Cross-Layer Media Access Control in Wireless Body Area Networks Using Fuzzy Logic and Evolutionary Computation
Over the past decade, advances in electronics, computer science, and wireless technologies have facilitated the rapid development of Wireless Body Area Networks (WBANs). WBANs consist of various sensors that are attached on or even implanted in the human body to improve health care and the quality of life. WBANs must provide high-quality communication in terms of both reliability and performance, in order to bring timely medical help to patients. Commonly used communication standard in WBANs is IEEE 802.15.4. However, due to poor channel quality in WBANs, this standard is limited in reliability and performance. To address this issue, cross-layer techniques for Media Access Control (MAC) have attracted substantial research attention in recent years. Aimed at developing cross-layer MAC technologies, Fuzzy Logic Controllers (FLCs) have been widely utilised to effectively and efficiently process information from different layers in WBANs. However, existing FLCs have mostly focused on improving communication reliability while ignoring the importance of network performance. To improve both the reliability and performance of MAC protocols in WBANs, this thesis introduces a new design of cross-layer FLC, called Cross-Layer Fuzzy logic based Backoff system (CLFB), to improve reliability by reducing the collision rate and increasing the packet delivery ratio. CLFB can also enhance the network performance in terms of throughput in WBANs while maintaining packet delays at a reasonable level by considering both channel condition and application requirements. Through the proper use of FLCs as an extension of the standard exponential back-off algorithms, CLFB is experimentally shown to achieve a high level of adaptability. This thesis also proposes an evolutionary approach to automate the design of FLCs for CLFB in WBANs. With the goal of improving network reliability while keeping the communication delay at a low level, we have particularly studied the usefulness of three coding schemes with different levels of flexibility, which enables us to represent alternative design of FLCs as candidate solutions in evolutionary algorithms. The influence of fitness functions that measure the effectiveness of each possible FLC design has also been examined carefully in order to identify useful FLCs. Moreover, we have utilised surrogate models to improve the efficiency of the design process. In consideration of practical usefulness, we have further identified two main design targets. The first target is to design effective FLCs for a specific network configuration. The second target covers a wide range of network settings. In order to examine the usefulness of our design approach, we have utilised and experimentally evaluated two popularly used evolutionary algorithms, i.e. Particle Swarm Optimisation (PSO) and Differential Evolution (DE). This thesis finally proposes a two-level control scheme at both the sensor level and the coordinator level to further improve communication quality in WBANs. The sensor-level FLC controls contention based channel access and the coordinator-level FLC controls contention free channel access. This two-level FLC architecture can effectively enhance the cooperation between sensors and the coordinator such that both the reliability and performance of the network can be significantly improved. We also studied the use of cooperative coevolutionary approach to automate the design of our twolevel control scheme. With the goal of effectively designing useful FLCs, we have particularly investigated different collaborator selection methods for our cooperative coevolutionary approach, which enable us to effectively select collaborators while evaluating the candidate FLC design in each sub-population. Specifically, we show that network knowledge can help our evolutionary design approach to select collaborators more effectively.