[pt] As técnicas de múltiplo acesso mais cogitadas nas propostas para padrões de 3 geração de sistemas celulares envolvem alguma forma de CDMA ( Code Division Multiple Access)na qual os usuários podem transmitir simultaneamente na mesma faixa de freqüência sendo separáveis no receptor pela ortogonalidade, ou quase ortogonalidade, entre os sinais utilizados pelos diferentes usuários para transmissão de informação.Os receptores multiusuário, propostos para uso na 3 geração de sistemas celulares, realizam a detecção do sinal do usuário desejado conjuntamente com os sinais interferentes, resultando em um ganho de desempenho em relação aos receptores CDMA convencionais, que tratam os sinais interferentes como um simples ruído de fundo.Este trabalho tem por objetivo estudar a viabilidade de um receptor CDMA multiusuário hpibrido síncrono, que combina a eficiência do recepor Decorrelator no combate a interferência de múltiplo acesso (IMA) e sua simpicidade de implementação com a robustez e alto desempenho do receptor ótimo. Para evitar a complexidade inerente a estrutura de detecção ótima é sugerido que o conjunto de sinais na saída do estágio Decorrelator seja particionado em grupos e detectados de forma paralela e ótima. Esta técnica é conhecida como detecção ótima por grupos. Também é proposto e explorada a utilização de uma métrica para a formação dos grupos, baseada na minimização da probabilidade de erro de bit do usuário com pior desenpenho no sistema. Ao longo deste trabalho são exibidos resultados analíticos e de simulação, que comprovam a utilidade e exelente relação entre desempenho e complexidade da estrutura proposta em canais AWGN. Por fim, a estrutura proposta é avaliada para um canal com desvanecimento de Rayleigh plano. / [en] The most discussed multiple access techiniques for the 3G of mobile wireless systems involve some form of CDMA - Code Division Multiple Access- where the users can transmit simultaneously their signals in the same frequency band separable in the receiver by the orthogonality or, quasi- orthogonality, properties of the transmitted signals. Multiuser detectors MUD perform the detection of the desired user signal jointly with the signals transmitted by the remaining users, resulting in a better performance than the conventional CDMA receivers, that treat the interference created by the signals of other users simply as background noise.This work has the objective of investigating the viability of syncrhronous hybrid multiuser detector, wich combines the implementation simplicity and efficiency of the decorrelating multiuser detector in mitigating the multiple access interference with the robustness and high performance of the optimum multiuser detector.In order to avoid the complexity of the optimum multiuser detection scheme it is proposed to combine the signals from the decorrelator output into groups and perform optimal detection of the signals in each group. This approach is known as optimal group detection. Also in this work, a strategy on how to perform the grouping is proposed and explored. This strategy is based on the minimization the bit error probability of the user with the worst performance.Simulation and analytical results show the excellent trade-off between performance and complexity of the proposed optimum multiuser group detector in AWGN channels. The receiver performance is also evaluated in Rayleigh flat fading channels.

This dissertation introduces three novel multiuser detection approaches in Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) systems by blind signal separation (BSS) techniques. The conventional methodologies for multiuser detection have to retransmit channel state information (CSI) constantly from the transmitter in MIMO ODFM systems at the cost of economic efficiency, because they require more channel resources to improve the communication quality. Compared with the traditional methodologies, the proposed BSS methods are relatively efficient approaches without the unnecessary retransmission of channel state information. The current methodologies apply the space-time coding or the spatial multiplexing to implement an MIMO OFDM system, which requires relatively complex antenna design and allocation in the transmitter. The proposed Spatial Division Multiple Access (SDMA) method enables different mobile users to share the same bandwidth simultaneously in different geographical locations, and this scheme requires only one antenna for each mobile user. Therefore, it greatly simplifies the antenna design and allocation. The goal of this dissertation is to design and implement three blind multiuser detection schemes without knowing the channel state information or the channel transfer function in the SDMA-based uplink MIMO OFDM system. The proposed scenarios include: (a) the BSS-only scheme, (b) the BSS-Minimum Mean Square Error (MMSE) scheme, and (c) the BSS-Minimum Bit Error Ratio (MBER) scheme. The major contributions of the dissertation include: (a) the three proposed schemes save the commercially expensive cost of channel resources; (b) the proposed SDMA-based uplink MIMO OFDM system simplifies the requirements of antennas for mobile users; (c) the three proposed schemes obtain high parallel computing efficiency through paralleled subcarriers; (d) the proposed BSS-MBER scheme gains the best BER performance; (e) the proposed BSS-MMSE method yields the best computational efficiency; and (f) the proposed BSS-only scenario balances the BER performance and computational complexity.

New technologies such as pervasive computing, ambient environment, and communication avid applications such as multimedia streaming are expected to impact the way people live and communicate in the wireless networks of the future. The introduction of these new technologies and applications is, however, a challenging task in wireless networks because of their high bandwidth requirements and Quality of Service (QoS) demands. A significant recent advance in wireless communication technology, known as Multiple-Input Multiple-Output (MIMO) provides unprecedented increase in link capacity, link reliability and network capacity. The main features of MIMO communication are spatial multiplexing, point-to-multipoint and multipoint-to-point transmission as well as interference suppression in contrast to the conventional single antenna (Single-In Single-Output, SISO) networks. In this thesis, we investigate the problem of scheduling flows for fair stream allocation (or, stream scheduling) in ad hoc networks utilizing MIMO antenna technology. Our main contributions include: i) the concept of stream allocation to flowsbased on their traffic demands or class, ii) stream allocation to flows in the network utilizing single user or multiuser MIMO communication, iii) achieving the proportional fairness of the stream allocation in the minimum possible schedule length, and iv)performance comparison of the stream scheduling in the network for single user and multiuser communication and the tradeoff involved therein. We first formulate demand based fair stream allocation as an integer linear programming (ILP) problem whose solution is a schedule that is guaranteed to be contention-free. We then solve this ILP in conjunction with binary search to find a minimum length contention-free schedule that achieves the fairness goals. Performance comparison results show the benefit of multiuser MIMO links over single user links which is predominant at higher traffic workloads in the network. We also implement a greedy heuristic for stream scheduling and compare its performance with the ILP-based algorithm in terms of the fairness goals achieved in a given schedule length. OPNET-based stochastic simulation confirms the benefits of MIMO-based stream scheduling over single antenna links, as shown by our theoretical analysis.

Computer Aided Design (CAD) applications have historically been based on a single user per application architecture. Although this architecture is still popular to date, it does have several drawbacks. First of all the single user CAD architecture inhibits a concurrent engineering design process where several designers can work on the same model simultaneously. This limitation introduces time inefficiency especially when a project involves geographically dispersed designers. A solution to these drawbacks could be a transition from the traditional single user CAD architecture to a multiuser collaborative architecture. Advances in computer networking technologies, especially relating to the Internet, have provided the needed tools to make this transition a reality, thus making it possible for designers to simultaneously work on geometric models from one or more networked computers regardless of the location of the user. This new paradigm is expected to improve collaboration and greatly reduce product design times and consequently reduce cost and improve productivity. The multi-user architecture will, however, also require reliable security mechanisms to ensure its successful deployment in an enterprise environment where protection of intellectual property is of critical importance. This thesis proposes a framework to implement authentication, authorization and secure data communications in a multiuser collaborative CAD software system. This framework has been tested on an emerging multiuser collaborative CAD system called v-CAx being developed at Brigham Young University.

The research presented in this dissertation is on the development and application of advanced reduced rank adaptive signal processing techniques for high data rate wireless code division multiple access (CDMA) communications systems. This is an important area of research in the field of wireless communications. Current systems are moving towards the use of multiple simultaneous users in a given channel to increase system capacity as well as spatial and/or temporal diversity for improved performance in the presence of multipath and fading channels. Furthermore, to accommodate the demand for higher data rates, fast signal processing algorithms are required, which often translate into blind signal detection and estimation and the desire for optimal, low complexity detection techniques. The research presented here shows how minimum mean square error (MMSE) receivers implemented via the multistage Wiener filter (MWF) can be employed at the receiving end of a CDMA system to perform multiuser detection (MUD) or interference suppression (IS) with no loss in performance and significant signal subspace compression better than any previous reduced rank techniques have shown. This is important for optimizing performance because it implies a reduction in the number of required samples, so it lessens the requirement that the channel be stationary for a time duration long enough to obtain enough samples for an accurate MMSE estimate. The structure of these receivers is derived for synchronous and asynchronous systems for a multipath environment, and then it is shown that implementation of the receiver in a reduced rank subspace results in no loss in performance over full rank methods. It is also shown in some instances that reduced rank exceeds full rank performance. Multiuser detectors are also studied, and the optimal reduced rank detector is shown to be equivalent to a bank of parallel single user detectors performing interference suppression (IS). The performance as a function of rank for parallel and joint multiuser detectors are compared. The research is then extended to include joint space-code (i.e. a joint multiuser detector) and joint space-time processing algorithms which employ receiver diversity for low complexity diversity gain. Non-linear techniques, namely serial interference cancellation (SIC) and parallel interference cancellation (PIC), will also be studied. The conventional matched filter correlator will be replaced by the MWF, thereby incorporating IS at each stage of the interference canceller for improved performance. A closed form expression is derived for the probability of error, and performance gains are evaluated. It will be further shown how the receiver structure can be extended when space-time codes are employed at the transmitter for additional diversity gain with minimal impact on complexity. The MMSE solution is derived and implemented via the MWF with some examples. It is believed that these new techniques will have a significant impact on the design of fourth generation (4G) and beyond cellular CDMA systems. / Ph. D.

The focus of this dissertation is the study of advanced processing techniques for multiuser interference cancellation in direct sequence code division multiple access communications. Emphasis is placed on the development of efficient techniques that are practical to implement. The work begins with a study of several sub-optimal multiuser detection techniques under a variety of conditions. Multistage parallel interference cancellation is identified as a practical and robust approach for mitigating multiple access interference. In order to reduce the effect of biased decision statistics inherent to parallel cancellation, a low-complexity modification to parallel interference cancellation that significantly improves performance is derived. Based on this approach, two real-time DSP implementations are devised, one fully coherent and one non-coherent. Multi-symbol differential detection is then explored as an alternative for improving the performance of the non-coherent approach. Additionally, dual-antenna diversity techniques are also investigated as a means for improving performance in multipath environments. / Ph. D.

This research investigates the application of multiuser detection to Code Division Multiple Access for cellular communications. This investigation focuses on the use of multiuser receivers at the base station of mobile radio systems. The first two chapters are dedicated to multiuser detection in general. An extensive literature survey is performed on the research concerning multiuser receivers to date. Six major receiver structures are chosen for extensive simulation studies. The bit error rate performance of these receivers is investigated in several system environments. Additionally, practical issues are considered such as computational complexity and robustness to code tracking errors. From this work, one receiver structure is identified for further study, namely multistage interference cancellation. The theoretical performance of this receiver is analyzed using a standard Gaussian Approximation and an Improved Gaussian Approximation for AWGN and fading environments. Additionally, the resistance of the receiver to interference energy levels is explored. Parameter estimation is an important issue for interference cancellation. Simple methods of improving parameter estimation are examined, as is the effect of parameter estimation error on system performance. A baseband hardware implementation is detailed and several design challenges are presented. Results are given for the performance of the implemented receiver and shown to match well with theory and computer simulation. Finally, the implications of this research are discussed. / Ph. D.

Parallel interference cancellation for DS-CDMA has been shown to suffer from biased amplitude estimates if a matched-filter estimator is used. The bias magnitude is proportional to the number of interfering users. For heavy system loads, the bias has been shown to adversely effect the accuracy of the interference cancellation process, thereby impairing BER after cancellation. Empirical simulation work has demonstrated that weighting down interference estimates can improve BER performance. This thesis substantiates these BER improvements by modelling and analyzing a soft interference cancellation technique which mitigates the effects of the bias by minimizing BER after cancellation in a bit-synchronous parallel interference cancellation CDMA receiver. We analyze system decision metrics with down-scaled interference estimates and determine both the mean and variance of the biased decision statistics. From these two metric moments, system BER is evaluated, and the optimal interference scaling function which minimizes BER is derived. We demonstrate BER performance enhancements by simulating this soft interference cancellation technique in systems under perfect power control and in the near-far situation. We further discuss the applicability of the results to asynchronous systems. / Master of Science

Research efforts have shown that capacity in a DS/CDMA cellular system can be increased through the use of digital signal processing techniques that exploit the nature of the multiple access interference (MAI). By jointly demodulating the users in the system, this interference can be characterized and reduced thus decreasing the overall probability of error in the system. Numerous multiuser structures exist, each with varying degrees of complexity and performance. However, the size and complexity of these structures is large relative to a conventional receiver. This effort demonstrates a practical approach to implementing parallel interference cancellation applied to DBPSK DS/CDMA on an FPGA-based configurable computing platform. The system presented acquires, tracks, cancels, and demodulates four users independently and performs various levels of interference cancellation. The performance gain of the receiver in a four-user environment under various levels of noise and cancellation are presented. / Master of Science

Due to its significant advantage in spectral efficiency, multiple-antenna communication technology will undoubtedly be a major component in future wireless system implementations. However, the full exploitation of this technology also requires perfect feedback of channel state information (CSI) to the transmitter-- something that is not practically feasible. This motivates the study of limited feedback systems, where CSI feedback is rate limited. This thesis focuses on the optimal design of limited feedback systems for three types of communication channels: the relay channel, the single-user point-to-point channel, and the multiuser broadcast channel. For the relay channel, we prove the efficiency of the Grassmannian codebooks as the source and relay beamforming codebooks, and propose a method for CSI exchange between the relay and the destination when global CSI is not available at destination. For the single-user point-to-point channel, we study the joint power control and beamforming problem and address the channel magnitude and direction quantization codebook design problem. It is shown that uniform quantization of the channel magnitude (in dB scale) is asymptotically optimal regardless of the channel distribution. The analysis further derives the optimal split of feedback bandwidth between the magnitude and direction quantization codebooks. For the multiuser broadcast channel, we first prove the sufficiency of a product magnitude-direction quantization codebook for managing the multiuser interference. We then derive the optimal split of feedback bandwidth across the users and their magnitude and direction codebooks. The optimization results reveal an inherent structural difference between the single-user and multiuser quantization codebooks: a multiuser codebook should have a finer direction quantization resolution as compared to a single-user codebook. It is further shown that the users expecting higher rates and requiring more reliable communication should provide a finer quantization of their CSI. Finally, we determine the minimum required total feedback rate based on users' quality-of-service constraints and derive the scaling of the system performance with the total feedback rate.