Delay and Energy-Efficient STDMA for Grid Wireless Sensor Networks: ORCHID

In this article, we study the issue of delay optimization and energy efficiency in grid wireless sensor networks (WSNs). We focus on STDMA (Spatial reuse - Time Division Multiple Access) scheduling, where a predefined cycle is repeated, and where each node has fixed transmission opportunities during specific slots of the cycle (defined by colors). We assume a STDMA algorithm that takes advantage of the regularity of grid topology to also provide a spatially periodic coloring ("tiling" of the same color pattern). In this setting, the key challenges are: 1) minimizing the average routing delay by ordering the slots in the cycle 2) being energy efficient. The solution we propose is called ORCHID. It proceeds in two steps. In the first step, ORCHID starts from a colored grid and builds a hierarchical routing based on these colors. In the second step, ORCHID builds a color ordering by considering jointly both routing and scheduling so as to ensure that any node will reach a sink in a single STDMA cycle. Simulation results show the excellent performance of ORCHID in terms of delays and energy compared to shortest-delay path routing.