In Internet-of-Things (IoT) edge inference systems, Softmax is a core component in multi-class classification heads and attention mechanisms. In resource-constrained IoT edge devices, conventional numerically stable implementations require multiple passes over the data, which increases memory band-width pressure and leads to hardly predictable variable processing latency. This paper presents Conditional Online Softmax, a control-flow-predictable algorithm that fuses reduction and normalization into a single pass to cut memory traffic while preserving numerical stability. We pair it with a branch-free vectorized kernel implemented using RISC-V Vector Extension (RVV) intrinsics, which better exploits instruction-level parallelism on low-core-count edge processors. On a standard RISC-V edge platform, Conditional Online Softmax achieves a 7% speedup over the numerically stable safe Softmax, and a 43% speedup over the original online Softmax. Furthermore, manual RVV vectorization delivers a 2:9× speedup compared to GNU Compiler Collection (GCC) auto-vectorization. The resulting design reduces cache misses and improves latency predictability, making it a drop-in replacement for Softmax in streaming and micro-batch pipelines commonly found in IoT gateways and sensor nodes. The implementation is portable across vector length (VLEN) and standard element width (SEW) settings and requires no specialized accelerators, which simplifies its deployment in practical IoT edge inference systems.

This work proposes two approaches, i.e., Fed-KD and Fed-TKD, to enhance communication efficiency with federated learning (FL) in Industrial Internet of Things (IIoT) radar sensing systems, which face the challenge in transmitting large volumes of sensitive data while still ensuring privacy. Fed-KD leverages knowledge distillation to transfer knowledge from complex teacher networks to simpler student models in order to reduce communication overhead in bandwidth-constrained environments. Fed-TKD improves communication efficiency further by applying tensor decomposition to reduce parameter redundancy. Experimental results using an industrial IoT radar imagery dataset show that both methods can significantly reduce communication costs while maintaining a high model accuracy, making them especially suitable for privacy-preserving industrial IoT sensing applications. Furthermore, experimental results with both IID and non-IID data distributions confirm the robustness of the proposed methods in heterogeneous environments.

In this paper, we investigate fairness-aware intelligent reflecting surface (IRS)-assisted multiple-user uplink communication systems. Current IRS-assisted communication methods do not consider the fairness issues of communication performance for multiple users. We formulate the IRS phase shift and receive beamforming design as an α-fair utility maximization problem by considering different fairness metrics, including zero fairness and proportional fairness, followed by an iterative algorithm to solve this problem. To overcome the high computational complexity of the iterative algorithm, we introduce a deep learning-based framework, which leverages convolutional neural networks with residual and attention mechanisms for real-time phase shift prediction. Extensive numerical simulations demonstrate that the proposed method achieves performance comparable to the traditional approach while significantly reducing computational overhead, making it a promising solution for future fairness-aware IRS-assisted communication systems.

Carrier frequency offset, Doppler shift, and phase noise degrade the orthogonality of subcarriers in Orthogonal Frequency Division Multiplexing (OFDM) systems, leading to inter-carrier interference (ICI). While a parallel cancellation (PC) schemes have been proposed to mitigate ICI, namely, Space-Time Parallel Cancellation (STPC). STPC has a lower bit error rate (BER) at the cost of double transmission time in time division multiplexing (TDM) mode when Doppler velocity exists in comparing with space-time code (STC) OFDM. In this paper, a new 2-path STPC algorithm is developed without TDM operation at the transceiver and is enhanced by integrating convolutional coding with an optimal code vector and Viterbi decoding for forward error correction (FEC). The proposed scheme guarantees a lower error floor with increasing bandwidth, as the redundancy introduced by FEC. Performance evaluations demonstrate that our uncoded and coded new STPC-OFDM scheme achieves superior bit error rate (BER) performance compared to the uncoded and coded STC-OFDM approach, making it a robust and bandwidth-efficient solution for OFDM-based communication systems.

With the increasing prevalence of Internet of Things (IoT) devices, security vulnerabilities and malware infections have emerged as significant risks. To address these challenges, advanced vulnerability detection tools are essential for enhancing IoT security assessments. In this study, we analyzed common vulnerabilities and evolving attack methodologies to develop improved detection techniques. Our research focuses on two key areas: 1) comprehensive vulnerability detection and 2) malware infection testing strategies. Through on-site testing and detailed analysis, we identified prevalent security flaws in IoT devices and developed a suite of tools tailored for detecting these vulnerabilities. Additionally, we discovered that some devices exhibit inherent immunity to specific malware strains, emphasizing the need for novel malware infection detection strategies. Real-world evaluations uncovered previously unknown vulnerabilities and weaknesses, revealed widespread susceptibility to DoS attacks, and demonstrated that not all devices are vulnerable to malware infections. These findings confirm the effectiveness of our approach in identifying risks and enhancing IoT security.

Deep learning (DL) has been widely utilized in automatic modulation classification (AMC), and its performance depends largely on the presence of high-quality datasets. Motivated by this fact, this work addresses the AMC challenges in data-limited IoT environments, proposing a framework combining few-shot meta-learning and federated learning for resource-constrained devices, where edge nodes use meta-learning for training with global updates via federated averaging (FedAvg). The system aggregates samples from multiple nodes while still maintaining data security. Simulations involved 11 modulation types with varying SNRs, 100 client nodes, and 10 rounds of federated learning. The iterative process includes loading pre-trained parameters, performing local training, averaging local parameters, and updating global parameters. The obtained results show 70% post-training testing accuracy, with a consistently good performance during federated iterations. The results demonstrated the effectiveness of the proposed framework in data-scarce IoT scenarios, offering a robust performance across varying signal qualities while minimizing energy consumption and communication overhead, which is crucial for IoT device longevity and network scalability, highlighting framework’s potential for real-world applications in distributed modulation recognition.

This paper explores the fairness issues on physical layer security (PLS) in intelligent reflecting surface (IRS) assisted multiple-user uplink systems. Due to unknown instantaneous eavesdropper channel state information (CSI), it is not possible to acquire exact secrecy rate of a PLS system. In this paper, we introduce secrecy outage probability (SOP) as a security metric, instead of secrecy rate as used in most existing works, and formulate a minimization problem of maximum (min-max) SOP among multiple users. To solve this problem, we propose two independent approaches: one is alternating optimization (AO) and the other is a deep learning based (DL) scheme. The AO scheme decouples the problem into two sub-problems to alternately optimize phase shift matrix and receiver beamforming vectors, which gives a near-optimal performance but with a high complexity. The DL scheme, on the other hand, works based on neural networks through offline training, which is used for online generation of phase shift matrix and receiver beamforming vectors with a lower complexity. As traditional self-supervised neural networks cannot achieve a good solution to the max-min problem, we design a multiple-stage booster (MSB) framework to solve this problem. Simulations demonstrated that SOP is improved significantly with the proposed schemes compared to benchmark schemes. In particular, the AO scheme outperforms the DL-based approach slightly at the cost of a relatively high computational complexity.

Orthogonal time frequency space (OTFS) schemes have received significant attention recently, especially for high mobility scenarios. OTFS is a new 2-dimensional (2D) modulation and multiplexing technique proposed as an alternative to orthogonal frequency division multiplexing (OFDM) for next generation standard and beyond. In implementation, the Inverse Symplectic Fast Fourier Transform (ISFFT) performs as a pre-processing block while the Symplectic Fast Fourier Transform (SFFT) works as a post-processing block to OFDM systems. While OTFS is a robust orthogonal modulation scheme, the received delay-Doppler (DD) domain signal faces inter-carrier interference (ICI) and interference to other users as the exactness of decomposing the channel, though theoretically orthogonal, is still at best an approximation of reality in time varying environment. At high carrier frequencies as in the 6GHz band used for Wi-Fi 6E systems, the residual carrier frequency offset (CFO) of practical crystals induces a non-negligible ICI, leading to performance degradation. By integrating the ICI parallel cancellation (PC) scheme into the zero-padding (ZP) OTFS system, we present two novel two-path PC OTFS systems with two architectures to mitigate the CFO). Without estimating the residual CFO, both systems improve bit error rate (BER). Simulation shows several dB of BER improvement for two-path PC OTFS systems over the regular OTFS system when residual CFO exists.

The Internet of Things (IoT) network is a complex system interconnected by different types of devices, e.g., sensors, smartphones, computers, etc.. Community detection is a critical component to understand and manage complex IoT networks. Although several community detection algorithms were proposed, they in general suffer several issues, such as lack of optimal solutions and scalability, and difficulty to be applied to a dynamic IoT environment. In this work, we propose a framework that uses Distributed Community Detection (DCD) algorithms based on Gromov-Wasserstein (GW) metric, namely GW-DCD, to support scalable community detection and address the issues with the existing community detection algorithms. The proposed GW-DCD applies Gromov-Wasserstein metric to detect communities of IoT devices embedded in a Euclidean space or in a graph space. GW-DCD is able to handle community detection problems in a dynamic IoT environment, utilizing translation/rotation invariance properties of the GW metric. In addition, distributed community detection approach and parallel matrix computations can be integrated into GW-DCD to shorten the execution time of GW-DCD. Finally, a new metric, i.e., Gromov-Wasserstein driven mutual information (GWMI), is derived to measure the performance of community detection by considering internal structure within each community. Numerical experiments for the proposed GW-DCD were conducted with simulated and real-world datasets. Compared to the existing community detection algorithms, the proposed GW-DCD can achieve a much better performance in terms of GWMI and the runtime.

Physical layer covert communication is a crucial secure communication technology that enables a transmitter to convey information covertly to a recipient without being detected by adversaries. Unlike typical cryptography and physical layer security systems that concentrate on protecting the sent signal content, covert communications seek to conceal the existence of legitimate transmission. Thus, with beyond fifth-generation (B5G) wireless communications, covert communications can operate in tandem or as a supplement to conventional security techniques. We provide an extensive overview of the basic theories and several strategies in physical layer covert communications in this article. In particular, we go into great detail about the basic theories of physical layer covert communications, such as channel models, codes, secret keys, and covertness metrics, as well as various covert schemes in progressively more complicated scenarios, such as covert communications in single-antenna and multiantenna three-node systems and covert communications in jammer-and relay-aided systems. In addition, we identify the challenges and future directions for research on covert communications in B5G wireless networks.

Due to the constraints on power supply and limited encryption capability, data security based on physical layer security (PLS) techniques in backscatter communications has attracted a lot of attention. In this work, we propose to enhance PLS in a full-duplex symbiotic radio (FDSR) system with a proactive eavesdropper, which may overhear the information and interfere legitimate communications simultaneously by emitting attack signals. To deal with the eavesdroppers, we propose a security strategy based on pseudo-decoding and artificial noise (AN) injection to ensure the performance of legitimate communications through forward noise suppression. A novel AN signal generation scheme is proposed using a pseudo-decoding method, where AN signal is superimposed on data signal to safeguard the legitimate channel. The phase control in the forward noise suppression scheme and the power allocation between AN and data signals are optimized to maximize security throughput. The formulated problem can be solved via problem decomposition and alternate optimization algorithms. Simulation results demonstrate the superiority of the proposed scheme in terms of security throughput and attack mitigation performance.

Reconfigurable intelligent surface (RIS) is a programmable metasurface composed of sub-wavelength meta-atoms and a controller. Recent research works have shown that multi-layer RISs can form over-the-air neural networks. This enables support for a variety of tasks, including image recognition, mobile communication coding-decoding, and real-time multi-beam focusing. In this work, we present a RISs-based over-the-air joint source channel coding (JSCC) scheme, named as AirJSCC, where multi-layer RISs are used to realize wave-based complex-valued neural networks (CVNNs). AirJSCC transforms the computation process inherited in JSCC into the transmission of wireless signals through RISs, and thus many benefits, such as light-speed computation and high parallel processing capability, can be achieved. To the best of our knowledge, this is the first deep JSCC scheme that incorporates RISs and CVNNs. Simulation results demonstrate that AirJSCC achieves better image reconstruction performance compared with the baseline scheme, even under low signal-to-noise ratio (SNR) and limited bandwidth, and exhibits robustness against varying channel conditions.

Space–air–aqua integrated network (SAAIN) is an emerging maritime network architecture to support reliable and timely communications. Considering the computation capability and information security in maritime transportation systems, this work proposes a double-edge secure offloading scheme, where both base-station (BS) and satellites provide secure mobile-edge computing services for delay-sensitive applications. Specifically, maritime mobile users may offload their computation tasks adaptively to the BS or satellites securely relayed by unmanned aerial vehicles (UAVs). To minimize offloading delay, we formulate an optimization problem to allocate the transmit power cooperatively via UAVs’ trajectory optimization. Moreover, jamming UAVs are deployed to protect the offloading process. For such a nonconvex optimization problem, two iterative algorithms are proposed to determine the transmit power and design the UAVs’ trajectories. Numerical results show the effectiveness of the proposed scheme in terms of offloading delay.

Wireless energy harvesting (WEH) enabled Device-to-Device (D2D) communication emerges as an effective technique to improve spectral and energy efficiencies. However, D2D users are usually power-constrained devices that may be monitored or attacked by adversaries easily. To confuse randomly distributed adversaries in WEH-enabled D2D underlaying cellular networks, power beacons (PBs) can be used to send jamming signals in idle time slots. A time slot is divided into two sub-slots, i.e., WEH sub-slot and covert transmission sub-slot. The energy collected by a D2D transmitter in the first sub-slot is used to support possible covert transmission in the second sub-slot. The performance of the proposed scheme is measured by covert throughput, which is defined as D2D communication rate with several constraints, e.g., wireless energy harvesting, covertness, and cellular link communication requirements. The closed-form expressions of energy outage probability, covert probability, and desired link connection outage probabilities are derived. The minimum covert probability is used to measure covertness. Moreover, an alternating optimization algorithm is adopted to maximize covert throughput. The analytical results are compared with Monte-Carlo simulation results to verify the analytical approach. In addition, the impacts of different parameters (e.g., density of PBs and covert signal transmit power) on covert performance are evaluated.

This article investigates physical layer security (PLS) in reconfigurable intelligent surface (RIS)-assisted multiple-input multiple-output multiple-antenna-eavesdropper (MIMOME) channels. Existing researches ignore the problem that secrecy rate can not be calculated if the eavesdropper’s instantaneous channel state information (CSI) is unknown. Furthermore, without the secrecy rate expression, beamforming and phase shifter optimization with the purpose of PLS enhancement is not available. To address these problems, we first give the expression of secrecy outage probability for any beamforming vector and phase shifter matrix as the RIS-assisted PLS metric, which is measured based on the eavesdropper’s statistical CSI. Then, with the aid of the expression, we formulate the minimization problem of secrecy outage probability that is solved via alternately optimizing beamforming vectors and phase shift matrices. In the case of single-antenna transmitter or single-antenna legitimate receiver, the proposed alternating optimization (AO) scheme can be simplified to reduce computational complexity. Finally, it is demonstrated that the secrecy outage probability is significantly reduced with the proposed methods compared to current RIS-assisted PLS systems.

In this paper, we propose a multi-roadside unit (RSU) assisted mobile crowd computing framework for intelligently connected vehicle (ICV) networks, where vehicles within RSUs’ coverage act as workers to provide their computation and communication resources for computing resource limited vehicle user equipments (VUEs). Physical (PHY) layer security is used to secure computation task offloading and results feedback in time-varying vehicular channels. Artificial noise (AN) assisted adaptive wiretap coding is adopted to enhance the security of offloading links. With PHY security, the intended receiver can decode secret message while eavesdropper cannot. A modified exhaustive two-dimensional (2D) search algorithm is proposed to optimize transmission rate and secrecy rate in an effective secrecy throughput maximization problem, and a multi-agent twin delayed deep deterministic policy gradient algorithm (MATD3) is utilized to assign VUEs’ tasks without a central controller, where a reward function is defined according to the computing costs, including execution time, energy consumption, and price paid for computing. Finally, simulations verify the effectiveness of the proposed framework.

The COVID-19 pandemic has caused a high rate of infection, and thus effective epidemic prevention measures of avoiding the second spread of COVID-19 in hospitals are major challenges for healthcare workers. Hospitals, where medicines are collected, are vulnerable to the rapid spread of COVID-19. Using the remote health monitoring technology of the Internet of Things (IoT) to automatically monitor and record the basic medical information of patients, reduce the workload of healthcare workers, and avoid direct contact with healthcare workers to cause secondary infections is an important research topic. This research proposes a new artificial intelligence solution based on the IoT, replacing existing medicine stations and recognizing medicine bags through the state-of-the-art optical character recognition (OCR) model and PP-OCR v2. The use of OCR in identification of medicine bags can replace healthcare workers in data recording. In addition, this research proposes an administrator management and monitoring system to monitor the equipment and provide a mobile application for patients to check the latest status of medicine bags in real time, and record their medication times. The results of the experiments indicate that the recognition model works very well in different conditions (up to 80.76% in PP-OCR v2 and 94.22% in PGNet), which supports both Chinese and English languages.

In order to meet mobile users’ unprecedented communication demands and the goal of global seamless communication, space–air–ground-integrated networks (SAGINs) have attracted lots of attention in recent years. The existing works related on air segment mainly discussed unmanned aerial vehicles (UAVs), airships, and balloons near the space. However, they neglected many other valuable resources, such as civil aircrafts (CAs). Moreover, communication problems for remote areas and emergency scenarios (such as disasters and hot-spot areas) have not been solved thoroughly. Motivated by these facts, we introduce CAs to enhance the current SAGIN and present a novel architecture called “CAs augmented space–air–ground-integrated vehicular networks” (CAA-SAGIVNs). The proposed network architecture makes breakthrough in three main aspects: 1) a normal network architecture; 2) collaboration with multiple sky access platforms (SAPs); and 3) service-oriented fair allocation. Although CAA-SAGIVN can bring out many benefits, it also faces more challenges due to its high mobility and cross-layer characteristics. Therefore, we provide an exhaustive review of state-of-the-art works on modeling, mobility management, solutions of service-oriented allocation in SAGIN. On the basis of the preliminary investigation and discussion, some open issues are identified as possible future research directions.

Although 5G networks have enabled mobile users to get a better experience, task scheduling remains challenging for massive Internet of Things (IoT) devices in remote areas. This paper investigates the task scheduling problem for delay-aware IoT applications in civil aircraft-augmented space-air-ground integrated networks (CAA-SAGIN), where the normalized sky access platforms (SAPs) can collect and forward the terrestrial tasks. Specifically, we first propose an access control scheme for a non-preemptive priority queuing system and a transmission control scheme with cross-layer optimization. Secondly, considering the uncertain distribution of the transmission numbers and generated data, we formulate a robust two-stage stochastic optimization problem of delay minimization. With the proposed robust task scheduling with risk aversion (RTS-RA) algorithm, the original problem can be decomposed into two subproblems, which can be further transformed into tractable semi-definite program (SDP) problems respectively. Simulation results show that the cross-layer optimization scheme can achieve a good tradeoff between delay and throughput. Also, the RTS-RA algorithm outperforms the exiting offloading schemes in terms of end-to-end delay, transmitted data, and energy consumption with lower computational complexity.

Ultra-dense network (UDN) is one of the most important techniques to support massive number of different types of user equipments (UEs). It is essential to understand the impact of densification on various system performance metrics in future cellular networks. Two contradictory conclusions were made from the existing research works. Some works indicated that the network densification has reached its upper limit and deploying more UEs may only reduce UDN performance; whereas the others claimed that more UEs can still be supported by deploying more small cell base stations (SBSs). The existence of above different conclusions is due mainly to the difficulties to find an effective means to take into account different path loss models in UDNs. In this paper, we aim to propose a unified analytical approach, which is effective to simplify UDN performance analysis based on different path loss models with a low computation complexity. Using the proposed approach with idle mode of SBSs enabled, we can identify the impact of the densities of SBSs and UEs on various performance metrics, i.e., coverage probability and average area spectral efficiency (ASE). The conclusions will be supported by the analytical and simulation results verifying the effectiveness of the proposed unified analytical approach.

In this paper, we propose a secure computationoffloading scheme (SCOS) in intelligently connected vehicle (ICV) networks, aiming to minimize overall latency of computing via offloading part of computational tasks to nearby servers in small cell base stations (SBSs), while securing the information delivered during offloading and feedback phases via physical layer security. Existing computation offloading schemes usually neglected time-varying characteristics of channels and their corresponding secrecy rates, resulting in an inappropriate task partition ratio and a large secrecy outage probability. To address these issues, we utilize an ergodic secrecy rate to determine how many tasks are offloaded to the edge, where ergodic secrecy rate represents the average secrecy rate over all realizations in a time-varying wireless channel. Adaptive wiretap code rates are proposed with a secrecy outage constraint to match time-varying wireless channels. In addition, the proposed secure beamforming and artificial noise (AN) schemes can improve the ergodic secrecy rates of uplink and downlink channels even without eavesdropper channel state information (CSI). Numerical results demonstrate that the proposed schemes have a shorter system delay than the strategies neglecting time-varying characteristics.

Sparse code multiple access (SCMA), which helps improve spectrum efficiency (SE) and enhance connectivity, has been proposed as a non-orthogonal multiple access (NOMA) scheme for 5G systems. In SCMA, codebook design determines system overload ratio and detection performance at a receiver. In this paper, an SCMA codebook design approach is proposed based on uniquely decomposable constellation group (UDCG). We show that there are N+1 ( N ≥ 1) constellations in the proposed UDCG, each of which has M (M ≥ 2) constellation points. These constellations are allocated to users sharing the same resource. Combining the constellations allocated on multiple resources of each user, we can obtain UDCG-based codebook sets. Bit error ratio (BER) performance will be discussed in terms of coding gain maximization with superimposed constellations and UDCG-based codebooks. Simulation results demonstrate that the superimposed constellation of each resource has large minimum Euclidean distance (MED) and meets uniquely decodable constraint. Thus, BER performance of the proposed codebook design approach outperforms that of the existing codebook design schemes in both uncoded and coded SCMA systems, especially for large-size codebooks.

Ultra-dense network (UDN) is one of the enabling technologies in the fifth generation (5G) wireless communications, and the application of joint transmission (JT) is extremely important to deal with severe inter-cell interferences in UDNs. However, most of the current works done in performance analysis on JT schemes in the literature were based largely on simulation results due to the difficulties in quantitatively identifying the number of desirable and interfering transmitters in a UDN setup. In this work, we are motivated to propose an analytical approach to investigate the performance of JT schemes with a unified approach based on stochastic geometry, which is in particular useful for studying different JT schemes as well as conventional transmission schemes without JT. Using the proposed approach, we can unveil the statistical characteristics (i.e., expectation, moment generation function, and variance) of desirable signal and interference powers of a given user equipment (UE), and thus the system performances, such as average signal-to-interference-plus-noise ratio (SINR) and area spectral efficiency, can be evaluated analytically. The simulation results are illustrated to verify the effectiveness of the proposed approach.

Long Term Evolution - Licensed Assist Access (LTELAA) was proposed in 3GPP TR 36.889 to support unlicensed data access assisted by licensed spectrum. However, owing to wide applications of unlicensed wireless technologies such as WiFi, we need to deal with coexistence and collision issues. In traditional LBT protocol, LTE-LAA UE performs CCA to sense whether a channel is idle or not and wait until the channel becomes clear before transmission. In this way, LTE-LAA UE may suffer a low probability of successful first-trial transmission and need to schedule for retransmissions. To tackle this problem, we proposed a spectrum map assisted unlicensed resource allocation scheme for LTE-LAA SBSs to enhance successful transmission probability. Once LTE-LAA UEs have more opportunities to access the unlicensed channels, they can save cost for licensed resources and improve the overall spectrum utilization efficiency. Simulation results show that the proposed scheme can improve the sum rates of LTE-LAA SBS and LTE-A MBS, and significantly reduce channel collisions with other WiFi APs and LTE-LAA UEs.

Nowadays, for different purposes and contexts, we interact with a lot of different smart devices in our daily lives. Most of these devices are connected to the Internet and are therefore commonly referred to as the Internet of Things (IoT). Mobile edge computing (MEC) has recently evolved as an emerging technique by moving the computing and storage resources from the cloud to the edge of the network. The MEC supports IoT devices to improve their efficiency and scalability; helps to reduce latency delay for real-time applications, bandwidth bottlenecks, and energy consumption; and delivers contextual information processing. MEC offers many features and capabilities, such as access to a multitude of network interface (from 4G and 5G to Wi-Fi), support for device mobility, device context, geo-location awareness, and geographical distribution. Such attributes can support the real-time processing requirements of the Internet of Everything application, such as patient care, disaster management and detection (e.g., earthquakes), and flood monitoring. However, to fully exploit the potential of MEC in the IoT applications, many challenges need to be addressed, such as issues related to IoT Big Data, effective management of data storage and computing, privacy and security concerns, and innovative and emerging communication paradigm (e.g., 5G), require new architectures, applications, and methods.

This paper presents a probabilistic decision-making framework for rear-end collision avoidance systems, focusing on modeling the impact of major collision-causing factors on the occurrence of accidents. Decisions on when and how to assist drivers are made using a Bayesian network approach according to collision risk evaluation results, given a prior probabilistic knowledge. The structure of the Bayesian network model is learnt using a K2 algorithm with a practical dataset. To provide adequate response time for drivers, we also predict collision probability in the next monitoring interval using a Kalman filter model. The prediction accuracy is evaluated with different use cases and compared to real scenarios with the obtained dataset. To make our framework more relevant to practical applications, we also discuss the corresponding safety control strategies by classifying collision risk into high and low levels. In addition, the proposed model is evaluated through experiments including simulations and road tests. In the simulations, the algorithms are tested in different scenarios with various configurations of weather conditions, driver response capability, and vehicular dynamics. In order to demonstrate collision avoidance performance, the proposed model is also compared to existing schemes. In the road tests, the algorithm is embedded into an unmanned vehicle with predefined parameters to evaluate the impact of computational complexity on rear-end collision avoidance. Numerical results show that the proposed model provides an accurate estimation for car-following collision risk with a relatively low complexity, taking into account the impacts of vehicle dynamics, driver reaction capacity, and external environment on rear-end collisions.

In wireless powered communication networks (WPCNs), base station (BS) and power beacons (PBs) can offer supplement power for uplink transmission of user equipments (UEs). However, the aggregate power consumption of massively deployed PBs may exceed that of a BS. We propose a non-uniform deployment scheme for PBs in WPCNs, where a cell is divided into inner and outer areas, such that BS and PBs can cooperate to power UEs. To be more specific, a BS located in the center of a cell provides downlink power supply for the inner area UEs and uplink information decoding for all the UEs in the cell; while the PBs power UEs in the outer area. With multiple antennas, maximum ratio transmission and maximum ratio combining are adopted for downlink energy beamforming and uplink information reception. Considering a finite area of the network, we derive the distribution of the distance from a non-center-located UE to its nearest PB in the outer area. An optimization problem is formulated to minimize total average power consumption while satisfying BS average transmission power constraint and coverage probability threshold. Moreover, coverage probability is derived for performance evaluation. The numerical results show that the power consumption of the proposed scheme is reduced significantly compared to PB-only WPCNs.

Virtual multiple-input multiple-output (V-MIMO) is an effective way to achieve a diversity gain in uplink channels. A well-designed user pairing scheme in V-MIMO systems can exploit multiuser diversity to yield a significant performance gain. This paper investigates joint user pairing and power allocation problems in V-MIMO systems to maximize energy efficiency (EE) while guaranteeing spectrum efficiency (SE) under a total power consumption constraint. We formulate such a joint problem as a complex nonlinear optimization problem, which can be solved in two steps in M ×K V-MIMO systems, i.e., power allocation with known user group and joint user pairing and power allocation. A multi-level water-filling method is used, and an iterative algorithm based on the outputs from the first step is employed in the second step. Finally, we propose a joint user pairing and power allocation scheme to find a user group that can achieve the maximum EE with power allocation. Simulation results and performance comparisons illustrate improved performance of the proposed scheme. Compared to the existing user pairing approaches, the proposed scheme achieves a significantly high EE gain while guaranteeing SE of a V-MIMO system.

In a traditional complementary coded CDMA (CC-CDMA) system, signals are spread using the direct sequence (DS) technique to suppress multipath interference (MI) and multiple access interference (MAI). However, DS spreading suffers from very poor bandwidth efficiency. This paper proposes a scrambling-based CC-CDMA system, namely complementary coded scrambling multiple access (CCSMA). A CCSMA system offers a unified platform to integrate CC-CDMA, multiple-input multiple-output (MIMO), and scrambling technique seamlessly. Utilizing complementary codes in scrambling modulation, a CCSMA system is able to suppress MI and MAI effectively. With the help of space domain scrambling, the CCSMA system offers much higher bandwidth efficiency than a CC DS-CDMA system. Bit error rate and system capacity are evaluated explicitly in this paper, verifying that CCSMA is a viable multiple access technique for providing high capacity in downlink MIMO channels.

In this paper, we investigate profit maximization problem in wireless powered communication (WPC) systems, where the profit is defined as the difference between the revenue earned from uplink data transmission and total energy cost of the system. Since the efficiencies of energy harvesting (EH) circuits and battery storage are both non-linear, conventional linear models will lead to suboptimal resource allocation. In this paper, we propose to maximize the profit of a WPC system under nonlinear energy harvesting and storage models. In order to solve the non-convex optimization problem, we develop a piecewise linear EH model, which is more accurate than conventional linear EH models and existing non-linear EH models. Numerical results show that the power and time allocation design based on the non-linear energy harvesting and storage models outperforms that based on existing linear models in terms of the profit.

Artificial noise (AN) can be used to confuse eavesdroppers in a physical layer security system. One of the main issues concerned in AN schemes is how to improve secrecy capacities. Most existing AN schemes were proposed based on an assumption that the number of transmit antennas t is larger than that of receiver antennas r, such that they can utilize all r eigen-subchannels of a multiple-output multiple-input (MIMO) system to send messages, and use remaining t - r null spaces for transmitting AN signals. These AN signals null out legitimate receivers and degrade eavesdropper channels. However, transmitting messages in all eigen-subchannels is not always a good strategy. In particular, when the number of transmit antennas is constrained or even smaller than those of receivers, the secrecy capacities of legitimate receivers will be impaired significantly if using all eigen-subchannels for message transmission. To improve secrecy capacity, we propose an AN scheme where messages are encoded in s (which is a variable) strongest eigen-subchannels based on ordered eigenvalues of Wishart matrices, while AN signals are generated in remaining t - s spaces. We derive the average secrecy capacity of a single-user MIMO wiretap channel in the presence of an eavesdropper with multiple antennas. We show that the numerical results are in a good agreement with simulation results. The secrecy capacity of the proposed AN scheme can be improved by approximately 20% ~ 40% if compared with existing AN schemes.

Heterogeneous network (HetNet) plays an important role in the upcoming 5G communications. A HetNet in futuristic communications is envisaged to provide full access data services, such that users will always have the best service with their available radio resources. To realize a full access HetNet, statistical signal transmission (SST) can be an important enabling technique. SST exploits statistical features of ordinary signals as a data transmission scheme to carry information data. Due to its inherent unique properties, SST signal is more robust than ordinary signals in conventional quasi-static channels. However, detection of SST signals in fast time-varying channels will face challenges. Motivated by this fact, this paper aims to analyze the impact of fast time-varying channels to SST, and reveals negative effects of burst of deep fading channel-state (BDFC) and feature dissipation with out-of-phase (FDOP). Accordingly, we design an automatic re-transmission scheme and a fragmental feature capturing method to mitigate the aforementioned issues, respectively. Based on these studies, we propose a channel condition aware detection scheme for SST, aiming to enhance SST detection performance in fast time-varying channels. Numerical results show that with the proposed scheme, a maximum gain of more than 10 dB can be obtained under either BDFC or FDOP condition. Furthermore, it is also demonstrated that the proposed scheme is robust with imperfect channel coherence time information, which is attractive for practical applications.

In a cooperative network, users share information with each other to achieve a common target. Due to the concerns of privacy and cost, users may be reluctant to share genuine information with each other, which incurs the information trustworthiness problem. Most of the existing research attempts have proposed various mechanisms targeting to enhance the information credibility in various scenarios. However, the users' information sharing inclinations have not been well considered and modeled, which closely determine the information credibility in the network. In this paper, we study a trustworthy situation in cooperative networks and utilize the concept of reputation to model users' behaviors. Specifically, we propose two reputation learning methods based on both the peer-to-peer Bayesian learning and the social non-Bayesian learning models, and also propose a trustworthiness learning method based on the posterior estimation. Finally, simulations are conducted to verify the correctness and effectiveness of our theoretical analysis.

In this article, a network model incorporating both line-of-sight (LOS) and non-line-of-sight (NLOS) transmissions is proposed to investigate impacts of blockages in urban areas on heterogeneous network coverage performance. Results show that co-existence of NLOS and LOS transmissions has a significant impact on network performance. We find in urban areas, that deploying more BSs in different tiers is better than merely deploying all BSs in the same tier in terms of coverage probability.

With the growth of wireless transmission rate on user terminals, the backhaul network capacity becomes a bottleneck for improving the performance of future 5G ultra-dense cellular networks. Based on the wireless multi-hop relay technology, the backhaul network capacity of 5G ultra-dense cellular networks with multi-gateways is analyzed in this paper. Moreover, a minimum average hop number (MAN) algorithm is developed to improve the backhaul network capacity and energy efficiency of wireless backhaul networks for 5G ultra-dense cellular networks. Simulation results indicate there exist a stationary backhaul network capacity and a maximum energy efficiency of wireless backhaul networks when the density of small cell BSs is larger than the specified threshold.

Power amplifier (PA) is one of important components in wireless communication system. However, its non‐linear and memory characteristic cause harmful distortion, which significantly decreases the power efficiency and wastes electronic energy since the power of PA needs back off. To overcome the non‐linear and memory problems, a novel method based on distortion‐characteristic estimation algorithm is developed in this study. The main innovation of this study is manifested in two aspects: first, the authors use well designed training signals to estimate the distortion characteristic of the PA; second, a novel PA characteristic estimation algorithm is developed in order to calculate the optimal state of the ideal predistorter that behaves like the inverse characteristic of PA theoretically. Since this novel method does not require the error between original signals and feedback signals, it can avoid the significant errors caused by the time mismatch between source signals and feedback signals, and can increase the power efficiency of the PA. The simulation results validate the performance increase and the synchronisation‐error immunity over the existing simplified filter look up table method.

Exploring potentially useful information from huge amount of textual data produced by microblogging services has attracted much attention in recent years. An important preprocessing step of microblog text mining is to convert natural language texts into proper numerical representations. Due to the short-length characteristics of microblog texts, using term frequency vectors to represent microblog texts will cause “sparse data” problem. Finding proper representations of microblog texts is a challenging issue. In this paper, we apply deep networks to map the high-dimensional representations of microblog texts to low-dimensional representations. To improve the result of dimensionality reduction, we take advantage of the semantic similarity derived from two types of microblogspecific information, namely the retweet relationship and hashtags. Two types of approaches, including modifying training data and modifying the training objective of deep networks, are proposed to make use of microblog-specific information. Experiment results show that the deep models perform better than traditional dimensionality reduction methods such as latent semantic analysis and latent Dirichlet allocation topic model, and the use of microblog-specific information can help to learn better representations.

Smart metering utility network (SUN) is an emerging wireless technology for achieving intelligent control and information transfer in smart grid applications. However, the operation of SUNs on unlicensed bands is very sensitive to external interferences generated from other networks working on the same spectrum bands, such as wireless local area networks (WLANs). This paper aims to analyze packet error rate (PER) performance of a victim SUN receiver under the interferences of one or more WLAN transmitting nodes. To deal with a scenario leveraged by multiple random variables, such as offset between desired SUN data packet and WLAN packet, duration of WLAN data packet, and time interval between two consecutive WLAN packets, we propose a segmented packet collision model with the help of the probability theory. We first divide all the packets into small segments and then calculate the collision probability of each segment to acquire a more accurate bit error rate (BER). Then, based on different BERs in distinct segments of the desired SUN packet, average PER of the victim SUN receiver is obtained. Finally, the proposed analytical model is validated by simulations as an effort to evaluate PER performance of a SUN receiver under the WLAN interferences.

This paper aims to analyze coexistence performance and study the impact of WLAN interferences on SUNs. To quantitatively reveal packet error rate (PER) performance of SUNs under multiple WLAN interferers, a segmentation scheme for desired data packet with multiple interferers is developed, based on which a packet collision model in a multi-interferer environment is proposed. The new packet collision model captures interference scenarios in every time segment of a desired packet using the proposed segmentation scheme. It paves a way to make a precise estimation of the PER of SUNs under multiple WLAN interferers. Furthermore, we design a channel agility scheme for coexistence of SUNs and WLANs via improving the existing ones, and analyze its performance in interference identification and avoidance. The proposed scheme performs interference detection for every data packet, and thus, an SUN is able to avoid WLANs interferences in a timely manner.

Abstract Machine‐type communication (MTC) has attracted much attention due to the fact that it provides pervasive connections for billions of MTC devices, forming a basis for the Internet of things. Most works in the literatures on machine‐to‐machine (M2M) communications focused on media access layer (MAC) layer or other upper layer applications, such as e‐health, energy management and entertainment services. On the other hand, physical (PHY) layer plays a pivotal role in M2M communications. To accommodate a large number of MTC devices, M2M should be made efficient enough in terms of its power consumption and spectrum utilisation. Copyright © 2016 John Wiley & Sons, Ltd.

Abstract Visible light communication (VLC) is a newly emerging technology, which integrates communications and lighting purposes, and has become a very active research topic in the areas of wireless communications. It is expected to become an important part of the next generation wireless communications because of its unique features in using unlicensed spectrum, support of high data rate, and its resistance to electromagnetic interferences. In this survey paper, we begin with a review on the basis of photometry, which is used to establish channel models of VLC systems. Then, we will continue to address various issues on the fundamental characteristic features of VLC systems, the impact of indoor environments on system performance, and the analysis and discussions of five different types of typical VLC channel models and other related parameters in VLC channel models. Finally, in terms of the future works, we will show the possible follow‐up research focuses and directions as an effort to identify some new research topics on VLCs. Copyright © 2016 John Wiley & Sons, Ltd.

Abstract Smart grid technology has attracted a lot of attention in the communication and power engineering communities because of its high efficiency and environmental friendly power production and delivery. Smart grid communication networks act as not only a bridge between the end power users and the utility but also a backbone of the entire smart grid system. It is a challenging task to deploy smart grid communication networks because of interference‐intensive environment. The goal of this paper is to explore various issues on deployment of smart grid communication networks. The first half of this paper provides a survey on smart grid communication networks. The background of smart grid communication networks is introduced, and the IEEE 802.15.4g standard for smart utility networks is reviewed. In addition, the issues on network topology and routing protocols of smart utility networks are addressed, followed by the discussions on optimizing gateway deployment. Various methods for addressing coexistence issues in smart grid communication networks are presented. The second half of this paper is to propose a channel discovery algorithm exploiting the inherent CSMA/CA mechanism in the protocols. Because of the use of CSMA/CA mechanism and up‐to‐date channel status table, the proposed algorithm can quickly find an available channel for transmission when interferences occur. Copyright © 2014 John Wiley & Sons, Ltd.

Because of random deployment patterns of femtocells, interference scenarios in a heterogeneous cellular network can be very complicated because of its changing network topology. Especially when each eNodeB occupies a fixed bandwidth, interference management becomes much more difficult. The benefit of dynamic management for local resource optimation is limited. Recently, resource virtualization has been proposed as a dynamic resource management scheme to optimize network performance. In fact, resource virtualization is viewed as a more flexible model, in which mobile network service providers can control physical resources in a global scope. This paper presents a joint resource virtualization and allocation scheme for its applications in heterogeneous macro-femto-cellular networks. The proposed scheme involves two major processes. First, it virtualizes physical resources as logical resources. Second, it carries out logical resource allocation optimization globally and aggregates logical and physical resources for resource allocation. The proposed scheme takes into account spectrum reuse and frequency domain interference jointly in order to achieve a high spectral efficiency and provide rate-on-demand services to all users. Simulation results demonstrate the effectiveness of the proposed scheme. Copyright © 2016 John Wiley & Sons, Ltd.

Machine to machine (M2M) communication has raised significant interests. However, due to the massive number of machine type communication (MTC) devices that are anticipated to communicate using cellular networks, there is a major problem on efficient accommodation of the heavy Random Access (RA) loads from the MTC devices. Use of small cells has been specified to provide network densification by 3GPP. In this paper we investigate the use of small cells to support RA and the allocation of Zadoff-Chu sequences to the small cells, which are used to generate preambles for the RA procedure. Small cells can be deployed on demand to handle mainly RA loads from MTC devices, which may generate much less data traffic compared to human devices. It is demonstrated that, with small cell support, more random channel access opportunities are provided and this can effectively support a massive number of machine devices. Using both simulations and analytical model the proposed implementation is evaluated and compared to two existing random access schemes without small cell support (the basic random access scheme and the access class barring (ACB) scheme). It is observed that the capacity of the networks in terms of the number of supported machine devices with small cell support can be increased significantly. The proposed implementation shows large potential to handle random channel access for massive machine devices.

Due to the ideal correlation properties of complementary codes, a code division multiple access (CDMA) communication system using complementary codes as its signature codes can offer multiple access and multi-path interferences resistant performance. Compared to the other complementary codes, I/Q column-wise complementary (IQCC) codes can support much more users with a given flock size, and at the same time it can still maintain perfect correlation properties via quadrature phase-shift keying (QPSK) spreading scheme. In this paper, we analyze the downlink bit error rate (BER) performance of an IQCC-based CDMA communication system. Utilizing the derived BER expression, we demonstrate the multiple access and multi-path interferences resistant performance of the system under both flat and frequency-selective fading channels.

Cognitive radio (CR) is a critical technology for improving spectrum utilization and solving the radio spectrum scarcity problem. In CR devices, spectrum sensing is important to implement opportunistic spectrum access. Many spectrum sensing schemes have been proposed, including uncooperative, cooperative, centralized, and distributed algorithms. However, they aimed to obtain a global consensus sensing result, which may not always be possible in large-scale cognitive radio networks (CRNs) due to heterogeneous spectrum availability in different areas. Hence, some new spectrum sensing schemes should be designed to discover idle heterogeneous spectrum in CRNs. In this paper, we propose an intelligent cooperative spectrum sensing algorithm based on a non-parametric Bayesian learning model, namely the hierarchical Dirichlet process, which groups spectrum sensing data without the need to know the number of hidden spectrum states, and discovers a common sparse spectrum within each group. Furthermore, a concisely distributed information exchange scheme is designed, where intra-cluster and inter-cluster spectrum information is shared for global spectrum cognition. Experimental results show that the proposed algorithm can exploit the spatial relationship among sensed data to achieve a better spectrum sensing performance in terms of detection probability and false alarm probability.

Complete complementary codes (CCC) refer to a set of two-dimensional matrices, which have zero non-trivial aperiodic auto- and cross- correlation sums. A modern application of CCC is in interference-free multicarrier code-division multiple-access (MC-CDMA) communications. In this paper, we first show that in asynchronous “fractional-delay” uplink channels, CCC-MC-CDMA systems suffer from orthogonality loss, which may lead to huge interference increase when a conventional correlator based receiver is deployed. Then, by exploiting the correlation properties of CCC, we present a fractional-delay-resilient receiver which is comprised of a chip-spaced correlating array. Analysis and simulations validate the interference-free achievability of the proposed CSCA receiver in strong interference scenarios.

Mobile Stream Control Transmission Protocol (mSCTP) has been proposed to support transport layer mobility management mostly based on the radio signal strength. It could result in aggregation of mobile stations accessed via a base station with the strongest signal in the same signal overlapped area and consequently decrease the transmission probability in wireless networks. In this paper, we propose a cross layer handoff mechanism based on mSCTP, named CL-mSCTP, which combines the layer 4 heartbeat successful contention probability with the link signal strength detection to accomplish the dynamic handoff management. The simulation results show that the CL-mSCTP achieves much better performance in terms of throughput and the balancing of traffic loading than mSCTP does.

Network virtualization provides a promising way to run multiple virtual networks (VNs) simultaneously on a shared infrastructure. It is critical to efficiently map VNs onto substrate resources, which is known as the VN embedding problem. Most existing studies restrict this problem in a single substrate domain, whereas the VN embedding process across multiple domains (i.e., inter-domain embedding) is more practical, because a single domain rarely controls an entire end-to-end path. Since infrastructure providers (InPs) are usually reluctant to expose their substrate information, the inter-domain embedding is more sophisticated than the intra-domain case. In this paper, we develop an efficient solution to facilitate the inter-domain embedding problem. We start with extending the current business roles by employing a broker-like role, virtual network provider (VNP), to make centralized embedding decisions. Accordingly, a reasonable information sharing scheme is proposed to provide VNP with partial substrate information meanwhile keeping InPs' confidential information. Then we formulate the embedding problem as an integer programming problem. By relaxing integer constraints, we devise an inter-domain embedding algorithm to handle online VN requests in polynomial time. Simulation results show that our solution outperforms other counterparts and achieves 80%-90% of the benchmarks in an ideal scenario where VNP has complete knowledge of all substrate information.

Trust evaluation plays an important role in securing wireless sensor networks (WSNs), which is one of the most popular network technologies for the Internet of Things (IoT). The efficiency of the trust evaluation process is largely governed by the trust derivation, as it dominates the overhead in the process, and performance of WSNs is particularly sensitive to overhead due to the limited bandwidth and power. This paper proposes an energy-aware trust derivation scheme using game theoretic approach, which manages overhead while maintaining adequate security of WSNs. A risk strategy model is first presented to stimulate WSN nodes' cooperation. Then, a game theoretic approach is applied to the trust derivation process to reduce the overhead of the process. We show with the help of simulations that our trust derivation scheme can achieve both intended security and high efficiency suitable for WSN-based IoT networks.

ABSTRACT This paper analyzes the performance bounds of a wireless relay system consisting of several relay stations working on both amplifier‐and‐forward (AF) and decode‐and‐forward (DF) protocols. We want to study the outage probability behavior of the proposed mixed AF and DF relay systems under independent Nakagami‐m fading channels. In particular, we will derive the lower and upper bounds of outage probability of the mixed AF and DF relay systems based on maximal ratio combining diversity reception. The results give optimal configuration of AF and DF relays under a specific channel condition, thus helping us to design an optimized mixed AF and DF relay system in a generic fading environment. The trade‐off between complexity and performance is discussed in this paper. In addition, we will use computer simulations to verify the effectiveness of the proposed mixed AF and DF relay configurations. Finally, the power allocation issues for such a mixed AF and DF relay system will also be discussed. Copyright © 2012 John Wiley & Sons, Ltd.

ABSTRACT Space–time coded multiple‐input multiple‐output (MIMO) technology is an important technique that improves the performance of wireless communication systems significantly without consuming bandwidth resource. This paper first discusses the characteristics and limitations of traditional symbol‐level space–time coding schemes, which work largely on the basis of an assumption that signals are sent to a block‐fading channel. Therefore, the symbol‐level space–time coding schemes rely on symbol‐level signal processing. Taking advantage of orthogonal complementary codes, we propose a novel MIMO scheme, in this paper, based on chip‐level space–time coding that is different from the traditional symbol‐level space–time coding. With the help of space–time–frequency complementary coding and multicarrier modem, the proposed scheme is able to achieve multipath interference‐free and multiuser interference‐free communications with simple a correlator detector. The proposed chip‐level space–time coded MIMO works well even in a fast fading channel in addition to its flexibility to achieve diversity and multiplexing gains simultaneously in varying channel environments. Copyright © 2012 John Wiley & Sons, Ltd.

Owing to ideal correlation properties of complementary codes (CCs), a complementary coded code division multiple access (CC-CDMA) system outperforms traditional CDMA systems in terms of its interference resistance in both synchronous and asynchronous transmissions. This paper shows that a normal CC-CDMA system loses its interference-free feature over uplink frequency selective fading channels due to the fact that it is incapable to reconstruct ideal complementary cross-correlation at a receiver. This work proposes a uplink pre-equalized CC-CDMA system to resist multi-user interference and near-far effect over asynchronous frequency selective fading channels with perfect channel state information (CSI). Simulation results show the superiority of the proposed scheme in terms of its interference and near-far effect resistance. In addition, the impact of imperfect CSI is also studied in this work.

IEEE 802.15.4 standard is a relatively new standard designed for low power low data rate wireless sensor networks (WSN), which has a wide range of applications, e.g., environment monitoring, e-health, home and industry automation. In this paper, we investigate the problems of hidden devices in coverage overlapped IEEE 802.15.4 WSNs, which is likely to arise when multiple 802.15.4 WSNs are deployed closely and independently. We consider a typical scenario of two 802.15.4 WSNs with partial coverage overlapping and propose a Markov-chain based analytical model to reveal the performance degradation due to the hidden devices from the coverage overlapping. Impacts of the hidden devices and network sleeping modes on saturated throughput and energy consumption are modeled. The analytic model is verified by simulations, which can provide the insights to network design and planning when multiple 802.15.4 WSNs are deployed closely.

In this study, the authors propose a new pulse waveform design methodology for ultra‐wideband (UWB) applications, namely spectrum shifted Gaussian waveform (SSGW) approach. Specifically, UWB pulses can be generated by superposition of Gaussian sub‐carriers, which can be used to fit Federal Communication Commission (FCC) spectrum masks specified for UWB applications. In this way, it can match with the FCC masks much better than those generated using the traditional methods based on Gaussian or Hermite pulses and their higher order derivatives. The SSGW works with a low computational complexity and offers a unique agility in design of time‐domain pulse shapes to fit different power spectrum density masks. In principle, the SSGW approach can also be used for pulse waveform design in many other bandwidth efficient communication systems, not necessarily limited only to UWB systems.

Peer-to-Peer (P2P) networks have been widely used in various Internet applications. However, P2P networks also cause serious concerns of copyrighted contents piracy: P2P helps to pirate copyrighted contents, which impedes wide application of P2P networks. In this paper, we investigate several important issues on the ways to prevent pirated content propagation in P2P networks. We propose a system to stop pirated content propagation by utilizing several attacks to BitTorrent (BT). First, we design a system that can handle a large number of concurrent connections to reduce bandwidth consumption. Second, we build two mathematical models for BT attacks, including leechers behavior model and fake-block behavior model, and we analyze the performance of the system using these models. Third, we optimize BT clients by taking into account different implementations and counter-measure designs. We conducted several experiments in real networks, the results of which verified that our system is suitable for most current BT clients, and BT clients' download duration is extended at least three times longer, which is much better than the results reported in the literature.

In this work, we investigate the properties of energy-efficiency (EE) and spectrum-efficiency (SE) for video streaming over mobile ad hoc networks by developing an energy-spectrum-aware scheduling (ESAS) scheme. To describe a practical mobile scenario, we use a random walk mobility model, in which each node can choose its mobility direction and velocity randomly and independently. Through rigorous analysis and extensive simulations, we demonstrate that the node mobility is beneficial to EE but not to SE. The contributions of this work are twofold: 1) We propose an ESAS scheme with a dynamic transmission range, which significantly outperforms the previous minimum-distortion video scheduling in terms of joint EE and SE performance; 2) We derive an achievable EE-SE tradeoff range and a tight upper/lower bound with respect to energy-spectrum efficiency index for various node velocities. We believe that this work helps to shed insights on the fundamental design guidelines on building an energy and spectrum efficient mobile video transmission system.

Traditional α -fairness resource allocation in wireless multimedia communications assumes that the quality of experience (QoE) model (or utility function) of each user is available to the base station (BS), which may not be valid in many practical cases. In this paper, we consider a blind scenario where the BS has no knowledge of the underlying QoE model. Generally, this consideration raises two fundamental questions. Is it possible to set the fairness parameter α in a precisely mathematical specific α -fairness resource allocation schememanner? If so, is it possible to implement a specific α -fairness resource allocation scheme online? In this work, we will give positive answers to both questions. First, we characterize the tradeoff between the performance and fairness by providing an upper bound of the performance loss resulting from employing α -fairness scheme. Then, we decompose the α-fairness problem into two subproblems that describe the behaviors of the users and BS and design a bidding game for the reconciliation between the two subproblems. We demonstrate that, although all users behave selfishly, the equilibrium point of the game can realize the α-fairness efficiently, and the convergence time is reasonably short. Furthermore, we present numerical simulation results that confirm the validity of the analytical results.

Online anomaly detection (AD) is an important technique for monitoring wireless sensor networks (WSNs), which protects WSNs from cyberattacks and random faults. As a scalable and parameter-free unsupervised AD technique, $(k)$-nearest neighbor (kNN) algorithm has attracted a lot of attention for its applications in computer networks and WSNs. However, the nature of lazy-learning makes the kNN-based AD schemes difficult to be used in an online manner, especially when communication cost is constrained. In this paper, a new kNN-based AD scheme based on hypergrid intuition is proposed for WSN applications to overcome the lazy-learning problem. Through redefining anomaly from a hypersphere detection region (DR) to a hypercube DR, the computational complexity is reduced significantly. At the same time, an attached coefficient is used to convert a hypergrid structure into a positive coordinate space in order to retain the redundancy for online update and tailor for bit operation. In addition, distributed computing is taken into account, and position of the hypercube is encoded by a few bits only using the bit operation. As a result, the new scheme is able to work successfully in any environment without human interventions. Finally, the experiments with a real WSN data set demonstrate that the proposed scheme is effective and robust.

Employing user pairing schemes in uplink virtual multiple-input multiple-output (V-MIMO) systems can significantly improve system capacity owing to multi-user diversity. Most of the existing user pairing algorithms were developed under the assumption that a scheduler always possesses the perfect channel state information (CSI). In this paper, taking into account the effects of channel estimation error (CEE), the authors propose a robust user pairing algorithm based on a closed-form average sum rate under CEE. In addition, the authors further propose an adaptive user selection scheme as an effort to maximise the sum rates owing to the rate constraint under CEE. The gain of the proposed method is shown in simulation results.

In a cooperative ad hoc network, cooperating nodes are randomly distributed and hence coordinating them in a centralized manner becomes costly and inefficient. Developing relaying schemes that allow network nodes to cooperate distributively is thus critically important. While instantaneous channel state information (CSI) available at the receiver is commonly employed as the basis for making cooperation decisions, most of the previous works rely on a centralized mechanism to collect the required CSI. Besides, performance analysis on cooperative networks is usually limited to the symmetric topology. In this paper, we propose a fully distributed relaying scheme, called distributed threshold relaying (DTR) for cooperative wireless ad hoc networks. In DTR, each relay measures signal-to-noise ratios (SNRs) of source-relay and relay-destination channels using broadcast messages sent from source and destination, respectively. The decision on cooperation is made at each relay by performing two threshold tests. In addition, the cooperation among multiple relays is coordinated via a distributive medium access control (MAC) protocol based on a discrete random backoff algorithm. We derive closed-form expressions for the average outage probability and the average delay for independent but not necessarily identically distributed (i.n.i.d.) flat Rayleigh fading channels. Using numerical results and comparing DTR with well-known opportunistic relaying (OR) and selection relaying (SR), we can show the tradeoff relation between reliability and delay in distributed cooperative relaying.

In this paper, we consider a secrecy relaying communication scenario where all nodes are equipped with multiple antennas. An eavesdropper has the access to the global channel state information (CSI), and all the other nodes only know the CSI not associated with the eavesdropper. A new secrecy transmission protocol is proposed, where the concept of interference alignment is combined with cooperative jamming to ensure that artificial noise from transmitters can be aligned at the destination, but not at the eavesdropper due to the randomness of wireless channels. Analytical results, such as ergodic secrecy rate and outage probability, are developed, from which more insightful understanding of the proposed protocol, such as multiplexing and diversity gains, can be obtained. A few special cases, where outage probability cannot be decreased to zero regardless of SNR, are also discussed. Simulation results are provided to demonstrate the performance of the proposed secrecy transmission protocol.

Combating time and frequency selectivity in wireless channels is one of the most challenging tasks in next generation wireless networks. In this paper, we propose an adaptive estimation algorithm to estimate Doppler shifts in a direct sequence code division multiple access (DS-CDMA) radio system with multiple Doppler subpaths. By modeling doubly selective channels using a basis expansion model (BEM), an expectation-maximization (EM) algorithm based adaptive estimation method is developed to extract accurate Doppler shift information. The Cramer-Rao lower bound (CRLB) analysis is conducted to study the performance bound of the proposed estimation algorithm. Based on the estimated Doppler shift results, a frequency domain equalizer (FDE) based receiver architecture is developed to exploit Doppler diversity in the frequency domain. Our analysis and simulation results demonstrate that this receiver architecture features a low complexity while still achieving a good performance compared with traditional CDMA receivers.

We study load balancing of paging schemes for multitier hierarchical cellular networks, in which different tiers of cells overlay each other to provide multiple coverage in cellular service areas. Each mobile terminal (MT) can be paged in any tier of a multitier hierarchical cellular network. Paging requests are balanced in different waiting queues of different tiers, and the load balancing among them is achieved probabilistically among N tiers. The studied paging schemes are the Hierarchical Pipeline Paging scheme, the Hierarchical Sequential Paging scheme, and the Hierarchical Blanket Paging scheme. We study two optimization problems using the N-tier load balancing: 1) given a paging delay constraint, to minimize the total paging cost under the constraint that the total delay is upper bounded by a predefined total delay, and 2) given a bound on the total delay, to minimize the total paging cost under a paging delay constraint.

Abstract One of the most challenging issues in wireless sensor networks is to meet the requirements of coverage and connectivity under given energy constraints. Most existing coverage and connectivity algorithms work to form tree networks when sensor nodes do not have location information of themselves. However, a tree topology network does not perform well in terms of energy efficiency and scalability if compared with a cluster network. In this paper, a novel topology control algorithm called Adaptive Random Clustering (ARC) is proposed to form a cluster network with required coverage and connectivity without location information. The performance of its coverage intensity and connectivity is analyzed based on the characteristics of cluster topology, and their proper parameters are determined. ARC inherits an excellent energy efficiency from cluster topology and avoids the collisions and overhearing of data packets. A good scalability can be achieved as only a limited number of channels are needed in ARC for a large‐scale network. Furthermore, ARC can adjust the number of active nodes adaptively according to the required coverage to balance the energy consumption. Simulation results demonstrate that required coverage and connectivity can be satisfied and network lifetime is prolonged significantly. Copyright © 2010 John Wiley & Sons, Ltd.

Multi-scale direct sequence ultra-wideband (MS/DS-UWB) is a new multiple access technique for UWB systems which utilizes the dyadic scaled orthogonal wavelets spread by direct sequence (DS) codes. It has dyadic scaled multi-band structure exposing the scalability of the spectrum. This paper presents the probability of error analysis for the MS/DS-UWB system for Nakagami-m small scale fading which is suitable for UWB sensor networks. The performance of a MS/DS-UWB is compared with a multi-processing gain (MPG) DS-UWB system. It is observed that the MS/DS-UWB outperforms substantially the MPG/DS-UWB scheme over a Nakagami-m fading channel.

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In this study, the authors introduce a new multiple access technique for ultra-wideband (UWB) communications called multi-scale (MS) direct sequence (DS)-UWB (MS/DS-UWB). In the MS/DS-UWB design, the dyadic-scaled orthogonal wavelets spread by DS codes create a dyadic-scaled multi-band structure which exhibits the scalability of the spectrum. Hence, it can be incorporated with cognitive radio by intelligent spectrum adaptation when part of the UWB spectrum cannot be used. MS/DS-UWB is a multi-rate system, where services are assigned to different orthogonal sub-bands with respect to their data rates. Compared with the conventional single-band DS-UWB, the increased pulse durations (or scaled sub-bands) relax the system design. Besides, it eliminates the disadvantages associated with orthogonal frequency division multiplexing-based UWB systems. The authors analyse the probability of error performance of the MS/DS-UWB system and compare it with a multi-processing gain (MPG) DS-UWB system over UWB channels. Numerical results show that the MS/DS-UWB performs much better than that of MPG/DS-UWB due to the low cross correlation among the scaled wavelets.

Network coding can deliver multiple data streams simultaneously and make full use of broadcast nature of wireless channels. The authors propose two diversity-enabled network-coding (NC) schemes to optimise wireless uplink scheduling. The existing scheduling protocols normally have to allow the users with relatively low channel gains to transmit, and it can maintain fairness but reduce congregated throughput. The main idea of the proposed scheme is to always schedule users with the best channel condition, while the use of NC encourages the scheduled users to help others which have not been served previously. Delay and capacity performance for different network coded scheduling schemes are analysed. Round-robin and pure opportunistic scheduling are evaluated for performance comparison. In order to show the effectiveness of the proposed schemes, NC schedulers are applied to a time division duplexing code division multiple access wireless cellular networks. System-level simulation was carried out based on the third generation partnership project specifications. Per-sector average throughput and cumulative distribution function of user average throughput are adopted as the performance metrics. Analytical and simulation results show that the proposed NC schedulers can achieve a better tradeoff between fairness and throughput than those without NC.

To improve power efficiency in vehicle-to-roadside infrastructure (V2I) communication networks, this paper proposes a joint power and sub-carrier assignment policy under delay aware quality of service (QoS) requirements. Due to the real-time nature of the V2I transmissions, the proposed policy should satisfy delay aware QoS requirements with a minimized power consumption. In particular, we develop a cross-layer framework in which orthogonal frequency division multiplexing (OFDM) is employed at the physical layer and the proposed power and sub-carrier assignment policy works at the data link layer. Under the assumption that the instantaneous channel state information (CSI) of all users is known, the optimization problem can be formulated and solved with the help of a time-sharing factor. The obtained results show that both the optimal power allocation and the optimal sub-carrier assignment depend on the delay aware QoS requirements of each user. We also theoretically prove that the proposed power allocation policy converges to the classical water-filling policy if we do not consider the QoS requirements. Experimental results reveal that the proposed policy offers a superior performance over the existing resource allocation policies.

The correlation lower bounds, as well as the set size upper bounds, of low-correlation-zone complementary sequences are presented in this paper. They can be treated as an extension of Tang-Fan-Matsufuji bounds in and , which considered only the conventional (non-complementary) low-correlation-zone sequence sets.

Traditional multimedia scheduling approaches assumed perfect control channels where each node has access to the knowledge of its neighbors. However, in practice the control channels are always constrained and nodes can only exchange limited information with their neighbors. In this paper, we investigate how imperfect neighbor information affects the multimedia scheduling. First, we formulate the optimal multimedia scheduling problem with the constraints on network information. Specifically, a constrained factor is introduced to capture the profile of control channels. Then, we consider two cases of the constrained factor distribution: 1) the class with finite mean and variance, and 2) a general class that does not employ any parametric representation. In each case, we investigate the relationship between the control gain and scheduling performance based on available network and multimedia information. We show that the control gain can be chosen properly such that the optimal distributed multimedia scheduling can be achieved with an exponential convergence rate. In addition, an explicit equation for asymptotic convergence rate is derived for each case. Finally, we use computer simulations to verify the analytical results.

Abstract Recently, vehicularad hocnetwork (VANET) has emerged as a promising approach for road safety and traffic efficiency improvement through a variety of vehicle applications enabled by communications between vehicles such as emergency braking warning, etc. However, due to its unique characteristics, such as intermittent connectivity due to high‐speed mobility of the network nodes (or vehicles), also known as vehicular delay tolerant network, it poses a major challenge to the realization of those applications. In this paper, we propose a new roadside unit (RSU) aided bundle forwarding protocol for vehicular delay tolerant networks. Furthermore, with the assistance from those RSUs deployed at some critical points on the road, for example, intersections, the proposed protocol can increase the network performance in terms of delivery ratio. At the same time, since vehicle‐to‐vehicle (V2V) and vehicle‐to‐RSU (V2R) privacy‐preserving authentications are guaranteed, the black (gray) hole attacks can be avoided. Extensive simulations demonstrate the effectiveness of the proposed protocol. Copyright © 2010 John Wiley & Sons, Ltd.

Abstract Spatial diversity in wireless networks can be attained by exploiting the broadcast nature of wireless transmission without the need of multiple antennas in individual device, leading to the implementation of cooperative communication. While most prior works focused on the single source—destination scenario, it should be more realistic to consider how to induce cooperation among multiple source‐destination pairs assisted by multiple relays. In such a case, multiple access interference (MAI) may present due to asynchronous transmissions of the users and relays. In this paper, a cooperative network architecture based on orthogonal complementary (OC) codes inherently immune to MAI is proposed. To efficiently utilize the scarce radio spectrum and codes, a centralized medium access control (MAC) protocol is proposed to coordinate the code assignment and channel access among users and relays. We theoretically analyze the bit error rate (BER) performance of the proposed OC coded cooperative network over multipath Rayleigh fading channel. The performance gain resulted from different numbers of relays is investigated, and compared with a time division multiple access (TDMA) based cooperative scheme. We show that the proposed OC coded cooperative network performs well in the presence of timing offset, and thus is well suited for asynchronous uplink transmission with cooperative relaying. Copyright © 2009 John Wiley & Sons, Ltd.

Binary exponential backoff (BEB) algorithm, the defacto standard contention resolution algorithm providing collision avoidance over ad hoc wireless networks, has several technical limitations. For instance, high collision rate problem, unfair channel access, and throughput degradation are several widely known issues. In this paper, a self-adaptive backoff algorithm, namely slow start backoff (SSB) algorithm, is proposed and analyzed for ad hoc networks. Numerical results show that the proposed SSB algorithm outperforms the legacy BEB algorithm. In addition to the theoretical analysis, simulations were conducted to verify the analysis, showing that the proposed scheme works very well in ad hoc wireless networks.

In IEEE 802.11 standard, the use of multi-channel is one of the most important characteristic features contributing to the improvement of the overall network throughput. However, the channel utilization based on the use of multi-channel is relatively low because most medium access control (MAC) protocols were designed based primarily on the application of non-overlapping channels. This paper analyzes the channel utilization for the schemes using both non-overlapping and overlapping channels. Based on the analysis, we propose a scheme to exploit overlapping channels with collision avoidance mechanism. The proposed scheme makes use of the non-overlapping channels to increase the network throughput and it also takes into account the transmissions on different channels in order to maximize the throughput. Simulation results verify the effectiveness of the proposed scheme.

Joint exploration of intra/inter-video coding versatility in signal processing domain and signature/hash diversity in the information security domain has not been well investigated in the literature. In this paper, we show multimedia stream authentication quality can be improved significantly by exploring these diversities. Specifically, we propose a new rate distortion framework by considering intra/inter-signature/hash diversity to simultaneously provide video service quality, communication rate efficiency and video content integrity for wireless video streaming. This research aims to provide a unique fusion of inter/intra video coding versatility and signature/hash authentication diversity to provide video authentication and traffic control. The results based on simulations demonstrate the effectiveness of the proposed scheme in achieving video robust authentication quality with limited rate constraint.

To improve the performance of a relay system with multiple channels, the following issues should be addressed. Namely, how to allocate the power at source and relay to subchannels, how to pair subchannels of the first and second hops, and which users should be scheduled to which subchannel pair. Considering these issues in the design of an optimal joint resource allocation scheme in orthogonal channels, in this paper we study a multi-user network with single regenerative relay node. A stochastic optimization problem to maximize system ergodic throughput with joint transmission power constraint and user average data rate is formulated. To satisfy user average data rate request, a scheme with a weighted factor associated to each user at each time slot is proposed. The stochastic approximation method is utilized to estimate this weighted factor and the proof of optimality is given. With the help of this weighted factor, the problem is converted into a deterministic optimization problem in each time slot and the Lagrange dual method can be employed to derive the optimal solution. Finally, the Stochastic Optimal Programming (SOP) is used to evaluate the performance by computer simulations.

Information technology systems face considerable challenges in seamless integration of telemetry and control information. These are essential to various autonomic management functions related to power, thermal, reliability, predictability, survivability, and adaptability. Sensors and control agents supporting this telemetry are a part of large multiprocessor environments that are scattered across the platform. The conventional approaches to support distributed observability and control using wired solutions are static, expensive, and non-scalable. We present an alternative approach for this unique environment that replaces static wired sensors with dynamically reconfigurable wireless sensors. It employs a genetic algorithm based approach to optimize sensor node function assignment, clustering decisions, resource distribution, and route establishment for improved control quality. Based on this new, wireless sensor network approach, we evaluate the average data-flow delay characteristics between sensor and control end-points. We also investigate the "quality of control'' by measuring the conformity of the controlled objective to platform policy specifications (like power limits).

Interference coordination is the key to reduce intercell interference in a TD-LTE system. Because of implementation complexity, partial and soft frequency reuse schemes are hardly suitable to TD-LTE systems. The decode-and-forward relay scheme, which may generate some additional interference, is presented to extend coverage and improve performance of TD-LTE systems. To mitigate the relay related interference, a directional relay topology is proposed in this paper. The radio resource is reused between the isolated RSs to improve the spectrum efficiency. The joint interference coordination and directional relay can suppress most inter-cell and inter-relay interferences and improve the capacity. The signal to interference plus noise ratio (SINR) distributions and throughput for the traditional non-relay scenario and the proposed directional relay scenario are derived. The analysis and simulation results show the effectiveness of the proposal scheme.

In this paper, we propose an innovative low-cost biometric authentication scheme based on uniformed Gaussian mixture model (GMM) for secure communications within wireless body area sensor networks (WBASNs), to obviate the cost-prohibitive key exchange problem. In this approach, the sender Inter Pulse Interval (IPI) information is selected as the biometric key for authentication, and a statistical based signing scheme is developed using the uniformed GMM. Experimental analysis shows that the proposed approach effectively protects the communication channels in WBASN with a low computational complexity, significantly removing the need for secret key exchange and critical time synchronization overheads.

Cooperative base station for downlink multiple-input–multiple-output (MIMO) system is known as a critical radio technology for worldwide interoperability for microwave access (WiMAX) communications. This study proposes to use zero-forcing beamforming for cooperative base stations in a downlink multicell MIMO system. It is well known that beamforming requires perfect knowledge of channel state information (CSI) at the transmitter, and in practice the perfect CSI may not be available because of channel estimation errors. In this study, using approximate capacity loss analysis the authors are able to analyse the effect of channel estimation errors on system capacity in a cooperative base station system with zero-forcing beamforming. A power allocation policy is proposed to reduce the capacity loss under per base station power constraints. Numerical results show that the approximate capacity loss is very close to the real capacity loss, which can be reduced with the help of the power allocation policy.

In this paper, we propose a scalable transport scheme for delivering on-demand video streams over broadband wireless networks in next-generation network/IP multimedia subsystem (NGN/IMS) architecture. The proposed transport scheme makes use of fine-granular-scalability (FGS) encoded videos to accommodate high bandwidth variation of wireless networks. We formulate the bandwidth allocation to FGS-encoded streams as a resource allocation problem and develop a quality-aware bandwidth allocation scheme, called QABA. With QABA, the proposed transport scheme can dynamically adjust the bit rate allocated to different streams to maximize the overall perceptual quality when available network bandwidth varies with time. QABA is theoretically proven to be able to find the optimal bandwidth allocation for all on-demand streams. To validate the effectiveness of QABA, extensive trace-based simulations are performed.

In this article, a video multicast protocol for multi-homed mobile terminals is proposed as an alternative stream control transmission protocol (SCTP) for partially reliable multicast services. It works with overlay peer-to-peer video multicast facility in the application layer. For a multi-homed mobile terminal, an error burst may occur when a handover is in process in the primary path switching procedure. The key issue concerned in this protocol is the ability to predict packet loss and to retransmit the lost packets as soon as a mobile terminal completes its primary path switching procedure. This property controls the delay sensitivity of transmissions. Conversely, the protocol can tolerate partial loss in video transmission as long as the loss is limited to a relatively short error burst. In addition, it reduces the message overhead significantly and provides a scalable communication mechanism for multicast applications. The performance improvement of the proposed protocol comes from 1) the estimation of temporal velocity of mobile terminals with lost packet prediction in a long error burst, and 2) the requirement for each mobile terminal to indicate which packets can be safely discarded from its agent.

Wireless multimedia sensor networks (WMSNs) support many acoustic applications for audio surveillance, animal tracking/vocalization, human health monitoring, etc. However, resource constraints in sensor networks (such as limited battery power, bandwidth/computation capability, etc.) pose challenges for the quality and security of audio data transmission and processing. The security is a critical issue since audio information can be accessed or even manipulated in WMSNs. In order to ensure security, audio quality and energy efficiency, we propose an index-based selective audio encryption scheme for WMSNs. The scheme protects data transmissions by incorporating both resource allocation and selective encryption based on modified discrete cosine transform (MDCT). In this proposed scheme, the audio data importance is leveraged using the MDCT audio index, and wireless audio data transmission proceeds with energy efficient selective encryption. The simulation results show that the proposed approach offers a significant gain in terms of energy efficiency, encryption performance and audio transmission quality.

Selective encryption for video streaming was proposed for efficient multimedia content protection. However, the issues on joint optimization of video quality, content protection, and communication energy efficiency in a wireless sensor network (WSN) have not been fully addressed in the literature. In this paper, we propose a scheme to optimize the energy, distortion, and encryption performance of video streaming in WSNs. The contribution of this work is twofold. First, a channel-aware selective encryption approach is proposed to minimize the extra encryption dependency overhead at the application layer. Second, an unequal error protection (UEP)-based network resource allocation scheme is proposed to improve the communication efficiency at the lower layers. Simulation experiments demonstrate that the proposed joint selective encryption and resource allocation scheme can improve the video transmission quality significantly with guaranteed content protection and energy efficiency.

Media integrity, transmission quality, and energy efficiency are critical for secure wireless image streaming in a wireless multimedia sensor network (WMSN). However, conventional data authentication and resource allocation schemes cannot be applied directly to WMSN due to the constraints on limited energy and computing resources. In this paper, we propose a quality-driven scheme to optimize stream authentication and unequal error protection (UEP) jointly. This scheme can provide digital image authentication, image transmission quality optimization, and high energy efficiency for WMSN. The contribution of this research is two-fold as summarized below. First, a new resource allocation-aware greedy stream authentication approach is proposed to simplify the authentication process. Second, an authentication-aware wireless network resource allocation scheme is developed to reduce image distortion and energy consumption in transmission. The scheme is studied by unequally protected image packets with the consideration of coding and authentication dependency. Simulation results demonstrate that the proposed scheme achieves a performance gain of 3 ~ 5 dB in terms of authenticated image distortion.

Cognitive radio is emerging as a promising technique to improve the utilization of the radio frequency spectrum. In this paper, we consider the problem of spectrum sharing among primary (or "licensed") users (PUs) and secondary (or "unlicensed") users (SUs). We formulate the problem based on bandwidth auction, in which each SU makes a bid for the amount of spectrum and each PU may assign the spectrum among the SUs by itself according to the information from the SUs without degrading its own performance. We show that the auction is a noncooperative game and that Nash equilibrium (NE) can be its solution. We first consider a single-PU network to investigate the existence and uniqueness of the NE and further discuss the fairness among the SUs under given conditions. Then, we present a dynamic updating algorithm in which each SU achieves NE in a distributed manner. The stability condition of the dynamic behavior for this spectrum-sharing scheme is studied. The discussion is generalized to the case in which there are multiple PUs in the network, where the properties of the NE are shown under appropriate conditions. Simulations were used to evaluate the system performance and verify the effectiveness of the proposed algorithm.

Special issue on ''quality of service and security in wireless and mobile networks''Recently, wireless and mobile communication systems have become increasingly popular as an inexpensive and promising means for ubiquitous communications and mobile computing.This trend will continue to affect the way in which we live, work, and play with innovative services based on the pervasive and mobile communications support.A preliminary condition for those services to become widely used is the effective support of quality of service (QoS) and security properties.For this reason, QoS and security management of wireless and mobile communication systems are very important issues for the future generation of wireless and mobile networks and services, and the design and management of solutions for QoS and security are some of the most challenging research tasks, in scenarios like the wireless and mobile systems, characterized by new assumptions and limitations.The main limitations typically involve the resources like battery energy, computation, and bandwidth.The new assumptions include the unprecedented dynamics caused by mobility, heterogeneity, variable channel conditions, the traffic characterization of new emerging applications, the open communication nature, and the distributed management architecture of wireless communication systems, like the wireless ad hoc, mesh, vehicular, and sensor networks.This special issue brings together nine contributions, selected from a total of 25 submissions, that address the issues of architectures, technologies, and protocols for wireless and mobile communication systems.The selected papers propose and analyze some of the most interesting and innovative solutions dealing with the QoS and Security management in wireless mobile networks, from different viewpoints, including the QoS-based channel time allocation and scheduling in IEEE 802.15.3 wireless PANs, QoS analysis of IEEE 802.15.4Low rate WPAN technology in industrial application scenarios, QoS routing with obstacle avoidance properties in wireless sensor

Abstract Mental health care is a major cost to all EU nations, and in many case, it results in additional costs to a country's economy due to the lost of productivity and concerns at the societal level. Cognitive behaviour therapy (CBT) is by far the treatment of choice for mental disorders such as depression, anxiety disorders, eating disorders, chronic insomnia, etc. Although effective, CBT suffers from a lack of accessibility and personalisation. In this paper, we proposed an ontology‐enhanced ubiquitous monitoring and treatment model for the purpose of assisting people to overcome the challenges of mental disorders. We first presented a context ontology for mental disorders as the basis upon which semantics‐enhanced methods are developed for gathering, formalising and manipulating patients' data. We implemented the proposed framework to facilitate online CBT, for treating depression at the current stage, which combines together talk/chat/messaging services, helps in retrieving neurofeedback, and supports collaborative diagnosis when necessary. An online statistic report was also integrated into our system. Finally, the paper discussed the proposed framework by comparing it against relevant research in the field and elaborated on possible further research directions. Copyright © 2009 John Wiley & Sons, Ltd.

Opportunistic relaying has been recently proposed to eliminate the need of symbol synchronization and challenging code design in distributed space-time coded (DSTC) cooperation yet achieving the same diversity multiplexing tradeoff. In opportunistic relaying, the relay terminal that possesses the best relay-destination channel quality is selected by the destination for cooperative forwarding. Combining opportunistic relaying with the automatic-retransmission request (ARQ) protocol that has already been used to recover the transmission error can further improve the communication reliability in error-prone wireless channels. However, the relay selection process taking place in the scale of the channel coherence time gives rise to additional delay. Furthermore, each relay terminal needs to perform error detection on a per frame basis to determine its eligibility for cooperative relaying. The caused overhead may be fast accumulated as the data rate increases. In this paper, we propose a fast relay selection protocol that selects the assisting relay without using any extra signaling than existing ARQ protocols. Only channel state information (CSI) available at the receiver side is required to perform the selection. We theoretically analyze the proposed scheme, and verify the accuracy of the theoretical analysis through simulations. Numerical results show that, the proposed scheme outperforms the classical ARQ protocols in terms of outage probabilities, using the same amount of signaling.

While original network coding is proposed over the data link layer, recent work suggests that it can also be implemented on the physical layer. In fact it is more natural for wireless networks because of its omnidirectional transmission. In this paper, we investigate the symbol-error-rate (SER) for binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK), but the approaches can be generalized to other constellation schemes. The closed-form SER results are derived for physical layer network coding over AWGN channels. The theoretical analysis is also validated by numerical simulation.

A cross layer Unequal Error Protection (UEP) scheme based upon the multimedia position-value (P-V) partitioning has been recently proposed to only improve energy-distortion performance in Wireless Multimedia Sensor Networks (WMSN). There is also a great potential to adapt this UEP scheme into multimedia stream integrity protection. However, performance gains in multimedia transmissions with considerations of energy, distortion, and authentication in WMSN is a challenge. This difficulty results from the media codec dependency on both the content of multimedia stream authentication and protection on the integrity process itself. In this article, we propose a novel scheme which optimally matches the application layer multimedia codec dependency to both lower layer resource allocation requirements and the upper layer stream authentication. The goal of this scheme is to optimize the integrity and quality performance gain within energy constraint. The contribution of the proposed approach is two folds. Firstly, a stream authentication scheme matched with P-V codec dependency is proposed which significantly reduces additional authentication dependency overhead. Secondly, a new resource allocation scheme is proposed to improve energy-distortion-authentication performance with regard to codec dependency. Simulation studies demonstrate that the proposed scheme significantly impacts achieving multimedia transmission quality with energy efficiency and authentication assurance.

Uncertain capacity in newly emerged Cognitive Radio Networks(CRN) renders specific performance analysis for the secondary users (SUs), particularly in the aspects of Quality-of-Service (QoS). The effective bandwidth concept is employed to analyze the SUs' performance. Considering the time-varying arrival and service process, we analytically obtain the QoS metrics, such as the approximated delay-violation probability and mean time delay. The accuracy of the approximation is demonstrated via simulation. With the extension to multiple secondary users, we then consider the inevitable sensing errors in CNR in a cross-layer framework, and introduce the promising collaborative sensing method among SUs. By integrating to upper link layer, we then study the collaborative gains in terms of mean time delay.

Capacity and power consumption are two main bottlenecks in the wide deployment of Voice-over-IP (VoIP) over IEEE 802.11 wireless LAN, when mobile terminals are used, the impact on the Voice-over-IP (VoIP) by power consumption becomes more critical. In this paper, we analyze the capacity of VoIP traffic in 802.11 WLAN and do the relevant simulations with and without sleep strategy. We find that capacity decreases when the network adopts sleep strategy. After a detailed analysis by p-persistent model of the IEEE 802.11 protocol, we provide a dynamic sleep strategy, namely Collision Detective Dynamic Sleep Strategy (CDDSS) to improve both the capacity and energy saving rate of the VoIP over 802.11 WLANs. In our CDDSS, mobile station (MS) adjusts sleep interval and packetization interval adaptively according to the average collision probability pc of the network. Our simulation results show that CDDSS cannot only improve the capacity of VoIP in 802.11 WLANs, but also reduce considerable energy consumption.

We propose a reduced-complexity genetic algorithm for intrusion detection of resource constrained multi-hop mobile sensor networks. Traditional intrusion detection mechanisms have limited applicability to the sensor networks due to scarce battery and processing resources. Therefore, an effective scheme would require a power efficient and lightweight approach to identify malicious attacks. The goal of this paper is to evaluate sensor node attributes by measuring the perceived threat and its suitability to host local monitoring node (LMN) that acts as trusted proxy agent for the sink and capable of securely monitoring its neighbors. Security attributes in conjunction with genetic algorithm jointly optimizes the placement of monitoring nodes (i.e., LMN) by dynamically evaluating node fitness by profiling workloads patterns, packet statistics, utilization data, battery status, and quality-of-service compliance.

Ultra-wide band (UWB) is a radio technology that uses a very large bandwidth. A two-dimensional exclusion mechanism and routing strategy are mentioned in. We extend the exclusion region to three-dimension and modify the interference from other nodes as Gaussian noise. The conclusion is also correct under mobility. We also present a new model combining exclusion mechanism and interference mitigation. Then we propose an optimal routing algorithm described as MTI. We raise a geometrical method unlike the simulation in to demonstrate that the MER routing minimize the total interference and is the optimal routing.

In this paper, we propose a cross-layer design (CLD) scheme combining rate adaptation and four types of hybrid automatic repeat request (HARQ) for multiband orthogonal frequency division multiplexing (MB-OFDM) ultra wideband (UWB) systems following the ECMA-368 standard. The time varying property is incorporated into standard UWB channel models for the purpose of investigating rate adaptation. To accurately accommodate fast time-varying channel conditions, we propose to embed the selected rate information into the acknowledgement (ACK) frames. It is shown that the proposed CLD scheme combining rate adaptation and HARQ can provide higher throughput than the HARQ-only schemes. Among the four types of HARQ schemes, HARQ Type-III has the best throughput performance while Type I has the worst.

By using cognitive radio technology, opportunistic spectrum access has the potential to solve the underused spectrum problem. In this paper, by first introducing a specific cognitive radio scheme, we analyze the secondary user's capacity and the collision probability. Employing this scheme, we build two call admission control models for three classes of service, namely, handoff voice calls, new voice calls and data calls. We show that they achieve improved blocking probability and throughput by exploiting the cognitive radio scheme. Further, we show a tradeoff between collision probability and blocking/throughput in the call admission control.

Multi-scale direct sequence ultra wide band (MS/DS-UWB) is a new multiple access technique for UWB communications. It utilizes the dyadic (i.e., with a power of two) scaled orthogonal wavelets spread by direct sequence codes creating dyadic scaled multi-band structure exposing the scalability of the spectrum. Services are assigned to different orthogonal sub-bands with respect to their data rates. Comparing to the conventional DS-UWB, increased pulse durations reduce the complexity of the conventional DS-UWB system. We show that the spectral structures of MS/DS-UWB using the four scales of Daubechies-5 and Daubechies-10 wavelets fit in to the FCC UWB mask. In terms of the probability of error performance over an asynchronous awgn channel, the proposed design promises better performances compared to a single band UWB design when multi-rate services are available.

It is widely believed that IEEE 802.11 standard is aimed mainly for fixed indoor wireless local area networks and is not suited for mobile applications, even though the IEEE 802.11b systems may work in either infrastructure mode or ad hoc mode. The impact of node mobility on ad hoc network performance has already been studied intensively, but these studies mostly do not consider temporal fluctuations of the mobile wireless channel due to the Doppler shift. An investigation of the mobility impact on the performance of IEEE 802.11b ad hoc systems with Rician/Rayleigh fading under different node velocities is presented. A comprehensive and in-depth analysis of the impacts of a multitude of different signal distortions on an IEEE 802.11b system performance is also presented. Specifically, the authors study the bit-error rate performances with respect to node velocities for different modulation schemes. The simulation results show that, owing to its extremely low implementation and deployment cost, the current IEEE 802.11b standard has its potential to be deployed in a mobile ad hoc environment if the line-of-sight path between transmitter and receiver exists.

Throughput capacity of a random wireless network has been studied extensively in the literature. Most existing studies were based on the assumption that each transmission involves only one transmitter in order to avoid interference. However, recent studies on physical-layer network coding (PLNC) have shown that such an assumption can be relaxed to improve throughput performance of a wireless network. In PLNC, signals from different senders can be transmitted to the same receiver in the same channel simultaneously. In this paper, we investigate the impact of PLNC on throughput capacity of a random wireless network. Our study reveals that, although PLNC scheme does not change the scaling law, it can improve throughput capacity by a fixed factor. Specifically, for a one-dimensional network, we observe that PLNC can eliminate the effect of interference in some scenarios. A tighter capacity bound is derived for a two-dimensional network. In addition, we also show achievable lower bounds for random wireless networks with network coding and PLNC.

The 16 articles in this special issue report cutting-edge research achievements in the field of applications of Networks for Healthcare.

IEEE 802.16 and Ethernet Passive Optical Network (EPON) are two promising broadband access technologies for high-capacity wireless access networks and wired access networks, respectively. They each can be deployed to facilitate connection between the end users and the Internet but each of them suffers from some drawbacks if operating separately. To combine the bandwidth advantage of optical networks with the mobility feature of wireless communications, we propose a convergence of EPON and 802.16 networks in this paper. First, this paper starts with presenting the converged network architecture and especially the concept of virtual ONU-BS (VOB). Then, it identifies some unique research issues in this converged network. Second, the paper investigates a dynamic bandwidth allocation (DBA) scheme and its closely associated research issues. This DBA scheme takes into consideration the specific features of the converged network to enable a smooth data transmission across optical and wireless networks, and an end-to-end differentiated service to user traffics of diverse QoS (Quality of Service) requirements. This QoS-aware DBA scheme supports bandwidth fairness at the VOB level and class-of-service fairness at the 802.16 subscriber station level. The simulation results show that the proposed DBA scheme operates effectively and efficiently in terms of network throughput, average/maximum delay, resource utilization, service differentiation, etc.

In this paper, a resource allocation and call admission control scheme based on game theory for satellite networks operating above 10 GHz is introduced, taking into account the propagation conditions and large propagation delay. The resource allocation scheme based on a bargaining model, which may be completed in a very short time, seems to be suitable for providing emergency services. A distributed call admission control algorithm exploiting the predictability of the satellite channel to guarantee quality of service (QoS) is also presented. According to the suggested scheme, each user unilaterally decides whether to accept a new call and, if all users unanimously agree, the new call is admitted. The performance of the proposed scheme is investigated using Markov chain modeling. Finally, analytical results concerning bounds for the call blocking probability and the system throughput are presented.

The transmission of confidential information over LMDS networks is investigated in the present paper from an information-theoretic standpoint. The major factor impairing the link performance at these networks is rain attenuation, a physical phenomenon exhibiting both spatial and temporal variation. Based on an accurate channel modeling, analytical expressions of the probability of non-zero secrecy capacity and the outage probability for this type of networks are provided, giving for the first time an information-theoretic approach of the problem of secure transmission for LMDS networks. Useful conclusions are drawn through extended numerical results suggesting the increase of operational frequency and the use of more directional antennas to improve security in transmission of confidential messages for this type of networks.

Previous research on sensor network security mainly considers homogeneous sensor networks, where all sensor nodes have the same capabilities. Research has shown that homogeneous ad hoc networks have poor performance and scalability. The many-to-one traffic pattern dominates in sensor networks, and hence a sensor may only communicate with a small portion of its neighbors. Key management is a fundamental security operation. Most existing key management schemes try to establish shared keys for all pairs of neighbor sensors, no matter whether these nodes communicate with each other or not, and this causes large overhead. In this paper, we adopt a Heterogeneous Sensor Network (HSN) model for better performance and security. We propose a novel routing-driven key management scheme, which only establishes shared keys for neighbor sensors that communicate with each other. We utilize Elliptic Curve Cryptography in the design of an efficient key management scheme for sensor nodes. The performance evaluation and security analysis show that our key management scheme can provide better security with significant reductions on communication overhead, storage space and energy consumption than other key management schemes.

In most application scenarios of wireless sensor networks (WSN), sensor nodes are usually deployed randomly and do not have any knowledge about the network environment or even their ID's at the initial stage of their operations. In this paper, we address the clustering problems with a newly deployed multi-hop WSN where most existing clustering algorithms can hardly be used due to the absence of MAC link connections among the nodes. We propose an effective clustering algorithm based on a random contention model without the prior knowledge of the network and the ID's of nodes. Computer simulations have been used to show the effectiveness of the algorithm with a relatively low complexity if compared with existing schemes.

In this paper a Markov chain based analytical model is proposed to evaluate the slotted CSMA/CA algorithm specified in the MAC layer of IEEE 802.15.4 standard. The analytical model consists of two two-dimensional Markov chains, used to model the state transition of an 802.15.4 device, during the periods of a transmission and between two consecutive frame transmissions, respectively. By introducing the two Markov chains a small number of Markov states are required and the scalability of the analytical model is improved. The analytical model is used to investigate the impact of the CSMA/CA parameters, the number of contending devices, and the data frame size on the network performance in terms of throughput and energy efficiency. It is shown by simulations that the proposed analytical model can accurately predict the performance of slotted CSMA/CA algorithm for uplink, downlink and bi-direction traffic, with both acknowledgement and non-acknowledgement modes.

RFID Tag detection/recognition is one of the most critical issues for successful deployment of RFID systems in diverse applications. The main factors influencing tag detection by RFID reader antenna include tag position, relative position of reader, read field length, etc. In this paper, we analyze the characteristics of tag detection for a carton box object on a wooden pallet by an experimental approach based on tag signal strength, and we propose a method for predicting detection related directly to the strength of tag signal using an intelligent machine learning technique called support vector machine (SVM). The use of the proposed method is able to save time and cost by quick prediction of tag detection. Extensive experiments showed that the proposed approach can predict tag recognition for a carton box object with an accuracy at 95% for various reader heights and read field lengths. The proposed approach is effective for determining the best tag detection influencing factor conditioned on the target object with the help of detectability prediction.

This paper presents a novel k-hop compound metric based clustering (KCMBC) scheme, which uses the host connectivity and host mobility jointly to select cluster-heads. KCMBC is a fast convergent and load balancing clustering approach that is able to offer significant improvement on scalability for large-scale ad hoc networks. On the other hand, since host mobility has been taken into account in terms of the average link expiration time, the clusters constructed by KCMBC are more stable than many other schemes. Simulation results show that the clusters created by using the KCMBC approach retain modest but more uniform cluster size, and cluster-head life-time can be increased by KCMBC up to 50%. Moreover, the control overheads for cluster formation using the KCMBC scheme are kept relatively low if compared to other clustering schemes.

Code-division multiple access (CDMA) technology has been applied to many wireless communication systems. CDMA system suffers from both multiple access interference (MAI) and inter-symbol interference (ISI) over a multipath channel. To suppress MAI and ISI, this paper proposes the spreading codes with interference free windows. In particular, we will develop several upper bounds on the efficiency of generalized spreading codes (for both unitary and complementary codes) in terms of the width of their interference free windows.

Cooperative relay is an effective means to improve spectral efficiency in wireless networks. In this paper, we propose a cooperative relay network protocol based on adaptive cooperative coding (ACC), in which the level of cooperation (LoC) is made adaptive to follow the variation of channel propagation conditions. We derive the strict performance upper bound and asymptotic bound expressions for the ACC protocol. Moreover, we develop a sub-optimal power allocation algorithm based on the derived asymptotic bound to further improve the performance of the ACC protocol. The optimization is achieved in two scenarios based on different link channel qualities. The cooperative diversity gain and power allocation gain of the ACC protocol are evaluated and their numerical results are provided.

Resource allocation and call admission control (CAC) are two key issues in satellite networks for multimedia applications with quality of service (QoS) guarantees. This paper introduces a new priority-based CAC scheme for multiclass services in satellite networks operating above 10 GHz, where the dominant factor limiting performance is rain attenuation, a phenomenon exhibiting both spatial and temporal variation. The proposed scheme exploits the predictability of the satellite channel and accepts a new call if sufficient resources exist for existing and new users to guarantee their QoS requirements. At the same time, it assigns higher priority to real-time users compared to non-real time ones. The performance of the proposed scheme is investigated using Markov chain to represent the satellite channel states. Analytical expressions concerning the call blocking probability under clear sky and rain conditions are derived.

Cognitive radio (CR) has been considered as a promising technology to improve the spectrum utilization. In this paper we analyze the capacity of a CR network with average received interference power constraints. Under the assumptions of uniform node placements and a simple power control scheme, the maximum transmit power of a target CR transmitter is characterized by its cumulative distribution function (CDF). We study two CR scenarios for future applications. The first scenario is called the CR based central access network, which aims at providing broadband access to CR devices. In the second scenario, the so-called CR assisted virtual multiple-input multiple-output (MIMO) network, CR is used to improve the access capability of a cellular system. The uplink ergodic channel capacities of both scenarios are derived and analyzed with an emphasis on understanding the impact of numbers of primary users and CR users on the capacity. Numerical and simulation results suggest that the CR based central access network is more suitable for less-populated rural areas where a relatively low density of primary receivers is expected; while the CR assisted virtual MIMO network performs better in urban environments with a dense population of mobile CR users.

Transmissions of large sized images can be a bottleneck for a wireless sensor network (WSN) due to its limited resources. Security can be another concern. This paper proposes a collaborative transmission scheme for image sensors to utilize inter-sensor correlations to decide the transmission and security sharing patterns based on the path diversities. Our proposed approach for secret image sharing on multiple node-disjoint paths for image delivery is to achieve high security without any key distribution and management, and thus the key management related problems do not exist. The energy efficiency is another major contribution made in this paper. This scheme does not only allow each image sensor to transmit optimal fractions of overlapped images through appropriate transmission paths in an energy-efficient way, but also provides unequal protection to overlapped image regions by path selections and adaptive bit error rate (BER) requirement. The simulation results show that the proposed scheme can achieve considerable gains in terms of network lifetime extension, image transmission security enhancement, image quality improvement, and energy efficiency for wireless sensor networks.

Distributed Source Coding (DSC) is an effective means in reducing data redundancy in Wireless Sensor Networks (WSNs). However, the issues on designing multirate DSC with multirate transmission in WSNs have not been well addressed in the literature. In this paper, we propose a cross-layer approach to achieve optimal DSC data quality while assuring energy efficiency and latency requirement through adjusting multirate DSC data dependencies and network multirate transmission parameters jointly. Specifically, the DSC coding dependency among multirate source coding sensors are fine-tuned to balance compression efficiency and transmission robustness. Transmission rate and retransmission limit on each wireless link are optimized accordingly to achieve Unequal Error Protection (UEP) among inter-dependent DSC streams. Simulation studies demonstrate that the proposed scheme ensures satisfactory DSC data quality and energy efficiency.

Block ciphers encrypt a fixed size block of plaintext at a time to produce a block of ciphertext. Stream ciphers encrypt stream data, such as voice or Telnet traffic, one bit or more bits at a time. The cipher feedback mode is a stream cipher implemented by a block cipher via multiple stages, and in each stage one bit or a number of bits of plaintext are encrypted at a time. In this paper, we study error performance of the stream-based cipher feedback mode in an unreliable wireless channel in terms of throughput. We model performance of the cipher feedback mode in terms of the probability that part of or the whole ciphertext can not be successfully decrypted, and the throughput by adopting the cipher feedback mode. We explicitly derive the optimal number of stages in the cipher feedback mode to achieve the optimal throughput, given an error rate in a wireless network. We also prove that for the cipher feedback mode, the whole ciphertext is successfully decrypted if and only if the whole ciphertext is successfully transmitted.

We propose a hierarchical pipeline paging (HPP) for multi-tier hierarchical cellular networks, in which different tiers overlay with one another to provide overlapped coverage of cellular service, and each mobile terminal can be paged in any tier of a network. Paging requests (PRs) are queued in different waiting queues, and multiple PRs in each waiting queue are served in a pipeline manner. We study HPP, hierarchical sequential paging (HSP), and hierarchical blanket paging (HBP) schemes analytically in terms of discovery rate, total delay, paging delay, and cost. It is shown that HPP scheme outperforms both HBP and HSP schemes in terms of discovery rate while maintaining the same cost as HSP scheme. The HPP scheme outperforms HSP scheme in terms of total delay and has a lower total delay than HBP scheme when traffic load is high.

Abstract In this paper, a cross‐layer analytical framework is proposed to analyze the throughput and packet delay of a two‐hop wireless link in wireless mesh network (WMN). It considers the adaptive modulation and coding (AMC) process in physical layer and the traffic queuing process in upper layers, taking into account the traffic distribution changes at the output node of each link due to the AMC process therein. Firstly, we model the wireless fading channel and the corresponding AMC process as a finite state Markov chain (FSMC) serving system. Then, a method is proposed to calculate the steady‐state output traffic of each node. Based on this, we derive a modified queuing FSMC model for the relay to gateway link, which consists of a relayed non‐Poisson traffic and an originated Poisson traffic, thus to evaluate the throughput at the mesh gateway. This analytical framework is verified by numerical simulations, and is easy to extend to multi‐hop links. Furthermore, based on the above proposed cross‐layer framework, we consider the problem of optimal power and bandwidth allocation for QoS‐guaranteed services in a two‐hop wireless link, where the total power and bandwidth resources are both sum‐constrained. Secondly, the practical optimal power allocation algorithm and optimal bandwidth allocation algorithm are presented separately. Then, the problem of joint power and bandwidth allocation is analyzed and an iterative algorithm is proposed to solve the problem in a simple way. Finally, numerical simulations are given to evaluate their performances. Copyright © 2008 John Wiley & Sons, Ltd.

Abstract The access control model is a core aspect of trusted information systems. Based on the role based access control (RBAC) model, we put forward the concept of thehomonymous role, which extends the role control categories in RBAC, balances the control granularity and the storage space requirements, and executes the fine‐grained access control. Instead of the traditional global access control policies (GACP), we propose thehomonymous control domain(HCD) mechanism to enable the coexistence of multiple types of access control policies in a single system, thereby improving the control granularity and flexibility. The HCD mechanism facilitates the discretionary supporting of independent access control policies for its homonymous user. The HCD mechanism and the traditional access control mechanism can be linked to construct a two‐layer access control policy mechanism for a system. Notably, we also consider the temporal characteristic in HCD, which is a critical feature of modern access control models. Furthermore, we analyze the conflicts between the HCD and GACP mechanisms. Finally, we design and implement our HCD on FreeBSD to demonstrate the advantages of the two‐layer access control mechanism. Copyright © 2008 John Wiley & Sons, Ltd.

Extensive studies on supporting voice traffic over wireless 802.11 networks have been carried out in the literature. Most of them were focused only on one hop infrastructure mode. This paper addresses the issue of voice over multihop wireless networks. Through simulation considering a simple topology, we identify the voice traffic bottleneck in 802.11 MAC and thus propose a burst queue (BQ) scheme to cooperate with 802.11b and 802.11e MAC protocols. Extensive simulation results under variety of circumstances show that BQ scheme can support 50%~100% more voice calls and obtain 20%~50% lower delay at the cost of a little higher loss ratio which is tolerable to voice flows. Even for the grid and random topology, our BQ scheme can provide smaller packet loss ratio, and shorter end-to-end delay.

Multimedia delivery over Wireless Sensor Networks (WSNs) is energy critical. In this paper, we study an energy constrained cross layer optimization scheme to improve digital image quality in cross polarization discrimination (XPD) based Multiple Input Multiple Output (MIMO) WSNs. The contribution of this research can be summarized in three parts. First, link layer energy consumption and packet loss ratio in multirate XPD based MIMO WSN are modeled. Secondly, XPD based modulation optimization cross link and physical layer is proposed, achieving considerable energy efficiency. Finally, a new Position-Value (P- V) based Unequal Error Protection (UEP) scheme is studied and discussed in XPD MIMO WSNs, with more efficient resource allocation compared to traditional approaches. Simulation results show the proposed modulation optimization scheme improves energy efficiency considerably, and position based UEP achieves significant distortion-energy gain compared with traditional layer based resource allocation schemes in XPD MIMO WSNs.

Security is critical for wireless sensor networks deployed in military, homeland security and other hostile environments. In this paper, we study a security issue related with broadcast in sensor networks. Due to the broadcast nature of wireless communications, often it is more efficient to broadcast packets to sensor nodes. Typically, broadcast authentication is achieved by digital signatures. Since digital signature operations are expensive for small sensor nodes, an attacker can launch a serious denial of service (DoS) attack. That is, an attacker may forge a large number of broadcast messages with digital signatures, and then force sensor nodes to verify these signatures, which can cause them run out of power. In this paper, we present an effective and efficient scheme that can defend such DoS attack on broadcast authentication. Our performance evaluation shows that the scheme is much more secure and efficient than an existing scheme.

In this paper we study the system level performance of cognitive radio (CR) networks under average received interference power constraints. Under the assumption of uniform node placements and a simple power control scheme, we derive the closed-form expression for the cumulative distribution function (CDF) of the maximum allowable transmit power of the target CR transmitter. We further study two CR network scenarios: a CR based central access network and a CR assisted virtual multiple-input multiple-output (MIMO) network. The average uplink capacities of both networks are derived and analyzed, with an emphasis on understanding the effect of the numbers of primary users and CR users on the capacity. Numerical and simulation results demonstrate that the CR based central access network is more suitable for less-populated rural areas where a lower density of primary receivers is expected, while the CR assisted virtual MIMO network performs better in urban environments with a dense population of mobile CR users.

We propose a reduced-complexity genetic algorithm for dynamic deployment of resource constrained multi-hop mobile sensor networks. The goal of this paper is to achieve optimal coverage and improved battery life using dynamic power scaling (DPS) and improved fitness function. DPS exploits idle times, packet delay guarantees, performance and workload data using additional controls related to sensor power states and transmission power. The dynamic power scaling in conjunction with genetic algorithm jointly optimizes power states and topologies by dynamically monitoring workloads, packet arrivals, utilization data and quality-of-service compliance. This results in minimization of the power consumption of the sensor system while maximizing the sensor objectives.

A Wireless Multimedia Sensor Network (WMSN) is an emerging networking paradigm that allows retrieving video and audio streams, still images, and generic sensing data. Different from conventional wireless sensor networks, a WMSN requires higher network bandwidth and throughput to deliver multimedia contents effectively using energy-constrained devices. In this paper, we propose a clustered on-demand multi-channel MAC protocol (COM-MAQ) to support energy-efficient, high- throughput, and reliable data transmission in WMSNs. The operation of proposed protocol consists of three sessions: request session, scheduling session, and data transmission session. For COM-MAC to achieve high energy efficiency, first, a scheduled multi-channel medium access is used within each cluster so that cluster members can operate in a contention-free manner within both time and frequency domains to avoid collision, idle listening and overhearing. Second, to maximize the network throughput, a traffic-adaptive and QoS-aware scheduling algorithm is executed to dynamically allocate time slots and channels for sensor nodes based on the current data traffic information and QoS requirements. Finally, to enhance transmission reliability, a spectrum-aware ARQ is incorporated to better exploit the unused spectrum for a balance between the reliability and retransmission. Simulation results indicate that COM-MAC can achieve increased network throughput at the cost of a small control and energy overhead.

This paper presents an adaptive call admission control scheme for multi-class services in wireless networks. We propose a closed-loop feedback control mechanism, which is composed of three model blocks: priority-based queueing model, adaptive fuzzy service degradation control model, and optimal service degradation allocation model. Based on the established service differentiation scheme, we present a priority-based queueing model, which provides a general method to calculate the handoff call dropping probability and new call blocking probability. The priority-based queueing model regulates each class of service to compete for bandwidth resources with a certain probability, which is proportional to its priority coefficient. In the meanwhile, the adaptive fuzzy service degradation control model degrades the existing services in the system, in order to release appropriate amount of recourses to admit more handoff calls. We design a fuzzy service degradation controller to dynamically control the future degree of degradation, according to the input current degree of degradation and handoff call dropping probability. Finally we propose an optimal service degradation allocation model. The intuition is that the optimization problem can be regarded as an analogy of the energy distribution and stability problem of a potential system. Optimization problem is then formulated, and nonlinear programming technique is employed to solve the optimal scheme of service degradation.

A secure image transmission mechanism is necessary when malicious intruders intend to access and modify content delivery over wireless networks. To assure data integrity, authentication is required for multimedia data delivered over wireless image sensor networks. Watermarking technique is an effective vehicle to assert and assure the image data authentications. There have been recent works reported on watermarking, but few with the consideration of energy cost in terms of the data communication and processing, which is a key constraint to many embedded systems and wireless sensor networks. Most watermarking systems only target at minimizing watermarked image distortion and increasing robustness at the source coding site for lossy image processing. The watermarked image distortion caused by error-prone wireless environments during transmission has not been fully considered. In this paper, an innovative energy-aware adaptive watermarking scheme for realtime image delivery is proposed in wireless multimedia sensor networks. This new scheme allocates network resource to protect the watermarked image transmission while embedding watermark coding redundancies into the images. Dynamic watermark thresholds are applied to be adaptive to the network condition (packet loss ratio) and the inter-frame correlation is exploited to reduce processing delay. The simulation results show that the proposed adaptive watermark system can achieve considerable energy efficiency and assure the data integrity.

Joint consideration of energy efficiency, multimedia quality and media authenticity has largely been overlooked in the studies of secure wireless multimedia streaming. In this paper, we propose an authentication - resource allocation framework for secure transmission of multimedia streaming over energy constrained wireless networks. The contributions of this research can be summarized in three aspects. Firstly, a hash chain based stream level authentication scheme compatible for JPEG2000 compression standard is proposed. Secondly, link layer energy- distortion is modeled, and the energy-distortion convex hull is efficiently identified, which significantly reduces optimization complexity. Finally, an authentication oriented resource allocation scheme is proposed based on the proposed authentication scheme and link layer energy-distortion convex hull, where the authenticated image quality is optimized according to energy budget constraints.

Presents the introductory welcome message from the conference proceedings.

The problem of wireless resource management in broadband cognitive OFDMA networks is addressed. The objective is to maximise the multiple cognitive users' weighted rate sum by jointly adjusting their rate, frequency and power resource, under the constraints of multiple primary users' interference temperatures. First, based on two interpretations of the interference temperatures, the problem studied is formulated as two nonlinear and non-convex optimisation problems. Secondly, these two problems are analysed, and a centralised greedy algorithm is proposed to solve one problem, as well as a centralised algorithm based on Lagrangian duality theory for the other. The two centralised algorithms are shown to be optimal and both have polynomial time complexities. Finally, it is shown that the two centralized algorithms can be distributively implemented by introducing the idea of virtual clock. And the distributed algorithms can be interpreted as an interesting distributed negotiated secondary market approach. It is believed that the work will provide a good reference for the emerging cognitive network protocol design.

The 17 papers in this special issue focus on the theory and applications of cognitive radio.

Abstract Security is of ultimate importance to the global communication and information networks. Both academia and industry have seen the importance of the R&D activities in security and communication networks, and a great deal of resources has been invested by both governments and private sectors around the world to enhance the security of global information and communication networks, which virtually cover all regions of the world. Therefore, the timely launch of Security and Communications Networks is of great interest to the world community. In this inaugural issue, worldwide experts contribute their papers, covering various important research areas in security. Copyright © 2008 John Wiley & Sons, Ltd.

Abstract The ever‐increasing demand for the services of wired and wireless data networks has led to network security being a prerequisite for all successful data transactions. This has accelerated research activities in academia, private sectors, and governments to respond to the challenges of providing complete solutions for secure communications and networks. Security and Communication Networks provides a distinctive global platform to explore the research advances in the communications network security from academia, industry, and governments. In this second issue, six thought provoking papers discussing various element of security research in communications and networks are presented. Copyright © 2008 John Wiley & Sons, Ltd.

Image transmissions in Wireless Multimedia Sensor Networks (WMSNs) are often energy constrained. They also have requirement on distortion minimization, which may be achieved through Unequal Error Protection (UEP) based communication approaches. In related literature with regard to wireless multimedia transmissions, significantly different importance levels between image-pixel-position information and image-pixel-value information have not been fully exploited by existing UEP schemes. In this paper, we propose an innovative image-pixel-position information based resource allocation scheme to optimize image transmission quality with strict energy budget constraint for image applications in WMSNs, and it works by exploring these uniquely different importance levels among image data streams. Network resources are optimally allocated cross PHY, MAC and APP layers regarding inter-segment dependency, and energy efficiency is assured while the image transmission quality is optimized. Simulation results have demonstrated the effectiveness of the proposed approach in achieving the optimal image quality and energy efficiency. The performance gain in terms of distortion reduction is especially prominent with strict energy budget constraints and lower image compression ratios.

In this letter, we study the user pairing algorithms for uplink virtual multiple-input multiple-output (V-MIMO) systems, and propose a new double proportional fair (D-PF) algorithm for user pairing of the uplink V-MIMO systems. The proposed D-PF algorithm uses the PF criterion to decide the first user, and a modified PF criterion is suggested to choose the pairing user. Simulation results show that the proposed D- PF algorithm outperforms the conventional determinant formula (DF) algorithm in terms of both throughput and fairness performances. Moreover, if compared with the conventional single PF (S-PF) algorithm, the proposed D-PF algorithm achieves a fairly good trade-off between the throughput and fairness, being able to trade with 5% throughput loss for 15% fairness enhancement.

This paper introduces a new space-time coding scheme, namely space-time complementary coding (STCC) scheme. With the help of pair-wise complementary codes, an STCC MIMO system can achieve both spatial diversity and multiplex transmission. The scheme requires neither bandwidth expansion nor feedback information from receiver, and it offers a nearly interference-free and multipath-resist performance due to a widely open interference-free window of the pair-wise complementary codes. The analytical results reveals that it provides a uniquely high diversity-multiplex gain product for its applications in flat or frequency-selective fading channel.

In this paper, we consider a cognitive radio system in fading wireless channels and propose an opportunistic power control strategy for the cognitive users, which serves as an alternative way to protect the primary user's transmission and to realize spectrum sharing between the primary user and the cognitive users. The key feature of the proposed strategy is that, via opportunistically adapting its transmit power, the cognitive user can maximize its achievable transmission rate without degrading the outage probability of the primary user. If compared with the existing cognitive protocols, which usually try to keep the instantaneous rate of the primary user unchanged, our strategy relieves the cognitive users from the burden of detecting and relaying the message of the primary user and relaxes the system synchronization requirements. The achievable rate of a cognitive user under the proposed power control strategy is analyzed and simulated, taking into account the impact of imperfect channel estimation. A modified power control strategy is also proposed to reduce the sensitivity of our strategy to the estimation errors. Its effectiveness is verified by the simulations.

According to the latest version of the IEEE 802.11 standard, the backoff parameters of its collision avoidance mechanism are far from optimal, especially in a heavy load or error-prone WLAN environment. This strategy has a high collision probability and channel utilization is degraded in bursty arrivals or congested scenarios. Besides, the standard backoff mechanism may treat noise corruption as packet collisions. In this paper, we identify the relationship between backoff parameters, contention level, and channel BER in order to propose a simple, but yet well-performing distributed algorithm that allows a station to dynamically adjust its contention window size based on turn-around-time measurement of channel status. In addition to theoretical analysis, simulations are conducted to evaluate its performance. The proposed scheme works very well in providing a substantial performance improvement in heavy loaded and error-prone WLAN environments.

This paper investigates the random channel access mechanism specified in the IEEE 802.16 standard for the uplink traffic in a point-to-multipoint (PMP) network architecture. An analytical model is proposed to study the impacts of the channel access parameters, bandwidth configuration and piggyback policy on the performance. The impacts of physical burst profile and non-saturated network traffic are also taken into account in the model. Simulations validate the proposed analytical model. It is observed that the bandwidth utilization can be improved if the bandwidth for random channel access can be properly configured according to the channel access parameters, piggyback policy and network traffic.

It has been widely known that one of the key design parameters in orthogonal frequency division multiplexing (OFDM) systems is the distribution of peak-to-average power ratio (PAPR). Recently some theoretical approaches to determine the PAPR distribution have been proposed based on the assumption that all subcarriers are allocated with equal power. However, this assumption may not be valid due to the following facts. First, in all realistic OFDM systems, usually only a subset: of subcarriers are used to carry information (active subcarriers) and the rest (inactive subcarriers) are set to zero. Second, due to the efficiency concerns transmission power should be allocated to active subcarriers. Third, power allocation may vary depending on different constellations used by different active subcarriers and their signal-to-noise-ratios. In this paper, we propose a general approach to identify PAPR distribution in OFDM systems. Specifically, a more accurate analytical expression of the PAPR distribution is derived with the help of Extreme Value Theory for Chi-squared-2 process in OFDM systems with unequal power distribution strategy. To validate the analytical results, extensive simulations have been conducted, showing a very good match between the identified PAPR distribution and that of real OFDM systems.

Spreading code plays an extremely important role on the overall performance of a CDMA system. The correlation properties and available number of spreading codes determine the interference-resist capability as well as system capacity. In this paper, we analyze the characteristics and limitations of traditional and recently reported spreading codes. Based on the analysis, we propose a new code design approach which will be used to generate inter-group complementary (IGC) codes. The correlation functions of the IGC codes possess definite and bi-valued interference-free windows. In addition, a corresponding code assignment algorithm and spreading scheme will be introduced to take advantage of the desirable properties of the IGC codes for their applications in CDMA systems. Both theoretical analysis and simulation results show that an IGC-CDMA system is interference-resistant and capable to offer a high spectral efficiency if compared with the ones based on other spreading codes.

Key management has become a challenging issue in the design and deployment of secure wireless sensor networks. A common assumption in most existing distributed key management schemes is that all sensor nodes have the same capability. However, recent research works have suggested that connectivity and lifetime of a sensor network can be substantially improved if some nodes are given greater power and transmission capability. Therefore, how to exploit those heterogeneity features in design of a good distributed key management scheme has become an important issue. This paper proposes a unified framework for distributed key management schemes in heterogeneous wireless sensor networks. Analytical models are developed to evaluate its performance in terms of connectivity, reliability, and resilience. Extensive simulation results show that, even with a small number of heterogeneous nodes, the performance of a wireless sensor network can be improved substantially. It is also shown that our analytical models can be used to accurately predict the performance of wireless sensor networks under varying conditions.

Cognitive radio (CR) is a dynamic spectrum access technology as a solution to spectrum under-utilization problem in some licensed bands. Operating over an exceedingly wide spectrum, CR systems usually adopt multi-carrier modulation (MCM) to implement flexible channelization. Consequently, efficient channel allocation scheme becomes extremely important to an MCM based CR (MCM-CR) system. In this paper, a maximum likelihood detection model is developed to detect the presence and locations of licensed users (LUs) signals in the frequency domain. Performance of the detection model, including the optimal detection region, detection probability and false alarm probability, is analyzed. A one-order two-state Markovian chain model is proposed to predict channel status information. In particular, a novel subcarrier allocation scheme for MCM-CR systems is proposed, taking into account the confidence of channel estimation, quality of services (QoS) of rental users (RUs) and throughput. To validate the analytical results, simulations have been conducted to show effectiveness of the proposed scheme.

In this paper, an innovative frequency domain joint estimation algorithm for synchronization parameter and channel impulse response (CIR) in direct sequence code division multiple access (DS-CDMA) systems is proposed. The algorithm is based on the expectation-maximization (EM) method. It can provide accurate estimation of channel state information and synchronization parameter for a DS-CDMA receiver even with a simple equalization module (e.g., an one-tap multiplier based frequency domain equalizer (FDE)), and a radio receiver with this approach performs better than a costly multi-tap multiplier based equalizer, such as the time domain equalizer (TDE). A generic receiver architecture based on the frequency domain equalization for a composite radio, which works in both CDMA and orthogonal frequency division multiplexing (OFDM) modes, is also proposed. The Cramer-Rao lower bound (CRLB) of the proposed estimator and its optimization scheme are derived. This architecture can be implemented with an iterative approach, and the results demonstrate that this adaptive receiver performs very well with a relatively low cost.

The advancement in Distributed Source Coding (DSC) for mission-driven wireless sensor networks (WSNs) has opened a new vista for its wide applications in multiple correlated sensor networks such as real time target tracking and environment monitoring. The features of the DSC applications offer many potential opportunities for sensor networks to utilize multirate transmissions for network performance enhancement. In this paper, we propose a methodology for cross-layer optimization between routing and DSC in WSNs, and we introduce a multirate based routing scheme for mission-driven DSC applications that can considerably extend network lifetime. The proposed scheme adopts the rate assignment based on residual energy and employs a joint rate and energy scheduling mechanism to meet the end-to-end transmission rate demand, information precision requirement, and energy constraints in the networks adopting DSC. This approach is different from the traditional multirate research in link adaptation, where the focus was to increase channel throughput based on rate adaptation from variable channel conditions. The objective of this work is not to utilize multirate for increasing channel throughput, but to exploit multirate capability for routing optimization in DSC based applications with consideration for energy consumption. We also introduce the concept of "Energy Usage Scheduling (EUS)" to optimize the energy usage based on the rate constraints and DSC application needs. Simulation results show that the proposed scheme can achieve a significantly longer network lifetime than that reported in the literature.

The generation of multiple uncorrelated Rayleigh fading waveforms is often demanded for simulating wideband fading channels, multiple-input multiple-output (MIMO) channels, and diversity-combined fading channels. In this letter, an improved deterministic sum-of-sinusoids (SoS) channel simulator with a new parameter computation method is proposed to simulate a large number of uncorrelated Rayleigh fading processes. Compared with the existing SoS channel simulators, the proposed deterministic SoS model yields a much better simulation efficiency while still preserving satisfactory approximations to the desired statistical properties of the reference model.

The (lower-bound) symbol error rate (SER) performance of cooperative digital communication schemes is investigated, where multiple dual-hop relays are invoked for achieving the cooperative diversity with the aid of the maximal ratio combining (MRC). A range of closed-form expressions are obtained for the probability density functions (PDFs) of the relay-channels' output signal-to-noise ratio (SNR). Accurate and approximated closed-form expressions are derived for computing the SER. Our study shows that, with the aid of the closed-form expressions obtained in this contribution, the SER performance of the relay-assisted digital communications schemes, which may employ various classes of coherent modulations, can be readily evaluated in the context of a variety of fading scenarios that the cooperative signals might experience.

In this paper, we perform a tele-traffic queueing analysis for OFDM subcarrier allocation in wireless multiservice networks. For this purpose two call admission control algorithms, the batch blocking scheme and the partial blocking scheme, are proposed. Call connection requests are classified into two different types, narrow-band and wide-band. For either class of calls, the traffic process is characterized as batch arrival, as each call may request multiple subcarriers to satisfy its quality-of-service (QoS) requirements. The batch size is a random variable which obeys a probability mass function (pmf) with a realistic maximum value. In addition, the service times for various call classes are different. Consequently, an OFDM-based broadband wireless multiservice network can be formulated as a multiclass multiserver batch arrival queueing system. The methodology and results are further generalized into a multiple-class scenario with service priority provision. Formulae are developed for evaluation of the following performance metrics: the probability that a call will be blocked, the average number of subcarriers used, and bandwidth utilization. Numerical results are presented to demonstrate the interactions between key parameters and performance metrics. The analytical model was validated by the simulation results, showing the fact that it can be used as an efficient tool for design of future-generation broadband wireless access networks.

Random access channel (RACH) is usually used for initial channel access, bandwidth request, etc. In this paper, we analyze the throughput and access delay performance of the RACH in an orthogonal frequency division multiple access (OFDMA) system. A closed-form expression of the RACH throughput is presented and its mean access delays under both binary exponential and uniform backoff policies have also been derived. Moreover, to further improve the utilization of RACH in an OFDMA system, we propose a novel message transmission scheme in a shared RACH. The message in the proposed scheme can be sent implicitly in the cyclic-shifted preambles which make the transmission much more reliable in a contention based RACH. Meanwhile, performance of the preamble detection has also been improved by the redundant information conveyed in messages. Finally, simulation results will demonstrate performance gains of the proposed scheme.

Abstract Long propagation delay is one of the most important characteristics in underwater wireless sensor networks (UWSNs) and poses a great challenge for medium access control (MAC) protocol design, especially for contention‐based MAC protocols due to intolerable delay caused by unpredictable retransmissions in UWSNs. Recently, some MAC protocols for UWSNs have been suggested in the literature, and most of them are based on random access with their capabilities to compensate propagation delay. However, two issues should still be resolved in these protocols: (a) they need to make measurements to realize duty cycle synchronization and (b) packet collisions exist, which not only reduce the throughput (thus increasing delays) but also waste energy. In this paper, we propose a novel MAC protocol explicitly designed for UWSNs, which makes the best use of the propagation delay to resolve collision problem and reduce overhead of control‐packet to save energy. The proposed MAC protocol can assure that the number of retransmissions is not more than one. To validate the analytical results, simulations have been conducted to show that the proposed MAC protocol can offer a low energy consumption while avoiding collisions in UWSNs. Copyright © 2008 John Wiley & Sons, Ltd.

Abstract In order to support the quality‐of‐service (QoS) requirements for real‐time traffic over broadband wireless networks, advanced techniques such as space‐time diversity (STD) and multicarrier direct‐sequence code division multiple access (MC‐DS‐CDMA) are implemented at the physical layer. However, the employment of such techniques evidently affects the QoS provisioning algorithms at the medium access control (MAC) layer. In this paper, we propose a space‐time infrastructure and develop a set of cross‐layer real‐time QoS‐provisioning algorithms for admission control, scheduling, and subchannel‐allocations. We analytically map the parameters characterizing the STD onto the admission‐control region guaranteeing the real‐time QoS. Our analytical analyses show that the proposed algorithms can effectively support real‐time QoS provisioning. Also presented are numerical solutions and simulation results showing that the STD can significantly improve the QoS provisioning for real‐time services over wireless networks. Copyright © 2007 John Wiley & Sons, Ltd.

Security is critical for sensor networks used in military, homeland security and other hostile environments. Previous research on sensor network security mainly considers homogeneous sensor networks, i.e., all sensor nodes are the same. Research has shown that homogeneous ad hoc networks have poor performance and scalability. Furthermore, many security schemes designed for homogeneous sensor networks have high communication overhead, computation overhead, and/or large storage requirement. To achieve better security and performance, we adopt a heterogeneous sensor network (HSN) model. In this paper, we present an efficient key management scheme that takes advantage of the powerful high-end sensors in HSN. The performance evaluation and security analysis show that the key management scheme provides better security with low complexity and significant reduction on storage requirement, compared to existing sensor key management schemes.

Time synchronization is critical for many sensor network operations, such as mobile object tracking and sensor node scheduling. Many existing time synchronization schemes for sensor networks do not considering security issue, and they are vulnerable to several attacks. Furthermore, most existing synchronization schemes are designed for homogeneous sensor networks, where all sensors have the same capability. Research has shown that homogeneous ad hoc networks have poor fundamental limits and performance. To improve performance and security of sensor networks, we adopt a Heterogeneous Sensor Network (HSN) model. In this paper, we present a secure and efficient time synchronization scheme for HSN by utilizing powerful high-end sensors. We implement the synchronization scheme in real sensors and our experiments show that the scheme achieves high accuracy. The security analysis demonstrates that our scheme is resilient to various attacks.

In this paper, an iterative EM-based frequency domain joint estimation algorithm of synchronization parameter and channel impulse response (CIR) in direct sequence code division multiple access (DS-CDMA) system is proposed. This algorithm is based on the expectation-maximization (EM) method. A complete Cramer-Rao lower bound (CRLB) analysis of the proposed algorithms is also given in this paper. Because this algorithm can provide accurate estimation of channel conditions and synchronization parameter for DS-CDMA receivers, even with a simple equalization module, for example, a one-tap multiplier based frequency domain equalizer (FDE), good performance can be achieved at a low cost.

In this paper, we propose an algorithm for frequency domain adaptive Doppler shift estimation in the presence of multiple Doppler subpaths in DS-CDMA systems. By modeling doubly selective channel with a basis expansion model (BEM), the proposed method works based on expectation-maximization (EM) algorithm and can accurately estimate Doppler shifts. A complete Cramer-Rao lower bound (CRLB) analysis of the estimation algorithms is also provided.

Many applications need the support of multi-privileged group communications, which use multiple data streams for members with various access privileges. We propose an ID-based hierarchical key graph scheme (IDHKGS) to manage multi- privileged group communications. The proposed scheme employs a key graph, on which each node is assigned a unique ID according to access relations between nodes. When a user joins/leaves a group or changes access privileges, other users in the group can deduce the new keys using one-way function by themselves according to the ID of joining/leaving/changing node on the graph. Compared with the existing MGKMS scheme, the proposed scheme can greatly reduce the rekeying overhead at the expense of a slightly larger storage overhead.

In image transmission applications over wireless sensor networks (WSN), energy efficiency and image quality are both important factors for joint optimization. In this paper, we propose a new position-value (P-V) oriented cross layer optimization approach to minimize energy consumption with distortion bounds for image transmission applications in WSN, by exploring importance level diversity for position-magnitude information. To put more effort on transmitting important position information and relatively less effort on unimportant magnitude information, desirable BER, ARQ retry limit, transmission rate and distortion reduction of image are jointly optimized cross PHY, MAC and APP layers, and image quality is assured while energy consumption is minimized. Simulation results demonstrate the effectiveness of the proposed approach in achieving energy efficiency while maintaining image quality.

We propose a group-based location service (GrLS) for mobile ad hoc networks (MANETs). The novelty of GrLS is in its exploitation of group mobility to improve the protocol efficiency. As an important mobility scenario, group mobility has many popular applications. GrLS provides different location management strategies for individual nodes and groups of nodes. An individual node recruits its own location servers and updates its location to them. On the other hand, in a set of nodes with group mobility, only the group leader recruits location servers and updates its location to a specific home region called group home region. Other members of the group are waived from performing network-wide location updates. Since the location update cost normally dominates other costs for all practical purposes, the overhead of location service is significantly reduced. Simulation results also show that GrLS can achieve higher location query success ratio with much lower overhead than existing protocols that do not consider group mobility.

The packet-pair dispersion techniques are the most common probing-based approach to measuring the bottleneck capacity of a path. In practice, the dispersion measurement, and therefore the bandwidth estimation, could be seriously distorted by the cross traffic queuing between or in front of the probe packet pair. Almost all the existing packet-pair techniques depend on heuristic filtering methods to find a final capacity estimate. In this paper, we take a different perspective to exploit the cross- traffic effect. We develop a queueing model to describe the output packet-pair dispersions interfered by the cross traffic, based on which a new measurement technique to estimate the available bandwidth is derived. Another important contribution is that we for the first time reveal that the statistics of the cross traffic, e.g. the marginal distribution and the autocovariance function of the arrival process, can also be inferred from the stochastic behavior of the output packet dispersions. Efficiency of the proposed available bandwidth estimation and traffic characterization techniques are demonstrated by computer simulations.

The many-to-one traffic pattern dominates in sensor networks, where a large number of sensor nodes send data to one sink. A sensor node may only communicate with a small portion of its neighbors. Most existing key management schemes for sensor networks are designed to establish shared keys for all pairs of neighbor sensors, no matter whether they communicate with each other or not, and this causes large overhead. To achieve better security and performance, we adopt a heterogeneous sensor network (HSN) model. In this paper, we propose a novel routing-driven key management scheme, which only establishes shared keys for neighbor sensors that may communicate with each other. Work has demonstrated the feasibility of implementing elliptic curve cryptography on small sensor nodes. We utilize elliptic curve cryptography to design an efficient key management scheme for HSN. The performance evaluation and security analysis show that our key management scheme can provide better security with significant saving on sensor storage space and energy consumption than some existing key management schemes.

Orthogonal frequency division multiplexing (OFDM) signals are very sensitive to radio frequency (RF) impairments. One of the major impairments is the mismatching errors between the in-phase (I) and quadrature (Q) branches in the up and down-conversions. The I/Q mismatching errors can include phase and amplitude mismatching errors. These mismatching errors make the signal constellation expand and rotate, which severely degrades the performance of OFDM systems. In this paper, a closed-form expression for the bit error rate (BER) of the OFDM system in I/Q mismatching error environments is derived as a function of phase and amplitude mismatching error parameters. This enables a convenient and useful evaluation of the performance of OFDM communication systems with I/Q mismatching errors. The derived analytical expression is validated by the fact that computer simulation results closely match with the analytical derivation results.

This paper addresses context issues in pervasive mobile computing and presents a semantic context model to support location based cooperative mobile applications. In this paper, a mobile learning prototype has been discussed as a scenario for evaluation of the context model. A Bluetooth based wireless solution is introduced for the purpose of integrating this model with the current e-learning system at UCE Birmingham. The aim of our research is to investigate some essential issues on a suitable context pattern applicable to contextual cooperative mobile computing.

We propose a reduced-complexity genetic algorithm for secure and dynamic deployment of resource constrained multi-hop mobile sensor networks. Mobility and security are relatively expensive operations since they involve both communication and computation. Furthermore, these operations have to co-exist with optimal node and route assignments. The goal of this paper is to achieve optimal secure coverage and improved battery life using dynamic re-locatability. The genetic algorithm is used to adaptively configure optimal position and security attributes by dynamically monitoring network traffic, packet integrity, and battery usage. This results in minimization of the power consumption of the sensor system while maximizing the sensor objectives (coverage and exposure).

Most of the current 802.11b research activities are conducted under stationary environment. As the need for high-speed connectivity in mobile environments increases, it becomes necessary to understand the impact of mobility on performance characteristics of 802.11b. In this paper, we present an investigation of the mobility impact on performance of the 802.11b system with Rician/Rayleigh fading under different client velocities. This includes analysis of the Doppler shift caused by the velocity of transmitter and receiver, the multipath interference due to reflections and diffractions from terrains in the radio service coverage area, and other serious impairing factors. We have also studied the bit error rate performances for various velocities with different data rates. Our analysis includes our analytical and simulation data verified with the results from our 3.5 mile Federal Railroad Administration test bed on the BNSF railroad track in Nebraska. Our results show that the current implementation of 802.11b standard has promising potential to be used in a mobile environment if the line-of-sight path between transmitter and receiver exists.

In this paper, an innovative frequency domain joint estimation algorithm of synchronization parameter and channel impulse response (CIR) in Direct Sequence Code Division Multiple Access (DS-CDMA) system is proposed. This algorithm is based on the expectation-maximization (EM) method. Because this algorithm can provide accurate estimation of channel conditions and synchronization parameter for DS-CDMA receivers, even with a simple equalization module, for example, a one-tap multiplier based frequency domain equalizer (FDE), a receiver architecture following this approach can still provide better performance than the costly multi-tap multiplier based equalizer, such as the time domain equalizer (TDE). Performance is well maintained under fast fading channel for mobile stations at high velocity. Moreover, a generic receiver architecture purely based on frequency domain equalization for a composite radio environment, that is CDMA and Orthogonal Frequency-Division Multiplexing (OFDM) systems, is also proposed in this paper. This architecture can be readily implemented by software defined radio (SDR) technology. Our results manifest that this adaptive receiver architecture is very promising and can achieve good performance with low cost.

In this paper, we propose a frequency domain adaptive estimation of Doppler shifts for multiple Doppler subpaths in direct sequence code division multiple access (DS-CDMA) systems. By modeling the doubly selective channel using a basis expansion model (BEM), the proposed expectation-maximization (EM) algorithm-based estimation method can obtain accurate information of Doppler shifts. Based on the estimated information of Doppler shifts, a FDE-based receiver architecture is developed to exploit Doppler diversity in frequency domain. Simulation results demonstrate that this receiver structure features a low computational complexity while achieving good performance compared with traditional receiver structures in DS-CDMA systems.

Recent advances in distributed source coding (DSC) for mission-driven wireless sensor networks (WSN) are related to the coding for multiple correlated sensors in applications such as real time target tracking and environment monitoring. The characteristic of these DSC applications provides significant potential opportunities in the associated sensor network to utilize multirate transmissions for enhancing network performance. In this paper, we study a methodology for interplay optimization between routing and DSC in WSN, and propose a novel multirate based routing scheme for mission-driven DSC applications that considerably extends network lifetime. The proposed scheme adopts the rate assignment based on the residual energy, and employs a joint rate and energy scheduling mechanism to meet the end-to-end transmission rate constraint, information precision requirement, and the energy constraints in the network for DSC. Simulation results show that this multirate based routing scheme achieves significantly longer network lifetime compared to other existing research in WSN for DSC applications.

Mobile sinks can be a viable solution to solve the problem that energy consumption is not balanced for all the sensor nodes in wireless sensor networks (WSNs). Frequent location updates from the sink (a current possible solution) can lead to both rapid energy consumption of the sensor nodes and increased collisions in wireless transmissions. This paper proposes a "local update-based routing protocol (LURP)" to help resolve this problem. When the sink node moves, it only needs to broadcast its location information within a local area rather than among the entire network. Both theoretical analysis and simulation studies show that LURP consumes less energy in each sensor node and also decreases the collisions in wireless transmissions, which can be used in large-scale WSNs.

Overlay multicast, which performs topology construction and data relaying in the application layer, has recently emerged as a promising vehicle for data distribution. In most of the existing systems, only a single stream is assumed for each overlay, and multiple streams, if needed, are distributed separately. In addition, while the overlay node performs relay functions, they generally do not filter the content to match the heterogeneous bandwidth constraints. We consider a more general model, in which a multicast session may consist of multiple data streams, such as video and audio, which are to be delivered to the same set of nodes. The services to these steams are elastic through layer dropping, shaping, or transcoding. In this paper, we focus on an important resource allocation problem in such an adaptive multi-stream multicast framework: Assume that each stream has an associated utility function, how to maximize the total utility of all the streams in a multicast session. The problem involves optimization not only in individual nodes, but also in the global overlay structure. We show that, given a total bandwidth of the streams, there is an efficient and optimal solution for the allocation problem at a single node. The problem however can be much more complicated in a hierarchical structure like tree. Yet we show that, if the utility is concave with bandwidth, i.e., the marginal improvement diminishes with bandwidth increase, then the upstream constraint can be ignored and a local optimal solution is simply the global optimal solution.

We consider a sensing task scheduling problem in two-level hierarchical sensor networks. To minimize the execution time of a given task, we propose efficient scheduling strategies following the divisible load scheduling paradigm. The proposed scheduling strategies minimize the finish time by eliminating transmission collisions and idle gaps between two successive data transmissions. In-network data aggregation for sensor data is further considered at data fusion nodes. Fused data are produced by some fusion functions on original data from local clusters. The scheduling strategies consist of two phases: intra-cluster scheduling and inter-cluster scheduling. Intra-cluster scheduling deals with assigning different fractions of a sensing workload among source nodes in each cluster; inter-cluster scheduling involves the distribution of fused data among all fusion nodes. Closed-form solutions to the problem of task scheduling are derived. Energy model is described for each kind of sensor nodes, considering data acquisition, communication, and processing. Finally, simulation results are presented to demonstrate the impacts of different system parameters such as the number of sensor nodes, measurement, communication, and processing speed, on the finish time and energy consumption.

In mobile ad hoc networks (MANETs), each node should not only work for itself, but should be cooperative with other nodes. Under such environment, some nodes may misbehave for individual interests. Currently two categories of trust management frameworks, reputation-based framework and trust establishment framework, are used to guarantee nodes to perform normal behavior. However, in reputation-based framework, it is unreasonable that only one parameter, trust value, is considered. Meanwhile, the trust establishment framework is vulnerable under the selective misbehavior attack, by which the attacker performs different behaviors to different nodes. To solve these problems, we propose an objective trust management framework (OTMF) for MANETs, by which one node evaluates the trustworthiness of another node objectively based not only on direct observations, but on second-hand information. To compare the OTMF with the existing frameworks, we provide performance evaluation. The evaluation results show that the OTMF can obtain more reliable trust than the reputation-based framework and can prevent the selective misbehavior attack more effectively than the trust establishment framework.

Signal processing applications such as Distributed Source Coding (DSC) in Wireless Sensor Network (WSN) are often multirate in nature, which provide additional significant opportunities in WSN design for achieving energy efficiency. In this paper, we analyze the DSC traffic for signal processing applications in multirate WSN and propose a novel rate oriented approach with MAC and PHY cross layer utilization to achieve high energy efficiency. Simulation results show that the proposed approach achieves significant energy efficiency by a factor of three while maintaining the transmission quality in WSN. This approach can be easily extended to other WSN MAC and PHY designs, providing multirate service to application layer to allow achieving higher energy efficiency

Peer-to-peer (P2P) file sharing protocols for mobile ad hoc networks (MANETs) is becoming an important area of research nowadays because of the constantly escalating demands from consumers to go mobile and the need to share contents between users. Current P2P file sharing protocols for MANETs use either aggressive or reactive flooding techniques for their path selection and recovery. Both techniques represent extreme measures to deal with path failures and they either do too much or very little work in response to path failures. In this paper we present a new path selection and recovery mechanism (reactive with unicast probing messages) that is a balance between the above two techniques in terms of its control overhead and the resulting path quality. By introducing a new unicast messaging mechanism to the reactive flooding technique, a requesting node can probe its set of available paths for their roundtrip times and use the best one for its file transfer. Our new technique is based on unicast messaging which proves to be effective and requires low control overhead as per the results of our simulations.

A number of turbo equalization (TE) methods with reduced complexity have recently been introduced, in which the maximum a posteriori probability (MAP) equalizer is replaced by suboptimal and low complexity ones. These methods can save the computational complexity significantly, but with sacrificed system performance. In this paper, the authors first use the extrinsic information transfer (EXIT) chart tool to visually explain why these suboptimal methods have the worse performance. Then, the authors propose a novel iterative method for TE which takes advantage of both parallel and serial concatenation turbo-like schemes. In the novel iterative method, the EXIT chart switches between the two schemes as a balance. The transmitter side is also changed correspondingly by utilizing the concept of repetition code to acquire a structure similar to turbo codes. It is shown from both analytical and simulation results that the proposed iterative method for TE results in the excellent system performance while its realization complexity is kept relatively low.

In this paper the impact of capture effect on the IEEE 802.11 networks has been investigated. In order to have insight into capture effect in the MAC mechanism, a new Markov chain model with capture effect has been built for describing the backoff scheme in the IEEE 802.11 networks. Based on the new iterative algorithm used for calculating the transmission probability, a new throughput model considering the impact of capture effect on the back-off scheme has been proposed. The numerical simulation results show that the capture effect has more impact on the basic access mechanism than that on the RTS/CTS access mechanism.

Channel quality indicator (CQI) is essential for the transmitter to select the proper modulation and coding scheme (MCS). However, the feedback of the CQI also decreases the spectrum efficiency. This paper proposes a reduced CQI feedback signaling for the multi-stream downlink multiple-input multiple-output (MIMO) system. In addition, power loading algorithms are applied to further increase the throughput of the system. The proposed scheme decreases the overhead of the CQI feedback remarkably, while achieves good throughput performance.

In this paper, we present a novel 2-way handshake authentication protocol to locally authorize intra-domain roaming users for efficient authentication in mobile IP networks, which is based on authentication, authorization and accounting (AAA) architecture. We develop a detailed procedure to establish local security associations (SAs) for re-authentication using commitment schemes. By considering the traffic and mobility patterns of a mobile user (MU), as well as the message transmission time between the MU and its home AAA server, we provide a performance study for comparing the authentication latency of existing authentication protocol with our approach. The result shows that our protocol outperforms the existing authentication protocol.

Distributed source coding (DSC) based signal processing applications involve data transmissions at multirates in wireless sensor network (WSN). In this paper, we propose a novel cross layer approach in WSN to achieve energy efficiency using multirates desirable for the DSC based signal processing applications and current network conditions. Our approach can be summarized into two parts: first, energy efficient multirate MAC-PHY supporting DSC applications; and second, rate oriented interaction of MAC-PHY and DSC applications. Multirate MAC-PHY utilizes desirable bit error rate (BER), channel attenuation, corresponding modulation schemes, and multirate requirements, to determine the minimum desirable transmission power. Rate oriented interaction provides optimal transmission rates feedback to interact with DSC applications, making DSC applications adaptively adjust information coding rate according to network conditions. This interactive approach achieves further energy saving significantly by making the DSC application aware of WSN conditions. Besides DSC, this work can be easily extended to other multirate signal processing applications in WSN.

While TCP (transmission control protocol) is an efficient transport protocol in the wired Internet, it performs poorly when used in wireless environments. This is because TCP couples the error and flow control by using packet loss to infer the network congestion and thus the random loss in wireless Internet can inevitably mislead TCP dropping its flow-control window unnecessarily, even if the network is not congested at all. To overcome this problem, we propose the second-order rate-based flow control and the decoupled window-based error-control schemes for high-throughput transport protocols over the wireless networks. The second-order rate control minimizes congestive losses by using the explicit congestion notification (ECN)-bit feedback to adapt the rate-gain parameter to the variations of the round-trip time (RTT) and cross-traffic flows. The error-control scheme detects and selectively retransmits the lost packets caused by either congestion or random-noise/handoffs on wireless links, which is decoupled from the flow control such that the rate control is independent of the random loss of wireless links. Using the fluid analysis, we establish the rate-control model, and derive expressions for throughput, losses, and link-transmission efficiency. Through extensive simulations, the proposed transport protocol is shown to possess the TCP-compatibility in bandwidth while coexisting with TCP-Reno traffics in the wired Internet. Our simulations also verify the analysis, and demonstrate the significant superiority of our scheme to TCP in terms of increasing the average throughput over wireless links and the robustness to the variation of wireless random-loss probability while minimizing the losses and retransmissions.

Orthogonal frequency division multiplex (OFDM) has become one of the most popular air-link technologies for future broadband wireless communications. To further improve its bandwidth efficiency and system performance, adaptive resource allocation and smart antenna techniques have been widely used in the OFDM system. However, the use of fully adaptive beamforming in an OFDM system significantly increases the complexity of the medium access control layer design and thus affects the implementation of adaptive resource allocation. A novel cross-layer adaptive resource allocation strategy with hybrid adaptive array and switched-beam smart antennas suitable for the OFDM systems has been proposed. With the help of different smart antennas schemes based on different users' quality of service requirements, the strategy effectively reduces the complexity of adaptive resource allocation in an OFDM system, while still maintaining a satisfactory system performance.

This paper proposes an approach to analyze pulse waveform dependent bit error rate (BER) performance of a DS-CDMA ultra wideband (UWB) radio, which operates in a frequency selective fading channel. The analysis takes into account almost all real operational conditions, such as asynchronous transmissions, RAKE receiver, multiple access interference (MAI), multipath interference (MI), log-normal shadowing, and noise. The main objective of the paper is to reveal the relationship between time domain characteristics of pulse waveforms and BER of a UWB radio. It is shown through analysis (also validated by simulation) that the normalized mean squared auto-correlation function (ACF) of the pulse waveforms can be used as an effective merit figure to judge the suitability for their applications in a DS-CDMA UWB radio. In fact, the normalized mean squared auto-correlation function (ACF) governs the average inter-chip interference caused by imperfect auto-correlation function of the pulse waveforms. The paper concludes that, as long as the power spectral density (PSD) functions of the pulse waveforms fit the FCC spectral mask, the pulse waveforms' normalized mean squared ACF should be minimized to ensure an acceptable BER.

Research on sensor network routing focused on efficiency and effectiveness of data dissemination. Few of them considered security issues during the design time of a routing protocol. Furthermore, previous research on sensor networks mainly considered homogeneous sensor networks where all sensor nodes have the same capabilities. It has been shown that homogeneous ad hoc networks have poor fundamental performance limits and scalability. To achieve better performance, we adopt a heterogeneous sensor network (HSN) model. In this paper, we present a secure and efficient routing protocol for HSNs - two tier secure routing (TTSR). TTSR takes advantage of powerful high-end sensors in an HSN. Our security analysis demonstrates that TTSR can defend typical attacks on sensor routing. Our performance evaluation shows that TTSR has higher delivery ratio, lower end-to-end delay and energy consumption than a popular sensor network routing protocol.

Mobility databases such as home location register and visitor location register are adopted to support mobility management in personal communications services networks. If a visitor location register fails or crashes, the subscribers' services will be seriously degraded due to the loss or corruption of location information. In this paper, we optimize utilization of demand re-registration messages for an adaptivep-persistent backoff database failure restoration scheme. An analytical model is developed and validated with simulations to obtain the optimal utilization using appropriate parameters so that the failed visitor location register is restored with the fastest speed. Some interesting aspects on the performance are studied and their deep insights are observed, such as effects of message sizes on choices of system parameters, effects of the inaccurate estimated number of stations, etc. One observation is that optimizations of utilization and successful transmission probability are two different goals, and a value to achieve the optimal successful transmission probability does not necessarily ensure optimal utilization. Furthermore, we also propose a scheme how to handle the problem with inaccurate (estimated) number of stations.

Abstract This paper introduces an algebraic approach to generate the super‐set of perfect complementary (PC) codes suitable for new generation CDMA applications, characterized by isotropic multiple access interference (MAI) free and multipath interference (MI) free properties. The code design methodology proposed in this paper takes into account major impairing factors existing in real applications, such as MAI, MI, asynchronous transmissions, and random signs in consecutive symbols, such that a CDMA system using the generated codes can insure a truly interference‐free operation. Two important facts will be revealed by the analysis given in this paper. First, implementation of an interference‐free CDMA will never be possible unless using complementary code sets, such as the PC code sets generated in this paper. In other words, all traditional spreading codes working on an one‐code‐per‐user basis are not useful for implementation of an MAI‐free and MI‐free CDMA system. Second, to enable the interference‐free CDMA operation, the flock size of the PC codes should be made equal to the set size of the codes, implying that a PC code set can support as many users as the flock size of the code set. A systematic search has been carried out to generate the super‐set of various PC codes with the help of carefully selected seed codes belonging to distinct sub‐sets. This paper will also propose an implementation scheme based on multi‐carrier CDMA architecture and its performance is compared by simulations with the ones using traditional spreading codes. Copyright © 2006 John Wiley & Sons, Ltd.

Sensor networks are envisioned to have important applications in military and homeland security. For sensor networks deployed in such hostile environments, security is critical to ensure privacy, integrity, authenticity, and availability of communications. Routing is a fundamental operation in sensor networks. Past researches on sensor network routing focused on efficiency and effectiveness of data dissemination. Few of them considered security during the design phase of the routing protocols. Furthermore, previous researches on sensor networks mainly considered homogeneous sensor networks, i.e., all sensor nodes are the same. Research has shown that homogeneous ad hoc networks have poor performance. We adopt a heterogeneous sensor network (HSN) model for better performance and security. In this paper, we present an efficient secure routing protocol for HSN which takes advantage of the powerful high-end sensors. The security analysis demonstrates that the secure routing protocol can defend typical routing attacks. The simulation shows that the secure routing protocol has better performance than a popular routing protocol - directed diffusion.

Security is critical for sensor networks deployed in hostile environments. Previous research on sensor network security mainly considers homogeneous sensor networks, i.e., all sensor nodes have the same capabilities. Many security schemes designed for homogeneous sensor networks suffer from high communication/computation overhead, and/or large storage requirement. We adopt a heterogeneous sensor network (HSN) model to overcome these problems. In this paper, we present an efficient asymmetric pre-distribution (AP) key management scheme that takes advantage of the powerful high-end sensors (H-sensors) in an HSN. The AP scheme utilizes the large storage of H-sensors and pre-load each H-sensor with a relatively large number of keys. The AP scheme dramatically reduces the total storage space for key pre-distribution. The performance evaluation and security analysis show that the AP scheme provides better security than existing key management schemes, while achieving significant reduction on sensor storage.

In this paper, we study various security aspects in the IEEE 802.15.3 wireless personal area networks (WPANs) with security analysis. We present these security features in a more natural and concise manner than the overly complicated standard.

Under the DSRC architecture, we propose a cluster-based multi-channel communication scheme to provision the quality of service (QoS), especially the real-time delivery of safety traffics, over the vehicle ad hoc networks (VANET). Our scheme integrates the clustering algorithm, contention-free, and contention-based MAC to support the real-time transmission of safety-messages. We develop the analytical model to conduct the safety message delay analyses. Our analyses show that our scheme can guarantee the real-time transmission of safety messages. The analytical results are also verified by our simulation experiments.

Advanced encryption standard has been proposed and implemented for several years as a new cipher to replace data encryption standard. There are many researches and applications using this new cipher. However, there is a need to provide its performance analysis in scenarios of network applications. In this paper, we provide an overhead analysis of advanced encryption standard and its performance in wireless networks.

This paper begins with a review of CDMA based 2-3G systems from a network perspective. In particular, the challenges faced by current CDMA technologies are addressed. We will show that many advanced operational requirements in future 4G networks, such as high-speed burst traffic, cross layer network design, seamless integration of different networks with and without infrastructure, will make the current CDMA technologies unsuitable due to their rigid and voice-centric physical (PHY) layer architecture. To respond to the call for CDMA innovation, a new physical layer architecture, namely Orthogonal Complementary Coded CDMA with Offset Stacking (OCC-CDMA/OS) spreading scheme, is proposed in this paper for its possible applications in futuristic wireless networks. One of the most important features of OCC-CDMA/ OS PHY architecture is its unique elasticity, which facilitates implementation of a fully-adaptive CDMA transceiver to work harmonically with various advanced upper-layer designs in 4G networks. Many other technical issues on CDMA based 4G networks will also be discussed. It is concluded that OCC-CDMA/OS has a great potential for its applications in future 4G wireless.

Providing packet fair queueing is difficult in CDMA2000 1xEV-DV networks since there are multiple sessions associated with each user and the packets from each session can be served by multiple code channels. Essentially, this is a multi-session multi-server problem, which so far has received little research. The above problem becomes more complicated in CDMA2000 1xEV-DV networks, where Adaptive Modulation and Coding (AMC) schemes are employed for a higher network resource utilization. This means that the underlying service data rate of each code channel will be dynamically changing with the current perceived channel quality and the current number of available code channels. However, current research on packet fair queueing in wireless networks mostly use fixed service data rates. Therefore, how to design a packet fair queueing scheme with multiple servers and time-varying service data rates is very important for QoS provisioning in CDMA2000 1xEV-DV networks. In this paper, we propose a new packet fair queueing algorithm, where it seamlessly integrates the fair queueing with the CDMA2000 1xEV-DV AMC scheme. Both theoretical analyses and simulation results show that the proposed fair queueing algorithm can provide fairness among different sessions as well as satisfy their QoS requirements.

Sensor nodes are unreliable small devices with limited energy supply, and sensors may run out of power or fail over time. A sensor network should have self-healing capability to handle coverage holes, routing voids and disconnections caused by random sensor locations and sensor failures. To provide self-healing capabilities in sensor networks while keeping the cost low, we propose to deploy a small number of mobile sensors in addition to a large number of static sensors. A mobile sensor may receive multiple requests from different areas. In this paper, we present a distributed decision making algorithm based on fuzzy logic. The distributed algorithm has low computation requirement and is robust to incomplete information. Our extensive simulations demonstrate the self-healing capabilities provided by mobile sensors, and the good performance of the distributed decision making algorithm.

In this paper we propose a reduced-complexity genetic algorithm for optimization of multi-hop sensor networks. The goal of the system is to generate optimal number of sensor-clusters with cluster-heads. It results in minimization of the power-consumption of the sensor-system while maximizing the sensor objectives (coverage and exposure). The genetic algorithm is used to adaptively create various components such as cluster-members, cluster-heads, and next-cluster. These components are then used to evaluate the average fitness of the system based on the sequence of communication links towards the sink.

Power control is a critical issue to implement Mobile Ad Hoc networks. This paper presents a novel power control protocol, namely the optimistic power MAC control (OPCM) protocol for its possible application in Mobile Ad Hoc networks. The OPCM protocol works by increasing power level in the retransmission stage to guarantee the DATA reception, rather than controlling the power in the initial transmission stage. It will be shown through comprehensive computer simulations that the proposed OPCM protocol is very energy efficient, while still being able to maintain a high throughput in a Mobile Ad Hoc network.

For time-sensitive applications requiring frequent data collections from a remote wireless sensor network, it is a challenging task to design an efficient routing scheme that can minimize delay and also offer good performance in energy efficiency, network lifetime and throughput. In this paper, we propose a new routing scheme, called Shortest Hop Routing Tree (SHORT), to achieve those design objectives through effectively generating simultaneous communication pairs and identifying the shortest hop (closest neighbor) for packet relay. Compared with the existing popular schemes such as PEGASIS, BINARY and PEDAP-PA, SHORT offers the best "energy x delay" performance and has the capability to achieve a very good balance among different performance metrics.

This paper introduces a new S-T coding scheme, namely Space-Time Complementary Coding (STCC) scheme. With the help of pair-wise complementary codes, the proposed STCC scheme can achieve transmitter diversity and parallel transmission at the same time. It will be shown that the STCC scheme can easily be generalized to achieve arbitrary combination of diversity gain and parallel transmission order. The new scheme requires neither bandwidth expansion nor feedback information from the receiver, and it offers a nearly interference-free performance. The analytical study will be carried out in this paper to show that it provides a very high diversity-multiplexing product gain for its operation in both flat and frequency-selective fading channels.

The application of multi-hop wireless Ad Hoc networks (WANETs) has been facing a great challenge to implement cooperative control over user transmission rates under the resource restrictions. In this paper, we propose an efficient bandwidth allocation scheme called Active-Time based Bandwidth Allocation Scheme (ATBAS) that can ensure fair bandwidth allocation among the users in a multi-hop WANET model. This scheme operates at each hop and fairly allots each competing flow a share of channel time, according to which each hop then computes the updating rate for each flow traversing the hop and records their updated rates into each data packet's special control header. The source of each flow in this scheme can eventually adjust its sending rate to reach its fair share. An algorithm is developed to implement the above mechanism. We will show through simulation results that our proposed algorithm is able to fairly distribute bandwidth among multi-hop flows.

In this paper, a cross-layer analytical framework is proposed to analyze the throughput and packet delay of a two-hop wireless link. The adaptive modulation and coding (AMC) is employed at the physical layer and finite-length buffers is considered at the network layer. Firstly, we model the wireless channel as a finite state Markov chain (FSMC). Then, a method is proposed to calculate the steady-state output traffic distribution of the source node. Based on this, a modified queuing FSMC model, which utilizes the non-Poisson distribution of the entering traffic, is proposed to analyze the performance at the mesh gateway. The results of the performance analysis are discussed and verified by numerical simulations. In addition, the proposed analytical framework can be easily extended to multi-hop wireless networks

Network selection is one of the most significant challenges for the next-generation wireless heterogenous networks. Many network selection schemes have been proposed, which can mainly be divided into two types: distributed schemes and hybrid schemes. Distributed schemes did not take into consideration the load unbalance problem in the whole heterogeneous system, which was not solved successfully in hybrid schemes either. In this paper, we propose a low complexity, centralized network selection scheme, aiming to optimally distribute the end users to the networks of the heterogeneous wireless system, in the sense of maximizing the global spectrum efficiency. Two suboptimal algorithms are proposed to solve this optimization problem, which are based on the measures of absolute bandwidth request (ABR) and relative bandwidth request (RBR) respectively. The overall bandwidth occupation results and the outage probabilities of the two algorithms are evaluated by means of numerical simulation

In wireless ad hoc networks, the major quality of service (QoS) concern and challenge in the design and analysis of contention-based medium access control (MAC) protocols is to achieve good throughput and access delay performance in the presence of hidden terminals, which are defined as the terminals out of the radio coverage area of an intended transmitter but within that of the receiver. We propose and analyze in this paper a new dual-channel random access protocol, called "Double Sense Multiple Access" (DSMA), for improving QoS support in wireless ad hoc networks. By separating the transmissions of ready-to-send (RTS) and data packets into two time-slotted channels and by introducing a novel double sense mechanism, DSMA completely solves the hidden terminal problem and can guarantee the success transmission of data packets. By taking into account the most complex network scenario in which all transmitters are hidden terminals with respect to each other, key QoS metrics such as throughput, blocking probability and access delay are derived mathematically for the proposed DSMA protocol. These analytical results are verified by extensive computer simulations.

This paper introduces an approach to generate generalized pairwise complementary (GPC) codes, which offer a uniform interference free windows (IFWs) across the entire code set. The GPC codes work in pairs and can fit extremely power efficient quadrature carrier modems. The characteristic features of the GPC codes include: the set size is 2K, the processing gain is 4NK, and the IFW's width is 8N identically for all codes in a set, where K is the times to perform Walsh-Hadamard expansions and N is element code length of seed complementary codes. Therefore, by using different N, the IFW width of a GPC code set can be adjusted with its set size unchanged. Each GPC code set consists of two code groups, with each having K codes, and they have sparsely and uniformly distributed autocorrelation side lobes and cross-correlation levels outside the IFWs.

Ultra-wideband (UWB) systems have been envisaged as optimal candidates to provide high data rate communications within short range. In this paper, a novel cross-layer approach suitable for TH-UWB systems is proposed, named punctured hopping (PH). PH scheme permits to limit the throughput reduction of the basic full rate turbo coding scheme, while maintaining the same code free distance. The proposed coding scheme performs parity bits puncturing. However, different from the rate compatible punctured turbo codes (RCPTC) scheme, it works on the idea that the less significant bits in a coded frame are not completely discarded as only half of their impulses are punctured. Thus preserving the error correcting capabilities jointly with a better spectral efficiency. The performance of the proposed PH coding scheme has been examined using theoretical analysis in additive white Gaussian noise channel and simulations in a realistic multipath fading channel. It has been compared with the uncoded, the basic full rate, and the RCPTC systems in order to underline its effectiveness. The results show a significant performance gain in comparison with the RCPTC and, hence, a significant reduction of the performance loss in comparison with the basic full rate turbo coding scheme, while maintaining the same throughput of the RCPTC.

Abstract Past researches on sensor network routing have been focused on efficiency and effectiveness of data dissemination. Few of them consider security issues during the design time of routing protocols. Security is very important for many sensor network applications. Studies and experiences have shown that considering security during design time is the best way to provide security for sensor network routing. In this paper, we propose an efficient key management scheme and a novel secure routing protocol—Secure cell relay (SCR) for sensor networks. We also present an effective key setup scheme for sensor nodes deployed in the later stage. We analyze the security of SCR under various attacks and show that SCR is very effective in defending against several sophisticated attacks, including selective forwarding, sinkhole, wormhole, Sybil, hello flooding, and clone attacks. SCR is an energy‐efficient routing protocol with acceptable security overhead. Our simulations demonstrate that with all the security primitives, SCR still has lower energy consumption and higher delivery ratio than a popular routing protocol—directed diffusion. Copyright © 2006 John Wiley & Sons, Ltd.

Abstract A mobile ad hoc network (MANET) is the cooperative engagement of a collection of wireless mobile nodes without the aid of any established infrastructure or centralized administration. The conventional security solutions to provide key management through accessing trusted authorities or centralized servers are infeasible for this new environment since mobile ad hoc networks are characterized by the absence of any infrastructure, frequent mobility, and wireless links. In this paper, we propose an on‐demand, fully localized, and hop‐by‐hop public key management scheme for MANETs. It can be performed by generating public/private key pairs by nodes themselves, issuing certificates to neighboring nodes, holding these certificates in their certificate repositories, and providing authentication service adaptive quickly to the dynamic topology of the network without relying on any servers. Also, our scheme can be performed successfully as long as there is a physical communication line between two nodes, and it is accustomed well to the on‐demand routing for MANETs. Copyright © 2006 John Wiley & Sons, Ltd.

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To efficiently and effectively achieve the hierarchical access control for multimedia networks, in this paper we propose a distributed key management scheme whereby each SG (service group) maintains an SG server and give detailed analysis. In the proposed scheme, the server for a SG is utilized to manage the key tree and provide the related session keys for all the users in this SG. A detailed case study is provided, and we show that the communication overhead can be greatly reduced by O (n/sub O/ /spl middot/ log/sub d/n/sub O/), where n/sub O/ is the number of users in a SG, compared with employing an integrated key graph to the hierarchical access control problem. At the same time, the storage overhead for all users can be reduced by O(N), where N is the total number of users.

The IEEE 802.15.4 specification defines medium access control (MAC) layer and physical layer for wireless sensor networks. Furthermore, security mechanisms are also defined in the specification. This paper first surveys security services provided in the IEEE 802.15.4 wireless sensor networks. Then, some security enhancements are proposed to prevent same-nonce attack, denial-of-service attack, reply-protection attack, ACK attack, etc.

Viruses spreading over the Internet can cause significant damage and the loss of network security. On the other hand, the anti-virus process also plays an important part affecting the dynamics of the virus spreading. The spreading dynamics of most viruses depend on the underlying network topology. While much research attention has been paid in developing the anti-virus software/tools, the dynamics and propagating model of the virus and anti-virus spreading in the topology-aware networks is neither well understood, nor thoroughly studied. To remedy this deficiency, we model and analyze the spreading characteristics of viruses as coexisting with the anti-virus spreading process in the two-layer small-world topology. Applying the fluid analysis, we derive the analytical solutions to the two-layer model. The simulations experiments confirm the validity of our fluid analyses in characterizing both virus and anti-virus spreading dynamics.

Complete complementary code based direct sequence code division multiple access (CC/DS-CDMA) system has been proposed recently as a potential candidate for beyond the third generation (B3G) mobile communications. This paper addresses the issue on variable capacity property (VCP) of a CC/DS-CDMA system with multiple time slots, pertaining to the multi-rate channel assignment schemes of a CC/DS-CDMA system. In particular, three traffic classes and two dynamic code flock assignment schemes, namely random assignment (RA) and compact assignment (CA), are considered in our study. Simulation results of blocking probability, mean and variance of system capacity, and throughput under different schemes are derived and compared. It is concluded that the VCP of CC/DS-CDMA systems is sensitively affected by different code flock assignment schemes. In general, CA can offer lower blocking probability and more stable system capacity; whereas RA can offer larger mean system capacity and higher throughput when offered traffic is heavy.

This paper studies a multiple access control protocol, hierarchy schedule sensing protocol, for CDMA wireless data centric networks characterized by high terminal density in a small area. In particular, this paper focuses on the impact of packet collision with capture effect on performance of a network based on the protocol, where a common-coded permission frame beacon is used to schedule-transmission requests to avoid collisions. To further reduce scheduling delay, hierarchical group-based coding is introduced to the scheme. The performance of such a network is analyzed considering packet collisions, capture effect, multipath-interference (Ml), and multiuser interference (MUI) with the help of a Markovian chain model. The average packet collision-rate with and without capture effect is explicitly derived, the performance of the protocol is compared with traditional receiver-based code protocol to show that, despite its low complexity, it is a fairly robust multiple access control protocol featured by uniformly high throughput and low delay.

This paper introduces an algebraic approach to generate the super-set of perfect complementary (PC) codes suitable for interference-free CDMA applications. The code design methodology proposed here takes into account major impairing factors in real applications, such as MAI, multipath effect, asynchronous up link transmissions and random signs in consecutive symbols. Two important conclusions are drawn from the analysis in this paper. First, implementation of an interference-free CDMA will never be possible unless using some proper complementary code sets, such as the PC code sets proposed in this paper. Second, to enable interference-free CDMA operation, the block size of the PC codes must be made equal to their set size. The operational advantages of a CDMA system based on the PC codes is also discussed and its performance is compared with traditional CDMA systems by simulations, showing that the PC code based CDMA architecture offers a performance limited only by noise.

This paper introduces an algebraic approach to generate the super-set of perfect complementary (PC) codes suitable for interference-free CDMA applications. The code design methodology proposed herein takes into account major impairing factors in real applications, such as MAI, multipath effect, asynchronous uplink transmissions and random signs in consecutive symbols. Two important conclusions are drawn by the analysis given in this paper. First, implementation of an interference-free CDMA will never be possible unless using proper complementary code sets, such as the PC code sets proposed in this paper. Second, to enable interference-free CDMA operation, the flock size of the PC codes must be made equal to their set size. The so generated CDMA code sets can be applied to implement a new CDMA architecture offers a. performance limited only by noise.

This paper focuses on performance analysis of a CDMA wireless data network based on hierarchy schedule-sensing (HSS) protocol, in which a common-code permission frame beacon is used to schedule request transmissions to avoid packet collisions. To further reduce scheduling delay, hierarchical group-based coding is adopted in the scheme. The performance of such a network is evaluated considering packet collisions with and without packet capture effect using a two-dimensional Markovian chain model. In particular, this paper will study the impact of multiple packet-collision effect on network performance with the help of the average packet-collision rate derived explicitly in the paper. The obtained results show that, despite its low implementation complexity, the HSS protocol is a fairly robust medium-access control protocol featuring uniformly high throughput and short delay even under very high traffic load, which especially suits data-centric wireless networks with relatively small coverage area and great node density.

Future wireless communication systems ought to offer a high spectral efficiency and an inherent flexibility to carry diverse services with variable rate requirements. The rate change should be made scalable, continuous and real-time based to match exactly to the required data rate for conservation of precious bandwidth. The rate change in multiples of two, as implemented in the existing 3G W-CDMA standard, is undesirable. It is also preferred that the multiple access technique itself should support asymmetrical traffic in up- and down-links to fit connection modes of the Internet services. This paper is a review of our recent research effort to propose an innovative CDMA architecture based on complete complementary codes, which can satisfy major requirements for the future mobile Internet applications. The architecture uses a new "offset stacked" spreading technique, based on which both high spectral efficiency and scalable rate-matching are guaranteed. To measure spectral efficiency of the systems, "spreading efficiency" (SE) is defined as the information bit(s) conveyed by each chip. All traditional CDMA-based standards offer an SE of 1/N, where N is the processing gain. On the contrary, the SE of the proposed architecture can be made as high as one. As the bandwidth of a CDMA system is determined by its chip width, a high SE means a high spectral efficiency. This paper illustrates that the proposed scheme is well-performing and has a great potential for applications in future wireless communications beyond 3G.

A new pilot-aided multiuser detection scheme, single code cyclic shift (SCCS) detector, is proposed in this paper for synchronous CDMA multiuser signal reception in a mobile communication environment. The salient feature of the proposed detector is that a receiver can decode multiuser signals even without explicit knowledge of all signature codes currently active in the system. The transmitting signal consists of orthogonal carrier modulated pilot and data signals; the former consists of periodically repeated pilot symbols encoded by the same signature codes as the one spreading the latter. A matched filter bank, consisting of M correlators that match to distinct cyclicly shifted versions of a "single" signature code, is employed for "channel cyclic shift correlation function" estimation, followed by the multiuser detection algorithm based on the channel information obtained earlier. The performance of the proposed SCCS detector is evaluated and compared to decorrelating detector by analysis and simulation considering various multipath channels with different profiles. The results demonstrate that a synchronous CDMA joint detection can be implemented even without knowing all signature codes of the system.

Abstract To implement 4G wireless that should deliver a much higher data‐rate than that possible in the current 3G systems, development of innovative air‐link technologies is the key. This article reviews our on‐going research activities in the design of ‘isotropic air‐interface technologies’ capable of substantially improving air‐link performance of a CDMA wireless system. To enable bidirectional high‐speed data transmission in the future 4G systems, the problem associated with incompatible up‐ and down‐link performance in 2–3G systems has to be addressed. The importance of the issue becomes even more evident due to the fact that the future wireless needs to support ‘mobile server’ applications, where a mobile terminal may act as a data source such that up‐link traffic can possibly exceed that in down‐link. In this article, two alternative methods enabling isotropic air‐interface will be discussed, either using ‘isotropic spreading techniques’ that ensure a homogeneous link‐performance regardless of its operation mode (synchronous or asynchronous), or relying on a precision up‐link synchronization control techniques to pave a way for successful application of orthogonal codes in up‐link channels, which otherwise are asynchronous and destroy the ‘orthogonality’ amongst the codes. The isotropic spreading techniques can minimize multiple access interference (MAI) and multipath‐interference (MI), the prevalent impairing effects in the current 2–3G systems. This article will introduce a promising complete complementary CC/DS‐CDMA scheme based on isotropic spreading techniques, which carries several extremely attractive properties such as MAI‐free, MI‐free, near–far effect resistance and low hardware complexity. The performance of the proposed CC/DS‐CDMA scheme is limited only by noise. To implement up‐link synchronization control, smart antenna and GPS‐based techniques play an important role and a working example based on TD‐SCDMA standard will be illustrated. Finally, we will also discuss the TDD techniques and its significance to implement isotropic air‐interface technologies for the future 4G systems. Copyright © 2003 John Wiley & Sons, Ltd.

This paper introduces an asynchronous orthogonal decision-feedback detector (AODFD) for asynchronous CDMA multiuser detection. The AODFD based on entire message-length detection is studied first. A realizable scheme, sliding-window AODFD, is then proposed and its performance is analyzed. In spite of its simple structure, the sliding-window AODFD can perform as desirably as ADDFD, which has a much higher complexity. The reduced complexity of the sliding-window AODFD is due to the use of orthogonal matched-filtering and a short window size. Different from ADDFD that requires computational intensive z-transformed matrix inversion and spectral factorization, AODFD uses the agile Gram-Schmidt procedure. It is possible for AODFD to adopt a simple updating algorithm and parameter updating is no longer always necessary when users leave the system. The comparisons are also made with other orthogonal-based detectors and the bit error probability results show that the AODFD is an attractive multiuser detector.

Mobile communication always suffer serious frequency-selective fading due to multipath. Traditional spreading codes are characterized by the fact that the time-domain orthogonality is based on a fixed chip width across code length. They may not perform satisfactorily under frequency-selective fading. This paper proposes a new class of CDMA code, wavelet-packet orthogonal codes capable to retain time-domain orthogonality as well as to provide intra-code subband diversity to mitigate frequency-selective fading. The new codes are constructed by congregating several wavelet waveforms with various dilations and shifts. The combination of the wavelet waveforms in different nodes in a wavelet-packet full binary tree enables frequency diversity capability. Owing to the length of even chips, they can facilitate implementing multi-rate streaming and multi-bit spreading. The performance study is carried using the statistics correlation distribution convolution (SCDC) algorithm and the results reveal that wavelet-packet codes with multi-band components, combined with RAKE, perform pretty well under frequency-selective fading.