Analysis and design of ΣΔ Modulators for Radio Frequency Switchmode Power Amplifiers

Power amplifiers are an integral part of every basestation, macrocell, microcell and mobile phone, enabling data to be sent over the distances needed to reach the receiver’s antenna. While linear operation is needed for transmitting WCDMA and OFDM signals, linear operation of a power amplifier is characterized by low power efficiency, and contributes to unwanted power dissipation in a transmitter. Recently, a switchmode power amplifier operation was considered for reducing power losses in a RF transmitter. A linear and efficient operation of a PA can be achieved when the transmitted RF signal is ΣΔ modu- lated, and subsequently amplified by a nonlinear device. Although in theory this approach offers linearity and efficiency reaching 100%, the use of ΣΔ modulation for transmitting wideband signals causes problems in practical implementation: it requires high sampling rate by the digital hardware, which is needed for shaping large contents of a quantization noise induced by the modulator but also, the binary output from the modulator needs an RF power amplifier operating over very wide frequency band. This thesis addresses the problem of noise shaping in a ΣΔ modulator and nonlinear distortion caused by broadband operation in switchmode power amplifier driven by a ΣΔ modulated waveform. The problem of sampling rate increase in a ΣΔ modulator is solved by optimizing structure of the modulator, and subsequent processing of an input signal’s samples in parallel. Independent from the above, a novel technique for reducing quan- tization noise in a bandpass ΣΔ modulator using single bit quantizer is presented. The technique combines error pulse shaping and 3-level quantization for improving signal to noise ratio in a 2-level output. The improvement is achieved without the increase of a digital hardware’s sampling rate, which is advantageous also from the perspective of power consumption. The new method is explored in the course of analysis, and verified by simulated and experimental results. The process of RF signal conversion from the Cartesian to polar form is analyzed, and a signal modulator for a polar transmitter with a ΣΔ-digitized envelope signal is designed and implemented. The new modulator takes an advantage of bandpass digital to analog conversion for simplifying the analog part of the modulator. A deformation of the pulsed RF signal in the experimental modulator is demonstrated to have an effect primarily on amplitude of the RF signal, which is correctable with simple predistortion

Arquiteturas paralelas avançadas para transmissores 5G totalmente digitais

The fifth generation of mobile communications (5G) is being prepared and should be rolled out in the early coming years. Massive number of Radio-Frequency (RF) front-ends, peak data rates of 10 Gbps (everywhere and everytime), latencies lower than 10 msec and huge device densities are some of the expected disruptive capabilities. At the same time, previous generations can not be jeopardized, fostering the design of novel flexible and highly integrated radio transceivers able to support the simultaneous transmission of multi-band and multi-standard signals. The concept of all-digital transmission is being pointed out as a promising architecture to cope with such challenging requirements, due to its fully digital radio datapath. This thesis is focused on the proposal and validation of fully integrated and advanced digital transmitter architectures that excel the state-of-the-art in different figures of merit, such as transmission bandwidth, spectral purity, carrier agility, flexibility, and multi-band capability. The first part of this thesis introduces the concept of all-digital RF transmission. In particular, the foundations inherent to this thematic line are given, together with the recent advances reported in the state-of-the-art architectures.The core of this thesis, containing the main developments achieved during the Ph.D. work, is then presented and discussed. The first key contribution to the state-of-the-art is the use of cascaded Delta-Sigma (∆Σ) architectures to relax the analog filtering requirements of the conventional All-Digital Transmitters while maintaining the constant envelope waveform. Then, it is presented the first reported architecture where Antenna Arrays are directly driven by single-chip and single-bit All-Digital Transmitters, with promising results in terms of simplification of the RF front-ends and overall flexibility. Subsequently, the thesis proposes the first reported RF-stage All-Digital Transmitter that can be embedded within a single Field-Programmable Gate Array (FPGA) device. Thereupon, novel techniques to enable the design of wideband All-Digital Transmitters are reported. Finally, the design of concurrent multi-band transmitters is introduced. In particular, the design of agile and flexible dual and triple bands All-DigitalTransmitter (ADT) is demonstrated, which is a very important topic for scenarios that demand carrier aggregation. This Ph.D. contributes withseveral advances to the state-of-the-art of RF all-digital transmitters.A quinta geração de comunicações móveis (5G) está a ser preparada e deve ser comercializada nos próximos anos. Algumas das caracterı́sticas inovadoras esperadas passam pelo uso de um número massivo de font-ends de Rádio-Frequência (RF), taxas de pico de transmissão de dados de 10 Gbps (em todos os lugares e em todas as ocasiões), latências inferiores a 10 mseg e elevadas densidades de dispositivos. Ao mesmo tempo, as gerações anteriores não podem ser ignoradas, fomentando o design de novos transceptores de rádio flexı́veis e altamente integrados, capazes de suportar a transmissão simultânea de sinais multi-banda e multi-standard. O conceito de transmissão totalmente digital é considerado como um tipo de arquitetura promissora para lidar com esses requisitos desafiantes, devido ao seu datapath de rádio totalmente digital. Esta tese é focada na proposta e validação de arquiteturas de transmissores digitais totalmente integradas e avançadas que ultrapassam o estado da arte em diferentes figuras de mérito, como largura de banda de transmissão, pureza espectral, agilidade de portadora, flexibilidade e capacidade multibanda. A primeira parte desta tese introduz o conceito de transmissores de RF totalmente digitais. Em particular, os fundamentos inerentes a esta linha temática são apresentados, juntamente com os avanços mais recentes do estado-da-arte. O núcleo desta tese, contendo os principais desenvolvimentos alcançados durante o trabalho de doutoramento, é então apresentado e discutido. A primeira contribuição fundamental para o estado da arte é o uso de arquiteturas em cascata com moduladores ∆Σ para relaxar os requisitos de filtragem analógica dos transmissores RF totalmente digitais convencionais, mantendo a forma de onda envolvente constante. Em seguida, é apresentada a primeira arquitetura em que agregados de antenas são excitados diretamente por transmissores digitais de um único bit inseridos num único chip, com resultados promissores em termos de simplificação dos front-ends de RF e flexibilidade em geral. Posteriormente, é proposto o primeiro transmissor totalmente digital RF-stage relatado que pode ser incorporado dentro de um único Agregado de Células Lógicas Programáveis. Novas técnicas para permitir o desenho de transmissores RF totalmente digitais de banda larga são também apresentadas. Finalmente, o desenho de transmissores simultâneos de múltiplas bandas é exposto. Em particular, é demonstrado o desenho de transmissores de duas e três bandas ágeis e flexı́veis, que é um tópico essencial para cenários que exigem agregação de múltiplas bandas.Apoio financeiro da Fundação para a Ciência e Tecnologia (FCT) no âmbito de uma bolsa de doutoramento, ref. PD/BD/105857/2014.Programa Doutoral em Telecomunicaçõe

An RF Carrier Bursting System using Partial Quantization Noise Cancellation

This paper introduces a novel method for bandpass cancellation of the quantization noise occurring in high efficiency, envelope pulsed transmitter architectures - or carrier bursting. An equivalent complex baseband model of the proposed system, including the Sigma Delta-modulator and cancellation signal generation, is developed. Analysis of the baseband model is performed, leading to analytical expressions of the power amplifier drain efficiency, assuming the use of an ideal class B power amplifier. These expressions are further used to study the impact of key system parameters, i.e. the compensation signal variance and clipping probability, on the class~B power amplifier drain efficiency and signal-to-noise ratio. The paper concludes with simulations followed by practical measurements in order to validate the functionality of the method and to evaluate the performance-trend predictions made by the theoretical framework in terms of efficiency and spectral purity

Investigation of Time Domain Modulation and Switching-Mode Power Amplifiers Suitable for Digitally-Assisted Transmitters

Innovation in wireless communication has resulted in accelerating demand for smartphones using multiple communications protocols such as WiFi, Bluetooth and the many cellular standards deployed around the world. The variety of frequency, bandwidth and power requirements associated with each standard typically calls for the implementation of separate radio frequency (RF) front end hardware for each standard. This is a less-than-ideal solution in terms of cost and device area. Software-defined radio (SDR) promises to solve this problem by allowing the RF hardware to be digitally reconfigurable to adapt to any wireless standard. The application of machine learning and cognition algorithms to SDR will enable cognitive radios and cognitive wireless networks, which will be able to intelligently adapt to user needs and surrounding radio spectrum conditions. The challenge of fully reconfigurable transceivers is in implementing digitally-controlled RF circuits which have comparable performance to their fixed-frequency counterparts. Switching-mode power amplifiers (SMPA) are likely to be an important part of fully reconfigurable transmitters since their switching operation provides inherent compatibility with digital circuits, with the added benefit of very high efficiency. As a step to understanding the RF requirements of high efficiency and switching PAs, an inverse class F PA in push-pull configuration is implemented. This configuration is chosen for its similarity to the current mode class D (CMCD) topology. The fabricated PA achieves a peak drain efficiency of over 75% with 42.7 dBm (18.6 W) output power at 2.46 GHz. Since SMPAs cannot directly provide the linearity required by current and future wireless communications standards, amplitude information must be encoded into the RF signal in a different way. Given the superior time resolution of digital integrated circuit (IC) technology, a logical solution is to encode this information into the timing of the signal. The two most common techniques for doing so are pulse width modulation and delta-sigma modulation. However, the design of delta-sigma modulators requires simulation as part of the design process due to the lack of closed-form relationships between modulator parameters (such as resolution and oversampling) and performance figures (such as coding efficiency and signal quality). In particular, the coding efficiency is often ignored although it is an important part of ensuring transmitter efficiency with respect to the desired signal. A study of these relationships is carried out to observe the tradeoffs between them. It is found that increasing the speed or complexity of a DS modulated system does not necessarily translate to performance benefits as one might expect. These observations can have a strong impact on design choices at the system level

Novas arquiteturas para transmissores digitais flexíveis e de banda larga

Next generation of wireless communication (5G) devices must achieve higher data rates, lower power consumption and better coverage by making a more efficient use of the RF spectrum and adopting highly exible radio architectures. To meet these requirements, the development of new radio devices will be far more complex and challenging than their predecessors. The future of radio communications have a twofold evolution, being one the low power consumption and the other the adaptability and intelligent use of the available resources. Conventional approaches for the radio physical layer are not capable to cope with the new demand for multi-band, multi-standard radio signals and present an inefficient and expensive solution for simultaneous transmission of multiple and heterogeneous radio signals. Digital radio transmitters have been presented as a solution for a newer and more exible architecture for future radios. All-digital transmitters use a completely digital implementation of the entire radio datapath from the baseband processing to the digital RF up-conversion. This concept bene ts from the use of highly integrated hardware together with a strong radio digitalization, motivated by the exibility and high performance from cognitive and software defi ned radio. However, such devices are still far from a massive deployment in most of communication scenarios due to some limiting factors that hinder their use. This PhD thesis aims to the development of novel radio architectures and ideas based on all-digital transmitters capable of improving the adaptability and use intelligently the available resources for software de ned and cognitive radio systems. The focus of this thesis is on the improvement of some of the common limitations for all-digital transmitters such as power efficiency, bandwidth, noise-shaping and exibility while using efficient and adaptable digital architectures. In the initial part of the thesis a review of the state-of-the-art is presented showing the most common digital transmitter architectures as well as their major bene ts and key limitations. A comparative analysis of such architectures is made considering their power and spectral efficiency, exibility, performance and cost. Following this initial analysis, the work developed on the course of this PhD is presented and discussed. The initial focus is on the improvement of all-digital transmitters bandwidth trough the study and use of parallel processing techniques capable of greatly improve common bandwidth values presented in the state-of-the-art. The presented work has resulted in several publications where FPGA-based architectures use parallel digital processing techniques to improve the system's bandwidth by a factor higher than 10. Other fundamental contribution of this thesis is focused on the pulsedtransmitters coding efficiency. In this section of the thesis, a method is presented showing the reduction of the quantization noise created by low amplitude resolution digital transmitters using multiple combined pulsedtransmitters to cancel the noise in speci c frequencies. This work has resulted in two main publications that showed how to increase the coding efficiency of the pulse-transmitters as well as the overall efficiency of the transmission system. Lastly, new-noise shaping methods are presented in order to develop new and more exible architectures for all-digital transmitters. The methods presented use new quantization processes that allow for the shaping of the quantization noise produced in pulsed-transmitters while using very simple and adaptable architectures. With these new techniques, it is possible to adjust the noise frequency distribution and deliberately change the noise shape in order to change some of the transmitter's characteristics such as central frequency or bandwidth. The work presented on this thesis has shown promising improvements to the all-digital transmitters' state-of-the-art, either in simulations and laboratory prototype measurements. It has contributed to advance the state-of-the-art in agile and power efficient all-digital RF transmitters with multi-mode and multi-channel capabilities and the improvement of the transceiver's bandwidth enabling the development of true software de ned and cognitive radio systemsA próxima geração de comunicações sem os (5G) exigirá taxas de transmissão mais elevadas, maior efi ciência energética e uma melhor cobertura fazendo um uso mais efi ciente do espectro de radiofrequência e adotando o uso de arquiteturas rádio mais flexíveis. Para cumprir tais requisitos, o desenvolvimento de novos dispositivos rádio será substancialmente mais complexo do que nas gerações anteriores. O futuro das comunicações rádio depende maioritariamente de dois fatores; o baixo consumo de potência e o uso inteligente dos recursos e tecnologias disponíveis. As abordagens convencionais para a camada física dos sistemas rádio não são as mais adequadas para lidar com a necessidade de dispositivos multi-banda e que usem múltiplos standards, por serem soluções inefi cientes e demasiado caras para esse efeito. Os transmissores rádio completamente digitais têm vindo a ser apresentados na literatura como uma solução inovadora e mais flexível para a implementação dos futuros sistemas de rádio. Os transmissores completamente digitais apresentam uma implementação da cadeia de processamento rádio, desde a banda-base até à conversão para RF, completamente constituída por lógica digital. Este conceito tira partido da vasta integração alcançada nas arquiteturas digitais, juntamente com a flexibilidade proveniente da digitalização das arquiteturas rádio que já se encontra em curso com a evolução dos rádios cognitivos e definidos por software. No entanto, devido a algumas limitações inerentes à tecnologia, este tipo de transmissores ainda não é amplamente utilizado na maioria dos sistemas. Esta tese de doutoramento propõe e avalia novas arquiteturas para transmissores completamente digitais, bem como novas técnicas de processamento de sinal que possam beneficiar das tecnologias de implementação existentes (e.g. FPGAs) por forma a construir novos transmissores digitais de forma eficiente e flexível. O objetivo desta tese é reduzir as limitações atuais ainda presentes neste tipo de transmissores, nomeadamente as relacionadas com a eficiência, largura de banda, cancelamento de ruído e falta de flexibilidade. Na parte inicial desta tese é realizada a revisão do estado da arte das diversas topologias de transmissores digitais bem como as suas principais vantagens e limitações técnicas. É também feita uma análise comparativa das diversas técnicas apresentadas em termos da sua eficiência energética, flexibilidade, desempenho e custo. De seguida, é apresentado o trabalho desenvolvido no contexto desta tese de doutoramento, seguindo-se uma discussão focada na resolução das atuais limitações deste tipo de transmissores. A primeira parte foca-se no uso de técnicas de processamento paralelo de sinal, por forma a suportar sinais de largura de banda mais elevada que os reportados no atual estado da arte. O trabalho desenvolvido e publicado baseia-se no uso de arquiteturas implementadas em FPGA que contribuíram para um aumento da largura de banda num fator de aproximadamente dez vezes. Outra das contribuições fundamentais desta tese consiste no aumento da eficiência do sistema através da melhoria da eficiência de codificação do sinal pulsado produzido. Com base no uso de múltiplos transmissores pulsados, e apresentado um esquema de combinação construtiva e destrutiva de sinais para a redução do ruído de quantização proveniente das técnicas de processamento de sinal pulsado usadas. Este trabalho resultou em duas importantes publicações que mostram que a melhoria da eficiência de codificação do sinal pode ser utilizada de forma a obter uma maior eficiência energética do transmissor. Por ultimo, são apresentadas diversas técnicas para a conversão dos sinais banda-base em sinais RF pulsados. As propostas apresentadas permitem o uso de uma arquitetura de hardware simplista, mas configurável por software, o que a torna bastante flexível. Com o uso desta arquitetura e possível alterar em pleno funcionamento a frequência central bem como a largura de banda e resposta do conversor pulsado. O trabalho apresentado nesta tese demonstra alguns dos melhoramentos no estado da arte para transmissores r adio completamente digitais, baseando os resultados obtidos não apenas em simulações mas também na implementação e medidas realizadas sobre protótipos laboratoriais. O trabalho desenvolvido no âmbito desta tese contribuiu com avanços na implementação de transmissores ageis, eficientes, com maior largura de banda e capazes de transmissão em múltiplas bandas com recurso a múltiplos protocolos, abrindo caminho para o desenvolvimento de novos rádios cognitivos e definidos por softwareFCT, FSEPrograma Doutoral em Engenharia Eletrotécnic

Digital Radio Encoding and Power Amplifier Design for Multimode and Multiband Wireless Communications

The evolution of wireless technology has necessitated the support of multiple communication standards by mobile devices. At present, multiple chipsets/radios operating at predefined sets of modulation schemes, frequency bands, bandwidths and output power levels are used to achieve this objective. This leads to higher component counts, increased cost and limits the capacity to cope with future communication standards. In order to tackle different wireless standards using a single chipset, digital circuits have been increasingly deployed in radios and demonstrated re-configurability in different modulation schemes (multimode) and frequency bands (multiband). Despite efforts and progress made in digitizing the entire radio, the power amplifier (PA) is still designed using an conventional approach and has become the bottleneck in digital transmitters, in terms of low average power efficiency, poor compatibility with modern CMOS technology and limited re-configurability. This research addresses these issues from two aspects. The first half of the thesis investigates signal encoding issues between the modulator and PA. We propose, analyze and evaluate a new hybrid amplitude/time signal encoding scheme that significantly improves the coding efficiency and dynamic range of a digitally modulated power amplifier (DMPA) without significantly increasing design complexity. The proposed hybrid amplitude/time encoding scheme combines both the amplitude domain and the time domain to optimally encode information. Experimental results show that hybrid amplitude/time encoding results in a 35% increase in the average coding efficiency with respect to conventional time encoding, and is only 6.7% lower than peak efficiency when applied to a Wireless Local Area Network (WLAN) signal with a peak to average power ratio equal to 9.9 dB. A new DMPA architecture, based on the proposed hybrid encoding, is also proposed. The second half of this thesis presents the design, analysis and implementation of a CMOS PA that is amenable to the proposed hybrid encoding scheme. A multi-way current mode class-D PA architecture has been proposed and realized in 130 nm CMOS technology. The designed PA has satisfied the objectives of wide bandwidth (1.5 GHz - 2.7 GHz at 1 dB output power), and high efficiency (PAE 63%) in addition to demonstrating linear responses using the proposed digital encoding. A complete digital transmitter combining the encoder and the multi-way PA was also investigated. The overall efficiency is 27% modulating 7.3 dB peak to average power ratio QAM signals

Concurrent Dual Band Radio-over-Fiber Transmission Using 1-bit Envelope Delta-Sigma Modulation

With the growing demand for bandwidth and transmission speed, mobile communication network designs must stay adaptable, efficient and cost-effective. A key integration has been Radio-over-Fiber (RoF) transmission systems that provide a cheaper option and low loss for high frequency signal transfer. For the optical transmitter, delta-sigma modulation (DSM) can be a beneficial addition. The partnership simplifies the Digital-Radio-over-Fiber setup by removing the need for additional converters and prompts adjustments based on system need. Main factors in delta-sigma modulators are the amount of quantization bits and the order of the modulator. Changing quantization bits to a single bit allows the system to use less processing bandwidth and less error experienced from optical transmission. High order structures provide more noise shaping to shift noise away from the band of interest. Still, such setups are prone to linearity problems due to clock jitter from multiple feedback loops. Different adaptations of delta-sigma modulation have been designed to combat the problems, but a key standout is the implementation of an envelope delta-sigma modulation (EDSM). Envelope delta-sigma modulation’s separate processing of envelope and phase delivers time alignment and noise shaping counter the negative implications from high order DSMs. Combining envelope delta-sigma modulation with RoF transmission is an attractive option, but research has yet to delve into carrier aggregation with these setups. This thesis explores concurrent dual band 64-QAM 20 MHz LTE Radio-over-Fiber using 1-bit envelope delta-sigma modulation. It expands transmitter functionality by concurrent signal integration. Inside the EDSM is a 4th order bandpass delta-sigma modulator custom tailored one of two carrier frequencies. The two frequencies come from two different LTE bands to show interband compatibility. The carrier frequencies are 2.112 GHz from LTE band 1 and 2.64 GHz from LTE band 7. Simulation and experimental results confirm the functionality of the proposed envelope delta-sigma modulation RoF system in single and dual band for LTE standards (error vector magnitude < 8%). Experimental results confirm that EDSM is more resilient to RoF transmission than BP-DSM. However, the EVM values for BP-DSM are better for carrier aggregated transmission

Techniques for Wideband All Digital Polar Transmission

abstract: Modern Communication systems are progressively moving towards all-digital transmitters (ADTs) due to their high efficiency and potentially large frequency range. While significant work has been done on individual blocks within the ADT, there are few to no full systems designs at this point in time. The goal of this work is to provide a set of multiple novel block architectures which will allow for greater cohesion between the various ADT blocks. Furthermore, the design of these architectures are expected to focus on the practicalities of system design, such as regulatory compliance, which here to date has largely been neglected by the academic community. Amongst these techniques are a novel upconverted phase modulation, polyphase harmonic cancellation, and process voltage and temperature (PVT) invariant Delta Sigma phase interpolation. It will be shown in this work that the implementation of the aforementioned architectures allows ADTs to be designed with state of the art size, power, and accuracy levels, all while maintaining PVT insensitivity. Due to the significant performance enhancement over previously published works, this work presents the first feasible ADT architecture suitable for widespread commercial deployment.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Characterization and modelling of software defined radio front-ends

Doutoramento em Engenharia ElectrotécnicaO presente trabalho tem por objectivo estudar a caracterização e modelação de arquitecturas de rádio frequência para aplicações em rádios definidos por software e rádios cognitivos. O constante aparecimento no mercado de novos padrões e tecnologias para comunicações sem fios têm levantado algumas limitações à implementação de transceptores rádio de banda larga. Para além disso, o uso de sistemas reconfiguráveis e adaptáveis baseados no conceito de rádio definido por software e rádio cognitivo assegurará a evolução para a próxima geração de comunicações sem fios. A ideia base desta tese passa por resolver alguns problemas em aberto e propor avanços relevantes, tirando para isso partido das capacidades providenciadas pelos processadores digitais de sinal de forma a melhorar o desempenho global dos sistemas propostos. Inicialmente, serão abordadas várias estratégias para a implementação e projecto de transceptores rádio, concentrando-se sempre na aplicabilidade específica a sistemas de rádio definido por software e rádio cognitivo. Serão também discutidas soluções actuais de instrumentação capaz de caracterizar um dispositivo que opere simultaneamente nos domínios analógico e digital, bem como, os próximos passos nesta área de caracterização e modelação. Além disso, iremos apresentar novos formatos de modelos comportamentais construídos especificamente para a descrição e caracterização não-linear de receptores de amostragem passa-banda, bem como, para sistemas nãolineares que utilizem sinais multi-portadora. Será apresentada uma nova arquitectura suportada na avaliação estatística dos sinais rádio que permite aumentar a gama dinâmica do receptor em situações de multi-portadora. Da mesma forma, será apresentada uma técnica de maximização da largura de banda de recepção baseada na utilização do receptor de amostragem passa-banda no formato complexo. Finalmente, importa referir que todas as arquitecturas propostas serão acompanhadas por uma introdução teórica e simulações, sempre que possível, sendo após isto validadas experimentalmente por protótipos laboratoriais.This work investigates the characterization and modeling of radio frequency front-ends for software defined radio and cognitive radio applications. The emergence of new standards and technologies in the wireless communications market are raising several issues to the implementation of wideband transceiver systems. Also, reconfigurable and adaptable systems based on software defined and cognitive radio models are paving the way for the next generation of wireless systems. In this doctoral thesis the fundamental idea is to address the particular open issues and propose appropriate advancements by exploring and taking profit from new capabilities of digital signal processors in a way to improve the overall performance of the novel schemes. Receiver and transmitter strategies for radio communications are summarized by concentrating on the usability for software defined radio and cognitive radio systems. Available instrumentation and next steps for analog and digital radio frequency hardware characterization is also discussed. Wideband behavioral model formats are proposed for nonlinear description and characterization of bandpass sampling receivers, as well as, for multi-carrier nonlinear systems operation. The proposed models share a great flexibility and have the freedom to be simply expanded to other fields. A new design for receiver dynamic range improvement in multi-carrier scenarios is proposed, which is supported on the useful wireless signals statistical evaluation. Additionally, receiver-side bandwidth maximization based on higher-order bandpass sampling approaches is evaluated. All the proposed designs and modeling strategies are accompanied by theoretical backgrounds and simulations whenever possible, being then experimentally validated by laboratory prototypes

Time and frequency domain algorithms for speech coding

The promise of digital hardware economies (due to recent advances in VLSI technology), has focussed much attention on more complex and sophisticated speech coding algorithms which offer improved quality at relatively low bit rates. This thesis describes the results (obtained from computer simulations) of research into various efficient (time and frequency domain) speech encoders operating at a transmission bit rate of 16 Kbps. In the time domain, Adaptive Differential Pulse Code Modulation (ADPCM) systems employing both forward and backward adaptive prediction were examined. A number of algorithms were proposed and evaluated, including several variants of the Stochastic Approximation Predictor (SAP). A Backward Block Adaptive (BBA) predictor was also developed and found to outperform the conventional stochastic methods, even though its complexity in terms of signal processing requirements is lower. A simplified Adaptive Predictive Coder (APC) employing a single tap pitch predictor considered next provided a slight improvement in performance over ADPCM, but with rather greater complexity. The ultimate test of any speech coding system is the perceptual performance of the received speech. Recent research has indicated that this may be enhanced by suitable control of the noise spectrum according to the theory of auditory masking. Various noise shaping ADPCM configurations were examined, and it was demonstrated that a proposed pre-/post-filtering arrangement which exploits advantageously the predictor-quantizer interaction, leads to the best subjective performance in both forward and backward prediction systems. Adaptive quantization is instrumental to the performance of ADPCM systems. Both the forward adaptive quantizer (AQF) and the backward oneword memory adaptation (AQJ) were examined. In addition, a novel method of decreasing quantization noise in ADPCM-AQJ coders, which involves the application of correction to the decoded speech samples, provided reduced output noise across the spectrum, with considerable high frequency noise suppression. More powerful (and inevitably more complex) frequency domain speech coders such as the Adaptive Transform Coder (ATC) and the Sub-band Coder (SBC) offer good quality speech at 16 Kbps. To reduce complexity and coding delay, whilst retaining the advantage of sub-band coding, a novel transform based split-band coder (TSBC) was developed and found to compare closely in performance with the SBC. To prevent the heavy side information requirement associated with a large number of bands in split-band coding schemes from impairing coding accuracy, without forgoing the efficiency provided by adaptive bit allocation, a method employing AQJs to code the sub-band signals together with vector quantization of the bit allocation patterns was also proposed. Finally, 'pipeline' methods of bit allocation and step size estimation (using the Fast Fourier Transform (FFT) on the input signal) were examined. Such methods, although less accurate, are nevertheless useful in limiting coding delay associated with SRC schemes employing Quadrature Mirror Filters (QMF)