DS-CDMA

Abstract: Code Division Multiple Access (CDMA) is the most promising candidate for wideband data access. This is due to the advantage of soft limit on the number of active mobile devices. Many wireless mesh systems impose an upper bound on the BER performance which restricts the increase in number of mobile users. Capacity is further reduced in Multipath Fading Environment (MFE). This paper presents an effective method of improving the capacity of a CDMA based mesh network by managing the transmitted ...

Abstract: Code Division Multiple Access (CDMA) is the most promising candidate for wideband data access. This is due to the advantage of soft limit on the number of active mobile devices. Many wireless mesh systems impose an upper bound on the BER performance which restricts the increase in number of mobile users. Capacity is further reduced in Multipath Fading Environment (MFE). This paper presents an effective method of improving the capacity of a CDMA based mesh network by managing the transmitted ...

Resumo-Neste tutorial, apresentamos os equalizadores turbo e detalhamos seus princípios de funcionamento. Diferentemente dos receptores convencionais, nos quais equalização e decodificação de canal são feitas separadamente, os equalizadores turbo realizam estas duas tarefas conjuntamente através de um procedimento iterativo. Desta maneira, o equalizador pode se beneficiar da robustez introduzida pelo código corretor de erros para tentar eliminar a interferência intersimbólica de uma maneira mais efetiva. Discutiremos brevemente os principais algoritmos utilizados nos equalizadores turbo, mostrando alguns resultados de simulação. Por fim, citaremos algumas das recentes aplicações dos equalizadores turbo em sistemas de comunicações digitais. Palavras-Chave-Equalização turbo, códigos corretores de erros, algoritmos SISO.

The Probabilistic Data Association (PDA) method is extended to multiuser detection over asynchronous Code-Division Multiple Access (CDMA) communication channels. PDA models the undecided user signals as binary random variables. By approximating the Multiple Access Interference (MAI) as Gaussian noise with an appropriately evaluated covariance matrix, PDA updates the probabilities associated with user signals iteratively. A sliding window processing is used during the updates in order to avoid considering the entire data. Computer simulations show that the probability of group detection error of the proposed PDA method is very close to the performance lower bound. The computational complexity of the PDA detector is O((2h + 1)K 3) per time frame where K is the number of users and 2h + 1 is the width of the processing window. Relations between the proposed PDA detector and the PDA detector for synchronous CDMA is also shown.

Resumo-O objetivo deste trabalhoé apresentar classes de códigos turbo M-ários. O esquema de codificação turbo consiste em dois codificadores componentes multiníveis, concatenados em paralelo via um entrelaçador, mapeados através da constelação M-PSK (M-ary Phase Shift Keying) e transmitidos pelo canal de comunicação AWGN (Aditive White Gaussian Noise). O esquema de decodificação turboé formado por um algoritmo de decodificação M-ário, baseado na decodificação MAP (máximo a posteriori) via a decodificação iterativa. Os resultados das simulações mostram que os códigos turbo M-ário são mais eficientes que os códigos turbo binários. Palavras-Chave-Códigos Turbo M-ário, Algoritmo de Decodificação Iterativa.

Spreading codes play an important role in multiple access capacity of DS-CDMA.M-sequences , has been traditionally used as spreading codes, they are generated by shift registers , they are periodic in nature and less in number so this limits the security. This paper present an investigation on use of chaotic sequences ;that recently ,chaos theory has developed a new area that is relevant to future radio spectrum management and security. one of the chaotic systems is used as a generator of sequences is non -linear recursive equation (logistic map).

The paper explores the use of residue number systems (RNS) to incorporate incremental redundancy (IR) and link adaptation (LA) in wireless communication systems. This also explores the use of RNS to increase the transmission data rate and to enhance the privacy of a wireless network. By exploiting IR and LA further, one can design an appropriate coding scheme for transmission at the currently experienced SNR level. The proposed method is implemented in existing wireless local area network (WLAN) standards and simulated extensively to find its suitability. The results of simulation studies are discussed in details.

This paper presents the results obtained by two linear multiuser detectors, namely the decorrelating detector and the minimum mean square error (MMSE) one, when the channel is affected by the flat Rayleigh fading and gray images are transmitted through the system. All the users transmit their information with the same power, but the spreading codes are correlated one another. We will investigate the fading effect on the received images and the impact of the cross-correlation of the spreading codes on the quality of the received images. In the end, several conclusions are highlighted, with respect to the level of performance achieved.

Resumo-Nos sistemas de comunicações móveis, os sinais se propagam por múltiplos percursos sobre canais variantes no tempo, ficando sujeitos a distorções provocadas pelo desvanecimento e desvios Doppler. Visando minimizar essas distorções, técnicas de codificação e de diversidade de transmissão podem ser empregas, tais como a Codificação Wavelet. Essa técnica baseia-se na ortogonalidade das linhas da matriz wavelet e possui a capacidade de gerar ganhos de diversidade, aumentando a robustez do sistema ao desvanecimento variante no tempo, sem comprometer a eficiência espectral do sistema. Neste trabalho, a codificação waveleté implementada sobre o ambiente GNU Radio Companion, o que permitirá uma análise de sua aplicabilidade em cenários de Rádio Definido por Software (RDS). O sistema desenvolvidoé validado através de uma análise de probabilidade de erro de bit (BER) versus E b /N0, sobre um modelo de canal com desvanecimento plano.

In this paper we derived the probability density function of the product of independent non-identically distributed Weibull RVs. The derivation is based on the use of the Mellin transformation along with the Residue theorem to evaluate the attained complex integral. The resultant integrands have only elementary functions, such as exponentials and algebraic power functions and can, therefore, be explicitly evaluated to any degree of accuracy using any symbolic mathematics based software. The distribution of the product of independent and identically distributed (i.i.d) Weibull RVs are also evaluated. The proposed method avoids the computational burden of residuals of a product of functions. Hence our results are expected to be of significant practical use to evaluate the error rate performance of any M-ary orthogonal faded systems in indoor and outdoor wireless environments.

This paper presents the results obtained by two linear multiuser detectors, namely the decorrelating detector and the minimum mean square error (MMSE) one, when the channel is affected by the flat Rayleigh fading and gray images are transmitted through the system. All the users transmit their information with the same power, but the spreading codes are correlated one another. We will investigate the fading effect on the received images and the impact of the cross-correlation of the spreading codes on the quality of the received images. In the end, several conclusions are highlighted, with respect to the level of performance achieved.

This letter contains two parts. In the first part, the probabilistic data association (PDA) method is extended to multiuser detection over symbol-asynchronous code-division multiple access (CDMA) communication channels. A direct extension as well as a sliding window processing method are introduced. While achieving near-optimal performance with (3) computational complexity in synchronous CDMA, being the number of users, it is shown that, in asynchronous CDMA, the probability of group detection error of the proposed PDA method is very close to the performance lower bound provided by an ideal clairvoyant optimal detector, and the computational complexity is only marginally increased to ([ ] 3) per symbol where and are the width and the sliding rate of the processing window, respectively. In the second part, due to the outstanding performance of the PDA detector in heavily overloaded asynchronous systems, it is observed that an optimally designed synchronous system can be easily outperformed by an arbitrarily designed asynchronous system. Hence, it is proposed to use asynchronous transmission deliberately, even when synchronous transmission is possible-asynchronous is better than synchronous! Index Terms-Asynchronous code-division multiple access (CDMA), code-division multiple access (CDMA), multiuser detection, overloaded system, probabilistic data association (PDA).

The successful use of CDMA technology is based on the construction of large families of encoding sequences with good correlation properties. This paper discusses PN sequence generation based on Residue Arithmetic with an effort to improve the performance of existing interference-limited CDMA technology for mobile cellular systems. All spreading codes with residual number system proposed earlier did not consider external interferences, multipath propagation, Doppler effect etc. In literature the use of residual arithmetic in DS-CDMA was restricted to encoding of already spread sequence; where spreading of sequence is done by some existing techniques. The novelty of this paper is the use of residual number system in generation of the PN sequences which is used to spread the message signal. The significance of cross-correlation factor in alleviating multi-access interference is also discussed. The RNS based PN sequence has superior performance than most of the existing codes that are w...

Spreading codes play an important role in multiple access capacity of DS-CDMA.M-sequences , has been traditionally used as spreading codes, they are generated by shift registers , they are periodic in nature and less in number so this limits the security. This paper present an investigation on use of chaotic sequences ;that recently ,chaos theory has developed a new area that is relevant to future radio spectrum management and security. one of the chaotic systems is used as a generator of sequences is non -linear recursive equation (logistic map).

In this paper, we present distributed cooperative and regret-matching-based learning schemes for joint transmit power and beamforming selection for multiple antenna wireless ad hoc networks operating in a multi-user interference environment. Under the total network power minimization criterion, a joint iterative approach is proposed to reduce the mutual interference at each node while ensuring a constant received signal-to-interference and noise ratio at each receiver. In cooperative and regret-matching-based power minimization algorithms, transmit beamformers are selected from a predefined codebook to minimize the total power. By selecting transmit beamformers judiciously and performing power adaptation, the cooperative algorithm is shown to converge to a pure strategy Nash equilibrium with high probability in the interference impaired network. The proposed cooperative and regret-matching-based distributed algorithms are also compared with centralized solutions through simulation results.

Many recent standards in telecommunications field are based on CDMA spread spectrum transmissions. In this paper, we describe a methodology for top-down design, modeling, and simulation of CDMA transmitter system using hardware description language VHDL-AMS. Details of VHDL-AMS implementation for each elementary block are shown. This paper together with the developed library of CDMA transmitter blocks are targeted towards engineers who work on behavioral modeling and simulation of complete CDMA systems using hardware description languages.

Resumo-A codificação wavelet de canal objetiva reduzir efeitos deletérios de canal de comunicação sem fio sobre um sinal transmitido e emprega comumente uma decodificação símbolo a símbolo. Este trabalho investiga o emprego de decodificador de Viterbi para códigos wavelet e compara, em termos da taxa de erro de bits, o desempenho do sistema operando sobre canal com desvanecimento Rayleigh, quando aplicada a decodificação de Viterbi, em relação à decodificação tradicional. Os resultados de simulação demonstraram que a decodificação de sequência proporciona desempenho superior à decodificação símbolo a símbolo.

Spreading codes play an important role in multiple access capacity of DS-CDMA.M-sequences , has been traditionally used as spreading codes, they are generated by shift registers , they are periodic in nature and less in number so this limits the security. This paper present an investigation on use of chaotic sequences ;that recently ,chaos theory has developed a new area that is relevant to future radio spectrum management and security. one of the chaotic systems is used as a generator of sequences is non -linear recursive equation (logistic map).

The objectives of this paper are to study, analyze and evaluate the performance of Direct Sequence-Wide Code Division Multiple Access (DS-WCDMA) systems in Mobile Rayleigh fading propagation channel. The Parameters which are considered for the evaluation performance are Additive white Gaussian noise (AWGN),Rayleigh propagation fading channel and the Code sequence .length. The evaluation of DS-WCDMA was derived for different type of digital modulation using Mat lab simulation link. The results were obtained in terms of bit error rates(BER)in graphical form.

Resumo Este artigo mostra a implementacao do algoritmo BCJR para utilizacao em codigos de bloco lineares, com e sem o uso da decodificacao iterativa. Sao feitas simulacoes computacionais, e geradas curvas de desempenho, relacionando valores das probabilidadesde erro com os de relacao sinalruido.

DS-Code division multiple access is considered as the third generation of cellular mobile used in interim standard 95(IS-95) [1]and it is currently being standardized for universal mobile telecommunication systems (UMTS). CDMA offers attractive features, such as frequency reuse, soft handoff, increased capacity, and multipath combating. In a CDMA system, several users simultaneously transmit information over a common channel using pre-assigned codes. The conventional single user detector consists of a bank of filters matched to the spreading codes. This detector suffers from two problems. First, multiple access interference (MAI) produced by the other co-channel users is a significant limitation to the capacity of this detector. The second problem is the near-far effect which occurs when the relative received power of interfering signals becomes larger. A potential solution is multiuser detection which exploits the information of signals of interfering users. In the present study performance of various linear detectors like matched filter detector, MMSE detector, and adaptive LMS detector are studied. These are the linear detectors that operate linearly on the received signal statistics and are suboptimal detectors. The matched filter bank is the conventional detector and offers the simplest way of demodulating CDMA signals .The detector resulting from the MMSE (minimum mean square error) criterion shows better performance over the conventional one for low SNR value. Adaptive LMS is employed to enhance the BER performance in MUD application.Several factors motivated the research to apply neural network as multiuser detector. NN are nonlinear classifier in addition to being adaptive and computationally efficient. The performance of two layer perceptron neural network using BP learning rule is used for multiuser detection of CDMA signals in AWGN channels. The neural network detectors show improvement of BER in the comparative analysis done in the present work. and offers further research scope for solving multiuser detection problems in CDMA application.

Resumo-Este artigo apresenta a segunda parte do estudo unificado da probabilidade de erro de bits dos esquemas de modulação M-PAM, M-QAM e R-QAM sob desvanecimento generalizado. As expressões obtidas para a BEP apresentadas na Parte I são usadas juntamente com as expressões obtidas para a função geratriz de momentos da SNR por ramo do receptor para cinco distribuições de probabilidade usadas para modelar os desvanecimentos Rayleigh, Hoyt, Rice, Nakagami e Lognormal.

Resumo-Este artigo apresenta a segunda parte do estudo unificado da probabilidade de erro de bits dos esquemas de modulação M-PAM, M-QAM e R-QAM sob desvanecimento generalizado. As expressões obtidas para a BEP apresentadas na Parte I são usadas juntamente com as expressões obtidas para a função geratriz de momentos da SNR por ramo do receptor para cinco distribuições de probabilidade usadas para modelar os desvanecimentos Rayleigh, Hoyt, Rice, Nakagami e Lognormal.

In this paper, we study the performance of L-branch maximal-ratio combining (MRC) receivers operating over independent Weibull-fading channels. Our main result is a very tight approximation of the probability density function (PDF) of the signal-to-noise ratio (SNR) at the output of the combiner. Based on this result, accurate approximation of significant performance criteria, such as outage probability and average symbol error rate (ASER) are derived. We also evaluate the average bit error rate (ABER) for several coherent and non-coherent modulation schemes, using a closed-form expression for the moment-generating function (MGF) of the output SNR for MRC receivers. In addition, we derive some analytical expressions for channel capacity under various adaptation policies such as optimal rate adaptation (ORA), Optimal simultaneous power and rate adaptation (OPRA), Channel inversion with fixed rate (CIFR), and Truncated channel inversion with fixed rate (TCIFR). The proposed mathematical analysis is complemented by numerous numerical results, which point out the effects of fading severity on the overall system performance. Computer simulations are also performed to verify the validity and the accuracy of the proposed theoretical approach.

In this paper, the performance analysis based on PDF approach of an L-branch equal gain combiner (EGC) over independent and not necessarily identical Weibull fading channels is presented. Several closed-form approximate expressions are derived in terms of only one Fox H-function as PDF, cumulative distribution function, and moments of the EGC output Signal-to-noise ratio (SNR), outage probability, amount of fading, channel capacity, and the average symbol error rate for various digital modulation schemes. All results are illustrated and verified by simulations using computer algebra systems.

Este artigo discute aspectos de implementação computacional e desempenho da metodologia Bayesiana Filtro de Partículas. Esta técnica de estimação é diretamente aplicada à estimativa dos coeficientes de canal em um sistema DS/CDMA. Os resultados da simulação para um canal de propagação Rayleigh sem linha de visada indicaram que o estimador Bootstrap PF é capaz de fornecer RMSE no intervalo de [10-3 ; 10-2 ] para uma grande faixa de nível de interferência de múltiplo acesso (MAI) e relação sinalruído (SNR), e ainda ser capaz de oferecer robustez para o efeito near-far. Palavras-chave: Estimação de parâmetros de canal. Estimador Bayesiano. Filtro de partículas. DS/ CDMA.

Abstract: In this paper we describe the design and Field Programmable Gate Array (FPGA) implementation of a baseband spread spectrum communication system using pseudo-chaotic sequences (PCS) for spreading digital data. The sequence generator and the DS-...

This paper focuses on a low-cost synchronization technique based on power criteria for impulsive ultra wideband scheme applied to power line communication systems. The advantage offered by this technique is that the knowledge of both channel pulse response and noise information are not required. To assess the proposed technique, we discuss performance results obtained with typical in-home, outdoor, and low-voltage power line channels covering the frequency band between 1.7 and 50 MHz and additive noises modeled as white Gaussian and impulsive Gaussian. The results show that the proposed technique achieves the best performance in terms of detection probability, false alarm probability, and bit error rate in comparison with previous synchronization techniques, which were design to work with the aforementioned data communication system. Besides, implementation analysis on field programmable gate array shows that the proposed technique demands less hardware resource utilization than previous techniques and its finite word length implementation with at least 16 bits word length results in negligible performance degradation.

Resumo Este artigo mostra a implementacao do algoritmo BCJR para utilizacao em codigos de bloco lineares, com e sem o uso da decodificacao iterativa. Sao feitas simulacoes computacionais, e geradas curvas de desempenho, relacionando valores das probabilidadesde erro com os de relacao sinalruido.

In this work, the performance analysis of Bit Error rate (BER) Vs Ratio of Bit Energy to Noise Power Spectral Density (Eb/No) of simulink based DS-SS system is presented using the BERTool module. We have simulated the DS-SS system simulink model with 16-QAM, 32-QAM 64-QAM modulation techniques and from the simulation results it is found that bit error rate is increasing as the value of M increases i.e bit rerror rate of 64-QAM is higher than the bit error rate of 32-QAM. Similarly bit error rate of 32-QAM is higher than the bit error rate of 16-QAM. Also DS-SS system with M-QAM modulation using different spreading sequences (PN, Gold, Kasami) is simulated for BER analysis and found that DS-SS system showed better BER performance with 32-QAM and 64-QAM modulation technique using Gold sequence and Kasami sequence respectively. From the simulation results, it is concluded that by choosing the type of modulation technique and spreading sequence can enhance the performance of DS-SS system.

Resumo-Este trabalho analisa o desempenho do algoritmo heurístico de busca local (LS-Local Search) baseado na distância Euclidiana mínima (1-opt LS) aliado ao detector de Viterbi, quando aplicados ao problema da detecção multiusuário (MuD-Multiuser Detection) em sistemas DS/CDMA (Direct Sequence/Code Division Multiple Access) codificados, sujeitos a canais com desvanecimento Rayleigh plano. A eficiência da estratégia propostaé analisada considerando-se o compromisso desempenho versus acréscimo na complexidade computacional e requisito de memória, quando comparadoà estratégia de detecção baseada somente no detector de Viterbi. Por fim, apresenta-se este acréscimo nos requisitos de memória e de processamento para se alcançar o desempenho, em termos de taxa de erro de bit (BER-Bit Error Rate), do decodificador de máxima verossimilhança (ML-Maximum Likelihood).

In wireless communication system spread spectrum modulation plays a vital role. Therefore generation of spreading sequence is one of the crucial issues in such type of modulation technique. For secure communication, chaotic spreading sequence is advantageous. This paper presents a design of orthogonal chaotic spreading sequence for application in Direct Sequence Spread Spectrum (DS SS) system for a faded wireless channel. Enhancing the security of data transmission is a prime issue which can better be addressed with a chaotic sequence. Generation and application of orthogonal chaotic sequence is done and a comparison with non orthogonal chaotic sequence is presented. The comparison factors are bit error rate (BER), mutual information, computational time and signal power, which finally dictates the efficiency of generated code. Again, a novel multiplier-less chaotic sequence generator is proposed for lower power requirement than the existing one.

This paper illustrates the effectiveness of the Mellin-transform (MT) method for the exact calculation of certain one-dimensional definite integrals occurring in studies of wireless communications systems. The MT method, though it often requires laborious calculations, is a straightforward and extremely powerful technique for the closed-form evaluation of definite integrals. Only basic information is provided with regard to the MT and the closely related class of Mellin-Barnes (MB) integrals. On this basis, the MT method is applied to several examples of integrals occurring in studies of fading channels and wireless transmission systems. Closed-form expressions are obtained, which are either new or provided without proof in the literature. The new formulas are suitable for numerical evaluation.

The tracking performance of a transmit antenna weight adaptation algorithm applied in a fading channel is considered. The system uses a stochastic gradient algorithm incorporating gradient sign feedback from a receiving unit to adjust the transmit weights. The feedback is simply a bit indicating which of two normalized perturbed transmission weights delivers greater power to the receiver. A reasonable performance measure is defined and an analytic estimate of the performance in an AR1 vector fading channel is derived and compared to simulations

Neste trabalho são discutidos conceitos fundamentais relativo à comunicação de múltiplo acesso por divisão de código (CDMA). Especificamente, analisa-se o impacto da seleção de sequências de espalhamento sobre o desempenho e capacidade, em termos de número máximo de usuários suportados, em sistemas CDMA por sequência direta (DS/CDMA), considerando ambientes com ruído branco Gaussiano, carregamento de sistema variável e múltiplas antenas (MISO-CDMA).

Resumo-Neste artigo apresentamos uma medida de complexidade de decodificação de códigos convolucionais baseada em ciclos de máquina consumidos pela execução das operações aritméticas do algoritmo de Viterbi (VA) operando com decisão abrupta. Através da simulação destas operações definimos uma complexidade computacional do VA que mede de forma mais adequada a complexidade de treliça em esquemas de decodificação implementados por software. Implementações baseadas nos módulos de treliça convencional e mínimo são consideradas. Uma relação entre a complexidade definida neste trabalho e a complexidade de treliça definida por McEliece e Lin é investigada. A arquitetura adotada é a família de processadores digitais de sinais TMS320C55xx da Texas Instruments. Palavras-Chave-Códigos convolucionais, complexidade de decodificação, modulo de treliça, algoritmo de Viterbi.

Resumo-Este artigo apresenta estudo comparativo entre classes de entrelac ¸adores, aplicados a pares de códigos convolucionais não necessariamente unicamente decodificáveis sobre o canal aditivo para dois usuários binários (2-BAC), em presenc ¸a de ruído aditivo gaussiano branco (RAGB). São apresentadas curvas de desempenho com resultados de simulac ¸ões computacionais com codificadores convolucionais para cada usuário na estrutura turbo, usando diferentes tipos de entrelac ¸adores. Verifica-se nos casos estudados para dois usuários que, para probabilidades de erro de bit da ordem de P b = 10 -4 , entrelac ¸adores do tipo DRP, S-aleatório e polinomial apresentaram um ganho de aproximadamente 1, 0dB na relac ¸ão sinal ruído em relac ¸ão ao entrelac ¸ador retangular de Berrou e Glavieux. Palavras-Chave-Códigos convolucionais, canal aditivo com dois usuários binários, códigos turbo, entrelac ¸adores, decodificac ¸ão iterativa.

Resumo-Apresentamos códigos LDPC otimizados para canais com desvanecimento Nakagami-m. Através do algoritmo Density Evolution associado ao algoritmo de busca Differential Evolution, foram encontrados códigos LDPC para o canal em questão. Simulac ¸ões computacionais mostram que os novos códigos encontrados possuem desempenho superior a outros códigos construídos para outros modelos de desvanecimento, quando avaliados sob o efeito de desvanecimento Nakagami-m.

Resumo-Técnicas de criptofonia são utilizadas para transformar sinais de voz em sinais ininteligíveis. Para sistemas de comunicações móveis que empregam Codificadores/Decodificadores (CODECs) comerciais, a encriptação digital nãoé uma opção adequada por não ser viável antes dos mesmos ou por requerer modificações de hardware e software. Se a encriptação for aplicada antes do CODEC, isto poderá resultar em uma baixa qualidade de voz, i.e., o CODEC tentaria codificar o sinal cifrado como se fosse sinal de voz. Alternativamente, alguns cifradores analógicos podem ser empregados sem causar grandes alterações no desempenho de codificação. Este trabalho investiga a degradação causada em sistemas de criptofonia analógicos no domínio da freqüência pelos efeitos de multipercursos de canais rádio presentes em telefonia celular. São apresentados resultados objetivos de qualidade pela metodologia PESQ (Perceptual Evaluation of Speech Quality) para sinais criptofonados e aplicados ao CODEC.

Primeiramente, a DEUS pela saúde, perseverança e iluminação, inteligência suprema que sempre me ajudou e ensinou. Aos meus pais, Josefa Farias e Jessé Pedro, e ao meu irmão Jessé Júnior, pelo amor, pelo carinho e pelo apoio que sempre me dedicaram, que me ajudaram a terminar mais essa etapa. A minha irmã, em especial, porque além de ser minha irmã foi também amiga e companheira durante todo o mestrado, me acompanhou nas horas difíceis e me ajudou sempre que podia, compartilhando seus conhecimentos científicos. Ao meu esposo e amigo Ageu Cordeiro, pelo carinho, pelo apoio, pela ajuda e dedicação exclusiva, pela paciência que teve comigo em todos os momentos difíceis, por compartilhar toda a minha ansiedade durante a realização deste trabalho. Ao professor Humberto César, pelo apoio, pela orientação e por todos os conselhos que me deu durante todo o desenvolvimento desse trabalho. Ao professor Luiz Guedes Caldeira, que tive o prazer de ter como colaborador em todo o desenvolvimento deste trabalho, pela dedicação, pelas cobranças e por ter me presenteado com sua amizade e confiança, sempre se mostrando disponível e paciente nos momentos de necessidade. Aos professores e amigos do PPGEEC-UFRN e do IFPB que contribuíram para o sucesso desta dissertação. Aos meus amigos Roberto Rannieri, Bruna Alice, Adelson, Jannayna, Náthelee e Luciano, pela sincera amizade que começou no mestrado e que durará por toda a vida. Aos grandes amigos do grupo TECFOTON, Leonardo Caetano, Hugo Michel, João Kleber, Marinaldo Sousa e George Dennes, que tive o prazer de compartilhar o dia a dia na UFRN. À Capes e ao CNPQ pelo suporte financeiro.

The interference reduction capability of cell sectoring and power control algorithms have been considered separately as means to decrease the interference in Code Division Multiple Access (CDMA) cellular systems. In this paper, we present Switched-Beam (SB) and Rotatable Equal Sectoring (RES) techniques for CDMA cellular systems in a 2D urban environment. In the SB technique by using a number of fixed, independent, or directional antennas we increase the downlink capacity of the CDMA systems. Also in the RES method, the equal sectors of the base stations are rotating together to decrease the inter-cell and intra-cell interferences. Also in this paper we use centralized power control to overcome the near-far problem. Simulation results indicate that the proposed techniques considerably increase the capacity of the CDMA cellular systems compared to ordinary Equal Sectoring (ES) method.

Resumo-Múltiplo Acesso por Códigos Esparsos (SCMA)é um poderoso candidato para as futuras gerações de comunicações sem fio. Diversos algoritmos de decodificação foram propostos para o SCMA. O conhecido Message-Passing Algorithm (MPA) apresenta um bom desempenho, mas com complexidade exponencial. Propomos uma modificação no Log-MPA convencional para reduzir a complexidade da decodificação. Uma estrutura deárvoreé associadaà equação de decodificação e, evitandose adequadamente símbolos pouco confiáveis,é obtida umá arvore podada, levando a uma menor complexidade, com um ajuste arbitrário sobre o compromisso entre desempenho e complexidade da detecção conjunta.

In this work, a new family of spatiotemporal chaotic codes is proposed as an alternative to the Gold codes conventionally used in asynchronous DS/CDMA systems. In addition to their synchronization advantage over the temporal-only chaotic codes, these new codes are shown to have improved performance compared to the Gold codes. The performance criteria used in this study are Multiple Access Interference (MAI), Signal to Noise Ratio (SNR) and Bit Error Probability.

In this work, the performance analysis of Bit Error rate (BER) Vs Ratio of Bit Energy to Noise Power Spectral Density (Eb/No) of simulink based DS-SS system is presented using the BERTool module. We have simulated the DS-SS system simulink model with 16-QAM, 32-QAM 64-QAM modulation techniques and from the simulation results it is found that bit error rate is increasing as the value of M increases i.e bit rerror rate of 64QAM is higher than the bit error rate of 32-QAM. Similarly bit error rate of 32-QAM is higher than the bit error rate of 16-QAM. Also DS-SS system with M-QAM modulation using different spreading sequences (PN, Gold, Kasami) is simulated for BER analysis and found that DS-SS system showed better BER performance with 32QAM and 64-QAM modulation technique using Gold sequence and Kasami sequence respectively. From the simulation results, it is concluded that by choosing the type of modulation technique and spreading sequence can enhance the performance of DS-SS system....

Set of binary sequences with good pair wise cross-correlation finds application in code division multiple access (CDMA) system. Normally in these schemes, since crosscorrelation has a minimum value but not zero, if the number of simultaneous users increases at a time, there is degradation in the quality of the recovered signal. Chaotic sequences are highly uncorrelated even when their initial values differ by a small value and hence can be used as address sequences in spread spectrum communication. In chaotic spread spectrum systems, different user may be assigned different sequences generated with different initial conditions. Chaotic functions exhibit cryptographically desirable qualities for secured data communication also. From implementation point of view binary sequences are desirable. This investigation deals with generation of chaotic sequences and deriving discrete sequences over Z4 and binary sequences derived from sequences over Z4, using binary conversion and three polyn...