It is generally considered that concurrent transmissions should be avoided in order to reduce collisions in wireless sensor networks. Constructive Interference (CI) envisions concurrent transmissions to positively interfere at the receiver. CI potentially allows orders of magnitude reductions in energy consumptions and improvements on link quality. In this paper, the authors theoretically introduce a sufficient condition to construct CI with IEEE 802.15.4 radio for the first time. Moreover, they propose Triggercast, a distributed middleware, and show it is feasible to generate CI in TMote Sky sensor nodes. To synchronize transmissions of multiple senders at the chip level, Triggercast effectively compensates propagation and radio processing delays, and has 95th percentile synchronization errors of at most 250ns.