<p>This thesis proposed and presented technical details of new probabilistic metrics of trust in the links wireless ad hoc networks for unobservable communications. In covert communication networks, only the end nodes are aware of the communication characteristics of the overall path. The most widely used protocols of ad hoc networks was overviewed. The routing protocols of ad hoc networks with trust considerations and select Destination Sequence Routing, a protocol that can be used in distributed ad hoc network settings for path discovery, was also reviewed. It establishes a path through which all packets sent by a source must pass to the destination. The end nodes are responsible for examining the statistics of the received packets and deriving inferences on path feature variations which are used for computing new trust metrics. When a path is judged not trustworthy based on the metrics, the Destination Sequence Routing is informed to undertake a new trusted path discovery between the end points. The thesis added a new feature based on the quality of service parameters of the path to create trust in the links in recognition of attacks.</p>

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.

The advance of multi-media applications has prompted researchers to undertake the task of finding suitable methods to carry multi-media data through ad-hoc wireless networks. The task of finding suitable methods for this particular application is rather difficult to achieve since ad-hoc networks exhibit highly dynamic link connectivity and are prone to error from radio transmission through the physical environment. In addition to all of the above, the sole concern of the current protocols is the connectivity of wireless nodes. The connectivity is maintained by a set of network messages. Communicating nodes in an ad hoc network select only the shortest available path. In the event of traffic congestion, packets are simply dropped from the queue. The notion of multi-path scheme has been investigated by other researchers, and DSR is still the most popular choice to implement network adaptation for multi-media traffic. Most of the existing network adaptation methods however lack the fairness in multi-path selection. A new network adaptation module with the aid of fuzzy logic and depth first search can provide a better guarantee in packet delivery and better use of network resources. In addition to the new module, a comparative study has been done in this project for suitable update methods for the unused multi-paths. This is a weakness in the use of DSR multi-path scheme because DSR only updates active traffic paths. The simulation results in this thesis show that the new network adaptation module is capable of improving the performance of the network communications. / Thesis ([MEng(ElectricalEngineering)])--University of South Australia, 2004.

With the integration of wireless technologies and multimedia services, transmitting high quality images and video has become one of the main objectives for next generations mobile network systems. Shannon's classic separation theorem states that, under ideal conditions, source coding and channel coding can be treated separately without sacrificing any performance for the whole system. However, this theorem holds true only under ideal conditions. Practical communication systems do not meet such requirements. Therefore, joint source and channel coding may reduce distortion, as well as complexity and delay. In this thesis, different schemes of joint source-channel coding and decoding for error resilient image transmission over noisy channels are examined. Unequal error protection (UEP) is one of the techniques used in joint source and channel coding. / A JPEG image is partitioned into DC components and AC components according to their respective sensitivity to channel noise. The highly sensitive DC components are better protected with a low coding rate, while the less sensitive AC components use a high coding rate. Simulation results show that the proposed UEP scheme slightly outperforms conventional equal error protection (EEP). A novel turbo diversity scheme (TDS) applied to JPEG coded images is proposed. Turbo codes have a built-in structure that is suitable for diversity techniques used to improve the quality of communications over a multi-path channel. The same image data is encoded by two separate turbo encoders and sent over two independent channels. The received data is then passed to a single turbo decoder. By utilising the built-in structure of the turbo encoder, the transmitted JPEG data encoded by a rate half code is recovered at the receiver using a more powerful rate third code yielded by the TDS. / An iterative source-channel decoding scheme applied to JPEG coded images is investigated. Huffman codes used as the variable-length coding scheme in JPEG coding can be represented by an irregular VLC-trellis structure. State transition probabilities can be derived from the irregular trellis and can be used as a priori information to help iterative decoding between source and channel a posteriori probability (APP) decoders. Iterative decoding of JPEG coded images only gives a small coding gain due to the poor distance property of the original JPEG Huffman codes. We propose to replace the Huffman codes used in JPEG coding with error resilient source codes with larger free distance. After accounting for the penalty due to the increased average codeword length, the new scheme achieves a 4 dB coding gain over the conventional system for a range of SNRs. While the focus of this thesis is on joint source-channel coding, two other related topics are also examined, namely, capacity and normalisation of intersymbol interference (ISI) channels and parallel data convolutional codes. / Previously published results showed a minimum Eb/N0 of -4.6 dB, 3 dB below the capacity of a flat channel, is obtained using the water-pouring capacity formulas for the 1+D channel. However, these results did not take into account that the channel power gain can be greater than unity when water-pouring is used. We present a new generic power normalization method of ISI channel frequency spectra, namely peak bandwidth normalisation, to facilitate the fair capacity comparison of various ISI channels. A final contribution presented in this thesis regards the proposed parallel data convolutional codes (PDCCs). The encoder inputs consist of the original block of data and its interleaved version. We propose a novel single self-iterative soft-input/soft-output (SISO) decoder structure for the decoding of PDCC. It has the advantage of needing only one APP decoder. Although the performance is not very encouraging, the novelty of the self-iterative idea behind the design is worth exploiting. / Thesis (PhDTelecommunications)--University of South Australia, 2003.

Thesis (PhD)--University of South Australia, 2002.

Telecommunication has experienced significant changes over the past few years and its paradigm has moved from wired to wireless communications. The wireless channel constitutes the basic physical link between the transmitter and the receiver antennas. Therefore, complete knowledge of the wireless channel and radio propagation environment is necessary in order to design efficient wireless communication systems. This PhD thesis is devoted to studying the spatial and temporal statistics of the wireless channel in cellular environments based on a geometry-based stochastic physical channel modeling approach. Contributions in this thesis report include the following: ??? A new physical channel model called the eccentro-scattering model is proposed to study the spatial and temporal statistics of the multipath signals in cellular environments. ??? Generic closed-form formulas for the probability density function (pdf) of angle of arrival (AoA) and time of arrival (ToA) of the multipath signals in each cellular environment are derived. These formulas can be helpful for the design and evaluation of modern communication systems. ??? A new Gaussian scattering model is proposed, which consists of two Gaussian functions for the distribution of scatterers around base station (BS) and mobile station (MS) and confines these scatterers within a scattering disc. ??? The effect of mobile motion on the spatial and temporal statistics of the multipath signals in cellular environments is discussed. Three motion scenarios are considered for the possible trajectory of the mobile unit. Furthermore, two different cases are identified when the terrain and clutter of mobile surrounding have additional effect on the temporal spread of the multipath signals during motion. ??? The physical channel model is employed to assess the performance of a RAKE receiver in cellular environments. ??? Comparisons between uniform scattering and Gaussian scattering, which are the two assumptions for the distribution of scatterers usually used in the derivation of the pdf of AoA, are also presented. ??? An overview of earlier physical channel models and comparisons between these models and with the proposed model are presented.

This dissertation concerns the problem of scheduling real-time traffic in wireless TDMA channels. The most important characteristic of real-time traffic is that it has straight end-to-end delay constraint. We begin the investigation by studying a scheduling principle which naturally achieves the best delay performance in stationary channel conditions. Since the resulting scheduling algorithm maintains equal flow delays across the whole system, it is termed the equal-delay policy. There are a number of advantages associated with this scheduling method. First, it is very simple and practical to implement in real system. Secondly, it can be easily modelled mathematically and admits an analytical solution, which is very important for the construction of an admission control algorithm, we present a mathematical model describing the dynamics of the scheduling system, as well as devising a tractable analytical solution to the problem. A third advantage of the equal-delay policy is that it can be easily extended to support flows with multiple delay constraints. We propose a multiple-class scheduling scheme based on similar allocation concepts as the equal-delay technique. The extended scheme can similarly be mathematically modelled and analytically characterized. A natural objection to the above proposed techniques is that wireless transmission resources can be under-utilized, since the scheduling algorithm pays no attention to the changing individual channel conditions. The reduction in channel utilization can also adversely affect the delay performance, We explain this phenomenon and study the impacts for a variety of different channel characteristics, Specifically, we propose an alternative channel-aware scheduling policy, which aims to maximize channel utilization while keeping a minimum probability of delay violation, The proposed channel-aware policy achieves near-optimal delay performance. However, unlike in the equal-delay case, the channel-aware policy is not practical to implement in a real system. The complicated system dynamics associated with the channel-aware scheme also hamper the development of a mathematical model and analytical solution for admission control. On the other hand, we observe from simulation results that under most circumstances, the equal-delay scheme achieves close to the pertonnance obtained by the channel-aware technique, With the additional benefits of simplicity and admitting analytical analysis. the equal-delay policy appears to be a more practical and suitable choice for scheduling real-time traffic in wireless networks.

Due to the enormous capacity and performance gains associated with the use of antenna arrays in wireless multi-input multi-output (MIMO) communication links, it is inevitable that these technologies will become an integral part of future systems. In order to assess the potential of such beam-oriented technologies, direct representation of the dispersion of multipath fading channel in angular and temporal domains is required. This representation can only be achieved with the use of spatial channel models. This thesis thus focuses on the issue of spatial channel modeling for cellular systems and on its use in the characteri- zation of multipath fading channels. The results of this thesis are presented mainly in five parts: a) modeling of scattering mechanisms, b) derivation of the closed-form expressions for the spatio-temporal characteristics, c) generalization of the quantitative measure of angular spread, d) investigation of the effect of mobile motion on the spatio-temporal characteris- tics, and e) characterization of fast fading channel and its use in the signature sequence adaptation for direct sequence code division multiple access (DS-CDMA) system. The thesis begins with an overview of the fundamentals of spatial channel modeling with regards to the specifics of cellular environments. Previous modeling approaches are dis- cussed intensively and a generalized spatial channel model, the 'Eccentro-Scattering Model' is proposed. Using this model, closed-form mathematical expressions for the distributions of angle and time of multipath arrival are derived. These theoretical results for the picocell, microcell and macrocell environments, when compared with previous models and available measurements, are found to be realistic and generic. In macrocell environment, the model incorporates the effect of distant scattering structures in addition to the local ones. Since the angular spread is a key factor of the second order statistics of fading processes in wireless communications, the thesis proposes a novel generalized method of quantifying the angular spread of the multipath power distribution. The proposed method provides almost all parameters about the angular spread, which can be further used for calculating more accurate spatial correlations and other statistics of multipath fading channels. The degree of accuracy in such correlation calculations can lead to the computation of exact separation distances among array elements required for maximizing capacity in MIMO systems or diversity antennas. The proposed method is also helpful in finding the exact standard deviation of the truncated angular distributions and angular data acquired in measurement campaigns. This thesis also indicates the significance of the effects of angular distribution truncation on the angular spread. Due to the importance of angular spread in the fading statistics, it is proposed as the goodness-of-fit measure in measurement campaigns. In this regard, comparisons of some notable azimuthal models with the measurement results are shown. The effect of mobile motion on the spatial and temporal characteristics of the channel is also discussed. Three mobile motion scenarios are presented, which can be considered to be responsible for the variations of the spatio-temporal statistical parameters of the multipath signals. Two different cases are also identified, when the terrain and clutter of the mobile surroundings have an additional effect on the temporal spread of the channel during mobile motion. The effect of increasing mobile-base separation on the angular and temporal spreads is elaborated in detail. The proposed theoretical results in spatial characteristics can be extended to characterizing and tracking transient behavior of Doppler spread in time-varying fast fading channels; likewise the proposed theoretical results in temporal characteristics can be utilized in designing efficient equalizers for combating inter-symbol interference (ISI) in time-varying frequency-selective fading channels. In the last part of the thesis, a linear state-space model is developed for signature sequence adaptation over time-varying fast fading channels in DS-CDMA systems. A decision directed adaptive algorithm, based on the proposed state-space model and Kalman filter, is presented. The algorithm outperforms the gradient-based algorithms in tracking the received distorted signature sequence over time-varying fast fading channels. Simulation results are presented which show that the performance of a linear adaptive receiver can be improved significantly with signature tracking on high Doppler spreads in DS-CDMA systems.

Due to the enormous capacity and performance gains associated with the use of antenna arrays in wireless multi-input multi-output (MIMO) communication links, it is inevitable that these technologies will become an integral part of future systems. In order to assess the potential of such beam-oriented technologies, direct representation of the dispersion of multipath fading channel in angular and temporal domains is required. This representation can only be achieved with the use of spatial channel models. This thesis thus focuses on the issue of spatial channel modeling for cellular systems and on its use in the characteri- zation of multipath fading channels. The results of this thesis are presented mainly in five parts: a) modeling of scattering mechanisms, b) derivation of the closed-form expressions for the spatio-temporal characteristics, c) generalization of the quantitative measure of angular spread, d) investigation of the effect of mobile motion on the spatio-temporal characteris- tics, and e) characterization of fast fading channel and its use in the signature sequence adaptation for direct sequence code division multiple access (DS-CDMA) system. The thesis begins with an overview of the fundamentals of spatial channel modeling with regards to the specifics of cellular environments. Previous modeling approaches are dis- cussed intensively and a generalized spatial channel model, the 'Eccentro-Scattering Model' is proposed. Using this model, closed-form mathematical expressions for the distributions of angle and time of multipath arrival are derived. These theoretical results for the picocell, microcell and macrocell environments, when compared with previous models and available measurements, are found to be realistic and generic. In macrocell environment, the model incorporates the effect of distant scattering structures in addition to the local ones. Since the angular spread is a key factor of the second order statistics of fading processes in wireless communications, the thesis proposes a novel generalized method of quantifying the angular spread of the multipath power distribution. The proposed method provides almost all parameters about the angular spread, which can be further used for calculating more accurate spatial correlations and other statistics of multipath fading channels. The degree of accuracy in such correlation calculations can lead to the computation of exact separation distances among array elements required for maximizing capacity in MIMO systems or diversity antennas. The proposed method is also helpful in finding the exact standard deviation of the truncated angular distributions and angular data acquired in measurement campaigns. This thesis also indicates the significance of the effects of angular distribution truncation on the angular spread. Due to the importance of angular spread in the fading statistics, it is proposed as the goodness-of-fit measure in measurement campaigns. In this regard, comparisons of some notable azimuthal models with the measurement results are shown. The effect of mobile motion on the spatial and temporal characteristics of the channel is also discussed. Three mobile motion scenarios are presented, which can be considered to be responsible for the variations of the spatio-temporal statistical parameters of the multipath signals. Two different cases are also identified, when the terrain and clutter of the mobile surroundings have an additional effect on the temporal spread of the channel during mobile motion. The effect of increasing mobile-base separation on the angular and temporal spreads is elaborated in detail. The proposed theoretical results in spatial characteristics can be extended to characterizing and tracking transient behavior of Doppler spread in time-varying fast fading channels; likewise the proposed theoretical results in temporal characteristics can be utilized in designing efficient equalizers for combating inter-symbol interference (ISI) in time-varying frequency-selective fading channels. In the last part of the thesis, a linear state-space model is developed for signature sequence adaptation over time-varying fast fading channels in DS-CDMA systems. A decision directed adaptive algorithm, based on the proposed state-space model and Kalman filter, is presented. The algorithm outperforms the gradient-based algorithms in tracking the received distorted signature sequence over time-varying fast fading channels. Simulation results are presented which show that the performance of a linear adaptive receiver can be improved significantly with signature tracking on high Doppler spreads in DS-CDMA systems.

In a wireless network, owing to the time-varying nature of wireless channels, different mobile users typically experience peaks and troughs in channel quality at different times. This diversity in channel quality is known as multiuser diversity. With the aid of rate adaptation, multiuser diversity can be exploited by allowing the mobile user with the best channel to use the channel resource. However, in order to achieve this in most practical systems, the mobile users in the network must feed back their channel state information (CSI) to the transmitting user. In large networks, this feedback overhead can outweigh the multiuser diversity gain. In this thesis dissertation, a centralised wireless medium access control (MAC) scheme, namely Multiuser Diversity with Capture (MDC), is discussed as a solution to obviate the overhead problem. MDC explicitly employs the capture effect in radio receivers to reduce network overhead by allowing multiple mobile stations (MSs) with channels better than a nominal response threshold to simultaneously compete for the wireless channel. Owing to the capture effect, the base station (BS) can determine which MS has the best channel. In comparison with the Medium Access Diversity (MAD) scheme in the literature, the proposed MDC possesses the strong merit that the feedback overhead is independent of the number of MSs in the network. Several aspects of the MDC scheme are investigated in detail. An application of the MDC scheme based on the physical layer and parts of the MAC layer of the IEEE 802.11a standard is considered. A general analytical framework for the goodput performance of MDC is derived. Using this framework, the exact closed form solution for the expected goodput of MDC with rate adaptation over Rayleigh fading channels is calculated. The fairness performance of MDC in networks where some MSs experience better average channel conditions than others is also addressed. MSs with low average channel states tend to use the channel less often in MDC than MSs with high average channel states. This issue is tackled with Fairer Multiuser Diversity with Capture (FMDC), a variant of the MDC scheme designed to share the channel resource more equitably across all of the MSs in the network. In FMDC, instead of using the network-wide response threshold to decide whether to compete for the channel, each MS only competes for the channel when their channel state is greater than a threshold factor multiplied by their average channel state. Finally, the problem of adaptive optimisation of the response threshold for MDC and the threshold factor for FMDC is also considered. In the proposed solution, the response threshold and the threshold factor are adapted heuristically according to the estimated goodput performance of the system. The adaptive heuristic has importance in practical systems because the BS usually does not know the characteristics of the time varying channels of the MSs in the network.