHz band. A mobile radio field measurement and coverage prediction model evaluation procedure based on the utilization of a geographical information system (GIS) is described. Coordination of mean signal strength values with detailed point-to-point topographic characteristics through the GIS is used to obtain accurate and reliable information in the form of an integrated propagation and topographic database. The proposed procedure can be used to evaluate or even modify current coverage prediction models over composite urban areas.