This paper examines the number of communication modes, that is, the degrees of freedom (DoF), in a wireless line-of-sight channel comprising a small continuous linear intelligent antenna array in the near field of a large one. The framework allows for any orientations between the arrays and any positions in a two-dimensional space, assuming that the transmitting array is placed at the origin. Therefore, apart from the length of the two continuous arrays, four key parameters determine the DoF and are hence considered in the analysis: the Cartesian coordinates of the center of the receiving array and two angles that model the rotation of each array around its center. The paper starts with the calculation of thedeterministicDoF for a generic geometric setting, which extends beyond the widely studied paraxial case. Subsequently, a stochastic geometry framework is proposed to study thestatisticalDoF, as a first step towards the investigation of system-level performance in near field networks. Numerical results applied to millimeter wave networks reveal the large number of DoF provided by near-field communications and unveil key system-level insights. A comparison of the proposed method with the singular value decomposition-based method is illustrated to validate the proposed approach.

This paper investigates the signal-to-interference-plus-noise ratio (SINR)-based performance of a cellular network consisting of a fluid antenna (FA)-equipped user and small base stations (SBS), spatially distributed according to a binomial point process (BPP). By accurately capturing the shadowing effect in the calculation of received power through large-scale fading, the SINR is expressed in terms of the best selected port and the maximum average received power from the serving BS. To this end, a maximum average received power user association policy is adopted. Subsequently, by expressing the well-known Jakes' model in terms of the Gaussian copula, performance analysis in terms of coverage probability and delay coverage rate (DCR) is conducted in the presence of a single dominant interferer under correlated Nakagamim fading between the FA's ports. Numerical results demonstrate that i) by increasing the number of FA's ports, the performance improves only for the larger-sized FAs, ii) even by increasing the size of a FA with fixed number of ports, the dominant interferer poses a bound on the achieved coverage performance until eventually saturated and iii) by increasing the number of the FA's port, the enhancement in the DCR performance is more profound for the smaller amounts of the transmitted data.

The application of integrated sensing and communications (ISACs) technology in air-ground networks, which include unmanned aerial vehicles (UAVs), offers unique opportunities for improving both sensing and communication performances. However, this type of networks are also sensitive to the peculiar characteristics of the aerial communications environment, which include shadowing and scattering caused by man-made structures. This paper investigates an aerial ISAC network and proposes a UAV-selection strategy for improved communication performance. We first derive analytical expressions for the received signal-to-interference ratio for both communication and sensing functions. These expressions are then used to analyze the outage probability of the communication part and the ergodic radar estimation information rate of the sensing part. The presented results reveal the impact of shadowing severity and interference on the system’s performance.

A novel stochastic geometry framework is proposed in this paper to study the downlink coverage performance in a millimeter wave (mmWave) cellular network by jointly considering the polar coordinates of the Base Stations (BSs) with respect to the typical user located at the origin. Specifically, both the Euclidean and the angular distances of the BSs in a maximum power-based association policy for the user equipment (UE) are considered to account for realistic beam management considerations, which have been largely ignored in the literature, especially in the cell association phase. For completeness, two other association schemes are considered and exact-form expressions for the coverage probability are derived. Subsequently, the key role of angular distances is highlighted by defining the dominant interferer using angular distance-based criteria instead of Euclidean distance-based, and conducting a dominant interferer-based coverage probability analysis. Among others, the numerical results reveal that considering angular distance-based criteria for determining both the serving and the dominant interfering BS, can approximate the coverage performance more accurately as compared to utilizing Euclidean distance-based criteria. To the best of the authors’ knowledge, this is the first work that rigorously explores the role of angular distances in the association policy and analysis of cellular networks.

In millimeter wave (mmWave) networks, the communication link performance is heavily dependent on the directional characteristics of the transceiver beams. Under practical beam management considerations, the transmitter and receiver beams may not be perfectly aligned. We consider such a situation in the downlink of a mmWave cellular network. In this case, the received powers from the base stations (BSs) at the user equipment (UE) of interest will depend upon both the Euclidean and the angular distances (of the BSs to that UE). We develop a novel stochastic geometry framework to study maximum power-based association in this setting. To the best of our knowledge, this is the first work that rigorously explores the role of angular distances in the association policy and analysis of cellular networks. We derive exact expressions for the distributions of signal-to-interference-plus-noise ratio (SINR) and the link rate, as well as the average achievable rate. Among others, our analysis reveals that as the receiver beam becomes more directional (i) the instantaneous achievable rate improves at the expense of a significantly higher variance, and (ii) both the desired received power and the interference power decrease because of the reduced misalignment error and the reduction of the number of interfering BSs falling within in the 3 dB beamwidth of the receiver antenna pattern, respectively, and (iii) the probability of achieving a higher target rate than the average increases.

In millimeter wave (mmWave) networks, the interference characteristics are heavily dependent on the directional transceiver beams and the corresponding half power beamwidths. The angular distance of the desired transmitter and the interferers as seen by a reference receiver, plays a key role on the interference levels. In this paper, stochastic geometry tools are utilized to capture the effect of a reference receiver's gain at azimuthal angle$\phi$, on the performance of a mmWave network. Accordingly, performance analysis in terms of coverage probability is conducted under three interference scenarios: i) a worst-case dominant interference scenario, in which the dominant interfering base station (BS) is the BS that is both the nearest in azimuthal angle,$\phi$, measured from the direction of the serving BS, and the second nearest in Euclidean distance,$r$, from the receiver, ii) a dominant interference scenario, in which the dominant interferer is the nearest BS in azimuthal angle,$n$, and uniformly distributed in an annulus formed by the the serving distance and a ball radius$R$, and iii) the aggregate interference scenario. By defining the signal-to-interference-plus-noise ratio (SINR) for the three interference scenarios, the coverage probability is derived in exact form. Numerical results show that azimuthal angles play a key role in the definition of dominant interference and consequently in the coverage performance in mmWave cellular networks.

Unmanned aerial vehicle (UAV)-enabled communications have been proposed as a critical part of the beyond fifth-generation (5G) cellular networks. This type of communications is frequently characterized by line-of-sight (LoS) and dynamic propagation conditions. However, in various scenarios, the presence of large obstacles in the LoS path is unavoidable, resulting in shadowed fading environments. In this paper, a new channel model is proposed, in which the effects of mobility and shadowing are simultaneously considered. In particular, the performance of a UAV-based communication system operating in a shadowed double-scattering channel is analyzed. The new channel model is generic, since it models various fading/shadowing conditions, while it is in terms of easy-to-evaluate mathematical functions. Moreover, a low complexity UAV selection policy is proposed, which exploits shadowing-related information. The proposed scheme offers a reduction of the signal processing complexity, without any important degradation on the performance, as compared to alternatives approaches. In this context, a new analytical framework has been developed for investigating the performance of the new strategy. Finally, the main outcomes of this paper are also validated by empirical data, collected in an air-to-ground measurement campaign.

Double-scattering propagation models have been applied in various wireless communication environments, including vehicle-to-vehicle communications and cooperative amplify-and-forward relaying. In all these scenarios, when obstacles exist between the transmitter (Tx) and the receiver (Rx), the average received power varies in a random manner, resulting to the shadowing effect. The combination of double-scattering and shadowing represents a composite fading scenario, which models worse than Rayleigh fading conditions. In this paper, we propose a new shadowed double-scattering distribution that can model various fading and shadowing conditions. To this aim, the inverse-gamma (IG) distribution is employed for modeling the shadowing coefficient and the Nakagami- m for the multipath fading. For the resulting double-Nakagami-IG distribution, an analytical stochastic framework has been developed for presenting important statistical properties. Moreover, the theoretical results have been compared with empirical ones, based on a measurement campaign that has been performed in an unmanned aerial vehicle (UAV)-to-ground communication scenario. It is shown that the proposed new distribution can efficiently model such communication environments, since it provides a notable fit to the empirical data.

Mobile-to-mobile communications are offering clear benefits including spectrum and energy efficiency and low latency. However, these systems have to deal with various constraints with the most notable being related to the power consumption as well as the various peculiarities of the time varying wireless medium. In order to improve these systems' performance, antenna selection (AS) techniques have been proposed as an efficient approach to satisfy the hardware/power limitations that exist. However, these systems' performance is directly affected by the fast varying wireless medium, since AS is frequently performed using outdated estimates of the signal-to-noise ratio. In this paper, we propose a transmit AS scheme, whose operation is based on the shadowing side information (SSI) that is acquired in every stationarity region. By adopting SSI as an AS criterion, the negative consequences of the outdated channel state information (CSI) can be alleviated, since large-scale fading varies more slowly as compared to small-scale fading. The performance of this scheme is analyzed using the measures of the outage probability and the average bit error probability. It is shown that the proposed scheme outperforms the corresponding one that is based on (outdated) CSI, especially in scenarios with good channel conditions.

Antenna selection (AS) is considered as an ideal approach for improving the performance of vehicle-to-vehicle (V2V) communications, while satisfying the hardware and power limitations that exist in vehicles. However, the promised gain over single antenna schemes is affected by the fast varying wireless channel, since AS is frequently performed using outdated versions of the signal-to-noise ratio. In this paper, we propose an AS scheme that exploits the stationarity of large scale fading. In particular, by adopting shadowing information as an AS criterion, the negative consequences of the outdated channel state information (CSI) can be alleviated, since large-scale fading varies more slowly as compared to small-scale fading. The performance of the proposed technique is analyzed using the criterion of outage probability. It is shown that the new scheme outperforms the corresponding one that is based on outdated CSI, especially in scenarios with mild channel conditions, in terms of fading/shadowing severity. Moreover, empirical results have been used to verify the effectiveness of the considered shadowing model.

Security assurance is defined as the degree of confidence that the security requirements of an IT system are satisfied. In view of the emerging paradigm of connected vehicles i.e., dynamic Cyber-Physical systems of highly-equipped infrastructure-connected vehicles, specifying the involved assurance becomes highly-critical yet challenging; vehicles increasingly exploit various communication means to exchange rich data of relevance with the infrastructure resulting in a large attack surface. Both the complexity and uncertainty are increased rendering the so-far generic methods for security assurance costly-to-apply. In this position paper we introduce a security assurance framework tailored for connected vehicles, as explored by the EU-funded H2020 SAFERtec project. We put under the microscope two instances of vehicle-to-infrastructure communications and relying on an innovative modeling methodology we identify the involved security and privacy requirements. We then present the way to enhance the processes of the credible yet generic Common Criteria approach to gain evidence that the above requirements are met. The experimental evaluation of the framework is carried-out over a reference implementation of a prototype vehicle connected to road-side units and cloud-based services. The expectations are that our work assists to effectively construct assurance arguments increasing trust in connected vehicles.

Important statistical properties of the double-generalized Gamma (dGG) distribution are studied in this paper. The dGG distribution is suitable for modeling non-homogeneous double-scattering radio propagation fading conditions, which can be frequently observed in vehicle-to-vehicle (V2V) communications. In this context, important statistical metrics for the bivariate dGG distribution, such as the joint probability density function, cumulative distribution function, and the moments, are derived for the first time, while simplified expressions for the corresponding marginal statistical metrics are presented. Moreover, the second-order statistics of this distribution are also analytically studied. The derived analytical framework has been employed to analyze the performance of a transmit antenna selection system operating in V2V communication channels modeled by the dGG distribution. In this scenario, the impact of outdated channel state information (CSI) to the system's performance is investigated in terms of various metrics, including the level crossing rate and the average fade duration. Furthermore, simplified asymptotic closed-form expressions for the outage probability have been derived to examine the achievable diversity and coding gains. Based on our analysis, insightful discussions are provided. It is shown that the diversity gain is independent from the number of transmit antennas when the available CSI becomes outdated.

In this paper, we investigate the performance of a spectrum sharing decode-and-forward multiple-relay network (SSDMN) with direct link under the joint impact of two practical and detrimental imperfections, called transceiver hardware impairments (HIs) and channel estimation errors (CEEs). Here, we consider the hardware distortions induced by all the non-ideal secondary nodes and residual interferences originating from imperfect channel estimations. By taking these imperfections into account, we derive an exact closed-form expression for the outage probability of the considered SSDMN employing selection cooperation over independent and non-identical Rayleigh fading channels. We identify three important ceiling effects invoked by HIs, namely relay cooperation ceiling (RCC), direct link ceiling (DLC), and overall system ceiling. Specifically, we highlight the impact of these prevailing effects on the system's performance and provide some useful insights about the tolerable level of HIs while designing the practical systems for a given rate requirement. We also illustrate that a potential direct link and a relaying link can, respectively, partially compensate for the incurred performance losses due to RCC and DLC effects. Moreover, we investigate the asymptotic behavior of outage probability to highlight the impact of HIs and CEEs on the achievable diversity and coding gains.

In this paper, important statistical properties of the double- generalized Gamma (dGG) distribution, a generic modelthat has been used for modeling the double-scattering radio propagation fading conditions, are obtained and investigated. In this context, useful statistical metrics, such as the probability density function, cumulative distribution function, and the moments, are derived. Moreover, the second-order statistics of this distribution are also analytically studied for the first time. Based on them, exact expressions for the outage probability, the average symbol error probability, and the capacity of a transmit antenna selection system operating in a vehicle-to-vehicle (V2V) communication environment are obtained. In addition, the level crossing rate and the average fade duration are studied. Simplified asymptotic closed-form expressions have been also obtained and used to investigate the diversity and coding gains.

In this paper, we analyze the performance of a cognitive amplify-and-forward multi-relay network (CAFMRN) with direct link under the joint impact of transceiver hardware impairments (HIs) and channel estimation errors (CEEs). Herein, we consider hardware distortions induced by all the non-ideal secondary nodes and residual interferences originating from imperfect channel estimations. Taking these imperfections into account, we derive a new closed-form expression for the outage probability of the considered CAFMRN employing selection cooperation over independent and non-identical Rayleigh fading channels. Furthermore, we derive an asymptotic outage behavior to assess achievable diversity order. We illustrate that HIs may invoke ceiling effects into the system, namely relay cooperation ceiling (RCC) and overall system ceiling (OSC). Specifically, we showcase that the RCC ceases the full cooperation, whereas, the OSC may further aggravate the system outage performance under a high-rate requirement. By modeling the CEEs as decreasing function of signal-to-noise-ratio, we demonstrate that CEEs pose critical impact on the coding gain only and not on the diversity gain of the system. Finally, numerical and simulation results are presented to validate our theoretical findings.

Transmit antenna selection (TAS) schemes have been adopted in various communication systems to provide improved performance with relatively reduced complexity. The quality-of-service (QoS) in these systems depends on the availability of perfect channel state information (CSI). In practice, it is difficult to acquire perfect CSI, since it may get outdated due to feedback process. In this paper, we investigate the impact of outdated CSI in a TAS system that operates in a vehicle-to-vehicle (V2V) communication environment. The V2V channel is modeled by the double-Weibull (DW) distribution, a widely adopted distribution for modelling scenarios where both the transmitter and the receiver are in motion. In this context, we introduce and derive important statistical metrics for the bivariate DW distribution, such as the probability density function, the cumulative distribution function, and the moments. Based on them, exact expressions for the outage probability and the average bit error probability of the TAS scheme are derived under the assumption of outdated CSI. Moreover, simplified asymptotic closed-form expressions have been obtained. These expressions are then used to study the performance of the system under consideration.

We consider a hybrid satellite-terrestrial spectrum sharing system wherein multiple terrestrial secondary networks coexist with a primary satellite network. We propose an amplify-and-forward based cooperative spectrum sharing protocol by employing an opportunistic secondary network selection (OSNS) scheme. The best secondary network is selected to minimize the outage probability of primary satellite system and to achieve spectrum access. With overlay approach, the selected secondary network allocates its partial power to relay the satellite signal and utilizes the remaining power to transmit its own signal. We derive closed-form expressions of outage probability for both primary and secondary networks and examine their achievable diversity order over hybrid fading channels. Numerical and simulation results validate our analysis and highlight the performance gains of the considered system with OSNS scheme.

In this paper, the joint effect of multiple jamming signals and noise at the eavesdropper in wiretap channels is studied in terms of the secrecy outage probability, where all nodes are deployed with multiple antennas. Specifically, the transmitter employs a transmit antenna selection (TAS) technique; the legitimate receiver uses either a selection combining (SC) or a maximal-ratio combining (MRC) scheme; and the eavesdropper adopts the MRC technique. Exact closed-form expressions for the secrecy outage probability are derived for both SC and MRC schemes at the legitimate receiver, which are applicable for arbitrary number of power distributed jamming signals. An asymptotic secrecy outage analysis is conducted, which shows that the diversity gains for all combining schemes equal to the product of the number of antennas at the transmitter and the legitimate receiver. Representative numerical examples are presented to illustrate the effects of the key system parameters on the secrecy outage performance. Finally, the proposed analysis is validated through Monte Carlo simulation results.

In this paper, we study a hybrid satellite-terrestrial spectrum sharing system (HSTSSS) in which multiple terrestrial secondary networks cooperate with a primary satellite network for dynamic spectrum access. For complexity-aware HSTSSS design, we propose an amplify-and-forward-based overlay spectrum sharing protocol using partial and opportunistic secondary network selection schemes. The secondary network selection aims to minimize the outage probability of the primary satellite system and, thereby, to explore spectrum sharing opportunities. With the overlay approach, the selected secondary network allocates part of its power to relay the satellite signal and utilizes the remaining power to transmit its own signal. Considering Shadowed-Rician fading for satellite links, and Nakagami-m as well as Rician fading for terrestrial links, we derive closedform expressions for the outage probability of both primary and secondary networks, and examine their achievable diversity orders. Numerical and simulation results validate our analysis and highlight the performance gains of the proposed schemes for an HSTSSS with and without a direct satellite primary communication link.

In this paper, we investigate the performance of spatial modulation (SM) for vehicle-to- infrastructure (V2I) and vehicle-to-vehicle (V2V) communications over a generalized multiple scattering channel. This channel model provides a realistic statistical description of an intervehicular communication environment and includes as special cases the Rician, Rayleigh, and double-Rayleigh channels. We derive an accurate closed form expression for the pairwise error probability of SM, which is used to obtain the error rate performances of SM over various propagation scenarios. The performance analysis reveals the impact of the number of receive antennas on the error rate performance of SM transmission over multiple scattering channels. Various numerically evaluated results are presented to demonstrate the correctness of the proposed analysis.

In this paper, we study the influence of interference in a inter-vehicular communication system. The interfering links are modeled using the multiple scattering radio channel, which fits quite well with experimental data for mobile-to-mobile communications. For this scenario, we derive the signal-to-interference ratio statistics of a single input single output system. Moreover, the analytical results are generalized by taking into account a diversity reception scenario with two well-known schemes, namely selection diversity and maximal ratio combining. In addition, we analytically study the realistic scenario of outdated (imperfect) channel estimates at the receiver side. The outage probability (OP) is considered as the performance metric for all scenarios considered. Based on the numerical evaluation of the derived analytical expressions for the OP, we depict the impact of interference, diversity schemes and outdated estimates on the system's quality of service. Simulation results are also included to validate the analytical results.

Vehicle-to-vehicle communication systems are characterized by a highly time varying channel. For this reason, many relay selection schemes demand the continuous monitoring of all available channel links, resulting to an unnecessary increase of the system overhead processing. In this paper, an alternative (lower complexity) cooperative relaying selection scheme is adopted. For this new scheme, the influence of interference as well as the existence of outdated channel state information in a decode-and-forward relaying scenario is investigated. To this aim the bivariate double-Nakagami probability density function is introduced and used to model the correlation between the channel states during the selection and the data transmission instances. The performance is analysed using the criteria of outage probability and the average symbol error probability. It is shown that with the proposed approach a significant reduction in feedback load processing is achieved, with only a slight loss in performance.

In this paper, the correlated double-Rayleigh distribution is introduced and analyzed. This distribution has been found to provide an accurate statistical description of the vehicle-to- vehicle multipath propagation. Novel infinite series expressions for the joint probability density and the cumulative distribution functions are derived, while the joint moments are also obtained in closed form. Although the new statistical metrics can be employed in a wide range of applications, this paper focuses on the performance of dual-branch selection diversity receivers. In this context, the outage probability (OP) and the average bit error rate (ABER) of dual- branch selection diversity operating over double- Rayleigh fading channels are analytically evaluated. Moreover, an asymptotic analysis is also included, resulting to simpler closed-form expressions for the OP and ABER. Finally, the applicability of the analytical results were verified using the measurements based WINNER2 models.

Reconfigurable antenna arrays provide important advantages such as reduction on the number of the physical antennas or reduced hardware complexity in terms of receiving radio frequency chains. Both these properties are critical in vehicle-to-vehicle (V2V) communication systems, which are characterized by inflexible dimensions and cost constraints. In this paper, a new pattern reconfigurable antenna array selection scheme is proposed that aims to reduce the complexity of the traditional antenna selection techniques. For this scheme, an analytical statistical framework is developed for characterizing the output signal-to-noise ratio (SNR). The analysis is general enough to take into consideration the impact of correlation on the antenna patterns. Special cases are also studied assuming independence between the patterns, while the diversity and coding gains are also evaluated. The performance is analysed using the criteria of outage probability and the average bit error probability. Moreover, the new scheme is applied in a V2V communication scenario based on the IEEE 802.11p standard, while it is compared with other well-known receivers under different channel model conditions.

In this paper, we investigate the performance of space shift keying (SSK) transmission over a generalized multiple scattering channel. This channel model provides a realistic statistical description of an intervehicular communication environment and includes as special cases the Rician, Rayleigh, and double-Rayleigh channels. We derive an accurate closed-form expression for the pairwise error probability of SSK, which is used to obtain the uncoded and coded error rate performances of SSK over various propagation scenarios. An asymptotic analysis for high values of the signal-to-noise ratio is carried out to determine the diversity and coding gains of the proposed system. The performance analysis reveals the impact of the number of receive antennas and the use of soft and hard decision decoding on the error rate performance of the SSK transmission over multiple scattering channels. Various numerically evaluated results accompanied by Monte Carlo simulations are presented to demonstrate the correctness of the proposed analysis.

In this work, relay selection policies for a network where a terrestrial source communicates with a terrestrial destination through multiple relay nodes which reside in the stratosphere are presented. More specifically considering realistic three-dimensional (3-D) non-isotropic scattering channel modelling combined with opportunistic relay selection for various fading conditions. Moreover, the stratospheric nodes are battery-dependent and relay selection aims at power reduction. As Channel State Information (CSI) overhead is introduced by the selection process, proactive and reactive versions are presented based on instantaneous CSI knowledge. In this setting, the selection policies in terms of outage probability, throughput and power reduction are evaluated and comparisons with statistical channels and with a relaying scheme that does not perform power adaptation are provided.

Beamspace MIMO (BS-MIMO) is a newly proposed transmission technique that can successfully overcome the two significant disadvantages of conventional MIMO. More specifically, BS-MIMO using Electronically Steerable Passive Array Radiators (ESPAR) achieves similar multiplexing and beamforming capabilities using a single RF chain. Moreover, it is proved that BS-MIMO benefits from the use of small sized antenna arrays outperforming in terms of capacity conventional MIMO systems that operate poorly due to spatial correlation. However, the open issues towards an implementable, fully functional BS-MIMO system are still plenty. This study is part of a series of research work that focuses on real-world BS-MIMO system design. The application of limited feedback techniques in BS-MIMO is introduced and the performance of the developed algorithm is evaluated. These results are also used to derive conclusions regarding the adaptivity requirements for the parasitic elements of the ESPAR antenna. Moreover, a practical V-BLAST reception scheme using receiver radiation pattern adaptation is also presented. Finally the performance of this reception scheme is compared with ideal reception in BS-MIMO.

This paper proposes a three-dimensional (3-D) three-concentric-cylinders model for multiple-input-multiple-output (MIMO) mobile-to-mobile (M-to-M) relay fading channels in amplify-and-forward (AF) communication networks. Based on this model, a parametric reference model for wideband MIMO M-to-M relay fading channels in urban macrocells is developed. From the reference model, the space-time-frequency correlation function among the links of the underlying MIMO communication channels is derived, under realistic 3-D non-isotropic scattering conditions. The numerical results depict that the correlation is significantly affected by the multipath elevation angles. These results also demonstrate that the time and frequency dispersion of a wide sense stationary uncorrelated scattering relay-based channel are not statistically independent. The proposed channel model provides an important framework and guidelines for the system design and performance analysis of multi-antenna M-to-M communication systems in relay-based wideband channels with non-line-of-sight double-bounce connections in the transmission links from the source to the destination via the relay.

Single-RF front-end MIMO systems have drawn the attention of the research community due to their considerable advantages against the conventional MIMO systems. MIMO multiplexing capabilities are implemented in the beamspace domain using parasitic antenna arrays with one active and many parasitic elements, like the Electronically Steerable Passive Array Radiator (ESPAR) antenna. The multiplexing is achieved in terms of linear combination of diverse symbols with orthonormal basis patterns. Initially, an analytical method was proposed to derive the basis patterns, which remain orthogonal in rich scattering environments. Recently, the numerical calculation of channel-aware orthonormal patterns was introduced. In this paper, we extend the presented method in 3-dimensional channels and radiation patterns. The proposed technique is based on the singular value decomposition of the wireless channel, as seen in the beamspace domain, and it adaptively computes 3D orthonormal basis patterns at both transmitter and receiver sides. It is expected that the 3D analysis will facilitate the compensation of the body effects in the handheld terminal performance.

This paper proposes a theoretical framework for the evaluation of the capacity of multiple-input multiple-output (MIMO) mobile-to-mobile (M-to-M) fading channels in dual-hop amplify-and-forward (AF) single-relay wireless communications networks. The evaluation is based on a recently introduced three-dimensional (3-D) geometry-based reference model for these channels. From this model, 3-D deterministic and statistical sum-of-sinusoids (SoS) based simulation models are also developed, under 3-D non-isotropic scattering conditions. The performance of the simulation models is verified through simulation results.

Despite the well known benefits of Multiple Input - Multiple Output (MIMO) systems, their remarkable implementation complexity limits the wide application to modern transceiver architectures. In order to simplify the inclusion of MIMO technology in mobile devices, this paper studies the capabilities of parasitic antenna structures on supporting MIMO transmission. In contrast to conventional antenna arrays the proposed MIMO architecture utilizes only a single radio - frequency chain, which reduces dramatically the implementation complexity and cost. The results show that the proposed MIMO architecture is able to achieve even better performance as compared to more complex MIMO systems with typical antenna arrays.

High-altitude platforms (HAPs) are considered as an alternative technology for next generation broadband wireless communications. This paper proposes three- dimensional (3-D) sum-of-sinusoids (SoS) simulation models for wideband HAP multiple-input-multiple- output (MIMO) channels. The starting point of the design procedure is a non-realizable reference model. The performance of the proposed models is studied in terms of the space-time-frequency correlation function (STFCF). The results show that the simulation models approximate the reference model with high precision.

This paper proposes a new system architecture for continuous monitoring wireless sensor networks, utilizing recent developments in the areas of cooperative beam-forming, radio frequency integrated circuit technology, and spread-spectrum techniques. In the proposed system, nodes are activated only when they are triggered to send their own data to the fusion center directly, without burdening the network with MAC layer collisions and energy-consuming routing algorithms, thus guaranteeing long lifetime cycles. The nodes consist of a simple active reflector circuit which is synchronized by an incident wave transmitted by a distant base station. Since the nodes can be deployed in large numbers, the network acts as a single antenna array with high directive gain pointing towards the direction of the fusion center. At the receiver, data from each node can be de-multiplexed with the use of a frequency shift keying direct sequence spread spectrum modulation scheme that enables multiple access without destroying the beamforming characteristics of the network.

The objective of this study is to obtain a comprehensive understanding of UWB propagation in a Hercules C-130 military airplane. An extended measurement campaign took place in the body of the airplane and many interesting results were extracted. In particular, we initially present results for the path-loss factor that expresses the distance dependence for various bandwidths in the frequency band allocated by FCC for UWB signals (i.e., 3.1GHZ to 10.6GHz). Then, we evaluate time dispersion parameters, namely, maximum excess delay, mean excess delay and r.m.s. delay spread and we present the behaviour of these parameters versus the distance of transmit and receive antennas. Moreover, we provide information on the number of significant multipath components and the percentage of the total energy they carry. Finally, we model the dependence of the number of multipath components on the delay spread of the channel.

Multiple input-multiple output (MIMO) systems have drawn remarkable interest in recent years, since they are known to boost the spectral efficiency of wireless communication systems compared to single antenna architectures. However, the implementation cost and circuit complexity of such systems conflicts the guaranteed high performance gain. Recent research efforts focus on the investigation of cheaper and low complexity transceivers, introducing a novel beamspace MIMO (BS-MIMO) architecture. BS-MIMO systems have shown equivalent performance with the corresponding conventional systems, while they have been also evaluated in closed loop environments. In this paper we extend the closed loop BS MIMO approach presenting advanced limited feedback techniques which outperform predecessor feedback schemes.

Beamforming capabilities of electronically steerable passive array radiator (ESPAR) antennas have attracted considerable attention in recent literature. In this paper we consider a different beamforming approach, presenting an efficient optimization algorithm which enables such antennas to create specific transmit radiation patterns in real time, rather than patterns aiming at signal to interference ratio maximization. Moreover, we investigate the proposed algorithm over a novel multiple input - multiple output architecture, showing that even cost and size sensitive mobile handsets may efficiently operate in closed loop communication environments.

A framework for implementing spatial multiplexing using an electronically steerable parasitic array radiator (ESPAR) antenna is presented. While traditional multi-element arrays (MEA) show great potential for meeting the increasing demand for higher data rates, they come at the expense of computationally demanding signal processing, larger size, and higher consumption of DC power, making traditional MEA impractical at userspsila hand-held devices. In this work, we introduce a novel methodology for using ESPAR antennas as a compact, cheap and less power-hungry MEA, for spatially multiplexing BPSK symbols, while still providing comparable spectral efficiency and performance as traditional MEA. The main idea is to exploit the strong mutual coupling among the ESPAR antenna elements at the transmitter side to create a linear combination of orthogonal patterns (beams), onto which the symbols are directly mapped, instead of the traditional approach of sending the symbols toward different locations within the antenna domain. On the other side, the receiver (base-station (BS) or access point (AP)) is equipped with a traditional uniform linear array (ULA), and decodes the received signal using the Vertical Bell labs layered space time (VBLAST) algorithm.

Recently, a novel beamspace multiple input multiple output (BS-MIMO) transmission technique has appeared, which increases the spectral efficiency of open-loop communication systems while using compact antenna structures with a single radio-frequency (RF) front-end at the transmitter. In this paper, we extend the aforementioned architecture proposing an efficient limited feedback technique for capacity optimization, considering electronically steerable parasitic array radiator (ESPAR) antennas at the transmitter. The performance of the resulting closed-loop BS-MIMO is evaluated against equivalent traditional MIMO systems that require a much larger number of active antenna elements to operate. The results are very promising, paving the way for integrating the proposed system in cost and size sensitive wireless handheld devices such as mobile terminals and mobile personal digital assistants.

Low-cost and low-power sensor nodes forming wireless sensor networks (WSNs) have become suitable for a wide range of applications during recent years. These networks, due to their special functional characteristics, demand the implementation of energy-aware techniques in all layers. Recently a MIMO - based structure has been proposed to offer enhanced energy savings in WSNs under certain circumstances. Based on that idea several alternatives have been examined, mainly focusing on 2times2 MIMO systems. In this paper, we summarize two MIMO schemes and compare their performance in terms of energy efficiency. Moreover, we investigate more complex MIMO systems, emphasizing on 4times4 structures, and prove that depending on the scenario, such systems may offer remarkable energy gains.

In this paper we introduce a new perspective to the implementation of wireless MIMO transmission systems with increased bandwidth efficiency. Unlike traditional spatial multiplexing techniques in MIMO systems, where additional information can be sent through the wireless channel by feeding uncorrelated antenna elements with diverse bitstreams, we use the idea of mapping diverse bitstreams onto orthogonal bases defined in the beamspace domain of the transmitting array far-field region. Using this approach we show that we can increase the capacity of wireless communication systems using compact parasitic antenna architectures and a single RF front end at the transmitter, thus paving the way for integrating MIMO systems in cost and size sensitive wireless devices such as mobile terminals and mobile personal digital assistants.

Low-cost and low-power sensor nodes forming Wireless Sensor Networks (WSNs) have become suitable for a wide range of applications during recent years. These networks, due to their special functional characteristics, demand the implementation of energy-aware techniques in all layers. Recently a MIMO-based structure has been proposed to offer enhanced energy savings in WSNs under certain circumstances. In this paper, we present a detailed analysis of the dissipated power during a sensor node's operation, to prove that as microelectronics develop the MIMO - based architecture will outperform the equivalent SISO structure for almost any case, in terms of energy efficiency. Moreover, we introduce a simple Cooperative Node (CN) Selection algorithm to achieve additional energy gains in the MIMO approach along with enhanced network lifetime. We examine the scalability of the algorithm on different channel conditions and varying network density, and investigate the effect of the power dissipation analysis on its efficiency.

Antenna subarray formation is a novel RF pre-processing technique that alleviates the performance degradations of conventional antenna selection schemes. With this method, each RF chain is not allocated to a single antenna element, but instead to a combined and complex-weighted response of a subarray of antenna elements. In this paper, we introduce a simple, novel, analytical, suboptimal algorithm for receive antenna subarray formation that is based only on the long-term channel statistics and aims at maximizing the capacity of the resulted system by exploiting spatial correlation. As extensive computer simulations demonstrate, the proposed scheme is an advantageous RF pre-processing technique that manages to reduce the number of necessary RF chains without updating the subarray formation process for each channel instance.

Antenna subarray formation is a novel RF preprocessing technique that reduces hardware complexity of MIMO systems while offering increased data rates with respect to conventional antenna selection schemes. With this method, each RF chain is not allocated to a single element but instead to the combined and weighted output of a subarray of elements. In this paper we study the impacts of hardware and signal processing nonidealities on the capacity performance of the proposed technique when it is applied to the receiver only. We show that it is robust to quantization, phase and calibration errors and also to channel estimation errors. Thus, it is proven that the suggested scheme could succeed in practice.

This paper presents results on the parameters of multipath components extracted from wideband measured MIMO channel matrices. Experimental data were collected with channel sounding measurements at 5.2 GHz in an urban environment, under LoS propagation conditions, for short range fixed wireless scenarios. The multidimensional Unitary ESPRIT algorithm was used to estimate the double directional angular and delay characteristics of the propagating waves. The results presented refer to the number of identified waves, their power, the channel's angular spectra and aim to provide information in the context of parametric MIMO channel characterization. Additionally, based on the parameter estimates, MIMO matrices are synthesized and their capacity performance is evaluated in comparison to the directly measured data

MIMO systems with reduced hardware complexity have attracted researchers' attention due to their high efficiency and low cost. Sub-optimum algorithms for antenna subset selection have been intensively studied in the literature. In this paper we present a new technique to maximize the capacity of multiple antenna wireless systems with reduced available RF chains. The technique is based on the adaptive formation of subarrays, i.e. the grouping of antenna elements and the application of appropriate element weights. The elements of each subarray and their weights are dynamically selected by an evolutionary optimization technique using the link capacity as a cost function

Wireless sensor networks (WSN) belong to a special category of networks where the nodes are size-limited and operate on small batteries that in most cases cannot be replaced. As a result ensuring the viability of these networks is usually a much more important task than achieving greater performance in terms of quality of services (QoS), capacity (C) or bandwidth efficiency. In this paper we consider the energy consumption according to the modulation schemes used in order to transmit the data. The trade-offs needed in order to achieve energy-efficiency and especially the case of the trade-off between bandwidth and energy consumption are examined. Furthermore, a model to measure the energy dissipation per transmitted bit will be presented including not only the analog but also the digital blocks of the transceiver. The description of the energy model is followed by the evaluation of the energy consumption introduced by M-ary frequency shift keying modulation (M-ary FSK), depended upon values of parameters such as the compromise in terms of bandwidth and transmission time

This paper presents results from fixed outdoor wideband multiple input multiple output (MIMO) channel measurements performed at 5.2 GHz. Rooftop to rooftop, short range environments, under line of sight (LoS) propagation conditions were examined using 8-element uniform linear arrays. The scope is to provide MIMO channel characterization results beneficial for either stochastic or physical channel description. In particular, delay dispersion measures, Doppler spectrum, Rice factor and spatial fading correlation results are reported. Furthermore, with the use of a two dimensional parameter estimation algorithm, multipath components' parameters are extracted and indicative double directional power spectra are provided.

In this paper, wideband MIMO channel measurements conducted in Athens, Greece are described. Short range scenarios with fixed transmitter and receiver, in urban like environments under LOS propagation conditions have been measured, using an 8×8 vector channel sounder operating at 5.2 GHz. Based on the measured MIMO channel matrices, results of mean and outage normalized capacity calculations are presented. These results correspond to systems with various numbers of antenna elements and various ULA antenna configurations.

This paper presents an antenna selection method for multiple-input multiple-output wireless systems. By exploitation of the channel transfer matrix, the antenna selection criterion is the maximization of the instantaneous capacity achieved using a specific number of transmitting and receiving antenna array elements. For each environment, the proposed method applies a genetic algorithm which seeks the most advantageous subset of antenna elements. The results are based on measured and simulated channels and show that the proposed method selects array configurations that yield superior performance compared to the arrays usually employed. Furthermore, comparative analysis results are presented, with respect to a state-of-the-art algorithm.

This paper reports the measurement and analysis of a wideband radio channel at 1900 MHz within a special propagation environment such as the Olympic Stadium of Athens. RMS delay spread values are evaluated as well as the coherence bandwidth of the channel is derived by the frequency correlation function. The mean delay spread is found to be 0.314 /spl mu/s and 0.731 /spl mu/s for LOS and NLOS conditions respectively. The mean coherence bandwidth that characterizes the entire area reaches 3.463 MHz providing wide transmission data rates. The correlation bandwidth is found to be inversely proportional to the RMS delay spread and modeled in a minimum mean square error sense.

This paper presents the results of the generalized radial basis function neural networks applications for the prediction of propagation path loss in urban and suburban environment. We have studied two types of neural network based models; the first one is used for path loss prediction while the second one is a hybrid prediction model based on error control. The performances of the neural models are compared to the path loss values measured in the city of Kavala and in Oia village on Santorini Island, Greece, based on the absolute mean error, standard deviation and root mean square error between predicted and measured values.

Indoor radio communication systems gain increasing interest of cellular network operators. A prerequisite to the design of these systems is the knowledge of indoor radio propagation characteristics. This knowledge should include information, concerning the in-building structure that strongly affects the signal transmission. The results presented in this paper provide characterization, of signal behavior in indoor wireless environment. Narrowband propagation measurements were conducted within a thirteen-story building (OTE-Hellenic Telecommunication Organization premises) in Athens, Greece. The purpose of the measurement campaign is to the indoor signal behavior and characterization considering she specific information. A description of the measurement environment and experimental setup is given. Path loss exponents and absolute path loss values are estimated for each measured case. Statistical analysis of the measured data is also presented.

The paper presents an analytical model for the prediction of multipath fading signal of land mobile satellite (LMS) channels, when non-GEO satellites are used. These satellites introduce a Doppler shift to the received mobile antenna signal, due to the movement with respect to the scatterers and the mobile receiver. The effects of this movement are investigated with reference to the received electric field, the baseband, and the envelope power spectra as a function of the satellite orbital parameters. The predicted power spectrum for the electric field is smooth and finite preserving the well known U-shape, as predicted by previous models, but with different central and limiting frequencies, dependent on the satellite and the mobile relative position.

An indoor communication channel is characterised by large propagation losses, slow time variations, a multipath nature and fading. We present a new cellular concept for indoor communications. A concentric circular model is the basic cell structure which consists of two or three concentric zones, operating in different frequencies, under the CDMA environment. Each user of this CDMA system employs a direct sequence spread spectrum waveform and does not occupy the entire allocated spectrum of the cell, but occupies a part of it, corresponding to each of the two or three frequency zones. Via a geometric analysis of the cell, we derived formulas for the system capacity and for the performance of this new cellular structure.

This paper presents the comparison of path loss prediction models. Theoretical models developed using ray-tracing techniques, and measurements based models are plotted versus distance for alternative configurations in a microcellular environment such as Athens city centre. The measurement set up, the data analysis and the conclusions of the comparison are given.

This paper presents the results of the propagation measurements performed in the centre of Athens for the complete characterisation of channel behaviour. The objective of the measurement campaign is to derive a prediction model for the path loss estimation for a microcell environment. For all types of measurements the variation of signal strength with distance was recorded for comparison with models extracted using ray tracing techniques. Some preliminary results were obtained and integrated using additional rays option for accurate path loss estimation. The paper also describes the conclusions of the comparison between theoretical and experimental results.>