[pt] Este trabalho propõe um receptor multiusuário híbrido para utilização em um sistema DS/CDMA com modulação M-PSK e codificação convolucional. O receptor proposto é composto por um receptor decorrelator ao final do qual é realizada uma decodificação convolucional por grupos. Resultados de desempenho obtidos por simulações para grupos de dois usuários são apresentados e comparados com o desempenho do receptor convencional, formado por um banco de filtros casados, e com o desempenho do receptor decorrelator já proposto na literatura. / [en] This work proposes a hybrid multiuser receiver for a DS/CDMA system employing M-PSK modulation and convolutional coding. The proposed receiver is composed by a decorrelator receiver followed by a group convolutional decoder. Perfomance results obtained by simulation for groups of two users are presented and compared to the performance of a conventional receiver, composed of a bank of matched filters, and to the decorrelator receiver itself.

Please read the abstract in the section 00front of this document / Dissertation (M Eng (Electronic Engineering))--University of Pretoria, 2006. / Electrical, Electronic and Computer Engineering / unrestricted

The main topic of this thesis is integer quadratic programming with applications to problems arising in the areas of automatic control and communication. One of the most widespread modern control methods is Model Predictive Control (MPC). In each sampling time, MPC requires the solution of a Quadratic Programming (QP) problem. To be able to use MPC for large systems, and at high sampling rates, optimization routines tailored for MPC are used. In recent years, the range of application of MPC has been extended to so-called hybrid systems. Hybrid systems are systems where continuous dynamics interact with logic. When this extension is made, binary variables are introduced in the problem. As a consequence, the QP problem has to be replaced by a far more challenging Mixed Integer Quadratic Programming (MIQP) problem, which is known to have a computational complexity which grows exponentially in the number of binary optimization variables. In modern communication systems, multiple users share a so-called multi-access channel. To estimate the information originally sent, a maximum likelihood problem involving binary variables can be solved. The process of simultaneously estimating the information sent by multiple users is called Multiuser Detection (MUD). In this thesis, the problem to efficiently solve MIQP problems originating from MPC and MUD is addressed. Four different algorithms are presented. First, a polynomial complexity preprocessing algorithm for binary quadratic programming problems is presented. By using the algorithm, some, or all, binary variables can be computed efficiently already in the preprocessing phase. In numerical experiments, the algorithm is applied to unconstrained MPC problems with a mixture of real valued and binary valued control signals, and the result shows that the performance gain can be significant compared to solving the problem using branch and bound. The preprocessing algorithm has also been applied to the MUD problem, where simulations have shown that the bit error rate can be significantly reduced compared to using common suboptimal algorithms. Second, an MIQP algorithm tailored for MPC is presented. The algorithm uses a branch and bound method where the relaxed node problems are solved by a dual active set QP algorithm. In this QP algorithm, the KKT systems are solved using Riccati recursions in order to decrease the computational complexity. Simulation results show that both the proposed QP solver and MIQP solver have lower computational complexity compared to corresponding generic solvers. Third, the dual active set QP algorithm is enhanced using ideas from gradient projection methods. The performance of this enhanced algorithm is shown to be comparable with the existing commercial state-of-the-art QP solver \cplex for some random linear MPC problems. Fourth, an algorithm for efficient computation of the search directions in an SDP solver for a proposed alternative SDP relaxation applicable to MPC problems with binary control signals is presented. The SDP relaxation considered has the potential to give a tighter lower bound on the optimal objective function value compared to the QP relaxation that is traditionally used in branch and bound for these problems, and its computational performance is better than the ordinary SDP relaxation for the problem. Furthermore, the tightness of the different relaxations is investigated both theoretically and in numerical experiments. / This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of the Linköping University's products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org. By choosing to view this material, you agree to all provisions of the copyright laws protecting it.

[pt] Esta tese apresenta novas estruturas e algoritmos adaptativos para detecção às cegas de sinais DS-CDMA. São investigados receptores cegos com restrições lineares baseados nas funções custo de mínima variância (CMV) e módulo constante (CCM). Algoritmos adaptativos do tipo Affine-Projection para estimação dos parâmetros do receptor são desenvolvidos e seu desempenho em estado estacionário é analisado. Também são apresentados algoritmos adaptativos para estimação às cegas do canal de comunicações. Em seguida, novas estruturas de canceladores de interferência são propostas. Primeiramente um cancelador de interferência paralelo (PIC) linear baseado na função custo CCM é proposto. Em seguida é desenvolvido um novo esquema não-supervisionado de cancelamento sucessivo de interferência (SIC), baseado no conceito de arbitragem paralela. Por fim, é apresentado um esquema híbrido (HIC) que combina a estrutura SIC com uma estrutura multi- estágio, resultando em melhores estimativas para detecção e desempenho uniforme para os usuários do sistema. / [en] This thesis presents new structures and adaptive algorithms for blind detection of DS-CDMA signals. Linearly constrained minimum variance (CMV) and constant modulus (CCM) receivers are investigated. Blind adaptive Affine- Projection like algorithms for receiver parameter estimation are derived and its steady-state performance is analyzed. Blind adaptive channel estimation algorithms are also presented. This work also proposes new interference cancellation structures. Firstly, a blind linear parallel interference canceller (PIC) based on the CCM cost function is proposed. Secondly, a new non-supervised serial interference canceller (SIC) based on the parallel arbitration concept is developed. Finally, an hybrid interference cancellation scheme (HIC) which combines SIC and multiple PIC stages is presented.

Since direct-sequence code-division multiple-access (DS-CDMA) is an interference-limited random multiple-access scheme, the reduction of co-channel interference with either interference suppression or interference cancellation multiuser receivers and/or power control to prevent detrimental near-far situations is vital for improved performance. Up to date, some contributions investigated randomly-spread asymptotically - large number of users and large bandwidth - large CDMA systems with multiuser receivers and power control via random matrix theoretic and free probability theoretic tools especially over Gaussian and single-path fading channels. As complement within this thesis, we analyze also within the generalized random spreading framework but at finite system sizes and without power control the capacity achievable with linear multichannel multiuser receivers / i.e. RAKE, zero-forcing decorrelator, linear minimum mean-squared error (LMMSE) multiuser receivers, within a single-cell setting over generalized time-varying GWSSUS - Rayleigh/Ricean - fading channels via random matrix theoretic tools. Assuming maximal-ratio combining (MRC) of resolved frequency - multipath - diversity channels due to wideband transmission, the signal-to-interference ratios (SIRs) with multichannel multiuser receivers that set the basis for further derivations are statistically characterized. The information-theoretic ergodic and outage sum-rates spectral efficiencies are then derived and analyzed.

[pt] Este trabalho tem como finalidade estudar o problema da detecção de usuários no canal reverso de sistemas MIMO de larga escala, que são caracterizados pelo elevado número de elementos de transmissão e recepção, com foco na complexidade computacional e no desempenho em termos de taxa de erro destes sistemas. Inicialmente, os algoritmos de detecção da família Likelihood Ascent Search (LAS) são investigados e é desenvolvido um novo algoritmo de detecção, denominado de Random-List Based LAS (RLBLAS), capaz de atingir melhores taxas de erros com menor complexidade computacional do que os demais detectores considerados. Posteriormente, técnicas de detecção e decodificação iterativas (Iterative Detection and Decoding - IDD) em sistemas MIMO foram analisadas de forma a propor uma estratégia IDD de complexidade computacional reduzida a fim de viabilizar a sua aplicação em cenários massivos. Finalmente, o problema da contaminação por pilotos em sistemas MIMO multicelulares de larga escala, um dos principais limitadores do desempenho desse tipo de sistema, é estudado e estratégias de detecção com cooperação parcial entre as estações base componentes do sistema que visam mitigar os efeitos da contaminação por pilotos são propostas. As análises e afirmações realizadas durante a presente tese são sustentadas por resultados de simulações de Monte Carlo dos sistemas de comunicações em diversos cenários distintos, incluindo os casos em que são considerados os efeitos de correlação entre as antenas de transmissão/recepção, os efeitos de sombreamento e os erros de estimação dos estados dos canais de comunicações envolvidos. / [en] This work focuses on the multi-user multi-cellular large-scale MIMO reverse channel detection problem, where the number of transmitting and receiving antenna elements grows to the order of hundreds. In these scenarios, one major issue is the computational complexity of such systems. Therefore, this thesis aims to propose low-complexity techniques with good BER performance for the reverse channel detection of MIMO systems. Initially, the detection algorithms of the Likelihood Ascent Search (LAS) family are investigated and a new LAS based detector is proposed. This new detector, named Random-List Based LAS (RLB-LAS), is capable of achieving better BER with lower complexity then the other considered detectors. Next, iterative detection and decoding (IDD) techniques are analyzed in order to propose an IDD strategy applied to the detection and decoding of the reverse MIMO channel with reduced complexity to make possible its application to massive scenarios. Finally, the pilot contamination problem in multi-cellular large-scale MIMO systems, one of the major bounds on BER performance of these systems, are studied and some cooperative strategies are proposed in order to reduce the effects of this type of impairments. The analysis and statements of this thesis are supported by Monte Carlo simulation results of the considered systems in different scenarios, including the cases where the effects of transmitting and receiving antenna correlation, log-normal shadowing, and the estimation errors on the channel state information acquisition are considered.

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.

WCDMA is typically characterised as a system capable of providing mobile users with data rates up to 2 Mb/s and beyond. It has been termed an ultra high-speed, ultra high-capacity radio technology that will be able to carry a new range of fast, colourful media, such as colour graphics, video, animations, digital audio, Internet and e-mail that consumers will be able to access over their mobiles devices. This current study has researched on the various existing Multiuser detection (MUD) processes or proposals conducted by various research institutions around the world. It has identified the advantages that the past work offers, and it is these advantages that form the basis of the current research into the improvement techniques. The proposed Partial Parallel Pipelined Multiuser Detector (PPPMUD or P3MUD) has come about from two main flavours or directions of research. The first one seeks to promote the Soft Parallel Interference Cancellation technique as an effective bias mitigation technique. This bias occurring in the second stage decision statistics, exhibits a very harmful effect on system Bit Error Rate, (BER), particularly for large system loads. This current study goes further by carefully analysing the Soft Cancellation Factor, SCF behaviour to eventually derive and determine the optimum SCF value which exhibits positive characteristics when varied with the increasing system load (number of users). This optimum SCF value is called the universal SCF or SCFUNV, as it is theoretically supposed to perform favourably under various system loads. A favourable or acceptable performance would be characterised by low observed or measured BER during the system processing stages. A further enhancement to the operational performance of the SCFUNV algorithm is the SCFUNV Compensator, which is basically a compensation mechanism created by modelling the behaviour of the SCF values, and adjusts the SCFUNV depending on the system load, (number of simultaneous users). Thus, the SCFUNV is adaptively adjusted in order to perform acceptably under all load conditions. The second direction of research, as regards improvements in MUD techniques, involves the conventional Bit-Streaming, Pipelined Multiuser Detector. This came about due to the computational complexity as well as matrix inversions which affected earlier asynchronous multiuser detection techniques. This detector has a pipelined architecture which avoids multishot (block-based) detection and instead, processes the bits in a streaming fashion. The architecture consists of a matched filter followed by three stages of parallel interference cancellation, (PIC). This present study extends that research by outlining the advantages of incorporating the soft parallel interference cancellation technique, by way of the universal soft cancellation factor, (SCFUNV), into the conventional pipelined multiuser detector architecture to form the P3MUD architecture, which includes the compensator. The contributions of the proposed P3MUD system is that the observed BER output simulations are promising, with an observed overall decrease in the error rate for the P3MUD process, as compared to the conventional pipelined detection method. This decrease signifies an improvement offered by the proposed (P3MUD) algorithm. Further observed analysis indicates the possibility of decreasing the number of parallel interference cancellation stages from three to at least two, after the matched filter detection stage, without an observable change in system BER. Hence, the proposal of the two-stage P3MUD. / Dissertation (MEng (Electronic))--University of Pretoria, 2006. / Electrical, Electronic and Computer Engineering / unrestricted

Single Carrier Frequency Division Multiple Access (SC-FDMA) is a multiple access transmission scheme that has been adopted in the 4th generation 3GPP Long Term Evolution (LTE) of cellular systems. In fact, its relatively low peak-to-average power ratio (PAPR) makes it ideal for the uplink transmission where the transmit power efficiency is of paramount importance. Multiple access among users is made possible by assigning different users to different sets of non-overlapping subcarriers. With the current LTE specifications, if an SC-FDMA system is operating at its full capacity and a new user requests channel access, the system redistributes the subcarriers in such a way that it can accommodate all of the users. Having less subcarriers for transmission, every user has to increase its modulation order (for example from QPSK to 16QAM) in order to keep the same transmission rate. However, increasing the modulation order is not always possible in practice and may introduce considerable complexity to the system. The technique presented in this thesis report describes a new way of adding more users to an SC-FDMA system by assigning the same sets of subcarriers to different users. The main advantage of this technique is that it allows the system to accommodate more users than conventional SC-FDMA and this corresponds to increasing the spectral efficiency without requiring a higher modulation order or using more bandwidth. During this work, special attentions wee paid to the cases where two and three source signals are being transmitted on the same set of subcarriers, which leads respectively to doubling and tripling the spectral efficiency. Simulation results show that by using the proposed technique, it is possible to add more users to any SC-FDMA system without increasing the bandwidth or the modulation order while keeping the same performance in terms of bit error rate (BER) as the conventional SC-FDMA. This is realized by slightly increasing the energy per bit to noise power spectral density ratio (Eb/N0) at the transmitters.

Single Carrier Frequency Division Multiple Access (SC-FDMA) is a multiple access transmission scheme that has been adopted in the 4th generation 3GPP Long Term Evolution (LTE) of cellular systems. In fact, its relatively low peak-to-average power ratio (PAPR) makes it ideal for the uplink transmission where the transmit power efficiency is of paramount importance. Multiple access among users is made possible by assigning different users to different sets of non-overlapping subcarriers. With the current LTE specifications, if an SC-FDMA system is operating at its full capacity and a new user requests channel access, the system redistributes the subcarriers in such a way that it can accommodate all of the users. Having less subcarriers for transmission, every user has to increase its modulation order (for example from QPSK to 16QAM) in order to keep the same transmission rate. However, increasing the modulation order is not always possible in practice and may introduce considerable complexity to the system. The technique presented in this thesis report describes a new way of adding more users to an SC-FDMA system by assigning the same sets of subcarriers to different users. The main advantage of this technique is that it allows the system to accommodate more users than conventional SC-FDMA and this corresponds to increasing the spectral efficiency without requiring a higher modulation order or using more bandwidth. During this work, special attentions wee paid to the cases where two and three source signals are being transmitted on the same set of subcarriers, which leads respectively to doubling and tripling the spectral efficiency. Simulation results show that by using the proposed technique, it is possible to add more users to any SC-FDMA system without increasing the bandwidth or the modulation order while keeping the same performance in terms of bit error rate (BER) as the conventional SC-FDMA. This is realized by slightly increasing the energy per bit to noise power spectral density ratio (Eb/N0) at the transmitters.