Cross - Layer Optimization in Energy Constrained Networks

In this thesis we consider wireless networks with a hard energy constraint, where each node is powered by a small battery. Under this hard constraint, reducing energy consumption is the most important design consideration for such networks. Since all layers of the protocol stack contribute to the energy per bit consumed in its end-to-end transmission, energy minimization requires a joint design across all these layers as well as incorporating the underlying hardware characteristics. In this thesis, we show that a joint optimization of the hardware, link, Multiple Access (MAC), and routing designs is a beneficial and feasible approach to implement an energy efficient wireless network. We consider both the interference-free case (with TDMA-based orthogonal MAC) and the interference-limited case (with non-orthogonal MAC). For the first case, we start with a point-to-point link, where we show that dramatic energy savings is possible when the tradeoff between the transmission energy and the circuit processing energy is explored by optimizing the transmission time and the constellation size. The results tell us that for short-range applications, bursty transmissions are preferred to minimize total energy consumption. We then consider a multiple access scenario, where multiple sensor nodes are sending data to a central node. By jointly designing the MAC layer and the link layer relative to the hardware power consumption, we propose a variable-length TDMA scheme to minimize the total energy consumption. We then extend the joint optimization across the MAC and routing in addition to the hardware and link designs. We show that if link adaptation is not allowed, the energy minimization problem is a Linear Programming (LP) problem and can be efficiently solved. The solution tells us how to optimally route the traffic to minimize the total energy consumption across the network. If link adaptation is allowed, the total energy consumption can be further reduced while the optimization problem can be relaxed to a convex problem and efficient algorithms exist to obtain a near-optimal solution. For the interference-limited case with non-orthogonal channel usage, we decompose the cross-layer problem into two subproblems: (1) link scheduling; and (2) optimal rate adaptation and routing. The iteration between the two sub-problems will lead to an energy-efficient solution. We then investigate cooperation between nodes to further reduce energy consumption. In particular, we show that by allowing multiple nodes to cooperate, we can construct