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Developing robust techniques for fast growing multi-user networks in pervasive environments is an important challenge. Multi-carrier communication is an established technique for achieving superior performance in multi-path frequency selective fading channels. In this thesis, new multi-carrier multiple access techniques using collaborative and cooperative approaches are proposed for multi-user systems in fading environments.

The development of reliable natural language interfaces to relational databases can accelerate the progress of many areas of applications, such as the need for interactive relational database interfaces for non-technical users. This thesis presents a novel development of C-BIRDs framework. The C-BIRD framework was developed in two phases. The first phase involved the development of a Static Approach-Based C-BIRD framework, which is based on a combination of a Goal-Oriented Conversational Agent (GOCA) and a knowledge tree (KT). GO CAs have proven their capability to disambiguate the user's needs through natural language conversations. KT is used to overcome the lack of connectivity between the GOCA and the relational database, through organizing the domain knowledge in a knowledge tree. In addition the Static Approach-Based C-BIRD framework, a number of strategies were employed based on scripting structures in order to enhance the reasoning capabilities towards answering user queries. The second phase involved the development of a Dynamic Approach- Based C-BIRD framework which is based on information extraction (lE) in order to dynamically create an SQL statement that answers user queries. lE component utilised a number of SQL query templates, which are made of relational database semantically understandable patterns such as table and column names. In addition, the dynamic approach used the conversational agent to disambiguate the dynamically generated SQL queries by confirming these queries with the user by means of SQL template specific strategies scripts. The Static Approach-Based C-BIRD prototype showed excellent results in terms of successfully mapping natural language conversations into SQL statements (i.e. task success, in which 5 tasks performed by 20 participants with an overall result of 91 %). The dynamic approach also showed very good results in terms of task success; 5 tasks performed by 20 participants with an overall result came to 74%. In summary, the proposed static approach C-BIRD framework offered a novel methodology to develop reliable conversational interfaces to relational databases in which engineered queries can be answered. In addition, the dynamic approach introduced a novel way to map natural language utterances into SQL statements and confirming the results with the user, before providing the final answer. Ultimately, the user experiences a real-time and friendly conversational interface with the relational.

Caching has been used in client-server database systems to improve the performance of applications. Much of the current work has concentrated on caching techniques at the server side, since the underlying assumption has been that clients are “thin” with application level processing taking place mainly at the server side. There are also a new class of “thick client” applications where clients need to access the database at the server but also perform substantial amount of processing at the client side; here client-side caching is needed to provide good performance for applications. This thesis presents a transactional cache consistency scheme suitable for systems with client-side caching. The scheme is based on the optimistic approach to concurrency control. The scheme provides serializability for committed transactions. This is in contrast to many modern systems that only provide the snapshot isolation property which is weaker than serializability. A novel feature is that the processing load for validating transactions at commit time is shared between clients and the database server, thereby reducing the load at the server. Read-only transactions can be validated at the client-side, without communicating with the server. Another feature is that the scheme permits disconnected operation, allowing clients with cached objects to work offline. The performance of the scheme is evaluated using simulation experiments. The experiments demonstrate that for mostly read only transaction load – for which caching is most effective - the scheme outperforms the existing concurrency control scheme with client-side caching considered to be the best, and matches the performance of the widely used scheme that only provides snapshot isolation. The results also show that the scheme in a disconnected environment provides reasonable performance.

This thesis explores the design of Crossmodal Display, a new kind of display-based interface that aims to help prevent information overload and support information presentation for multiple simultaneous people who share a physical space or situated interface but have different information needs and privacy concerns. By exploiting the human multimodal perception and utilizing the synergy of both existing public displays and personal displays, crossmodal displays avoid numerous drawbacks associated with previous approaches, including a reliance on tracking technologies, weak protection for user‟s privacy, small user capacity and high cognitive load demands. The review of the human multimodal perception in this thesis, especially multimodal integration and crossmodal interaction, has many design implications for the design of crossmodal displays and constitutes the foundation for our proposed conceptual model. Two types of crossmodal display prototype applications are developed: CROSSFLOW for indoor navigation and CROSSBOARD for information retrieval on high-density information display; both of these utilize coordinated crossmodal cues to guide multiple simultaneous users‟ attention to publicly visible information relevant to each user timely. Most of the results of single-user and multi-user lab studies on the prototype systems we developed in this research demonstrate the effectiveness and efficiency of crossmodal displays and validate several significant advantages over the previous solutions. However, the results also reveal that more detailed usability and user experience of crossmodal displays as well as the human perception of crossmodal cues should be investigated and improved. This thesis is the first exploration into the design of crossmodal displays. A set of design suggestions and a lifecycle model of crossmodal display development have been produced, and can be used by designers or other researchers who wish to develop crossmodal displays for their applications or integrate crossmodal cues in their interfaces.