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This paper studies the impact of unmanned aerial vehicle (UAV) wobbling on the coherence time of the wireless channel between UAVs and a ground user equipment (UE), using a Rician multi-path channel model. We consider two different scenarios for the number of UAVs: (i) single UAV scenario (SUS), and (ii) multiple UAV scenario (MUS). For each scenario, we model UAV wobbling by two random processes, i.e., the Wiener and sinusoidal processes, and characterize the channel autocorrelation function (ACF) which is then used to derive the coherence time of the channel. For the MUS, we further show that the UAV-UE channels for different UAVs are uncorrelated from each other. A key observation in this paper is that even for small UAV wobbling, the coherence time of the channel may degrade quickly, which may make it difficult to track the channel and establish a reliable communication link. [ABSTRACT FROM AUTHOR]

Driven by the massively growing applications in Internet of Things (IoT) and the new requirements of the six generation (6G) wireless network, the amount of terrestrial mobile devices (TMDs) and data traffic in wireless communications have increased explosively. Consequently, the integrated terrestrial-satellite relay networks (ITSRNs) have emerged to address the issue of the huge wireless traffic with limited allocated resources, which attracts extensive research attention to the field of IoT. Limited improvement of system throughput is realized by configuring terrestrial relays (TR) in the previous works. In this paper, by introducing an aerial relay (AR), we propose a novel structure of ITSRN and investigate the system throughput maximization and energy consumption minimization problems by jointly optimizing power allocation and AR altitude. Specifically, the system implementation includes two periods: satellite to relay (S-R) period and satellite to devices (S-D) period. To maximize the overall system throughput, we propose a joint optimization scheme of the power allocation and altitude of AR. Due to its non-convexity, a bilevel programming method is proposed to obtain the global optimal solution, where the closed-form expressions of power allocation and the optimal altitude of AR are derived. Subsequently, we investigate the energy consumption minimization problem during S-R period, which is formulated as a convex problem. We propose a low complexity algorithm and derive the optimal closed-form solution based on the rigorous mathematical analysis. The effectiveness of the proposed algorithms and solutions are verified by extensive simulations. [ABSTRACT FROM AUTHOR]

This paper presents a novel active power control scheme for SRMs that are used as S/G units at MEAs; the proposed method uses a vector-controlled SAF, facilitating the acceleration of the SRM during starter mode and the efficient power control of the SRG at various operational points during generator mode. Moreover, an energy storage unit is incorporated in the SAF, aiming at enhancing the SRM's starting torque production and taking full advantage of the SRG's energy generation capability, through the appropriate phases’ magnetization regulation that is imposed by the developed adaptive SAF vector control. In addition, the proposed scheme provides enhanced FRTC during both modes of operation. The mathematical analysis is verified through MATLAB/Simulink simulations, while the performance of the S/G concept under study is evaluated via real-time CHIL tests, with the use of the dSPACE 1202 (MicroLabBox) platform and the 16-bit dsPIC30F4011 microcontroller. [ABSTRACT FROM AUTHOR]

From the perspective of statistical performance, this correspondence presents an analytical framework to study the interference coordination schemes in two-tier networks with closed-access small-cell base stations (SBSs). We assume the SBSs adopt the combination of power control, access control, and coordinated beamforming to suppress the intertier interference to the macro users (MUEs). Using stochastic geometry, we derive the expressions for the average data rate of users in different tiers. Based on the analytical results, we discuss the optimal transmission schemes of SBSs to maximize the average data rate of small-cell users while guaranteeing the performance of the MUEs. We find that access control is always a suboptimal scheme under the network model in this paper. [ABSTRACT FROM AUTHOR]

This paper proposes a unified performance analysis framework of asymmetric dual-hop radio-frequency/free-space optical (RF/FSO) transmission systems. The generalized independent and identically distributed (i.i.d.) $\alpha -\mu$ distribution is considered for each branch in the RF link, whereas the FSO link is assumed to experience Málaga ($\mathcal {M}$) fading with pointing error impairments. With amplify-and-forward (AF) and decode-and-forward (DF) relaying, the imperfect channel state information (CSI) of both links is considered. For the considered system, we first derive exact and novel analytical expressions for the probability density function (PDF) and cumulative distribution function (CDF) of the end-to-end signal-to-noise ratio (SNR). Furthermore, we obtain key performance metrics such as outage probability (OP), average bit-error-rate (ABER), and effective capacity (EC). The important effects of channel parameters on the system are disclosed from mathematical analysis and are further validated by Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

Incipient fault detection and diagnosis (FDD) is a key technology for enhancing safety and reliability of high-speed trains. This paper develops a real-time incipient FDD method named deep principal component analysis (DPCA) for electrical drive in high-speed trains. This method can effectively detect incipient faults in electrical drive before they develop into faults or failures. This scheme adopting multivariate statistics is composed of multiple data processing layers to extract more accurate signal features of electrical drive, which exhibits several salient advantages: 1) It can establish precise data models containing both systematic and noise information of electrical drive, which are helpful for incipient fault detection; 2) the incipient faults are described by multicharacteristics which can improve the fault diagnosis ability; 3) it can be easily implemented even if the system models and parameters of electrical drive are unknown. The effectiveness and feasibility of the proposed FDD scheme are authenticated via a mathematical analysis and validated via two experiments. Results of two experiments show that the missing alarm rate and detection delay by using the proposed DPCA-based FDD method are less than 10% and 0.68 s, respectively. In comparison with the standard PCA-based FDD method, the proposed DPCA-based FDD method can show its superiorities by the detailed performance comparisons. [ABSTRACT FROM AUTHOR]

Reconstructing emergency communication networks (ECNs) quickly after a disaster occurs is critical so that people can share information and confirm their safety. In recent studies, deployable base stations (DBSs) have demonstrated their ability to reconstruct an ECN. However, considering limited resources, it is impossible to deploy DBSs in the whole disaster area. The above shortage can be covered by deploying small-cell networks (i.e., low-power transmission base stations) in areas with high communication demand, e.g., in refuges. Considering the above two-tier ECN, in this paper, we study its performance and optimization issue with the objective of minimizing the number/density of DBSs while guaranteeing acceptable coverage probabilities for both communication tiers. The majority of current research focuses on scenarios where the base stations follow a homogeneous Poisson point process of coverage probability. It is difficult to transfer the results to other applications, e.g., when communication resources are shared, such as by refugees following a disaster. In such cases, the distribution of users is closer to that of a Poisson cluster process. We then investigate the optimization method to minimize the number/density of DBSs. We used Monte Carlo methods with various parameter choices to evaluate the results and to determine the accuracy of our evaluation. [ABSTRACT FROM PUBLISHER]

Hardware impairments (HI) always exist in wireless devices, especially the low-cost ones. However, due to the complicated mathematical analysis, previous studies on wireless networks usually neglected the HI, particularly for full-duplex relaying (FDR) systems. In this paper, we consider the impact of the HI on the outage probabilities (OPs) and ergodic capacities (ECs) of both one-way (OW) and two-way (TW) amplify-and-forward (AF) FDR systems. We also take into account the residual self-interference (RSI) due to the imperfect self-interference cancellation (SIC), which appears at the FDR node of the considered HI-OW-AF-FDR system and all nodes of the considered HI-TW-AF-FDR system. Particularly, we derive the exact expressions of the outage probabilities and ergodic capacities for the HI-OW-AF-FDR and HI-TW-AF-FDR systems using mathematical analysis. Numerical results show that the OPs and ECs of the two systems are significantly affected by both HI and RSI, however the impact of the HI is more serious. It is recommended that besides SIC techniques, effective methods to reduce the HI should also be used to guarantee the system performance. [ABSTRACT FROM AUTHOR]

A number of methods have been proposed for the response-time analysis (RTA) of controller area networks (CANs), which are the industry standard for in-vehicle network protocols. However, most of these methods are unsuitable for analyzing nonperiodically transmitted messages because they assume a constant transmission period for each message. Moreover, the calculated evaluation criteria are limited and comprise many over-pessimistic estimations, such as the worst-case response time (WCRT). To tackle these problems, this paper proposes an RTA method for CANs with randomly occurring messages, which replaces the constant transmission period with the average number of transmission instances per unit time. This approach finds the approximate probability distribution of the response time to any message in a CAN, providing a more general measure than the WCRT. To this end, a mathematical model based on M/G/1 queueing theory is developed. The model accurately emulates the behavior of nonperiodic CAN messages and enables RTA with minimal pessimistic estimations. When tested against the benchmarks, the proposed method accurately obtained response time distributions within several seconds to several tens of seconds. [ABSTRACT FROM AUTHOR]

Ultra-dense networks (UDNs) envision the massive deployment of heterogeneous base stations (BSs) to meet the desired traffic demands. Furthermore, UDNs are expected to support the diverse devices e.g., personal mobile devices and unmanned aerial vehicles. User mobility and the resulting excessive changes in user to BS associations in such highly dense networks may however nullify the capacity gains foreseen through BS densification. Thus, there exists a need to quantify the effect of user mobility in UDNs. In this paper, we consider a three-dimensional $N$ -tier downlink network and determine the association probabilities and inter/intra tier handover rates using tools from stochastic geometry. In particular, we incorporate user and BSs’ antenna heights into the mathematical analysis and study the impact of user height on the association and handover rate. The numerical trends show that the intra-tier handovers are dominant for the tiers with shortest relative elevation w.r.t. the user and this dominance is more prominent when there exists a high discrepancy among the tiers’ heights. However, biasing can be employed to balance the handover load among the network tiers. [ABSTRACT FROM AUTHOR]

In multicell networks, unbalanced cell loading can lead to decreased system stability and reduced fairness among serviced users. In this paper, we propose theoretical-analysis-based distributed load balancing (DLB) over dynamic overlay clustering implemented over a multicell network. The proposed system is divided into two parts: DLB and overlay clustering. First, for DLB, we define the long-term expected load after deriving the long-term expected rate in terms of proportional fairness. We then introduce two algorithms: DLB for load dispersion and DLB for edge-rate enhancement (ERE). These algorithms operate in a distributed manner based on mathematical analyses and load balancing characteristics. Second, through overlay clustering, load balancing within each cluster is consecutively performed on neighboring clusters, which enables the algorithm to optimally approximate in a distributed manner. The simulation results show that approximately 90% of the near-optimal performance in terms of load variation and ERE can be achieved with low complexity by using the proposed schemes. In addition, we discuss aspects and tradeoffs of the load balancing system. [ABSTRACT FROM AUTHOR]

In this paper, we derive closed-form exact and asymptotic expressions for the symbol error rate (SER) and channel capacity when communicating over Fox's $H$-function fading channel. The SER expressions are obtained for numerous practically employed modulation schemes in case of single-branch and three multiple-branch diversity receivers: maximal ratio combining (MRC), equal gain combining (EGC), and selection combining (SC). The derived exact expressions are given in terms of the univariate and multivariate Fox's $H$-functions for which we provide a portable and efficient Python code. Since Fox's $H$-function fading channel represents the most generalized fading model ever presented in the literature, the derived expressions subsume most of those previously presented for all the known simple and composite fading models. Moreover, easy-to-compute asymptotic expansions are provided to easily study the behavior of the SER and channel capacity at high values of the average signal-to-noise ratio (SNR). The asymptotic expansions are also useful in comparing different modulation schemes and receiver diversity combiners. Numerical and simulation results are also provided to support the mathematical analysis and prove the validity of the obtained expressions. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of imperfect channel state information on the performance of the cooperative non-orthogonal multiple access (NOMA) transmission scheme, in which the cell-center user acts as a decode-and-forward relay to assist the information transmission. The cell-center user adopts a simultaneous wireless information and power transfer (SWIPT) technique to harvest energy for information forwarding. Based on two channel error models, a maximization problem is formulated to jointly design the robust beamforming and the power splitting ratio while satisfying the minimum data rate for the far user and the successfully decoding requirement for the information of the far user at the near user. For the worst case design under the deterministic error model, the successive convex approximation and the semidefinite relaxation technique are utilized to approximate the non-convex problem to an iterative convex problem. For the outage-constrained design under the stochastic error model, Bernstein-type inequality-based and large deviation inequality-based approaches are used to safely approximate the probabilistic constraints of the inner-level problem to the deterministic constraints, and a golden section search algorithm is employed to find out the optimal single variable of the outer-level problem. Furthermore, the rank proof is provided to prove that the relaxation is tight. [ABSTRACT FROM AUTHOR]

In this paper, the outage performance of opportunistic relaying systems under the constraint of limited feedback is investigated. The outage probability for amplify-and-forward (AF) and selective decode-and-forward (DF) relaying with limited feedback are both derived in closed-form expressions. On this basis, the diversity gain of the system is proved to be only two for both AF and selective DF relaying, regardless of the number of participating relays or feedback accuracy, which differs from the full diversity gain achieved by opportunistic relaying with perfect channel state information (CSI). On the other hand, mathematical analysis shows that the outage performance loss can be evaluated in closed form, and the outage probability approaches asymptotically to that of the perfect CSI opportunistic relaying with the increasing accuracy of feedback. With these theoretical results, a feedback method is developed to assign feedback bits within a given outage performance requirement. Simulation results confirm the theoretical analysis. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate the issues of joint user pairing and subchannel allocation (JUP-SA) in a multisubchannel multiuser nonorthogonal multiple access (NOMA) system. We first investigate the uplink and propose a JUP-SA scheme so that the minimum achievable diversity order of users is maximized. By deriving the upper and lower bounds of the outage probability of the worst-performance user, the achievable diversity order of the scheme is obtained. Following this, the downlink of the system is considered and a JUP-SA scheme that maximizes the minimum diversity order is presented. The closed-form expression of the worst-case outage probability is then derived and validated by simulation results. Numerical results show that both schemes in uplink and downlink NOMA systems can achieve the same diversity order as that of exhaustive search. [ABSTRACT FROM AUTHOR]

We investigate the cell coverage optimization problem for the massive multiple-input multiple-output (MIMO) uplink. By deploying tilt-adjustable antenna arrays at the base stations (BSs), cell coverage optimization can become a promising technique that is able to strike a compromise between covering cell-edge users and pilot contamination suppression. We formulate a detailed description of this optimization problem by maximizing the cell throughput, which is shown to be mainly determined by the user distribution within several key geometrical regions. Then, the formulated problem is applied to different example scenarios. For a network with hexagonal cells and uniformly distributed users, we derive an analytical lower bound of the ergodic throughput in the objective cell; based on this, it is shown that the optimal choice for the cell coverage should ensure that the coverage of different cells does not overlap. For a more generic network with sector-shaped cells and nonuniformly distributed users, we propose an analytical approximation of the ergodic throughput. After that, a practical coverage optimization algorithm is proposed, where the optimal solution can be easily obtained through a simple 1-D line searching within a confined searching region. Our numerical results show that the proposed coverage optimization method is able to greatly increase the system throughput in macrocells for the massive MIMO uplink transmission, compared with the traditional schemes where the cell coverage is fixed. [ABSTRACT FROM PUBLISHER]

Since a novel diversity technique called a space–time line code (STLC) was invented, previous studies have focused on the analysis of a rate-1 STLC assuming two receive antennas. For the state-of-the-art STLCs with transmit antenna selection (TAS), in this study, the ergodic capacities of TAS-STLC systems with various code rates are mathematically derived, not only for a TAS-STLC system with two receive antennas but also for those with three or four. To this end, the probability density function of the instantaneous signal-to-noise ratio was characterized. The accuracy of the mathematical analysis was validated by a numerical simulation. This study is significant for investigating the fundamental performance of TAS-STLC systems. [ABSTRACT FROM AUTHOR]

Utilization of drones as aerial access points (AAPs) is a promising concept to enhance network coverage and area capacity promptly and on demand. The emerging millimeter-wave (mmWave) communication technology may in principle deliver higher data rates, thus, making the use of AAPs more effective. By extending the conventional (planar) stochastic geometry considerations, we construct a novel three-dimensional model for drone-based mmWave communication that captures the high directionality of transmissions as well as the random heights of the communicating entities. Choosing signal-to-interference ratio as our primary parameter of interest, we assess system performance with an emphasis on the impact of the “vertical” dimension in aerial mmWave connectivity. We also demonstrate that accurate performance assessment is only possible with simplified models for certain ranges of input parameters. [ABSTRACT FROM AUTHOR]

The information-centric networking (ICN) is a promising and significant approach towards the host-to-content interaction. Motivated by the fact that nodes are inhomogeneously distributed to a certain extent in wireless networks, e.g., the distribution of vehicles exhibits cluster phenomenon in some bustling places of urban areas, we study the impacts of inhomogeneous node distribution and local node cooperation on information-centric wireless networks with infrastructure support. The distribution of nodes is assumed to be inhomogeneous as a multiclustering topology. The base stations are assumed to have access to all the contents, which are requested following a popularity distribution. We propose a scheme where each node retrieves the requested content from other nodes or base stations. Based on the number of clusters and that of base stations, we divide the network topology into two cases, i.e., the cluster-dense case and cluster-sparse case, and evaluate the corresponding throughput and delay performance. Finally, under the Zipf's law for content popularity distribution, we investigate the optimal caching problem in the two cases, and obtain the optimized throughput and delay performance. [ABSTRACT FROM AUTHOR]