Página 1 dos resultados de 102 itens digitais encontrados em 0.008 segundos

‣ Contribuições ao estudo de sistemas opticos de multiplo acesso por divisão de codigos

Jose Santo Guiscafre Panaro
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 25/02/2005 Português
Relevância na Pesquisa
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O presente trabalho investiga códigos adequados para esquemas CDMA em redes de comunicação por meios ópticos como, por exemplo, fibras ópticas e redes sem fio por infravermelho. O trabalho desenvolve uma base teórica do comportamento de sistemas CDMA em canais unipolares submetidos à interferência estritamente positiva e sinalização binária, e antecipa as suas implicações. Além disso, verifica-se o desempenho de códigos unipolares apropriados para comunicação óptica propostos anteriormente, comparando-os com uma nova classe de códigos ópticos com salto temporal (OTH). Finalmente, para sistemas ópticos de múltiplo acesso por divisão de código (O-CDMA) com códigos de espalhamento ortogonais no tempo e no comprimento de onda, efetua-se uma análise de desempenho para sistemas tanto com sinalização do tipo OOK (on-off keying) quanto OOO (on-off orthogonal keying); This work concerns optical fiber and infrared networks using code-division multiple access techniques. A theoretical basis for the behavior of the unipolar CDMA systems under strictly positive interference and binary signaling is developed, and their consequences are predicted. In addition, the performance of unipolar codes, appropriate for optical communications is verified and compared with a new class of optical timehopping codes (OTH) proposed in this work. Finally...

‣ Design, development and performance analysis of DSSS-based transceiver for VLC

Terra, D.; Kumar, N.; Lourenço, N.; Alves, L. N.; Aguiar, R.L.
Fonte: Institute of Electrical and Electronics Engineers (IEEE) Publicador: Institute of Electrical and Electronics Engineers (IEEE)
Tipo: Conferência ou Objeto de Conferência
Português
Relevância na Pesquisa
47.694287%
This paper addresses and implements a Direct Sequence Spread Spectrum (DSSS) transceiver for Visible Light Communication (VLC) systems based on FPGA. A transceiver was implemented including a transmitter capable of driving an array of light emitting diodes (LED) and a Pseudo-Noise (PN) matched decorrelator. The receiver architecture uses a discrete FIR correlator for data synchronization and acquisition. In this paper, a novel and simple PN code with a 10 bit sequence length is developed. This code offers a similar performance to the popular Barker code; however, it has a simple design. The used FPGA resources are presented along with a performance analysis. © 2011 IEEE.

‣ Visible light communication for intelligent transportation in road safety applications

Kumar, N.; Terra, D.; Lourenço, N.; Alves, L. N.; Aguiar, R.L.
Fonte: Institute of Electrical and Electronics Engineers (IEEE) Publicador: Institute of Electrical and Electronics Engineers (IEEE)
Tipo: Conferência ou Objeto de Conferência
Português
Relevância na Pesquisa
48.54236%
This paper discusses the implementation of a light emitting diode based visible light communication system, for intelligent transportation in road safety applications. The signal processing of both, transmitter and receiver are realized in a field programmable gate array using two Spartan-3E development kits. The implemented modulation scheme is based on direct sequence spread spectrum techniques. The performance of the overall system is evaluated and results are presented. It is found that the system is well suited for traffic broadcast at low data rate and medium range. The simulation results also show that spread spectrum modulation techniques mitigate the effect of noise produced by ambient artificial light sources. © 2011 IEEE.; FCT, Government of Portugal - VIDAS, Grant (PTDC/EEATEL/ 75217/2006)

‣ Visible light communications in intelligent transportation systems

Kumar, Navin; Lourenço, Nuno; Terra, Domingos; Alves, Luis Nero; Aguiar, Rui Luís Andrade
Fonte: IEEE Publicador: IEEE
Tipo: Conferência ou Objeto de Conferência
Português
Relevância na Pesquisa
47.70999%
Emerging intelligent transportation systems (ITS) are based on several technologies. A new concept of integrating visible light communications (VLC) in ITS is introduced in this paper.VLC in ITS is a cost effective method of implementation. This paper presents a VLC broadcast system considering LED-based traffic lights. It discusses the integration of traffic light road side unit (RSUs) within the existing ITS architecture. A prototype demonstrator of the designed VLC systems is also presented. A robust modulation technique based on direct sequence spread spectrum (DSSS) sequence inverse keying (SIK) has been implemented to minimize the effect of noise sources. Experimental results show data communication range of over 40m with 200mm custom designed traffic light, even during bright sun light.

‣ The Theoretical Limits of Watermark Spread Spectrum Sequence

Jiang, Nan; Wang, Jian
Fonte: Hindawi Publishing Corporation Publicador: Hindawi Publishing Corporation
Tipo: Artigo de Revista Científica
Publicado em 23/03/2014 Português
Relevância na Pesquisa
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At present, the spread spectrum (SS) sequences used in watermark include i.i.d. random sequences and the sequences used in SS communications. They appear earlier than digital watermark. Almost no researchers pay attention to whether they are really fit for watermark. In this paper, we compare the SS watermark channel and the traditional SS communication channel. We find out that their correlation property is different. Considering cropping and translation attacks, we define watermark auto- and cross-correlation and propose Loose Autocorrelation and Tight Cross-Correlation (LAC&TCC) properties for SS watermark. The LAC&TCC properties are that, whether or not synchronized, the autocorrelation is equal or close to 1 and the cross-correlation is equal or close to 0. Accordingly, the peak correlation is divided into the peak autocorrelation R a(l) and the peak cross-correlation R c(l). We establish the lower bound of R c(l) and the higher bound of R a(l), respectively. The two bounds indicate that, no matter how small the cover is reserved, the extractor can always find a threshold to distinguish auto- and cross-correlation in theory.

‣ Throughput analysis for code division multiple accessing of the spread spectrum channel

Fonte: Laboratory for Information and Decision Systems, Massachusetts Institute of Technology Publicador: Laboratory for Information and Decision Systems, Massachusetts Institute of Technology
Formato: [5] p.; application/pdf
Português
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Joseph Y.N. Hui.; Bibliography: p. [5]; "September 1984." Reprinted from IEEE Journal on Selected Areas in Communications, vol. SAC-2, no.4, July 1984.; "... supported by the National Science Foundation under Grant NSF/ECS 78-19880..." "... Advanced Project Agency under Grant ONR-N00014-75-C1183."

‣ Non-binary spread-spectrum multiple-access communications / Derek Paul Rogers.

Rogers, Derek Paul
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado Formato: 50753 bytes; application/pdf
Publicado em //1995 Português
Relevância na Pesquisa
78.41249%
This thesis investigates non-binary spread-spectrum multiple-access communications. The research considers wide generation techniques, system performance, how performance is influenced by the different properties of the codes, and how those properties relate to the code generation technqiue. The research refines the code design philosophy and investigates this by developing a novel code generation technique.; Thesis (Ph.D.)--University of Adelaide, Dept. of Electrical and Electronic Engineering, 1995?; Bibliography: leaves 181-201.; ix, 201 leaves : ill. ; 30 cm.; Title page, contents and abstract only. The complete thesis in print form is available from the University Library.

‣ A multiuser receiver for code division multiple access communications over multipath channels

Fawer, Urs; Aazhang, Behnaam; Fawer, Urs; Aazhang, Behnaam
Fonte: Universidade Rice Publicador: Universidade Rice
Tipo: Artigo de Revista Científica
Português
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Journal Paper; A multiuser communication system is considered where K users share a channel with multipath propagation by using code division for multiple access. Data modulation is carried out by binary phase shift keying and direct sequence spread spectrum signaling. The micro-cellular communication media is modeled as a frequency selective fading channel with multipath propagation. The multipath diversity of the received signals from the K users is exploited by a bank of K RAKE correlators. Algorithms based on the maximum likelihood rule have been developed for estimating the complex channel coefficients as well as for detection of the desired data packets from the sufficient statistics provided by the RAKE correlators. The performance of the resulting multiuser detector is evaluated analytically and via Monte Carlo simulations. The results indicate that the estimator of the channel coefficients has a variance close to the Cramer-Rao lower bound, and that the proposed multiuser detector is capable of eliminating the near-far effect as well as processing the signals propagated through multiple paths.

‣ New Spread Spectrum Techniques for Multiple Antenna Transmit Diversity

Rajan, Dinesh; Erkip, Elza; Aazhang, Behnaam; Rajan, Dinesh; Erkip, Elza; Aazhang, Behnaam
Fonte: Universidade Rice Publicador: Universidade Rice
Tipo: Conference paper
Português
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Conference Paper; We propose a new scheme for multiple antenna transmission in the context of spread spectrum signaling. The new scheme consists of using shifted Gold sequences to modulate independent information on the multiple atennas. We show that this new scheme greatly improves the throughout over crrently known multiple-antenna methods. We also find the optimal power allocation strategy among multiple transmit antennas for a fixed feedback rate.

‣ Empirical detection for spread spectrum and code division multiple access (CDMA) communications

Lee, Yuan Kang
Fonte: Universidade Rice Publicador: Universidade Rice
Português
Relevância na Pesquisa
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In this thesis, the method of "classification with empirically observed statistics"--also known as empirical classification, empirical detection, universal classification, and type-based detection--is configured and applied to the despreading/detection receiver operation of a spread-spectrum (SS), code division multiple access (CDMA) communications system. In static and Rayleigh-fading environments, the empirical detector is capable of adapting to unknown noise environments in a superior manner than the linear matched-filter despreader/detector, and done with reasonable amounts of training. Compared to the optimum detector, when known, the empirical detector always approaches optimal performance, again, with reasonable amounts of training. In an interference-limited channel, we show that the single-user likelihood-ratio detector, which is the optimum single-user detector, can greatly outperform the matched filter in certain imperfect power-control situations. The near optimality of the empirical detector implies that it, too, will outperform the matched filter in these situations. Although the empirical detector has the added cost of requiring chip-based phase synchronization, its consistent and superior performance in all environments strongly suggests its application in lieu of the linear detector for SS/CDMA systems employing long...

‣ Direct-sequence spread-spectrum acoustic communications with CRV Decomposition

Angelopoulos, Pavlos.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
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Approved for public release; distribution is unlimited.; Direct-Sequence Spread-Spectrum (DS-SS) is among the preferred modulation techniques for military applications. DS-SS offers three greatly desired characteristics. It allows for the development of Low Probability of Detection (LPD) and Low Probability of Intercept (LPI) systems and has a very good performance in fading channels. This thesis investigates the performance of the "Cross-Product RV (CRV) decomposition" as the basis of blind-equalization algorithms. The CRV is a rank-revealing decomposition alternative to the Eigenvalue Decomposition (EVD) that can provide a recursively updated estimate of the signal and noise subspace at a reduced computational cost. The CRV updating algorithm is implemented in MATLAB and evaluated in a previously proposed communication scheme intended for use in an underwater acoustic network called Seaweb. The underwater channel is modeled with the Monterey-Miami Parabolic Equation Model (MMPE) for various multipath perturbations. The receiver performance is examined using Monte Carlo simulation. Bit-error rates versus signal-to-noise ratio are presented for various, noise assumptions, and receiver synchronization assumptions.

‣ Direct-sequence spread-spectrum modulation for utility packet transmission in underwater acoustic communication networks

Duke, Peter S.
Fonte: Monterey, Calif. Naval Postgraduate School Publicador: Monterey, Calif. Naval Postgraduate School
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
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Approved for public release, distribution is unlimited; This thesis investigates the feasibility and performance of using Direct-Sequence Spread-Spectrum (DSSS) modulation for utility-packet transmission in Seaweb underwater wireless acoustic communications networks. Seaweb networks require robust channel-tolerant utility packets having a low probability of detection (LPD) and allowing for multi-user access. MATLAB code simulated the DSSS transmitter and receiver structures and a modeled channel impulse response represented the underwater environment. The specific modulation scheme implemented is direct-sequence, differentially encoded binary phase-shift keying (DS-DBPSK) with quadrature spreading. Performance is examined using Monte Carlo simulation. Bit error rates and packet error rates for various signal-to-noise ratios and channel conditions are presented and the use of a RAKE receiver, forward error-correction coding and symbol interleaving are examined for improving system performance.

‣ OPNET implementation of spread spectrum network for voice and data distribution

Standfield, Roger D
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Português
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Approved for public release; distribution is unlimited; This thesis presents an OPNET model and simulation of a single cell wireless communications system within a proposed expeditionary warfare communications network. The focus of this thesis is to model and implement data and voice traffic generation, slofted ALOHA medium access control protocol, and direct sequence spread spectrum code division multiple access (CDMA) mechanisms in OPNET. The RF channel is modeled as both a Rayleigh fading channel and a non- fading noise limited channel. Simulation results evaluating the induced BER and multiple access implementation are presented

‣ Four frequency-shift keying (4-FSK) spread spectrum modulator and demodulator

Murray, Terrence J.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
47.84697%
Approved for public release; distribution is unlimited.; This thesis explores the potential use of a four frequency-shift keying (4-FSK), spread spectrum modulator and demodulator (MODEM) in a low orbit satellite. In this first approach a short maximal length sequence of 127 chips would be used to spread the four frequencies. After successful implementation, the design could be extended to longer codes which would provide for greater processing gain. This MODEM was not preselected for use in satellite communications based on the merits of 4-FSK, but was assigned as one of four possible digital communication designs. A MODEM would be selected for use in the Petite Amateur Navy Satellite after a thorough design review.

‣ Spread spectrum signal characteristic estimation using exponential averaging and an ad-hoc chip rate estimator

Weber, John Baltzer
Fonte: Monterey California. Naval Postgraduate School Publicador: Monterey California. Naval Postgraduate School
Formato: xxii, 132 p. : ill. (some col.) ; 28 cm.
Português
Relevância na Pesquisa
78.27269%
This dissertation investigates two methods of spread spectrum (SS) signal characteristic estimation for the two principle types of SS systems, frequency-hopped (FH) and direct sequence SS. The exponential averaging detector is used to detect and estimate the hopped frequencies for a SS-FH signal in the presence of interference signals as well as additive-white-Gaussian-noise (AWGN). The detection method provides an estimate of the AWGN plus interference spectrum using exponential averaging and then generates an estimate of the desired signal spectrum by combining the estimated AWGN plus interference spectrum with the composite (desired signal plus interference plus AWGN) spectrum. Finally, this dissertation evaluates the detector's performance as a function of the exponential coeeficient, the combining method, the probability of false alarm, signal-to-AWGN ratio, and signal-to-interference ratio. The second method of SS signal characteristic estimation uses a signal ad-hoc chip rate estimator (ACRE). The ACRE is used to estimate the chip rate of a half-sine pulse shaped SS direct-sequence signal. The Acre is explained in relation to its similarities and contrasts to the chip rate detector. The components and performance of the ACRE are presented for standard-ACRE. ACRE with additional filtering...

‣ Effects of fading and data modulation on noncoherent m-sequence acquisition schemes

Vincent, Patrick J.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
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Approved for public release; distribution is unlimited.; In direct-sequence spread-spectrum systems, successful communications require phase synchronization of the m-sequence in the incoming signal with a locally generated m-sequence at the receiver. Many acquisition schemes which extract the phase of an incoming m-sequence have been studied, but most of them assume coherent demodulation (which is usually not available during acquisition) and or independent samples (which introduce a loss in the effective signal to noise ratio (SNR)). This thesis investigates the performance of two m-sequence acquisition schemes in the presence of fading and data modulation. A fixed sample size test and a truncated sequential test are studied without the usual assumptions of coherent demodulation or independent samples. The effects of fading and data modulation on our schemes' probability of false alarm, probability of detection and test length are thoroughly explored. We find that channel fading in effect induces a loss of signal SNR, but the desired power of the tests can be restored by suitable adjustments in the decision processor. We find that the effects of data modulation are less severe, but more problematic to correct; http://archive.org/details/effectsoffadingd00vinc; Lieutenant...

‣ Terrestrial communication between wireless sensor networks using beam-forming and space division multiple access

Taylor, Chris Edward
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Tese de Doutorado Formato: xvi, 99 p. : ill. (chiefly col.) ;
Português
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In this thesis, methods for forming a communications link of Wireless Sensor Networks (WSN) by enabling each WSN to act as a smart antenna are presented. Each WSN is simulated as a set of randomly placed sensor nodes within a planar area. The proposed method involves a searching WSN, a receiving WSN and a link budget for establishing the link. The searching WSN has the task of transmitting a search beam in order to find adjacent WSNs. Like a lighthouse this is done in a rotating beam style search using the sensor nodes as an aperiodic array. Results show that for a random array, we can achieve a specific beamwidth and gain as a function of the number of elements and area. We also demonstrate that for a given required gain level we can spatially thin the array without significant loss of gain or the effects of grating lobes. The receiving WSN uses a spread spectrum based space division multiple access (SDMA) receiver. This receiver is simulated to determine the direction of arrival from the searching WSN and to extract the location information from the searching WSN's signal with additive white Gaussian noise. From the DOA and the location information within the arriving signal, the WSN has sufficient knowledge to respond to the query of the searching WSN and form the communications link.; US Navy (USN) author.

‣ Adaptive reception for underwater communications

Dessalermos, Spyridon.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Formato: xx, 164 p. : col. ill. ; 28 cm.
Português
Relevância na Pesquisa
47.70999%
This research concerns the development of an adaptive receiver for underwater communication. In this type of wireless link, the radio channel is replaced by an underwater acoustic channel, which is strongly dependent on the physical properties of the ocean medium and its boundaries, the link geometry and the ambient noise. Traditional acoustic communications have involved a priori matching of the signaling parameters to the expected characteristics of the channel. To achieve more robust communications, as well as high quality of service, it is necessary to develop a type of adaptive receiver for the acoustic link. This process involves estimating the channel scattering function by processing the received Direct Sequence Spread Spectrum (DSSS) acoustic signal. This information is used by an innovative receiver introduced for usage in the underwater environment. The research also involves the development of a robust acquisition system for the acoustic communication signal. The major design goal is the receiver's robustness on a Doppler distorted fast varying multipath fading channel, for applications in multiuser environments. Another important design goal is low complexity in signal processing. The proposed receiver is able to adapt in the acoustic channel variations. Experimental results prove the effectiveness of the receiver.; Hellenic Navy author

‣ Compressive Sensing for Spread Spectrum Receivers

Fyhn, Karsten; Jensen, Tobias Lindstrøm; Larsen, Torben; Jensen, Søren Holdt
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/02/2013 Português
Relevância na Pesquisa
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With the advent of ubiquitous computing there are two design parameters of wireless communication devices that become very important power: efficiency and production cost. Compressive sensing enables the receiver in such devices to sample below the Shannon-Nyquist sampling rate, which may lead to a decrease in the two design parameters. This paper investigates the use of Compressive Sensing (CS) in a general Code Division Multiple Access (CDMA) receiver. We show that when using spread spectrum codes in the signal domain, the CS measurement matrix may be simplified. This measurement scheme, named Compressive Spread Spectrum (CSS), allows for a simple, effective receiver design. Furthermore, we numerically evaluate the proposed receiver in terms of bit error rate under different signal to noise ratio conditions and compare it with other receiver structures. These numerical experiments show that though the bit error rate performance is degraded by the subsampling in the CS-enabled receivers, this may be remedied by including quantization in the receiver model. We also study the computational complexity of the proposed receiver design under different sparsity and measurement ratios. Our work shows that it is possible to subsample a CDMA signal using CSS and that in one example the CSS receiver outperforms the classical receiver.; Comment: 11 pages...

‣ Pseudorandom Sequences in Spread-Spectrum Communications Generated by Cellular Automata

Ordaz-Salazar,F.C.; González-Salas,J.S.; Campos-Cantón,E.; Rosu,H.C.
Fonte: UNAM, Centro de Ciencias Aplicadas y Desarrollo Tecnológico Publicador: UNAM, Centro de Ciencias Aplicadas y Desarrollo Tecnológico
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/2012 Português
Relevância na Pesquisa
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Dynamical systems methods have been recently used in spread-spectrum digital communication systems. The expansion of the spectrum using a pseudorandom sequence with a higher frequency than the information signal is the key feature for its robustness against the signal traveling interference through the channel. In this work, we propose to generate pseudorandom sequences by employing cellular automata and we check these sequences have the necessary properties which are required in modern communication systems. The computed sequences obtained by the cellular automata are tested in a quadrature phase shift keying (QPSK) spread-spectrum communication system. The efficiency of the system is analyzed by computing the bit error rate under different signal to noise ratio conditions. These results are compared with systems that employ Golden code and other typical pseudorandom sequences.