Wireless networking offers great potentials for the development of new applications in real-time monitoring and control. However, current design processes do not simultaneously consider energy efficiency, system requirements, and standards compatibility. Modeling, optimization, and integration of communication and control protocols are essential to achieve efficient overall operations. We propose a holistic design framework, which includes physical channels, medium access control (MAC), multi-hop routing, and control applications. Accordingly, we provide the following contributions.
First, we investigate the performance of the IEEE 802.15.4 MAC through an accurate Markov chain model and its simplified representation. The effects of traffic load, number of devices, and MAC parameters on reliability, delay, and energy consumption are determined analytically and experimentally. We show that the delay distribution is different with respect to commonly used models in networked control systems design. Moreover, we introduce an adaptive mechanism to minimize the energy consumption while fulfilling reliability and delay constraints.
Second, we extend the analysis to multi-hop networks, including heterogeneous traffic distribution and limited carrier sensing range. Due to the contention-based channel access, routing decisions based on reliability or delay typically direct traffic toward nodes with high packet generation rates, leading to unbalanced performance and higher energy consumption. A load balancing metric is proposed for the IETF routing protocol for low-power and lossy networks. Furthermore, a mechanism to optimally select routes and MAC parameters is implemented.
Third, we include a realistic channel model in the analysis. Multi-path and shadowing are modeled by a Nakagami-lognormal distribution. A moment matching approximation is used to derive the statistics of aggregate signals. The impact of fading on MAC and routing is determined for various traffic regimes, distances among devices, and signal-to-(interference plus noise)-ratio settings. The results show that a certain level of fading actually improves the network performance.
Fourth, we propose TREnD, a cross-layer protocol that takes into account tunable application requirements. Duty cycling, data aggregation, and power control are employed to provide energy efficiency and an optimization problem is solved to select the protocol parameters adaptively. TREnD is implemented on a test-bed and it is compared to existing protocols. Experimental results show load balancing and adaptation for static and dynamic scenarios.
Finally, the analytical models developed in the thesis are formalized into a contract-based design framework. We consider a building automation example with a feedback control system over a heterogeneous network. We include the effects of delays and losses in the controller synthesis and we compare various robust control strategies. The use of contracts allows for a compositional design that handles performance, heterogeneity, and reconfigurability in a systematic and efficient way.
KTH Royal Institute of Technology, 2012. , ix, 195 p.