This paper investigates a network architecture based on unmanned aerial vehicles (UAVs), wherein an aerial jammer (AJ) is deployed to counter unauthorized interception attempts by an aerial eavesdropper (AE). To enhance both network security and energy efficiency, an optimization framework is introduced that aims to maximize the secure energy efficiency (SEE) through the joint adjustment of transmit power, jamming power, and the flight trajectories of legitimate UAVs. The fractional optimization problem is reformulated into a subtractive-form optimization problem using Dinkelbach’s method. Given the non-convex nature of both the objective function and constraints, block coordinate descent (BCD) and successive convex approximation (SCA) techniques are employed. Simulation results demonstrate the effectiveness of this approach in improving SEE and the influence of channel fading on the physical-layer security (PLS).

This paper investigates physical-layer security (PLS) in UAV-assisted communication networks featuring an untrusted amplify-and-forward aerial relay. To counter potential eavesdropping threats, a destination-aided jamming strategy is employed. The proposed solution adopts a hybrid framework that integrates particle swarm optimization (PSO) and a dual-layer deep neural network (DNN). In this framework, PSO is responsible for optimizing the three-dimensional (3-D) placement of the UAV relay by minimizing the total aggregate path loss, while the DNN tackles the transmit antenna selection (TAS) problem at the source to maximize the average secrecy rate. The effectiveness of the suggested approach is assessed by comparing diverse relay placement techniques and antenna selection methods. The simulation results demonstrate that the proposed framework consistently outperforms conventional machine learning (ML)-based approaches and random baselines. Interestingly, single-antenna selection achieves slightly better secrecy rates than dual-antenna configurations. Overall, the proposed method provides a scalable and computationally efficient solution for enhancing the security of UAV-enabled communications in scenarios involving untrusted relays.

Reconfigurable Intelligent Surfaces (RIS) and UAV-assisted mmWave communications have emerged as pivotal technologies to enhance connectivity and network reliability, particularly for next-generation 6G systems. However, harnessing their full potential poses significant challenges, particularly in optimizing the UAV’s trajectory and power allocation to maximize the Signal-to-Noise Ratio (SNR) while simultaneously mitigating interference with neighboring networks. To address these challenges, we propose a reinforcement learning (RL) based trajectory and transmit power optimization scheme that combines UAV with RIS. The proposed scheme consists of spatially distributed stationary UAV (S-UAV-mm) that directly serve their associated UEs, alongside a moving UAV (M-UAV-mm), that communicates with its dedicated users through a RIS, thereby ensuring optimal coverage for its UEs while minimizing interference to neighboring groups. Simulation results confirm that the RL algorithm converges to an optimal policy, yielding an average SNR improvement of at least 73.3% and achieving robust interference mitigation with an 87.5% enhancement through closed-loop power control.

In this paper, multiple relay selection (MRS) schemes for untrusted unmanned aerial vehicle (UAV)-enabled networks are proposed. In this context, various machine learning (ML) models are employed to improve secrecy performance by optimally selecting multiple aerial relays from the available ones, all of which are regarded as untrusted. Notably, these ML models can cope with the quickly changing and random positioning of the aerial relays, effectively decoupling the intricate coupling relationship between the secrecy rate, channel coefficients, and inter-node distances. The results indicate that the ML-based MRS schemes obtain sufficient accuracy and better decoupling than a respective exhaustive searching (ES) approach, while also maintaining a lower computational complexity.

An appealing alternative towards dealing with mobility and coverage challenges in future wireless networks, are hybrid satellite-terrestrial networks (HSTNs). The combination of network coding (NC) and non-orthogonal multiple access (NOMA) techniques, have recently gained popularity due to their performance gains in improving the quality of a wireless transmission and exploiting the accessible spectrum. This paper introduces a joint NOMA-NC approach, integrated to an HSTN comprising a satellite, a terrestrial base station (BS), and multiple ground-based mobile terminals (MTs). The suggested method facilitates a cluster of end-users to be concurrently served through NOMA from a satellite using optimal power allocation, whereas the BS utilizes random linear NC to augment the MTs’ reception in the event of errors. The simulation results confirm the effectiveness of the proposed methodology in improving system sum throughput, demonstrating substantial enhancements compared with both OMA with NC, and standalone NOMA and OMA approaches. The resilience of the suggested method is proved, improving the sum throughput even at low power transmissions and in the worst case channel conditions.

This paper proposes a security-aware computation offloading framework tailored for mobile edge computing (MEC)-enabled Internet of Things (IoT) networks operating in environments with aerial eavesdroppers (AEs) and ground eavesdroppers (GEs). It is envisaged that multiple ground nodes (GNs) should perform computation tasks partly locally and partly remotely by offloading a portion of these tasks to MEC servers. To facilitate this paradigm, an unmanned aerial vehicle (UAV) is deployed, serving as both an aerial MEC server and a relay for forwarding part of the tasks to a ground access point (AP) for computing. The computation offloading is further reinforced by incorporating a reconfigurable intelligent surface (RIS) unit in close proximity to the AP. Within this context, this paper provides an analysis of the secrecy outage probability (SOP) and formulates an optimization problem aimed at maximizing the minimum secure computation efficiency (SCE) by jointly optimizing transmit power allocation, time slot scheduling, task allocation, and RIS’s phase shifts. Given the non-convex nature of the problem, an iterative algorithm is introduced to address the fractional objective function and coupled optimization variables by employing Dinkelbach- and block coordinate descent (BCD)-based methods, respectively. The obtained results confirm the efficacy of the optimized scheme.

This paper proposes a scheme for rapidly deploying a UAV-enabled emergency cellular network (UECN) in disaster scenarios, such as earthquakes or floods, to support rescue operations. The unsupervised placement of UAV aerial base stations (ABSs) is achieved through machine learning (ML) techniques. Specifically, the k-medoids algorithm is utilized to cluster ground users in the disaster area and determine the minimum number of ABSs and their position. ABS's altitude is defined based on its capacity capabilities and propagation environment. The UECN cooperates with undamaged cellular infrastructure via joint coordinated multipoint transmission and reception (CoMP) with neighbouring functional terrestrial macrocell base stations (TBSs) to improve end-user signal quality. Finally, the proposed scheme is comparatively evaluated through simulation, and the results demonstrate its efficiency even in the presence of users and ABSs positioning errors.

This paper presents the preliminary results of a propagation study at D-band based on deterministic modeling for an indoor line-of-sight (LOS) environment. The proposed channel model is based on multiple rays between a fixed transmitter and a moving receiver. The results reveal that the antenna characteristics significantly affect the propagation along the corridor, which is also reflected to the produced power delay profiles of the channel. Furthermore, the existence of side-corridors also influences the channel parameters, reducing the root-mean-square (rms) delay and angular spread values. At 12-m distance away from the transmitter the rms delay spread varies between 0.1 and 4.3 ns, whereas the azimuth and elevation rms angular spreads (AS) are in the range of 0.7°-12.4° and 2.1°-7.0°, respectively, depending on the selected scenario and antenna configuration.

This paper presents prediction path loss models in an urban environment for cellular networks with the help of machine learning methods. For this goal, Support Vector Regression (SVR), Random Forest (RF) and K-Nearest Neighbor (KNN) algorithms are exploited and assessed. The training and testing procedure is carried out with the help of a path loss dataset generated by simulated results considering a Long Term Evolution (LTE) network utilizing a digital terrain model. The simulation takes into account an urban environment for both line-of-sight (LOS) and non-LOS (NLOS) propagation condition. The results reveal that all the evaluated algorithms forecast path loss with a remarkable accuracy, providing root-mean-square errors on the order of 2. 1-2.2dB for LOS and 3. 4-4.1dB for NLOS locations, respectively. Among the examined algorithms, KNN shows the best performance, thus being an appealing option to predict path loss in urban areas. For comparison purposes, the COST231 Walfisch-Ikegami empirical model was applied, which presents the worst performance, providing the highest errors under-predicting path loss, especially in NLOS locations.

Buffer-aided relays can improve the diversity of multi-hop networks, however, they increase packet delays. Thus, various delay-aware protocols have been developed, but without considering the transmission diversity. Moreover, most works adopt ideal assumptions, such as symmetric links, perfect channel state information (CSI), and error-free feedback channels. So, we propose a low-complexity (LoCo) link selection algorithm, herein called LoCo-Link. The proposed algorithm may experience delays during CSI updates, and hence, by using outdated CSI its performance may deteriorate. To alleviate this issue, we next propose a distributed version of LoCo-Link (d-LoCo-Link) dealing with outdated CSI. In both algorithms, the source performs broadcasting toward multiple relays; when the packets are transmitted by a relay to the destination, they are discarded from all other relays. This coordination relies on feedback channels. For non error-free feedback channels, we propose a scheme in which the relays listen to the transmission of the best relay and drop duplicate packets. Results show that LoCo-Link surpasses other algorithms, by decreasing the delay in asymmetric networks. Moreover, d-LoCo-Link avoids diversity losses due to outdated CSI, while the effect of non error-free feedback channels is mitigated by taking advantage of the inter-relay channels.

Nowadays, various techniques and systems have been proposed for the implementation of wireless indoor localization systems. Recently, significant focus has been given to Multiple Input-Multiple Output (MIMO) implementations of Ultra-Wideband (UWB) systems, in order to enhance the localization precision while introducing high communication performance. The main disadvantages of such approaches are the high deployment cost and computational complexity, due to the increased data processing. These facts are compensated by adopting the Time Reversal (TR) transmission scheme, providing scaling capabilities and Spatial Focusing (SF) to the system, at the same time. In this paper, a 2 × 2 Multi-Antenna Time-Reversal Indoor Positioning (MATRIP) system model is proposed to track the mobile Blind Node (BN) in a harsh indoor environment, measuring its time-range from one fixed-position Anchor Node (AN). This kind of implementation overcomes the restrictions imposed by the legacy localization systems, regarding the minimum number of ANs of the system. The efficiency of the MATRIP system is demonstrated through simulation results evaluating the average localization error versus the Signal to Noise Ratio (SNR), the Focusing Gain (FG), and the channel capacity.

While buffer-aided relaying improves the diversity of a multi-hop network, its deployment introduces time-delays, thus rendering buffering unreliable for delay-intolerant applications. To alleviate excessive delays, various studies propose delayaware protocols, but at the expense of reduced diversity, and consequently, increased outage probability. Attempts to maintain the diversity of the system while trying to reduce delays, however, may lead to even higher delays, especially in asymmetric topologies. In this work, we propose a Low-Complexity (LoCo) link selection algorithm, herein called LoCo - Link, that aims at reducing packet delays and enhancing the performance of practical asymmetric two-hop networks. The complexity of LoCo - Link is derived and compared with other state-of-the-art relay selection policies. The performance of the proposed algorithm is evaluated in terms of outage probability, average throughput and average delay, focusing on scenarios with asymmetric links.

In this paper, we propose novel relay selection policies that aim at reducing the average delay by incorporating the buffer size of the relay nodes into the relay selection process. More specifically, we propose two delay-aware protocols that are based on the max - link relay selection protocol. First, a delay-aware only approach while it reduces the delays considerably it starves the buffers and increases the outage probability of the system. Towards this end, we propose a delay- and diversity-aware buffer-aided relay selection policy that aims at reducing the average delay considerably and at the same time maintaining good diversity. The protocols are analyzed by means of Markov Chains and expressions for the outage, throughput and delay are derived. The performance and use of our proposed algorithms is demonstrated via extensive simulations and comparisons.

The knowledge of direction information of the transmitted signals requires angle-based position estimation techniques, as well as the orientation of the individual nodes of the system. When using omnidirectional antennas, this information is difficult to be calculated in order to find the Blind Node's (BLN or BN) position. Also, the combination of Ultra Wide-Band (UWB) systems with angle-based techniques is not the best option when tracking down an unknown target, especially indoors, due to the large bandwidth of the UWB signals and the huge number of paths. This paper proposes a geometrical angle-based positioning approach for UWB omnidirectional systems which also combines Fingerprint (FP) and Time-of-Flight (ToF), aiming to calculate the BN's position with the minimum positioning error. The efficiency of the proposed positioning scheme is demonstrated through simulation results in terms of error probability and comparisons with other state-of-the-art positioning schemes.

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.

In this paper, we present a relay selection scheme which combines the spectral efficiency of successive opportunistic relaying with the robustness of single-link relay selection. More specifically, we propose a scheme that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The new relay selection policy is analyzed in terms of outage probability and diversity by modeling the evolution of relay buffers as a Markov Chain. We construct the state transition matrix of the Markov Chain and obtain its stationary distribution, which in turn, yields the outage probability. The proposed scheme outperforms relevant state-of-the-art relay selection schemes in terms of throughput, diversity, energy efficiency and average delay, as demonstrated via representative numerical examples.

The problem of speed control of a permanent magnet synchronous motor is investigated in the presence of time delays due to wireless signal transmission among the controller, the sensors and the actuators. In the present paper, the decoupling controller approach proposed in [1] is extended to compensate the presence of the delays and to achieve satisfactory performance despite their presence. In particular, the research focuses mainly on the stability of the time delay closed loop system.

In this work, we study a cooperative network with multiple full-duplex buffer-aided relays. A hybrid cooperative relaying policy is proposed that employs power adaptation and consists of two alternative schemes: (i) full-duplex transmission through the relay which requires the least total power expenditure and loop interference is mitigated through power adaptation; (ii) buffer-aided max - link selection with power adaptation, when full-duplexity is not feasible. Aiming to reduce the overhead of channel state information (CSI) acquisition and processing, we propose a suboptimal distributed method for relay selection, for which the network performance is not degraded significantly. We show that power adaptation offers reduced overhead of CSI acquisition. Numerical results and comparisons with other state-of-the-art relaying schemes are provided and performance evaluation in terms of throughput, power minimization and switching rate, show the benefits of the proposed hybrid scheme.

In this paper we consider a simple cooperative network consisting of a source, a destination and a cluster of decode-and-forward relays characterized by the half-duplex constraint. At each time-slot the source and (possibly) one of the relays transmit a packet to another relay and the destination, respectively. When the source and a relay transmit simultaneously, inter-relay interference is introduced at the receiving relay. In this work, with the aid of buffers at the relays, we mitigate the detrimental effect of inter-relay interference through either interference cancellation or mitigation. More specifically, we propose the min-power opportunistic relaying protocol that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The min-power relay-pair selection scheme, apart from minimizing the energy expenditure, also provides better throughput and lower outage probability than existing works in the literature. The performance of the proposed scheme is demonstrated via illustrative examples and simulations in terms of outage probability and average throughput.

Cooperative relaying with Distributed Space-Time Coding (DSTC) and cooperative Opportunistic Relaying (OR) are the two main relaying techniques that are typically employed in current wireless systems. However, OR is less complex and proven to be outage optimal compared to DSTC. Thus, in this paper we propose a Multi-Mode Relaying (MMR) technique that is based on OR and combined with the Successive Relaying (SR) technique in order to deal with the half-duplex constraint of the relay nodes. MMR, by employing Multi-Mode Decode-and-Forward (MMDF) relay nodes and incorporating two interference mitigation algorithms, not only combines the merits of OR and SR but also is able to alleviate the increased inter-relay interference caused by SR. The performance of MMR is evaluated through simulation and the results show that it is able to increase the average end-to-end capacity of the wireless path as well as the percentage of high quality packets received at the terminal of the user.

In this work, we present a successive opportunistic relaying protocol with an inter-relay interference (IRI) mitigation scheme, in a quasi-static environment with Rayleigh fading. In order to recover the half duplex loss of the transmission, we adopt a successive relaying (SR) strategy, coupled with reactive relay selection considering the Signal to Noise Ratio (SNR) and the Signal to Interference plus Noise Ratio (SINR) of each hop, after every source transmission. The main degrading factor in this scheme is the IRI introduced by the best relay, which is selected to forward the previous frame, while the rest of the relays are receiving the current source frame. Trying to cancel the effect of IRI, we employ interference cancelation (IC) to subtract the interference signal depending on the capacity of the interference channel.

The present paper studies the performance of a high-speed third-generation (3.5G) networking technology (Enhanced Uplink), in collaboration with well known personal area networks (i.e. Zigbee and Bluetooth) and wireless local area network (WLAN) technology, for delivering pervasive healthcare applications. The association of personal and local area networks along with the 3.5G network is a critical factor for successful development of pervasive e-health services perceived by end-users. In this paper, we propose a methodology for performance assessment based on the joint uplink transmission of voice, real-time video and biomedical data, such us electrocardiogram, vital signals and heart sounds. Various scenarios were concerned for point of care applications in real-time and near-real-time modes and in random locations, where no other system but 3.5G is available. The accomplishment of quality of service (QoS) was explored through the delay and the jitter of the networks' parameters, attributing the joint network system for best performance in the context of the desired e-health services.

This paper is focused on a detailed coverage and performance analysis of wireless metropolitan area networks based on the IEEE 802.16 standard, which is the basis of WiMAX technology. A coverage study and performance evaluation of a mobile WiMAX system in an urban and suburban environment with the help of a software simulation tool is presented. All the required parameters are given to the program focusing only in suitable propagation models e.g. ITU-R 525/526, or COST231-HATA. The total coverage of the area was 67% from which 52% provides QAM modulation offering higher data rates to the users. Very promising results were also extracted, when utilizing multiple antennas. It is also found that in a 4 2 system the average received power at the limits of each cell is enhanced about 6.1 dB in comparison with the SISO case.

The present paper studies the performance of a high-speed third- generation (3.5G) networking technology, HSPA (High Speed Packet Access), for delivering crisis management and emergency e-health applications under urgent situations via satellite backhauling. The performance of a joint network comprised by wireless sensors, the 3.5G and satellite networks is a critical factor for successful development of emergency services perceived by end-users during a disaster situation. In this paper, we propose an emergency system architecture for performance assessment based on the joint transmission of voice, real-time video, vital data and healthcare records file transfer between the disaster-site and the safe-site. Three scenarios were concerned in terms of real-time, non-real- time and emergency applications in random locations, where no other system but 3.5G is available in case of crisis or disaster. The accomplishment of quality of service (QoS) was explored through a step-by-step improvement of the HSPA and satellite system's parameters, attributing the network system for best performance in the context of the desired emergency services.

This paper presents an experimental power angle profile measurement procedure, as well as theoretical capacity results of a single-input-multiple-output (SIMO) system which operates in the millimeter wave band and especially at 60 GHz. Power angle profiles (PAPs) were measured in different indoor locations while detailed angle-of-arrival (AoA) shape factors describing the spatial properties of the millimeter wave channel were derived. The measurement results were related to the site-specific environments and showed correlation between the propagation environments and the multipath channel structure. The 10% outage capacity was found theoretically between 5.7 and 6.2 b/s/Hz for a 1 × 4 SIMO system, and in the range of 4.3 and 4.6 b/s/Hz for a 1 × 2 SIMO system.

Abstract—Land Mobile Satellite (LMS) systems can exploit multiple-input-multiple-output (MIMO) techniques in order to achieve high transmission rates. This paper evaluates theoretically the capacity of single-input-multiple-output (SIMO) system utilizing uniform linear arrays at the receiver terminal for satellite applications. The theoretical study is performed at three different frequency bands. Sufficient capacities are achieved varying between 5 and 6.2 b/s/Hz at L and S bands being almost independent from the elevation angle. In Ku band the capacity is highly variable with the elevation angle, taking values between 2 and 5.9 b/s/Hz, whereas the maximum capacity values are observed in a blocked environment. At L and S bands the capacity is uncorrelated with distance for element spacing up to 1.5 cm and varies between 5.5 and 6 b/s/Hz. In Ku band the capacity is highly variable with the element spacing and maximizes for distances between 1 and 1.5 cm.

This paper evaluates theoretically the capacity of single-input-multiple-output (SIMO) system utilizing uniform linear arrays at the receiver terminal for satellite applications. The theoretical study is performed at three different frequency bands. Overall, very sufficient capacities are achieved, varying between 3 and 8 b/s/Hz. Higher capacity values are achieved for low and medium elevation angles. In the heavy shadowing scenario the capacity drops significantly comparable with the light shadowing case. It reduces, in average, 1.4-1.7 b/s/Hz. Finally, the channel capacity is uncorrelated with element separation after a specific value of lambda.

Abstract It has been generally agreed that the security of electronic patient records and generally e‐health applications must meet or exceed the standard security that should be applied to paper medical records, yet the absence of clarity on the proper goals of protection has led to confusion. The primary purpose of this study was to investigate appropriate security mechanisms, which will help clinical professionals and patients discharge their ethical and legal responsibilities by selecting suitable systems and operating them safely and in short order. Thus, in this paper we propose a security model based on XML with the intention of developing a fast security policy mostly intended for mobile healthcare information systems. The proposed schema consists of a set of principles based on XML security models through the use of partial encryption, signature and integrity services and it was implemented by means of a web‐based m‐health application in a centralized three‐tier architecture utilizing wireless networks environment. Several experiments took place with the aim of measuring the client response time implementing a number of m‐health scenarios. The results showed that the response times required for the fulfilment of a client request with the XML security model are smaller compared to those corresponding to the conventional security mechanisms such as the application of SSL. By selectively applying confidentiality and integrity services either to the medical information as a whole or to some sensitive parts of it, the obtained results clearly demonstrate that XML security mechanisms overwhelm those of SSL and they are suitable for deployment in m‐health applications. Copyright © 2008 John Wiley & Sons, Ltd.

This paper describes a wide-area tele-medicine platform, specially suited for homecare services, based on the DVB-RCS and Wi-Fi communication technologies. Implementation of DVB-RCS, utilizing dynamic assignment techniques mandated in the DVB-RCS specification, has been specifically designed and tuned for multimedia and high-speed data transfer. The monitored patients can be practically anywhere, even in geographically dispersed and isolated areas, where normally there is no terrestrial communications infrastructure capable of supporting similar services. The presented platform combines medical data acquisition and transfer, patient remote monitoring and teleconference services. Possible operational scenarios simulated and results regarding available data rates, quality of service (QoS) provision, and prioritization of tele-monitoring, videoconference and medical data transfer are provided and discussed in the paper.

This paper describes a technique which can be used for wideband satellite signal power measurements. This method consists in measuring signal's power within its spectral mask for a certain period of time. A series of measurements has been carried out not only for evaluating the derived results of the proposed method, but also for the computation of building penetration loss for indoor satellite coverage. The results of this measurement campaign showed site and frequency dependence and they were evaluated by comparing results obtained using this method for signals of both vertical and horizontal polarization as well. Penetration loss for satellite signals can be modeled using regression models for both polarizations.

Performance degradation of analog FM and PAL-G signals due to spread spectrum (SS) interference is investigated. An analytical expression between RF TV protection ratio (CIR) and baseband unweighted signal-to-noise ratio (SNR/sub V/), for cochannel spread spectrum and noise interference, is derived. Also, we express the audio frequency signal-to-noise ratio (AF SNR) output due to SS overlay for various types of spreading scenarios and for different carrier frequency differences (/spl Delta/f).