Energy-efficient synchronization strategy: For each sensor, construct an internal clock as a periodic timer that evolves from 0 to 2π repeatedly and emits a UWB (ultra-wide bandwidth) monocycle pulse when its phase reaches 2π; A sensor stays in sleep mode when the phase of its internal clock is within the interval [0, D). When in the sleep mode, a sensor shuts down its transceiver but the phase of its timer still evolves freely towards 2π; For each sensor, when the phase of its internal timer exceeds D, it turns on its transceiver and switches to active mode in order to be able to receive pulses; For each sensor in active mode, upon receiving a UWB monocycle pulse, it modifies the phase of its internal clock according to the given coupling strength l and its delay-advance phase response function.