The cyclic prefix (CP) is usually adopted in single carrier frequency domain equalization (SC-FDE) system to avoid inter-block interference (IBI) and inter-symbol interference (ISI) in multipath fading channels. In addition, the use of CP also converts the linear convolution between the transmitted signal and the channel into a circular convolution, leading to significant decrease in receiver equalization. However, the use of CP reduces the bandwidth efficiency. Therefore the SC-FDE system without CP is investigated in this thesis. A number of schemes have been proposed to improve the performance of systems without CP, where both IBI and ICI are dramatically increased. Unfortunately, most of the existing schemes have extremely high computational complexity and are difficult to realize. In this thesis, a novel low-complexity CP reconstruction (CPR) scheme is proposed for interference cancellation, where the successive interference cancellation (SIC) and QR decomposition (QRD) are adopted. In addition, the system performance is further improved by using the fact that the interferences of different symbols are not the same. Simulation experiments are conducted to verify the system performance of the proposed scheme. It is shown that the proposed scheme can effectively reduce the interference, while maintain a low computational complexity.

In the single carrier systems with cyclic-prefix (CP), the use of CP does not only eliminate the inter-block interference (IBI), but also convert linear convolution of the transmitted signal with the channel into circular convolution, which leads to the computation complexity of the frequency domain equalization (FDE) at the receiver is reduced. Unfortunately, the use of CP considerably decreases the bandwidth utilization. In order to increase the bandwidth utilization, the single carrier systems with frequency domain equalization (SC-FDE) is investigated. When FDE is used in a single carrier system without CP, the IBI is induced by the modulated symbols and then the bit-error rate (BER) is increased. To reduce the interference and then improve the system performance, a novel interference cancellation scheme is proposed in this thesis. After FDE, it is shown that interference is induced from the right end of a time domain signal block and most of the interference is located at both ends of an equalized time domain signal block. Based on this observation, the modulated symbols which induce the interference are detected according to the maximum-likelihood (ML) principle and then the interference is regenerated and eliminated. For simplifying the computation complexity, we further propose a successive interference cancellation scheme, which is implemented by using the Viterbi algorithm. The simulation results demonstrate that the proposed scheme improves BER performance significantly in SC-FDE systems. In addition, the proposed architecture has comparable BER performance with the SC-CP systems when the multi-path channel is exponentially decayed.

The work reported in this thesis is divided in to two parts. In the first part, we report a closed-form bit error rate (BER) performance analysis of orthogonal frequency division multiple access (OFDMA) on the uplink in the presence of carrier frequency offsets (CFOs) and/or timing offsets (TOs) of other users with respect to a desired user. We derive BER expressions using probability density function (pdf) and characteristic function approaches, for a Rician faded multi-cluster multi-path channel model that is typical of indoor ultrawideband channels and underwater acoustic channels. Numerical and simulation results show that the BER expressions derived accurately quantify the performance degradation due to non-zero CFOs and TOs. Ultrawideband channels in indoor/industrial environment and underwater acoustic channels are severely delay-spread channels, where the number of multipath components can be of the order of tens to hundreds. In the second part of the thesis, we report low complexity equalization algorithms for cyclic-prefixed single carrier (CPSC)systems that operate on such inter-symbol interference(ISI) channels characterized by large delay spreads. Both single-input single-output and multiple-input multiple-output(MIMO) systems are considered. For these systems, we propose a low complexity graph based equalization carried out in frequency domain. Because of the noise whitening effect that happens for large frame sizes and delay spreads in the frequency domain processing, improved performance compared to time domain processing is shown to be achieved. Since the graph based equalizer is a soft-input soft-output equalizer, iterative techniques(turbo-equalization) between detection and decoding are shown to yield good coded BER performance at low complexities in convolutional and LDPC coded systems. We also study joint decoding of LDPC code and equalization of MIMO-ISI channels using a joint factor graph.

Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier Block Transmissions (SCBT) are located at the two opposite edges of block transmission concept. In this thesis a system which lies between OFDM and SCBT is proposed. The new system, namely Block Transmissions on Orthogonal Carriers (BTOC), can be considered as a hybrid form of OFDM and SCBT. BTOC system is investigated under the redundant filterbank precoders and equalizers framework. Peak to average power ratio (PAPR) of BTOC is formulated and compared with the PAPRs of OFDM and SCBT. Effect of frequency offset for BTOC is investigated and comparison between OFDM, SCBT, and BTOC is presented. Simulation results of Zero Padded OFDM (ZP-OFDM), SCBT, and BTOC are included.

Pour un terminal mobile alimenté sur batterie, le rendement de l’amplificateur de puissance (AP) doit êtreoptimisé. Cette optimisation peut rendre non-linéaire la fonction d’amplification de l’AP. Pour compenser lesdistorsions introduites par le caractère non-linéaire de l’AP, un détecteur numérique fondé sur un modèle deVolterra peut être utilisé. Le comportement de l’AP et le canal étant modélisé par le modèle de Volterra, uneapproche par filtrage de Kalman (FK) permet d’estimer conjointement les noyaux de Volterra et les symbolestransmis. Dans ce travail, nous proposons de traiter cette problématique dans le cadre d’une liaison montantedans un contexte radio intelligente (RI). Dans ce cas, des contraintes supplémentaires doivent être prises encompte. En effet, étant donné que la RI peut changer de bande de fréquence de fonctionnement, les nonlinéaritésde l’AP peuvent varier en fonction du temps. Par conséquent, nous proposons de concevoir une postdistorsionnumérique fondée sur une modélisation par modèles multiples combinant plusieurs estimateurs àbase de FK. Les différents FK permettant de prendre en compte les différentes dynamiques du modèle.Ainsi, les variations temporelles des noyaux de Volterra peuvent être suivies tout en gardant des estimationsprécises lorsque ces noyaux sont statiques. Le cas d’un signal monoporteuse est adressé et validé par desrésultats de simulation. Enfin, la pertinence de l’approche proposée est confirmée par des mesures effectuéessur un AP large bande (300-3000) MHz. / For a battery driven terminal, the power amplifier (PA) efficiency must be optimized. Consequently,non-linearities may appear at the PA output in the transmission chain. To compensatethese distortions, one solution consists in using a digital post-distorter based on aVolterra model of both the PA and the channel and a Kalman filter (KF) based algorithm tojointly estimate the Volterra kernels and the transmitted symbols. Here, we suggest addressingthis issue when dealing with uplink cognitive radio (CR) system. In this case, additionalconstraints must be taken into account. Since the CR terminal may switch from one subbandto another, the PA non-linearities may vary over time. Therefore, we propose to designa digital post-distorter based on an interacting multiple model combining various KF basedestimators using different model parameter dynamics. This makes it possible to track thetime variations of the Volterra kernels while keeping accurate estimates when those parametersare static. Furthermore, the single carrier case is addressed and validated by simulationresults. In addition, the relevance of the proposed approach is confirmed by measurementscarried on a (300-3000) MHz broadband PA.

Actualment estem entrant a una nova era de la informació amb gran demanda de sistemes de comunicació sense fils. Nous serveis com dades i video requereixen transmissions fiables d'alta velocitat, fins i tot en escenaris d'alta mobilitat. A més a més, la dificultat d'assignar el limitat espectre radioelèctric juntament amb la necessitat d'incrementar el temps de vida de les bateries dels terminals mòbils, requereix el diseny de transceptors que usin la potència i l'ampla de banda disponibles de manera eficient. Les comunicacions multiportadora basades en OFDM són capaces de satisfer la majoria d'aquests requeriments. Però, entre altres reptes, reduir la sensibilitat a la amplificació no-lineal és un factor clau durant el diseny. En aquesta tesi doctoral s'analitza la sensibilitat dels sistemes multiportadora basats en OFDM a l'amplificació no-lineal i es consideren formes eficients per superar aquest problema. La tesi s'enfoca principalment al problema de reduir les fluctuacions de l'envolupant del senyal transmès. En aquest sentit es presenta també un estudi de les mètriques de l'envolupant del senyal, PAPR i CM. A més a més, basant-nos en l'anàlisis presentat es proposen noves tècniques per sistemes OFDM i MC-SS. Per MC-SS, també es tracta el diseny d'una tècnica de postprocessament en forma de detector multiusuari per canals no-lineals. / Actualmente estamos entrando en una nueva era de la información donde se da una gran demanda de sistemas de comunicación inalámbricos. Nuevos servicios como datos y vídeo requieren transmisiones fiables de alta velocidad, incluso en escenarios de alta movilidad. Además, la dificultad de asignar el limitado espectro radioeléctrico junto con la necesidad de incrementar el tiempo de vida de las baterías de los terminales móviles, requiere el diseño de transceptores que usen eficientemente la potencia y el ancho de banda disponibles. Las comunicaciones multiportadora basadas en OFDM son capaces de satisfacer la mayoría de dichos requerimientos. Sin embargo, entre otros retos, reducir su sensibilidad a la amplificación no-lineal es un factor clave durante el diseño. En esta tesis se analiza la sensibilidad de los sistemas multiportadora basados en OFDM a la amplificación no-lineal y se consideran formas eficientes para superar dicho problema. La tesis se enfoca principalmente al problema de reducir las fluctuaciones de la envolvente. En este sentido también se presenta un estudio de las métricas de la señal, PAPR y CM. Además, basándonos en el análisis presentado se proponen nuevas técnicas para OFDM y MC-SS. Para MC-SS, también se trata el diseño de un detector multiusuario para canales no-lineales. / We are now facing a new information age with high demand of wireless communication systems. New services such as data and video require achieving reliable high-speed transmissions even in high mobility scenarios. Moreover, the difficulty to allocate so many wireless communication systems in the limited frequency band in addition to the demand for long battery life requires designing spectrum and power efficient transceivers. Multicarrier communications based on OFDM are known to fulfill most of the requirements of such systems. However, among other challenges, reducing the sensitivity to nonlinear amplification has become a design key. In this thesis the sensitivity of OFDM-based multicarrier systems to nonlinear amplification is analyzed and efficient ways to overcome this problem are considered. The focus is mainly on the problem of reducing the envelope fluctuations. Therefore, a study of the signal metrics, namely PAPR and CM, is also presented. From the presented analysis, several new techniques for OFDM and MC-SS are proposed. For MC-SS, the design of a post-processing technique in the form of a multiuser detector for nonlinearly distorted MC-SS symbols is also addressed.

[pt] Este trabalho apresenta uma análise detalhada de uma recente proposta de combinação da transmissão em blocos com portadora única e a técnica CDMA, referida como CS-CDMA (Chip Spread CDMA). Uma característica marcante desta técnica de transmissão é que diferentemente do que ocorre nos sistemas DS-CDMA, a ortogonalidade entre os códigos dos diferentes usuários é mantida mesmo quando a transmissão é feita através de um canal multipercurso seletivo em frequência, suposto invariante no tempo, permitindo assim que os usuários possam ser identicamente desacoplados na recepção. Além desta vantagem, resultados de desempenho indicaram uma significativa superioridade deste sistema sobre o tradicional DS-CDMA. Entretanto comparações existentes, consideraram apenas o up-link, do sistema e adotaram algumas premissas, que incluem, a utilização de códigos ortogonais pelos diversos usuários e a suposição de canais de transmissão invariantes no tempo. O presente trabalho apresenta uma análise detalhada da técnica (CS-CDMA e uma análise comparativa dos sistemas em condições menos favoráveis. Os sistemas operam em ambientes invariante e variante no tempo, com códigos ortogonais e não ortogonais e em dois cenários diferentes, down- link e up-link. Os resultados consideram recepção com equalização no domínio da frequência utilizando equalizadores do tipo ZF (Zero Forcing) e MMSE (Minimum Mean Squared Error). Simulações foram realizadas no intuito de se avaliar o desempenho dos dois sistemas considerados. Curvas de probabilidade de erro foram obtidas e ilustram e comparam tais desempenhos em diferentes situações e cenários de interesse. / [en] This dissertation proposes detailed analysis of a recent combined mechanism for transmission in blocks with a single carrier and CDMA technique known as Chip Spread CDMA (CS-CDMA). An important feature of this transmission technique is that unlike what happens in Direct Sequence (DS-CDMA) systems, the orthogonality between codes of different users is maintained even when the transmission, considered time-invariant, is made through a selective multipath channel frequency, which ideally allows users to be uncoupled in reception. However, existing comparisons only consider the up-link transmission and adopt certain assumptions, which include orthogonal codes for different users and time invariant channels. This technique have shown a significant superiority as compared with the traditional DS-CDMA. A comparison of (CS-CDMA systems in more realistic conditions are presented in this work. Various environments were tested in the presence of BPSK modulation systems, as well as invariant and time-varying transmission. Comparison between scenarios down-link and up-link are also presented. The results consider equalized reception in the frequency domain using the ZF (Zero Forcing) equalizers and MMSE (Minimum Mean Squared Error). Simulations were carried out in order to evaluate the performance of the two systems considered. Error probability curves were obtained to illustrate and compare the performances in different situations and scenarios.

Abstract This thesis concentrates on joint optimization of transmit power allocation and receive filtering in multiuser, multi-antenna communications. Due to the increasing number of wireless devices, the design of energy-efficient communication links is becoming increasingly important. In cellular mobile communications, reducing the average power consumption in uplink transmission is beneficial for users in order to extend battery life and, hence, energy efficiency in general. However, the power consumption of the high power amplifier (HPA) at the transmitter depends on the peak power of the transmission. This thesis focuses on power allocation problems for single-carrier (SC) frequency division multiple access (FDMA) and orthogonal FDMA (OFDMA) transmission assuming iterative reception. The goal in the first scheme presented in this thesis is to reduce the average power consumption by designing a power allocation method that takes into account the convergence properties of an iterative receiver in multiuser uplink communications. The proposed scheme can guarantee that the desired quality of service (QoS) is achieved after a sufficient number of iterations. Reducing the peak-to-average power ratio (PAPR) in any transmission system is beneficial because it allows the use of inexpensive, energy-efficient power amplifiers. The goal in the second scheme presented in this thesis is to control the PAPR of the transmitted signal. Hence, in addition to the QoS constraint, the instantaneous PAPR constraint is derived for SC-FDMA and OFDMA transmission. Moreover, a statistical approach is considered in which the power variance of the transmitted waveform is controlled. The QoS and PAPR constraints are considered jointly and, therefore, the proposed power allocation strategy jointly takes into account the channel quality and the PAPR characteristics of the power amplifier. However, the PAPR constraint can be adopted to any SC-FDMA or OFDMA framework and it is not restricted to the scheme presented in this thesis. The objective of the optimization problems considered throughout the thesis is to minimize the sum power. The majority of the derived constraints are non-convex and therefore, two alternative successive convex approximations (SCAs) are derived for all the non-convex constraints considered. The numerical results show that the proposed power allocation strategies can significantly reduce the average transmission power of users while allowing flexible PAPR control. Hence, the proposed methods can be used to extend battery life for users and especially improve the QoS at the cell edges. / Tiivistelmä Väitöskirjassa tutkitaan lähettimessä tapahtuvan tehoallokoinnin sekä vastaanottimessa tapahtuvan signaalin suodatuksen yhteisoptimointia monikäyttöön suunnatussa langattomassa moniantennikommunikaatiossa. Langattomien laitteiden lukumäärän kasvaessa energiatehokkuuden merkitys tiedonsiirtolinkkien suunnittelussa korostuu. Soluihin perustuvassa langattomassa tietoliikenteessä keskimääräisen tehonkulutuksen pienentäminen ylälinkkilähetyksessä (käyttäjältä tukiasemaan) on tärkeää käyttäjän kannalta, sillä se pidentää laitteen akun kestoa. Lähettimen tehovahvistimen (high power amplifier (HPA)) tehonkulutus on kuitenkin verrannollinen lähetyksen huipputehoon. Väitöskirjassa luodaaan uusia menetelmiä sekä vertaillaan tehoallokointia yhden kantoaallon taajuustason monikäyttöön (single carrier frequency division multiple access (SC-FDMA)) ja ortogonaalisen taajuustason monikäyttöön (orthogonal FDMA (OFDMA)) perustuvissa lähetysteknologioissa. Työn ensimmäisessä osiossa tavoitteena on keskimääräisen tehonkulutuksen pienentäminen monen käyttäjän ylälinkkikommunikaatiossa suunnittelemalla tehoallokointimenetelmä, joka ottaa huomioon iteratiivisen vastaanottimen konvergenssiominaisuudet. Työssä ehdotettu menetelmä takaa vastaanotetun informaation halutun laadun (quality of service (QoS)) riittävän monen vastaanottimessa tehdyn iteraation jälkeen. Huipputehon ja keskitehon suhteen (peak to average power ratio (PAPR)) pienentäminen missä tahansa lähetyksessä on hyödyllistä, sillä sen ansiosta voidaan käyttää energiatehokkaampia ja halvempia tehovahvistimia. Työn jälkimmäisessä osiossa tavoitteena on kontrolloida lähetetyn signaalin huipputehon ja keskitehon suhdetta. Työn ensimmäisessä osiossa esitetyn QoS-rajoitteen lisäksi tehoallokointia rajoitetaan symbolisekvenssikohtaisella PAPR-rajoitteella SCFDMA- ja OFDMA-lähetyksessä. Lisäksi esitetään tilastollinen menetelmä, jossa rajoitetaan lähetetyn signaalin tehon varianssia. Kun käytetään yhtäaikaisesti QoS- ja PAPR-rajoitteita, voidaan tiedonsiirtokanavaan suunnitella optimaalinen tehoallokointi ottaen huomioon tehovahvistimen epälineaarisuudet. Työssä esitetty PAPR-rajoite on kuitenkin geneerinen, ja se voidaan sovittaa mihin tahansa SCFDMA- tai OFDMA- optimointikehykseen. Työssä esitettävien optimointiongelmien tavoitteena on käyttäjien summatehon minimointi. Suurin osa työssä esiintyvistä ongelmista on ei-konvekseja, joten siinä esitetään kaksi vaihtoehtoista peräkkäinen konveksi approksimaatio (successive convex approximation (SCA)) -menetelmää kaikille ei-konvekseille rajoitteille. Numeeriset tulokset osoittavat, että esitetyt tehoallokointimenetelmät pienentävät merkittävästi keskimääräistä tehonkulutusta mahdollistaen lisäksi adaptiivisen PAPR-kontrolloinnin. Väitöskirjassa esitettyjen menetelmien avulla voidaan pidentää mobiilikäyttäjien akun kestoa sekä erityisesti parantaa solun reunakäyttäjien palvelun laatua.

[pt] Além de apresentar uma visão unificada das técnicas de transmissão digital em blocos e suas combinações com a técnica de múltiplo acesso CDMA (Code Division Multiple Access), esta dissertação examina uma nova possível técnica de transmissão para sistemas com múltiplo acesso, aqui chamada CS-CDMA (Chip-Spread Code Division Multiple Access), combinada com as técnicas de transmissão em blocos, em portadora única (SC-Single Carrier ), gerando o esquema SC CS-CDMA, e em múltiplas portadoras OFDM (Orthogonal Frequency Division Multiplexing), gerando o esquema MC CSCDMA. Comparações de desempenho com os sistemas de transmissão por blocos CDMA de portadora única conhecido como SC CDMA (Single Carrier Code Division Multiple Access) e de múltiplas portadoras conhecido como MC CDMA (Multi Carrier Code Division Multiple Access) são apresentadas. Uma característica marcante da técnica de transmissão enfocada é que, diferentemente do que ocorre nos sistemas CDMA, a ortogonalidade entre os códigos dos diferentes usuários é mantida mesmo quando a transmissão é feita atravéss de um canal multipercurso seletivo na frequência, permitindo assim que os usuários possam ser idealmente desacoplados na recepção. Além desta vantagem, e muito devido a ela, os resultados de desempenho, obtidos via simulação, indicam uma significativa superioridade do sistema proposto CSCDMA, quando os sinais são transmitidos em portadora única, sobre o tradicional CDMA. Os resultados consideram recepção com equalização no domínio da frequéncia utilizando equalizadores do tipo ZF (Zero Forcing) e MMSE (Minimum Mean Squared Error). / [en] In addition to presenting a unified view of digital blocks transmission technologies and their combinations with the technique of multiple access CDMA (Code Division Multiple Access), this dissertation addresses a new possible transmission technique for multiple access sytem, called here CS -CDMA (Chip-Spread Code Division Multiple Access), combined with blocks transmissions techniques, in single carrier (SC- Single Carrier ), generating scheme SC CS-CDMA, and in multi carrier OFDM (Ortogonal Frequency Division Multiplexing), generating escheme MC CS-CDMA. Performance comparisons with the traditional SC CDMA (Single Carrier Code Division Multiple Access) and MC CDMA (Multi Carrier Code Division Multiple Access) block transmission systemare presented. A key feature of the proposed transmission technique is that, differently from what occurs with CDMA, the original code orthogonality between users is preserved despite propagation through multipath frequency selective channels, thus ideally allowing the decoupling of the signals at the receiver side. Beyond this advantage, and mostly due to it, the performance results indicate a clear superiority of the proposed system CS-CDMA, over the traditional CDMA, when signals are transmitted in single carrier. The simulation results consider frequency domain ZF (Zero Forcing) and MMSE (Minimum Mean Squared Error).

Multiuser and cooperative orthogonal frequency division multiplexing(OFDM) systems are being actively researched and adopted in wireless standards, owing to their advantages of robustness to multipath fading, modularity, and ability to achieve high data rates. In OFDM based systems, perfect frequency and timing synchronization is essential to maintain orthogonality among the subcarriers at the receiver. In multiuser OFDM on the uplink, timing offsets (TOs) and/or carrier frequency offsets (CFOs) of different users, caused due to path delay differences between different users, Doppler and/or poor oscillator alignment, can destroy orthogonality among subcarriers at the receiver. This results in multiuser interference (MUI)and consequent performance degradation. In this thesis, we are concerned with the analysis and mitigation of the effect of large CFOs and TOs in multiuser OFDM systems, including uplink orthogonal frequency division multiple access (OFDMA),uplink single-carrier frequency division multiple access(SC-FDMA), and cooperative OFDM. Uplink OFDMA: In the first part of this thesis, we analytically quantify the effect of large CFOs and TOs on the signal-to-interference plus noise ratio(SINR) and uncoded bit error rate(BER) performance of uplink OFDMA on Rayleigh and Rician fading channels, and show analytical results to closely match with simulation results. Such an SINR/BER analysis for uplink OFDMA in the presence of both large CFOs as well as TOs has not been reported before. We also propose interference cancelling(IC) receivers to mitigate the performance degradation caused due to large CFOs and TOs of different users. SC-FDMA versus OFDMA: An issue with uplink OFDMA is its high peak-to-average power ratio(PAPR).Uplink SC-FDMA is proposed in the standards as a good low-PAPR alternative to uplink OFDMA; e.g., SC-FDMA has been adopted in the uplink of 3GPP LTE. A comparative investigation of uplink SC-FDMA and OFDMA from a sensitivity to large CFOs and TOs view point has not been reported in the literature. Consequently, in the second part of the thesis, we carry out a comparative study of the sensitivity of SC-FDMA and OFDMA schemes to large CFOs and TOs of different users on the uplink. Our results show that while SC-FDMA achieves better performance due to its inherent frequency diversity advantage compared to OFDMA in the case of perfect synchronization, its performance can get worse than that of OFDMA in the presence of large CFOs and TOs. We further show that use of low-complexity multistage IC techniques, with the knowledge of CFOs and TOs of different users at the receiver, can restore the performance advantage of SC-FDMA over OFDMA. Cooperative OFDM: Cooperative OFDM is becoming popular because of its ability to provide spatial diversity in systems where each node has only one antenna. In most studies on cooperative communications, perfect time synchronization among cooperating nodes is assumed. This implies that the transmissions from different cooperating nodes reach the destination receiver in orthogonal time slots. In practice, however, due to imperfect time synchronization, orthogonality among different nodes’ signals at the destination receiver can be lost, causing inter-symbol interference(ISI).In the third part of the thesis, we investigate cooperative OFDM communications using amplify-and-forward(AF) protocol at the relay, in the presence of imperfect timing synchronization. We derive analytical expressions for the ISI as function of timing offset for cooperative OFDM with AF protocol, and propose an IC receiver to mitigate the effects of timing offset induced ISI.