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[pt] Uma abordagem promissora para reduzir o consumo de energia é considerar a quantização grosseira no receptor. Neste estudo, investigamos novas técnicas de pré-codificação no espaço e no tempo para canais de downlink MIMO multiusuário limitados em banda com quantização de 1 bit e sobre amostragem no receptor, considerando a modulação de cruzamento zero. A modulação de instância de tempo de zero cruzamento proposta transmite a informação nas instâncias de tempo de cruzamento zero. Dois critérios de projeto para a instância de tempo de cruzamento zero são investigados, a saber, a distância mínima até o limiar de decisão e o erro quadrático médio entre o sinal recebido e o desejado. A maximização da distância mínima para o limiar de decisão pode ser formulada como um programa quadrático restrito quadraticamente. Como alternativa, um problema equivalente pode ser formulado com base na minimização de potência, o que reduz a complexidade computacional. Além disso, outro método é implementado onde a informação é transmitida nas instâncias de tempo de cruzamento zero em segmentos de forma de onda. Partindo da técnica convencional baseada no erro quadrático médio, um algoritmo mais sofisticado é desenvolvido, o que implica a extensão ativa da constelação para melhorar o desempenho em altaSNR. O problema estendido é resolvido com duas abordagens: formulando o problema como um programa de cone de segunda ordem e considerando um algoritmo de otimização alternada. Outro método baseado no algoritmo de descida de gradiente é implementado com a técnica do erro quadrático médio para reduzir ainda mais a complexidade computacional. Além disso, um limite inferior para a eficiência espectral é obtido. Os resultados numéricos mostram que os métodos de pré-codificação de cruzamento zero de instância de tempo propostos melhoram significativamente a taxa de erro de bit em comparação com os métodos de última geração. Finalmente, a maximização da distância mínima ao limiar de decisão e as técnicas de pré-codificação baseadas no erro quadrático médio são avaliadas considerando um canal de onda milimétrica seletivo em frequência. Os resultados numéricos mostram que ambas as técnicas de pré-codificação respondem bem à seletividade de frequência do canal. / [en] A promising approach to reduce energy consumption is to consider coarse quantization at the receiver. In this study, we investigate novel precoding techniques in space and time for bandlimited multiuser MIMO downlink channels with 1-bit quantization and oversampling at the receiver, considering zero-crossing modulation. The proposed time-instance zero-crossing modulation conveys the information into the time-instances of zero-crossings. Two design criteria for time-instance zero-crossing modulation are investigated, namely, the minimum distance to the decision threshold and the mean-square error between the received and the desired signal. The maximization of the minimum distance to the decision threshold can be formulated as a quadratically constraint quadratic program. As an alternative, an equivalent problem is formulated based on power minimization, which reduces computational complexity. Moreover, another method is implemented where the information is conveyed into the time-instances of zero-crossings using waveform segments. Departing from the conventional mean-square error based technique, a more sophisticated algorithm is developed, which implies active constellation extension to improve the performance at high SNR. The extended problem is solved with two approaches: by formulating the problem as a second-order cone program and by considering an alternating optimization algorithm. Another method based on the gradient descent algorithm is implemented with the mean-square error technique to reduce computational complexity further. Besides, a lower bound on the spectral efficiency is obtained. Numerical results show that the proposed time-instance zero-crossing precoding methods significantly improve the bit error rate compared to the state-of-the-art methods. Finally, the maximization of the minimum distance to the decision threshold and the mean-square error based precoding techniques are evaluated considering a frequency-selective millimeter wave channel. Numerical results show that both precoding techniques respond well to the frequency selectivity of the channel.

With the increasing demand for faster communication systems, soon data rates in the terabit regime (100 Gbit/s and beyond) are required, which yields new challenges for the design of analog-to-digital converters (ADCs) since high bandwidths imply high sampling rates. For sampling rates larger than 300MHz, which we now achieve with 5G, the ADC power consumption per conversion step scales quadratically with the sampling rate. Thus, ADCs become a major energy consumption bottleneck. To circumvent this problem, we consider digital receivers based on 1-bit quantization and oversampling. We motivate this concept by a brief comparison of the energy efficiency of a recently proposed system employing 1-bit quantization and oversampling to the conventional approach using high resolution quantization and Nyquist rate sampling. Our numerical results show that the energy efficiency can be improved significantly by employing 1-bit quantization and oversampling at the receiver at the cost of increased bandwidth. The main part of this work is concerned with the synchronization of fully digital receivers using 1-bit quantization and oversampling. As a first step, we derive performance bounds for phase, timing, and frequency estimation in order to gain a deeper insight into the impact of 1-bit quantization and oversampling. We identify uniform phase and sample dithering as crucial to combat the non-linear behavior introduced by 1-bit quantization. This dithering can be implemented by sampling at an irrational intermediate frequency and with an oversampling factor with respect to the symbol rate that is irrational, respectively. Since oversampling results in noise correlation, a closed form expression of the likelihood function is not available. To enable an analytical treatment we thus study a system model with white noise by adapting the receive filter bandwidth to the sampling rate. Considering the aforementioned dithering, we obtain very tight closed form lower bounds on the Cramér-Rao lower bound (CRLB) in the large sample regime. We show that with uniform phase and sample dithering, all large sample properties of the CRLB of the unquantized receiver are preserved under 1-bit quantization, except for a signal-to-noise ratio (SNR) dependent performance loss that can be decreased by oversampling. For the more realistic colored noise case, we discuss a numerically computable upper bound of the CRLB and show that the properties of the CRLB for white noise still hold for colored noise except that the performance loss due to 1-bit quantization is reduced. Assuming a neglectable frequency offset, we use the least squares objective function to derive a typical digital matched filter receiver with a data-and timing-aided phase estimator and a timing estimator that is based on square time recovery. We show that both estimators are consistent under very general assumptions, e.g., arbitrary colored noise and stationary ergodic transmit symbols. Performance evaluations are done via simulations and are compared against the numerically computable upper bound of the CRLB. For low SNR the estimators perform well but for high SNR they converge to an error floor. The performance loss of the phase estimator due to decision-directed operation or estimated timing information is marginal. In summary, we have derived practical solutions for the design of fully digital receivers using 1-bit quantization and oversampling and presented a mathematical analysis of the proposed receiver structure. This is an important step towards enabling energy efficient future wireless communication systems with data rates of 100 Gbit/s and beyond.

[pt] Sistemas de múltiplas antenas de larga escala são técnicas fundamentais para sistemas de comunicação sem fio do futuro, que deverão servir dezenas de usuários por estação rádio-base. Neste contexto, um problema chave é o aumento do consumo de energia à medida que o número de antenas cresce. Recentemente, CADs de baixa resolução têm atraído grande interesse de pesquisa. Em particular, CADs de 1 bit são adequados para sistemas de larga escala devido ao seu baixo custo e consumo de energia. Nesta tese, CADs de 1 bit são usados em três diferentes abordagens de projeto, que operam a taxa de Nyquist e a taxas superiores a taxa de Nyquist com estratégias de amostragem uniforme e dinâmica. Nos sistemas operando a taxa de Nyquist, algoritmos de estimação de canal que exploram o conhecimento da baixa resolução e um novo esquema de detecção e decodificação iterativas são propostos, em que códigos low-density paritycheck de bloco curto são considerados para evitar alta latência. Nos sistemas operando a taxas superiores a taxa de Nyquist com sobreamostragem uniforme, algoritmos eficientes de estimação de canal e de detecção com janela deslizante com exploração da baixa resolução são propostos. Além disso, são deduzidas expressões analíticas associadas aos limitantes de Cramér-Rao para os sistemas com sobreamostragem. Resultados numéricos ilustram o desempenho dos algoritmos de estimação de canal propostos e existentes e os limitantes teóricos deduzidos. Nos sistemas operando com sobreamostragem dinâmica, duas abordagens de projeto são desenvolvidas: uma técnica baseada na maximização da soma das taxas e uma técnica baseada na minimização do erro médio quadrático. Em seguida, três algoritmos de redução de dimensão são apresentados e investigados. Resultados de simulações mostram que os sistemas com sobreamostragem dinâmica têm melhor desempenho do que os sistemas com sobreamostragem uniforme em termos de soma das taxas alcançáveis e de taxa de erro de símbolos, enquanto o custo computacional das técnicas examinadas é comparável. / [en] Large-scale multiple-antenna systems are a key technique for future wireless communications, which will serve tens of users per base station (BS). In this scenario, one problem faced is the large energy consumption as the number of receive antennas scales up. Recently, low-resolution analogto-digital converters (ADCs) have attracted much attention. Specifically, 1-bit ADCs in the front-end are suitable for such systems due to their low cost and low energy consumption. In this thesis, 1-bit ADCs are applied in three different system designs, which operate at the Nyquist rate and faster than Nyquist rates along with uniform and dynamic strategies. In the Nyquist-sampling system, low-resolution-aware channel estimation algorithms and a novel iterative detection and decoding scheme are proposed, where short block length low-density parity-check codes are considered for avoiding high latency. In the faster than Nyquist rates with uniform oversampling system, lowresolution-aware channel estimation and sliding window based detection algorithms are proposed due to their low computational cost and high detection accuracy. Particularly, analytical expressions associated with the Bayesian Cramér-Rao bounds for the oversampled systems are presented. Numerical results are provided to illustrate the performance of the proposed channel estimation algorithms and the derived theoretical bounds. In the dynamic-oversampling system, two different system designs are devised, namely, sum rate and mean square error based. Three different dimension reduction algorithms are presented and thoroughly investigated. Simulation results show that the systems with the proposed dynamic oversampling outperform the uniformly oversampled system in terms of the computational cost, achievable sum rate and symbol error rate performance.

[pt] Eficiência energética e espectral são características importantes para comunicações militares e internet das coisas (IoT). Nesta tese, métodos e sistemas de quantização de 1-bit com modulação de fase contínua (CPM) são estudados e propostos para resolver as necessidades de sistemas de comunicações modernos com baixo consumo energético. Nesse contexto, o método de superamostragem em relação a duração de um símbolo é promissor, pois a informação está contida ao longo da transição de fase de sinais CPM, que não são estritamente limitados em banda. Consequentemente, a perda de taxa alcançável causada pela quantização de 1-bit pode ser reduzida consideravelmente, até mesmo para esquemas com maior ordem de modulação. Este estudo investiga diferentes abordagens para melhorar o desempenho do modelo de sistema proposto. Um esquema de codificação de canal é projetado com mapeamento de bits adaptado ao problema de quantização grosseira, fazendo uso de um soft-in soft-out (SISO) turbo receiver. Formas de onda CPM com duração de símbolo significamente menor que o inverso da banda do sinal são propostas, nomeadas de faster-than-Nyquist CPM. Um fator maior de superamostragem é aplicado com uma estratégia de seleção de amostras em um modelo de amostragem adaptativa. Finalmente, resultados numéricos confirmam melhor desempenho em taxa de erro de bit, eficiência espectral e taxa alcançável para os métodos propostos, em comparação às técnicas recentemente utilizadas. / [en] Energy and spectral efficiency are appealing features for military communications and internet of things (IoT). On this thesis, systems and schemes with 1-bit quantization and continuous phase modulation (CPM) are studied and proposed to address the needs for modern and power efficient communications. In this context, oversampling with respect to the symbol duration is promising because the information is conveyed in the phase transitions of the CPM signals, which are not strictly bandlimited. With this, the loss in achievable rate caused by the coarse quantization can be greatly reduced, even for higher order modulation schemes. This study investigates different approaches to enhancing the performance of the proposed system model. A channel coding scheme is designed with a tailored bit mapping, by means of employing a soft-in soft-out (SISO) turbo receiver. CPM waveforms with symbol durations significantly shorter than the inverse of the signal bandwidth are proposed, termed faster-than-Nyquist CPM. Higher oversampling is applied with a sample selection strategy for a nonuniform adaptive oversampling model. Finally, numerical results confirm better performance on bit error rate, spectral efficiency and achievable rate for the proposed methods in comparison with state of the art techniques.

<p dir="ltr"> The widespread adoption of Convolutional Neural Networks (CNNs) in real-world applications, particularly on resource-constrained devices, is hindered by their computational complexity and memory requirements. This research investigates the application of pruning and quantization techniques to optimize CNNs for arrhythmia classification using the MIT-BIH Arrhythmia Database. By combining magnitude-based pruning, regularization-based pruning, filter map-based pruning, and quantization at different bit-widths (4-bit, 8-bit, 2-bit, and 1-bit), the study aims to develop a more compact and efficient CNN model while maintaining high accuracy. The experimental results demonstrate that these techniques effectively reduce model size, improve inference speed, and maintain accuracy, adapting them for use on devices with limited resources. The findings highlight the potential of these optimization techniques for real-time applications in mobile health monitoring and edge computing, paving the way for broader adoption of deep learning in resource-limited environments.</p>

Given a geometric graph G = (V,E), where V is the set of vertices and E is the set of edges and a source-target pair {s,t} is a subset of V, a local geometric routing algorithm seeks a route from s to t using only local neighborhood relationships. This thesis proposes a local geometric routing algorithm that uses only a single state bit as message overhead and guarantees delivery of messages in three different classes of edge-augmented planar graphs: convex subdivisions, quasi planar convex subdivisions (allow some augmented edges on a spanning convex subdivision) and 2-augmented triangulations (allow some augmented edges on a spanning triangulation). The proposed algorithm is origin oblivious (does not require the knowledge of the origin vertex s) and predecessor oblivious (does not require the knowledge of the predecessor vertex).

Given a geometric graph G = (V,E), where V is the set of vertices and E is the set of edges and a source-target pair {s,t} is a subset of V, a local geometric routing algorithm seeks a route from s to t using only local neighborhood relationships. This thesis proposes a local geometric routing algorithm that uses only a single state bit as message overhead and guarantees delivery of messages in three different classes of edge-augmented planar graphs: convex subdivisions, quasi planar convex subdivisions (allow some augmented edges on a spanning convex subdivision) and 2-augmented triangulations (allow some augmented edges on a spanning triangulation). The proposed algorithm is origin oblivious (does not require the knowledge of the origin vertex s) and predecessor oblivious (does not require the knowledge of the predecessor vertex).

Sustainably realizing applications of the future with high performance demands requires that energy efficiency becomes a central design criterion for the entire system. For example, the power consumption of the analog-to-digital converter (ADC) can become a major factor when transmitting at large bandwidths and carrier frequencies, e.g., for ultra-short range high data rate communication. The consumed energy per conversion step increases with the sampling rate such that high resolution ADCs become unfeasible in the sub-THz regime at the very high sampling rates required. This makes signaling schemes adapted to 1-bit quantizers a promising alternative. We therefore quantify the performance of bandlimited 1-bit quantized wireless communication channels using techniques like oversampling and faster-than-Nyquist (FTN) signaling to compensate for the loss of achievable rate. As a limiting case, we provide bounds on the mutual information rate of the hard bandlimited 1-bit quantized continuous-time – i.e., infinitely oversampled – additive white Gaussian noise channel in the mid-to-high signal-to-noise ratio (SNR) regime. We derive analytic expressions using runlength encoded input signals. For real signals the maximum value of the lower bound on the spectral efficiency in the high-SNR limit was found to be approximately 1.63 bit/s/Hz. Since in practical scenarios the oversampling ratio remains finite, we derive bounds on the achievable rate of the bandlimited oversampled discrete-time channel. These bounds match the results of the continuous-time channel remarkably well. We observe spectral efficiencies up to 1.53 bit/s/Hz in the high-SNR limit given hard bandlimitation. When excess bandwidth is tolerable, spectral efficiencies above 2 bit/s/Hz per domain are achievable w.r.t. the 95 %-power containment bandwidth. Applying the obtained bounds to a bandlimited oversampled 1-bit quantized multiple-input multiple-output channel, we show the benefits when using appropriate power allocation schemes. As a constant envelope modulation scheme, continuous phase modulation is considered in order to relieve linearity requirements on the power amplifier. Noise-free performance limits are investigated for phase shift keying (PSK) and continuous phase frequency shift keying (CPFSK) using higher-order modulation alphabets and intermediate frequencies. Adapted waveforms are designed that can be described as FTN-CPFSK. With the same spectral efficiency in the high-SNR limit as PSK and CPFSK, these waveforms provide a significantly improved bit error rate (BER) performance. The gain in SNR required for achieving a certain BER can be up to 20 dB. / Die nachhaltige Realisierung von zukünftigen Übertragungssystemen mit hohen Leistungsanforderungen erfordert, dass die Energieeffizienz zu einem zentralen Designkriterium für das gesamte System wird. Zum Beispiel kann die Leistungsaufnahme des Analog-Digital-Wandlers (ADC) zu einem wichtigen Faktor bei der Übertragung mit großen Bandbreiten und Trägerfrequenzen werden, z. B. für die Kommunikation mit hohen Datenraten über sehr kurze Entfernungen. Die verbrauchte Energie des ADCs steigt mit der Abtastrate, so dass hochauflösende ADCs im Sub-THz-Bereich bei den erforderlichen sehr hohen Abtastraten schwer einsetzbar sind. Dies macht Signalisierungsschemata, die an 1-Bit-Quantisierer angepasst sind, zu einer vielversprechenden Alternative. Wir quantifizieren daher die Leistungsfähigkeit von bandbegrenzten 1-Bit-quantisierten drahtlosen Kommunikationssystemen, wobei Techniken wie Oversampling und Faster-than-Nyquist (FTN) Signalisierung eingesetzt werden, um den durch Quantisierung verursachten Verlust der erreichbaren Rate auszugleichen. Wir geben Grenzen für die Transinformationsrate des Extremfalls eines strikt bandbegrenzten 1-Bit quantisierten zeitkontinuierlichen – d.h. unendlich überabgetasteten – Kanals mit additivem weißen Gauß’schen Rauschen bei mittlerem bis hohem Signal-Rausch-Verhältnis (SNR) an. Wir leiten analytische Ausdrücke basierend auf lauflängencodierten Eingangssignalen ab. Für reelle Signale ist der maximale Wert der unteren Grenze der spektralen Effizienz im Hoch-SNR-Bereich etwa 1,63 Bit/s/Hz. Da die Überabtastrate in praktischen Szenarien endlich bleibt, geben wir Grenzen für die erreichbare Rate eines bandbegrenzten, überabgetasteten zeitdiskreten Kanals an. Diese Grenzen stimmen mit den Ergebnissen des zeitkontinuierlichen Kanals bemerkenswert gut überein. Im Hoch-SNR-Bereich sind spektrale Effizienzen bis zu 1,53 Bit/s/Hz bei strikter Bandbegrenzung möglich. Wenn Energieanteile außerhalb des Frequenzbandes tolerierbar sind, können spektrale Effizienzen über 2 Bit/s/Hz pro Domäne – bezogen auf die Bandbreite, die 95 % der Energie enthält – erreichbar sein. Durch die Anwendung der erhaltenen Grenzen auf einen bandbegrenzten überabgetasteten 1-Bit quantisierten Multiple-Input Multiple-Output-Kanal zeigen wir Vorteile durch die Verwendung geeigneter Leistungsverteilungsschemata. Als Modulationsverfahren mit konstanter Hüllkurve betrachten wir kontinuierliche Phasenmodulation, um die Anforderungen an die Linearität des Leistungsverstärkers zu verringern. Beschränkungen für die erreichbare Datenrate bei rauschfreier Übertragung auf Zwischenfrequenzen mit Modulationsalphabeten höherer Ordnung werden für Phase-shift keying (PSK) and Continuous-phase frequency-shift keying (CPFSK) untersucht. Weiterhin werden angepasste Signalformen entworfen, die als FTN-CPFSK beschrieben werden können. Mit der gleichen spektralen Effizienz im Hoch-SNR-Bereich wie PSK und CPFSK bieten diese Signalformen eine deutlich verbesserte Bitfehlerrate (BER). Die Verringerung des erforderlichen SNRs zur Erreichung einer bestimmten BER kann bis zu 20 dB betragen.

[pt] IoT refere-se a um sistema de dispositivos de computação inter-relacionados que visa transferir dados através de uma rede sem exigir interação humanohumano ou humano-para-computador. Esses sistemas de comunicação modernos, exigem restrições de baixo consumo de energia e baixa complexidade no receptor. Nesse sentido, o conversor analógico-digital representa um gargalo para o desenvolvimento das aplicações dessas novas tecnologias, pois apresenta alto consumo de energia devido à sua alta resolução. A pesquisa realizada em relação aos conversores analógico-digitais com quantização grosseira mostrou que esses dispositivos são promissores para o projeto de futuros sistemas de comunicação. Para equilibrar a perda de informações, devido à quantização grosseira, a resolução no tempo é aumentada através da superamostragem. Esta tese considera um sistema com quantização de 1 bit e superamostragem no receptor com um canal de downlink MIMO multiusuário com banda ilimitada e apresenta, como principal contribuição, a nova modulação de cruzamento de zeros que implica que a informação é transmitida no instante de tempo zero-crossings. Este método é usado para a pré-codificação temporal através da otimização do design da forma de onda para dois pré-codificadores diferentes, a maximização temporal da distância mínima até o limiar de decisão com forçamento a zero espacial e a pré-codificação MMSE no espácio-temporal. Os resultados da simulação mostram que a abordagem de cruzamento de zeros proposta supera o estado da arte em termos da taxa de erro de bits para os dois pré-codificadores estudados. Além disso, essa nova modulação reduz a complexidade computacional, permite dispositivos de complexidade muito baixa e economiza recursos de banda em comparação com o método mais avançado. Análises adicionais mostram que a abordagem do cruzamento de zeros é benéfica em comparação com o método mais avançado em termos de maior distância mínima até o limiar de decisão e menor MSE para sistemas com limitações de banda. Além disso, foi desenvolvido um esquema de mapeamento de bits para modulação de cruzamento por zero, semelhante à codificação de Gray para reduzir ainda mais a taxa de erro de bits. / [en] The IoT refers to a system of interrelated computing devises which aims to transfer data over a network without requiring human-to-human or humanto- computer interaction. This Modern communication systems demand restrictions of low energy consumption and low complexity in the receiver. In this sense, the analog-to-digital converter represents a bottleneck for the development of the applications of these new technologies since it has a high energy consumption due to its high resolution. The research carried out concerning to the analog-to-digital converters with coarse quantization has shown that such devices are promising for the design of future communication systems. To balance the loss of information, due to the coarse quantization, the resolution in time is increased through oversampling. This thesis considers a system with 1-bit quantization and oversampling at the receiver with a bandlimited multiuser MIMO downlink channel and introduces, as the main contribution, the novel zero-crossing modulation which implies that the information is conveyed within the time instant of the zero-crossings. This method is used for the temporal precoding through the waveform design optimization for two different precoders, the temporal maximization of the minimum distance to the decision threshold with spatial zero forcing and the space-time MMSE precoding. The simulation results show that the proposed zero-crossing approach outperforms the state-of-theart in terms of the bit error rate for both precoders studied. In addition, this novel modulation reduces the computational complexity, allows very low complexity devices and saves band resources in comparison to the state-ofthe- art method. Additional analyses show that the zero-crossing approach is beneficial in comparison to the state-of-the-art method in terms of greater minimum distance to the decision threshold and lower MSE for systems with band limitations. Moreover, it was devised a bit-mapping scheme for zero-crossing modulation, similar to Gray-coding to further reduce the bit error rate.