Doctor of Philosophy

dissertationWireless communications pervade all avenues of modern life. The rapid expansion of wireless services has increased the need for transmission schemes that are more spectrally efficient. Dynamic spectrum access (DSA) systems attempt to address this need by building a network where the spectrum is used opportunistically by all users based on local and regional measurements of its availability. One of the principal requirements in DSA systems is to initialize and maintain a control channel to link the nodes together. This should be done even before a complete spectral usage map is available. Additionally, with more users accessing the spectrum, it is important to maintain a stable link in the presence of significant interference in emergency first-responders, rescue, and defense applications. In this thesis, a new multicarrier spread spectrum (MC-SS) technique based on filter banks is presented. The new technique is called filter bank multicarrier spread spectrum (FB-MC-SS). A detailed theory of the underlying properties of this signal are given, with emphasis on the properties that lend themselves to synchronization at the receiver. Proposed algorithms for synchronization, channel estimation, and detection are implemented on a software-defined radio platform to complete an FB-MC-SS transceiver and to prove the practicality of the technique. FB-MC-SS is shown through physical experimentation to be significantly more robust to partial band interference compared to direct sequence spread spectrum. With a higher power interfering signal occupying 90% of its band, FB-MC-SS maintains a low bit error rate. Under the same interference conditions, DS-SS fails completely. This experimentation leads to a theoretical analysis that shows in a frequency selective channel with additive white noise, the FB-MC-SS system has performance that equals that obtained by a DS-SS system employing an optimal rake receiver. This thesis contains a detailed chapter on implementation and design, including lessons learned while prototyping the system. This is to assist future system designers to quickly gain proficiency in further development of this technology

A Two-stage approach to harmonic rejection mixing using blind interference cancelling

Current analog harmonic rejection mixers typically provide 30–40 dB of harmonic rejection, which is often not sufficient. We present a mixed analog-digital approach to harmonic rejection mixing that uses a digital interference canceler to reject the strongest interferer. Simulations indicate that, given a practical RF scenario, the digital canceler is able to improve the signal-to-interference ratio by 30–45 dB

Filter Bank Multicarrier Modulation for Spectrally Agile Waveform Design

In recent years the demand for spectrum has been steadily growing. With the limited amount of spectrum available, Spectrum Pooling has gained immense popularity. As a result of various studies, it has been established that most of the licensed spectrum remains underutilized. Spectrum Pooling or spectrum sharing concentrates on making the most of these whitespaces in the licensed spectrum. These unused parts of the spectrum are usually available in chunks. A secondary user looking to utilize these chunks needs a device capable of transmitting over distributed frequencies, while not interfering with the primary user. Such a process is known as Dynamic Spectrum Access (DSA) and a device capable of it is known as Cognitive Radio. In such a scenario, multicarrier communication that transmits data across the channel in several frequency subcarriers at a lower data rate has gained prominence. Its appeal lies in the fact that it combats frequency selective fading. Two methods for implementing multicarrier modulation are non-contiguous orthogonal frequency division multiplexing (NCOFDM)and filter bank multicarrier modulation (FBMC). This thesis aims to implement a novel FBMC transmitter using software defined radio (SDR) with modulated filters based on a lowpass prototype. FBMCs employ two sets of bandpass filters called analysis and synthesis filters, one at the transmitter and the other at the receiver, in order to filter the collection of subcarriers being transmitted simultaneously in parallel frequencies. The novel aspect of this research is that a wireless transmitter based on non-contiguous FBMC is being used to design spectrally agile waveforms for dynamic spectrum access as opposed to the more popular NC-OFDM. Better spectral containment and bandwidth efficiency, combined with lack of cyclic prefix processing, makes it a viable alternative for NC-OFDM. The main aim of this thesis is to prove that FBMC can be practically implemented for wireless communications. The practicality of the method is tested by transmitting the FBMC signals real time by using the Simulink environment and USRP2 hardware modules

Experimental Verification of a Harmonic-Rejection Mixing Concept using Blind Interference Canceling

Abstract—This paper presents the first practical experiments\ud on a harmonic rejection downconverter, which offers up to 75 dB of harmonic rejection, without an RF filter. The downconverter uses a two-stage approach; the first stage is an analog multipath/ multi-phase harmonic rejection mixer followed by a second stage providing additional harmonic rejection based on blind adaptive interference canceling in the discrete-time domain. The aim is to show its functional operation and to find practical performance limitations. Measurement results show that the harmonic rejection of the downconverter is insensitive to frontend nonlinearities and LO phase noise. The canceler cannot cope with DC offsets. The DC offsets are removed by highpass filters. The signal paths used to obtain an estimate of the interference must\ud be designed to provide as much attenuation of the desired signal as possible

Spectrum Adaptation in Cognitive Radio Systems with Operating Constraints

The explosion of high-data-rate-demanding wireless applications such as smart-phones and wireless Internet access devices, together with growth of existing wireless services, are creating a shortage of the scarce Radio Frequency (RF) spectrum. However, several spectrum measurement campaigns revealed that current spectrum usage across time and frequency is inefficient, creating the artificial shortage of the spectrum because of the traditional exclusive command-and-control model of using the spectrum. Therefore, a new concept of Cognitive Radio (CR) has been emerging recently in which unlicensed users temporarily borrow spectrum from the licensed Primary Users (PU) based on the Dynamic Spectrum Access (DSA) technique that is also known as the spectrum sharing concept. A CR is an intelligent radio system based on the Software Defined Radio platform with artificial intelligence capability which can learn, adapt, and reconfigure through interaction with the operating environment. A CR system will revolutionize the way people share the RF spectrum, lowering harmful interference to the licensed PU of the spectrum, fostering innovative DSA technology and giving people more choices when it comes to using the wireless-communication-dependent applications without having any spectrum congestion problems. A key technical challenge for enabling secondary access to the licensed spectrum adaptation is to ensure that the CR does not interfere with the licensed incumbent users. However, incumbent user behavior is dynamic and requires CR systems to adapt this behavior in order to maintain smooth information transmission. In this context, the objective of this dissertation is to explore design issues for CR systems focusing on adaptation of physical layer parameters related to spectrum sensing, spectrum shaping, and rate/power control. Specifically, this dissertation discusses dynamic threshold adaptation for energy detector spectrum sensing, spectrum allocation and power control in Orthogonal Frequency Division Multiplexing-(OFDM-)based CR with operating constraints, and adjacent band interference suppression techniques in turbo-coded OFDM-based CR systems

Channel estimation techniques for filter bank multicarrier based transceivers for next generation of wireless networks

A dissertation submitted to Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering (Electrical and Information Engineering), August 2017The fourth generation (4G) of wireless communication system is designed based on the principles of cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) where the cyclic prefix (CP) is used to combat inter-symbol interference (ISI) and inter-carrier interference (ICI) in order to achieve higher data rates in comparison to the previous generations of wireless networks. Various filter bank multicarrier systems have been considered as potential waveforms for the fast emerging next generation (xG) of wireless networks (especially the fifth generation (5G) networks). Some examples of the considered waveforms are orthogonal frequency division multiplexing with offset quadrature amplitude modulation based filter bank, universal filtered multicarrier (UFMC), bi-orthogonal frequency division multiplexing (BFDM) and generalized frequency division multiplexing (GFDM). In perfect reconstruction (PR) or near perfect reconstruction (NPR) filter bank designs, these aforementioned FBMC waveforms adopt the use of well-designed prototype filters (which are used for designing the synthesis and analysis filter banks) so as to either replace or minimize the CP usage of the 4G networks in order to provide higher spectral efficiencies for the overall increment in data rates. The accurate designing of the FIR low-pass prototype filter in NPR filter banks results in minimal signal distortions thus, making the analysis filter bank a time-reversed version of the corresponding synthesis filter bank. However, in non-perfect reconstruction (Non-PR) the analysis filter bank is not directly a time-reversed version of the corresponding synthesis filter bank as the prototype filter impulse response for this system is formulated (in this dissertation) by the introduction of randomly generated errors. Hence, aliasing and amplitude distortions are more prominent for Non-PR. Channel estimation (CE) is used to predict the behaviour of the frequency selective channel and is usually adopted to ensure excellent reconstruction of the transmitted symbols. These techniques can be broadly classified as pilot based, semi-blind and blind channel estimation schemes. In this dissertation, two linear pilot based CE techniques namely the least square (LS) and linear minimum mean square error (LMMSE), and three adaptive channel estimation schemes namely least mean square (LMS), normalized least mean square (NLMS) and recursive least square (RLS) are presented, analyzed and documented. These are implemented while exploiting the near orthogonality properties of offset quadrature amplitude modulation (OQAM) to mitigate the effects of interference for two filter bank waveforms (i.e. OFDM/OQAM and GFDM/OQAM) for the next generation of wireless networks assuming conditions of both NPR and Non-PR in slow and fast frequency selective Rayleigh fading channel. Results obtained from the computer simulations carried out showed that the channel estimation schemes performed better in an NPR filter bank system as compared with Non-PR filter banks. The low performance of Non-PR system is due to the amplitude distortion and aliasing introduced from the random errors generated in the system that is used to design its prototype filters. It can be concluded that RLS, NLMS, LMS, LMMSE and LS channel estimation schemes offered the best normalized mean square error (NMSE) and bit error rate (BER) performances (in decreasing order) for both waveforms assuming both NPR and Non-PR filter banks. Keywords: Channel estimation, Filter bank, OFDM/OQAM, GFDM/OQAM, NPR, Non-PR, 5G, Frequency selective channel.CK201

A Blind Interference Canceling Technique for Two-Stage Harmonic Rejection in Down-mixers

This paper presents practical experiments on a harmonic rejection down-mixer, which offers up to 75 dB of harmonic rejection, without an RF filter. The down-mixer uses a two-stage approach; the first stage is an analog multi-path/multiphase harmonic rejection mixer followed by a second stage providing additional harmonic rejection based on blind adaptive interference canceling in the discrete-time domain. The aim is to show its functional operation. The canceler cannot cope with DC offsets. The DC offsets are removed by highpass filters. The signal paths used to obtain an estimate of the interference must be designed to provide as much attenuation of the desired signal as possible. Front-end nonlinearities and DC offsets are discussed

Transmissores reconfiguráveis para rádios definidos por software

Doutoramento em Engenharia ElectrotécnicaFlexible radio transmitters based on the Software-Defined Radio (SDR) concept are gaining an increased research importance due to the unparalleled proliferation of new wireless standards operating at different frequencies, using dissimilar coding and modulation schemes, and targeted for different ends. In this new wireless communications paradigm, the physical layer of the radio transmitter must be able to support the simultaneous transmission of multi-band, multi-rate, multi-standard signals, which in practice is very hard or very inefficient to implement using conventional approaches. Nevertheless, the last developments in this field include novel all-digital transmitter architectures where the radio datapath is digital from the baseband up to the RF stage. Such concept has inherent high flexibility and poses an important step towards the development of SDR-based transmitters. However, the truth is that implementing such radio for a real world communications scenario is a challenging task, where a few key limitations are still preventing a wider adoption of this concept. This thesis aims exactly to address some of these limitations by proposing and implementing innovative all-digital transmitter architectures with inherent higher flexibility and integration, and where improving important figures of merit, such as coding efficiency, signal-to-noise ratio, usable bandwidth and in-band and out-of-band noise will also be addressed. In the first part of this thesis, the concept of transmitting RF data using an entirely digital approach based on pulsed modulation is introduced. A comparison between several implementation technologies is also presented, allowing to state that FPGAs provide an interesting compromise between performance, power efficiency and flexibility, thus making them an interesting choice as an enabling technology for pulse-based all-digital transmitters. Following this discussion, the fundamental concepts inherent to pulsed modulators, its key advantages, main limitations and typical enhancements suitable for all-digital transmitters are also presented. The recent advances regarding the two most common classes of pulse modulated transmitters, namely the RF and the baseband-level are introduced, along with several examples of state-of-the-art architectures found on the literature. The core of this dissertation containing the main developments achieved during this PhD work is then presented and discussed. The first key contribution to the state-of-the-art presented here consists in the development of a novel ΣΔ-based all-digital transmitter architecture capable of multiband and multi-standard data transmission in a very flexible and integrated way, where the pulsed RF output operating in the microwave frequency range is generated inside a single FPGA device. A fundamental contribution regarding the simultaneous transmission of multiple RF signals is then introduced by presenting and describing novel all-digital transmitter architectures that take advantage of multi-gigabit data serializers available on current high-end FPGAs in order to transmit in a time-interleaved approach multiple independent RF carriers. Further improvements in this design approach allowed to provide a two-stage up-conversion transmitter architecture enabling the fine frequency tuning of concurrent multichannel multi-standard signals. Finally, further improvements regarding two key limitations inherent to current all-digital transmitter approaches are then addressed, namely the poor coding efficiency and the combined high quality factor and tunability requirements of the RF output filter. The followed design approach based on poliphase multipath circuits allowed to create a new FPGA-embedded agile transmitter architecture that significantly improves important figures of merit, such as coding efficiency and SNR, while maintains the high flexibility that is required for supporting multichannel multimode data transmission.Transmissores de rádio flexíveis baseados no conceito do Rádio Definido por Software (SDR) estão a receber uma crescente importância de investigação essencialmente devido à proliferação sem precedentes de novos standards de comunicações wireless que trabalham em frequências diferentes, usando esquemas de modulação e codificação dissimilares, estando direcionados para os mais diversos fins. Neste novo paradigma de comunicações wireless, a camada física do transmissor rádio tem de ser capaz de suportar a transmissão simultânea de sinais provenientes de diferentes standards, operando em diferentes bandas de frequências e com diferentes ritmos de transmissão, o que na prática é muito difícil ou muito ineficiente de implementar utilizando abordagens convencionais. Contudo, os últimos desenvolvimentos nesta área incluem novas arquiteturas de transmissão inteiramente digitais onde o datapath do rádio é digital desde a banda base até ao RF. Tal conceito tem uma elevada flexibilidade e representa um passo importante para o desenvolvimento de transmissores baseados em SDR. No entanto, a implementação de tal rádio para cenários de comunicação reais é uma tarefa desafiadora, onde algumas limitações chave estão ainda impedindo uma maior adopção deste conceito. Esta tese tem como principal objetivo o de investigar algumas destas limitações, propondo e implementando arquiteturas inovadoras de transmissão inteiramente digitais com inerente elevada flexibilidade e integração, e onde melhorar importantes figuras de mérito, tais como a eficiência de codificação, a relação sinal-ruído, a largura de banda utilizável e o ruído dentro e fora da banda também serão abordadas. Na primeira parte deste trabalho é introduzido o conceito de transmissão de dados RF utilizando uma abordagem totalmente digital, baseada em modulação por impulsos. Uma comparação entre diversas tecnologias de implementação é também apresentada, permitindo afirmar que as FPGAs actuais oferecem um compromisso interessante entre desempenho, eficiência de energia e flexibilidade, tornando-as uma escolha interessante como uma tecnologia de implementação com elevado potencial para transmissores completamente digitais baseados em moduladores pulsados. Após esta discussão são apresentados os conceitos fundamentais inerentes aos moduladores pulsados e introduzidos os avanços relativos a transmissores RF modulados por pulsos, juntamente com vários exemplos de arquiteturas do estado da arte encontrados na literatura. Em seguida, o núcleo desta tese contendo os principais desenvolvimentos alcançados durante este trabalho de doutoramento é apresentado e discutido. O primeiro contributo fundamental para o estado da arte aqui apresentado consiste no desenvolvimento e integração em FPGA de uma nova arquitetura de transmissão inteiramente digital, baseada em moduladores ΣΔ e dotada de uma elevada flexibilidade e integração, sendo capaz de transmitir dados de multiplos standards e em multiplas bandas de RF. Uma segunda contribuição chave relativa à transmissão simultânea de vários sinais RF é então introduzida, sendo apresentadas e descritas novas arquiteturas de transmissão de sinal RF inteiramente digitais, as quais tiram proveito de serializadores de dados multi-gigabit disponíveis em FPGAs atuais de alto desempenho. Melhorias adicionais a esta abordagem permitiram desenvolver uma arquitetura de transmissão com duas fases de conversão na frequência, a qual permite a transmissão concorrente de sinais multistandard e multicanal com ajuste fino na frequência. Por ultimo, foram ainda investigadas diversas técnicas que visam reduzir duas limitações fundamentais inerentes aos actuais transmissores completamente digitais, nomeadamente, a baixa eficiência de codificação dos moduladores pulsados e o elevado fator de qualidade combinado com elevados requisitos de adaptabilidade na frequencia do filtro de reconstrução do sinal RF a transmitir. A abordagem seguida baseada em multiplos caminhos polifásicos permitiu desenvolver uma nova arquitetura de transmissão integrada em FPGA que melhora de forma significativa importantes figuras de mérito, tais como a eficiência de codificação e SNR, enquanto mantém a elevada flexibilidade que é necessária para suportar a transmissão de dados multimodo e multicanal