published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Wireless networked control systems use shared wireless links to communicate between sensors and controllers, and require a channel access policy to arbitrate access to the links. Existing multiple access protocols perform this role in an agnostic manner, by remaining insular to the applications that run over the network. This approach does not give satisfactory control performance guarantees. To enable the use of wireless networks in emerging industrial applications, we must be able to systematically design wireless networked control systems that provide guaranteed performances in resource-constrained networks. In this thesis, we advocate the use of state-based channel access policies. A state-based policy uses the state of the controlled plant to influence access to the network. The state contains information about not only the plant, but also the network, due to the feedback in the system. Thus, by using the state to decide when and how frequently to transmit, a control system can adapt its contribution to the network traffic, and enable the network to adapt access to the plant state. We show that such an approach can provide better performance than existing methods. We examine two different state-based approaches that are distributed and easy to implement on wireless devices: event-based scheduling and adaptive prioritization. Our first approach uses events to reduce the traffic in the network. We use a state-based scheduler in every plant sensor to generate non-coordinated channel access requests by selecting a few critical data packets, or events, for transmission. The network uses a contention resolution mechanism to deal with simultaneous channel access requests. We present three main contributions for this formulation. The first contribution is a structural analysis of stochastic event-based systems, where we identify a dual predictor architecture that results in separation in design of the state-based scheduler, observer and controller. The second contribution is a Markov model that describes the interactions in a network of event-based systems. The third contribution is an analysis of the stability of event-based systems, leading to a stabilizing design of event-based policies. Our second approach uses state-based priorities to determine access to the network. We use a dominance protocol to evaluate priorities in a contention-based setting, and characterize the resulting control performance. An implementation and evaluation of this channel access mechanism on sensor nodes is also presented. The thesis finally examines the general networked control problem of jointly optimizing measurement and control policies, when a nonlinear measurement policy is used to perform quantization, event-triggering or companding. This contribution focuses on some of the fundamental aspects of analyzing and synthesizing control systems with state-based measurement policies in a more generalized setting. We comment on the dual effect, certainty equivalence and separation properties for this problem. In particular, we show that it is optimal to apply separation and certainty equivalence to a design problem that permits a dynamic choice of the measurement and control policies. / <p>QC 20140408</p>

Mobile communications have developed since the radio communications that were in use 50 years ago. With the advent of GSM, mobile communications was brought to the average citizen. More recently, COMA technology has provided the user with higher data rates and more reliable service, and it is apparent that it is the future of wireless communication. With the introduction of 3G technology in South Africa, it is becoming clear that it is the solution to the country's wireless communication requirements. The 3G and next-generation technologies could provide reliable communications to areas where it has proven difficult to operate and maintain communications effectively, such as rural locations. It is therefore important that the se technologies continue to be researched in order to enhance their capabilities to provide a solution to the wireless needs of the local and global community. Whilst COMA is proving to be a reliable communications technology, it is still susceptible to the effects of the near-far problem and multiple-access interference. A number of multiuser detectors have been proposed in literature that attempt to mitigate the effects of multiple-access interference. A notable detector is the blind MOE detector, which requires only the desired user 's spreading sequence , and it exhibits performance approximating that of other linear multiuser detectors. Another promising class of multiuser detector operate using an iterative principle and have a joint multiuser detection and error-correcting coding scheme. The aim of this research is to develop a blind iterative detector with FEC coding as a potential solution to the need for a detector that can mitigate the effects of interfering users operating on the channel. The proposed detector has the benefits of both the blind and iterative schemes: it only requires the knowledge of the desired user ' s signature, and it has integrated error-correcting abilities. The simulation results presented in this dissertation show that the proposed detector exhibits superior performance over the blind MOE detector for various channel conditions. An overview of spread-spectrum technologies is presented, and the operation of OS-COMA is described in more detail. A history and overview of existing COMA standards is also given . The need for multiuser detection is explained, and a description and comparison of various detection methods that have appeared in literature is given. An introduction to error coding is given , with convolutional code s, the turbo coding concept and method s of iterative detection are described in more detail and compared, as iterat ive decoding is fundamental to the operation of an iterative COMA detector. An overview of iterative multiuser detection is given , and selected iterative methods are described in more detail. A blind iterative detector is proposed and analysed. Simulation results for the propo sed detector, and a comparison to the blind MOE detector is presented, showing performance characteristics and the effects of various channel parameters on performance. From these results it can be seen that the proposed detector exhibits a superior performance compared to that of the blind MOE detector for various channel conditions. The dissertation is concluded, and possible future directions of research are given. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

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Conventional Minimum-Variance (MV) channel estimation is affected by two sources of error, namely the finite number of samples used to estimate the covariance matrix and the asymptotic bias due to interference and additive noise. On the other hand, widely linear (WL) filtering has been shown to improve the estimation of improper complex signals. Researchers have recently demonstrated that the application of WL processing principles can significantly improve the performance of subspace-based channel estimation algorithms. However, in contrast to MV estimation algorithms, subspace-based algorithms assume knowledge of the total number of users in the system, and must be coupled with sophisticated user enumeration algorithm at the expense of increased complexity. In this work, in an effort to combine the practical advantages of MV channel estimation algorithms with the performance of WL filters we propose a widely linear version of the MV channel estimator in the context of multicarrier(MC) CDMA systems employing real modulation. We use numerical simulations to demonstrate that the widely linear minimum-variance algorithm yields more accurate channel estimates compared to the conventional MV algorithm. By considering two simplified transmission/reception models, we also show analytically that the widely linear estimator on average reduces both types of error.

Modern communications has become far more than point-to-point calling and wireless communications is part of every-day life. Driven by ever growing demand for high data rate communication, multiple-access techniques are of interest for allowing multiple users to share limited resources, such as frequency, time and space. Commercially introduced in 1995, Code-Division Multiple-Access (CDMA) quickly became one of the world's fastest-growing wireless technologies. However, CDMA is subject to some limiting factors, such as multiple-access interference (MAI), which dramatically affects the capacity of the wireless system and degrades performance. Fortunately, these effects can be alleviated by applying advanced signal processing techniques such as multiuser detection (MUD), which potentially provides a large increase in system capacity, enhances spectral efficiency, and relaxes requirements for power control. / Further improvements of MUD can be obtained through joint multiuser detection/decoding. However this is a very complex approach. Inspired by Turbo codes and iterative decoding, Turbo-MUD and iterative multiuser decoding have been proposed. The main objective of this research is to analyse the existing iterative techniques applied to Turbo multiuser decoding for coded CDMA systems and propose new decoder structures to improve the system performance. / In this thesis, we observe that many of the iterative multiuser decoding algorithms in the literature are focused on exchanging information obtained within the most current iteration. However, if correlations over iterations are low, then in principle the bit error rate (BER) performance can be improved by combining signal estimates over iterations. Inspired by this idea, iterative maximal ratio combining (MRC) is proposed in this thesis for application to iterative decoding structures. With this approach all previous estimates are recursively weighted and combined to refine the current signal estimates. The derivation of the corresponding weighting factors is based on the statistics of the decoder outputs over iterations, which leads to maximizing the resultant signal-to-noise ratio (SNR) for each current signal estimate. It is shown that the recursive MRC scheme can be widely applied to many existing iterative structures and provide significantly improved system performance with acceptable computational complexity. In addition, the analytic and numerical results illustrate that the resulting performance gain from the application of MRC is inversely proportional to the correlation of the decoder estimates across iterations. The more correlated the signal estimates over consecutive iterations are, the slower system convergence will be, if MRC is employed over all iterations. MRC over only a few initial iterations where correlation across those iterations is low provides faster convergence. A truncated MRC is suggested, which provides better performance while maintaining low computational complexity. Simulation results based on monte carlo averaging demonstrate that the system performance for the proposed techniques is better than many existing algorithms in the literature. / Thesis (MA(Telecommunications))--University of South Australia, 2005.

Thesis (Ph.D.)--University of South Australia, 1996.

The introduction of cellular wireless systems in the 1980s has resulted in a huge demand for personal communication services. This demand has made larger capacity systems necessary. This has been partially satisfied by the introduction of second generation digital systems. New third generation systems are now under going standardisation and will require even more efficient utilisation of the spectrum if the high bandwidth features and larger capacity are to become a reality. Motivated by these growing requirements we discuss methods of achieving large improvements in spectral efficiency and performance. Multiple-user communications over a channel can only be achieved with some form of diversity. In this work we point out that the efficient utilisation of the dimensions of space, time, and frequency will ultimately maximise the system capacity of a multiple-user system. We apply our receiver techniques solely to the base-station design where capacity limitations are currently present. We note however, that some of these techniques could also be applied at the mobile terminal receiver. We primarily focus our attention on the direct-sequence code-division multiple-access (DS/CDMA) channel, since this channel is inherently interference limited by other users in the cell of interest. We exploit a new powerful channel coding technique named " turbo coding" for its iterative decoding approach. We show how we can substitute the inner convolutional code of a turbo code encoder with the CDMA channel. By " iterative detection/decoding" or " turbo equalisation" at the receiver we achieve performance results which show the interference from other users to approach complete removal. We develop and analyse a new, low complexity, iterative interference canceller/decoder. This receiver has complexity per user linear with the memory of the channel and independent of the number of users in the system. We extend this receiver to more realistic channels that are asynchronous and include multi-path, and include spatial diversity by using an antenna array at the receiver. The CDMA channel we study exclusively uses randomly generated spreading codes. With this channel model we still achieve single user performance (no interference from other users) with a 10logL gain from L antenna elements and a gain of up to 10logP from P multi-path components. With any new receiver design, sensitivity to channel parameter errors is of paramount interest. We find that the sensitivity of our receiver is low with respect to the parameter errors induced. This is as we desire for a realisable receiver design. Finally we investigate the application of this new iterative interference canceller/decoder receiver to a number of other interference channels. These include the intersymbol interference (ISI) channel, partial response signalling (PRS), and continuous phase modulation (CPM). For these channels excellent performance improvement is generally achieved by the utilisation of the iterative interference canceller/decoder solution. / Thesis (PhD)--University of South Australia, 1999