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In this dissertation, we discuss various aspects of scheduling and resource allocation in multi-user wireless systems. This work starts from how to utilize advanced physical-layer technology to improve the system performance in a multi-user environment. We show that by using superposition coding (SPC) and successive interference cancellation, the system performance can be greatly improved with utility-based scheduling. Several observations are made as the design guideline for such system. Scheduling algorithms are designed for a system with hierarchical modulation which is a practical implementation of SPC. However, when the utility-based scheduling is designed, it is based on the assumption that the system is saturated, {\em i.e.}, users in the system always have data to transmit. It is pointed out in the literature that in a system with stochastic traffic, even if the arrival rate lies inside the capacity region, the system in terms of queue might not be stable with the utility-based scheduling. Motivated by this, we have studied the stability region of a general utility-based scheduling in a multi-user system with stochastic traffic. We show that the stability region is generally less than the capacity region, depends on how to interpret an intermediate control variable, and the resultant stability region may be even non-convex and exhibits undesirable properties which should be avoided. As the utility-based scheduling cannot achieve throughput-optimal, we turn our attentions to the throughput-optimal scheduling algorithms, whose stability region is identical to the capacity region. The limiting properties of an overloaded wireless system with throughput-optimal scheduling algorithms are studied. The results show that the queue length is unstable however the scheduling function of the queue length is stable, and the average throughput of the system converges. Finally we study how to schedule users in a multi-user wireless system with information-theoretic security support, which is focused on the secrecy outage probability. The problem is essentially about how to schedule users, and allocate resources to stabilize the system and minimize the secrecy outage probability. We show that there is a tradeoff between the arrival rate of the traffic and the secrecy outage probability. The relative channel condition of the eavesdropper also plays an important role to the secrecy outage probability. In summary, we showed utility-based scheduling using SPC can improve the system performance greatly, but the utility-based scheduling has limitations: the stability region might not have desired properties. On the contrary throughput-optimal scheduling has its own drawbacks: the traffic cannot be handled properly if the system is overloaded. The further study on the secrecy outage probability gives guideline on how to design a scheduler in a system with information-theoretic security support. / Graduate

In this thesis, a comprehensive investigation of the state-of-the-art guidelines for the conducted schemes in the near vicinity of the human body has been provided. The primary objectives of this work have been mainly focused on the electromagnetic (EM) wave exposure at both the microwave and millimetre-wave (mm-wave) frequency bands, along with their thorough analyses for a number of wireless technologies, such as the smart meters (SMs) and multiple-input multiple-output (MIMO) antennas. This work has undertaken a detailed theoretical and numerical modelling, as well as experimental measurements to propose the frameworks for the exposure conditions regarding the advanced wireless systems and applications. The employed numerical methods have been validated using the finite integration technique (FIT)-based simulations. The specific absorption rate (SAR) distributions have been determined using the anatomically realistic human models at various frequency bands. In this regard, each human model is assigned with the age- and frequency-dependent (AFD) dielectric properties, based on the novel expressions. This has then resulted in the representation of dispersive and age-dependent dielectric properties, in order to potentially improve the accuracy of the current assessment methods. Moreover, a number of generalised exposure conditions involving the standing and sleeping postures have been assessed for the home area network operating at the microwave band. The obtained results on the study on the SMs have been partly used by the Public Health England to provide the practical guidelines for the improvement of the services provided for the public. Furthermore, an assessment of the MIMO mobile handset has been performed in two exposure conditions, involving calling and body worn postures. Some aspects of this MIMO study has shown higher absorption levels in comparison with the basic restriction limits. A new SAR estimation approach has also been adopted to address the compliance assessment issues at the mm-wave bands. Finally, this work has identified a number of key factors that cause the high absorption levels in the human body and has provided insight into the efficient techniques in order to reduce such effects.

Ever increasing demand for higher data rates and better Quality of Service (QoS) for a growing number of users requires new transceiver algorithms and architectures to better exploit the available spectrum and to efficiently counter the impairments of the radio channel. Multiple-Input Multiple-Output (MIMO) communication systems employ multiple antennas at both transmitter and at the receiver to meet the requirements of next-generation wireless systems. It is a promising technology to provide increased data rates while not involving an equivalent increase in the spectral requirements. However, practical implementation of MIMO detectors poses a significant challenge and has been consistently identified as the major bottleneck for realizing the full potential that multiple antenna systems promise. Furthermore, in order to make judicious use of the available bandwidth, the baseband units have to dynamically adapt to different modes (modulation schemes, code rates etc) of operations. Flexibility and high throughput requirements often place conflicting demands on the Very Large Scale Integration (VLSI) system designer. The major focus of this dissertation is to present efficient VLSI architectures for configurable MIMO detectors that can serve as accelerators to enable the realization of next generation wireless devices feasible. Both, hard output and soft output detector architectures are considered.

Στην παρούσα διπλωματική εξετάζονται τρόποι μετάδοσης δεδομένων στο ασύρματο κανάλι διαλείψεων με χρήση μίας ή περισσότερων κεραιών στον πομπό ή και στο δέκτη. Αρχικά, παρουσιάζεται το μοντέλο του ασύρματου καναλιού. Δίνεται έμφαση στο φαινόμενο των διαλείψεων μικρής κλίμακας, καθώς και στον τρόπο με τον οποίο επηρεάζει τη μετάδοση. Στη συνέχεια, περιγράφονται τεχνικές μετάδοσης στο ασύρματο κανάλι. Πρώτα εξετάζεται η μετάδοση όταν πομπός και δέκτης διαθέτουν μία κεραία ο καθένας (SISO). Στη συνέχεια, μελετάται η μετάδοση με χρήση πολλών κεραιών στο δέκτη (SIMO) ή στον πομπό (MISO) και εισάγεται η έννοια του κέρδους ποικιλότητας (diversity gain) που προσφέρει η μετάδοση SIMO και MISO. Τέλος, παρουσιάζονται τεχνικές μετάδοσης όταν τόσο ο πομπός όσο και ο δέκτης διαθέτουν πολλές κεραίες (MIMO). Σε αυτήν την περίπτωση, εκτός από κέρδος ποικιλότητας δημιουργούνται και πρόσθετοι χωρικοί βαθμοί ελευθερίας. Παρουσιάζονται, επίσης, διάφορες αρχιτεκτονικές πομπού και δέκτη όπως Maximal Ratio Combining (MRC), Maximal Ratio Transmission (MRT), Singular Value Decomposition (SVD) και V-BLAST με δέκτη μέγιστης πιθανοφάνειας (ML), V-BLAST με γραμμικό δέκτη επιβολής μηδενισμών (linear ZF) και V-BLAST με γραμμικό δέκτη ελαχιστοποίησης μέσου τετραγωνικού σφάλματος (linear MMSE). Ακόμη, εισάγεται η έννοια της διαδοχικής εξάλειψης παρεμβολών (SIC) και της ταξινόμησης σε δέκτες SIC (SIC with ordering). Με χρήση προσομοιώσεων συγκρίνεται η απόδοση των τεχνικών μετάδοσης και των αρχιτεκτονικών πομποδέκτη για κανάλια επίπεδων διαλείψεων ως προς το ρυθμό μετάδοσης, την πολυπλοκότητα, τις απαιτήσεις σε γνώση του καναλιού και το ρυθμό σφάλματος συμβόλων και ψηφίων (BER/SER). / Data transmission strategies for the wireless channel are presented that employ one or more antennas at the transmitter and/or the receiver. First, a model is introduced for the wireless channel, focusing on small-scale fading and its effect on transmission. Then strategies for transmission over the wireless channel are described. The single-input single-output (SISO) case where both the transmitter and the receiver employ a single antenna is examined first. Then transmission with multiple antennas at the receiver (SIMO) or the transmitter (MISO) is studied and the concept of diversity gain, which is achieved in SIMO and MISO transmission, is introduced. Finally, transmission strategies are presented that require many antennas at both the transmitter and the receiver. In this case, new spatial degrees of freedom become available, in addition to the diversity gain. Furthermore, transceiver architectures are studied, including Maximal Ratio Combining (MRC), Maximal Ratio Transmission (MRT), Singular Value Decomposition (SVD) and Vertical Bell-Labs Layered Space-Time (V-BLAST) transmission with Maximum-Likelihood (ML) detection, V-BLAST with linear Zero Forcing (ZF) equalization, and V-BLAST with linear Minimum Mean Square Error (MMSE) equalization. The concept of Successive Interference Cancellation (SIC) is also introduced, followed by ordering in SIC detectors (SIC with ordering). Using simulations, the performance of the transmission techniques and the transceiver architectures over flat fading channels is compared with respect to the achievable data rate, the complexity, the requirements for channel state information and the Symbol- and Bit-Error Rate (SER/BER).

As a result of rapid progress in research and development, today's wireless world exhibits several heterogeneous communication networks, such as cellular networks, satellite networks, wireless local area networks (WLAN), mobile ad hoc networks (MANET), and sensor networks. These networks are complementary to each other. Hence, their integration can realize a unified wireless system that has the best features of the individual networks. This has spurred much research interest in designing integrated next-generation of wireless systems (NGWS). While existing wireless networks have been extensively studied individually, the integrated wireless system brings new challenges in architecture design, system management, and protocol design. The different wireless networks use different communication technologies and are based on different networking paradigms. Therefore, it is challenging to integrate these networks such that their heterogeneities are hidden from each other and a harmonious inter-operation among them is achieved. The objective of this research is to design a scalable, secure, and robust architecture and to develop seamless mobility management protocols for NGWS. More specifically, an architecture that integrates the heterogeneous wireless systems is first proposed for NGWS. Next, a cross-layer (Layer 2 + 3) handoff management protocol is developed for NGWS. Afterward, analytical modeling is developed to investigate the handoff performance of the existing mobility management protocols for different types of applications. Finally, a framework for multi-layer mobility management is developed to support the seamless handoff support to all types of applications in NGWS.

The coding and feedback inaccuracies of the channel state information (CSI) in limited feedback multiple-input multiple-output (MIMO) wireless systems can severely impact the achievable data rate and reliability. The CSI is mathematically represented as a Grassmann manifold or manifold of unitary matrices. These are non-Euclidean spaces with special constraints that makes efficient and high fidelity coding especially challenging. In addition, the CSI inaccuracies may occur due to digital representation, time variation, and delayed feedback of the CSI. To overcome these inaccuracies, the manifold structure of the CSI can be exploited. The objective of this dissertation is to develop a new signal processing techniques on the manifolds to harvest the benefits of MIMO wireless systems. First, this dissertation presents the Kerdock codebook design to represent the CSI on the Grassmann manifold. The CSI inaccuracy due to digital representation is addressed by the finite alphabet structure of the Kerdock codebook. In addition, systematic codebook construction is identified which reduces the resource requirement in MIMO wireless systems. Distance properties on the Grassmann manifold are derived showing the applicability of the Kerdock codebook to beam-forming and spatial multiplexing systems. Next, manifold-constrained algorithms to predict and encode the CSI with high fidelity are presented. Two prominent manifolds are considered; the Grassmann manifold and the manifold of unitary matrices. The Grassmann manifold is a class of manifold used to represent the CSI in MIMO wireless systems using specific transmission strategies. The manifold of unitary matrices appears as a collection of all spatial information available in the MIMO wireless systems independent of specific transmission strategies. On these manifolds, signal processing building blocks such as differencing and prediction are derived. Using the proposed signal processing tools on the manifold, this dissertation addresses the CSI coding accuracy, tracking of the CSI under time variation, and compensation techniques for delayed CSI feedback. Applications of the proposed algorithms in single-user and multiuser systems show that most of the spatial benefits of MIMO wireless systems can be harvested. / text

We investigate in detail two multiuser opportunistic scheduling problems in centralized wireless systems: the scheduling of "delay-sensitive" flows with packet delay requirements of a few tens to few hundreds of milliseconds over the air interface, and the scheduling of "best-effort" flows with the objective of minimizing mean file transfer delay. Schedulers for delay-sensitive flows are characterized by a fundamental tradeoff between "maximizing total service rate by being opportunistic" and "balancing unequal queues (or delays) across users". In choosing how to realize this tradeoff in schedulers, our key premise is that "robustness" should be a primary design objective alongside performance. Different performance objectives -- mean packet delay, the tail of worst user's queue distribution, or that of the overall queue distribution -- result in remarkably different scheduling policies. Different design objectives and resulting schedulers are also not equally robust, which is important due to the uncertainty and variability in both the wireless environment and the traffic. The proposed class of schedulers offers low packet delays, less sensitivity to the scheduler parameters and channel characteristics, and a more graceful degradation of service in terms of the fraction of users meeting their delay requirements under transient overloads, when compared with other well-known schedulers. Schedulers for best-effort flows are characterized by a fundamental tradeoff between "maximizing the total service rate" and "prioritizing flows with short residual sizes". We characterize two regimes based on the "degree" of opportunistic gain present in the system. In the first regime -- where the opportunistic capacity of the system increases sharply with the number of users -- the use of residual flow-size information in scheduling will 'not' result in a significant reduction in flow-level delays. Whereas, in the second regime -- where the opportunistic capacity increases slowly with the number of users -- using flow-size information alongside channel state information 'may' result in a significant reduction. We then propose a class of schedulers which offers good performance in either regime, in terms of mean file transfer delays as well as probability of blocking for systems that enforce flow admission control. This thesis provides a comprehensive theoretical study of these fundamental tradeoffs for opportunistic schedulers, as well as an exploration of some of the practical ramifications to engineering wireless systems. / text

[pt] A crescente demanda dos usuários por serviços de banda larga tais como dados a alta velocidade, vídeo, aúdio e multimídia, tem levado ao uso de sistemas rádio ponto- multiponto em freqüências superiores a 10 GHz, que além de menos utilizadas, permitem oferecer as grandes larguras de banda necessárias. Nessas freqüências, as perdas de propagação do sinal devem-se principalmente ao efeito de chuvas. A atenuação por chuvas afeta diretamente a cobertura e o desempenho dos sistemas que operam nestas faixas de freqüência. No presente trabalho é desenvolvido um modelo empírico para a previsão da distribuição cumulativa da atenuação por chuvas, utilizando a base oficial de ITU-R e resultados experimentais de medidas realizadas no Brasil. Este modelo fornece resultados mais precisos do que os atualmente propostos na literatura técnica e permite avaliar o impacto das chuvas na cobertura de sistemas ponto-multiponto tipo LMDS (Local Multipoint Distribution System). Além disto, foram desenvolvidos dois modelos para a previsão da distribuição da atenuação diferencial por chuvas, fator essencial para a determinação de efeitops de interferência e seu impacto das chuvas no desempenho de sistemas LMDS. Finalmente, foram desenvolvidos modelos para a previsão da duração total e média, de eventos de atenuação por chuvas, estatísticas também relevantes na análise de performance de sistemas LMDS. / [en] The growing demand for broadband services such as high- speed data, video, audio and multimedia, has lead to the use of point-multipoint radio systems operating in frequencies above of 10 GHz, that are not yet allocated for other services and allow the use of the large bandwidths required by these services. At these frequencies, the main contribution for the propagation losses is due to rain attenuation. The rain directly affects the area coverage and the performance of systems operating at these frequencies. In the present work, an empirical model is developed for predicting the cumulative distribution of rain attenuation prediction, using the ITU-R data base and results of propagation measurements campaigns performed in Brazil. The model provides better results than others currently found in the technical literature and allows the evaluation of the impact of rain attenuation in the coverage of LMDS (Local Multipoint Distribution System/Local Multipoint Communications System). Also, two models were developed for predicting distributions of differential rain attenuation, an essential factor for interference effects and its impact in LMDS performance. Finally, models were developed for the prediction of total and average duration of rain attenuation events, also relevant for systems performance assessment.

Data intensive applications, requiring reliability and strict delay constraints, have emerged recently and they necessitate a different approach to analyzing system performance. In my work, I establish a framework that relates physical channel parameters to the queueing performance for a single-user wireless system. I then seek to assess the potential benefits of multirate techniques, such as hybrid-ARQ (Automatic Repeat reQuest), in the context of delay-sensitive communications. Present methods of analysis in an information theoretic paradigm define capacity assuming that long codewords can be used to take advantage of the ergodic properties of the fading wireless channel. This definition provides only a limited characterization of the channel in the light of delay constraints. The assumption of independent and identically distributed channel realizations tends to over-estimate the system performance by not considering the inherent time correlation. A finite-state continuous time Markov channel model that I formulate enables me to partition the instantaneous data-rate received at the destination into a finite number of states, representing layers in a hybrid-ARQ scheme. The correlation of channel has been incorporated through level crossing rates as transition rates in the Markov model. The large deviation principle governing the buffer overflow of the Markov model, is very sensitive to channel memory, is tractable, and gives a good estimate of the system performance. Metrics such as effective capacity and probability of buffer overflow, that are obtained through large deviations have been related to the wireless physical layer parameters through the model. Using the above metrics under QoS constraints, I establish the quantitative performance advantage of using hybrid-ARQ over traditional systems. I conduct this inquiry by restricting attention to the case where the expected transmit power is fixed at the transmitter. The results show that hybrid-ARQ helps us in obtaining higher effective capacity, but it is very difficult to support delay sensitive communication over wireless channel in the absence of channel knowledge and dynamic power allocation strategies.