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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]

The distribution of the interservice time, which is the interval between the end of a call served and the start of the succedent call served by a portable, is a crucial parameter for modeling and cost analysis for location management in personal communication service (PCS) networks. The interservice time differs from the interarrival time due to the busy-line effect. However, in the literature, most of the cost analyses for location management were carried out by ignoring the busy-line effect, i.e., by identifying the interservice time with the interarrival time, because, so far, there is no analytical result for the interservice time distribution considering the busy-line effect. In this paper, we develop an analytical model to investigate the busy-line effect on the interservice time distribution. Closed-form analytical formulas for the interservice time are derived. A numerical study shows that a big discrepancy exists between the interarrival time and the interservice time. Based on the analytical results for the interservice time, the impact of the busy-line effect on modeling of portable movements in PCS networks is further evaluated. An analytical result for the distribution of the number of cell-boundary crossings during the interservice time is obtained. Both analytical results and numerical examples indicate that the busy-line effect has influence on the distribution of the number of cell-boundary crossings during the interservice time. Therefore, cost analysis for location management without considering the busy-line effect may lead to unreliable results. The results presented in this paper pave the way for modeling location management and portable movements in PCS networks. [ABSTRACT FROM AUTHOR]

Based on the basic principle of spread spectrum systems, this paper proposes a hybrid automatic-repeat-request (HARQ) scheme with interference mitigation for multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. The proposed hybrid media access control (MAC) and physical layer (PHY) design orthogonally spreads signals in the transmission domain to separate the desired signals from interferences. It not only improves the signal-to-noise ratio (SNR) but also suppresses intercarrier interference (ICI) and interantenna interference (IAI). Almost without increasing complexity, mathematical analyses and numerical simulations show that the proposed scheme outperforms the conventional Chase combining HARQ. [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]

This paper tackles a power minimization problem of interleave-division multiple-access (IDMA) systems over a fading multiple-access channel. The problem is minimizing the total power received by the receiver while keeping the bit error rates (BERs) of all users below a predefined value. The original formulation of the problem has highly nonlinear and implicitly defined functions, which render most existing optimization methods incapable. A new formulation is proposed in this paper, whose solution can effectively be obtained by a branch-and-bound (B&B) technique. An algorithm is devised based on B&B, and its effectiveness is also demonstrated by numerical experiments of systems with a moderate numbers of users. [ABSTRACT FROM AUTHOR]

In this paper, we propose a fast and efficient adaptive transmission algorithm for the Mobile WiMAX system. In our algorithm, we select the position of the boundary between the downlink and uplink subframes, the positions of the service, flows within the frame, the coding and modulation schemes, and the transmission power values. Our algorithm gains high spectral efficiency, satisfies quality-of-service (QoS) requirements for the service flows, and has low computational complexity. The algorithm includes a frame boundary position optimization procedure, downlink and uplink optimization procedures, as well as a procedure that selects a transmission power value that enables satisfaction of the QoS requirements. All these procedures are novel, efficient, and fast. We evaluate the adaptive transmission algorithm by performing a system-level simulation of the Mobile WiMAX system with multisector topology. We show that our algorithm has high spectral efficiency and low computational complexity, enabling QoS provisioning. [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]

Interlayer interaction of a layered error-recovery mechanism to act against channel errors for wireless-data communications is examined in this paper. Error-control designs are implemented at two different layers in the protocol stack. A finite number of retransmissions is performed at the lower layer by a mix-mode automatic-repeat request as the primary error removal. At the upper layer, a time-out mechanism is employed, together with a packet-level coding technique, as the secondary error elimination. Performance of the proposed scheme is analyzed and verified by simulations. Through numerical experiments, we have demonstrated how the system performance is influenced by layer parameters. It is discovered that error-correcting capability needs to function at the upper layer to ensure a good system performance. [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, the end-to-end performance of a wireless relay transmission system that employs amplify-and-forward (AF) relays and operates in an interference-limited Nakagami- $m$ fading environment is studied. The wireless links from one relay node to another experience Nakagami- $m$ fading, and the number of interferers per hop is Poisson distributed. The aggregate interference at each relay node is modeled as a shot-noise process whose distribution follows an $\alpha$-stable process. For the considered system, analytical expressions for the moments of the end-to-end signal-to-interference ratio (SIR), the end-to-end outage probability (OP), the average bit-error probability (ABEP), and the average channel capacity are obtained. General asymptotic expressions for the end-to-end ABEP are also derived. The results provide useful insights regarding the factors affecting the performance of the considered system. Monte Carlo simulation results are further provided to demonstrate the validity of the proposed mathematical 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]

In this paper, we investigate the secrecy rate of finite-alphabet communications over multiple-input–multiple-output–multiple-antenna eavesdropper (MIMOME) channels. Traditional precoding designs based on Gaussian input assumption may lead to substantial secrecy rate loss when the Gaussian input is replaced by practical finite-alphabet input. To address this issue, we investigate linear precoding designs to directly maximize the secrecy rate for MIMOME systems under the constraint of finite-alphabet input. By exploiting the theory of Karush–Kuhn–Tucker (KKT) analysis and matrix calculus, we first present necessary conditions of the optimal precoding design when instantaneous channel-state information (CSI) of the eavesdropper is known at the transmitter. In this light, an iterative algorithm for finding the optimal precoding matrix is developed, utilizing a gradient decent method with backtracking line search. Moreover, we find that the beamforming design in MIMONE systems, which is a secrecy-capacity-achieving approach for Gaussian signaling, no longer provides the maximum secrecy rate for finite-alphabet input data. This case is substantially different from the Gaussian input case. In addition, we derive the closed-form results on the precoding matrix, which maximizes the secrecy rate in the low signal-to-noise ratio (SNR) region, and reveal the optimal precoding structure in the high-SNR region. A novel jamming signal generation method that draws on the CSI of the eavesdropper to additionally increase the secrecy rate is further proposed. The precoding design with only statistical CSI of the eavesdropper available at the transmitter is also considered. Numerical results show that the proposed designs provide significant gains over recent precoding designs through a power control policy and the precoding design with the Gaussian input assumption in various scenarios. [ABSTRACT FROM PUBLISHER]

In high-voltage dc power systems of advanced aircraft, most of the electrical motor drives and power electronic converters behave as constant power loads (CPLs) when tightly regulated and usually cause negative impedance instability problems. This paper presents stability criteria for an electrical system constituted of a dc power supply, multistage LC filters, and an ideal CPL. The derived criteria are based on the Brayton–Moser mixed potential theory and can be extended to actual CPLs. The dynamic negative incremental impedance of the CPL is used to place design constraints on the filter parameters to prevent instabilities. Simulation and experimental results confirm that the proposed criteria would guarantee system stability under different voltage disturbances. [ABSTRACT FROM PUBLISHER]

The role of the push-based architecture in modern wireless telecommunication systems has been gaining momentum over the past few years. However, these systems pose a challenging problem: the creation of the optimal common broadcast schedule that best fits the needs of the totality of the available clients. To this end, various approaches have been proposed, with one of the most well known and widely accepted approaches being the Broadcast Disks method. This method ensures the periodicity and proportionality attributes of the broadcast schedule but leaves ample room for performance optimization, which is a task traditionally assigned to heuristic algorithms. This paper presents a mathematical analysis of the Broadcast Disks method, which establishes a new analytical performance-optimization procedure. Comparison with other related well-known algorithms yielded better performance in every client test case. Finally, the analysis showed that even small deviations from optimal parameter values may lead to significant performance degradation in terms of the clients' mean waiting time. [ABSTRACT FROM AUTHOR]

In the context of next-generation wireless systems, it is expected that services targeted at mass content distribution will become widely popular. Multicast services, such as audio/video streaming and mobile television, have the characteristic that the same information has to be transmitted to a group of recipients, which may include user terminals as well as relay stations, in the case of relaying networks. This paper deals with the problem of multigroup multicast beamforming for multiantenna wireless cellular networks, which assumes that multiple multicast groups can share the same resource. The intergroup interference, which appears due to the resource sharing, needs to be suppressed by the beamforming algorithms. In this paper, new linear transmit beamforming techniques are proposed, which aim to provide a reasonable tradeoff between performance and computational complexity. These techniques correspond to extensions of known algorithms, such as zero forcing (ZF), minimum mean square error (MMSE), and signal-to-interference-plus-noise ratio (SINR) balancing, to the multicast case. Furthermore, new "multicast- aware" techniques that take into account the peculiarities of multicast transmission are proposed to improve performance. To provide the necessary mathematical framework for the analysis, a novel general multigroup multicast system model is proposed as well. [ABSTRACT FROM AUTHOR]

The IEEE 802.11 contention-based distributed coordination function has been used in ad hoc networks, and so far, a comprehensive mathematical analysis of such multihop systems has not been reported in the literature due to complicating factors such as hidden terminals and the unreachability problem. Iii this paper, we propose a new analytical model based on the parallel space-time Markov chain for IEEE 802.11 medium access control (MAC) in multihop ad hoc networks. The proposed scheme is able to model the hidden-terminal and unreachability phenomena. Extensive evaluation is conducted to demonstrate the accuracy of the proposed framework. [ABSTRACT FROM AUTHOR]

Geometric dilution of precision (GDOP) has been widely used as an accuracy metric for navigation and tracking systems. Since high accuracy in a positioning system requires both accurate measurement of the range and a good geometric relationship between the mobile device and the measuring points, the analysis of GDOP is an essential feature in determining the performance of a positioning system. In this paper, we perform GDOP analysis to obtain concise analytical expressions for a number of scenarios, which are generally applicable to geometries where the mobile device is surrounded by base stations. Comparison of the analytical results with simulations using the typical geometries of indoor positioning systems shows good agreement, except when the mobile position is close to a base station. This effect is a consequence of the ranging errors being a significant proportion of the range in short-range tracking systems and discontinuities in GDOP at the base station. The results provide useful information for the design and testing of tracking systems, as well as for the determination of the geometric deployment of base stations for good GDOP in the coverage area. [ABSTRACT FROM AUTHOR]

Liu and Tureli proposed a blind carrier frequency offset (CFO) estimation method for orthogonal frequency-division multiplexing (OFDM) systems, making use of null subcarriers. The optimal subcarrier placement that minimizes the Cramer-Rao bound (CRB) of the CFO estimation was reported by Ghogho et al. In this paper, we study the optimality of the null subcarrier placement from another perspective. We first show that the SNR of the CFO estimation using null subcarriers is a function of the null subcarrier placement. We then formulate the CFO-SNR optimization for the null subcarrier placement as a convex optimization problem for small CFO values and derive the optimal placement when the number of subcarriers is a multiple of the number of null subcarriers. In addition, we show that the SNR-optimal null subcarrier placement also minimizes the theoretical mean square error in the high SNR region. When the number of subcarriers is not a multiple of the number of null subcarriers, we propose a heuristic method for the null subcarrier placement that still achieves good performance in the CFO estimation. We also discuss the optimality of the null subcarrier placement in practical OFDM systems, where guard bands are required at both ends of the spectrum. [ABSTRACT FROM AUTHOR]

Cross-layer optimization solutions have been proposed in recent years to improve the performance of wireless users that operate in a time-varying, error-prone network environment. However, these solutions often rely on centralized cross-layer optimization solutions that violate the layered network architecture of the protocol stack by requiring layers to provide access to their internal protocol parameters to other layers. This paper presents a new systematic framework for cross-layer optimization, which allows each layer to make autonomous decisions to maximize the wireless user's utility by optimally determining what information should be exchanged among layers. Hence, this cross-layer framework preserves the current layered network architecture. Since the user interacts with the wireless environment at various layers of the protocol stack, the cross-layer optimization problem is solved in a layered fashion such that each layer adapts its own protocol parameters and exchanges information (messages) with other layers that cooperatively maximize the performance of the wireless user. Based on the proposed layered framework, we also design a message-exchange mechanism that determines the optimal cross-layer transmission strategies, given the user's experienced environment dynamics. [ABSTRACT FROM AUTHOR]

In this paper, a novel prioritization mechanism for random access strategies in cellular networks is proposed. The proposed prioritized random access strategy sets different retransmission probabilities to users, depending on their priorities. First, a mathematical analysis method for simultaneously evaluating throughput and access delay, considering an infinite population model and different retransmission policies, is developed. Most of the previous related research has been done, considering both finite population and saturation conditions where all the nodes in the system always have a packet ready to be transmitted, and the transmission queue of each station is assumed to be always nonempty. This assumption is a good approximation for a local area network working at full capacity. However, in a cellular system, the assumptions of a finite population and every node in a cell always having a packet to transmit are not very realistic. Here, the analytical results consider a Poisson arrival process-which is more suitable for the traffic model in a cellular system-for the users in the cells. Then, considering slotted ALOHA as a random access strategy and the developed analysis method, the case where two different types of service exist (low- and high-priority users) is mathematically analyzed. With the proposed prioritized random access strategy, the average access delay achieved for the high priority users is always lower than that for the low-priority users. Additionally, a mathematical expression for the throughput is derived with and without service differentiation. Finally, two approaches for finding the optimum retransmission probabilities are developed; in the first approach, the number of. backlogged packets is required, whereas a simpler and efficient alternative method requires only the knowledge of the mean new packet arrival rate. For the performance analysis, four of the most common backoff algorithms are considered: 1) Uniform (UB); 2) Binary Exponential (BEB); 3) Negative Exponential (XB); and 4) Geometric (GB). [ABSTRACT FROM AUTHOR]

In this paper, we present a new unified bit-error-rate analysis of group-orthogonal multicarrier (GO-MC) systems for both uplink and downlink segments when using a maximum-likelihood (ML) multiuser and multisymbol detector, respectively. The proposed analysis is based on the union bound, and it is general enough to allow each of the transmitted symbols in the group to come from a different modulation alphabet and have different received power. Monte Carlo simulation results are also presented to assess the accuracy of the derived analytical expressions. A very good agreement between analytical and simulation results can be appreciated, highlighting the usefulness of our approach in the assessment and planning of generalized GO-MC systems. [ABSTRACT FROM AUTHOR]

In this paper, we investigate two reduced-complexity iterative soft interference cancellation minimum mean square error (SIC-MMSE) receivers for frequency-selective multiple-input-multiple-output (MIMO) channels. In the first receiver, the extrinsic information is exchanged between the SIC-MMSE equalizer and the channel decoding stages at each iteration. In the second receiver, the extrinsic information obtained from the SIC-MMSE equalizer is fed back to itself only up to a certain number of iterations and then passed to the channel decoder at the end of the last iteration only to reduce the computational complexity. Moreover, to better understand the convergence behavior of the proposed iterative receivers, we study the notion of extrinsic information transfer (EXIT) characteristics. Using simulations, we derive the extrinsic information trajectory on the EXIT chart at various bit-energy-to-noise-spectral-density ratio (Eb/No) ranges to predict the number of iterations required to converge and the turbo cliff region. The predicted behavior of the proposed receivers is then confirmed by the bit-error-rate (BER) performance curves. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel system that invokes jointly optimized iterative source and channel decoding for enhancing the error resilience of the adaptive multirate wideband (AMR-WB) speech codec. The resultant AMR-WB-coded speech signal is protected by a recursive systematic convolutional (RSC) code and transmitted using a non-coherently detected multiple-input-multiple-output (MIMO) differential space-time spreading (DSTS) scheme. To further enhance the attainable system performance and to maximize the coding advantage of the proposed transmission scheme, the system is also combined with multidimensional sphere-packing (SP) modulation. Furthermore, the convergence behavior of the proposed scheme is evaluated with the aid of extrinsic information transfer (EXIT) charts. The proposed system exhibits an Eb /No gain of about 1 dB, as compared with the benchmark scheme carrying out joint channel decoding and DSTS-aided SP demodulation in conjunction with separate AMR-WB decoding, when using only Isystem = 2 system iterations and when communicating over narrow-band correlated Rayleigh fading channels. [ABSTRACT FROM AUTHOR]

In this paper, we propose a hybrid location-update scheme, which combines the time- and movement-based location- update approaches. In the proposed scheme, a mobile user updates its location after the following two events have occurred: First, the mobile crossed n cell boundaries, and then, a time interval T elapsed. We also examine the inverse scheme, where an update is performed after the n boundary crossings occur subsequent to the time interval T. We evaluate our scheme by applying it to various mobility models, such as exponential cell-residence tunes with a constant mobility pattern, exponential cell-residence times with a gamma-distributed mobility pattern, hyperexponential cell-residence times, and cell-residence tunes with a small coefficient of variation. From our numerical analysis and simulation results, we learn that the movement-based-only method (i.e., T 0) performs well when the coefficient of variation of the cell-residence time is relatively small. However, when the coefficient of variation is large, the proposed hybrid scheme (i.e., T > 0, n > 0) outperforms each of the two approaches, namely, the time- and movement-based location-update schemes, when these schemes are individually applied. [ABSTRACT FROM AUTHOR]

In this paper, the optimal carrier frequency offset (CFO) and channel estimator for an orthogonal frequency-division multiple-access (OFDMA) uplink based on a joint maximum- likelihood (ML) criterion is derived. Direct implementation of the resultant estimation scheme is challenging due to the need for a multidimensional exhaustive search in multi-CFO estimation. To solve this problem, an optimization theorem is exploited, and a computationally efficient method using an importance sampling technique is proposed. Without the need to provide an initial estimate, the proposed estimator guarantees the generation of the global optimal solution. Simulation results clearly verify the effectiveness of the proposed estimation scheme. [ABSTRACT FROM AUTHOR]

Ongoing next-generation networks are expected to be deployed over current existing networks in the form of overlayed heterogeneous networks, particularly in hot-spot areas. Therefore, it will be necessary to develop an interworking technique, such as load balancing, to achieve increased overall resource utilization in the various heterogeneous networks. In this paper, we present a new load-balancing mechanism termed "soft" load balancing, where the Internet Protocol (IP) traffic of a user is divided into subflows, each of which flows into a different access network. The terminology of soft load balancing involves the use of both load-sharing and handover techniques. Through a numerical analysis, we obtain an optimal load-balancing ratio (LBR) to determine the volume of traffic delivered to each network over an overlayed multicell environment. Using the optimal LBR, a more reliable channel transmission can be achieved by efficiently reducing the outage probability for given user traffic. [ABSTRACT FROM AUTHOR]

In this paper, we present an efficient rate-guaranteed opportunistic scheduling (ROS) scheme, which optimizes the system throughput performance while satisfying the "fairness" constraint for resource allocation by exploiting time-varying channel states. We prove the optimality of the proposed scheduler via mathematical analysis and show that it has lower computational complexity compared with existing opportunistic scheduling algorithms. We define a new concept of guaranteed-rate node with loss, which is extended from the service curve with a loss model, to analyze the performance bounds of ROS, such as the statistic end-to-end packet delay bound. We validate the mathematical analysis and demonstrate the throughput performance improvement and a delay bound based on ns-2 simulations. [ABSTRACT FROM AUTHOR]

In this paper, a precoded and iteratively detected downlink muItiuser system is proposed, which is capable of operating in rank- deficient scenarios, when the number of transmitters exceeds the number of receivers. The literature of uplink space division multiple access (SDMA) systems is rich, but at the time of writing there is a paucity of information on the employment of SDMA techniques in the downlink. Hence, we propose a novel precoded downlink SDMA (DL-SDMA) muItiuser communication system, which invokes a low-complexity near-maximum-likelihood sphere decoder and is particularly suitable for the aforementioned rank-deficient scenario. Powerful iterative decoding is carried out by exchanging extrinsic information between the precoder's decoder and the outer channel decoder. Furthermore, we demonstrate with the aid of extrinsic information transfer charts that our proposed precoded DL-SDMA system has a better convergence behavior than its nonprecoded DL-SDMA counterpart. Quantitatively, the proposed system having a normalized system load of L8 = 1.333, i.e., 1.333 times higher effective throughput facilitated by having 1.333 times more DL-SDMA transmitters than receivers, exhibits a "turbo cliff" at an Eb/No of 5 dB and hence results in an infinitesimally low bit error rate (BER). By cciitrast, at Eb/No = 5 dB, the equivalent system dispensing with precoding exhibits a BER in excess of 10%. [ABSTRACT FROM AUTHOR]

A suboptirnum search algorithm that considers the users' power profile in conjunction with a primary stage of polynomial-expanded (PE) linear multiuser detector for mobile uplink is proposed to suppress interference level. The initial stage is improved by mathematical analysis using two new ideas based on the Rayleigh-Ritz and Gershgorin theorems for both synchronous and asynchronous users. The performance of the PE structure is further enhanced by a new suboptimum search algorithm as the second stage. The structure of the search algorithm is based on starting from a point close to the optimum search and performing sequential steps to modify it. The proposed structures of the PE method and the suboptimum search algorithm are well suited together and make them collaboratively work without encountering a high level of complexity, since they use similar information as the correlation matrix. The power profile of the users is also considered in the suboptimum search algorithm, which led to more than a 50% reduction of the complexity, while keeping the performance almost the same. The performance of the final structure is carried out by means of computer simulations, and it has been compared to a partial parallel interference cancellation method with optimized partial coefficients. One of the main features of the structure proposed in this paper is that it performs its operations on the canonical formulation of the system, which makes it highly suitable for a broad range of systems such as multicarrier code division multiple access and multiinput multioutput. [ABSTRACT FROM AUTHOR]

In this paper, we propose a robust space-time block coded (STBC)-based transmit diversity scheme with low decoding complexity against spatially transmit-correlated fading channels compared with the conventional STBC orthogonal frequency-division multiplexing (STBC-OFDM) schemes. The proposed scheme is comprised of Alamouti code for spatial diversity and a simple mapping process with subcarrier selection for frequency diversity. Proceeding from a detailed analysis of the rank and determinant of the channel correlation matrix, we present a proper subcarrier selection scheme satisfying the conditions for robustness against spatial transmit correlation. Furthermore, when the receiver employs a low-complexity decoder such as a zero-forcing decoder, the proposed scheme shows improved performance compared with the existing space-frequency coded OFDM schemes. [ABSTRACT FROM AUTHOR]

The movement-based scheme is a dynamic location management (LM) scheme that is particularly easy to implement. Each mobile simply counts the number of cell-boundary crossings and initiates location update when this number exceeds the predefined movement threshold. Although its LM cost is inferior to that of distance-based schemes, this cost can be decreased by sequential paging methods that take into account mobiles' movement statistics. In this paper, we derive the probability distribution of a mobile's number of cell-boundary crossings during the interval between two location registrations in a movement-based LM scheme for Poisson call arrivals and generally distributed cell residence time. Based on the derived statistics, we obtain the optimal movement threshold and develop an optimal sequential paging scheme. Numeric results are presented to show that the proposed optimal sequential paging scheme outperforms existing sequential paging schemes over a wide range of call-to-mobility ratios. [ABSTRACT FROM AUTHOR]

This paper presents the performance analysis of a conditional diversity combining scheme, which is called output-threshold maximum ratio combining (OT-MRC), over the independent identically distributed Nakagami-m with integer m and independent but not necessarily identically distributed Rayleigh fading channels. Based on the closed-form expressions for the statistics, the authors analyze the performance of the OT-MRC systems and compare it with that of the conventional MRC systems. They exploit the tradeoff between complexity and performance in this scheme over the Nakagami fading channels and see the effect of unbalance of the fading gains over multiple diversity branches on the performance of OT-MRC. They show, from some numerical examples, that the OT-MRC scheme, when compared with the MRC scheme, can reduce the complexity significantly without losing the required performance. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of two control parameters of the Universal Mobile Telecommunications System (UMTS) handoff algorithm on the total peak base station (BS) transmit power required per mobile station (MS) to satisfy a prescribed outage probability. Denoting the set of BSs supporting an MS in soft handoff (SH) as the active group, the first parameter n is the maximum allowed size of the active group. The second, denoted r, is the upper limit on the difference between the path loss of the dominant BS and that of any other member of the active group, where the term "dominant" refers to the BS with the lowest path loss to the MS. It was assumed that the MS is equipped with a Rake receiver capable of performing "maximal ratio combining" of the signals it receives from the transmitting BSs. General analytical derivations along with results derived for specific situations through numerical integration are presented. [ABSTRACT FROM AUTHOR]

In multi-service wireless networks, asymmetric bandwidth allocation has been proposed to satisfy the requirements of asymmetric traffic load introduced by some data applications. However, it is difficult to promptly adjust bandwidth allocation on uplink and downlink according to the dynamics of traffic load. Inappropriate call admission control (CAC) policies in this environment may admit superfluous real-time (RT) calls or non real-time (NRT) calls and thus lead to low bandwidth utilization. In this paper, we propose and evaluate two new CAC schemes to address the problems caused by the mismatch of bandwidth allocation and traffic changing in multi-service wireless networks with bandwidth asymmetry between uplink and downlink. By determining the admissible regions for the RT calls and the NRT calls, the proposed schemes prevent the calls of a specific class from overusing the bandwidth resources. Mathematical analysis and simulation experiments are employed to study and compare the performance of the proposed schemes and the existing schemes. The numerical results show that the proposed schemes can achieve better performances in terms of call blocking probability and bandwidth utilization compared to some existing schemes, even those performing well in bandwidth asymmetry wireless networks. [ABSTRACT FROM AUTHOR]

In this paper, a novel mathematical approach to evaluate the performance of adaptive or flexible resource allocation (FRA) strategies with uniform quality of service (QoS) provisioning in multiservices wireless cellular networks, in terms of the call blocking probabilities and transmission delay, is proposed. FRA strategies improve channel utilization by dynamically adjusting user transmission rates. Based on the available cell capacity, the bandwidth offered to users can be adjusted in accordance with the elasticities of their service types. When an FRA strategy provides similar QoS to all the calls of the same service type, it is known as an FRA with uniform QoS provisioning. In multiservices wireless cellular networks, calls can be identified as belonging to one of four service classes (i.e., conversational, streaming, interactive, and background). Transmission delay is one of the most important performance measurements of interactive and background service classes. Transmission delay, however, has not been addressed in previous studies on FRA with uniform QoS provisioning either in conjunction with interactive or background service classes. This is because such studies have been based on the Markov property of the negative exponential distribution of the service time which lacks time delay information. The analytical approach in this paper is based on the fact that in FRA strategies with uniform QoS provisioning, calls of all service types tend to use an average number of resources (i.e., the number of resources available for a service type divided by the number of active users of that service type). This feature facilitates the assessment of the mean service lime and the number of resources allocated to a call during its lifetime and, consequently, a call's transmission delay. The study also considers the fact that the probability density function (pdf) of the normalized transmission delay is almost a symmetrical function and has low variance. The accuracy of the proposed mathematical analysis is then corroborated by means of semianalytical methods and by discrete event computer simulation. [ABSTRACT FROM AUTHOR]

Safety-critical applications form the main motivation for intelligent transportation systems (ITSs). Studying the major concerns in such applications, i.e., delay and reliability, through mathematical analysis is extremely beneficial because it enables us to design optimized schemes. Such analysis is, however, challenging due to the dynamics of such a network. In this paper, we present a mathematical model to study delay and reliability of emergency message dissemination in vehicular networks. First, a basic model is presented using simplified assumptions about the channel. Then, we enhance the model using Rician distribution as a realistic fading model for vehicular environment. We make some interesting observations from the presented model. First, the end-to-end reliability has a fairly fast transition over time, which we formally prove in this observation. The second observation from the analytical model confirms the fact that using the vehicle density on the road is a good metric for setting the right forwarding probability in vehicles. We exploit this conclusion and propose a completely distributed forwarding strategy. Simulation studies indicate that our model does capture the delay characteristics of vehicular networks. It also affirms the effectiveness of our warning dissemination scheme in terms of delay and single-hop reliability in comparison with other well-known routing methods. We believe that this is a promising step toward accurate characterization of communication delay and reliability in vehicular networks. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate the outage performance of “partner cooperation” based on opportunistic Decode-and-Forward with constrained partial selection and reactive relaying strategies in dual-hop cooperative Nakagami-m fading links. The source/destination, which is based on the unique knowledge of local channel state information, selects the best relay to increase the chances of cooperation in both uplink and downlink communications when the direct link is also available. After deriving new expressions for the cumulative distribution functions of the variables of interest, the outage probability of the system is obtained in closed-form. We also derive the \epsilon-outage capacity in different particular cases, and the obtained results—when the channel model is reduced to a Rayleigh fading—either are new or correspond to those previously obtained in other works. Simulation results confirm the accuracy of our analysis for a large selection of system and fading parameters and provide a new insight into the design and optimization of cooperative configurations. [ABSTRACT FROM PUBLISHER]

In this paper, we introduce a cooperative collisionavoidance (CCA) scheme for intelligent transport systems. Unlike contemporary strategies, the envisioned scheme avoids flooding the considered vehicular network with high volumes of emergency messages upon accidental events. We present a clusterbased organization of the target vehicles. The cluster is based upon several criteria, which define the movement of the vehicles, namely, the directional bearing and relative velocity of each vehicle, as well as the intervehicular distance. We also design a risk-aware medium-access control (MAC) protocol to increase the responsiveness of the proposed CCA scheme. According to the order of each vehicle in its corresponding cluster, an emergency level is associated with the vehicle that signifies the risk of encountering a potential emergency scenario. To swiftly circulate the emergency notifications to collocated vehicles to mitigate the risk of chain collisions, the medium-access delay of each vehicle is set as a function of its emergency level. Due to its twofold contributions, i.e., the cluster-based and risk-conscious approaches, our adopted strategy is referred to as the cluster-based risk-aware CCA (C-RACCA) scheme. The performance of the C-RACCA system is verified through mathematical analyses and computer simulations, whose results clearly verify its effectiveness in mitigating collision risks of the vehicles arising from accidental hazards. [ABSTRACT FROM AUTHOR]

For the two-branch space-time (ST) block-coded direct-sequence code-division multiple-access (DS-CDMA) systems, the impacts of a time-varying multipath channel on the downlink transmission are analyzed. By considering the systems using the random binary spreading code (RBSC) and deterministic binary spreading code (DBSC), the effects of the multipath interference and multiuser interference are included in the analyses of the bit-error rate and bit-error outage. Also, for the performance analysis of the system employing the decision-feedback (DF) detector, the effect of error propagation is taken into account. It is known that enlarging the spreading factor can enhance the interference-rejection ability of a DS-CDMA system and, hence, can improve the performance. However, it also lengthens the symbol duration and, thus, stiffens the diversity penalty resulting from the channel variation within an ST-code-word duration. Thus, a moderate spreading factor should be chosen. In this paper, for the RBSC system using the simple-maximum-likelihood (SML) detector, we derive an optimum spreading factor that is optimum in the minimum-error-probability sense. Numerical results have revealed that the derived optimum spreading factor is a good estimate of the ones for the DBSC systems using the SML, zero-forcing, and DF detectors. Therefore, it is very useful for system designers in determining the system parameters. [ABSTRACT FROM AUTHOR]

In this paper, a highly efficient blind band-edge filter-based technique is introduced for the estimation of carrier frequency offsets that are much larger than the subcarrier spacing in orthogonal-frequency-division-multiplexing (OFDM) systems. The proposed scheme is shown to have a high accuracy and an acquisition range that is much larger than the subcarrier spacing limitation inherent to other synchronization techniques. The performance of the proposed system as well as other well-referred techniques is assessed over additive white Gaussian noise and frequency-selective fading channels using different pulse shaping filters. [ABSTRACT FROM AUTHOR]