18 research outputs found

    Decentralized random energy allocation for massive non-orthogonal code-division multiple access

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    © 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This work studies the spectral efficiency achievable when a very large number of terminals are connected simultaneously to a central node (uplink) through independent and identically-distributed flat-fading channels. Assuming that terminals only have statistical channel state information (CSI), the optimum random transmitted-energy allocation is formulated considering a non-orthogonal direct-sequence code-division multiple access (DS-CDMA) where all users transmit using the same modulation and error correcting code and the receiver implements successive interference cancellation (SIC). Focusing on low-power terminals, optimization is carried out by imposing constraints on both the average and peak peruser transmitted energy. Simulations have revealed that a limited number of random energy levels, whose number is determined by the channel power gain variance, is sufficient to achieve approximately the maximum spectral efficiency that would be obtained under direct optimization of the received energy profile.Peer ReviewedPostprint (author's final draft

    Cooperative diversity in CDMA networks

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    Spatial diversity is one of the well known diversity methods used in combating fading channels. Recently, cooperative diversity has been widely studied in literature as a spatial diversity technique. Different from multiple-input multiple-output (MIMO) systems, each user in the cooperative network is employed with a single transmit/receive antenna. In this thesis, we propose a cooperative diversity technique for asynchronous direct sequence code division multiple access (D8-CDMA) over frequency selective slow fading environment. First we assume the single cooperation relay case, where the bit-error-rate performance of the system is studied for both cases of perfect and imperfect inter-user channel (user-relay link). In order to mitigate the multi-access interference (MAI), decorrelator multiuser detectors are introduced at both relay and base station sides. Its effect on performance is studied and compared to the performance of the conventional matched filter receiver. Additionally, the performance of the system is studied and compared for different multi-path diversity scenarios in the inter-user and uplink channel. Furthermore, a coded multi-relay cooperation technique is proposed, where channel coding is introduced to minimize errors over the inter-user channel. All users are embedded with convolutional encoder and a Viterbi decoder. We study the performance of the coded system for different number of cooperating relays and over different multi-path diversity scenarios. Both simulation and analytical results are compared. Finally, we conclude that for a communication network to benefit from the cooperation diversity technique, a reliable communication link between active users and the cooperating relays should be secured (inter-user channel). We show that for an active user cooperating with V relays over a P -path frequency-selective fading channel, the expected diversity degree is P ( V +1

    High Capacity CDMA and Collaborative Techniques

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    The thesis investigates new approaches to increase the user capacity and improve the error performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation and collaborative spreading and space diversity techniques. Collaborative Coding Multiple Access (CCMA) is also investigated as a separate technique and combined with CDMA. The advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the uplink and downlink are proposed and evaluated. Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM) property of digital signals to blindly suppress interference during the despreading process and obtain amplitude estimation with minimum mean squared error for use in cancellation stages. Two new blind adaptive receiver designs employing successive and parallel interference cancellation architectures using the CM algorithm (CMA) referred to as ‘CMA-SIC’ and ‘BA-PIC’, respectively, are presented. These techniques have shown to offer near single user performance for large number of users. It is shown to increase the user capacity by approximately two fold compared with conventional IC receivers. The spectral efficiency analysis of the techniques based on output signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore, an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI without any knowledge of channel amplitudes and allows large number of users compared with equal gain and maximum ratio combining techniques normally used in practice. New user collaborative schemes are proposed and analysed theoretically and by simulations in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First, a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA are designed and analysed. Next, a new user collaborative scheme with successive interference cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce MAI and achieve improved diversity. To further improve the performance of C-SIC under high system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed. It is shown to minimize the residual MAI, leading to improved user capacity and a more robust system. It is known that collaborative diversity schemes incur loss in throughput due to the need of orthogonal time/frequency slots for relaying source’s data. To address this problem, finally a novel near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time encoding methods. At the receiver collaborative joint detection is performed to separate each paired users’ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth as inter-user channel SNR becomes high. A novel approach of ‘User Collaboration’ is introduced to increase the user capacity of CDMA for both the downlink and uplink. First, collaborative group spreading technique for the downlink of overloaded CDMA system is introduced. It allows the sharing of the same single spreading sequence for more than one user belonging to the same group. This technique is referred to as Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative coding is used for each group to form a composite codeword signal of the users and then a single orthogonal sequence is used for the group. At each user’s receiver, decoding of composite codeword is carried out to extract the user’s own information while maintaining a high SINR performance. To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions, Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative coding multiple access and space-time coding principles. A new scheme for uplink of CDMA using the ‘User Collaboration’ approach, referred to as CS-CDMA-UL is presented next. When users’ channels are independent (uncorrelated), significantly higher user capacity can be achieved by grouping multiple users to share the same spreading sequence and performing MUD on per group basis followed by a low complexity ML decoding at the receiver. This approach has shown to support much higher number of users than the available sequences while also maintaining the low receiver complexity. For improved performance under highly correlated channel conditions, T-user collaborative coding is also investigated within the CS-CDMA-UL system

    Reduced Complexity Sequential Monte Carlo Algorithms for Blind Receivers

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    Monte Carlo algorithms can be used to estimate the state of a system given relative observations. In this dissertation, these algorithms are applied to physical layer communications system models to estimate channel state information, to obtain soft information about transmitted symbols or multiple access interference, or to obtain estimates of all of these by joint estimation. Initially, we develop and analyze a multiple access technique utilizing mutually orthogonal complementary sets (MOCS) of sequences. These codes deliberately introduce inter-chip interference, which is naturally eliminated during processing at the receiver. However, channel impairments can destroy their orthogonality properties and additional processing becomes necessary. We utilize Monte Carlo algorithms to perform joint channel and symbol estimation for systems utilizing MOCS sequences as spreading codes. We apply Rao-Blackwellization to reduce the required number of particles. However, dense signaling constellations, multiuser environments, and the interchannel interference introduced by the spreading codes all increase the dimensionality of the symbol state space significantly. A full maximum likelihood solution is computationally expensive and generally not practical. However, obtaining the optimum solution is critical, and looking at only a part of the symbol space is generally not a good solution. We have sought algorithms that would guarantee that the correct transmitted symbol is considered, while only sampling a portion of the full symbol space. The performance of the proposed method is comparable to the Maximum Likelihood (ML) algorithm. While the computational complexity of ML increases exponentially with the dimensionality of the problem, the complexity of our approach increases only quadratically. Markovian structures such as the one imposed by MOCS spreading sequences can be seen in other physical layer structures as well. We have applied this partitioning approach with some modification to blind equalization of frequency selective fading channel and to multiple-input multiple output receivers that track channel changes. Additionally, we develop a method that obtains a metric for quantifying the convergence rate of Monte Carlo algorithms. Our approach yields an eigenvalue based method that is useful in identifying sources of slow convergence and estimation inaccuracy.Ph.D.Committee Chair: Douglas B. Williams; Committee Member: Brani Vidakovic; Committee Member: G. Tong zhou; Committee Member: Gordon Stuber; Committee Member: James H. McClella

    Performance evaluation of detection algorithms for MOMI OFDM systems

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    Includes abstract.Includes bibliographical references (leaves 79-86).Introduction of Multi Input Multi Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) as the base air interface method for Next Generation Network (NGN) will face a number of challenges from hostile channel conditions to interference from other users. This would result in an increase of detection complexity required for mobile systems. Complex detection will reduce the battery life of mobile devices because of the many calculations that have to be done to decode the signal. Very powerful detection algorithms exist but they introduce high detection complexity. NGN will employ different MIMO systems, but this research will consider spatially multiplexed MIMO which is used to improve the data rate and network capacity. In NGN different multi access modulation schemes will be used for uplink and downlink but they both have OFDM as the basic building block. In this work performance of MIMO OFDM is investigated in different channels models and detection algorithms. A low complexity detection scheme is proposed in this research to improve performance of MIMO OFDM. The proposed detection scheme is investigated for different channel characteristics. Realistic channels conditions are introduced to evaluate the performance of the proposed detection scheme. We analyze weaknesses of existing linear detectors and the enhancements that can be done to improve their performance in different channel conditions. Performance of the detectors is evaluated by comparison of Bit Error Rate (BER) and Symbol Error Rate (SER) against signal to noise ratio (SNR). This thesis proposes a detector which shows a higher complexity than linear detectors but less than Maximum Likelihood Detector (MLD). The proposed detector shows significant BER improvement in all channel conditions. For better performance evaluation this work also investigates performance of MIMO OFDM detectors in realistic channels like Kronecker and Weichselberger channel models

    A random access MAC protocol for MPR satellite networks

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    Dissertação apresentada para obtenção do Grau de Mestre em Engenharia Electrotécnica e de Computadores, pela Universidade Nova de Lisboa, Faculdade de Ciências e TecnologiaRandom access approaches for Low Earth Orbit (LEO) satellite networks are usually incompatible with the Quality of Service (QoS) requirements of multimedia tra c, especially when hand-held devices must operate with very low power. Cross-Layered optimization architectures, combined with Multipacket Reception (MPR)schemes are a good choice to enhance the overall performance of a wireless system. Hybrid Network-assisted Diversity Multiple Access (H-NDMA) protocol, exhibits high energy e ciency, with MPR capability, but its use with satellites is limited by the high round trip time. This protocol was adapted to satellites, in Satellite-NDMA, but it required a pre-reservation mechanism that introduces a signi cant delay. This dissertation proposes a random access protocol that uses H-NDMA, for Low Earth Orbit (LEO) satellite networks, named Satellite Random-NDMA (SR-NDMA). The protocol addresses the problem inherent to satellite networks (large round trip time and signi cant energy consumption) de ning a hybrid approach with an initial random access plus possible additional scheduled retransmissions. An MPR receiver combines the multiple copies received, gradually reducing the error rate. Analytical performance models are proposed for the throughput, delay, jitter and energy e ciency considering nite queues at the terminals. It is also addressed the energy e ciency optimization, where the system parameters are calculated to guarantee the QoS requirements. The proposed system's performance is evaluated for a Single-Carrier with Frequency Domain Equalization (SC-FDE) receiver. Results show that the proposed system is energy e cient and can provide enough QoS to support services such as video telephony

    Allocation designs for massive multiple access with interference cancellation

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    In the transition towards the next generation of wireless technology systems, the increasing number of devices curbs the potential of current wireless networks to cope with such increases in network density. Wireless communications via satellite constitute a cost effective option to achieve high transmission reliability in remote areas or to create resilient networks to be used in emergency situations. To counterbalance the growing network density, one of the main goals in the uplink is to increase the spectral efficiency of the network. By working on the application of non-orthogonal multiple access and the exploitation of the collision domain through interference cancellation, this dissertation tackles the problem of massive multiple access. A consensual scheme that meets the main goal and the aim of reducing the interaction between devices and the satellite in the control plane is Enhanced Spread Spectrum ALOHA, which combines spreading-based short-packet transmissions with successive interference cancellation (SIC) on the receiver's side. This combination opens up several design avenues in terms of energy and code allocation to users when a certain amount of channel state information is available to them. Motivated by this scheme, this thesis studies the best allocation strategies when the SIC receiver operates nonideally: firstly, it investigates a system model for a receiver that, inspired by the demodulator adopted in the Enhanced Spread Spectrum ALOHA system, deals with the problems of user ordering and iterative decoding with short packets; and secondly, it delves into the user-asymptotic regime and the application of the calculus of variations to derive the stationary point equations corresponding to the optimal allocation rules.The first part of this thesis investigates the impact of nonideal decoding and imperfect cancellation on the first iteration of a SIC receiver aided by redundancy-check error control. The system model characterises both non-idealities using known functions of the signal-to-interference-plus-noise ratio. The propagation of packet decoding success/failure events throughout the stages of the receiver is circumvented in the user-asymptotic regime, since the model takes a deterministic form. The asymptotically optimal energy and rate allocation is studied for a wide variety of cases. The second part of this thesis investigates an iterative SIC receiver and extends the allocation designs derived previously to iterations beyond the first. The derivation of a system model is challenging, since each iteration of the receiver operates with memory with respect to the previous ones, and due to the fact that the decoding operations for the same user in different iterations are statistically dependent. This thesis motivates and states a system model that solves said difficulties by adding minimal complexity to the one adopted previously. The user-asymptotic regime is investigated to reveal mathematical forms to the above model that allow for a thorough understanding of the adopted receiver. Finally, the chapter exploits the user-asymptotic model and conducts research to designing smooth allocation functions. The third part of this thesis studies the user-ordering problem for a SIC receiver to which the strengths received from all users are unknown. The thesis derives an accurate system model for a large-user SIC receiver, which proceeds to order users after estimating their symbol energies at the initial stage through preamble cross-correlations. Analytical findings are determined in the user-asymptotic regime. The asymptotically optimal energy allocation is shown to obey, in contrast to the practically exponential user-energy distributions obtained before, a piecewise constant function; fact that entails great computational advantages of its application.En la transició cap a la pròxima generació de sistemes tecnològics sense fils, el creixent nombre de dispositius frena el potencial de les xarxes sense fils actuals per fer front a tal augment en la densitat de xarxa. Les comunicacions sense fils via satèl·lit constitueixen una opció rentable per assolir una fiabilitat de transmissió alta en zones remotes o per crear xarxes que puguin ser utilitzades en situacions d'emergència. Per contrarestar la creixent densitat de xarxa, un dels objectius principals en l'enllaç ascendent és augmentar l'eficiència espectral d'aquesta. Aquesta tesi aborda el problema d'accés múltiple massiu combinant l'aplicació de tècniques d'accés múltiple no ortogonal amb esquemes de cancel·lació d’interferència. Un esquema consensuat que acompleix amb l’objectiu principal i amb la fita de reduir la interacció entre dispositius i satèl·lit en el pla de control és Enhanced Spread Spectrum ALOHA, que combina transmissions de paquets curts basades en l'eixamplament del senyal amb la cancel·lació successiva d'interferències (SIC) en recepció. Aquesta combinació obre diverses vies per l'assignació d'energia i codi als diferents usuaris quan aquests disposen d’informació sobre l'estat del canal. Motivat per l'esquema anterior, aquesta tesi estudia les millors estratègies d'assignació quan s'adopta un receptor SIC no ideal: en primer lloc, investiga un model de sistema per un receptor SIC que, inspirat en el desmodulador adoptat en el sistema Enhanced Spread Spectrum ALOHA, aborda els problemes d'ordenació d'usuaris i de descodificació iterativa amb paquets curts; i, en segon lloc, s’endinsa en el règim asimptòtic d'usuaris i en l'aplicació del càlcul de variacions per derivar les equacions de punt estacionari corresponents a les funcions d'assignació òptimes. La primera part d'aquesta tesi investiga l'impacte de la descodificació no ideal i de la cancel·lació imperfecta en la primera iteració d'un receptor SIC assistit per control d'errors. El model de sistema proposat caracteritza ambdues no idealitats fent ús de funcions conegudes de la relació senyal-a-soroll-més-interferència. La propagació dels esdeveniments d'èxit/fracàs en la descodificació de paquets al llarg de les etapes del receptor s'aborda en el règim asimptòtic d'usuaris, ja que el model pren forma determinista. Les funcions d'assignació s'estudien en el règim asimptòtic d'usuaris per varis casos. La segona part de la tesi investiga un receptor SIC iteratiu i estén les assignacions derivades en el capítol anterior per a iteracions del SIC més enllà de la primera. La derivació d'un model de sistema suposa un repte, ja que cada iteració del receptor opera amb memòria respecte a iteracions anteriors i degut a que les operacions de descodificació per a un mateix usuari en iteracions diferents són estadísticament dependents. Es proposa un model de sistema que resol tals dificultats afegint complexitat mínima al model adoptat anteriorment. S'investiga el règim asimptòtic d'usuaris amb l'objectiu d’evidenciar expressions matemàtiques del model que permetin la completa comprensió del receptor adoptat. Per últim, es dissenyen funcions d'assignació contínuament diferenciables fent ús del model asimptòtic anterior. La tercera i última part d'aquesta tesi estudia el problema d'ordenació d'usuaris aplicat a un receptor SIC que desconeix les potències rebudes de tots ells. Es deriva un model de sistema per un receptor que gestiona nombrosos usuaris i els ordena després d'estimar les energies de tots ells en l'etapa inicial mitjançant correlacions de preamble. Els resultats analítics s’obtenen en el règim asimptòtic d'usuaris. Es demostra que, contràriament a les distribucions pràcticament exponencials obtingudes anteriorment, l'assignació d'energia òptima derivada per a infinits usuaris presenta una estructura constant a trossos; fet que comporta grans avantatges computacionals en la seva aplicació.En la transición hacia la próxima generación de sistemas tecnológicos inalámbricos, el creciente número de dispositivos frena el potencial de las redes inalámbricas actuales para hacer frente a esos aumentos en la densidad de red. Impulsadas por las innovaciones en tecnología satelital, las comunicaciones inalámbricas vía satélite constituyen una opción rentable para lograr una alta fiabilidad de transmisión en zonas remotas o para crear redes reservadas para situaciones de emergencia. Para contrarrestar la creciente densidad de la red, uno de los objetivos principales en el enlace ascendente es aumentar la eficiencia espectral de la misma. En favor de este objetivo, se identifican tres técnicas no excluyentes: (i) la aplicación de técnicas de acceso múltiple no ortogonal, para hacer frente a la limitada disponibilidad de recursos ortogonales requeridos en el acceso múltiple convencional, (ii) la explotación del dominio de colisión por el receptor, mediante la cancelación de interferencias, y (iii) la utilización de satélites multihaz, que, usando la tecnología multiantena, permiten una reutilización más eficiente del dominio espacial. Esta tesis aborda el problema de acceso múltiple masivo trabajando en los dos primeros puntos. Un esquema consensuado que cumple con el objetivo principal y con el fin de reducir la interacción entre los dispositivos y el satélite en el plano de control es Enhanced Spread Spectrum ALOHA, que combina transmisiones de paquetes cortos basadas en el ensanchamiento de la señal con la cancelación sucesiva de interferencias (SIC) en recepción. Esta combinación abre diversas vías para la asignación de energía y código a los usuarios cuando estos disponen de cierta información sobre el estado del canal. Motivado por el esquema anterior, esta tesis reexamina resultados previos bajo análisis teóricos de capacidad y cancelación perfecta, y estudia las mejores estrategias de asignación cuando el receptor SIC opera de forma no ideal. Los análisis anteriores se amplían en dos frentes: en primer lugar, adoptando políticas de decodificación y cancelación adaptadas para paquetes cortos; y, en segundo lugar, explorando el desequilibrio de energía, tasa de transmisión y fiabilidad. Con respecto al primer punto, esta tesis investiga un modelo de sistema para un receptor SIC que, inspirado en el demodulador adoptado en el sistema Enhanced Spread Spectrum ALOHA, aborda los problemas de ordenación de usuarios y decodificación iterativa con paquetes cortos. En cuanto al segundo punto, esta tesis se adentra en el régimen asintótico de usuarios y en la aplicación del cálculo de variaciones para derivar las ecuaciones de punto estacionario correspondientes a las funciones de asignación óptimas. Una de las principales contribuciones de esta tesis es el descubrimiento de funciones discontinuas (continuamente diferenciables a trozos) como una clase de distribuciones de energía ordenada para maximizar la eficiencia espectral; un enfoque que ha demostrado ser abrumadoramente exitoso. En concreto, el modelo derivado en la presente tesis incorpora, progresivamente y a lo largo de tres capítulos independientes, aspectos prácticos del cancelador de interferencias adoptado: 1. La primera parte de esta tesis investiga el impacto de la decodificación no ideal y de la cancelación imperfecta en la primera iteración de un receptor SIC asistido por control de errores. El modelo de sistema caracteriza ambas no idealidades utilizando funciones conocidas de la relación señal-a-ruido-más-interferencia (SINR) bajo la suposición de interferencia gaussiana: las funciones tasa de error de paquete (PER) y energía residual. La propagación de los eventos de éxito/fracaso en la decodificación de paquetes a lo largo de las etapas del receptor SIC se sortea en el régimen asintótico de usuarios, puesto que el modelo de sistema adopta expresiones deterministas. La asignación de energía y código se estudia en el régimen asintótico de usuarios para una amplia variedad de casos, incluyendo conjuntos formados por un número finito o infinito de esquemas de modulación y corrección de errores para paquetes de longitud finita e infinita. 2. La segunda parte de esta tesis investiga un receptor SIC iterativo y extiende las asignaciones derivadas anteriormente para iteraciones del SIC más allá de la primera. La derivación de un modelo para tal sistema supone un reto, ya que cada iteración del receptor opera con memoria respecto a las anteriores y porque las operaciones de decodificación para un mismo usuario en distintas iteraciones son estadísticamente dependientes. Esta tesis propone justificadamente un modelo de sistema que resuelve dichas dificultades añadiendo complejidad mínima al adoptado con anterioridad. En concreto, el modelo usa funciones PER multivariable, cuyos argumentos corresponden a las SINRs que experimenta un usuario a lo largo de las iteraciones del receptor, y define biyecciones para relacionar los índices de los usuarios que permanecen decodificados sin éxito en cada iteración. Se investiga el régimen asintótico de usuarios para revelar expresiones matemáticas del modelo anterior que permitan un completo entendimiento del receptor adoptado. Por último, se investiga el diseño de funciones de asignación continuamente diferenciables con extremos libres haciendo uso del modelo asintótico anterior. 3. La tercera y última parte de esta tesis estudia el problema de ordenación de usuarios en un receptor SIC que desconoce las potencias recibidas de todos ellos. La tesis deriva un modelo de sistema para un receptor SIC que gestiona un gran número de usuarios y los ordena tras estimar sus energías en la etapa inicial mediante correlaciones de preámbulo. En el régimen asintótico de usuarios, se obtienen resultados analíticos en los que el rendimiento del sistema se rige por un kernel conocido. Se demuestra que, contrariamente a las distribuciones prácticamente exponenciales obtenidas anteriormente, la asignación óptima de energía derivada para un número infinito de usuarios obedece una función constante a trozos; hecho que conlleva grandes ventajas computacionales en su aplicación.Postprint (published version

    Agile wireless transmission strategies

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    Multi-Antenna Techniques for Next Generation Cellular Communications

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    Future cellular communications are expected to offer substantial improvements for the pre- existing mobile services with higher data rates and lower latency as well as pioneer new types of applications that must comply with strict demands from a wider range of user types. All of these tasks require utmost efficiency in the use of spectral resources. Deploying multiple antennas introduces an additional signal dimension to wireless data transmissions, which provides a significant alternative solution against the plateauing capacity issue of the limited available spectrum. Multi-antenna techniques and the associated key enabling technologies possess unquestionable potential to play a key role in the evolution of next generation cellular systems. Spectral efficiency can be improved on downlink by concurrently serving multiple users with high-rate data connections on shared resources. In this thesis optimized multi-user multi-input multi-output (MIMO) transmissions are investigated on downlink from both filter design and resource allocation/assignment points of view. Regarding filter design, a joint baseband processing method is proposed specifically for high signal-to-noise ratio (SNR) conditions, where the necessary signaling overhead can be compensated for. Regarding resource scheduling, greedy- and genetic-based algorithms are proposed that demand lower complexity with large number of resource blocks relative to prior implementations. Channel estimation techniques are investigated for massive MIMO technology. In case of channel reciprocity, this thesis proposes an overhead reduction scheme for the signaling of user channel state information (CSI) feedback during a relative antenna calibration. In addition, a multi-cell coordination method is proposed for subspace-based blind estimators on uplink, which can be implicitly translated to downlink CSI in the presence of ideal reciprocity. Regarding non-reciprocal channels, a novel estimation technique is proposed based on reconstructing full downlink CSI from a select number of dominant propagation paths. The proposed method offers drastic compressions in user feedback reports and requires much simpler downlink training processes. Full-duplex technology can provide up to twice the spectral efficiency of conventional resource divisions. This thesis considers a full-duplex two-hop link with a MIMO relay and investigates mitigation techniques against the inherent loop-interference. Spatial-domain suppression schemes are developed for the optimization of full-duplex MIMO relaying in a coverage extension scenario on downlink. The proposed methods are demonstrated to generate data rates that closely approximate their global bounds

    Iterative Detection for Overloaded Multiuser MIMO OFDM Systems

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    Inspired by multiuser detection (MUD) and the ‘Turbo principle’, this thesis deals with iterative interference cancellation (IIC) in overloaded multiuser multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Linear detection schemes, such as zero forcing (ZF) and minimum mean square error (MMSE) cannot be used for the overloaded system because of the rank deficiency of channel matrix, while the optimal approach, the maximum likelihood (ML) detection has high computational complexity. In this thesis, an iterative interference cancellation (IIC) multiuser detection scheme with matched filter and convolutional codes is considered. The main idea of this combination is a low complexity receiver. Parallel interference cancellation (PIC) is employed to improve the multiuser receiver performance for overloaded systems. A log-likelihood ratio (LLR) converter is proposed to further improve the reliability of the soft value converted from the output of the matched filter. Simulation results show that the bit error rate (BER) performance of this method is close to the optimal approach for a two user system. However, for the four user or more user system, it has an error floor of the BER performance. For this case, a channel selection scheme is proposed to distinguish whether the channel is good or bad by using the mutual information based on the extrinsic information transfer (EXIT) chart. The mutual information can be predicted in a look-up table which greatly reduces the complexity. For those ‘bad’ channels identified by the channel selection, we introduce two adaptive transmission methods to deal with such channels: one uses a lower code rate, and the other is multiple transmissions. The use of an IIC receiver with the interleave-division multiple access (IDMA) to further improve the BER performance without any channel selection is also investigated. It has been shown that this approach can remove the error floor. Finally, the influence of channel accuracy on the IIC is investigated. Pilot-based Wiener filter channel estimation is used to test and verify how much the IIC is influenced by the channel accuracy
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