11 research outputs found

    Sequence Design for Cognitive CDMA Communications under Arbitrary Spectrum Hole Constraint

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    To support interference-free quasi-synchronous code-division multiple-access (QS-CDMA) communication with low spectral density profile in a cognitive radio (CR) network, it is desirable to design a set of CDMA spreading sequences with zero-correlation zone (ZCZ) property. However, traditional ZCZ sequences (which assume the availability of the entire spectral band) cannot be used because their orthogonality will be destroyed by the spectrum hole constraint in a CR channel. To date, analytical construction of ZCZ CR sequences remains open. Taking advantage of the Kronecker sequence property, a novel family of sequences (called "quasi-ZCZ" CR sequences) which displays zero cross-correlation and near-zero auto-correlation zone property under arbitrary spectrum hole constraint is presented in this paper. Furthermore, a novel algorithm is proposed to jointly optimize the peak-to-average power ratio (PAPR) and the periodic auto-correlations of the proposed quasi-ZCZ CR sequences. Simulations show that they give rise to single-user bit-error-rate performance in CR-CDMA systems which outperform traditional non-contiguous multicarrier CDMA and transform domain communication systems; they also lead to CR-CDMA systems which are more resilient than non-contiguous OFDM systems to spectrum sensing mismatch, due to the wideband spreading.Comment: 13 pages,10 figures,Accepted by IEEE Journal on Selected Areas in Communications (JSAC)--Special Issue:Cognitive Radio Nov, 201

    MMP-DCD-CV based Sparse Channel Estimation Algorithm for Underwater Acoustic Transform Domain Communication System

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    In this paper, we propose a computationally efficient multipath matching pursuit (MMP) channel estimation algorithm for underwater acoustic (UWA) transform domain communication systems (TDCSs). The algorithm, referred to as the MMP-DCD-CV algorithm, is based on the dichotomous coordinate descent (DCD) iterations and cross validation (CV). The MMP-DCD-CV sparse channel estimator in each iteration searches for multiple promising path candidates most relevant to a residual vector and chooses the best candidate. The DCD iterations are used to solve the corresponding least squares problem with low complexity and numerical stability. The CV provides a stopping criterion of the algorithm without a priori information on the channel sparsity and noise level and examines whether the algorithm overfits its data, thus improving the estimation accuracy. The performance of the proposed algorithm is evaluated under simulated sparse UWA channels. The numerical results show that the algorithm achieves better performance than the original MMP algorithm, has lower complexity, and does not require prior knowledge on the channel sparsity and noise level. We also propose an UWA TDCS with sparse channel estimation based on the proposed MMP-DCD-CV algorithm. The proposed UWA communication system is tested by the Waymark simulator, providing the virtual signal transmission in the UWA channel, with a measured Sound Speed Profile and bathymetry. Numerical results demonstrate that the UWA TDCS with the proposed sparse channel estimator offers considerable improvement in system performance compared to other TDCS schemes

    Opportunistic communications in large uncoordinated networks

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    (English) The increase of wireless devices offering high data rate services limits the coexistence of wireless systems sharing the same resources in a given geographical area because of inter-system interference. Therefore, interference management plays a key role in permitting the coexistence of several heterogeneous communication services. However, classical interference management strategies require lateral information giving rise to the need for inter-system coordination and cooperation, which is not always practical. Opportunistic communications offer a potential solution to the problem of inter-system interference management. The basic principle of opportunistic communications is to efficiently and robustly exploit the resources available in a wireless network and adapt the transmitted signals to the state of the network to avoid inter-system interference. Therefore, opportunistic communications depend on inferring the available network resources that can be safely exploited without inducing interference in coexisting communication nodes. Once the available network resources are identified, the most prominent opportunistic communication techniques consist in designing scenario-adapted precoding/decoding strategies to exploit the so-called null space. Despite this, classical solutions in the literature suffer from two main drawbacks: the lack of robustness to detection errors and the need for intra-system cooperation. This thesis focuses on the design of a null space-based opportunistic communication scheme that addresses the drawbacks exhibited by existing methodologies under the assumption that opportunistic nodes do not cooperate. For this purpose, a generalized detection error model independent of the null-space identification mechanism is introduced that allows the design of solutions that exhibit minimal inter-system interference in the worst case. These solutions respond to a maximum signal-to-interference ratio (SIR) criterion, which is optimal under non-cooperative conditions. The proposed methodology allows the design of a family of orthonormal waveforms that perform a spreading of the modulated symbols within the detected null space, which is key to minimizing the induced interference density. The proposed solutions are invariant within the inferred null space, allowing the removal of the feedback link without giving up coherent waveform detection. In the absence of coordination, the waveform design relies solely on locally sensed network state information, inducing a mismatch between the null spaces identified by the transmitter and receiver that may worsen system performance. Although the proposed solution is robust to this mismatch, the design of enhanced receivers using active subspace detection schemes is also studied. When the total number of network resources increases arbitrarily, the proposed solutions tend to be linear combinations of complex exponentials, providing an interpretation in the frequency domain. This asymptotic behavior allows us to adapt the proposed solution to frequency-selective channels by means of a cyclic prefix and to study an efficient modulation similar to the time division multiplexing scheme but using circulant waveforms. Finally, the impact of the use of multiple antennas in opportunistic null space-based communications is studied. The performed analysis reveals that, in any case, the structure of the antenna clusters affects the opportunistic communication, since the proposed waveform mimics the behavior of a single-antenna transmitter. On the other hand, the number of sensors employed translates into an improvement in terms of SIR.(Catal脿) El creixement incremental dels dispositius sense fils que requereixen serveis d'alta velocitat de dades limita la coexist猫ncia de sistemes sense fils que comparteixen els mateixos recursos en una 脿rea geogr脿fica donada a causa de la interfer猫ncia entre sistemes. Conseq眉entment, la gesti贸 d'interfer猫ncia juga un paper fonamental per a facilitar la coexist猫ncia de diversos serveis de comunicaci贸 heterogenis. No obstant aix貌, les estrat猫gies cl脿ssiques de gesti贸 d'interfer猫ncia requereixen informaci贸 lateral originant la necessitat de coordinaci贸 i cooperaci贸 entre sistemes, que no sempre 茅s pr脿ctica. Les comunicacions oportunistes ofereixen una soluci贸 potencial al problema de la gesti贸 de les interfer猫ncies entre sistemes. El principi b脿sic de les comunicacions oportunistes 茅s explotar de manera eficient i robusta els recursos disponibles en una xarxa sense fils i adaptar els senyals transmesos a l'estat de la xarxa per evitar interfer猫ncies entre sistemes. Per tant, les comunicacions oportunistes depenen de la infer猫ncia dels recursos de xarxa disponibles que poden ser explotats de manera segura sense induir interfer猫ncia en els nodes de comunicaci贸 coexistents. Una vegada que s'han identificat els recursos de xarxa disponibles, les t猫cniques de comunicaci贸 oportunistes m茅s prominents consisteixen en el disseny d'estrat猫gies de precodificaci贸/descodificaci贸 adaptades a l'escenari per explotar l'anomenat espai nul. Malgrat aix貌, les solucions cl脿ssiques en la literatura sofreixen dos inconvenients principals: la falta de robustesa als errors de detecci贸 i la necessitat de cooperaci贸 intra-sistema. Aquesta tesi tracta el disseny d'un esquema de comunicaci贸 oportunista basat en l'espai nul que afronta els inconvenients exposats per les metodologies existents assumint que els nodes oportunistes no cooperen. Per a aquest prop貌sit, s'introdueix un model generalitzat d'error de detecci贸 independent del mecanisme d'identificaci贸 de l'espai nul que permet el disseny de solucions que exhibeixen interfer猫ncies m铆nimes entre sistemes en el cas pitjor. Aquestes solucions responen a un criteri de m脿xima relaci贸 de senyal a interfer猫ncia (SIR), que 茅s 貌ptim en condicions de no cooperaci贸. La metodologia proposada permet dissenyar una fam铆lia de formes d'ona ortonormals que realitzen un spreading dels s铆mbols modulats dins de l'espai nul detectat, que 茅s clau per minimitzar la densitat d鈥檌nterfer猫ncia indu茂da. Les solucions proposades s贸n invariants dins de l'espai nul inferit, permetent suprimir l'enlla莽 de retroalimentaci贸 i, tot i aix铆, realitzar una detecci贸 coherent de forma d'ona. Sota l鈥檃bs猫ncia de coordinaci贸, el disseny de la forma d'ona es basa 煤nicament en la informaci贸 de l'estat de la xarxa detectada localment, induint un desajust entre els espais nuls identificats pel transmissor i receptor que pot empitjorar el rendiment del sistema. Tot i que la soluci贸 proposada 茅s robusta a aquest desajust, tamb茅 s'estudia el disseny de receptors millorats fent 煤s de t猫cniques de detecci贸 de subespai actiu. Quan el nombre total de recursos de xarxa augmenta arbitr脿riament, les solucions proposades tendeixen a ser combinacions lineals d'exponencials complexes, proporcionant una interpretaci贸 en el domini freq眉encial. Aquest comportament asimpt貌tic permet adaptar la soluci贸 proposada a entorns selectius en freq眉猫ncia fent 煤s d'un prefix c铆clic i estudiar una modulaci贸 eficient derivada de l'esquema de multiplexat per divisi贸 de temps emprant formes d'ona circulant. Finalment, s鈥檈studia l'impacte de l'煤s de m煤ltiples antenes en comunicacions oportunistes basades en l'espai nul. L'an脿lisi realitzada permet concloure que, en cap cas, l'estructura de les agrupacions d'antenes tenen un impacte sobre la comunicaci贸 oportunista, ja que la forma d'ona proposada imita el comportament d'un transmissor mono-antena. D'altra banda, el nombre de sensors emprat es tradueix en una millora en termes de SIR.(Espa帽ol) El incremento de los dispositivos inal谩mbricos que ofrecen servicios de alta velocidad de datos limita la coexistencia de sistemas inal谩mbricos que comparten los mismos recursos en un 谩rea geogr谩fica dada a causa de la interferencia inter-sistema. Por tanto, la gesti贸n de interferencia juega un papel fundamental para facilitar la coexistencia de varios servicios de comunicaci贸n heterog茅neos. Sin embargo, las estrategias cl谩sicas de gesti贸n de interferencia requieren informaci贸n lateral originando la necesidad de coordinaci贸n y cooperaci贸n entre sistemas, que no siempre es pr谩ctica. Las comunicaciones oportunistas ofrecen una soluci贸n potencial al problema de la gesti贸n de las interferencias entre sistemas. El principio b谩sico de las comunicaciones oportunistas es explotar de manera eficiente y robusta los recursos disponibles en una red inal谩mbricas y adaptar las se帽ales transmitidas al estado de la red para evitar interferencias entre sistemas. Por lo tanto, las comunicaciones oportunistas dependen de la inferencia de los recursos de red disponibles que pueden ser explotados de manera segura sin inducir interferencia en los nodos de comunicaci贸n coexistentes. Una vez identificados los recursos disponibles, las t茅cnicas de comunicaci贸n oportunistas m谩s prominentes consisten en el dise帽o de estrategias de precodificaci贸n/descodificaci贸n adaptadas al escenario para explotar el llamado espacio nulo. A pesar de esto, las soluciones cl谩sicas en la literatura sufren dos inconvenientes principales: la falta de robustez a los errores de detecci贸n y la necesidad de cooperaci贸n intra-sistema. Esta tesis propone dise帽ar un esquema de comunicaci贸n oportunista basado en el espacio nulo que afronta los inconvenientes expuestos por las metodolog铆as existentes asumiendo que los nodos oportunistas no cooperan. Para este prop贸sito, se introduce un modelo generalizado de error de detecci贸n independiente del mecanismo de identificaci贸n del espacio nulo que permite el dise帽o de soluciones que exhiben interferencias m铆nimas entre sistemas en el caso peor. Estas soluciones responden a un criterio de m谩xima relaci贸n de se帽al a interferencia (SIR), que es 贸ptimo en condiciones de no cooperaci贸n. La metodolog铆a propuesta permite dise帽ar una familia de formas de onda ortonormales que realizan un spreading de los s铆mbolos modulados dentro del espacio nulo detectado, que es clave para minimizar la densidad de interferencia inducida. Las soluciones propuestas son invariantes dentro del espacio nulo inferido, permitiendo suprimir el enlace de retroalimentaci贸n sin renunciar a la detecci贸n coherente de forma de onda. En ausencia de coordinaci贸n, el dise帽o de la forma de onda se basa 煤nicamente en la informaci贸n del estado de la red detectada localmente, induciendo un desajuste entre los espacios nulos identificados por el transmisor y receptor que puede empeorar el rendimiento del sistema. A pesar de que la soluci贸n propuesta es robusta a este desajuste, tambi茅n se estudia el dise帽o de receptores mejorados usando t茅cnicas de detecci贸n de subespacio activo. Cuando el n煤mero total de recursos de red aumenta arbitrariamente, las soluciones propuestas tienden a ser combinaciones lineales de exponenciales complejas, proporcionando una interpretaci贸n en el dominio frecuencial. Este comportamiento asint贸tico permite adaptar la soluci贸n propuesta a canales selectivos en frecuencia mediante un prefijo c铆clico y estudiar una modulaci贸n eficiente derivada del esquema de multiplexado por divisi贸n de tiempo empleando formas de onda circulante. Finalmente, se estudia el impacto del uso de m煤ltiples antenas en comunicaciones oportunistas basadas en el espacio nulo. El an谩lisis realizado revela que la estructura de las agrupaciones de antenas no afecta la comunicaci贸n oportunista, ya que la forma de onda propuesta imita el comportamiento de un transmisor mono-antena. Por otro lado, el n煤mero de sensores empleado se traduce en una mejora en t茅rminos de SIR.Postprint (published version

    A General Framework for Analyzing, Characterizing, and Implementing Spectrally Modulated, Spectrally Encoded Signals

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    Fourth generation (4G) communications will support many capabilities while providing universal, high speed access. One potential enabler for these capabilities is software defined radio (SDR). When controlled by cognitive radio (CR) principles, the required waveform diversity is achieved via a synergistic union called CR-based SDR. Research is rapidly progressing in SDR hardware and software venues, but current CR-based SDR research lacks the theoretical foundation and analytic framework to permit efficient implementation. This limitation is addressed here by introducing a general framework for analyzing, characterizing, and implementing spectrally modulated, spectrally encoded (SMSE) signals within CR-based SDR architectures. Given orthogonal frequency division multiplexing (OFDM) is a 4G candidate signal, OFDM-based signals are collectively classified as SMSE since modulation and encoding are spectrally applied. The proposed framework provides analytic commonality and unification of SMSE signals. Applicability is first shown for candidate 4G signals, and resultant analytic expressions agree with published results. Implementability is then demonstrated in multiple coexistence scenarios via modeling and simulation to reinforce practical utility

    Resource Management in Cognitive Radio Networks

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    In the last decade, the world has witnessed rapid increasing applications of wireless networks. However, with the fixed spectrum allocation policy that has been used since the beginning of the spectrum regulation to assign different spectrum bands to different wireless applications, it has been observed that most of the allocated spectrum bands are underutilized. Therefore, if these bands can be opportunistically used by new emerging wireless networks, the spectrum scarcity can be resolved. Cognitive Radio (CR) is a revolutionary and promising technology that can identify and then exploit the spectrum opportunities. In Cognitive Radio Networks (CRNs), the spectrum can be utilized by two kinds of users: Primary Users (PUs) having exclusive licenses to use certain spectrum bands for specific wireless applications, and Secondary Users (SUs) having no spectrum licenses but seeking for any spectrum opportunities. The SUs can make use of the licensed unused spectrum if they do not make any harmful interference to the PUs. However, the variation of the spectrum availability over the time and locations, due to the coexistence with the PUs, and the spread of the spectrum opportunities over wide spectrum bands create a unique trait of the CRNs. This key trait poses great challenges in different aspects of the radio resource management in CRNs such as the spectrum sensing, spectrum access, admission control, channel allocation, Quality-of-Service (QoS) provisioning, etc. In this thesis, we study the resource management of both single-hop and multi-hop CRNs. Since most of the new challenges in CRNs can be tackled by designing an efficient Medium Access Control (MAC) framework, where the solutions of these challenges can be integrated for efficient resource management, we firstly propose a novel MAC framework that integrates a kind of cooperative spectrum sensing method at the physical layer into a cooperative MAC protocol considering the requirements of both the SUs and PUs. For spectrum identification, a computationally simple but efficient sensing algorithm is developed, based on an innovative deterministic sensing policy, to assist each sensing user for identifying the optimum number of channels to sense and the optimum sensing duration. We then develop an admission control scheme and channel allocation policy that can be integrated in the proposed MAC framework to regulate the number of sensing users and number of access users; therefore, the spectrum identification and exploitation can be efficiently balanced. Moreover, we propose a QoS-based spectrum allocation framework that jointly considers the QoS provisioning for heterogeneous secondary Real-Time (RT) and Non-Real Time (NRT) users with the spectrum sensing, spectrum access decision, and call admission control. We analyze the proposed QoS-based spectrum allocation framework and find the optimum numbers of the RT and NRT users that the network can support. Finally, we introduce an innovative user clustering scheme to efficiently manage the spectrum identification and exploitation in multi-hop ad hoc CRNs. We group the SUs into clusters based on their geographical locations and occurring times and use spread spectrum techniques to facilitate using one frequency for the Common Control Channels (CCCs) of the whole secondary network and to reduce the co-channel interference between adjacent clusters by assigning different spreading codes for different clusters. The research results presented in this thesis contribute to realize the concept of the CRNs by developing a practical MAC framework, spectrum sensing, spectrum allocation, user admission control, and QoS provisioning for efficient resource management in these promising networks

    Cognitive Radio Systems

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    Cognitive radio is a hot research area for future wireless communications in the recent years. In order to increase the spectrum utilization, cognitive radio makes it possible for unlicensed users to access the spectrum unoccupied by licensed users. Cognitive radio let the equipments more intelligent to communicate with each other in a spectrum-aware manner and provide a new approach for the co-existence of multiple wireless systems. The goal of this book is to provide highlights of the current research topics in the field of cognitive radio systems. The book consists of 17 chapters, addressing various problems in cognitive radio systems

    Spectrally-Temporally Adapted Spectrally Modulated Spectrally Encoded (SMSE) Waveform Design for Coexistent CR-Based SDR Applications

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    This work expands the applicability of the Spectrally Modulated, Spectrally Encoded (SMSE) framework by developing a waveform optimization process that enables intelligent waveform design. The resultant waveforms are capable of adapting to a spectrally diverse transmission channel while meeting coexistent constraints. SMSE waveform design is investigated with respect to two different forms of coexisting signal constraints, including those based on resultant interference levels and those based on resultant power spectrum shape. As demonstrated, the SMSE framework is well-suited for waveform optimization given its ability to allow independent design of spectral parameters. This utility is greatly enhanced when soft decision selection and dynamic assignment of SMSE design parameters are incorporated. Results show that by exploiting statistical knowledge of primary user spectral and temporal behavior, the inherent flexibility of the SMSE framework is effectively leveraged such that SMSE throughput (Bits/Sec) is maximized while limiting mutual coexistent interference to manageable levels

    Industrial Wireless Sensor Networks

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    Wireless sensor networks are penetrating our daily lives, and they are starting to be deployed even in an industrial environment. The research on such industrial wireless sensor networks (IWSNs) considers more stringent requirements of robustness, reliability, and timeliness in each network layer. This Special Issue presents the recent research result on industrial wireless sensor networks. Each paper in this Special Issue has unique contributions in the advancements of industrial wireless sensor network research and we expect each paper to promote the relevant research and the deployment of IWSNs

    Peak to average power ratio reduction and error control in MIMO-OFDM HARQ System

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    Currently, multiple-input multiple-output orthogonal frequency division multiplexing (MIMOOFDM) systems underlie crucial wireless communication systems such as commercial 4G and 5G networks, tactical communication, and interoperable Public Safety communications. However, one drawback arising from OFDM modulation is its resulting high peak-to-average power ratio (PAPR). This problem increases with an increase in the number of transmit antennas. In this work, a new hybrid PAPR reduction technique is proposed for space-time block coding (STBC) MIMO-OFDM systems that combine the coding capabilities to PAPR reduction methods, while leveraging the new degree of freedom provided by the presence of multiple transmit chairs (MIMO). In the first part, we presented an extensive literature review of PAPR reduction techniques for OFDM and MIMO-OFDM systems. The work developed a PAPR reduction technique taxonomy, and analyzed the motivations for reducing the PAPR in current communication systems, emphasizing two important motivations such as power savings and coverage gain. In the tax onomy presented here, we include a new category, namely, hybrid techniques. Additionally, we drew a conclusion regarding the importance of hybrid PAPR reduction techniques. In the second part, we studied the effect of forward error correction (FEC) codes on the PAPR for the coded OFDM (COFDM) system. We simulated and compared the CCDF of the PAPR and its relationship with the autocorrelation of the COFDM signal before the inverse fast Fourier transform (IFFT) block. This allows to conclude on the main characteristics of the codes that generate high peaks in the COFDM signal, and therefore, the optimal parameters in order to reduce PAPR. We emphasize our study in FEC codes as linear block codes, and convolutional codes. Finally, we proposed a new hybrid PAPR reduction technique for an STBC MIMO-OFDM system, in which the convolutional code is optimized to avoid PAPR degradation, which also combines successive suboptimal cross-antenna rotation and inversion (SS-CARI) and iterative modified companding and filtering schemes. The new method permits to obtain a significant net gain for the system, i.e., considerable PAPR reduction, bit error rate (BER) gain as compared to the basic MIMO-OFDM system, low complexity, and reduced spectral splatter. The new hybrid technique was extensively evaluated by simulation, and the complementary cumulative distribution function (CCDF), the BER, and the power spectral density (PSD) were compared to the original STBC MIMO-OFDM signal
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