  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, we theoretically
introduce a sufficient condition to construct CI with IEEE 802.15.4 radio for
the first time. Moreover, we 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 $95^{th}$
percentile synchronization errors of at most 250ns. Triggercast also
intelligently decides which co-senders to participate in simultaneous
transmissions, and aligns their transmission time to maximize the overall link
PRR, under the condition of maximal system robustness. Extensive experiments in
real testbeds reveal that Triggercast significantly improves PRR from 5% to 70%
with 7 concurrent senders. We also demonstrate that Triggercast provides on
average $1.3\times$ PRR performance gains, when integrated with existing data
forwarding protocols.
