Predicting channel excess attenuation is of utmost importance for the current satellite networks design. However, the adoption of data-driven models is hampered by the absence of reliable satellite propagation measurements in real conditions. In this framework, ESA initiated in 2015 a dedicated project for a large-scale measurement campaign using the Alphasat SCIEX Ka/Q band signals (ASALASCA). Here, we propose a data-driven model to predict in excess attenuation in the next future time-steps, by exploiting the real measurements (excess attenuation and rainfall rate) from the two experimental stations in Greece by NTUA operating at Ka band (19.704GHz).

This paper investigates the secure transmission in satellite communication systems, where a geostationary orbit (GEO) satellite serves an earth station while multiple eavesdroppers attempt to intercept the confidential message. Assuming that only the imperfect channel state information (CSI) of the wiretap channels are available, we propose a secure beamforming scheme to maximize the secrecy energy efficiency (SEE) of the earth station while satisfying the signal-noise-ratio (SNR) requirement at earth station, the secrecy constraints at eavesdroppers, and per-antenna power constraints at satellite antenna feeds. Since the formulated optimization problem is mathematically intractable, we propose a two-stage beamforming scheme to convert the original nonconvex problem into a solvable one and obtain the beamforming weight vectors. Numerical results are finally provided to verify the effectiveness of our proposed scheme.

The coexistence of a primary user (PU) with a secondary user (SU) is studied in an overlay cognitive radio network (CRN) which is proposed as an efficient technique to treat with the problem of spectrum underutilisation. A cooperative spectrum leasing process under the decode and forward protocol is considered among the PU and SU. Assuming users' specific Quality of Service requirements, a joint power and time allocation scheme for maximising the PU's cooperative effective capacity subject to minimum SU's and PU's effective capacities and maximum SU's instantaneous power transmission constraints is proposed. Its performance is close to the optimal one, that is computed using the exhaustive search scheme. A two‐step solution is applied considering the fundamentals of convex optimisation theory and closed‐form power and time expressions are extracted. As simulation results prove, the proposed scheme outperforms a less sophisticated scheme considering power allocation under constant time. Finally, the proposed algorithm is incorporated in a multi‐user CRN scenario where efficient matchings among the PUs and the SUs are investigated targeting to the increase of PUs' cooperative performance applying the matching theory.

This paper deals with the problem of precoding, scheduling, and link adaptation in next generation mobile interactive multibeam satellite systems. In contrast to the fixed satellite services, when the user terminals move across the coverage area, additional challenges appear. Due to the time varying channel, the gateway has only access to a delayed version of the channel state information (CSI), which can eventually limit the overall system performance. However, in contrast to general multiuser multiple-input multiple-output terrestrial systems, the CSI degradation in multibeam mobile applications has a very limited impact for typical fading channel and system assumptions. Under realistic conditions, the numerical results show that precoding can offer an attractive gain in the system throughput compared with the conservative frequency reuse allocations.

The need for increased data rates has led towards the development of advanced broadband satellite systems able to achieve higher capacity for providing Internet and backbone services in remote areas. Aside of the more conventional geostationary orbit (GEO), Medium Earth Orbit (MEO) satellite constellations are envisaged due to their advantages in terms of shorter link path and low latency; MEO satellite constellations are currently operating at Ka-band and are already showing signs of spectrum saturation in high demand areas in their coverage. In search of additional spectrum, in this paper, we employ the Q-band by investigating the performance of a multi-satellite MEO constellation network in terms of system capacity considering sophisticated propagation and physical layer tools. In particular, within the proposed Q-band solution, a higher throughput can be achieved with respect to a corresponding Ka-band system, by reaching up to 100% gain in terms of system capacity in a clear sky conditions.

Multiuser cooperative communications represent a research area of great interest constituting a key technology for future wireless networks. In this study, the authors consider a scenario of multi‐user, single‐relay network, where each user employs a part of the relay's transmission power taking into account the constraint of the total transmission power of the relay. The problem of relay power allocation to individual users with a view to optimising the long‐term network performance is investigated. An adequate utility function is defined, in terms of outage probability, evaluating the cooperative network's performance due to the relay's cooperation. Taking into consideration the distributed nature of cooperative networks along with the coupled constraint of relay's total transmission power, the specific problem is transformed from a network‐level to a user‐level optimisation problem using a modern game‐theoretic tool. More specifically, the initial power allocation problem is formulated as a state‐based potential game for which the corresponding Nash equilibrium exists. Finally, a gradient learning mechanism is proposed to achieve the stationary state Nash equilibrium of the game that constitutes also the global optimal solution of the initial problem. The performance of the new proposed scheme is investigated through numerical simulations for various network parameters.

In this paper, an overlay cognitive radio network is considered where each primary user leases access time to selected secondary users in exchange for their cooperation. To optimize the network performance in a distributed way and being inspired by matching theory, we model the cognitive radio network as a matching market and we deal with the pairing problem among the primary and the secondary users as a many-to-one matching problem with externalities. A modified scheme based on the well-known deferred acceptance mechanism is proposed, leading to a stable matching. The performance of the proposed scheme is evaluated and compared to the performance of a centralized scheme that results in optimal performance for the primary users and a simple pairing scheme that is based on the distance among the users. As is highlighted in the numerical analysis, the proposed mechanism leads to similar performance with the centralized scheme in terms of primary users' utility whereas, it outperforms the other two schemes from the perspective of secondary users.

Secure communications constitute a major concern in wireless networks' design. Toward this direction, physical layer security can achieve confidential data transmission from an information-theoretic viewpoint. This paper focuses on investigating the secure transmission of information over Nakagami-m fading channels in the presence of an eavesdropper. Specifically, we propose a distributed scheduler that opportunistically exploits the time-varying fading channel characteristics with a view to maximizing the expected secrecy throughput of a wireless link. The proposed multi-threshold scheduling policy postpones the communication up to an acceptable deadline until the secure-optimal time instant is found, based on the optimal stopping theory. The performance of the proposed scheduler is evaluated through simulations for various link parameters, while its achieved secrecy gain is examined and compared with other heuristic schemes.

In this paper, we analyze the problem of spectrum leasing in cognitive radio networks using principles from matching theory. More specifically, we deal with the pairing problem among the primary and the secondary users and we propose two different matching schemes that take into account the preferences of each user in terms of performance. The first scheme is a one-to-one matching scheme that is based on the deferred acceptance algorithm whereas the second one refers to a one-to-many matching algorithm where each primary user may cooperate with more than one secondary user. The performance of both proposed schemes is finally investigated for various network parameters and some useful conclusions are drawn.

New physical–mathematical models for the calculation of the outage probability of the maximal ratio combining and selection combining dual branch spatial diversity schemes for broadband fixed wireless access networks operating above 10 GHz are presented. At these frequency bands and considering line‐of‐sight conditions, rain attenuation is the dominant fading mechanism, which should be taken into account in the radio communications system design. The models are based on bivariate inverse Gaussian (IG) distribution and on the adoption of a spatial correlation coefficient for the convergent terrestrial wireless links. IG distribution has been shown that models accurately the rain attenuation fading for both temperate and tropical climatic regions. The models are validated with numerical results and some useful conclusions are drawn.

Spectrum leasing in cognitive radio networks has recently started to draw the attention of the researchers. According to this concept, the primary users own specific spectral resources and they decide how to lease them to the secondary users so as to optimize their performance. In this paper, we consider an overlay cognitive radio network where the secondary users are allowed to transmit simultaneously with the primary user in exchange for devoting a part of their power to relay the primary signal. Focusing on the distributed nature of cognitive radio networks, we propose an auction-based mechanism for the leasing process. The secondary users, who represent the bidders, decide the power that are willing to employ for relaying PU's data whereas the primary user has the role of the auctioneer deciding the duration of the leasing process as well as the leasing time allocated to each secondary user. Furthermore, three different scenarios are studied, depending on the channel state information available to the secondary users. The performance of the proposed mechanism is investigated through the numerical analysis and the advantages of complete knowledge of channel state information are highlighted.

The scope of this research paper is the proposal of voice quality prediction models, based on Adaptive Neuro-Fuzzy Inference Systems (ANFIS), for Adaptive Multi Rate (AMR) telephony service provided by GSM and UMTS rollout public land mobile networks. For the scope of our research we performed an experimental drive-test measurement campaign in order to evaluate objectively the End-to-End Service QoS (ESQoS) as well as radio System QoS (SQoS) parameters. Subsequently, the collected measurement data are used to train ANFIS empirical models. We then assess the prediction performance by depicting 2D/3D FIS surfaces and the impacts of SQoS to ESQoS. The prediction methodology can be successfully applied in Quality of Experience centric radio network planning, fine tuning and optimization processes by mobile operators.

The constantly increasing demand for capacity in modern communication systems such as wireless backhaul has led to the employment of frequencies above 10 GHz where the dominant fading mechanism is rain attenuation which is usually modelled as a lognormal random variable (when expressed in dB - log-lognormal in linear scale). In addition, in the context of green communications the use of cooperative diversity techniques can be considered as an important tool which improves the performance, reliability and efficiency of such systems. In this paper, the outage performance analysis of a cooperative system operating at frequencies above 10 GHz is presented, where the propagation medium consists of spatially correlated fading channels. In addition, the energy and spectral efficiency of various multiple-access protocols as well as different relay retransmission and receiver combining techniques are investigated. Moreover, it is proposed that the high correlation coefficient between various rain fading channels can be exploited by the relay to estimate the rain fading of the next hop. Extended numerical results present the impact of various system parameters on the end-to-end performance. Finally, comparison with an energy- and bandwidth- equivalent direct link highlights the system parameter regions where cooperative diversity offers gains over non-cooperation.

Local Multipoint Distribution Service (LMDS) systems operate at frequency bands above 20GHz. Code division multiple access (CDMA) schemes have been proposed for this technology. LMDS networks at millimeter wave bands are susceptible to tropospheric propagation phenomena and their Quality of Service (QoS) performance mainly suffers from rain attenuation and inter-cell interference. In this paper we propose a new Fuzzy Inference System (FIS) for the formulation of a downlink power control scheme, according to which the transmitted power is adjusted based on the output of the FIS. Furthermore, the power allocation is site-dependent because it depends on the long term statistical distribution of rainfall rate which influences the rain attenuation. Finally, we present simulations and comparison results of the proposed FIS downlink power control method and classic power control in terms of outage probability.

The ever increasing demand for multimedia services has led to the extensive use of fixed wireless outdoor networks, where the transmitted information signals arrive to the final destination through a relay node, creating a dual-hop transmission. Due to the increased demand for bandwidth and the spectral congestion at low frequencies, operation at frequencies above 10GHz satisfying the Line-Of-Sight condition is required. At this frequency range, the dominant fading mechanism exposing spatial and temporal stochastic properties is rain attenuation, typically modeled as a lognormal flat fading channel when expressed in dB (log-lognormal in linear scale). In this paper, the outage performance of a dual hop system using several radio relay types is presented and physical prediction models with analytical formulas are proposed, employing spatially correlated lognormal fading channels. Extended numerical results investigate the impact of various operational, geometrical and geographical radio relay parameters, as well as the impact of the employment of a relay channel estimator, on the end-to-end performance.

In broadband fixed wireless access (BFWA) networks mainly operating above 20 GHz, a clear line-of-sight (LOS) path is necessary between the base stations (BS) and the subscribers (S). In this letter, a Markov-based method to estimate the packet loss rate (PLR) statistical characteristics over a dynamically, due to propagation, varying LOS BFWA channel is presented. The method is verified by means of information theoretic metrics and tested against simulation results.

This paper investigates the applicability of multiple-input-multiple-output (MIMO) technology to broadband fixed wireless access (BFWA) systems operating in the 10-66 GHz frequency range. In order to employ the MIMO principle at these frequencies, the spatial channel benefits that may arise from the rainfall spatial inhomogeneity are more relevant since multipath is insignificant. Therefore, a special MIMO/BFWA channel may be implemented if every subscriber is equipped with multiple antennas and communicates with multiple base stations. The exact relationship between conventional MIMO and the proposed 10-66 GHz MIMO/BFWA channels is established. Then, emphasis is put on two different topics from the field of MIMO applications: (i) capacity enhancement for spatial multiplexed MIMO/BFWA systems; and (ii) interference reduction for MIMO/BFWA diversity systems employing receive antenna selection. More specifically, in the first case, a communication-oriented single-user capacity analysis of a 2 times 2 MIMO/BFWA spatial multiplexing system is presented, the relevant optimal power allocation policy is explored and useful analytical expressions are derived for the outage capacity achieved in the asymptotically low and high SNR regions. The effect of feedback on the capacity is investigated and quantified through Monte Carlo simulations. In the second case, a 2 times 2 MIMO/BFWA diversity system with receive selection combining is considered and its efficiency to mitigate intrasystem/intersystem cochannel interference over the downstream channel is studied from a propagation point of view. A general analytical prediction model for the interference reduction obtained by such a 2times2 MIMO/BFWA diversity system is presented along with a numerical validation.

Abstract Wireless Mesh Networks (WMNs) are considered for a plethora of complex applications, where different QoS specifications should be satisfied. In such applications, QoS issues concerning the delay and the reliability of data transmission are of utmost importance. In this paper, a distributed routing protocol for WMNs operating above 10 GHz to support services with distributed QoS is presented. As far as propagation is concerned, the major factor limiting the performance of millimetre wave radio systems is rain attenuation. The routing problem is formulated as a problem of linear programming, where the objective is to determine the path with minimum transmission delay while at the same time, satisfying a given set of constraints. Lagrangian Relaxation and dual decomposition are used to solve the problem in a distributed way. The proposed algorithm exhibits a low computational complexity, it is scalable and guarantees convergence to an optimal routing scheme. Its performance is verified through extended simulations employing an accurate spatial‐temporal channel model. Copyright © 2007 John Wiley & Sons, Ltd.

The reliable design of modern satellite communication networks requires the exact evaluation of the interference effects on the availability and performance of both the uplink and downlink. In this paper, the impact of uplink power control as fade mitigation technique in the adjacent satellite network interference is examined. Here, we present a novel physical-propagation model to predict the co-channel interference statistical distribution taking into account correlated fading channels. Additionally, the sensitivity of the operational and the geometrical parameters of the system under consideration are investigated through extended numerical results. Some useful conclusions are drawn.