Adaptive WHT Aided QAM for Fading Channels Subjected to Impulsive Noise

The Bit Error Rate (BER) expressions of Walsh-Hadamard Transform (WHT) aided Quadrature Amplitude Modulation (QAM) transmitted in impulsive noise environments are derived. It is found that there is a crossover point between conventional and WHT-aided schemes’ BER curves, above which WHT is beneficial in terms of the corresponding BER performance. The exact crossover SNR is analysed and its approximate closed-form expression is derived for a QPSK scheme based on the Chernoff bound. An adaptive WHT scheme is proposed for QAM systems, where the transmitter activates the WHT only when the instantaneous SNR at the receiver is higher than the crossover SNR. The theoretical BER of our adaptive WHT scheme is derived for QAM systems operating over Nakagami-m fading channels. It is found that our adaptive WHT scheme is effective in terms of mitigating the detrimental effects of impulsive noise over the entire SNR range

Sistemas MIMO auxiliados por grandes superfícies refletoras como novidade para tecnologias 6G

Orientador: Gustavo FraidenraichDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: Large Intelligent Surfaces (LIS) é uma tecnologia promissora para a sexta geração (6G) de comunicações móveis devido ao seu potencial para melhorar a relação sinal-ruído (SNR), aumentar a eficiência espectral e ainda possibilitar a redução do consumo de energia no estação rádio base (BS) durante a transmissão. O LIS é um painel formado por células que podem refletir ondas eletromagnéticas para fazer beamforming e remover a fase do canal, a superfície é formada por metamateriais que podem alterar a fase das ondas incidentes com um ângulo quantizado que pode ser controlado digitalmente por software permitindo que o sinal resultante da soma de todas as componentes refletidas pelo LIS possua uma fase adaptada para cancelar o efeito da fase do canal. Esta fase é estimada por algoritmos de aprendizado de máquina e quanto mais eficiente a estimativa melhor será o processo de ajuste de fase, mas devido às não idealidades do sistema, temos um erro de fase residual que neste trabalho é modelado pela distribuição de Von Mises. Dividimos nosso estudo em dois capítulos, o primeiro referindo-se a sistemas com apenas uma antena na BS, considerando a presença de uma linha direta de propagação de ondas eletromagnéticas a.k.a. line of sight (LoS) com desvanecimento Nakagami-m e nós ignoramos a possibilidade de um link direto com o usuário, já na segunda parte consideramos um arranjo de antenas na estação base e incluindo um link direto entre o usuário e a BS, mas negligenciando a LoS ao considerar canais com desvanecimento Rayleigh. Para o cenário da BS de uma antena, derivamos a probabilidade de erro de bit exata considerando modulação M-QAM e BPSK quando o número de elementos do LIS, n , é igual a 2 e 3 considerando que os coeficientes de desvanecimento do canal são Nakagami-m e o LIS tem um erro de fase com distribuição de Von Mises. Além disso, com base no teorema do limite central, e considerando um grande número de elementos refletores, apresentamos uma aproximação precisa e limites superiores para a taxa de erro de bit. Por meio de várias simulações de Monte Carlo, demonstramos que todas as expressões derivadas correspondem perfeitamente aos resultados simulados. No cenário de matriz de antenas, consideramos o Rayleigh flat fading para cada subcanal entre a BS, o LIS e o usuário e aplicamos um precoder na estação base para ter a transmissão de razão máxima (MRT). Com base no teorema do limite central (CLT), concluímos que o canal total tem um desvanecimento Gamma equivalente cujos parâmetros são derivados dos momentos estatísticos do canal entre o arranjo de antenas e LIS, e também do LIS para o usuário. Assumindo que o canal equivalente pode ser modelado como uma distribuição Gama, propomos expressões de forma fechada muito precisas para a probabilidade de erro de bit e um limite superior muito restrito. Para o caso em que o LIS não é capaz de realizar o cancelamento de fase perfeito, ou seja, sob erros de fase, é possível analisar o desempenho do sistema considerando as aproximações analíticas e os resultados simulados obtidos pelo método de Monte Carlo. As expressões analíticas para os parâmetros da distribuição Gama são muito difíceis de serem obtidas devido à complexidade das transformações não lineares de variáveis aleatórias com média diferente de zero e termos correlatos. Mesmo com o cancelamento de fase perfeito, todos os coeficientes de desvanecimento são complexos devido à ligação entre o usuário e a estação base que não é negligenciada neste estudoAbstract: Large intelligent surfaces (LIS) is a promising technology for the sixth generation (6G) of mobile communications due to its potential to improve the signal to noise ratio (SNR), increase spectral efficiency, and even make it possible to reduce energy consumption in the radio base station (BS) during transmission. The LIS is a panel formed by cells that can reflect electromagnetic waves to make beamforming and cancel the channel phase, the surface is formed by metamaterials that can change the phase of the incident waves with a quantized angle that can be controlled digitally by software allowing that the signal resulting from the sum of all components reflected by the LIS has a phase adapted to nullify the effect of the channel phase. The channel information is estimated by machine learning algorithms and more efficient phase estimations implies better phase adjustments at the LIS, but due to the system's non-idealities, we have a residual phase error that in this work is modeled by the Von Mises distribution. We divided our study into two chapters, the first referring to systems with a single antenna at the BS considering the existence of a straight and unobstructed line for electromagnetic wave propagation a.k.a. line of sight (LoS) with Nakagami-m fading and we ignore the possibility of a direct link between the user, in the second part we consider an antenna array at the base station and including a direct link between the user and the BS but neglecting the LoS by considering Rayleigh fading channels. For the single antenna BS scenario, we derive the exact bit error probability considering quadrature amplitude modulation (M-QAM) and binary phase-shift keying (BPSK) when the number of LIS elements, n, is equal to 2 and 3 considering that the channel fading coefficients are Nakagami-m. Also, based on the central limit theorem (CLT), and considering a large number of reflecting elements, we present an accurate approximation and upper bounds for the bit error rate. Through several Monte Carlo simulations, we demonstrate that all derived expressions perfectly match the simulated results. In the antenna array scenario, we consider Rayleigh flat fading for each subchannel between the BS, the LIS, and the user and we apply a precoder at the base station to have the maximum ratio transmission (MRT). Based on the CLT, we conclude that the overall channel has an equivalent Gamma fading whose parameters are derived from the moments of the channel fading between the antenna array and LIS, and also from the LIS to the single user. Assuming that the equivalent channel can be modeled as a Gamma distribution, we propose very accurate closed-form expressions for the bit error probability and a very tight upper bound. For the case where the LIS is not able to perform perfect phase cancellation, that is, under phase errors, it is possible to analyze the system performance considering the analytical approximations and the simulated results obtained using the well known Monte Carlo method. The analytical expressions for the parameters of the Gamma distribution are very difficult to be obtained due to the complexity of the nonlinear transformations of random variables with non-zero mean and correlated terms. Even with perfect phase cancellation, all the fading coefficients are complex due to the link between the user and the base station that is not neglected in this studyMestradoTelecomunicações e TelemáticaMestre em Engenharia Elétrica88882.329402/2019-01CAPE

Uncoded space-time labelling diversity : data rate & reliability enhancements and application to real-world satellite broadcasting.

Doctoral Degree. University of KwaZulu-Natal, Durban.Abstract available in PDF

Adaptive transmission techniques in wireless fading channels

Master'sMASTER OF ENGINEERIN

Approximations for Performance Analysis in Wireless Communications and Applications to Reconfigurable Intelligent Surfaces

In the last few decades, the field of wireless communications has witnessed significant technological advancements to meet the needs of today’s modern world. The rapidly emerging technologies, however, are becoming increasingly sophisticated, and the process of investigating their performance and assessing their applicability in the real world is becoming more challenging. That has aroused a relatively wide range of solutions in the literature to study the performance of the different communication systems or even draw new results that were difficult to obtain. These solutions include field measurements, computer simulations, and theoretical solutions such as alternative representations, approximations, or bounds of classic functions that commonly appear in performance analyses. Field measurements and computer simulations have significantly improved performance evaluation in communication theory. However, more advanced theoretical solutions can be further developed in order to avoid using the ex- pensive and time-consuming wireless communications measurements, replace the numerical simulations, which can sometimes be unreliable and suffer from failures in numerical evaluation, and achieve analytically simpler results with much higher accuracy levels than the existing theoretical ones. To this end, this thesis firstly focuses on developing new approximations and bounds using unified approaches and algorithms that can efficiently and accurately guide researchers through the design of their adopted wireless systems and facilitate the conducted performance analyses in the various communication systems. Two performance measures are of primary interest in this study, namely the average error probability and the ergodic capacity, due to their valuable role in conducting a better understanding of the systems’ behavior and thus enabling systems engineers to quickly detect and resolve design issues that might arise. In particular, several parametric expressions of different analytical forms are developed to approximate or bound the Gaussian Q-function, which occurs in the error probability analysis. Additionally, any generic function of the Q-function is approximated or bounded using a tractable exponential expression. Moreover, a unified logarithmic expression is proposed to approximate or bound the capacity integrals that occur in the capacity analysis. A novel systematic methodology and a modified version of the classical Remez algorithm are developed to acquire optimal coefficients for the accompanying parametric approximation or bound in the minimax sense. Furthermore, the quasi-Newton algorithm is implemented to acquire optimal coefficients in terms of the total error. The average symbol error probability and ergodic capacity are evaluated for various applications using the developed tools. Secondly, this thesis analyzes a couple of communication systems assisted with reconfigurable intelligent surfaces (RISs). RIS has been gaining significant attention lately due to its ability to control propagation environments. In particular, two communication systems are considered; one with a single RIS and correlated Rayleigh fading channels, and the other with multiple RISs and non-identical generic fading channels. Both systems are analyzed in terms of outage probability, average symbol error probability, and ergodic capacity, which are derived using the proposed tools. These performance measures reveal that better performance is achieved when assisting the communication system with RISs, increasing the number of reflecting elements equipped on the RISs, or locating the RISs nearer to either communication node. In conclusion, the developed approximations and bounds, together with the optimized coefficients, provide more efficient tools than those available in the literature, with richer capabilities reflected by the more robust closed-form performance analysis, significant increase in accuracy levels, and considerable reduction in analytical complexity which in turns can offer more understanding into the systems’ behavior and the effect of the different parameters on their performance. Therefore, they are expected to lay the groundwork for the investigation of the latest communication technologies, such as RIS technology, whose performance has been studied for some system models in this thesis using the developed tools

Fifty Years of Noise Modeling and Mitigation in Power-Line Communications.

Building on the ubiquity of electric power infrastructure, power line communications (PLC) has been successfully used in diverse application scenarios, including the smart grid and in-home broadband communications systems as well as industrial and home automation. However, the power line channel exhibits deleterious properties, one of which is its hostile noise environment. This article aims for providing a review of noise modeling and mitigation techniques in PLC. Specifically, a comprehensive review of representative noise models developed over the past fifty years is presented, including both the empirical models based on measurement campaigns and simplified mathematical models. Following this, we provide an extensive survey of the suite of noise mitigation schemes, categorizing them into mitigation at the transmitter as well as parametric and non-parametric techniques employed at the receiver. Furthermore, since the accuracy of channel estimation in PLC is affected by noise, we review the literature of joint noise mitigation and channel estimation solutions. Finally, a number of directions are outlined for future research on both noise modeling and mitigation in PLC