28 research outputs found

    Enhanced Channel Estimation Based On Basis Expansion Using Slepian Sequences for Time Varying OFDM Systems

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    The Channel estimation in OFDM has become very important to recover the accurate information from the received data as the next generation of wireless technology has very high data rate along with the very high speed mobile terminals as users. In addition the fast fading channels, ICI, multipath fading channels may completely destroy the data. Also it is required to use less complex method for estimation. We are proposing the method which compares the number of techniques and gives the results in BER Vs SNR graphs. The LS estimation technique is less complex as compared to MMSE estimation but gives fails in accuracy. Using Prolate function we can reduce the complexity in calculation of parameters. If compared with state of art approach where the complexity is O(N)3, the complexity using Prolate function is O(N)2.The function depends upon maximum delay and maximum Doppler frequency spread thus parameter calculation is reduced. The technique dose not calculate particular channel characteristics. Slepian sequences utilizes the bandwidth as the sharp pulses replace the regular rectangular pulses which causes spectral leakage and thus ICI. The simulation of BER Vs SNR using CP and UW with and without Prolate is proposed that increases spectral efficiency with reduced calculations replacing rectangular pulses by Slepian pulses which increase energy concentration by Sharpe pulses thus reduction in inter carrier interference caused by multipath fading. DOI: 10.17762/ijritcc2321-8169.150513

    Channels and parameters acquisition in cooperative OFDM systems

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    CODIV, FP7/ICT/2007/215477CADWIN, PTDC/EEA – TEL/099241/2008Portuguese Foundation for Science and Technology (FCT

    Timing synchronization in decode-and-forward cooperative communication systems

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    Cooperative communication systems have attracted much attention recently due to their desirable performance gain while using single antenna terminals. This paper addresses the joint timing and channel estimation problem, and furthermore the resynchronization of multiple timing offsets in a cooperative relay system. The estimations of timing and channel are conducted in two phases and the associated Cramér-Rao bounds (CRB) are derived for both phases. It is demonstrated that the conventional CRB is not valid for timing parameters under fading conditions, and a new bound called Weighted Bayesian CRB is proposed. With the timing and channel estimates, a general framework of the resynchronization filter design is developed in order to compensate the multiple timing offsets at the destination. The proposed methods are applied to different scenarios with varying degrees of timing misalignment and are numerically shown to provide excellent performances that approach the perfectly synchronized case. © 2009 IEEE.published_or_final_versio

    Design of distributed space-time block codes for relay networks

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    The fading effect often faced in wireless communications can cause severe attenuation in signal strength. To solve this problem, diversity techniques (in terms of spatial/time/frequency) have been considered. For example, spatial diversity can be achieved by using multiple antennas at the transmitter or the receiver or both. One important architecture that can efficiently exploit the multiple antennas is the space-time block coding (STBC). The realization of STBC requires more than one antenna at the transmitter. Unfortunately, the use of multiple antennas is not practical in many wireless devices due to the size limitation. Recently, the “cooperative diversity”, also known as “user diversity”, enables single-antenna mobiles in a multi-user environment to share their antennas and generate a virtual multiple-antenna transmitter that allows them to achieve transmit diversity. To apply concept of the STBC schemes to the cooperative communications, Laneman et al. suggest the use of “conventional” orthogonal STBC in a “distributed” fashion for practical implementation of user cooperation. The pioneering works on distributed STBC (DSTBC) assume flat fading channels. This can be achieved by using multi-carrier techniques such as orthogonal frequency division multiplex (OFDM) to divide a whole spectrum into a set of narrower bands. Hence, the channel can be considered flat in each sub-band. However, for current wireless communications with single-carrier transmission, the frequency selective channels cannot be avoided. Thus, in this dissertation, I will consider the application of DSTBC to frequency selective fading channels. In the first part of my thesis, I present a new design of DSTBC to achieve full rate transmission and channel decoupling property as in conventional STBC by using zero-padding (ZP). Several receiver techniques in frequency domain are studied for the signal detection of the proposed DSTBC. The extension from ZP to unique-word (UW) will be proposed in the second part. Exploiting the properties of the UW, I will present in the third part of my thesis a method of channel estimation for relay networks

    Nonlinear amplifier distortion in cooperative OFDM systems

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    OFDM (Orthogonal frequency division multiplexing) on lupaava langattoman tietoliikenteen teknologia johtuen sen hyvÀstÀ suorituskyvystÀ monitieympÀristössÀ. Yhteistoiminnallisen tiedonvÀlityksen tekniikka on nykyisin jatkuvan tutkimuksen kohteena. Se hyödyntÀÀ muiden pÀÀtteiden antenneja virtuaalisen moniantennijÀrjestelmÀn luomiseen mahdollistaen moniantennijÀrjestelmille ominaisia kapasiteettihyötyjÀ. TÀssÀ diplomityössÀ tutkitaan epÀlineaarista vahvistussÀröÀ, kun nÀitÀ molempia tekniikoita kÀytetÀÀn yhdessÀ. EnsimmÀiset kappaleet kÀsittelevÀt OFDM-jÀrjestelmien ja epÀlineaaristen OFDM-jÀrjestelmien sÀrön sekÀ yhteistoiminnallisen tiedonvÀlityksen taustoja. Yhteistoiminnallisten OFDM-jÀrjestelmien suorituskykyÀ mitataan simulaatioiden avulla epÀlineaarisen sÀrön vaikuttaessa. SuorituskykyÀ mitataan bittivirhesuhteena kÀyttÀen epÀyhteistoiminnallista ja lineaarista yhteistoiminnallista jÀrjestelmÀÀ vertailukohteena. LisÀksi sÀrötermi myös analysoidaan. Systeemimalli sisÀltÀÀ epÀlineaarisen vahvistuksen vÀlittimessÀ, jota mallinnetaan elektronisella tehovahvistimella. Lopuksi esitellÀÀn ja testataan tekniikka jÀrjestelmÀn suorituskyvyn parantamiseen optimoimalla maksimisuhdeyhdistintÀ. Se optimoidaan mallintamalla vahvistussÀröÀ normaalijakaumalla. LisÀksi esitellÀÀn ja testataan yhteistoiminnallisille jÀrjestelmille sopiva tehovahvistimen epÀlineaarisuuden poistotekniikan muunnelma, jolla saadaan lÀhellÀ lineaarista tapausta olevia tuloksia.Orthogonal frequency division multiplexing (OFDM) is a promising technique for wireless communications because of its good performance under multipath environments. The concept of cooperative communications is currently under constant research. It uses antennas of other terminals to create virtual multiple input multiple output (MIMO) systems, providing capacity gains similar to those of MIMO systems. This thesis studies the issue of nonlinear amplifier distortion when these two techniques are used together. The first chapters give a background on OFDM systems, nonlinear distortion in OFDM systems, and Cooperative Communications. The performance of OFDM cooperative systems under nonlinear distortion are measured by simulations. The performance is measured in terms of BER using a non-cooperative system and a linear cooperative system as references. In addition, the distortion term is also analysed. The system model includes a non-linear amplifier at the relay, modelled as a solid state power amplifier (SSPA). A technique for improving the performance of the system, by optimising the maximum ratio combiner (MRC), is introduced and tested. The MRC is optimised by modelling the distortion noise as Gaussian. Also, a modification to the power amplifier nonlinearity cancellation (PANC) technique, suitable to cooperative systems, is introduced and tested, showing results close to the linear case

    Enable Reliable and Secure Data Transmission in Resource-Constrained Emerging Networks

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    The increasing deployment of wireless devices has connected humans and objects all around the world, benefiting our daily life and the entire society in many aspects. Achieving those connectivity motivates the emergence of different types of paradigms, such as cellular networks, large-scale Internet of Things (IoT), cognitive networks, etc. Among these networks, enabling reliable and secure data transmission requires various resources including spectrum, energy, and computational capability. However, these resources are usually limited in many scenarios, especially when the number of devices is considerably large, bringing catastrophic consequences to data transmission. For example, given the fact that most of IoT devices have limited computational abilities and inadequate security protocols, data transmission is vulnerable to various attacks such as eavesdropping and replay attacks, for which traditional security approaches are unable to address. On the other hand, in the cellular network, the ever-increasing data traffic has exacerbated the depletion of spectrum along with the energy consumption. As a result, mobile users experience significant congestion and delays when they request data from the cellular service provider, especially in many crowded areas. In this dissertation, we target on reliable and secure data transmission in resource-constrained emerging networks. The first two works investigate new security challenges in the current heterogeneous IoT environment, and then provide certain countermeasures for reliable data communication. To be specific, we identify a new physical-layer attack, the signal emulation attack, in the heterogeneous environment, such as smart home IoT. To defend against the attack, we propose two defense strategies with the help of a commonly found wireless device. In addition, to enable secure data transmission in large-scale IoT network, e.g., the industrial IoT, we apply the amply-and-forward cooperative communication to increase the secrecy capacity by incentivizing relay IoT devices. Besides security concerns in IoT network, we seek data traffic alleviation approaches to achieve reliable and energy-efficient data transmission for a group of users in the cellular network. The concept of mobile participation is introduced to assist data offloading from the base station to users in the group by leveraging the mobility of users and the social features among a group of users. Following with that, we deploy device-to-device data offloading within the group to achieve the energy efficiency at the user side while adapting to their increasing traffic demands. In the end, we consider a perpendicular topic - dynamic spectrum access (DSA) - to alleviate the spectrum scarcity issue in cognitive radio network, where the spectrum resource is limited to users. Specifically, we focus on the security concerns and further propose two physical-layer schemes to prevent spectrum misuse in DSA in both additive white Gaussian noise and fading environments

    Contributions to the Performance Analysis of Intervehicular Communications Systems and Schemes

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    RÉSUMÉ Le but des systĂšmes de communication intervĂ©hicule (Inter-Vehicle Communication – IVC) est d'amĂ©liorer la sĂ©curitĂ© de conduite en utilisant des capteurs et des techniques de communication sans fil pour ĂȘtre en mesure de communiquer mutuellement sans aucune intervention extĂ©rieure. Avec l'utilisation de ces systĂšmes, les communications vĂ©hicule Ă  vĂ©hicule (V2V) peuvent ĂȘtre plus efficaces dans la prĂ©vention des accidents et la dĂ©congestion de la circulation que si chaque vĂ©hicule travaillait individuellement. Une des solutions proposĂ©es pour les systĂšmes IVC est l’utilisation des systĂšmes de communication coopĂ©rative, qui en principe, augmentent l'efficacitĂ© spectrale et Ă©nergĂ©tique, la couverture du rĂ©seau, et rĂ©duit la probabilitĂ© de dĂ©faillance. La diversitĂ© d'antenne (entrĂ©es multiples sorties multiples « Multiple-Input Multiple-Output » ou MIMO) peut Ă©galement ĂȘtre une alternative pour les systĂšmes IVC pour amĂ©liorer la capacitĂ© du canal et la diversitĂ© (fiabilitĂ©), mais en Ă©change d’une complexitĂ© accrue. Toutefois, l'application de telles solutions est difficile, car les communications sans fil entre les vĂ©hicules sont soumises Ă  d’importants effets d'Ă©vanouissements des canaux appelĂ©s (canaux sujets aux Ă©vanouissements de n*Rayleigh, « n*Rayleigh fading channels»), ce qui conduit Ă  la dĂ©gradation des performances. Par consĂ©quent, dans cette thĂšse, nous proposons une analyse de la performance globale des systĂšmes de transmission coopĂ©ratifs et MIMO sur des canaux sujets aux Ă©vanouissements de n*Rayleigh. Cette analyse permettra d’aider les chercheurs pour la conception et la mise en Ɠuvre de systĂšmes de communication V2V avec une complexitĂ© moindre. En particulier, nous Ă©tudions d'abord la performance de la sĂ©lection du relais de coopĂ©ration avec les systĂšmes IVC, on suppose que la transmission via « Amplify-and-Forward» (AF) ou bien «Decode-and-Forward» (DF) est assurĂ©e par N relais pour transfĂ©rer le message de la source Ă  la destination. La performance du systĂšme est analysĂ©e en termes de probabilitĂ© de dĂ©faillance, la probabilitĂ© d'erreur de symbole, et la capacitĂ© moyenne du canal. Les rĂ©sultats numĂ©riques dĂ©montrent que la sĂ©lection de relais rĂ©alise une diversitĂ© de l'ordre de (d≈mN/n) pour les deux types de relais, oĂč m est un paramĂštre Ă©vanouissement de Rayleigh en cascade. Nous Ă©tudions ensuite la performance des systĂšmes IVC Ă  sauts multiples avec et sans relais rĂ©gĂ©nĂ©ratifs. Dans cette Ă©tude, nous dĂ©rivons des expressions approximatives pour la probabilitĂ© de dĂ©faillance et le niveau d’évanouissement lorsque la diversitĂ© en rĂ©ception basĂ©e sur le ratio maximum de combinaison (MRC) est employĂ©e. En outre, nous analysons la rĂ©partition de puissance pour le systĂšme sous-jacent afin de minimiser la probabilitĂ© globale de dĂ©faillance. Nous montrons que la performance des systĂšmes rĂ©gĂ©nĂ©ratifs est meilleure que celle des systĂšmes non rĂ©gĂ©nĂ©ratifs lorsque l’ordre de cascade n est faible, tandis qu’ils ont des performances similaires lorsque n est Ă©levĂ©. Ensuite, nous considĂ©rons le problĂšme de la dĂ©tection de puissance des signaux inconnus aux n* canaux de Rayleigh. Dans ce travail, de nouvelles expressions approximatives sont dĂ©rivĂ©es de la probabilitĂ© de dĂ©tection moyenne avec et sans diversitĂ© en rĂ©ception MRC. En outre, la performance du systĂšme est analysĂ©e lorsque la dĂ©tection de spectre coopĂ©rative (CSS) est considĂ©rĂ©e sous diverses contraintes de canaux (par exemple, les canaux de communication parfaits et imparfaits). Les rĂ©sultats numĂ©riques ont montrĂ© que la fiabilitĂ© de dĂ©tection diminue Ă  mesure que l'ordre n augmente et s’amĂ©liore sensiblement lorsque CSS emploie le schĂ©ma MRC. Il est dĂ©montrĂ© que CSS avec le schĂ©ma MRC maintient la probabilitĂ© de fausse alarme minimale dans les canaux d’information imparfaite plutĂŽt que d'augmenter le nombre d'utilisateurs en coopĂ©ration. Enfin, nous prĂ©sentons une nouvelle approche pour l'analyse des performances des systĂšmes IVC sur n*canaux de Rayleigh, en utilisant n_T antennes d'Ă©mission et n_R antennes de rĂ©ception pour lutter contre l'effet d’évanouissement. Dans ce contexte, nous Ă©valuons la performance des systĂšmes MIMO-V2V basĂ©s sur la sĂ©lection des antennes d'Ă©mission avec un ratio maximum de combinaison (TAS/MRC) et la sĂ©lection combinant (TAS/SC). Dans cette Ă©tude, nous dĂ©rivons des expressions analytiques plus prĂ©cises pour la probabilitĂ© de dĂ©faillance, la probabilitĂ© d'erreur de symbole, et l’évanouissement sur n*canaux Rayleigh. Il est montrĂ© que les deux rĂ©gimes ont le mĂȘme ordre de diversitĂ© maximale Ă©quivalent Ă  (d≈mn_T n_R /n) . En outre, TAS / MRC offre un gain de performance mieux que TAS/ SC lorsque le nombre d'antennes de rĂ©ception est plus que celle des antennes d’émission, mais l’amĂ©lioration de la performance est limitĂ©e lorsque n augmente.----------Abstract The purpose of intervehicular communication (IVC) systems is to enhance driving safety, in which vehicles use sensors and wireless communication techniques to talk to each other without any roadside intervention. Using these systems, vehicle-to-vehicle (V2V) communications can be more effective in avoiding accidents and traffic congestion than if each vehicle works individually. A potential solution can be implemented in this research area using cooperative communications systems which, in principle, increase spectral and power efficiency, network coverage, and reduce the outage probability. Antenna diversity (i.e., multiple-input multiple output (MIMO) systems) can also be an alternative solution for IVC systems to enhance channel capacity and diversity (reliability) but in exchange of an increased complexity. However, applying such solutions is challenging since wireless communications among vehicles is subject to harsh fading channels called ‘n*Rayleigh fading channels’, which leads to performance degradation. Therefore, in this thesis we provide a comprehensive performance analysis of cooperative transmission and MIMO systems over n*Rayleigh fading channels that help researchers for the design and implementation of V2V communication systems with lower complexity. Specifically, we first investigate the performance of cooperative IVC systems with relay selection over n*Rayleigh fading channels, assuming that both the decode-and-forward and the amplify-and-forward relaying protocols are achieved by N relays to transfer the source message to the destination. System performance is analyzed in terms of outage probability, symbol error probability, and average channel capacity. The numerical results have shown that the best relay selection approach achieves the diversity order of (d≈mN/n) where m is a cascaded Rayleigh fading parameter. Second, we investigate the performance of multihop-IVC systems with regenerative and non-regenerative relays. In this study, we derive approximate closed-form expressions for the outage probability and amount of fading when the maximum ratio combining (MRC) diversity reception is employed. Further, we analyze the power allocation for the underlying scheme in order to minimize the overall outage probability. We show that the performance of regenerative systems is better than that of non-regenerative systems when the cascading order n is low and they have similar performance when n is high. Third, we consider the problem of energy detection of unknown signals over n*Rayleigh fading channels. In this work, novel approximate expressions are derived for the average probability of detection with and without MRC diversity reception. Moreover, the system performance is analyzed when cooperative spectrum sensing (CSS) is considered under various channel constraints (e.g, perfect and imperfect reporting channels). The numerical results show that the detection reliability decreases as the cascading order n increases and substantially improves when CSS employs MRC schemes. It is demonstrated that CSS with MRC scheme keeps the probability of false alarm minimal under imperfect reporting channels rather than increasing the number of cooperative users. Finally, we present a new approach for the performance analysis of IVC systems over n*Rayleigh fading channels, using n_T transmit and n_R receive antennas to combat fading influence. In this context, we evaluate the performance of MIMO-V2V systems based on the transmit antenna selection with maximum ratio combining (TAS/MRC) and selection combining (TAS/SC) schemes. In this study, we derive tight analytical expressions for the outage probability, the symbol error probability, and the amount of fading over n*Rayleigh fading channels. It is shown that both schemes have the same maximum diversity order equivalent to (d≈mn_T n_R /n). In addition, TAS/MRC offers a better performance gain than TAS/SC scheme when the number of receive antennas is more than that of transmit antennas, but the performance improvement is limited as n increases

    Sensors and Systems for Indoor Positioning

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    This reprint is a reprint of the articles that appeared in Sensors' (MDPI) Special Issue on “Sensors and Systems for Indoor Positioning". The published original contributions focused on systems and technologies to enable indoor applications

    Advanced Resource Management Techniques for Next Generation Wireless Networks

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    The increasing penetration of mobile devices in everyday life is posing a broad range of research challenges to meet such a massive data demand. Mobile users seek connectivity "anywhere, at anytime". In addition, killer applications with multimedia contents, like video transmissions, require larger amounts of resources to cope with tight quality constraints. Spectrum scarcity and interference issues represent the key aspects of next generation wireless networks. Consequently, designing proper resource management solutions is critical. To this aim, we first propose a model to better assess the performance of Orthogonal Frequency-Division Multiple Access (OFDMA)-based simulated cellular networks. A link abstraction of the downlink data transmission can provide an accurate performance metric at a low computational cost. Our model combines Mutual Information-based multi-carrier compression metrics with Link-Level performance profiles, thus expressing the dependency of the transmitted data Block Error Rate (BLER) on the SINR values and on the modulation and coding scheme (MCS) being assigned. In addition, we aim at evaluating the impact of Jumboframes transmission in LTE networks, which are packets breaking the 1500-byte legacy value. A comparative evaluation is performed based on diverse network configuration criteria, thus highlighting specific limitations. In particular, we observed rapid buffer saturation under certain circumstances, due to the transmission of oversized packets with scarce radio resources. A novel cross-layer approach is proposed to prevent saturation, and thus tune the transmitted packet size with the instantaneous channel conditions, fed back through standard CQI-based procedures. Recent advances in wireless networking introduce the concept of resource sharing as one promising way to enhance the performance of radio communications. As the wireless spectrum is a scarce resource, and its usage is often found to be inefficient, it may be meaningful to design solutions where multiple operators join their efforts, so that wireless access takes place on shared, rather than proprietary to a single operator, frequency bands. In spite of the conceptual simplicity of this idea, the resulting mathematical analysis may be very complex, since it involves analytical representation of multiple wireless channels. Thus, we propose an evaluative tool for spectrum sharing techniques in OFDMA-based wireless networks, where multiple sharing policies can be easily integrated and, consequently, evaluated. On the other hand, relatively to contention-based broadband wireless access, we target an important issue in mobile ad hoc networks: the intrinsic inefficiency of the standard transmission control protocol (TCP), which presents degraded performance mainly due to mechanisms such as congestion control and avoidance. In fact, TCP was originally designed for wired networks, where packet losses indicate congestion. Conversely, channels in wireless networks might vary rapidly, thus most loss events are due to channel errors or link layer contention. We aim at designing a light-weight cross-layer framework which, differently from many other works in the literature, is based on the cognitive network paradigm. It includes an observation phase, i.e., a training set in which the network parameters are collected; a learning phase, in which the information to be used is extracted from the data; a planning phase, in which we define the strategies to trigger; an acting phase, which corresponds to dynamically applying such strategies during network simulations. The next generation mobile infrastructure frontier relies on the concept of heterogeneous networks. However, the existence of multiple types of access nodes poses new challenges such as more stringent interference constraints due to node densification and self-deployed access. Here, we propose methods that aim at extending femto cells coverage range by enabling idle User Equipments (UE) to serve as relays. This way, UEs otherwise connected to macro cells can be offloaded to femto cells through UE relays. A joint resource allocation and user association scheme based on the solutions of a convex optimization problem is proposed. Another challenging issue to be addressed in such scenarios is admission control, which is in charge of ensuring that, when a new resource reservation is accepted, previously connected users continue having their QoS guarantees honored. Thus, we consider different approaches to compute the aggregate projected capacity in OFDMA-based networks, and propose the E-Diophantine solution, whose mathematical foundation is provided along with the performance improvements to be expected, both in accuracy and computational terms
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