Variable packet delays (VPDs) are the key limiting factor for achieving tight synchronization with Pulse-coupled Oscillators (PCO) schemes. In this paper, we argue that specific MAC frames should be designed to facilitate PCO synchronization in IEEE 802.15.4 systems. In particular, a customized PCO-MAC frame is proposed to reduce VPDs. We show that this customized MAC not only effectively reduces VPDs, but also improves the energy efficiency of the network as the radio on-off activity is reduced. As a proof of concept, two state-of-the-art PCO protocols are implemented using the proposed PCO-MAC. Experimental tests on networks of different sizes, using off-the-shelf Zolertia sensor nodes demonstrate its feasibility. Results show that a synchronization precision of a few clock ticks, comparable to existing non-PCO solutions for wireless sensor networks, can be achieved with PCO-MAC.
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