Grouping Tasks to Save Energy in a Cyclic Scheduling Problem: A Complexity Study.

This paper is motivated by the repetitive and periodic transmission of messages in Wireless Sensor Networks (WSNs) given by the ZigBee standard. In order to save energy, communication tasks are grouped in time. The WSN applications, such as control loops used in production lines, impose deadlines on the message delivery time. By introducing a grouping constraint, this paper extends the polynomial cyclic scheduling problem of building a periodic schedule with uniform precedence constraints and no resource constraints. We consider that each task belongs to one group, and each group is to be scheduled as a super-task in every period. We show two examples issued from different applications of such grouping constraints, using uniform constraints to model the deadlines expressed in the number of periods. We tackle the feasibility problem (the existence of a periodic schedule), which has shown to be independent of the processing times. We propose a graph formulation of the problem using the retiming technique. In the ZigBee case, we prove that the particular tree structure of the groups and of the uniform precedences based upon the communication tree, lead to a polynomial algorithm to solve the problem. But the general problem is proven to be NP-complete even if no additional resource constraints are considered, and unit processing times are assumed. This extension of the cyclic scheduling leads to a new range of grouping problems with applications, not only in communication networks but also in production and logistics scheduling.