ALBERT

Description: In cognitive radio networks, the secondary users (SUs) switch the data transmission to another empty spectrum band to give priority to primary users (PUs). In this paper, channel switching in cognitive radio mobile ad hoc networks (CR-MANETs) through an established route is modeled. The probability of channel availability in this route is calculated based on the PU's activity, SU's mobility, and channel heterogeneity. Based on the proposed model, the channel and link availability time are predicted. These predictions are used for channel assignment in the proposed channel allocation scheme. A handoff threshold as well as a life time threshold is used in order to reduce the handoff delay and the number of channel handoffs originating from the short channel usage time. When the channel handoff cannot be done due to the SU's mobility, the local flow handoff is performed to find an appropriate node in the vicinity of a potential link breakage and transfer the current data flow to it. The local flow handoff is performed to avoid possible flow disruption and also to reduce the delay caused by the link breakage. The study reveals that the channel heterogeneity and SU's mobility must be considered as important factors, which affect the performance of the handoff management in the CR-MANETs. The results emphasize on the improvement of the route maintenance probability after using the local flow handoff. It is also stated that the amounts of handoff requirement and handoff delay are decreased after using the predicted channel usage life time and handoff threshold time.

Description: Due to interference between co-located wireless networks, obtaining accurate channel assessment becomesincreasingly important for wireless network configuration. This information is used, amongothers, for cognitive radio solutions and for intelligent channel selection in wireless networks. Solutionssuch as spectrum analyzers are capable of scanning a wide spectrum range, but are not dedicatedfor channel occupation assessment because they are extremely costly and not able to perform continuousrecording for a time period longer than a few seconds. On the other hand, low-cost solutionslack the flexibility and required performance in terms of configuration and sensing efficiency. To remedythe situation, this paper presents an alternative for channel assessment on top of a commercialsoftware-defined radio platform. Although there exist software solutions for performing spectrumsensing on such platforms, to the best of our knowledge, continuous spectrum sensing and long-termrecording remain challenging. We propose a pioneering solution that is capable of seamless spectrumsensing over a wide spectrum band and guarantees sufficient flexibility in terms of configurations. Theproposed solution is validated experimentally. We demonstrate two advantages of seamless spectrumsensing: the capability of accurate channel occupancy measurement and detecting transient signalssuch as Bluetooth.

Description: Long-term monitoring of an environment is a fundamental requirement for most wireless sensornetworks. Owing to the fact that the sensor nodes have limited energy budget, prolonging theirlifetime is essential in order to permit long-term monitoring. Furthermore, many applications requiresensor nodes to obtain an accurate estimation of a point-source signal (for example, an animal call orseismic activity). Commonly, multiple sensor nodes simultaneously sample and then cooperate toestimate the event signal. The selection of cooperation nodes is important to reduce the estimationerror while conserving the network's energy. In this paper, we present a novel method for sensor dataacquisition and signal estimation, which considers estimation accuracy, energy conservation, andenergy balance. The method, using a concept of 'virtual clusters,' forms groups of sensor nodes withthe same spatial and temporal properties. Two algorithms are used to provide functionality. The'distributed formation' algorithm automatically forms and classifies the virtual clusters. The 'roundrobin sample scheme' schedules the virtual clusters to sample the event signals in turn. Theestimation error and the energy consumption of the method, when used with a generalized sensingmodel, are evaluated through analysis and simulation. The results show that this method can achievean improved signal estimation while reducing and balancing energy consumption.

Description: We study the problem of achieving maximum network throughput with fairness among the flows at the nodes in a wireless mesh network, given their location and the number of their half-duplex radio interfaces. Our goal is to find the minimum number of non-overlapping frequency channels required to achieve interference-free communication. We use our existing Select x for less than x topology control algorithm (TCA) to build the connectivity graph (CG), which enhances spatial channel reuse to help minimize the number of channels required. We show that the TCA-based CG approach requires fewer channels than the classical approach of building the CG based on the maximum power. We use multi-path routing to achieve the maximum network throughput and show that it provides better network throughput than the classical minimum power-based shortest path routing. We also develop an effective heuristic method to determine the minimum number of channels required for interference-free channel assignment.

Description: Radio frequency identification (RFID) is a promising wireless technology for using tiny, remotely powered chips as identifiers. The number of RFID applications is rapidly increasing because RFID technology is convenient, fast, and contactless. However, collisions occur when multiple tags simultaneously transmit their IDs. Therefore, an efficient anti-collision algorithm is needed to accelerate tag identification. In some applications, the reader may repeatedly identify staying tags, which constantly exist in the reader's range. The adaptive query splitting algorithm (AQS) was proposed for reserving information obtained from the last identification process to enable rapid re-identification of staying tags. However, since AQS is a non-blocking algorithm that allows newly arriving tags to use the slots reserved for staying tags, collisions among them are problematic. Thus, semi-blocking AQS (SBA) proposed in this study is designed to reduce the collisions between arriving tags and staying tags by applying a semi-blocking technique in which only a minority of arriving tags use the slots reserved for staying tags. By counting the number of minor arriving tags, SBA estimates the number of unrecognized arriving tags and generates proper queries to minimize their collisions. The identification delay of SBA is analyzed, and simulation results show that SBA significantly outperforms AQS.

Description: Using the impulse responses of these multipath channels, the bit error rate (BER) performance for binary pulse amplitude modulation impulse radio ultra-wideband communication system is calculated. The optimization location of receiving antenna is investigated by dynamic differential evolution (DDE) and genetic algorithm (GA) to minimize the outage probability. Numerical results show that the performance for reducing BER and outage probability by DDE algorithm is better than that by GA.

Description: According to the ongoing IEEE 802.11ac amendment, the wireless network is about to embrace the gigabit-per-second raw data rate. Compared with previous IEEE standards, this significant performance improvement can be attributed to the novel physical and medium access control (MAC) features, such as multi-user multiple-input multiple-output transmissions, the frame aggregation, and the channel bonding. In this paper, we first briefly survey the main features of IEEE 802.11ac, and then, we evaluate these new features in a fully connected wireless mesh network using an analytic model and simulations. More specifically, the performance of the MAC scheme defined by IEEE 802.11ac, which employs the explicit compressed feedback (ECFB) mechanism for the channel sounding, is evaluated. In addition, we propose an extended request-to-send/clear-to-send scheme that integrates the ECFB operation to compare with the IEEE 802.11ac-defined one in saturated conditions. The comparison of the two MAC schemes is conducted through three spatial stream allocation algorithms. A simple but accurate analytical model is derived for the two MAC schemes, the results of which are validated with simulations. The observations of the results not only reveal the importance of spatial stream allocations but also provide insight into how the newly introduced features could affect the performance of IEEE 802.11ac-based wireless mesh networks.

Description: In this paper, we derive a performance comparison between two training-based schemes for multipleinputmultiple-output systems. The two schemes are the time-division multiplexing scheme and therecently proposed data-dependent superimposed pilot scheme. For both schemes, a closed-form expressionfor the bit error rate (BER) is provided. We also determine, for both schemes, the optimalallocation of power between the pilot and data that minimizes the BER.

Description: Per-cell codebook-based limited feedback is highly desirable in coordinated multi-point (CoMP) systems owing to flexibility and compatibility. Except for the per-cell codeword indices, each user should feed back other information for the central unit to reconstruct global channel direction information (CDI) and estimate channel quality information (CQI) for downlink transmission. In this paper, we analyze the essential feedback information for per-cell codebook-based coherent CoMP systems. To show what should be fed back to estimate the CQI, we first derive a conservative CQI estimation to avoid the outage in transmission, which depends on the quantization accuracy and the norm of the global channel. We then analyze whether the per-cell small-scale fading channel norms are necessary to feed back in obtaining the global channel norm and reconstructing the global CDI. Next, we analyze whether the feedback of phase ambiguity (PA) is necessary. Both the analysis and simulation results show that the global channel norm should be fed back instead of the per-cell channel norms. The benefit of the PA feedback is evident only when the number of base stations is large and the number of antennas at each base station is small.

Description: As the geostationary orbit (GEO) is congested in the commercial Ku band, the adjacent satellites willreceive the interference from small aperture terminals which have a wide field of view. To reducethe interference toward the adjacent satellites, code division multiple access (CDMA) with spreadspectrum needs to be adopted in the satellite communications on-the-move system. However, themutual interference between CDMA users leads to the performance loss, which is not acceptable forthe power-sensitive small aperture terminals, and the computation complexity is still an intractableproblem when the conventional iterative minimum mean square error (MMSE) scheme is employedto suppress the multiple access interference (MAI) in satellite systems especially that is coded bylow-density parity-check (LDPC) code. Moreover, the long transmission delays of different userscaused by the long transmission distance require the robustness of the interference suppression forthe asynchronous case. In this paper, a joint chip-level algorithm for CDMA multiuser detectionand LDPC decoding with a new and simple iterative structure based on belief propagation (BP)for satellite systems is developed, which integrates the iterations of multiuser detection and theiterations of LDPC decoding with factor graphs. Through analysis and simulations, it is verified thatthe proposed algorithm is robust to the asynchronous CDMA satellite system, and with the similariterative complexity, the performance of the proposed algorithm outperforms that of the conventionaliterative MMSE scheme in the asynchronous CDMA satellite system.

Description: In this paper, we propose the use of bit-interleaved coded modulation in alternate-relaying decode-and-forward cooperative communication systems. At the destination, we exploit the interference signal, which results from the simultaneous transmission of data streams through both direct and one of the relay channels to develop an optimal detector. It is shown that the proposed detector can be implemented by parallel concatenating maximum \textit{a posteriori} (MAP) algorithms and demappers to the decoders. The detector exchanges soft information between the decoders and the MAP algorithms in an iterative way for performance improvement. The proposed optimal detector requires a long delay as the destination has to receive and store the entire frame before performing data detection. To avoid this, a sub-optimal detector is also proposed. Unlike the optimal detector, the sub-optimal one exploits two consecutive received packets to decode one packet. It turns out that the sub-optimal detector has less reduced delay, complexity, memory size, and bandwidth loss with a slight increase of the bit-error-rate. Extensive simulation results are presented to demonstrate the effectiveness of the proposed detectors.

Description: Opportunistic routing (OR) protocols for ad hoc networks basically consist of selecting a few forwardersbetween the source and destination and prioritizing their transmission. The performance ofOR protocols depends on how these two steps are performed. The aim was to reduce the numberof transmissions to deliver packets to the destination. In this paper, we first present a mathematicalmodel to compute the total number of packets including duplicate packets generated by OR protocols.We use the model to analyse well-known OR protocols and understand the reason behind theirincrease in number of transmissions. Next, we propose an OR scheme transmission-aware opportunisticad hoc routing (TOAR) protocol, which attempts to minimize retransmissions. Our proposedOR protocol uses tree structures to select forwarders and prioritize them. The use of tree structureshelps in identifying primary forwarders which carry packets farthest to the destination during eachtransmission round. TOAR also helps in choosing secondary forwarders which will transmit packetsmissed out by the forwarder. The optimized selection of forwarders results in significant reduction inretransmissions, a smaller forwarder list set, and improvement in goodput.

Description: A new protocol, called fully distributed space-time coded (FDSTC) protocol having information exchange between relays, is proposed and compared with the conventional distributed space-time coded (DSTC) protocol using non-regenerative relays (NR-relays) and regenerative relays (R-relays). Closed-form error probabilities are derived to verify the simulations. In terms of error performance, the FDSTC protocol gets significant average signal-to-noise ratio (SNR) gains (3.7 dB for NR-relays and 18.1 dB for R-relays). In addition, the impact of the relative distance of relays on the required SNR is reduced up to 70\%. The system diversity order using the FDSTC protocol is higher than that using the DSTC protocol (especially, the FDSTC protocol obtains full diversity with NR-relays). As a result, at the same spectral efficiency, FDSTC has better performance in terms of outage probability in high SNR regions. In terms of energy efficiency, the FDSTC protocol is shown to outperform DSTC for long-range transmissions.

Description: 4G cellular systems envisions a comprehensive all-IP-based solution, including voice, data, and streamed multimedia at much higher data rates and spectral efficiency. Emerging standards and technologies like LTE and M-WiMAX are actually leading towards this vision. The objective of this special issue is to address specific research research issues and solutions on different aspects of 4G wireless systems and beyond. The total six papers included in this special issue demonstrates research outcomes in different aspects of 4G wireless, like scheduling, resource allocation, cognitive and cooperative communications, multicast services and coverage and planning of small cells. We hope that the research results included in this special issue will serve as an important step for further research and development in 4G wireless communication systems.

Description: The modern society evolves into a sensorial network environment where individual sensor data can be transformed into cumulative and comprehensive representation for human. In a real time, it is independent of current location or behavior. The only limits to increase productivity and to create a smarter surrounding environment are personal habits and technology progress. The paper describes sensorial framework, which is dealing with the current aspects of technology, sociology, and usability in daily life usage of ubiquitous mobile devices with sensors, and arising computed and energy power. Nowadays, most of the common tasks of our lives are mainly influenced by network and social connectivity where infrastructural speed limits are provided by the information flow.

Description: Cognitive radio (CR) enriches wireless technology systems by harnessing spectrum white spaces. Such systems require continuous and reliable sensing to provide high-quality service to their users and to minimize the interference they may cause to legacy networks. As the simplicity of implementation of energy detectors and their incoherent requirements make them an ideal candidate for this type of application, this work provides a further mathematical analysis to the probability of detection over different fading channels. We derive a tight closed-form expression for the probability of detection over Nakagami channels. In addition, we introduce an accurate recursive algorithm to compute the probability of detection for an odd degree of freedom over additive white Gaussian noise channels, which has been overlooked in the literature so far. Finally, we present the simulation results which concur with the numerical results and which are also a perfect match with the mathematical expressions derived.

Description: We investigate the estimation of the time of arrival (ToA) of a radio signal transmitted over a flat-fading channel. The path attenuation is assumed to depend only on the transmitter-receiver distance and the path loss exponent (PLE) which, in turn, depends on the physical environment. All previous approaches to the problem either assume that the PLE is perfectly known or rely on estimators of the ToA which do not depend on the PLE. In this paper, we introduce a novel analysis of the performance of the maximum likelihood (ML) estimator of the ToA under an imperfect knowledge of the PLE. Specifically, we carry out a Taylor series expansion that approximates the bias and the root mean square error of the ML estimator in closed form as a function of the PLE error. The analysis is first carried out for a path loss model in which the received signal gain depends only on the PLE and the transmitter-receiver distance. Then, we extend the obtained results to account also for shadow fading scenarios. Our computer simulations show that this approximate analysis is accurate when the signal-to-noise ratio (SNR) of the received signal is medium to high. A simple Monte Carlo method based on the analysis is also proposed. This technique is computationally efficient and yields a better approximation of the ML estimator in the low SNR region. The obtained analytical (and Monte Carlo) approximations can be useful at the design stage of wireless communication and localization systems.

Description: In this paper, efficient power allocation strategies for multi-hop multi-branch amplify-and-forward networks are developed in generalized fading environments. In particular, we consider the following power optimization schemes: \emph{i)} minimizing of the all transmission powers subject to an outage constraint; and \emph{ii)} minimizing the outage probability subject to constraint on total transmit powers. In this work, we first derive asymptotically tight approximations for the statistics of the received SNR in the system under study with maximal ratio combining (MRC) and selection combining (SC) combiners. With the statistical characterization of the received SNR, we then carryout a thorough performance analysis of the system. Finally, the asymptotic expression of the outage probability is used to formulate the original optimization problems using geometric programming. The geometric programming can be readily transformed into nonlinear convex optimization problem and therefore solved efficiently and globally using the interior-point methods. Numerical results are provided to substantiate the analytical results and to demonstrate the considerable performance improvement achieved by the power allocation.

Description: The technology for through-wall human detection with ultra-wideband (UWB) radar was discussed. Due to the large amount of UWB radar data, compressive sensing theory was introduced and compressed UWB radar data can be collected. The singular value decomposition algorithm was used to acquire the singular values of compressed radar data. The compressed UWB radar data were collected at two statuses of human being for gypsum wall. The experimental results showed that the singular values with a human target were increased compared with those without a target.

Description: The theory of compressive sensing is briefly introduced, and some construction methods for measurement matrix are deduced. A measurement matrix based on Kronecker product is devised, and it has been proved in mathematical proof. Numerical simulations on 2-D image verify that the proposed measurement matrix has better performance in storage space, construction time, and image reconstruction effect when compared with commonly used matrices in compressive sensing. This novel measurement matrix offers great potential for hardware implementation of compressive sensing in image and high-dimensional signal.

Description: A comparison of 4 x 4 multiple-input multiple-output wireless local area network wireless communication characteristics for six different geometrical shapes is investigated. These six shapes include the straight shape corridor with rectangular cross section, the straight shape corridor with arched cross section, the curved shape corridor with rectangular cross section, the curved shape corridor with arched cross section, the L-shape corridor, and the T-shape corridor. The impulse responses of these corridors are computed by applying shooting and bouncing ray/image (SBR/Image) techniques along with inverse Fourier transform. By using the impulse response of these multipath channels, the mean excess delay, root mean square (RMS) delay spread for these six corridors can be obtained. Numerical results show that the capacity for the rectangular cross section corridors is smaller than those for the arched cross section corridors regardless of the shapes. And the RMS delay spreads for the T-and the L-shape corridors are greater than the other corridors.

Description: In this article, we study the medium access control (MAC) problem inunderwater wireless acoustic networks. We explore the random accessand handshaking (i.e., RTS/CTS) techniques in both single-channeland multi-channel network scenarios. We model and analyze these twoapproaches, and conduct extensive simulations to study theirperformance in various network conditions. Based on our results, weobserve that the performance of both approaches are affected by manyfactors such as data rate, propagation delay and packet size. Ourresults show that the RTS/CTS approach is more suitable for densenetworks with high date rate, whereas the random access approach ispreferred in sparse networks with low data rate. Our results alsodemonstrate that multi-channel techniques can potentially help uscombat the long delay feature of underwater acoustic channels.However, uncoordinated random channel access cannot fully exploitthe advantages of the multi-channel network settings and it performseven worse than the single-channel random access protocol. Only withcareful design and coordination such as multi-channel access withRTS/CTS handshaking process, can multi-channel MAC protocols greatlyimprove the system performance. We believe that this study willprovide useful guidelines for efficient MAC design in underwaterwireless acoustic networks.

Description: In order to mitigate the mutual interference between ultra-wideband (UWB) impulse radio and other existing wireless systems, a novel adaptive interference-avoiding UWB pulse, in the context of the appealing cognitive radio, is presented based on the radial basis function neural networks. Theoretical and implementation architecture for UWB pulse generator is addressed. Transmission performance in the presence of narrowband interference including tone interference is comprehensively analyzed. The presented generator can adaptively change the spectrum shape of UWB signals according to the authorized users' working states, and hence the mutual interference between UWB and other systems sharing the frequency bands can efficiently be avoided. The critical performance indication parameters, such as the SNR, SIR, and BER of UWB communication links, are also derived. Performance evaluations and comparisons between the proposed scheme and other pulse optimization methods have been provided based on the extensive numerical simulations. It is demonstrated that the presented pulse can indeed achieve much superior performance in terms of either single-link BER or anti-jamming capability.

Description: Non-line-of-sight (NLOS) error is a bottleneck problem influencing positioning accuracy. However, a large number of scatterers distribute randomly in the surrounding of the mobile station (MS) in the dense multipath environment, such as urban. In most cases, there is no obstacle between scatterer and MS. So, the geographic information of scatterers around MS can be used to restraint NLOS errors and improve the positioning accuracy. If scatterer can be regarded as the virtual base station (BS), the precondition of the positioning algorithm is easier to satisfy than the traditional positioning algorithm, such as circle positioning algorithm. The algorithm proposed in this article selects suitable scatterers with linear constraints by analyzing the Doppler frequency shift of received signals which reflected by scatterers. Thereby, the selected scatterers and only two real BSs form a complete positioning system. In addition, because MS is motionless in most scenarios, BS must be moving to acquire the Doppler frequency shift. The algorithm proposed in this article is adjusted for the scenarios. And the scatterers with linear constraint can also be utilize fully. Simulation results show the algorithm proposed in this article in two different scenarios, not only simplifies the traditional algorithm, but also achieves the higher positioning accuracy.

Description: This article explores the effects of quantization offeedback information on energy consumption in multiuser wirelesscommunication systems. In order to optimize the energy consumptionof the system, the article concentrates on the amount of transmitenergy, the additional energy due to quantization and theprobability of power outage. Closed form expressions for suchparameters are obtained, where the impact of the number ofquantization bits is explicitly outlined. An optimization problem isthen formulated to find the optimum number of quantization bits ableto minimize the consumption in the energy resources. Simulationsdemonstrate the good results obtainable with the presentedoptimization strategy, and provide effective validation of theanalytic solution presented in the article.

Description: Firewalls are network devices dedicated to analysing and filtering the traffic in order to separate network segments with different levels of trust. Generally theyare placed on the network perimeter and are used to separate the Intranet fromthe Internet. Firewalls are used to forbid some protocols, to shape thebandwidth resources, and to perform deep packet inspection in order to spotmalicious or unauthorized contents passing through the network. In a wirelessmulti-hop network the concept of perimeter is hard to identify and the firewallfunction must be implemented on every node together with routing. But whenthe network size grows, the rule-set used to configure the firewall may growaccordingly and introduce latencies and instabilities for the low-power meshnodes.We propose a novel concept of firewall in which every node filters the trafficonly with a portion of the whole rule-set in order to reduce its computationalburden. Even if at each hop we commit some errors, we show that the filteringefficiency measured for the whole network can achieve the desired precision, with a positive effect on the available network resources. This approach is different from the protection of a space behind a wall: We use the term waterwall to indicate a more distributed and homogeneous filtering function spreadamong all the nodes in the network.

Description: We propose a relaying scheme based on superposition coding (SC) with discrete adaptive modulation and coding (AMC) for a three-node wireless relay system, based on half duplex transmission, where each node decodes messages by successive interference cancelation (SIC). Unlike the previous works where the transmission rate of each link is assumed to achieve Gaussian channel capacity, we design a practical superposition-coded relaying scheme with discrete AMC while taking into account the effect of decoding errors at each stage of the SIC process at each node. In our scheme, hierarchical modulation (HM) is used to create an SC message composed of one basic and one superposed message with optimized power allocation. We firstly introduce the proposed scheme without forward error correction (FEC) for high signal-to-noise ratio (SNR) region and provide the optimal power allocation between the superimposed messages. Next, we extend the uncoded scheme to incorporate FEC to overcome bad channel conditions. The power allocation in this case is based on an approximated expression of the bit error rate (BER). Numerical results show the performance gains of the proposed SC relaying scheme with HM compared to conventional schemes, over a large range of SNRs.

Description: In this paper, a reduced complexity Log-MAP algorithm based on a non-recursive approximation of the max* operator is presented and studied for turbo trellis-coded modulation (TTCM) systems. In the algorithm, denoted as AvN Log-MAP, the max* operation is generalized and performed on n ¿ 2 arguments. The approximation is derived from the Jensen inequality. The non-recursive form of the max* calculations allows to achieve significant reduction in the decoding effort in comparison to the conventional Log-MAP algorithm. Bit-error rate performance simulation results for serial and parallel TTCM schemes in the additive white Gaussian noise and uncorrelated Rayleigh fading channels show that the AvN Log-MAP algorithm performs close to the Log-MAP. Performance and complexity comparisons of the AvN Log-MAP algorithm against the Log-MAP and several relevant reduced complexity turbo decoding algorithms proposed in the literature reveal, that it offers favorable low computational effort for the price of small performance degradation.

Description: In this article, we investigate the information-theoretical performance of a cooperative orthogonalfrequency division multiplexing (OFDM) system with imperfect channel estimation. Assuming thedeployment of training-aided channel estimators, we derive a lower bound on the achievable rate forthe cooperative OFDM system with amplify-and-forward relaying over frequency-selective Rayleighfading channels. The bound is later utilized to allocate power among the training and datatransmission phases. Numerical results demonstrate that the proposed power allocation schemebrings between 5 and 19% improvement depending on the level of signal-to-noise ratio and relaylocations.

Description: The article provides an infrared and visible image fusion algorithm based on directionlets transform. The registered original images were decomposed into the low-frequency and high-frequency coefficients by directionlets transform. Getting the mean of the low-frequency coefficients, applying the local variance maximum principle to the high-frequency coefficients, thereby the fusion coefficients of the fused image can be acquired. Finally, the fused image was obtained using inverse directionlets transform. The experiment indicates that the fusion algorithm can extract the original image features better. Compared with the traditional fusion algorithms, the fusion algorithm presented in this article provides better subjective visual effect, and the standard deviation and entropy value would be somewhat increased.

Description: In this article, we propose a closed-loop precoding method for the Generalized Space Shift Keying (GSSK) modulation scheme, suitable for Multiple-Input-Single-Output (MISO) systems and denoted as adaptive GSSK (AGSSK), which achieves transmit-diversity gains in contrast to GSSK. For the case of a perfect feedback channel, we analytically show that for three and four antennas at the transmitter and rates 1 and 2 bits per channel use (bpcu), respectively, a full transmit-diversity can be achieved without reducing the achievable rate. For higher number of transmit antennas and rates, the performance of the proposed scheme degrades due to the smaller average minimum Euclidean distance as the rate increases. Due to this, we, furthermore, propose an enhancing method for AGSSK which relies on the use of time-orthogonal shaping filters for the different constellation points. For the enhanced method, named as AGSSK with time-orthogonal signal design (AGSSK-TOSD), we analytically prove that it offers transmit-diversity gains which are greater than the number of active transmit antennas for any number of transmit antennas and supported rate. This is attained without any antenna subset selection technique, which alleviates the processing burden on the terminal side. Monte Carlo simulations show that AGSSK significantly outperforms GSSK in terms of average bit error probability (ABEP) and, moreover, for medium to high rates and practical signal-to-noise ratio (SNR) regions AGSSK-TOSD outperforms well-known feedback-based multiple-antenna schemes. This advantage of AGSSK-TOSD is further substantiated with an energy effficiency comparison over the conventional schemes for a target (uncoded) ABEP.

Description: Future wireless networks are designed to cope with drastically increasing user demands. However, network resources reach the limits of their capacity to user requirements. Recently, femtocell networks have attracted much attention to enhance the efficiency of wireless resource usage. In this article, a new adaptive power control scheme is developed for the femtocell network system. By using the concept of the weighted voting game, the proposed scheme adaptively adjusts a transmit power level while ensuring relevant tradeoff between system efficiency and fairness. This power control paradigm can provide the ability to practically respond to current communication conditions and suitable for real network operations. Simulation results are presented to verify the effectiveness of the proposed voting game-based scheme in comparison with the existing schemes.

Description: IEEE 802.16j multi-hop relay network systems provide the mobile wireless communication environment. In such network systems, the handover scanning procedure allows a mobile station (MS) to obtain the information about the handover target base stations (BSs) or relay stations (RSs). The network systems need more time to negotiate the association parameters and to handle scanning the BSs and the RSs when the number of BS and RS increases. It results in more overhead for the handover scanning procedure. In order to accelerate the handover process and reduce the transmission interruption, efficient handover scanning procedure schemes and corresponding algorithms must be developed and designed. A novel relative angle computing algorithm is proposed in this article to accelerate the handover process by taking into account the moving behavior of the MS, and the distances among the MS, the RS, and the BS. The main idea of this algorithm is to reduce the management information overhead and to estimate the potential moving path of the MS in the wireless mobile communication networks. By using the proposed scheme, we eliminate the unnecessary associations and scanning intervals, and reduce the handover scanning procedure efficiently. Simulation result demonstrate the superior performance of our proposed scheme and its ability to strike the appropriate performance in the handover overhead and the message delay for IEEE 802.16j multi-hop relay network systems.

Description: This article addresses the spectrum efficiency study of nested sparse sampling and coprime samplingin the estimation of power spectral density for QPSK signal. The authors proposed nested samplingand coprime sampling only showed that these new sub-Nyquist sampling algorithm could achieveenhanced degrees of freedom, but did not consider its spectrum efficiency performance. Spectralefficiency describes the ability of a communication system to accommodate data within a limitedbandwidth. In this article, we give the procedures of using nested and coprime sampling structure toestimate the QPSK signal's autocorrelation and power spectral density (PSD) using a set of sparsesamples. We also provide detailed theoretical analysis of the PSD of these two sampling algorithmswith the increase of sampling intervals. Our results prove that the mainlobe of PSD becomesnarrower as the sampling intervals increase for both nested and coprime sampling. Our simulationresults also show that by making the sampling intervals, i.e., N1 and N2 for nested sampling, and Pand Q for coprime sampling, large enough, the main lobe of PSD obtained from these twosub-Nyquist samplings are much narrower than the original QPSK signal. That is, the bandwidth Boccupancy of the sampled signal is smaller, which improves the spectrum efficiency. Besides thesmaller average rate, the enhanced spectrum efficiency is a new advantage of both nested sparsesampling and coprime sampling.

Description: This article examines how an entrepreneur's strategic choice influences corporate development leading to the initial public offering. The data come from Korean startups in information technology industry. We consider the dynamic interplay between an entrepreneur's attributes and government's institutional support as key factors. An empirical analysis of 615 listed companies shows that an entrepreneur's attributes play a significant role in market entry especially in information technology industry which tends to have shorter lead time to the initial public offering. Government certification and venture capital investment also facilitated corporate growth toward the public company status.

Description: The coexistence of heterogeneous networks within the same spectrum for enhancing the spectrum efficiency has attracted large interest lately in the research community. Furthermore, the research interest towards the deployment of small cells and multibeam satellites is increasing due to high capacity, easier deployment and higher energy efficiency. However, due to the scarcity of available spectrum and the requirement of additional spectrum for these systems, small cells need to coexist with macrocells and multibeam satellites need to coexist with monobeam satellites within the same spectrum. In this context, this contribution investigates an underlay spectral coexistence mechanism which exploits an interference alignment (IA) technique in order to mitigate the interference of cognitive transmitters towards the primary receivers in a normal uplink mode. More specifically, three types of IA techniques, namely static, uncoordinated and coordinated are investigated. The performance of the IA technique is evaluated and compared with primary only, resource division and no-mitigation techniques in terms of sum-rate capacity, primary to secondary rate ratio and primary rate protection ratio. It is shown that the coordinated IA technique perfectly protects the primary rate in both terrestrial and satellite coexistence scenarios.

Description: We address the problem of analyzing the performances of interference-limited cellular networks inlarge-scale shadowing environments. Focusing on the fractional frequency reuse (FFR) framework,we examine how to optimally assign mobile users in a cell either to the full frequency reuse part or tothe orthogonal part of the FFR band. Instead of using traditional Monte Carlo simulations, which donot provide sufficiently accurate results under important shadowing, we propose a fast and accuratenumerical method. We consider a fast-fading environment and we use the ergodic capacity as theperformance measure. Applying a distributed power control and scheduling strategy, we examineboth cases where access points have knowledge of partial- or full-channel state information (CSI); forthe latter, we also propose an approximated waterfilling strategy. The interest of our method lies inthe fact that it allows for a fast and accurate analysis of the performances of FFR. In addition, it takesinto account a broad range of shadowing environments.

Description: We consider multi-antenna base stations using orthogonalfrequency-division multiple access and space division multipleaccess techniques to serve single-antenna users. Some users, calledreal-time users, have minimum rate requirements and must be servedin the current time slot while others, called non real-time users,do not have strict timing constraints and are served on abest-effort basis. The resource allocation (RA) problem is to findthe assignment of users to subcarriers and the transmit beamformingvectors that maximize the total user rates subject to power andminimum rate constraints. In general, this is a nonlinear andnon-convex program and the zero-forcing technique used here makes itinteger as well, exact optimal solutions cannot be computed inreasonable time for realistic cases. For this reason, we present atechnique to compute both upper and lower bounds and show that theseare quite close for some realistic cases. First, we formulate thedual problem whose optimum provides an upper bound to all feasiblesolutions. We then use a simple method to get a primal-feasiblepoint starting from the dual optimal solution, which is a lowerbound on the primal optimal solution. Numerical results for severalcases show that the two bounds are close so that the dual method canbe used to benchmark any heuristic used to solve this problem. As anexample, we provide numerical results showing the performance gap ofthe well-known weight adjustment method and show that there isconsiderable room for improvement.

Description: Lately, wireless sensor network applications have emerged for provision of commercial services to end users. This results in commercial deployment of sensor networks which is as an important research area due to a number of design and quality of service challenges. An important technical challenge for sensor service provision to end users is managing dynamic network conditions such as unreliability of sensor nodes and network links which results in frequent service outages. This research is aimed at addressing this challenge. It presents a novel architecture which utilizes the availability of multiple sensor networks under different administrative domains, deployed in an area such that maximum network connectivity and high service availability are ensured. The architecture incorporates modifications and enhancements at the medium access control and the routing layers of sensor nodes for the collaborative operation of sensor networks. The design is based on IEEE 802.15.4 standard and ad hoc on demand distance vector routing protocol. The proposed architecture is mathematically analyzed with regards to overheads associated with the design such as routing and communication, and techniques to minimize these overheads are recommended. Through simulations using OMNET++, we show that the proposed architecture effectively provides connectivity for disconnected nodes achieving an overall increase in throughput for all the cooperating networks.

Description: Cell range expansion (CRE) is a technique to expand a pico cellrange virtually by adding a bias value to the pico received power,instead of increasing transmit power of pico base station (PBS), sothat coverage, cell-edge throughput, and overall network throughputare improved. Many studies have focused on inter-cell interferencecoordination (ICIC) in CRE, because macro base station's (MBS's)strong transmit power harms the expanded region (ER) user equipments(UEs) that select PBSs by bias value. Optimal bias value thatminimizes the number of outage UEs depends on several factors suchas the dividing ratio of radio resources between MBSs and PBSs. Inaddition it varies from UE to another. Thus, most articles use thecommon bias value among all UEs determined by trial-and-errorMethodIn this article, we propose a scheme to determine the biasvalue of each UE by using Q-learning algorithm where each UElearns its bias value that minimizes the number of outage UEs fromits past experience independently. Simulation results show that,compared to the scheme using optimal common bias value, the proposedscheme reduces the number of outage UEs and improves networkthroughput.

Description: In today's heterogeneous networking (HetNet) environments, where end users are provided with universal connectivity opportunities, femtocell deployments can become key players in the enhancement of critical performance indicators such as capacity, coverage, QoS, etc. In order to confront the up-to-date LTE femtocell challenges, we propose a context-aware framework that provides a controlled environment from the femtocell point of view, which is required for applicable functionality. More specifically, we aim to (a) control the local environment where the femtocell is placed within, by efficiently managing the total incoming traffic load and by continuously adjusting the distribution of the backhaul capacity among the coexisting networks and (b) control the macro--femto interference caused by macrocell users transmitting close to the femtocell by investigating the "femtocell as a relay" concept. Finally, the performance of the proposed framework is evaluated via simulation results showing that the overall performance of a HetNet environment can be leveraged in terms of QoS requirements, energy saving and data rate enhancement.

Description: This paper presents a variational model based segmentation approach for polarimetric synthetic aperture radar (PolSAR) images. The formulation for PolSAR image segmentation is based on a scaled Wishart distribution based continuous Potts model, which can partition the image domain into distinct regions with respect to the statistical property of PolSAR data. To make the segmentation efficient, a duality based optimization approach is utilized to minimize the energy functional. Moreover, an automatic initialization approach which takes the unsupervised H--a classification result of the polarimetric data as input is used to initialize the segmentation process. This approach can estimate the appropriate number of clusters and the corresponding classification map for the PolSAR data, which are used as the input of the following variational segmentation approach. In such a way, the proposed approach is carried out in a fully unsupervised way. Both of the polarimetric decomposition features and the statistical characteristics are used to get the final segmentation result, which helps to increase the accuracy. Experimental results demonstrate the effectiveness of the proposed approach. Without any artificial supervision, the proposed approach can produce superior segmentation results than results obtained with random initialized variational approach and Wishart--H--a classification approach.

Description: In this study, we propose an indoor navigation system that utilizes visible light communication technology, which employs LED lights and a geomagnetic correction method, aimed at supporting visually impaired people who travel indoors. To verify the effectiveness of this system, we conducted an experiment targeting visually impaired people. Although acquiring accurate positional information and detecting directions indoors is difficult, we confirmed that using this system, accurate positional information and travel direction can be obtained utilizing visible light communication technology, which employs LED lights, and correcting the values of the geomagnetic sensor integrated in a smartphone.

Description: A two-dimensional correlation interferometer algorithm based on dimension separation is proposed for wideband direction finding. The original two-dimensional angle searching is divided into 2 one-dimensional searching processes in the proposed algorithm. Therefore, the computational complexity is reduced. Meanwhile the introduced interpolation process ensures the direction finding precision. Simulation results demonstrate that compared with conventional correlation interferometer algorithm, the proposed one can offer higher direction finding speed.

Description: In the 60GHz millimeter-wave radar communication systems, the nonlinear power amplifier is inevitable. In order to combat this problem, a promising estimation algorithm based on the particle filtering (PF) is presented here. By employing the conception of Bayesian approximation and sequential importance sampling, this appealing Monte Carlo random sampling method can address this complicated statistic estimation problem. In sharp contrast to the classical linear equalization problem, nevertheless, in the considered situation the PF-based method may become invalid due to the hardware nonlinearity and the resulting non-analytical importance function. To remedy this difficulty, based on the linearization technique a novel PF framework is suggested, and we show in particular how to linearize the involved nonlinearity transform in the formulated discrete dynamic state-space modeling (DSM). The merit of this method is that it can efficiently deal with discrete DSMs that are practically nonlinear and non-Gaussian. Experimental simulations verify the superior performance of our presented PF-based detection scheme, which may properly be applied to 60GHz millimeter-wave radar communication systems.

Description: The impact of outdated channel state information (CSI) on the capacity of amplify-and-forward (AF) partial relay selection systems is studied in this paper. The closed-form expressions for the distribution of received signal-to-noise ratio (SNR) in a multi-relay cooperative communications system is first derived, with independent and identically distributed (i.i.d.) Rayleigh fading channels being assumed in each wireless link. After that, the theoretical closed-form expressions for both outage probability and channel capacity of partial relay selection are derived, with four classical adaptive transmission techniques, including the constant power with optimal rate adaption (ORA), the optimal power and rate adaption (OPRA), the channel inversion with fixed rate (CIFR) and truncated channel inversion with fixed rate (TIFR), being considered. Numerical analysis proves that the channel capacity of partial relay selection is impacted considerably by some critical parameters, including the number of relays, the channel correlation coefficient and the end-to-end SNR, etc. It's also exhibited in the numerical results that among the four adaptive transmission techniques, the diversity order of OPRA is larger than that of TIFR, and the OPRA outperforms TIFR with about 0.15 bits/s/Hz in terms of average channel capacity.

Description: Editorial Article

Description: We present the delay tolerant firework routing (DTFR) protocol, a protocol designed for use in disconnected delay tolerant networks that consist of a very large number of location-aware, highly mobile nodes. Networks with these properties appear frequently in many settings, notably in vehicular networks. Under DTFR, each data packet travels from the source to the estimated location of the destination using high-priority transmissions and a delay-tolerant variant of geographic forwarding. Once there, a number of packet replicas are created, and the replicas proceed to travel through the area where the destination is expected to be. Using simulations in an urban setting, we compare DTFR with two baseline protocols (Flooding and Spray andWait), one recently proposed state-of-the-art protocol (GeoDTN+Nav), and an idealistic protocol of our design which we term Bethlehem Routing (BR). For a wide range of environmental parameters, DTFR performs significantly better than the other realistic protocols, in terms of throughput and delay, and close to the upper performance bounds of BR. We also develop an analytical framework based on stochastic geometry tools, a number of simplifying assumptions, and a small number of judiciously chosen approximations. Using this framework, we develop approximate closed form expressions for the average end-to-end throughput and delivery delay of DTFR and BR.

Description: In vehicular ad hoc networks (VANETs), essentially the information of one-hop neighbors is important for data delivery. In a general way, each node broadcasts short packets, i.e., hello packets, to indicate its appearance and establishes a neighbor table for storing neighbor information through receiving others' hello packets. As a popular approach, it is named as a hello scheme. Determining the validity of the neighbor table, a hello scheme is vital to routing protocols in VANETs. However, a hello scheme with high accuracy and low overhead is severely challenged due to the highly dynamic topology and restricted vehicle mobility in VANETs. To address the issue, it is crucial to optimally configure two key parameters, called as hello interval (HI) and timeout interval (TI), respectively. In this article, a probability model of the hello scheme for VANETs is proposed. It is used to analyze factors affecting the two key parameters. Depending on derivation results, an effective local information-based adaptive hello scheme (LAH) is proposed subsequently. It utilizes the local information, i.e., the variation of neighbor table and received hello packets, to adjust HI and TI adaptively. According to different TI adjustment algorithms, four variants of LAH are designed as LAH-I, LAH-L, LAH-1, and LAH-2. In the end, a comparison between LAH schemes and existing three solutions is conducted to evaluate the performance. Results verify that the proposed LAH schemes are capable of obtaining higher accuracy of neighbor table and lower overhead.

Description: Coordination between base stations (BSs) is a promising solution for cellular wireless systems to mitigate intercell interference, improving system fairness, and increasing capacity in the years to come. The aim of this manuscript is to propose a new distributed power allocation scheme for the downlink of distributed precoded multicell MISO-OFDM systems. By treating the multicell system as a superposition of single cell systems we define the average virtual bit error rate (BER) of one single-cell system, allowing us to compute the power allocation in a distributed manner at each BS. The precoders are designed in two phases: first the precoder vectors are computed in a distributed manner at each BS considering two criteria, distributed zero-forcing and virtual signal-to-interference noise ratio; then the system is optimized through distributed power allocation with per-BS power constraint. The proposed power allocation scheme minimizes the average virtual BER over all user terminals and the available subcarriers. Both the precoder vectors and the power allocation are computed by assuming that the BSs have only knowledge of local channel state information. The performance of the proposed scheme is compared against other power allocation schemes that have recently been proposed for precoded multicell systems based on LTE specifications. The results also show that although our power allocation scheme is based on the minimization of the virtual uncoded BER, it also has significant gains in coded systems.

Description: Relay-assisted Automatic Repeat reQuest (ARQ), which allows the relays instead of the source tocarry out retransmissions of packets erroneously received at the destination, is an efficient coopera-tive transmission technique to improve link reliability with high spectrum efficiency in wireless net-works. In cognitive radio (CR) systems, however, this relay-assisted ARQ could induce large latency,since a successful transmission of a packet may consume two idle timeslot or temporal spectrumholes. To overcome this limitation, an active interference cancellation (IC)-aided distributed relay-assisted ARQ method is proposed in this article to serve heterogeneous elastic traffics with diversequality of service (QoS) demands in CR systems. Specifically, by applying our recently proposeddistributed beamforming-based IC method in the physical layer, cognitive relays can exploit spatialspectrum holes (SSHs) to retransmit while keeping the interference to primary users at a tolerablelevel. Meanwhile, at the MAC layer, two scheduling policies, namely probabilistic and queue-length-based scheduling, are proposed to obtain an efficient allocation of temporal and SSHs to direct trans-missions and retransmissions. The performance of the proposed schemes are analyzed and optimizedby adjusting the scheduling parameters. As a result, the secondary users can obtain significant QoSgains, as validated by theoretical and simulation results.

Description: Track-before-detect (TBD) based on the particle filter (PF) algorithm is known for its outstanding performance in detecting and tracking of weak targets. However, large amount of calculation leads to difficulty in real-time applications. To solve this problem, effective implementation of the PF-based TBD on the graphics processing units (GPU) is proposed in this article. By recasting the particles propagation process and weights calculating process on the parallel structure of GPU, the running time of this algorithm can greatly be reduced. Simulation results in the infrared scenario and the radar scenario are demonstrated to compare the implementation on two types of the GPU card with the CPU-only implementation.

Description: In this article, we propose a resource allocation algorithm for orthogonal frequency division multiplexing-based cellular system that serves users each of which requests a regular service or a multimedia broadcast multicast service (MBMS). We assume that the MBMSs are considered for video services such as mobile TV. In order to efficiently serve the users with heterogeneous channel conditions, we employed channel-adaptive video streams by using scalable video coding (SVC), where a realistic multicast configuration is considered for multicasting the SVC streams. Therefore, we designed the cellular system to serve constant and flexible bit rates for the regular services and the MBMSs, respectively. Thus, we consider three objectives when allocating the bandwidth resources, where total transmit power is regulated. First, the bit rate of each regular service has to be guaranteed. Second, the minimum bit rate of each MBMS for achieving the minimum video quality has to be guaranteed. Third, the average video quality of the users requesting MBMSs has to be maximized. In order to achieve the objectives, we propose to quantify the service quality of each service, and then find the resource allocation that maximizes the total service quality. By using the computer simulation results, we could verify that the three objectives can efficiently be achieved. Also, we could verify that the performance of the proposed algorithm is superior to that of the previous algorithm designed to maximize the total bit rate while guaranteeing the minimum bit rates.

Description: Inspired by recent advances in compressive sensing (CS), we introduce CS to the radar sensor network(RSN) using pulse compression technique. Our idea is to employ a set of stepped-frequency(SF) waveforms as pulse compression codes for transmit sensors, and to use the same SF waveformsas the sparse matrix to compress the signal in the receiving sensor. We obtain that the signal samplesalong the time domain could be largely compressed so that they could be recovered by a small numberof measurements. A diversity gain could also be obtained at the output of the matched filters. In addition,we also develop a maximum likelihood (ML) algorithm for radar cross section (RCS) parameterestimation and provide the Cramer-Rao lower bound (CRLB) to validate the theoretical result. Simulationresults show that the signal could be perfectly reconstructed if the number of measurementsis equal to or larger than the number of transmit sensors. Even if the signal could not be completelyrecovered, the probability of miss detection of target could be kept zero. It is also illustrated that theactual variance of the RCS parameter estimationb \theta satisfies the CRLB and our ML estimator is anaccurate estimator on the target RCS parameter.

Description: Synthetic aperture radar (SAR) automatic target recognition (ATR) is playing a very important role in military and civil field. Much work has been done to improve the performance of SAR ATR systems. It is well-known that ensemble methods can be used for improving prediction performance. Thus recognition using multiple classifiers fusion (MCF) has become a research hotspot in SAR ATR. Most current researchers focus on the fusion methods by parallel structure. However, such parallel structure has some disadvantages, such as large time consumption, features attribution conflict and low capability on confuser recognition. A hierarchical propelled strategy for multi-classifier fusion (HPSMCF) is proposed in this paper. The proposed HPSMCF has the characters both of series and parallel structure. Features can be used more effective and the recognition efficiency can be improved by extracting features and fusing the probabilistic outputs in a hierarchical propelled way. Meanwhile, the confuser recognition can be achieved by setting thresholds for the confidence in each level. Experiments on MSTAR public data demonstrate that the proposed HPSMCF is robust for variant recognition conditions. Compared with the parallel structure, HPSMCF has better performance both on time consumption and recognition rate.

Description: Reliable spectrum sensing is one of the most crucial aspects for the successful deployment of cog-nitive radio (CR) technology. For CR, it is not possible to transmit on a licensed band and sense itsimultaneously, therefore sensing must be interleaved with transmission. Spectrum sensing in CR ischallenged by a number of uncertainties, which degrade the sensing performance and in turn requiremuch more time to achieve the targeted sensing efficiency. Hence, the authors propose a spectrumsensing scheme which obtains reliable results with less mean detection time. First, the scheme deter-mines a better matched filter, or a combination of energy and cyclostationary detectors based on thepower and band of interest. In the combined energy and cyclostationary detector, an energy detectorwith a bi-threshold is used, and the cyclostationary detector is applied only if the energy of the signallies between two thresholds. Second, sensing is performed by the selection choice resulting from thefirst step. To evaluate the scheme's performance, the results are compared with those where only anenergy detector, matched filter, or cyclostationary detector are performed. The performance metricsare the probability of detection, probability of false alarm, and mean detection time.

Description: Femtocell is a promising solution for enhancing the indoor coverage and capacity in wireless net-works. However, for the small size of femtocell and potentially frequent power on/off, existing han-dover schemes may not be reliable enough for femtocell networks. Moreover, improper handoverparameters settings may lead to handover failures and unnecessary handovers, which make it neces-sary to enhance the mobility robustness for femtocells. In this article, we propose a gradient methodand cost function-based mobility robustness optimization scheme for long term evolution (LTE) fem-tocell self-organizing networks. Moreover, signalling overhead of the scheme is analyzed. Simulationresults show that the proposed scheme has a better performance than the fixed parameters method interms of reduced the number of handover failures and unnecessary handovers with limited signallingmodifications.

Description: We consider resource allocation for semi-elastic applications such as video conferencing. We proposeto model user utility as a sigmoid function of the average bit rate over multiple time slots. Thegoal is to maximize the total expected user utility over time through allocation of downlink powerand subcarriers in each time slot. We propose resource allocation that considers both the average rateachieved so far and the future expected rate, and show how the future expected rate can be estimated bymodeling the probability that a user will be allocated a subcarrier in a future time slot. The algorithmcan be implemented in a distributed fashion by an exchange of price and demand amongst users, thenetwork, and an intermediate power allocation module. To reduce the complexity, we also proposea greedy algorithm to maximize incremental utility in the current time slot. The performance isillustrated using numerical results.

Description: The combination of MIMO--OFDM is a very attractive solution for broadband wireless services. Thus, the two prominent fourth-generation (4G) cellular systems, WiMAX and LTE-advanced, have both adopted MIMO--OFDM transmission at the physical layer. OFDM signal however suffers from nonlinear distortions when passed through high-power amplifier (HPA) at the RF stage. This nonlinear distortion introduces out-of-band spectral broadening and in-band distortions on the transmitted signals. 4G cellular standards have placed strict limits on the allowable spectral broadening in their spectrum mask specifications, to insure that data transmission on a given channel is not interfering significantly with an adjacent channel user. In this article, we characterize the out-of-band spectral broadening introduced by HPA when MIMO--OFDM signals are transmitted over multiple relaying channels. Expressions for the power spectral density of MIMO--OFDM signals are derived over multiple relay channels, and the cumulative effects of HPA on the spectrum of the transmitted signals are estimated. It is shown that depending on the number of relays and the relaying configuration employed, it may happen that a transmitted MIMO--OFDM signal with the transmit spectrum mask initially within the allowable set limit at the source node arrives at the destination violating this limit due to the cumulative effects of the multiple HPA's in a multihop relaying channel

Description: An underwater acoustic sensing array was presented in this article. With the high-precision sampling clock generation and transmission system, the array can acquire signals synchronously in sub-microsecond level, which is important in offshore environment. Meanwhile, real-time data transmission and storage system was established. All of the data received in host computer can be saved and displayed immediately. Data acquisition experiment was implemented in freshwater reservoir near Tianjin city,China, and the results of the signal wave show that the acquisition and transmission system of hydrophone array can be used to get the underwater information by acoustic exploration.

Description: This article presents a hybrid adaptive multi-carrier architecture that, according to the channel condition, wisely switches between orthogonal frequency division multiplexing (OFDM) and orthogonal filtered multitone (FMT) modulations. This new scheme is referred to as hybrid-FMT (H-FMT). For H-FMT, the problem of jointly allocating the sub-channels powers, the overhead duration, and the modulation scheme, to the channel condition to (a) maximize the achievable rate, or (b) minimize the total transmitted power, is considered. Furthermore, we present an efficient implementation that requires minimal increased complexity w.r.t. baseline OFDM. The potential benefits given by the adoption of H-FMT are shown through extensive numerical results over typical WLAN channels.

Description: This paper studies the bit error rate (BER) performance of an adaptive decode-and-forward (DF) relaying scheme for a cooperative wireless network operating on independent and identically distributed (i.i.d.) or independent and non-identically distributed (i.n.d.) Rayleigh fading channels. The considered network is with one source, $K$ relays, and one destination in which binary frequency-shift keying modulation is employed to facilitate noncoherent communications. A relay decodes and retransmits the received signal to the destination only if its decision variable is larger than the corresponding threshold. Depending on the availability of the information of whether a particular relay retransmits or remains silent, the destination combines the received signals from all the relays and the received signal from the source or only the received signals from the retransmitting relays and the received signal from the source to detect the transmitted information. The average end-to-end BERs are determined in closed-form expressions. The thresholds employed at the relays are investigated to minimize the end-to-end BERs. Analytical and simulation results are provided to validate our theoretical analysis. The obtained results show that the studied scheme improves the BER performance significantly compared to the previous proposed piecewise-linear (PL) scheme. In addition, the information of whether a particular relay retransmits or remains silent at the destination does not really improve the BER performance of the network.

Description: In this paper, we present a comprehensive performance analysis of dual-hop multiple antenna channel state information (CSI) assisted amplify-and-forward (AF) relaying systems over Rayleigh fading channels employing arbitrary transmit antenna selection (TAS) and receiver maximum ratio combining (MRC) with feedback delay in the presence of co-channel interference (CCI) at both the relay and destination. Specifically, an upper bound on the cumulative distribution function (CDF) of the end-to-end signal-to-interference ratio (SIR) is proposed, based on which closed-form expressions for the outage probability and the average symbol error rate (SER) are derived. To gain further insights, simple and high informative expressions for the outage probability and the average SER are obtained at the high SIR regime, which readily enable us to characterize the achievable diversity order and coding gain of the system. Moreover, we present new analytical upper and lower bounds for the ergodic capacity of the system, which apply to the system with arbitrary number of antennas, CCI and feedback delay at any SIRs. Finally, to minimize the outage probability of the system, an optimum power allocation scheme is devised under the total transmission power constraint between the source and the relay. The findings suggest that the feedback delay limits the diversity order to one, while the CCI degrades the outage performance by affecting the coding gain of the system.

Description: This paper investigates mobile video delivery in a heterogeneous wireless network from a video server to a multi-homed client. Joint source-channel coding (JSCC) has proven to be an effective solution for video transmission over bandwidth-limited, error-prone wireless networks. However, one major problem with the existing JSCC approaches is that they consider the network between the server and the client as a single transport link. The situation becomes more complicated in the context of multiple available links because involving a low-bandwidth, highly lossy, or long-delay wireless network in the transmission will only degrade the video quality. To address the critical problem, we propose a novel flow rate allocation-based JSCC (FRA-JSCC) approach that includes three key phases: (1) forward error correction redundancy estimation under loss requirement, (2) source rate adaption under delay constraint, and (3) dynamic rate allocation to minimize end-to-end video distortion. We present a mathematical formulation of JSCC to optimize video quality over multiple wireless channels and provide comprehensive analysis for channel distortion. We evaluate the performance of FRA-JSCC through emulations in Exata and compare it with the existing schemes. Experimental results show that FRA-JSCC outperforms the competing models in improving the video peak signal-to-noise ratio as well as in reducing the end-to-end delay.

Description: Recently, the fingerprint-based wireless local area network (WLAN) positioning has gained significant interest. A probability distribution-aided indoor positioning algorithm based on the affinity propagation clustering is proposed. Different from the conventional fingerprint-based WLAN positioning algorithms, the paper first utilizes the affinity propagation clustering to minimize the searching space of reference points (RPs). Then, we introduce the probability distribution-aided positioning algorithm to obtain the target's refined position. Furthermore, because the affinity clustering can effectively lead to a reduction of the computational cost for the RP searching which is involved in the probability distribution-aided positioning algorithm, the proposed algorithm can lower the difficulty and minimize the power consumption when estimating the user's position. Experimental results conducted in the real environments show that our proposed algorithm will significantly improve the performance of the probability distribution-aided positioning algorithm in both the positioning accuracy and real-time ability.

Description: Two blind adaptive receivers, based on constrained constant modulus (CM) algorithms, are investigated for the detection of the multicarrier code division multiple access (MC-CDMA) system in the multiuser and multipath channels. As the data rate of the information bit is getting high enough, the transmitted signal of the MC-CDMA system becomes more sensitive to the frequency-selective fading. To mitigate this drawback, the parallel transmissions of multiple data symbols within one OFDM block are employed in the proposed systems to ensure that the length of the cyclic prefix (CP) is longer than the delay spread of the multipath channel. The first receiver is called a full-tap receiver (TDes receiver), which can carry out the interference suppression in the frequency-domain. Alternatively, a reduced-tap receiver (TRTap) is proposed on the basis of the derived cyclically shifting matrix. According to the derived matrix representation of the received signal, the analytical steady-state mean square errors (MSEs) for both receivers are also derived and compared. Simulation results are provided to illustrate the effectiveness of the proposed receivers and validate the accuracy of the theoretical analysis. As expected, the symbol error rate (SER) performance of the full-tap receiver is better than that of the reduced-tap receiver in severe interference environments. In contrast, the reduced-tap receiver can provide the satisfied SER performance just like the full-tap receiver in low-interference scenarios, even though the former one adopts only one-tap equalization.

Description: In most applications, wireless sensor networks are supposed to operate in an unattended manner fora long period after sensor nodes' deployment. However, in such networks, sensor nodes frequentlybecome faulty and unreliable because of the harsh environment of the observed area. Therefore,protocols used in wireless sensor networks must be designed to be robust. Moreover, because thebattery capacity of a node is limited, energy savings are crucial in wireless sensor networks. Tomeet the requirements of future diverse wireless sensor networks, a sophisticated any-to-any routingprotocol is thus required. As well as meeting the typical demands of wireless sensor networks, anany-to-any routing protocol needs to achieve low energy consumption, high scalability, robustness,and reliability. In this paper, we realize a potential-based any-to-any routing protocol (PBAR) bymerging potential-based upstream and downstream routing. In PBAR, sensor nodes can send data toa certain sensor node by routing the data via a sink node. In simulation experiments, we show that,given a suitable node density, PBAR attains a data delivery ratio greater than 99.7%. We also showthat the data delivery ratio recovers immediately after failure of 30% of sensor nodes or failure of asink node.

Description: Reliability analysis is an important issue in wireless sensor networks (WSNs). This paper aims to study the reliability of a data flow in event-driven wireless sensor networks with multiple sending transmission approach without acknowledgments. Initially, an event-driven wireless sensor network model is described in terms of limited node battery energy and shadowed fading channels. Then, in order to analyze the network reliability, the wireless link reliability and the node energy availability are investigated, respectively. Furthermore, the analytical expressions of the instantaneous network reliability and the mean time to failure (MTTF) are derived. Finally, the simulation results validate the correctness and accuracy of the analytical results.

Description: The ability to determine in real-time the geographic location of client nodes is an important tool inwireless networks, allowing instantaneous mobile tracking, implementation of location-awareservices and also efficient channel and power allocation planning. Among existing classicalcooperative localization techniques for wireless networks, the maximum likelihood estimator (MLE)is theoretically the best. However, the gradient-based algorithms that are commonly used formaximum likelihood estimation are quite sensitive to the initial values and cannot achieve thetheoretical optimal performance. In this paper, we propose a new iterative positioning algorithmbased on received signal strength information that employs a location ordering strategy and anumerical nonlinear optimization method. The algorithm performance is evaluated throughsimulation for different network scenarios. A real wireless network scenario is also implemented inorder to demonstrate the algorithm effectiveness. The proposed algorithm, while presenting asimplified implementation, can achieve better positioning estimates than the classical MLE approachbased on the conjugated gradient.

Description: In this paper, an efficient sphere detection (SD) scheme for multiple-input multiple-output (MIMO) systems is presented. The proposed SD scheme can substantially reduce computational complexity by introducing a pre-processing scheme and an adaptive threshold. In particular, our proposed pre- processing scheme is based on an iterative successive interference cancelation (SIC). The proposed adaptive threshold is used to judge whether a traversed path is correct or not and obtained by analyzing the distributions of the correct and erroneous estimates which have the minimum error pattern among all the SD searching paths. Simulation results show that our proposed scheme offers a substantial complexity reduction in terms of the number of visited nodes while keeping a significantly small performance degradation compared to conventional approaches.

Description: One of the most important objectives of wireless network service providers is to make the traffic as uniform as possible in different sectors of the network. In this paper, we analyze the uniformity of the traffic in a WiMAX network with the aid of a forecasting methodology. Taking into account the high volume of data transferred in a wireless network and the requirement of real time, we propose a forecasting methodology based on data mining. The theoretical basis of the proposed method is explained in detail. Its implementation is highlighted by diagrams, which explain each step of the algorithm. The method is applied on real data and the obtained results are discussed.

Description: This paper investigates the sparse channel estimation issue of multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Beginning with the formulation of least squares (LS) solution to sparse MIMO-OFDM channel estimation, a compressed channel sensing (CCS) framework based on the new smoothed l0-norm-regularized least squares (l2-Sl0) algorithm is proposed. Three methods, namely quasi-Newton, conjugate gradient, and optimization in the null and complement spaces of the measurement matrix, are then proposed to solve the l2-Sl0 unconstrained optimization problem. Moreover, the two former are also applied to solve the l2-Sl0 channel estimation. A number of computer simulation-based experiments are conducted showing a better reconstruction accuracy of the l2-Sl0 algorithm as compared with the smoothed l0-norm (Sl0) algorithm in the presence of noise. The proposed CCS approach can save nearly 25% pilot signals to maintain the same mean square error (MSE) and bit error rate (BER) performances as given by the conventional LS method.

Description: Recently, a variant of network coding at the physical layer named wireless network coding (WNC) has gained much attention due to it's simplicity and capacity improvement of a bi-directional link. In this paper, we design and analyze a bi-directional cognitive radio (CR) system with multiple pairs based on WNC while taking into account the imperfect spectrum sensing and interference from/to the CR system. In addition, we design a resource allocation framework consisting of a subcarrier allocation strategy with different priority assignments and optimal power allocation algorithm. We show that the quality of service within the CR system highly depends on a proper design of the spectrum sensing process to minimize the probability of missed detection, while the spectrum efficiency of the CR system increases with the number of pairs within the system to which we assign priorities.

Description: In this paper, a new scheme of combining cooperative diversity with network coding is proposed for wirelessuplink multi-source multi-relay networks. The existing network-coded cooperative scheme always conducts networkcoding operation at relays in moderate-to-high signal-to-noise ratio region. Distinct from it, the proposed schemedetermines either a direct cooperative mode or a network-coded cooperative mode at relays according to thechannel qualities of the broadcast phase. Compared with the existing network-coded cooperative scheme, theproposed scheme achieves a performance gain in terms of both diversity order and system ergodic capacity withoutextra bandwidth resource consumption. Both theoretical analysis and simulations verify the validity and superiorityof the proposed cooperative scheme.

Description: Within the context of linearly precoded MIMO-OFDM (combination of multiple antenna techniqueswith multicarrier transmission schemes such as orthogonal frequency division multiplexing) systemswith multiple-stream transmission, maximum likelihood detection (MLD) has been shown to offerlarge performance gains when compared to an all-linear setup (i.e., linear transmitter/receiver) wheneither perfect or imperfect channel state information at the transmitter (CSIT) is available. Unfortunately,these gains come at the cost of a higher complexity. In particular, the increase in computationalcost is more significant when the receiver is designed to operate with soft information and even moredramatic when, in order to optimise error rate performance, iterative decoding is allowed. In orderto exploit the best features of each detection technique, this paper proposes a method to selectivelychoose the detection strategy (ML or linear) for each individual subcarrier as a function of the instantaneouschannel conditions and CSIT accuracy. Numerical results show that a cautious and selectiveuse of ML detection substantially reduces complexity while still reaping most of the performanceadvantage.

Description: This paper proposes the multicast system for seamless live multimedia in a wireless network. The multicast technique for live multimedia and a part of handover mechanism are performed on the buffered multicasting/switching agent (BMSA) in a wireless local area network (WLAN). The rest of the handover mechanism is performed on access points (APs). This paper uses a new Internet Protocol (IP) header that is composed of three AP numbers (AP_#s) and 1-bit overwrite bit (OB) to perform seamless service. The three AP_#s support a smooth handover for solving the cutoff service when mobile nodes (MNs) move among other APs in WLAN. The overhead for a new IP header (2 bytes long) is negligible in comparison with multimedia service, and the overhead for the multicast group address (MGA) packet that is used in the transmission of MGA is only 11 bits. The MGA packet is composed of two AP_#s, an overwriting group address (OGA) bit, and MGA. We confirmed that the proposed multicast system can decrease the number of channels required, the delay of service, and the possibility of packet duplication because multicast is not provided on the multimedia server but on the BMSA in WLAN

Description: In this paper, the problem of training optimization for estimating a multiple-input multiple-output (MIMO) flat fading channel in the presence of spatially and temporally correlated Gaussian noise is studied in an application-oriented setup. So far, the problem of MIMO channel estimation has mostly been treated within the context of minimizing the mean square error (MSE) of the channel estimate subject to various constraints, such as an upper bound on the available training energy. We introduce a more general framework for the task of training sequence design in MIMO systems, which can treat not only the minimization of channel estimator's MSE but also the optimization of a final performance metric of interest related to the use of the channel estimate in the communication system. First, we show that the proposed framework can be used to minimize the training energy budget subject to a quality constraint on the MSE of the channel estimator. A deterministic version of the 'dual' problem is also provided. We then focus on four specific applications, where the training sequence can be optimized with respect to the classical channel estimation MSE, a weighted channel estimation MSE and the MSE of the equalization error due to the use of an equalizer at the receiver or an appropriate linear precoder at the transmitter. In this way, the intended use of the channel estimate is explicitly accounted for. The superiority of the proposed designs over existing methods is demonstrated via numerical simulations.

Description: Tone reservation (TR) is one of the attractive techniques to reduce peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing system. As conventional TR technique requires exhaustive searching over all the combinations of the given peak reduction tone (PRT) sets, it results in computational complexity that increases exponentially with the number of the subcarriers. In this paper, we aim to obtain a desirable PAPR reduction with low computational complexity. Since the process of searching the optimal PRT set can be categorized as combinatorial optimization with some variables and constraints, we propose a novel scheme, which is based on a nonlinear optimization approach named as invasive weed optimization method, to search the optimal combination of PRT set with low complexity. To validate the analytical results, extensive simulations have been conducted, showing that the proposed schemes can achieve significant reduction in computational complexity while keeping good PAPR reduction. As results of simulations, the proposed scheme shows almost the same PAPR reduction performance as compared with the genetic algorithm-based TR method which has been known to have the best performance and obtains near-optimal PRT sets.

Description: This work describes the design, implementation, and performance evaluation of an orthogonal frequency division multiple access (OFDMA) time-division duplexing (TDD) physical layer (PHY) compliant with the worldwide interoperability for microwave access (WiMAX) standard using a costeffective software-defined radio (SDR) platform containing field programmable gate array (FPGA) and digital signal processor (DSP) modules. We show that the proposed SDR architecture is capable of supporting the wide variety of configuration options described in the WiMAX standard while fulfilling the stringent requirements of WiMAX OFDMA TDD PHYs. The architecture allows for the implementation of all TDD functionalities in the downlink and the uplink at both the base station and the mobile station. The proposed design is shown to efficiently use the available FPGA and DSP resources. We also carried out specific experiments that take into account the frame and the downlink map messages detection over ITU-R wireless channel models to illustrate the performance of the proposed design. Finally, we discuss the utilization of the proposed hardware architecture to implement the wirelessMAN-advanced air interface.

Description: Broadcast is a fundamental operation in wireless networks. To this end, many past studies have studiedthe NP-hard, broadcast problem for always-on multi-hop networks. However, in wireless sensornetworks, nodes are powered by batteries, meaning, they have finite energy. Consequently, nodesare required to have a low duty cycle, whereby they switch between active and sleep state periodically.This means that a transmission from a node may not reach all of its neighbors simultaneously.Consequently, any developed broadcast protocols must consider collisions and the wake-up times ofneighboring nodes. Henceforth, this paper studies the minimum latency broadcast scheduling problemin duty cycled multi-hop wireless networks (MLBSDC), which remains NP hard. The MLBSDCproblem aims to find a collision-free schedule that minimizes the time in which the last node receivesa broadcast message. We propose a novel algorithm called CFBS that allows nodes in different layersof the broadcast tree to transmit simultaneously. We prove that CFBS produces a latency of at most(T + 1)H + TO(log2H). Here, T denotes the number of time slots in a scheduling period, and His the optimal broadcast latency obtained from the shortest path tree algorithm assuming no collision.We also show that the total number of transmissions is at most 4(T +2) times larger than the optimalvalue. The results from extensive simulation show that CFBS has a better performance than OTAB,the best broadcast scheduling algorithm to date. In particular, the broadcast latency achieved by CFBSis up to 3/20 that of OTAB.

Description: This paper studies the fairness, power allocation, and channel state information (CSI) quantization in multiuser systems utilizing multiple feedback bits per user. For a given set of schedulers, we obtain the system throughput with different power allocation strategies and any combination of different fading distributions. Moreover, the system performance under user different outage probability constraints is investigated. Assuming homogenous users, as a special case, the throughput is determined for fixed and random request networks. Considering nonidentical fading channels between the transmitter and receivers, the throughput is found under the K-significant average feedback bit allocation technique, and two suboptimal fairness schemes are investigated which satisfy different quality-of-service requirements. The results show that using optimal power allocation, the first quantization region (QR) of each user is the only QR for which no power may be allocated. The system outage probability vanishes as the number of users goes to infinity. Users hard outage probability constraints can be satisfied at the cost of one more QR in the channel quantizer. Finally, the proposed fairness schemes are more flexible than the standard proportional fair (PF) scheduling in dealing with the throughput-fairness tradeoff. However, their superiority over PF scheduling depends on the fairness constraint.

Description: In this paper, we address the challenging issue of multi-hop cooperative relaying over heterogeneous compound channels, where constituent channels can incorporate topographical variations and respec- tive distribution changes (due to environmental factors). Existing methods in the literature address various issues pertinent to receiver design by assuming a homogeneous framework. Since real-time signal level monitoring at the relay node is impractical in transparent relaying, statistical knowledge of the end-to-end channel is critical for an energy-efficient system design. As the received signal quality and hence the signal-to-noise ratio (SNR) depend on the channel condition, an exact charac- terisation of the end-to-end compound channel is essential for the implementation of transparent relay networks in a particular coverage area. This demands a realistic heterogeneous compound channel model, having distinct component distributions, that can accommodate variations in per-hop channel distribution. Exact probability density function and cumulative distribution function of the end-to- end compound channel are derived in closed form, through an inverse Mellin transform approach. As there can be many combinations of heterogeneous channels, we propose a unified approach for deriving statistical properties of interest. Two typical cases of heterogeneous channels, Nakagami × Weibull and Rayleigh × Weibull, are considered to validate the proposed generic analytical approach. Based on the derived exact statistics of these models, performance metrics, such as coefficient of vari- ation, average SNR, outage probability, and average SER, are evaluated. Through appropriate use of these metrics, optimal power boosting at the relays and quality stream selection for arbitrary diversity combiner units can be done. The pertinent analytical results are also validated through simulation studies to demonstrate the accuracy and applications of the results.

Description: This paper presents a new family of turbo codes called multi-non-binary turbo codes (MNBTCs) that generalizes the concept of turbo codes to multi-non-binary (MNB) parallel concatenated convolutional codes (PCCC). An MNBTC incorporates, as component encoders, recursive and systematic multi-non-binary convolutional encoders. The more compact data structure for these encoders confers some advantages on MNBTCs over other types of turbo codes, such as better asymptotic behavior, better convergence, and reduced latency. This paper presents in detail the structure and operation of an MNBTC: MNB encoding, trellis termination, Max-Log-MAP decoding adapted to the MNB case. It also shows an example of MNBTC whose performance is compared with the state-of-the-art turbo code adopted in the DVB-RCS2 standard.

Description: Rapid growth in the Internet usage and diverse military applications have led researchers to think of intelligent systems that can assist the users and applications in getting the services by delivering required quality of service in networks. Some kinds of intelligent techniques are appropriate for providing security in communication pertaining to distributed environments such as mobile computing, e-commerce, telecommunication, and network management. In this paper, a survey on intelligent techniques for feature selection and classification for intrusion detection in networks based on intelligent software agents, neural networks, genetic algorithms, neuro-genetic algorithms, fuzzy techniques, rough sets, and particle swarm intelligence has been proposed. These techniques have been useful for effectively identifying and preventing network intrusions in order to provide security to the Internet and to enhance the quality of service. In addition to the survey on existing intelligent techniques for intrusion detection systems, two new algorithms namely intelligent rule-based attribute selection algorithm for effective feature selection and intelligent rule-based enhanced multiclass support vector machine have been proposed in this paper.

Description: Game theory is the study of decision making in an interactive environment. Coalitional games fulfill the promise of group efficient solutions to problems involving strategic actions. Formulation of optimal player behavior is a fundamental element in this theory. This paper comprises a self-instructive didactic means to study basics of coalitional games indicating how coalitional game theory tools can provide a framework to tackle different problems in communications and networking. We show that coalitional game approaches achieve an improved performance compare to non-cooperative game theoretical solutions.

Description: In this paper, we consider medium access control of local area networks (LANs) underlimited-information conditions as befits a distributed system. Rather than assuming 'by rule'conformance to a protocol designed to regulate packet-flow rates (e.g., CSMA windowing), webegin with a non-cooperative game framework and build a dynamic, conditional, altruism term intothe net utility. The effects of altruism are analyzed at Nash equilibrium for the carrier sense multipleaccess (CSMA)-like random-access framework in the quasi-stationary (fictitious play) regime. Weconsider either power or throughput-based costs of networking, and the cases of identical orheterogeneous (independent) players. Unlike related work in this area, our objective is not tooptimize the performance of the system but to formulate a realistic economic model capturingaltruistic motivations. This model can then form the basis for higher-level incentive schemes that willencourage altruistic, instead of selfish, behavior. In a numerical study we consider the cases ofidentical or diverse players and show interesting relationships between the main parameters of ourmodel and comparisons with other alternatives discussed in the literature.

Description: A conventional angle diversity receiver uses multiple receiving elements that are oriented in different directions, where each element employs its own filter and nonimaging concentrator, such as a compound parabolic concentrator (CPC) or hemispheric lens. In this paper, a study of the design of a conventional receiver structure using angle diversity that offers improved performance with respect to the infrared channel characteristics is presented. To this end, a recently proposed model for the effective signal collection area of a conventional angle diversity receiver that more closely approximates real behaviour than the ideal model is used. The inclusion of this model in a Monte Carlo ray-tracing algorithm allows us to investigate the effects of conventional receiver parameters on the main infrared channel parameters, such as path loss and rms delay spread. Furthermore, in order to determine the number of receiver elements, the outage probability and the average error probability are also considered. Based on the results, a conventional angle diversity receiver composed of seven elements is proposed, with one of them oriented towards the ceiling, and six angled at a 56[degree sign] elevation with a 60[degree sign] separation in azimuth. For each element, a CPC with a 50[degree sign] field of view must be used.

Description: Timing synchronization is a major issue for any communication system since it is essential to ensure its stable operation and reliable performance. In this paper, we compare two low-complexity synchronization algorithms for impulse radio ultra-wideband (IR-UWB) system, employing orthogonal pulse shape modulation (PSM). The two widely adopted modulation schemes for IR-UWB systems are binary pulse amplitude modulation and binary pulse position modulation. However, the possibility of generating orthogonal UWB pulses in recent years has motivated the use of orthogonal PSM which is particularly attractive as high-order modulation and also due to its possible robustness against ISI, and therefore is the focus of this paper. Relying on the unique signal format, the first algorithm applies simple overlap-add operation followed by energy detection to achieve synchronization. This approach is semi non-data-aided (NDA) because a part of the signal is specifically reserved to help enable synchronization. The other algorithm, on the other hand, exploits the discriminating nature of well-designed polarity codes and employs a series of code word matching and averaging operations to achieve synchronization. This approach is full NDA as there is no need to interrupt the data transmission. Based on the judicious change in the phase of transmitted signal applied for synchronization purposes, the second algorithm can also be used to extract synchronized aggregate templates. These templates are then used in demodulation, resulting in a low-complexity non-coherent alternative to complex Rake receivers. The two compared timing algorithms rely on simple overlap-add operations and thus remain operational under practical UWB settings. Simulation results are provided to demonstrate the efficient performance of proposed timing estimators.

Description: A cognitive multi-access network in which a primary user and a secondary user transmit to a common receiver is considered. The secondary user senses the channel at the beginning of each time slot to determine whether the primary user is active or idle. The sensing is not perfect; hence, the secondary user can miss the detection of an active primary user or erroneously declare an idle primary user as active. The secondary user can vary its transmission rate and power from a time slot to the other. A joint rate and power scheduling algorithm is proposed that minimizes the probability of packet loss of the secondary user under a maximum probability of collision constraint at the primary user and a constraint on the average power transmitted by the secondary user. The case in which no retransmissions are allowed and the cases in which one or both users retransmit the collided packets are also considered. The problem is posed as a linear optimization problem that can be solved efficiently.

Description: Cooperative spectrum sensing in cognitive radio (CR) networks is studied in which each CR performs energy detection to obtain a binary decision on the absence/presence of the primary user. The problem of interest is how to efficiently report and combine the local decisions to/at the fusion center under fading channels. In order to reduce the required transmission bandwidth in the reporting phase, the paper examines nonorthogonal transmission of local decisions by means of on-off keying. Proposed and analyzed is a novel decoding-based fusion rule that essentially performs in three steps: (1) estimating minimum mean-square error of the transmitted information from cognitive radios, (2) making hard decisions of the transmitted bits based on the estimated information, and (3) combining the hard decisions in a linear manner. Simulation results support the theoretical analysis and show that the added complexity of the decoding-based fusion rule leads to a considerable performance gain over the simpler energy-based fusion rule when the reporting links are reasonably strong.

Description: Vehicular networks have attracted increasing attention from both the academy and industry. Applications of vehicular networks require efficient data communications between vehicles, whose performance is concerned with delivery ratio, delivery delay, and routing cost. The most previous work of routing in vehicular networks assumes oversimplified node mobility when evaluating the performance of vehicular networks, e.g., random mobility or artificial movement traces, which fails to reflect the inherent complexity of real vehicular networks. To understand the achievable performance of vehicular networks under real and complex environments, we first comprehensively analyze the affecting factors that may influence the performance of vehicular networks and then introduce four representative routing algorithms of vehicular networks, i.e., Epidemic, AODV, GPSR, and MaxProp. Next, we develop an NS-2 simulation framework incorporating a large dataset of real taxi GPS traces collected from around 2,600 taxis in Shanghai, China. With this framework, we have implemented the four routing protocols. Extensive trace-driven simulations have been performed to explore the achievable performance of real vehicular networks. The impact of the controllable affecting factors is investigated, such as number of nodes, traffic load, packet TTL, transmission range, and propagation model. Simulation results show that a real vehicular network has surprisingly poor data delivery performance under a wide range of network configurations for all the routing protocols. This strongly suggests that the challenging characteristics of vehicular networks, such as unique node mobility, constraints of road topology, need further exploration.

Description: In intelligent transportation systems, the cooperation between vehicles and the road side units is essential to bring these systems to fruition. Vehicular ad hoc networks (VANETs) are a promising technology to enable the communications among vehicles on one hand and between vehicles and road side units on the other hand. However, it is a challenging task to develop a reliable routing algorithm for VANETs due to the high mobility and the frequent changes of the network topology. Communication links are highly vulnerable to disconnection in VANETs; hence, the routing reliability of these ever-changing networks needs to be paid special attention. In this paper, we propose a new vehicular reliability model to facilitate the reliable routing in VANETs. The link reliability is defined as the probability that a direct communication link between two vehicles will stay continuously available over a specified time period. Furthermore, the link reliability value is accurately calculated using the location, direction and velocity information of vehicles along the road. We extend the well-known ad hoc on-demand distance vector (AODV) routing protocol to propose our reliable routing protocol AODV-R. Simulation results demonstrate that AODV-R outperforms significantly the AODV routing protocol in terms of better delivery ratio and less link failures while maintaining a reasonable routing control overhead.

Description: In this paper, we analyze the impact of nonlinear high-power amplifier (HPA) on the performance of cooperative network based on orthogonal frequency division multiplexing combined with time-division multiplexing (OFDM/TDM) using minimum mean-square-error frequency-domain combining (MMSE-FDC) in a frequency-selective fading channel. We design a novel MMSE-FDC weights while taking into account the nonlinearity of HPA at source and relay. Closed-form symbol error rate and outage probability expressions are derived while approximating the residual inter-slot interference after the MMSE-FDC as a random Gaussian variable. We discuss and address the nonlinear OFDM/TDM system design issues in cooperative network using the obtained simulation and theoretical results. We show that the OFDM/TDM with MMSE-FDC can be used to reduce the impact of nonlinear HPA on overall performance of cooperative network in comparison to OFDM while providing the target quality-of-service for reduced required signal-to-noise ratio. This is because OFDM/TDM with MMSE-FDC achieves frequency diversity in addition to cooperative diversity, while reduced peak-to-average power ratio makes it more robust on nonlinear degradation due to HPA saturation in comparison to conventional OFDM.

Description: In this contribution, the spatial cross-correlation of composite channels in a distributed antenna system (DAS) is studied for the case that a nine-antenna DAS is deployed in an indoor environment. As few measurement data of DAS propagation channels are available, the propagation graph modeling based on the electromagnetic wave reverberation theory is used to generate the synthetic impulse responses (IRs) for the composite channels induced by the different groups of antennas in the DAS. The simulations take into account the geographic parameters of the environment, the cluttering scatterers, the realistic visibility conditions, as well as the flexible locations for the antennas, and the user equipment. The uncorrelated scattering assumption is proved to be valid by using the real measurement data from the indoor environments. Based on this assumption, narrowband channel cross-correlation is computed by integrating the small-scale fading cross-correlation within the same delay bins in two channel IRs. The characteristics of the cross-correlation in four cases with different antenna grouping are investigated for the nine-antenna DAS. Part of the modeling results are shown to be consistent with the empirical counterparts for specific DAS constellations calculated using the real measurement data.

Description: Intersession network coding (NC) can provide significant performance benefits via mixing packets at wireless routers; these benefits are especially pronounced when NC is applied in conjunction with intelligent scheduling. NC, however, imposes certain processing operations, such as encoding, decoding, copying, and storage. When not utilized carefully, all these operations can induce tremendous processing overheads in practical settings. Our testbed measurements suggest that such processing operations can severely degrade the router throughput, especially at high bit rates. Motivated by this, we design network coding framework for rate adaptive wireless links (NCRAWL). The design of NCRAWL facilitates low overhead NC operations, thereby effectively approaching the theoretically expected throughput benefits of joint NC and scheduling. We implement and evaluate NCRAWL on a wireless testbed. Our experiments demonstrate that NCRAWL meets the theoretical predicted throughput gain while requiring much less CPU processing, compared to related frameworks.

Description: Indirect trust computation based on recommendations form an important component in trust-based access control models for pervasive environment. It can provide the service provider the confidence to interact with unknown service requesters. However, recommendation-based indirect trust computation is vulnerable to various types of attacks. This paper proposes a defense mechanism for filtering out dishonest recommendations based on a measure of dissimilarity function between the two subsets. A subset of recommendations with the highest measure of dissimilarity is considered as a set of dishonest recommendations. To analyze the effectiveness of the proposed approach, we have simulated three inherent attack scenarios for recommendation models (bad mouthing, ballot stuffing, and random opinion attack). The simulation results show that the proposed approach can effectively filter out the dishonest recommendations based on the majority rule. A comparison between the exiting schemes and our proposed approach is also given.

Description: Spectrum sensing is a powerful tool of the cognitive cycle to help circumvent the apparent spectrum scarcity faced by wireless transmission systems. To overcome the challenging issues faced by the localized sensing, multiple cognitive radios can cooperate to explore the multiuser diversity and generate a more reliable decision on the presence of a signal in the frequencies of interest. In such a cooperative sensing scenario, a common reporting channel is needed for the transmission of the information of each element. As the number of elements that participate in the sensing operation increases, so does the bandwidth demanded for the reporting channel, quickly becoming the limiting factor in this scenario. To tackle the issue of reducing the sensing report overhead, this paper introduces a new cooperative sensing scheme that introduces silence periods in the reporting and, relying on information theory principles, explores the information present in these periods to reduce by 50% the sensing reporting overhead while maintaining the same performance of standard reporting schemes. Numerical and experimental results confirm the theoretical analysis and show the predicted reduction in reporting overhead and performance preservation.

Description: In this paper, we present a fundamental study on the stationarity and ergodicity of eight classes of sum-of-cisoids (SOC) processes for the modeling and simulation of frequency-nonselective mobile Rayleigh fading channels. The purpose of this study is to determine which classes of SOC models enable the design of channel simulators that accurately reproduce the channel's statistical properties without demanding information on the time origin or the time-consuming computation of an ensemble average.We investigate the wide-sense stationarity, first-order stationarity of the envelope, mean ergodicity, and autocorrelation ergodicity of the underlying random processes characterizing the different classes of stochastic SOC simulators. The obtained results demonstrate that only the class of SOC models comprising cisoids with constant gains, constant frequencies, and random phases is defined by a set of stationary and ergodic random processes.The analysis presented here can easily be extended with respect to the modeling and simulation of frequency-selective single-input single-output (SISO) and multiple-input multiple-output channels. For the case of frequency-selective SISO channels, we investigate the stationarity and ergodicity in both time and frequency of 16 different classes of SOC simulation models.The findings presented in this paper can be used in the laboratory as guidelines to design efficient simulation platforms for the performance evaluation of modern mobile communication systems.

Description: We consider a cognitive radio network in which a multiple-accesssecondary system coexists with an automatic repeat request (ARQ)-based primary system under heavy primary traffic. To achievespectrum sharing without degrading the performance of the primarysystem, the secondary transmitters alternate between cooperation andaccess modes based on a credit system. In the cooperation mode, thesecondary transmitters serve as potential relays among which thebest one is selected to help forward the primary packet, thusaccumulating credits. These credits will then allow the secondarytransmitters to gain spectrum access by exploiting the ARQ mechanismof the primary system. Our results show that with a cluster ofclosely located secondary transmitters, the proposed spectrum sharing protocol achieves an equal average throughput for theprimary system compared to the case without spectrum sharing, whileproviding access opportunities for the secondary system.Furthermore, by increasing the number of secondary transmitters ordecreasing the distance between secondary transmitters and secondaryreceiver, an overall higher throughput can be achieved for thesecondary system, without affecting the analytical results for theupper bounds of primary throughput under cooperation mode, andsecondary throughput under access mode are also derived.

Description: The performance of underwater sensor networks (UWSNs) is greatly limitedby the low bandwidth and high propagation delay of acoustic communications.Deploying multiple surface-level radio-capable gateways can enhanceUWSN performance metrics, reducing end-to-end delays and distributingtraffic loads for energy reduction. In this paper, we study the problemof gateway placement for maximizing the cost-benefit of this UWSNarchitecture. We develop a mixed integer programming (MIP) gatewaydeployment optimization framework. We analyze the tradeoff betweenthe number of surface gateways and the expected delay and energy consumptionof the surface gateway architecture in the optimal case. We used anMIP solver to solve the developed optimization problem and integratedthe optimal results to serve as an input forour simulations to evaluate the benefits of surface gateway optimizationframework. We investigated the effect of acoustic channel capacityand the underwater sensor node deployment pattern on our solution.Our results show the significant advantages of surface gateway optimizationand provide useful guidelines for real network deployment.