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Wireless sensor networks based on the IEEE 802.15.4 standard is able to achieve low-power transmissions in low-rate and short-distance wireless personal area networks (WPANs). It employs the slotted carrier sense multiple access with collision avoidance (CSMA/CA) in the contention access period (CAP). Unfortunately the periodic broadcast beacon frame is transmitted without CSMA/CA so that it could be continuously collided. This type of collision causes lost synchronization and unable to join a network because the beacon frame cannot be normally received. This paper proposes a simple time shift scheme (STSS) based on IEEE 802.15.4 to keep original superframe structure and distribute transmission of beacon frame over active period to avoid beacon frame collisions. We use a simple function to allocate beacon frame transmission in the active period. The simulation results demonstrate that the proposed scheme significantly reduces the beacon collision and lost synchronization rate, and it also improves the throughput.